NewsNuclear Physics News is published on behalf of theNuclear Physics European Collaboration Committee(NuPECC), an Expert Committee of the EuropeanScience Foundation, with colleagues from Europe,America, and Asia.
Volume 17/No. 4
Vol. 17, No. 4, 2007, Nuclear Physics News 1
Editor: Gabriele-Elisabeth Körner
Editorial BoardT. Bressani, Torino S. Nagamiya, TsukubaR. F. Casten, Yale A. Shotter, VancouverP.-H. Heenen, Brussels (Chairman) H. Ströher, Jülich, JülichJ. Kvasil, Prague T. J. Symons, BerkeleyM. Leino, Jyväskylä C. Trautmann, Darmstadt
CorrespondentsArgentina: O. Civitaresse, La Plata; Australia: A. W. Thomas, Adelaide; Austria: H. Leeb, Vienna; Belgium:C. Angulo, Lauvain-la-Neuve; Brasil: M. Hussein, São Paulo; Bulgaria: D. Balabanski, Sofia; Canada: J.-M. Poutissou,TRIUMF; K, Sharma, Manitoba; C. Svensson, Guelph: China: W. Zhan, Lanzhou; Croatia: R. Caplar, Zagreb; CzechRepublic: J. Kvasil, Prague; Slovak Republic: P. Povinec, Bratislava; Denmark: K. Riisager, Århus; Finland: M. Leino,Jyväskylä; France: G. De France, GANIL Caen; M. MacCormick, IPN Orsay; Germany: K. Langanke, GSI Darmstadi;U. Wiedner, Bochum; Greece: E. Mavromatis, Athens; Hungary: B. M. Nyakó, Debrecen; India: D. K. Avasthi, NewDelhi; Israel: N. Auerbach, Tel Aviv; Italy: M. Ripani, Genova; L. Corradi, Legnaro; Japan: T. Motobayashi, RIKEN;Mexico: J. Hirsch, Mexico DF; Netherlands: G. Onderwater, KVI Groningen; T. Peitzmann, Utrecht; Norway: J. Vaagen,Bergen; Poland: B. Fornal, Cracow; Portugal: M. Fernanda Silva, Sacavém; Romania: V. Zamfir, Bucharest;Russia: Yu. Novikov, St. Petersburg; Serbia: S. Jokic, Belgrade; Spain: B. Rubio, Valencia; Sweden: P.-E. Tegner,Stockholm; Switzerland: K. Kirch, PSI Villigen; United Kingdom: P. Regan, Surrey; USA: D. Geesaman, Argonne;D. W. Higinbotham, Jefferson Lab; M. Thoenessen, Michigan State Univ.; H. G. Ritter, Lawrence Berkeley Laboratory;G. Miller, Seattle.
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Laboratory PortraitRadioactive Isotope Beam Factory at RIKEN (RIBF)
by Tohru Motobayashi and Yasushige Yano .................................................................................................. 5
Feature ArticlesAlpha Particle Condensation in Nuclear Systems
by Y. Funaki, H. Horiuchi, G. Röpke, P. Schuck, A. Tohsaki, and T. Yamada ............................................. 11
Mass Chain Evaluations for the Evaluated Nuclear Structure Data File (ENSDF)—An Urgent Appeal for European Participation
by F. G. Kondev, A. L. Nichols, and J. K. Tuli .............................................................................................. 19
Giant Resonance Overtones: Compression Modes of the Nucleus.by Mátyás Hunyadi and Muhsin N. Harakeh ................................................................................................ 24
Neutron Stars and Nuclei: Two Dense Systems.by M. Fallot, M. Grasso, E. Khan, and J. Margueron .................................................................................. 31
Impact and ApplicationHeavy-Ion Beam Pumped UV Laser
by Andreas Ulrich ......................................................................................................................................... 37
by Shoji Nagamiya, Ohru Motobayashit, and Makoto Oka .......................................................................... 40
Nuclear Physicists Meet in the Land of the Incasby Ricardo Alarcon........................................................................................................................................ 43
International Symposium on Physics of Unstable Nuclei (ISPUN07)by Dao Tien Khoa and Nguyen Van Giai ..................................................................................................... 45
Facilities and MethodNew Promises for the Determination of the Neutrino Mass? (A Brainstorming Meeting at GSI, Darmstadt)
by H.-Jürgen Kluge and Yuri Novikov ......................................................................................................... 48
News from EPS/NPB ........................................................................................................................................ 51
The views expressed here do not represent the views and policies of NuPECC except where explicitly identified.
The Canadian community ofnuclear and particle physicists has justpublished its vision for the nextdecade, outlining the present strengthsof and the future objectives for thenational research program (“Perspec-tive on Subatomic Physics in Canadafor 2006–2016”). In Canada, nuclearand particle physics are sponsored bya common program of NSERC calledsubatomic physics, and have a com-mon national laboratory, TRIUMF.The Long Range Planning Committee(LRPC) report takes an in-depth lookat the current set of activities and pro-poses bold recommendations for seiz-ing new scientific opportunities. Itthen describes the level of resourcesneeded to realize these objectives. Tocapture the most benefit, the studyconcluded, the level of support wouldneed to double over the course of (N)years. This situation is quite similar tothat in other countries, indicating ahealthy global program in nuclearphysics. A good number of othernations are making significant com-mitments to new facilities as well. Inthis article, I will be focusing on thenuclear physics elements.
The Canadian report looks particu-larly carefully at the development ofgraduate students in subatomic phys-ics. More Canadian students arechoosing careers in subatomic phys-ics! The study shows a strong growth(45%) in the number of graduate stu-dents in the period of the survey(2001–2005). The students are aboutequally split between the two sub-fields. This increase can be explainedby a large faculty renewal (35%) due
to retirement (young faculty tend to bemore research and graduate studenthungry), some special Canadian pro-grams to attract high profile facultyinto the country (such as CanadaResearch Chairs), and capital invest-ments in new facilities such as ISACat TRIUMF, SNO, and SNOlab. TheLRPC also examines the global stand-ing of the Canadian facilities. It findsthat these facilities are highly compet-itive and world-leading. Based onthese observations, the report makesfive recommendations to optimize thescientific return for Canada. The onerelevant for this audience: Full exploi-tation of the high intensity radioactivebeams for nuclear physics and nuclearastrophysics at ISAC and ISAC-II atTRIUMF.
Again, this projects a very positiveoutlook into the future. We here inCanada are extremely fortunate withthe facilities and experimental deviceswe have. Things become a little moredifficult when one looks into whatresources will be available to operatethe facilities, to build the upgrades fornew and more radioactive beams atTRIUMF, which one needs to opti-mally utilize the world-class facili-ties, and to pay for researchers and theaforementioned students, who areflocking in ever-increasing numbers,into our nuclear physics labs. In thecase of Canada, the LRPC investi-gated different scenarios. The findingsof the committee are very clear; if wewant to exploit the already-maderecent major capital investments in thehighest priority projects, substantialnew funds must be allocated. How can
this be achieved, and what is the rolethat we as individuals in Canada butalso the worldwide nuclear physicscommunity can play? The fact is,these are not unique problems toCanada.
On a recent visit from a delegationof the German Ministry of Researchand Education, the State Secretarywho is in charge of evaluating theavailable budget for nuclear physicsand facilities in Germany, asked me ifit is more sensible to have ten Ferraricars parked in the garage (note, Ferrarirepresents a state-of-the-art Formula 1race car, not the luxury item!), withlittle money for gas, or whether oneshould rather buy nine and save theextra money to drive the others more.The answer is probably: Buy ten andlook for extra money elsewhere. Andthis is where the worldwide commu-nity comes in. The European countrieshave since long worked together tocome up with coherent plans for all itscountries, but now one would hope,we can go a step further and try tobring even more communitiestogether—the Americas, Asia, andEurope. We need more overlookingplanning or communication to firstlycome up with sufficient argumentswhy we want to extend certain fieldsor facilities, and why this in not doneelsewhere, but secondly strengthenthese by providing arguments comingfrom people outside the country whosay they are interested and willing tocontribute. Planning and communica-tion on a global scale is more widelyspread in the high energy physicscommunity, and everything points
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2
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4 Nuclear Physics News, Vol. 17, No. 4, 2007
toward the fact that we are enteringthis phase in nuclear physics as well.We have to make sure not everybodyis buying the same ten Ferraris andthen looking elsewhere for help. Therehas to be a balance between competi-tion and complementarities.
Currently, TRIUMF, Canada’sNational Laboratory for Particle andNuclear Physics, is preparing its nextfive-year plan. TRIUMF operates infive-year cycles and the present oneends in 2010. For the new plan, majorupgrades and new facilities are beingconsidered and we will again face thesame dilemma: balancing new capitalinvestments—which one needs to staycompetitive—against support for theoperation of existing facilities—whichone needs to realize the full scientificpotential of these world-leading devices.TRIUMF is approaching these ques-tions in a truly global fashion by open-ing dialogues with other countries.
For example, TRIUMF is hosting ajoint workshop with the Oak RidgeNational Laboratory at the U.S. APSDNP meeting, to evaluate commongoals and strategies for an electron-driven photo-fission facility at ISAC.
In another example, TRIUMF ishosting a workshop in the spring of2008 to look at joint ventures withGermany’s GSI/FAIR and possibleCanadian participation through TRI-UMF’s next five-year plan. These areexamples of ad-hoc strategies fromone country (Canada), and certainlyother nations do similar things. How-ever, a more orchestrated approach isbound to be even more successful.The recent INPC in Tokyo was anideal occasion to bring together theworldwide community to initiate suchplanning. We should follow thatexample with INPC2010 in Vancou-ver, but my hope is that we will notwait that long: more global communi-cation and planning is possible rightnow! The IUPAP Working Group onInternational Cooperation in NuclearPhysics (ICNP) is an internationalbody with the goal of coordinating theglobal program. Also, as for globalplanning, the OECD, as another globalbody, recommends regional planning,although it is still not happening. Forexample, the U.S. and Canada couldget together on ISOL-based and frag-mentation-based isotopes production,
and develop a coherent plan. I thinkwe should all support these efforts.Moreover, we as individuals have towork toward this end within our owncountries to encourage our decisionmakers to enter the dialogue withother countries and to start regionalplanning (and eventually global plan-ning), at least for the next generationof facilities. Nuclear science is pres-ently in an excellent position on a glo-bal scale, but real work, for keeping itthat way, lies ahead of us.
JENS DILLING
TRIUMF Vancouver
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Vol. 17, No. 4, 2007, Nuclear Physics News 5
Radioactive Isotope Beam Factory at RIKEN (RIBF)
Introduction The RIKEN RI Beam Factory
(RIBF) has started its operation afterten years of construction [1]. It isdesigned to provide beams of variouskinds of unstable nuclei with theworld’s highest intensities. The insti-tute RIKEN was founded in 1917, andcovers a variety of research fields suchas physics, chemistry, medical sci-ence, biology, and engineering.RIKEN has a long history of con-structing accelerators. The first cyclo-tron in Japan was built in 1937 byYoshio Nishina in the former RIKENcampus, and the RIKEN’s 4th cyclo-tron completed in 1966 was the firstheavy-ion cyclotron in Japan, whichaccelerated light nuclei to the energyaround 7 MeV/nucleon.
Since 1987, RIKEN has providedintermediate-energy light- and heavy-ion beams by a four-sector ring cyclo-tron (RIKEN Ring Cyclotron, RRC)with K = 540 MeV coupled with twoinjectors, an azimuthally varying field(AVF) cyclotron with K = 78 MeV anda 16 MV variable-frequency linearaccelerator (RILAC). The maximumenergy for light heavy-ions such as16O is 135 MeV/nucleon. Vector- andtensor-polarized deuteron beams arealso available. To provide the beammatched to requirements from thenew RIBF accelerators, several impro-vements have been made. For exam-ple, a variable-frequency RFQ linac(FCRFQ) [2] and an 8-MV fixed-fre-quency booster linac (CSM) [3] wereinstalled before and after the RILAC.Thus, intense ions at several MeV/nucleon energies became available in
the RILAC experimental hall. In 2004,two events indicating the productionof the isotope 278113 by the 209Bi(70Zn,n) fusion reaction have been observed[4]. The experiment was performedwith an intense beam of 5 MeV/nucleon Zn from the RILAC. Thebeams have been used also for variousapplications to nuclear chemistry, bioand medical science, and materialsscience. Among them, production oflight-mass RI-beams by a projectile-fragment separator (RIPS [5]) is oneof the characteristic features of thefacility.
The new facility RIBF uses theaccelerators RILAC and RRC, andsuccessively boosts the beam energyup to 345 MeV/nucleon by threenewly-built cyclotrons, the fixed-fre-quency Ring Cyclotron (fRC) withK = 570 MeV, Intermediate stage RingCyclotron (IRC) with K = 980 MeV,
and Superconducting Ring Cyclotron(SRC) with K = 2600 MeV. The firstprimary beam, 345MeV/nucleon 27Al10+,was extracted on December 28, 2006.The first experimental result, produc-tion of a new neutron-rich isotope125Pd, was obtained in May 2007 with238U beams. The RIBF is located in theRIKEN Wako Campus in Wako-shi,Saitama, Japan (see Figure 1). A sche-matic view of the facility is shown inFigure 2.
RI Beams at RIKEN At RIKEN, RI beams have been pro-duced by the projectile fragmentationscheme since 1990. Due to the largeangular momentum acceptance andhigh bending power of the separatorRIPS [5], the RI beam intensities arehighest in the world for many lightneutron-rich nuclei. By using this
1The word “RI beam” is an abbreviation ofRadioactive Isotope beam.
Figure 1. View of the RIKEN Wako Campus. The picture was taken in May2005, when the RIBF was under construction.
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6 Nuclear Physics News, Vol. 17, No. 4, 2007
high-intensity capability, the veryneutron-rich fluorine isotope 31F wasfound to be particle-stable [6]. Besidesthese intermediate-energy RI beams,beams of light unstable nuclei at lowerenergies, typically 5 MeV/nucleon, arealso available in the CRIB facilityconstructed by CNS [7]. The CRIBuses in-flight direct reactions, such as(p,n) and (3He,n), to produce RIbeams, which are mainly used forstudying low-energy reactions ofastrophysical interest.
Studies Using the Fast RI Beam The RI beams from the RIPS have
been used for various experiments.Following the pioneering works atLBL in the 1980s [8], studies of inter-action cross-section have been per-formed in RI-beam facilities includingRIKEN and GSI, and neutron halo andneutron skin structures have beenestablished in some light neutron-richnuclei [9]. Certain neutrons haveextended spatial distribution outsideof the core where neutrons and pro-tons are equally distributed. This is thefirst indication that some neutrons canbe decoupled from protons in spite ofthe strong p–n interaction.
Disappearance of magic numbersis another interesting phenomena. The
first Coulomb excitation experimentwith fast RI beams has been per-formed for the neutron-rich 32Mgnucleus with the N = 20 magic number[10]. The extracted large B(E2) valuesupports the idea of disappearance ofthe N = 20 shell closure in 32Mg. Anew method of γ-ray spectroscopy,measurement of γ-rays from fast-moving excited nuclei in coincidence
with reaction products with particleidentification, was applied to thisexperiment, and many experimentsusing this technique have been per-formed so far for studying nuclearstructures of nuclei around the shellclosure at N = 8 and N = 20.
Recently, the decoupling of pro-tons and neutrons has been revealedalso in excitation of the 16C nucleus toits 2+ state. The 2+ lifetime measuredin a new recoil shadow method [11],inelastic scattering with 1H [12], andthe one with 208Pb [13] all point to ananomaly: Neutrons almost solely con-tribute to the 2+ state excitation whereasprotons have little contribution. Thispicture might be related to the lowelectric quadrupole moments mea-sured for 15B and 17B using theβ-NMR technique, which require littleneutron effective charge in a shell-model calculation [14].
Studies of particle-unbound statesare another highlight. The Coulomb
Figure 2. Bird’s-eye view of the RIBF.
Figure 3. Superconducting Ring Cyclotron (SRC).
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Vol. 17, No. 4, 2007, Nuclear Physics News 7
dissociation to simulate astrophysical(p,γ) reactions [15] and the neutronhalo structure [16] has been exten-sively studied.
These are examples of the studiesat RIKEN using the fast RI beams.Encouraged by their success, theRIBF project has been launched toextend the research opportunities pro-vided by the use of RI beams.
SRC, Big RIPS, and Zero Degree Spectrometer
The part of RIBF that has beencompleted is the cyclotron complexand the superconducting RI beamseparator Big RIPS [17] (see Figure 2).The Zero Degree spectrometer will becompleted in fall 2007. Figure 3shows the picture of the Supercon-ducting Ring Cyclotron (SRC), whichis the world first superconductingring cyclotron with the largest bend-ing power. It consists of six sector
magnets and four RF cavities. It hassuperconducting main- and trim-coils.The valley regions between the sectormagnets are covered with 1-m thicksoft iron slabs in order to reduce the straymagnetic-field that otherwise deflectsthe beam to the opposite direction.The total weight of the SRC amounts
to 8300 tons. The mean injection andextraction radii are 3.56 m and 5.36 m,respectively. The SRC can boost theenergy of output beams from the IRCup to 440 MeV/nucleon for light ionsand 345 MeV/nucleon for ions up touranium.
The primary beams are convertedto intense RI beams by the BigRIPS(see Figure 4 for the picture) with thehelp of in-flight fission and/or projec-tile fragmentation of heavy-ionsincluding uranium. A plan view of theBigRIPS is shown in Figure 5. TheBigRIPS consists of fourteen super-conducting quadrupole triplets and sixroom-temperature dipoles. It employsa two-stage separation scheme. Thefirst stage serves to produce and sepa-rate RI beams with a wedge-shapeddegrader inserted at the momentum-dispersive focus F1. The second-stageidentifies RI beam species event-by-event and tags the secondary beamthat still contains various differentions. The horizontal and verticalangular acceptances are designedrespectively to 80 mrad 100 mrad ver-tically, while the momentum accep-tance is 6%. These angular andmomentum acceptances enable oneto collect about half of the fission
Figure 4. Superconducting RI beam separator (BigRIPS).
Figure 5. Plan view of the Big RIPS and Zero Degree spectrometer togetherwith the two cyclotrons IRC and SRC of the RIBF accelerate complex.
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8 Nuclear Physics News, Vol. 17, No. 4, 2007
fragments produced by a 350 MeV/nucleon uranium beam.
The Zero Degree spectrometer(ZDS), also shown in Figure 5, ana-lyzes secondary reaction productsemitted in the beam direction. Itstypical use is for γ-ray measure-ments in coincidence with fast-mov-ing excited nuclei discussed earlier.The ZDS is used to identify thereaction product. For other applica-tions, such as β-decay measure-ments with stopped RIs, the ZDS isalso useful.
The RI beam yield estimated bythe code EPAX2 [18] with a primarybeam of 1 particle μA intensity and350 MeV/nucleon energy is illus-trated in Figure 6. For example, theintensities of the doubly magicnuclei 78Ni, 132Sn, and 100Sn are exp-ected to be 10, 100, and 1 particles/s,respectively, encouraging detailedstudies of nuclei far from the stabil-ity valley.
Status of RIBF As mentioned, after the first
extraction of the beam, the accelera-
tors have been tuned to improve theirperformance, and 238U ions were suc-cessfully accelerated in March 2007.Commissioning of RI beam produc-tion started in March and the firstattempt of new-isotope productionwith uranium beams was performed.A 345 MeV/nucleon 238U86+ beam wasdelivered to a beryllium productiontarget of 7-mm thick. The parametersof the BigRIPS were matched to neu-tron-rich fission products with theatomic number around 50. Figure 7shows a correlation plot for theatomic number Z and the mass-to-charge ratio A/Q. These quantities areobtained from the energy-loss, totalenergy, velocity (or time-of-flight)and magnetic rigidity measured bybeam-line counters with the help oftrack-reconstruction. The yield forZ = 46 (palladium) is plotted inFigure 8. Observed double peakstructures are due to the mixture ofions not fully stripped. A peak corre-sponding to the new isotope 125Pd,
Figure 6. Nuclear chart covered by the RIBF project. The thick solid curvesindicate the limit of RI productions of 1 particle per day. An expected r-processpath is also shown.
Z
A/Q
Z = 46
Figure 7. Correlation plot for the atomic number Z and the mass-to-charge ratioA/Q of fission products produced by the 238U+9Be interaction at 345MeV/nucleon.
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Vol. 17, No. 4, 2007, Nuclear Physics News 9
indicated by the arrow, is clearly seenwith 26 counts. The data were takenwith the beam current of 4 × 107 par-ticle/s on an average, which is about10−5 of the goal intensity, 1 particleμA, and the data acquisition time isabout one-day. This indicates highpotential of the Big RIPS separatorand future possibility of the RIBF inaccessing a large amount of unknownunstable nuclei.
Because the RI beam intensitiesachieved so far is not enough for sec-ondary reaction studies, variousefforts are ongoing including theimprovement of the beam transmis-sion in every stage of the acceleratorcomplex. The use of the 48Ca and 86Krbeams as well as 238U is currentlyassumed in the first series of experi-ments. Information on the RIBF statusincluding the call for proposal is foundon the Web page http://www.nish-ina.riken.jp/UsersGuide/.
Major Experimental Installations To fully exploit the research oppor-
tunity provided by the variety of RIbeams from the RIBF, several programsto construct experimental equipment arebeing considered or in progress.
The SHARAQ spectrometer [19] isa high-resolution spectrometer for miss-ing-mass measurement with RI beams.It is of a QQ-D-Q-D configuration. Byemploying dispersion-matching opticswith a specially designed beam linefrom the BigRIPS, the momentum reso-lution of 15,000 will be achieved. TheSHARAQ spectrometer is under con-struction and will be installed in 2007.
The SLOWRI [20] aims at con-ducting various experiments usingslow or trapped RIs. RI beams fromthe Big RIPS will be efficientlystopped and extracted by a gas-catchersystem with the RF ion-guidetechnique.
The SAMURAI is a spectrometer[21] with a large solid angle and alarge momentum acceptance dedi-cated to particle-correlation studies. Itis of a QQQ-D configuration, wherethe dipole D is a superconducting H-type magnet with 6.7 Tm rigidity. Itslarge gap (80 cm) is useful formeasurements of projectile-rapidityneutrons.
The e-RI ring is for electron-RIscattering experiments using a Self-Confining Radioactive Ion Target(SCRIT [22]). RI ions are transverselyconfined due to the attractive forcecaused by the electron beam itself. Amirror potential is applied externallyto achieve longitudinal confine-ment. Test experiments to examinethe confinement mechanism are ongo-ing at an existing electron ring.
The rare RI mass ring [23] isdesigned to measure the mass of rare(with the production rate of 1 particleper day, for example) exotic nuclides
in 10−6 accuracy. Each ion is injectedindividually to the ring by a triggersignal provided from a counter in theBig RIPS. The ring is precisely tunedto achieve the isochronous condition,and a time-of-flight of the ion in thering is measured.
To allow for running an RI-beambased experiment and a super heavyelement search simultaneously, con-struction of a new injector [24] isbeing considered. Another possibilityto extend the research opportunity isto build a beam line that brings backthe IRC beam to the RIPS separator.Various nuclear moment studies andcondensed matter research areplanned.
Summary The RIKEN RI Beam Factory
(RIBF), one of the new-generation RIbeam facilities, has started its operation.Potential of RI-beam production by theRIBF cyclotron complex coupled with
Figure 8. Yield distribution obtain by selecting the events in Figure 7 by a“Z= 46” gate. The arrow indicates the peak position for the new isotope 125Pd.
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10 Nuclear Physics News, Vol. 17, No. 4, 2007
the superconducting RI beam separatorBig RIPS was demonstrated by produc-tion of the new isotope 125Pd. When theRIBF reaches to its full performance, itprovides a fascinating opportunity toartificially produce and experimentallystudy almost all nuclides that have beencreated and are being created now in theuniverse. Continuous attempts toimprove the primary beam intensitytogether with construction of variousexperimental equipment will open anew domain of nuclear physics, andhopefully create a new view on atomicnucleus as well as on the element gene-sis in the universe, incorporated with theworld efforts for realizing RI-beamfacilities being constructed or planned.
References 1. Y. Yano, Nucl. Instr. Meth. B 261
(2007) 1009.
2. O. Kamigaito et al., Rev. Sci. Instr. 76(2005) 0133061-1.
3. O. Kamigaito et al., Rev. Sci. Instr. 70(1999) 4523.
4. K. Morita et al., J. Phys. Soc. Jpn. 73(2004) 2593.
5. T. Kubo et al., Nucl. Instr. Meth. B 70(1992) 309.
6. H. Sakurai et al., Phys. Lett. B 448(1999) 180.
7. Y. Yanagisawa, et al., Nucl. Instr.Meth. A 539 (2005) 74.
8. I. Tanihata et al., Phys. Rev. Lett. 55(1985) 2676.
9. T. Suzuki et al., Phys. Rev. Lett. 75(1995) 3241.
10. T. Motobayashi et al., Phys. Lett. B346 (1995) 9.
11. N. Imai et al., Phys. Rev. Lett. 92(2004) 062501.
12. H.J. Ong et al., Phys. Rev. C 73 (2006)024610.
13. Z. Elekes et al., Phys. Lett. B 686(2004) 34.
14. H. Ueno et al., Nucl. Phys. A 738, 211(2004).
15. for example, T. Motobayashi, Nucl.Phys. A 693 (2001) 258.
16. for example, T. Nakamura et al., Phys.Rev. Lett. 96 (2006) 252502.
17. T. Kubo et al., IEEE Trans. Appl.Superconductivity 17 (2007) 1069.
18. K. Suemmerer and B. Blank, Phys.Rev. C 61 (2000) 034607.
19. T. Uesaka et al., CNS Annual Report2004 (2005) 42.
20. M. Wada et al., Nucl. Instr. and Meth.A 532 (2004) 40.
21. Y. Sasamoto et al., CNS Annual Report2004 (2005) 85.
22. M. Wakasugi et al., Nucl. Instr. andMeth. A 532 (2004) 216.
23. Y. Yamaguchi et al., CNS AnnualReport 2004 (2005) 83.
24. O. Kamigaito et al., RIKEN Accel.Prog. Rep. 39 (2005) 261.
TOHRU MOTOBAYASHI AND
YASUSHIGE YANO
RIKEN Nishina Center
feature article
Vol. 17, No. 4, 2007, Nuclear Physics News 11
Alpha Particle Condensation in Nuclear Systems
Y. FUNAKI1, H. HORIUCHI
2, G. RÖPKE3, P. SCHUCK
4,5, A. TOHSAKI2, AND T. YAMADA
6 1The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-0198, Japan 2Research Center for Nuclear Physics (RCNP), Osaka University, Ibaraki, Osaka 567-0047, Japan 3Institut für Physik, Universität Rostock, D-18051 Rostock, Germany 4Institut de Physique Nucléaire, CNRS, UMR 8608, Orsay, F-91406, France 5Université Paris-Sud, Orsay, F-91505, France 6Laboratory of Physics, Kanto Gakuin University, Yokohama 236-8501, Japan
Introduction Quantum condensation of particles is one of the most
amazing phenomena exhibited by many-body systems.Familiar yet striking examples known for many decadesinclude superconductivity of metals at low temperaturesand superfluidity of liquid 4He. More recently the realiza-tion of Bose-Einstein condensation of ultracold atoms intraps has created an exciting new field of quantum many-body physics. Also, atomic nuclei and neutron stars canexperience quantum condensation of fermion pairs and dis-play superfluid properties. However, in nuclear physics themost tightly bound light cluster is not a pair but a quartet,namely the alpha particle. Can we then expect α-particlecondensation in nuclei? Before pursuing to this question,let us make some general remarks. It is a fact that quartet-ting is more pronounced in nuclei than in most other Fermisystems. And the dominance of quartetting can be traced tothe fact that nucleons can exist in four different internalstates: proton and neutron, each with spin up or down, allattracting each other. Therefore, a shell model picture inwhich the α-particle is the first doubly magic nucleus, witha filled 0S–level, is valid (see Fig. 2 below). Moreover, theα-particle is especially stiff, with its first excited state lyingquite high, at ~ 20 MeV. The search is now on for quartetsin systems other than nuclei. There has long been talk aboutbi-excitons [1], but with the rapid development of coldatom physics [2], one can hope that soon four different spe-cies of fermions will be trapped, giving rise to quartettingof atoms. Several theoretical papers and proposals alongthis line have already appeared [3]. Quartetting is likely toemerge as an important topic in quantum many-bodyphysics.
Let us return now to the nuclear case. The only nucleushaving a pronounced quartet-cluster structure in its groundstate is 8Be. In Fig. 1(a) we show the result of an exact cal-culation of the density distribution in the laboratory frame
based on a realistic N-N interaction, with the distribution inthe intrinsic, deformed frame shown in Fig. 1(b). Weobserve that the α-structure is very pronounced and leads tothe very low average density r ~ r0/3 apparent in Fig. 1(a),r0 = 0.17 fm−3 is the nuclear saturation density. The nucleus8Be is very large with an rms radius of ~3.7 fm, to be com-pared with the value R = r0 A
1/3 ~ 2.44 fm given by nuclearsystematics.
The nucleus 8Be is definitely unusual. One may askwhat happens when a third a-particle is brought alongside8Be. We know the answer: the 3-a system collapses to theground state of 12C which, within its small radius of 2.4 fm,is much denser than 8Be and cannot accommodate threea-particles just touching one another. Like virtually allother nuclei, 12C in its ground state is essentially a Fermi-gas,
Figure 1. Contours of constant density (taken from [4]) for8Be(0+). The left side (a) is in the laboratory frame whilethe right side (b) is in the intrinsic deformed frame.
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12 Nuclear Physics News, Vol. 17, No. 4, 2007
describable in mean-field approximation with effectiveforces. This does not exclude the presence of α-like corre-lations, but they would produce only a small correction. Onthe other hand, one may ask whether the dilute three aconfiguration 8Be-a or a−a−a can form an isomeric orexcited state of 12C. Continuing in this way, one could thinkof adding a fourth a-particle, a fifth, and so on. Theseprospects will be the main subject of the followingconsiderations.
a-Condensate States in Self-Conjugate 4n-Nuclei We will now show that α-particle condensation most
likely occurs in nα nuclei at energies near the n α-particlebreak-up threshold. Ample experimental data will allow usto identify at least one nucleus, namely 12C, in which suchan α-particle condensed state exists. We then discuss thelikelihood and the indications that such states are very natu-rally also present in other nuclei, particularly 16O. The phe-nomenon may in fact be quite general in nuclear systems[5]. We begin with strong arguments that the 0+
2 state at7.654 MeV in 12C is, indeed, a state of α-particle conden-sate nature.
First, it should be pointed out that the 0+2 state in
12C, like the ground state of 8Be, is actually particle-unstable, being situated about 400 keV above the threeα-break up threshold. This state only is stabilized bythe Coulomb barrier. It has a width of 8.7 eV and a cor-responding lifetime of 7.6 × 10−17 s. As is well known,this state is of great astrophysical and biological impor-tance owing to its crucial role in the synthesis of the12C present in the universe. Partly on anthropicgrounds, the astrophysicist Fred Hoyle predicted theexistence of this state in 1953. He argued that the highabundance of 12C in the universe, essential for life, can
only be explained if the triple alpha reaction α + α + α→ 8Be + α → 12C* is strongly accelerated by the pres-ence of a resonance state in 12C at the right energy [6].Hoyle deduced the position of this state very preciselyfrom thermodynamic equilibrium considerations, and itwas discovered experimentally three years later byWilly Fowler and his collaborators [7]. Now wellknown as the Hoyle state, it is notoriously difficult todescribe and reproduce within conventional nuclearstructure theory (see B. Barrett et al. [8]). For example,the most modern no-core shell-model calculations pre-dict the 0+
2 state in 12C to occur at around 17 MeV,more than twice the actual excitation energy [8]. Thisfact alone tells us that the Hoyle state must have a veryunusual structure. Should this state indeed consist ofthree loosely bound a-particles, one can easily under-stand that a shell-model approach would have great dif-ficulties in explaining its properties. The firsttheoretical idea on the structure of the Hoyle state thatwas widely discussed in the community came fromMorinaga. He postulated the “linear chain state” inwhich three α-particles are lined up one after the other[9]. At a later stage, other authors pointed out that thestate may instead be a loosely bound configuration ofthree α-particles or a two-alpha 8Be nucleus with onemore alpha orbiting around it. Hackenbroich et al. [10]and Horiuchi et al. [11] were forerunners in advocatingthis picture. In fact, the latter group was able to make a
0s
b
B
0S
Figure 2. Pictorial representation of the THSR wavefunction for n= 3 ( 12C). The three a-particles are trappedin the 0S-state of a wide harmonic oscillator (B) and thefour nucleons of each a are confined in the 0s-state of anarrow one (b). All nucleons are antisymmetrised.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Ground state Hoyle state
S1D1G1S2D2G2S3D3G3
Figure 3. Occupation probabilities of the k-th a-orbitswith S, D and G waves, which are denoted by Lk for theL-waves, for the ground and Hoyle states of 12C obtainedby diagonalizing the density matrix r(R, R¢).
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Vol. 17, No. 4, 2007, Nuclear Physics News 13
quite precise prediction of the position of the 0+2 state,
using the so-called Orthogonality Condition Model(OCM) [12]. Then, in the mid-seventies, two veryimportant works on the alpha-cluster structure of 12Cappeared. The Japanese physicists, M. Kamimura [13]and K. Uegaki [14], together with their collaborators,independently and almost simultaneously reproducedthe Hoyle state within microscopic theory, i.e., employ-ing a twelve-nucleon wave function together with aHamiltonian containing an effective nucleon-nucleoninteraction. At the time, these remarkable works didnot attract the widespread attention they deserved.Only recently the significance of the achievement of theKamimura and Uegaki groups is being appreciated morewidely. The two groups started from practically the sameansatz for the 12C wave function, having the followingthree a-cluster structure: .Here, A is the antisymmetriser acting on all nucleonsand fi is an intrinsic a-particle wave function of pre-scribed Gaussian form, i.e.
where the size parameter b is adjusted to fit the rms value ofthe free a-particle radius. The factor χ(R, s) is the three-body wave function for the center of mass motion of thethree a’s, with R and s the corresponding Jacobi coordi-nates. This function was determined using both a Generator
Coordinate Method (GCM) [14] and Resonating GroupMethod (RGM) [13], assuming Volkov I and Volkov IInucleon-nucleon forces. Thirty years back, the precise solu-tion of this complicated three-body problem was truly pio-neering work. The position of the Hoyle state as well asother experimental properties including inelastic formfactor and the transition probability were successfullyreproduced. Other states of 12C below and around theenergy of the Hoyle state were also well described. Impor-tantly, it was recognized that the three α’s in the Hoylestate form an α-gas like state, a feature which had alreadybeen pointed out by H. Horiuchi [11] prior to the works of[13] and [14]. All of these authors concluded from theirstudies that the linear chain state of three α-particles had tobe discarded as a description of the Hoyle state. The treat-ments of [13] and [14] were later extended to reaction the-ory in [15]. It should also be mentioned that a very recenteffort [16] has again reproduced all the features of theHoyle state, starting from a realistic bare N-N force.
Although several authors of the early papers stressedthe resemblance of the Hoyle state to an alpha-particlegas, two key aspects were not recognized at the time. Thefirst and fundamental aspect is that since, all three α’smove with their c.o.m. in the same S-wave orbit, one isdealing with an α-condensate state (albeit not in the mac-roscopic sense) and that α-particle condensation may be aquite general phenomenon in nuclear physics. The secondand practical aspect is that the complicated three bodywave function χ(R, s) can be replaced by a structurallyand conceptually very simple microscopic three α wavefunction of the condensate type which has practically 100percent overlap with the wave functions constructed pre-viously [17].
To establish and explain the latter feature, it isinstructive to exploit an analogy with the Cooper-pairBCS wave function of ordinary pairing. The componentof this wave function with given particle number can bewritten,
where f(r1, r2) is the Cooper-pair wave function, includingspin and isospin, to be determined variationally by the wellknown BCS equations. As before, A is the antisymmetriser.The condensate character of the BCS ansatz is born out bythe fact that we have a product of N/2 times the same pairwave function f. Formally it now is a simple matter to
⟨ ⟩ =r r R s1 12L | [( , )121 2 3C A φ φ φ
φ( , , ) exp ( ) ( )r r r r1 2 1 32 24
1 8− − = − −⎡⎣⎢
⎤⎦⎥>∑L b r rm nm n
⟨ ⟩ = −r rN1L | [ ( , ) ( , ) ( , )]BCS r rN NA φ φ φr r r r1 2 3 4 1
Figure 4. Momentum distribution r(k) (a) and k2 r(k) (b) ofthe a-particle for the 0+1 (black line) and 0+2 (grey line)states.
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14 Nuclear Physics News, Vol. 17, No. 4, 2007
generalize the pair-condensed state |BCS⟩ to α-particle con-densation. We write
where φα is the wave function common to all α-particles.Of course, finding the variational solution for φα (r1, r2, r3,r4) from (1) is considerably more complicated than findingthe Cooper pair wave function. However, in the case of theα-particle and more generally for quartets and for relativelysmall clusters, the complexity of the problem can bereduced dramatically. As already recognized in the earlyworks [13] and [14], an excellent variational ansatz is pro-vided by an intrinsic wave function of the α-particle ofGaussian form, with only the size parameter b to be deter-mined. In addition, and here lies the essential and crucialaspect of our wave function, even the center-of-massmotion of the various α-particles can very well bedescribed by a Gaussian wave function. This introduces anadditional size parameter B, with B >> b, to account for themotion over the whole nucleus. We therefore write
where R = (r1 + r2 + r3 + r4)/4 is the c.o.m. coordinate of oneα-particle and φ(r1 − r2, . . .) is the same intrinsic α-particle
wave function of Gaussian form as used in [13], [14] andwritten out above. Of course, in (1) the c.o.m. coordinateXcm of all α’s, i.e. of the whole nucleus, should also be elim-inated. This is easily achieved by replacing R by R−Xcm ineach of the α wave functions in (1), utilizing a helpful prop-erty of Gaussian functions [18]. The α-particle condensatewave function (1) with (2), as proposed and applied to 12Cand 16O in [19] (see also Ref. [10]) and henceforth calledTHSR-wave function, now depends only on two parame-ters, B and b. The expectation value of the microscopicHamiltonian
can be evaluated, and the corresponding two-dimensionalenergy surface [20] can be quantized using the two parame-ters B and b as Hill-Wheeler coordinates [21].
The Hamiltonian was taken to be that of Ref. [22],containing an effective nucleon-nucleon force of theGogny type, with parameters adjusted to fit α − α scatter-ing phase shifts. Our theory is therefore free of adjustableparameters.
Before presenting the results, we should note thatthe THSR wave function reproduces two important lim-its exactly: for B = b, it is a pure Slater determinant[23], and for B >> b, the density of α-particles is so lowthat the antisymmetriser in front of (1) can beneglected. In the latter case, Eq. (1) becomes an idealBose condensate of α-particles, i.e. a pure product state(see Fig. 2).
Results for Finite Nuclei As already pointed out, the THSR wave function con-
structed from the Hill-Wheeler equation based on Eqs. (1),(2), has practically 100 percent overlap with the wave func-tions of [13], [14], once the same Volkov force is used [17].It is therefore not astonishing that we arrive at very similarresults. For 12C we obtain two eigenvalues, correspondingto the ground state and the Hoyle state. Theoretical valuesfor positions of energy, rms values, and transition probabil-ities compared with the data, are given in Table 1. From thecomparison of the rms radii we see that the volume of theHoyle state is a factor 3 to 4 larger than that of the groundstate of 12C.
This is the dilute-gas aspect we highlighted at the out-set, the density of the Hoyle state at the center of thenucleus being reduced to r0/2 (!) [16]. Constructing an
⟨ ⟩ =
−
r r r r r
r r r r1 1 2 3 4
5 8 3
L
L L
r AN n
N N
| [ ( , , , )
( , , ) ( , )]
Φ α α
α
φ φφ
(1)
φ φα ( , , , ) ( , , )r r r r r r r r1 2 3 42
1 2 1 3
2 2
= − −−e R B L (2)
H ( , ) ( , ) | | ( , ) |B b B b H B bn n n n= ⟨ ⟨ ⟩Φ Φ Φ Φα α α α (3)Figure 5. Present result of the inelastic form factorcompared with experiment [32]. RGM result correspondsto Ref. [13].
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Vol. 17, No. 4, 2007, Nuclear Physics News 15
a-particle density matrix r(R, R′), integrating out of thetotal density matrix all intrinsic a-particle coordinates, anddiagonalizing the result, we find that the corresponding 0Sa-particle orbit is occupied to more than 70 percent by thethree a-particles [24], [25]. This is a huge percentage,affirming the almost ideal α-particle condensate nature ofthe Hoyle state. By contrast, even at zero temperature only10 percent of the particles in superfluid 4He belong to thecondensate (which is nevertheless a macroscopic supply ofcondensed particles). To add further perspective to the pic-ture, in the ground state of 12C the α-particle occupationsare equally shared between S, D and G orbits, thus invali-dating a condensate picture for the ground state. The occu-pation numbers for the ground and Hoyle states are shownin histogram format in Fig. 3. The difference between theHoyle state and the ground state is seen to be spectacular.In the Hoyle state the 0S-occupancy is at least an order ofmagnitude (!) higher than for any other orbit. This is one ofthe main typical features of Bose-Einstein condensation,even in strongly correlated Bose systems where there mayexist a strong depletion of the condensate, like in superfluid4He. On the other hand, the ground-state occupancies canbe explained quite well with the standard shell model [25].It should also be noted that that ground state of 12C is rea-sonably well reproduced by our theory (see Table 1). A fur-ther strong indication of the condensate-like behavior of thea-particles in the Hoyle state is their momentum distribu-tion, which is much narrower, almost δ-function-like, thanin the ground state (see Fig. 4).
An interesting fact in this respect is that in infinite nuclearmatter, a-particle Bose-Einstein condensation (BEC) onlyexists at low density and that there is no analogue to the BCSphase with a very large coherence length, which allows for
pairing also at high densities. a-particle condensation onlyexists in the BEC-phase, i.e. at low density! [26].
Let us now discuss what is, to our mind, the most com-pelling evidence that our description and interpretation ofthe Hoyle state is the correct one. Just as the authors of[13], we reproduce very accurately the inelastic form fac-tor 0+
1 → 0+2 of 12C. This is shown in Fig. 5. The agree-
ment with experiment as such is already quite impressive.Additionally, however, the following study was made. Weartificially varied the extension of the Hoyle state and studiedthe influence on the form factor [27]. It was found that theoverall shape of the form factor, and in particular the mini-mum, shows little variation. Rather, we found a strong depen-dence on the absolute magnitude of the form factor. A 20percent increase of the rms radius of the 0+
2 state decreases theheight of the first maximum by a factor of two! This strongsensitivity of the magnitude of the form factor makes us confi-dent that the agreement with the actual measurement is ineffect a proof that the calculated wide extension of the Hoylestate corresponds to reality. Additionally, it should be kept inmind that the reproduction of the strong monopole transition isby no means trivial, as explained in a recent work by Yamadaetal. [28].
The Hoyle state can be considered the ground state ofthe α-particle condensate configuration. On nuclear scalesit has a very long lifetime. Exciting one α-particle from thecondensate into the 0D orbit reproduces the experimentallymeasured position at 9.9 MeV of the 2+
2 state in 12C [29,30]. Without going into details, the 1.0 MeV width of thisstate is well reproduced [29] by the THSR ansatz. It istempting to imagine that the 0+
3 state at 10.3 MeV, whichexperimentally is almost degenerate with the 2+
2 state, isobtained by lifting one a-particle from the 0S into the 1S
Table 1. Comparison of the binding energies, rms radii (Rrms), and monopole matrix elements M(0+2 → 0+
1) for 12C obtained by solving the Hill-Wheeler (H. W.) equation based on (2) [17] and by RGM [13]. Volkov II as the effective two-nucleon force is adopted in the two cases, for which the 3α threshold energy E3α is calculated to be −82.04 MeV (experimental value: −84.9 MeV). Absolute values E are shown for the ground state in parentheses.
orbit. Initial theoretical studies [31] indicate that this viewmight indeed be realized. However, its width (~ 3 MeV) isvery broad, making a theoretical treatment rather delicate.Further investigations are necessary to validate this picture.At any rate, it would be very satisfactory if the triplet ofstates, with quantum numbers 0+
2, 2+2, 0+
3, could all beexplained from the a-particle point of view, since thosethree states of even parity are precisely the ones that cannotbe explained within a (no core) shell model approach [8].
In conclusion, so far as 12C is concerned, we haveassembled convincing evidence that the Hoyle state isdescribed remarkably well by the THSR ansatz of Eqs.(1), (2), and that it can legitimately be called an α-particlecondensate state. In saying this, we are aware that usingthe word “condensate” for only three particles mayamount to a certain abuse of the word. On the other hand,one must remember that also in the case of nuclear pair-ing, only a few Cooper pairs are sufficient to produceclear signatures of superfluidity in nuclei! (An example isseen in the chain of Sn-isotopes [33].) Still, with just threeα-particles condensed into the 0S-state, we would like tohave more.
We are thus led to ask about α-particle condensationin heavier nuclei? Once one accepts the idea that theHoyle state is essentially a state of three free α-particlesheld together only by the Coulomb barrier, it is hard tobelieve that analogous states should not also exist inheavier n α nuclei like 16O, 20Ne, 24Mg, . . . . Indeed, ourcalculations, using the THSR ansatz, systematicallyalways show a 0+-state close to the nα-particles disinte-gration threshold [20]. For example, in 16O weobtain three 0+-states in [19], namely the ground stateat E0 = −124.8 MeV (compared with the experimentalvalue −127.62 MeV), a second state at excitation energyE0+2 = 8.8 MeV and a third at E0+3 = 14.1 MeV. The fourα-particles threshold in 16O is at 14.4 MeV. Unfortu-nately, the experimental situation in 16O is far less com-plete than in 12C. However, there exists a 0+ state at14.04 MeV [34] with a strong E0 transition probability ofM = 3.3 fm2, whose magnitude is close to the E0 value ofthe calculated 0+
3 state, that is M = 2.5 fm2. As alreadymentioned, such an agreement is non-trivial [28]. Thisstate is, therefore, a very strong candidate to be the analogto the Hoyle state in 16O. Recently Wakasa [35] identifieda new 0+-state at 13.6 MeV in 16O, which might also havea strong α-condensate component [36]. Further work isnecessary to clarify the situation in 16O, but it is verylikely that one of the two 0+ states at 13.6 MeV or at
14.04 MeV will be the α condensate state. Similar statesin 20Ne, 24Mg, . . . , are yet to be discovered. Additionaltheoretical investigations along the lines of Ref. [16]would be opportune.
Outlook Further topics to be investigated in the future in the con-
text of α-particle condensation are numerous. An interest-ing question is how many α’s can maximally be in a selfbound α-gas state. In this respect, a schematic investigationusing an effective α − α interaction in an α-gas mean fieldcalculation of the Gross-Pitaevsky type was performed[37]. Because of the increasing Coulomb repulsion, theCoulomb barrier fades away and our estimate yields a max-imum of about eight a-particles that can be held together ina condensate. However, a few extra neutrons can have astrong additional binding effect (see 9Be and 10Be [38, 39])and may stabilize larger condensates.
Another exciting possibility is to observe expandinga-particle condensate states. Imagine that one excites 40Ca,via a heavy-ion collision, to about 60 MeV, i.e. to the totalα disintegration threshold. The α condensate, being formedwith a certain probability, will start expanding, since thereno longer exists any Coulomb barrier to confine it. Withmultiparticle detectors such as INDRA or CHIMERA, alldecaying α-particles could be detected in coincidence, andthe coherent state could be identified by its very low energyin the c.o.m. system. This would then be analogous to anexpanding atomic condensate after switching off the con-fining trap potential [40]. Experiments in this direction arebeing analysed at IPN-Orsay [41].
Another interesting idea concerning α-particle condensateswas put forward by von Oertzen and collaborators [5, 42].a-particles outside a strongly bound core (e.g. 40Ca) can form acondensate at the multi-α-particle threshold [5]. For the con-densate with a fixed particle number, the emission of two a’sand three α’s must be enhanced. In fact the observation of theemission of 12C in the 0+
2 state from the compound nucleus52Fe has been observed [43] and a very strong deviation fromstatistical model predictions is observed. Similar ideas havebeen advanced by Ogloblin [44], who hypothesizes a threea-particle cluster state on top of 100Sn, and earlier by Brenneretal. [45] who reports evidence of a gaseous a-particles in 28Siand 32S on top of an inert 16O core. Also, very interestingrecent experimental work on loosely bound a-structures inlight nuclei has been performed by T. Kawabata etal. [46].
In finite nuclei we never will have a macroscopic con-densate of α-particles. The situation in this respect is, as
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Vol. 17, No. 4, 2007, Nuclear Physics News 17
mentioned, the same as for ordinary nuclear superfluidity.However, there may exist the exciting possibility that incollapsing stars or protoneutron stars a macroscopic a-particlecondensate be formed. Shen and Toki et al [47] and Lat-timer and Swesty [48] give the possible a-phases in com-pact stars, see also [49]. In an earlier work, we predictedrather high critical temperatures for a-particle condensation[26] and, therefore, this must be considered as a possibility.It should, however, be kept in mind that although the criti-cal temperature is well understood in the low density limit,the theory needs to be extended to higher densities, wherethe α-particles dissolve.
Remarks and Conclusion In conclusion, we see that the idea of a-particle con-
densation in nuclei and other nuclear systems, whereseveral a-like clusters move coherently in the samec.o.m. orbit, has already triggered many new works andideas, in spite of the fact that so far strong identificationof such a state only exists in 12C, and potentially also in16O. The possibility that there exists a completely new(low density) nuclear phase where a-particles play therole of quasi-elementary bosonic constituents is surelyfascinating, and one may predict that many more a-par-ticle condensed states will be detected in the nearfuture.
It also can be expected that one soon will find con-densed quartets in other systems [3]. For example it maybe feasible to trap fermionic atoms in four different mag-netic substates, giving rise to a situation quite analogousto nuclear physics.
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Vol. 17, No. 4, 2007, Nuclear Physics News 19
Mass Chain Evaluations for the Evaluated Nuclear Structure Data File (ENSDF)—An Urgent Appeal for European Participation
F. G. KONDEV,1 A. L. NICHOLS,2 AND J. K. TULI3
1Nuclear Engineering Division, Argonne National Laboratory, Argonne, IL 60439, USA 2Nuclear Data Section, Department of Nuclear Sciences and Applications, International Atomic Energy Agency, Wagramer Strasse 5, A-1400 Vienna, Austria 3National Nuclear Data Center, Brookhaven National Laboratory, Upton, NY 11973-5000, USA
Introduction Reliable nuclear structure and decay data represent
the fundamental building blocks of nuclear physics andastrophysics research, and are also of vital importance ina significant number of applied nuclear fields such aspower generation and associated fuel cycle operations(e.g., fuel manufacture, transport, reprocessing, andwaste management), materials analysis, medical diagno-sis, and radiotherapy. There is a continuous demand forgood quality data formulated and recommended throughthe speedy assessment and incorporation of new andimproved measurements.
Systematic studies of the fundamental properties ofatomic nuclei represent the quest to understand the origins,evolution, and structure of the universe. Although varioustheoretical models predict the possible formation of over6,000 radionuclides, many of them have proven to be diffi-cult to produce in a laboratory environment. Althoughnearly 3,000 of these nuclides have been generated andcharacterized in the laboratory, more than 3,000 remainunknown and ill-defined. Most new discoveries can beidentified with the neutron-rich side of the valley of stabil-ity, where many properties of the resulting nuclei can beexpected to change significantly. These studies of neutron-rich nuclei are challenging the accepted frontiers of modernnuclear structure physics and astrophysics. Existing andfuture experimental facilities, such as FAIR (Europe), FRIB(USA), RIEKEN (Japan), and ISAC-TRIUMF (Canada),will provide a surge of new data to impact the field signifi-cantly. Such paths to new discoveries benefit greatly fromsystematic studies of the accumulated knowledge ofnuclear physics research, and from the ability to accessthese data promptly. Under these circumstances, the rapidavailability of comprehensive, up-to-date, and well-ordered
databases is an essential requirement for the nuclear phys-ics research community and applications specialists whoneed reliable data at the press of a key. Such credible data-bases also act as a bridge between science, technology, andsociety by making the results of basic nuclear physicsresearch available to a broad audience of users, and hencehaving a profound effect on the socioeconomical applica-tions of modern nuclear science.
Experimentally determined nuclear structure and decaydata for all known nuclei are evaluated and incorporatedinto the Evaluated Nuclear Structure Data File (ENSDF)database [1]. This database contains comprehensive nuclearstructure data from various nuclear reactions and decayprocesses, and recommends best values for a range ofnuclear properties that are derived by critical analysis of allexperimental information. The net result is a treasury ofdata of immediate use to the world-wide nuclear physicscommunity (Table 1).
Imagine, for example, a scientist who is interested instudying the properties of the doubly magic nucleus 24Mgand their relevance to various nucleosynthesis phenomena.With more than 1,900 journal articles published on the sub-ject (as retrieved from the bibliographical Nuclear ScienceReferences (NSR) database [2]), a preliminary reviewwould involve a large amount of time and effort to identifyand extract the required information. However, usingENSDF and with the simple operation of a computermouse, evaluated data can be accessed and displayed in amatter of seconds. Last year, more than 1 million electronicretrievals were made from ENSDF and other derivativedatabases, with the main users being scientists in the USAand European Union (Figure 1). Although the recom-mended data are sufficiently complete and precise for manyapplications, the contents can be used as the starting point
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20 Nuclear Physics News, Vol. 17, No. 4, 2007
toward more extensive studies. Thus, ENSDF is a primarysource of data for many specialized databases, sophisti-cated computer codes, search engines, and publications,some of which are indicated in Figure 2. ENSDF evalua-tions can be particularly useful in the identification of con-tradictory results that exist in the literature, and hence canstimulate new and improved measurements to assist inresolving such discrepancies. Last but not least, ENSDFdata are frequently used in the planning of various stages ofmany nuclear physics experiments, including dedicatedresearch and development activities, interpretation ofresults from new measurements, and in the preparation andreview of journal articles. ENSDF is a crucial database in awide range of important nuclear activities.
International Network of Nuclear Structure and Decay Data Evaluators
The compilation and evaluation of nuclear structure anddecay data stretches back to the 1930s, with the publicationof the first known tabulations of recommended nuclearparameters by Curie et al. [3] and Fea [4]. These tabulationswere followed in 1940 by what was essentially to becomethe popular Table of Isotopes as conceived originally byLivingood and Seaborg [5]. Clearly, this type of extremelyuseful work has developed and expanded considerablysince those times. These and other parallel evaluationefforts continued, and the familiar presentational style of
Table 1. Nuclear parameters in ENSDF: adopted levels, nuclear reaction and decay data sets.
Nuclide:
Q(β−) and Q(α) β− decay energy and α decay energy for the ground state
S(n) and S(p) neutron and proton separation energies
XREF cross-reference assignments for the various experimental data sets
Nuclear level:
E(level) excitation energy relative to theground state
Jπ spin and parity
T1/2or Γ half-life or total width in centre of mass
decay branching of ground states and isomers (T1/2 ≥ 0.1 s)
Q,μ static electric and magnetic moments
XREF flags indicate in which reaction/decay data sets the level is seen
configuration assignments
—
band assignments also possibly band parameters
isomer and isotope shifts
only literature reference is given
charge distribution for ground states—only literature reference is given
deformation parameters
—
B(E2), B(M1) electric and magnetic excitation probabilities
γ and E0 transitions:
level scheme placement of levels
Eγ measured γ-ray or E0 transition energy
Iγ relative photon intensity
normalization factor converts relative to absolute photon intensity per 100 decays of the parent
Mult, δ electric or magnetic multipole character; mixing ratio
CC internal-conversion coefficients (when significant)
B(E2)W, B(M1)W reduced transition probabilities in
Weisskopf units
Figure 1. Geographical usage of the nuclear data services ofNNDC, BNL, USA. Data source www.nndc.bnl.gov/usndp.
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Nuclear Data Sheets appeared in early 1966, while Ewbankand co-workers at the Oak Ridge National Laboratory inTennessee, USA, developed the database format in the1970s. Since 1980, the ENSDF master database has beenmaintained by staff at the National Nuclear Data Center(NNDC) of Brookhaven National Laboratory, sponsored bythe Office of Science, U.S. Department of Energy. Evalu-ated data from ENSDF have been used as the primary inputfor Nuclear Data Sheets, Table of Isotopes, Nuclear WalletCards, NuDat, and other products familiar to the nuclearphysics community (see Figure 2).
Nuclear structure and decay data are compiled and eval-uated by means of a collaborative program organizedthrough the International Network of Nuclear Structure andDecay Data Evaluators (NSDD) established in 1974 underthe auspices of the International Atomic Energy Agency(IAEA). This network began at a time when the workloadwas heavily reliant on American input. A more equitableinvolvement of other national laboratories and universitieswas envisaged, and partially achieved. At different timesprominent nuclear physicists, such as F. Ajzenberg-Selove,R. G. Helmer, C. W. Reich, S. Raman (USA), P. M. Endt,C. van der Leun, P. J. Twin and A. H. Wapstra (Europe)and many others, have been involved in the resulting com-pilation and evaluation activities. Several countries havecontributed over a long period of time, including Belgium,
Canada, China, France, Japan, Kuwait, Russia, and theUnited States of America. Recently, new evaluation groupshave emerged in other countries, such as Australia andIndia. The total NSDD evaluation effort is equivalent toabout 9 full-time equivalent scientists per annum (FTE),albeit approximately 12 FTE are required to maintain thedesired currency and quality of ENSDF.
Other specific nuclear properties are comprehensivelycompiled and evaluated by individual members of the net-work without limiting their efforts to a particular masschain. Most recently, these horizontal evaluations includeatomic masses [6], nuclear magnetic and electric moments[7], electric monopole strengths [8], and capture gammarays [9]. An additional database entitled XUNDL [10] pro-vides a rapid means of electronic access to the most recentpublications and pre-prints of experimental nuclear struc-ture data before they are included in the ENSDF database.An additional useful source of information is the biblio-graphical database entitled Nuclear Science References(NSR) [2].
There are many distinct advantages associated withmaintaining a healthy ENSDF database by means of a mul-tinational network:
(a) ensure the maintenance of a well-defined archive ofnuclear structure and decay data for future generations,
Figure 2. ENSDF: major data sources and derivatives.
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22 Nuclear Physics News, Vol. 17, No. 4, 2007
(b) valuable interplay between nuclear reactionand decay data studies to define the best nuclearparameters,
(c) constructive impact of recommended nuclear structuredata on developing nuclear theories,
(d) assistance in the resolution of contradictory results, (e) identification of requirements for and stimulation of
new measurements, (f) benefits to users in many applied areas—nuclear
medicine, analytical science, environmental monitoring,nuclear engineering, and so on.
Members of the NSDD network can take great personalpride in assisting such an impressive array of basic andapplied nuclear physicists with their everyday nuclear dataneeds.
Future Perspectives There is no doubt that future advances in nuclear phys-
ics and other related areas will require large and increas-ingly sophisticated databases that are based on moderncomputer technologies. Popular use and adoption of datawithin any electronic database is largely determined by thefollowing:
(a) reliability and credibility—data must be correctlyevaluated and incorporated into the database,
(b) comprehensive—the database should include allmeasured quantities and their uncertainties,
(c) up-to-date—results from all measurements should bepromptly incorporated into the database,
(d) accessibility—easy and rapid availability in user-required formats.
The main goal of the NSDD network is to improve theexisting nuclear physics databases in all these aspects.Furthermore, the development of specialized data mod-ules will continue, such as calculations of internal-conversion coefficients, log ft values, and so on that aretailored to the various needs of the nuclear physicsresearch community. Special attention will also be paidto the development of new evaluation methodologies andto the prompt introduction of modern computer technol-ogies in order to improve the quality and efficiency ofaccess to the existing databases. Improved impact of theresulting nuclear data evaluations in advanced researchareas will be sought by carrying out specialized horizon-tal evaluations and topical reviews in collaboration withscientists from the nuclear physics research community.All of this work will require dedicated effort by nuclear
data physicists and the full support of the internationalnuclear physics community.
Challenges The organizers of the International Network of Nuclear
Structure and Decay Data Evaluators have become aware inrecent years of an increasing problem in maintaining andupdating ENSDF evaluations with the necessary regularity.Evidence of a shortfall in effort has been detected over theprevious ten years as evaluators in Europe have retiredwithout any obvious replacements. Although someprogress has been made in recruitment through the commit-ment of nuclear physics institutes in India and elsewherefor this essential work, these welcome additions are notfully commensurate with the losses experienced in Europe,a region of the world that might have been expected toensure some re-generation of expertise in this vital area ofresearch and development.
The survival and maintenance of the quality of ENSDFdepends on the recruitment of new data evaluators toreplace the ageing nuclear physicists undertaking thisimportant work. Unless new blood can be introduced soon,there is a serious danger that the current loose confedera-tion of dedicated participants will fade away and as a con-sequence the core nuclear physics databases will becomehopelessly outdated. An urgent need has arisen for youngerscientists to join the NSDD evaluation network and to con-tribute to the nuclear data activities. There can be no doubtthat the assistance of the worldwide nuclear physicsresearch community is urgently required to ensure the sur-vival of ENSDF at the necessary level of credibility, reli-ability, and quality. Anyone with the necessary expertise,supportive infrastructure, and personal interest in undertak-ing mass chain evaluations for ENSDF should contactJagdish Tuli at NNDC, Brookhaven National Laboratory,USA.
Acknowledgements The authors express their gratitude to all colleagues
within the International Network of Nuclear Structureand Decay Data Evaluators for their enthusiasticefforts to maintain the quality of the existing nuclearstructure databases. F. G. Kondev and J. K. Tuli aresupported by the U.S. Department of Energy, Office ofNuclear Physics, Office of Science, under contractsDE-AC02-06CH11357 and DE-AC02-98CH10886,respectively.
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References 1. ENSDF database (Evaluated Nuclear Structure Data File),
www.nndc.bnl.gov/nsr 3. M. Curie, A. Debierne, A. S. Eve, H. Geiger, O. Hahn, S. C.
Lind, St. Meyer, E. Rutherford, and E. Schweidler, The radio-active constants as of 1930, Rev. Mod. Phys. 3 (1931) 427.
4. G. Fea, Tabelle riassuntive e bibliografia delle transmutazioniartificiali, Nuovo Cimento 12 (1935) 368.
5. J. J. Livingood and G. T. Seaborg, A table of induced radioac-tivities, Rev. Mod. Phys. 12 (1940) 30.
6. A. H. Wapstra, G. Audi, and C. Thibault, The AME2003atomic mass evaluation, Nucl. Phys. A729 (2003) 129.
7. N. J. Stone, Table of nuclear magnetic dipole and electricquadrupole moments, At. Data Nucl. Data Tables 90(2005) 75.
8. T. Kibédi and R. H. Spear, Electric monopole transitionsbetween 0+ states for nuclei throughout the periodic table, At.Data Nucl. Data Tables 89 (2005) 77.
9. H. D. Choi, R. B. Firestone, R. M. Lindstrom, G. L. Molnár, S.F. Mughabghab, R. Paviotti-Corcuera, Z. Révay, A. Trkov, V.Zerkin, and C. Zhou, Database of prompt gamma rays fromslow neutron capture for elemental analysis, InternationalAtomic Energy Agency, Vienna, Austria (2007), ISBN 92-0-101306-X.
M. FALLOT,1 M. GRASSO,2,3 E. KHAN,2 AND J. MARGUERON2
1Subatech, 4 rue Alfred Kastler BP 20722, F-44307 Nantes Cedex 3, France 2Institut de Physique Nucléaire, Université Paris-Sud, IN2P3-CNRS, F-91406 Orsay, France 3Dipartimento di Fisica e Astronomia and INFN, Via Santa Sofia 64, I-95123 Catania, Italy
Introduction: Neutron Stars, Nuclei, and Nuclear Matter
Neutron stars are the remnants of core collapse superno-vae [1,2]. They are the most compact stellar objects afterblack holes. Some of their properties, such as masses, rota-tion frequencies, and emission of radiations are measurable,whereas other signals like gravitational wave emission areplanned to be in the next years. The properties that are notdirectly linked to observations, such as the internal compo-sition or temperature, require the development of theoreti-cal models. Fortunately, some of the missing informationcan be obtained from the study of the other dense nuclearsystems, atomic nuclei, which are accessible to experimen-tal facilities. Traditionally, the link between neutron starsand bulk nuclei is made via the nuclear matter: an ideal infi-nite system equally composed of interacting neutrons andprotons where Coulomb interaction has been switched off.For instance, the central density of heavy nuclei is veryclose to the equilibrium density of nuclear matter, calledthe saturation density ρ0. Moreover, the nuclear matter con-cept can be extended to isospin asymmetries. Asymmetricnuclear matter is rather similar to the nuclear matter foundin neutron stars. Coulomb potential energy at those densi-ties is usually small compared to kinetic energy and themain interaction between particles is driven by the nuclearforce. Recently, more direct relations between neutron-richnuclei and neutron star matter have been proposed. Indeed,some of the exotic neutron-rich nuclei produced in nuclearfacilities are also located in the outer crust of neutron stars,while the inner crust is composed by drip-line nucleiimmersed in a neutron gas. Before entering into this discus-sion, we should present in more detail the physics of neu-tron stars.
Discovery and Observation of a Large Variety of Neutron Star Systems
Landau as well as Baade and Zwicky suggested theexistence of neutron stars in the early 1930s. Their exist-ence remained conjectural until 1968 when Jocelyn Bell
and her thesis advisor Anthony Hewish discovered radiopulsars, characterized by radio emission with a periodicitythat lies between a few seconds and few tens of millisec-onds. Radio pulsars are interpreted as spinning neutronstars with an intense magnetic field misaligned with therotation axis. Radio waves are thought to be emitted by theelectrons accelerated along the polar magnetic fields.Hence, the radio waves are not isotropically emitted butfocussed and the rotating neutron star is emitting a pulsedsignal like the lighthouses that guide the boats along thecoasts (Figure 1). The vast majority of radio pulsars are iso-lated neutron stars because in binary systems the accretiondisk tends to screen the signal. In addition to radio emis-sion, neutron stars are also found in interacting binary sys-tems that emit intense X-rays. In such binaries, a neutronstar closely orbits a normal optically visible star and drawsgas away from it. The infalling accreted gas is heated tomillions of degrees and emits X-rays. Rapidly rotating andrelatively young radio pulsars are also found in the visiblespectrum (Crab pulsar, Vela pulsar). Some neutron stars arealso strong high-energy (greater than tens of MeV) gamma-ray sources. Besides, the strongest known magnetic fieldsof the present Universe have been found in neutron starswhere surface magnetic fields are of the order of 107 T. Infew young neutron stars, much more intense magneticfields have been observed and may exceed 1011 T. Theusual dynamo effect is here unable to produce such intense
Figure 1. Rotating pulsar with its magnetic fields and thefocussed radio beams.
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magnetic fields. A possible phase transition to strongly spinpolarised matter could be responsible, but this is still aspeculation in dense matter. From time to time, probablydue to the twist of its magnetic field, magnetars emit a giantflare like the one that reached the Earth on December 27,2004 and has interrupted all radio broadcasts for a fewseconds.
Measurements of masses and radii of neutron stars stillrepresent an observational challenge. The most accuratemeasurements of masses are obtained in binaries of neutronstars applying the Kepler laws. Observed values are typi-cally around 1.4 solar masses. The time derivative of therotation velocity, associated to the luminosity, provides anestimate of the moment of inertia that, combined with thevalue of the mass, gives a measure of the radius. This leadsto a typical radius of a tenth of kilometers. These measure-ments cannot reach the accuracy required to disentanglebetween the models used to describe neutron stars. Othermethods are then proposed, like the one based on blackbody radiation but it has been found that neutron stars mayhave a non-uniformly distributed surface temperature. Thiscomplicates the interpretation of the black body emission.
Up to now, about 1,500 neutron stars have been identi-fied so far and, as shown, they participate to a large varietyof observed systems that are characterized by their electro-magnetic emission going from visible spectrum to gammarays. Could those emission processes provide informationabout the internal composition of neutron stars?
The Equation of State of Dense Stellar Matter On the theoretical side, the mass and the radius are
determined by solving the hydrostatic equilibriumequation. In the framework of the general relativity theequilibrium of a spherical object is described by theTolman-Oppenheimer-Volkov equations, and for complete-ness, the equation of state (EoS) is required. The densityincreases from 106 g/cm3 at the surface (starting point of thecrust), to several times the saturation density (ρ0 is 3·1014 g/cm3) in the core. The number of neutrons in neutron starsexceeds by far that of protons. The net isospin asymmetryδ = (N − Z)/(N + Z) can reach 0.95 in the interior of the stars.The equation of state relies on the composition of densematter in the star for which very scarce information areavailable: Where are localized the phase transitionsbetween matter composed of neutrons, protons, and elec-trons, and more massive hadrons such has hyperons? Thereis a global consensus that nuclear matter will convert to quarkmatter, but at which density? Does mesons (pions, kaons)
condensation occur? Several equations of state have beenderived in order to investigate the observational conse-quences of the composition of dense matter. The maximummasses and the radii predicted by those models can be quitedifferent (Figure 2).
On the experimental side, investigations on the atomicnuclei like the measurement of giant monopole resonances,masses, and central densities allow one to probe the equa-tion of state around the saturation density. Heavy ion colli-sions, hot giant resonances, and exotic nuclei properties,attempt to explore more extreme regions of the phase dia-gram. However, the improvement of EoS’s at lower andhigher densities than ρ0 and for strong isospin asymmetriesis still required (Figure 3). In the latter case, the densitydependence of the symmetry energy is convenient toexplore the relation between isospin symmetric and asym-metric equation of state: it can be shown that the densitydependence of the symmetry energy is equivalent to theisospin dependence of the incompressibility modulus. Thesymmetry energy therefore plays a central role in determin-ing the structure and the evolution (cooling) of the stars.The future facilities producing exotic nuclei will allow oneto test this isospin dependency for values of the asymmetryparameter, δ, larger than 0.2. This asymmetry is smallerthan the asymmetry in neutron star, but may provide at leastadditional constraints for the theoretical models.
Figure 2. Mass-radius diagram for typical EoS, depictedwith observational constraints (see J. M. Lattimer and M.Prakash, Ap. J. 550 (2001) 426 for notations and furtherexplanations).
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Anatomy of the Star Neutron stars are quasi-spherical objects composed of
six major regions: the inner and outer cores (~99% of themass) where nuclear matter is homogeneous and that areusually sufficient to understand the main properties of neu-tron stars; the inner and outer crust (1–2 km width) com-posed of inhomogeneous nuclear matter (nuclei or nuclearclusters), which screens the core from observations (evenfrom neutrinos), the envelope (few meters), which influ-ences the transport and the release of thermal energy fromthe surface, and finally the atmosphere (few centimeters),which plays an important role in shaping the emergent pho-ton spectrum (Figure 4).
At the surface defined by the interface between theouter crust and the envelope, 56Fe atoms are arranged as in asolid. Going toward the interior, the atoms are ionized andin the outer part of the crust one can find nuclei with num-bers of nucleons up to A = 200 arranged in a Coulomb lat-tice in the presence of an electron gas. Due to electroncapture processes, these nuclei become richer in neutronswith increasing density (109 to 1011 g/cm3). Neutrons startto leak out of nuclei at densities above the neutron drip den-sity—the equivalent of the neutron drip line in a stellarenvironment (finite pressure, beta-equilibrium): 4 1011g/cm3 inthe inner crust. Nuclei are located at the sites of a crystalimmersed in a super-fluid of neutrons and relativisticleptons. The lattice can be modelized by its elementaryconstituents, the Wigner-Seitz (WS) cells, each of them
containing the most probable nuclear cluster, the neutronand the electron gases. For densities higher than 1013g/cm3,the nuclear clusters are close enough to begin a dissolutionprocess and deformed structures appear. They are com-monly called the pasta phases because the matter isarranged in noodle shapes like lasagne or spaghetti, orSwiss cheese. At this stage, the proton fraction hasdecreased down to 0.1. This process results in the formationof homogeneous nuclear matter.
In the inner core, where the density is greater than thesaturation density, exotic particles such as strange hyperonsand/or Bose condensates (pions or kaons) may becomeabundant. It is possible that a transition to a mixed phase ofhadronic and deconfined quark matter develops.
Cooling, Glitches, and Vortices: The Life of a Neutron Star
With time neutron stars evolve and new phenomenaoccur. In the following we report on some of those phe-nomena that are directly related to the properties of nuclearmatter and in particular to the pairing properties of nuclearmatter.
Being at the end point of stellar evolution, neutron starsdo not produce energy but lose the gravitational energygained during the core collapse by neutrino emission.
Figure 3. Equation of state for various values of the isospind (from D. T. Khoa etal. Nucl. Phys. A602 (1996) 98).
Figure 4. The basic structure of a neutron star (from G.Röpke, Univ. Rostock).
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Indeed, because of their very weak coupling with matter,neutrinos and anti-neutrinos mainly produced by beta decayand inverse beta decay could carry out the energy of thecore and cool down the temperature. This is called theURCA process [3], by reference to the name of a casinoexisting in the mid 1950s in Rio di Janeiro, known by thepromoter of this process, G. Gamow. According to him, theefficiency of this casino in spoiling the money of the gam-blers was comparable to the URCA process in coolingdown the star. Later on, it was discovered that the URCAprocess is strongly suppressed by energy and momentumconservation unless a minimum amount of proton, around11% of the baryonic density, is present [4]. This minimalamount is strongly correlated with the symmetry energy as.Relativistic models, having large values of as, satisfy thiscriterion around the saturation density while most non-rela-tivistic models, with a lower as value, do not. The differ-ence between non-relativistic and relativistic modelspredictions should then be investigated. Anyhow, theURCA process is too efficient to explain the slowing downof the surface temperature with time that is observed for adozen of stars: neutron stars are visible by thermal emissionduring a few millions of years. Several improvements havebeen proposed: superfluidity leading to the presence of aneutron gap may quench cooling from the URCA process.A modified URCA process is also considered where addingan additional nucleon as a spectator of the process allowsmomentum and energy conservation. Other processes arealso considered like neutrino bremsstrahlung, pair breakingemission, and so on. It should be noted that the specific heatin the crust is also important in cooling modelization. Itdepends on the excitation spectrum, which is different inthe super-fluid phase than in the normal phase [5].
Neutron stars are also fast rotating stellar objects and weknow, from Earth laboratory experiments on Helium 4 forinstance, that a rotating super-fluid produces vortices. Inthe case of finite nuclei, surface effects forbid the formationof vortices. In other words the rotational energy needed istoo high and nuclei vaporize at lower energies. In the caseof neutron stars, gravitational pressure maintains the nucle-ons together and vortices can be formed in the core as wellas in the inner crust. Those vortices link together two layersof the star (core and crust) and impose a rigid rotation. Asthe neutron star releases energy, the vortex must bedestroyed from time to time and this is the possible originof observed “giant glitches.” A “giant glitch” is a brutalvariation of the rotation period of the star. One possiblescenario that explains the existence of the glitches is that
the neutron fluid forms vortices that can pin on the nuclearclusters in the crust. The unpinning would generate theangular momentum transfer from the core to the crust,which is at the origin of the glitches. The pinning forcedepends on the neutron pairing gap in the crust.
Hence, cooling and giant glitches require accurate mod-elization of the pairing gap in the crust of neutron starsmade of non-homogeneous matter. Most of the actual mod-els are based on the Wigner-Seitz approximation since theseminal work of Negele and Vautherin [6]. This allowsstraightforward application of the Hartree-Fock BCS orHartree-Fock-Bogoliubov models built for the descriptionof atomic nuclei. It has recently been shown that thosemodels are valid if the density of states around the Fermisurface is averaged over a few 100 keV by temperatureeffects or energy exchanged during reaction processes [7].For temperature below a few 100 keV, it is necessary toimprove the modelization of the continuum states. For that,based on the ideas developed in condensed matter, firstband theory type approaches have been built [8] and repre-sent certainly the new generation of models. Nevertheless,experimental probes of the pairing gap in nuclei are neces-sary but still very difficult. With respect to the importanceof such knowledge, nuclear physics investigations shouldbe pushed in this direction.
Nuclei: A Possible Laboratory for Neutron Stars Recently, several empirical relationships have been
found that are directly correlated to some properties ofnuclei to neutron star physics. For instance, the neutronskin thickness nuclei has been linked to the pressure of pureneutron matter at sub-nuclear densities [9] and conse-quently to the neutron star radius [10]. Indeed, the pressureis related to the derivative of the symmetry energy [11] andthe neutron skin thickness of nuclei is an observable thatyields some information about low-density neutron-richmatter and, in particular, about the density dependence ofthe symmetry energy. In neutron stars, this question isessential: the density dependence of the symmetry energydetermines the proton fraction and the threshold density atwhich direct URCA process occurs, as discussed in the pre-vious section. Moreover, it governs the threshold densitiesof other particles such as hyperons, pions, kaons, quark,and so on, which trigger phase transitions and cooling pro-cesses.
This example illustrates that articulations can be drawnin which nuclear physics experiments could bring useful
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constraints. The perspectives offered by the next generationof radioactive beam facilities are in this sense very attrac-tive. Identifying the experimental methods and choosingthe relevant observables for the future exotic beam facilitiesis a strong challenge and requires an important interplaybetween experimental and theoretical fields. This is themotivation of a series of workshops called Exotic Nucleiand Neutron Stars [12]. These workshops associate nuclearexperimentalists, theoreticians, and astrophysicists in fiveworking groups. The aim is to draw physics cases for theexperiments relevant to neutron star properties. After thesecond meeting, held in May 2007 at the Institut de Phy-sique Nucléaire d’Orsay, some tasks have already beendefined. We briefly mention some of them.
• Working group 1: collective excitations in exoticnuclei. Among the different ways to measure neutronskins, collective vibrational modes furnish very effi-cient constraints on the models used to compute theneutron skins. Some of them are even more directlyrelated to the neutron skin thickness such as the spindipole mode. In GSI a pioneering work has beenrecently performed relating the neutron skin thicknessto soft dipole modes [13]. The study of collectivemodes in exotic nuclei is also relevant because theexcitation spectra of neutron-rich nuclei in the crust ofthe stars can influence the cooling of the star. Further-more, the incompressibility modulus of nuclear mattercan be deduced from Isoscalar Giant Monopole Reso-nance and Giant Dipole Resonance properties; theirstudy in exotic nuclei will constitute essential piece ofinformation to constraint the symmetry energy and itsisospin dependence. As a complementary constraint onthe models, charge radius measurements using laserspectroscopy in very neutron rich nuclei will be per-formed accurately.
• Working group 2: pairing in exotic nuclei. The mainquestion lies on the pairing interaction itself [14]. Howis pairing generated? What is the contribution of phononcoupling to the pairing? How do medium effects such asisoscalar or isovector densities influence the pairingfield? The answer may come from a global and uniquedescription of pairing effects going from halo nuclei toheavy nuclei via low density neutron matter in the crustof neutron stars. The experimental study of two-neutrontransfer would constitute an interesting tool, but theoret-ical developments are required to analyze such data.For instance, to check the dependence of the results
(energies and cross-sections for the rotation and vibra-tion pairing modes) on the properties of the chosen pair-ing interaction.
• Working group 3: EoS dependence on density and tem-perature. Probing the phase diagram away from standardnuclei constitute the Graal of experimental investigationssuch as fusion-evaporation and multi-fragmentationexperiments. Those are also privileged tools to access tolevel densities at finite temperature. Experiments arealready conducted in this aim, but future facilities willallow to probe the nuclear EoS in more asymmetric mat-ter, together with 4π arrays such as FAZIA [15]. Nuclearmodels predict different isospin dependence and furthertheoretical investigations are required in the comingyear(s).
• Working group 4: nucleosynthesis in neutron star merg-ers. Both type II supernovae and neutron star mergersare candidates to be the location of nucleosynthesisthrough rapid neutron capture process [16]. The pathsare known to be dependent on nuclear inputs such as thesymmetry energy. Again, the density dependence ofsymmetry energy is fundamental to furnish precise andreliable predictions. A lot of measurements and theoret-ical calculations are required for r-process study. Opti-cal potential determinations should be performed atvery low energy. From the theoretical point of viewmany topics are of relevant importance, such as thedetermination of neutron capture and beta-decay ratesand the study of fission processes.
• Working group 5: hyper-nuclei. The presence ofhyperons in dense matter softens the EoS. Hyperonscontribute more to the energy density than to the pres-sure because they have larger masses and smallerFermi momenta. Their presence also enhances the neu-trino cooling of the core because they can participatein rapid URCA reactions such as λ⇒p + e + ve. Fur-thermore, they increase proton to neutron ratio andtrigger the URCA process involving nucleons. Unfor-tunately, very few experimental data exist in this fieldto discriminate the different theoretical predictions.Many data are expected in the future thanks to experi-ments such as HyPhi in GSI or J-PARC in Japan.Among those data is expected the production of a largevariety of σ hyper-nuclei as well as hyper-nuclei hav-ing several hyperons. Together with mean-field mod-els, those data should help in understanding thedifference between σ-N and λ-N interaction as well asthe λ-λ interaction.
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36 Nuclear Physics News, Vol. 17, No. 4, 2007
The next stage of the working groups is to propose con-crete experiments on present or future facilities, which willbe discussed during the next workshop. In order to preparethese proposals, predictions are needed to investigate therelevance of future measurements. For instance, the sensi-tivity of two neutron transfer cross-sections on the pairingfunctional has to be established. Both experimental and the-oretical contributions are necessary. Any interested physi-cist is encouraged to contact the authors of this article, inorder to join the next workshop.
We would like to stress that the activity of these work-shops and working groups relies on the important contribu-tions of our speakers and group coordinators: DidierBeaumel (IPN, Orsay, France), Brandon Carter (LUTH,Meudon, France), François De Oliveira (Ganil, France),Hans Emling (GSI, Germany), Lydie Giot (Subatech,France), Stephane Goriely (IAA ULB, Brussels, Belgium),Francesca Gulminelli (LPC, France), François Le Blanc(IPN, Orsay, France), Nicolas Le Neindre (IPN, Orsay,France), Marek Lewitowicz (Ganil, France), PatriciaRoussel-Chomaz (Ganil, France), Alan Shotter (TRIUMF,Vancoucver, Canada), Take Saito (GSI, Germany), andHeinrich Johannes Wörtche (KVI, The Netherlands).
References 1. N. K. Glendenning, Compact Stars, Springer-Verlag,
New York (1997). 2. H. Bethe, Rev. Modern Phys. 62 (1990) 801. 3. G. Gamow and M. Schoenberg, Phys. Rev. 59 (1941) 539. 4. J. M. Lattimer, C. J. Pethick, M. Prakash, and P. Haensel,
Phys. Rev. Lett. 66 (1991) 2701. 5. P. M. Pizzochero et al., Astro. J. 569 (2002) 381; C. Mon-
rozeau, J. Margueron, and N. Sandulescu, Phys. Rev. C(2007), in press.
6. J. W. Negele and D. Vautherin, Nucl. Phys. A207 (1973)298.
7. N. Chamel, S. Naimi, E. Khan, and J. Margueron, Phys. Rev.C (2007).
8. B. Carter, N. Chamel, and P. Haensel, Nucl. Phys. A748(2005) 675.
9. S. Typel and B. A. Brown, Phys. Rev. C64 (2007) 027301. 10. C. J. Horowitz and J. Piekarewicz, Phys. Rev. C64 (2001)
062802. 11. J. M. Lattimer and M. Prakash, Phys. Rep. 333 (2000) 121. 12. http://snns.in2p3.fr/ 13. A. Klimkiewicz et al., submitted to Phys. Rev. Lett. 14. H.-J. Schulze et al., Phys. Rev. C63 (2001) 044310. 15. http://fazia.in2p3.fr/documents/
LoI_SPIRAL2_ThermoDyn_v7.pdf 16. S. Goriely et al., Nucl. Phys. A758 (2004) 587.
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40 Vol. 17, No. 4, 2007, Nuclear Physics News
International Nuclear Physics Conference INPC2007
The 23rd International NuclearPhysics Conference, INPC2007, washeld from June 3 (Sunday) throughJune 8 (Friday) at Tokyo InternationalForum in Japan. This conference is thelargest conference in nuclear physicsand it is held every three years. InJapan, the previous one was held in1977, exactly 30 years ago. In spite ofan anticipated rainy season in Japan atthis time of year, we fortunately hadvery comfortable weather with clearskies throughout the conferenceperiod. About 800 participantsattended this conference from 38countries.
Yukawa Session The conference started with a pub-
lic symposium on June 3 for the cen-tennial celebration of the birth ofProfessor Hideki Yukawa. Seveninvited speakers—H. Sato, T.Yamazaki, T. D. Lee, A. Zichichi, A.Arima, Y. Nambu, and J. P. Schiffer(shown in Figure 1)—presented avariety of public lectures, coveringhistorical stories behind the birth ofthe Yukawa theory on nuclear interac-tions to frontier sciences after theYukawa theory. Half the conferenceparticipants plus 400 public audiencemembers spent the entire afternoonlistening to these interesting lectures.
Opening Ceremony On June 4 the Opening Ceremony
of the INPC2007 was held under thepresence of the Emperor and Empressof Japan. After the welcome greeting bythe conference chair (Shoji Nagamiya),the President of the Japanese PhysicalSociety (Masako Bando), the Presi-dent of the Science Council of Japan(Ichiro Kanazawa), and the President
of IUPAP (A. Astbury) gave shortspeeches. Then, the Emperor deliv-ered an impressive speech (Figure 2).He mentioned first how the Japanesepeople were encouraged and pleasedby the news of the Noble Prize forHideki Yukawa at the time when Japa-nese people had been devastated bythe War. Then, he referred to Dr.Nishina who built the first cyclotron inJapan and sympathized with him onthe occasion when his cyclotron wasthrown into the Tokyo Bay after theWar. Finally, he touched both positiveand negative sides of science by refer-ring to nuclear power, and spoke ofhis strong desire that the research innuclear physics must contribute toworld peace and the happiness of thehumankind. The entire message iswritten in Ref. [1].
After the Opening Ceremony aTea Party was held by invitingselected participants. They chattedwith the Emperor and Empress at avery warm atmosphere (see Figure 3).
Because it is not common to havethe Majesty at a scientific conference,the INPC2007 was selected as themost important conference in 2007among conferences that were spon-sored by the Science Council of Japan.
Scientific Program The scientific session of the con-
ference was started by an opening talkdelivered by W. Wiese. Then, almostall subjects in nuclear physics werediscussed in subsequent scientificsessions. About 30 plenary speakersspoke about the most recent progressin the fields covering neutrinos, hot
Figure 1. Speakers at the Yukawa session held on the first day. (Photo for T.Yamazaki is missing from this poster photo.)
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Vol. 17, No. 4, 2007, Nuclear Physics News 41
and dense QCD matter, hadron struc-ture, nuclear structure, nuclear reac-tions, astrophysics, applications, andso on.
In parallel sessions, over 1,000contributions were submitted, out ofwhich about 300 were selected for oralpresentations. Among submitted con-tributions 300 papers were for nuclearstructure and 200 for nuclear reac-tions, showing that two traditionalfields are still growing in a healthymanner. Many new results werereported in the fields of hot and densenuclear matter, hadrons in nuclei,astrophysics, and neutrinos. Further-more, on the fourth day of the confer-ence an exciting new result wasreported from a new accelerator inJapan, the RI Beam Factory atRIKEN.
All the talks can be downloadedfrom http://inpc2007.riken.jp/
IUPAP Prize, and So On A few years ago the IUPAP (Inter-
national Union for Pure and Applied
Physics) decided to create the “IUPAPYoung Scientists Prize.” It was alsoagreed that methods of how to grantthis prize should be decided individu-ally by each commission. The C12commission, which is a commissionfor nuclear physics, decided to grantthree prizes to three young physicistsat every INPC.
At INPC2007 three physicists, asintroduced in Figure 5, received thefirst IUPAP prizes, and they gaveinvited talks in a plenary session. At alarge conference like INPC it is notcommon to select young scientists asplenary speakers. Therefore, our trialto set aside a session for the threeawardees was well received by theparticipants of the conference.
Figure 3. Tea party with the Emperor and Empress.
Figure 2. Speech by the Emperor of Japan at the Opening Ceremony of INPC2007.
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42 Nuclear Physics News, Vol. 17, No. 4, 2007
In addition, the Elsevier publishingcompany decided to give two prizes,one for the most impressive oral pre-sentation and the other for the mostimpressive poster presentation. Theaward ceremony for these two prizeswas held at the same time as theIUPAP Session.
RI Beam Factory and J-PARC In Japan, two new accelerators will
be open to international users. One isthe RI Beam Factory at RIKEN,which started its operation from theend of 2006, and the other is theJ-PARC high-intensity proton accelera-
tor at KEK and JAEA, which will startto operate from the end of 2008. Theorganizers feel strongly that it isextremely important to open these twonew facilities to international commu-nities. Having these accelerators was,therefore, a strong motivation forJapanese nuclear physicists to hostthis important conference in Tokyo atthis time. Pre- and post-conferencesymposia were thus devoted to explor-ing scientific opportunities at thesetwo facilities. Tours of the two facili-ties were organized before the confer-ence. It was extremely well receivedfrom those who participated in thepre-symposia.
Unification of the Field Along with the expansion of
nuclear physics, the field has alsobeen fragmented. Accordingly, topicalconferences have become popular inrecent years, more than a generalnuclear physics conference. There-fore, when we decided to organize thisINPC conference in Tokyo, the firstgoal was to set to reunification of frag-mented sub-fields. This effort is veryimportant, because nuclear physics isan exciting field and it has a commongoal to study many-body isolated sys-tems in vacuum.
At INPC2007, this goal wasachieved rather successfully, primarily
Figure 4. Scenery for a plenary session.
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Vol. 17, No. 4, 2007, Nuclear Physics News 43
because of excellent speakers at theconference. All the organizers feel
strongly that this trend to reunitesubfields in nuclear physics must
be kept in future conferences atINPC.
At the end of the conference, itwas announced that the next INPCconference would be held in Vancou-ver in 2010.
Reference 1. S. Nagamiya, AAPPS Bulletin 17
(2007) 2.
SHOJI NAGAMIYA
J-PARC Center
OHRU MOTOBAYASHIT
RIKEN Nishina Center
MAKOTO OKA
Tokyo Institute of Technology
Nuclear Physicists Meet in the Land of the Incas From June 11 to 16, 2007, nuclear
physicists from around the world metin the historic city of Cusco, Peru tocelebrate the VII Latin AmericanSymposium on Nuclear Physics andApplications. And a celebration itwas! Cusco, the ancient capital of theIncas, provided an awe-inspiring set-ting for the symposium. The city wel-comed the scientists with open armsbeginning with an inauguration cere-mony that took place at the ParaninfoUniversitario, a beautiful colonialcloister at the Plaza de Armas. Thiswas followed by an exclusive visit tothe Incan Museum of Cusco and cul-minated in the evening with a recep-tion at the Palacio Municipal wherethe participants were escorted aroundthe Plaza de Armas by an outstandingmusic band of students. What fol-lowed was a truly exciting week thatgave nuclear physicists the opportu-
nity to enjoy a momentous scientificprogram in the sacred land of the Incacivilization, in which Cusco shinedthrough its ancient ruins, monuments,and the music and smiles of the Incanpeople.
This conference continued theseries initiated in Caracas, Venezuela(1995, 1997), with subsequent meetingsin San Andrés, Colombia (1999),Ciudad de México, Mexico (2001),Santos, Brazil (2003), and Iguazu,Argentina (2005). The Symposiumprovided a forum for the promotion ofNuclear Physics and its applicationsamong Latin American laboratoriesand the international community. Thesymposium series has evolved from ameeting on Nuclear Structure andHeavy-Ion Reactions in 1995 to thepresent format, where all majorresearch frontiers of nuclear scienceare represented together with a strong
emphasis in the applications of thefield and its broad impacts on societyat large. Our aim is to hold an eventthat clearly shows the vitality and
Figure 5. Three IUPAP Prize winners: R. J. Fries (left), Y. A. Litvinov (middle),and K. Sekiguchi (right).
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44 Nuclear Physics News, Vol. 17, No. 4, 2007
significance of our field both to phys-ics itself as well as to the well-beingof society through its numerous andpowerful applications. More than 120scientists and students representing atotal of 20 countries that includedmost of Latin America, the UnitedStates, Canada, the European Union,Croatia, Australia, and Russiaattended the Cusco Symposium. Thesessions were held at the Departmentof Physics of the Universidad Nacio-nal San Antonio Abad del Cusco(UNSAAC). The meeting consisted ofplenary and parallel sessions and cov-ered six main topics: Applications ofNuclear Physics, QCD in NuclearPhysics, Fundamental Symmetriesand Neutrinos, Nuclear Structure,Nuclear Astrophysics, and Advancesin Nuclear Physics.
Thanks to grant support from theNSF, a total of 16 graduate studentsfrom U.S. institutions participated in
the Symposium. They were joinedby a similar number of students trav-eling from Latin American countriesand by local Peruvian students. Theypresented 20-minute talks on theirresearch and each submitted a 4-page paper for publication in theupcoming AIP proceedings. A spe-
cial student session was held wherethey discussed issues like futureinteractions and opportunities forgraduate school in the United States.The Symposium was also a time foracknowledgments as a special ses-sion was devoted to honor the scien-tific career of Professor Ettore
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Gadioli of the University of Milano.Professor Gadioli has a long-stand-ing relationship with Latin Ameri-can nuclear physicists and he wasinstrumental in launching the firstSymposium in 1995.
The last day of the Symposium,June 16, was reserved for an orga-
nized trip to Machu Picchu, probablythe most familiar symbol of the IncaEmpire and recently recognized as oneof the wonders of the modern world.This exciting trip was the perfect giftafter a week of talks, events, excur-sions, cold weather, altitude sickness,and many more adventures. The Sym-
posium series will continue in 2009with Chile already selected as themost likely host.
RICARDO ALARCON
Arizona State University
International Symposium on Physics of Unstable Nuclei (ISPUN07)
The second International Sympo-sium on Physics of Unstable Nuclei(ISPUN07) was held from July 3 to7, 2007, in a sea side resort close toHoi An, an ancient town on the cen-tral cost of Vietnam that is protectedby UNESCO as a World Heritage.The first meeting of this series tookplace in 2002 at Halong Bay, also afamous destination in Vietnam, withthe aim to encourage nuclear physicsresearch in that part of the continentand to give an opportunity for thenew generation of Vietnamese phys-icists to meet their colleagues fromabroad. An early attempt to hold aconference in Hanoi in 1994 has alsobeen quite successful and got a goodresponse from the internationalnuclear physics community. Thesuccess of ISPUN02 led to the ideaof having a regular cycle of suchmeetings in Vietnam, focusing oncurrent issues of nuclear structureand reactions studies with unstablenuclei.
ISPUN07 was organized by Insti-tute for Nuclear Science & Technique(INST) and Institute of Physics andElectronics (IPE) in Hanoi, and it hasstrongly benefited from the encour-agement and support from variousorganizations both at home and
abroad: the Natural Science Councilof Vietnam and Vietnam AtomicEnergy Commission, GANIL, CEASaclay, GSI Darmstadt, RIKEN, andthe EU Asia-Link network on nuclearphysics and astrophysics. In particu-lar, the involvement of the latter net-work has allowed the attendance of anumber of Asian students (6 fromChina and 3 from Vietnam) who par-ticipated actively in the Asia-Link net-work workshop held on July 2 in HoiAn, and in ISPUN07 after that. Itshould be noted that several of themdid not hesitate to travel all the way bytrain from Beijing or Lanzhou (a 3-day trip each way) to Hoi An for thispurpose.
The attendance was quite satisfac-tory with a total of 90 participants,about 80% coming from abroad andthe rest from Vietnamese universitiesand institutes in Hanoi, Ho Chi MinhCity, and Dalat. The ISPUN07 scien-tific program has been rather heavywith a total of 66 oral presentations. Inaddition, a poster display was set upfor the whole duration of the meeting.Nevertheless, the atmosphere wasquite relaxed and there were quitegood working conditions in beautifulnatural surroundings, with the weatherconditions varying from heavy tropi-
cal downpours to hot sunny days. Inthe middle of the week a half-dayexcursion around the ancient Hoi Anby rickshaw (the local taxi) wasorganized for the whole group of par-ticipants in order to give everyone achance to discover the ancient city ofHoi An which was open to the Worldin the 17th century by the Portuguesemissionaries and settled down by theJapanese and Chinese merchants lateron in the 18th and 19th centuries. Theexcursion was concluded by a relaxingboat ride along the Thu Bon River,down to where the river merges intothe sea.
The scientific program of the sym-posium can be divided into three mainparts: construction of the new facili-ties and instruments, present status,and new problems in the experimentalstudies of unstable nuclei, status, andprogress of theoretical approaches.Concerning the new facilities andinstruments, there were several gen-eral presentations. The RIBF atRIKEN was presented, with the firstbeam extracted in December 2006 andthe first experiment with the RI beamseparator Big RIPS performed in May2007, where results on the new iso-tope 125Pd have been obtained. TheSPIRAL2 project at GANIL is
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expected to produce radioactivebeams of unprecedented intensities(two orders of magnitude above theexisting facilities) in the mass rangefrom A = 60 to A = 140, at energiesfrom a few keV up to 25 MeV/nucleon. The FAIR facility at GSI hasan ambitious program of studyingexotic nuclei by means of direct reac-tions at low momentum transfer. TheEXL set-up is a high efficiency andhigh resolution universal detectionsystem, and its first feasibility studywith prototype detectors at the ESRstorage ring of GSI was presented. Abrief presentation of the Radioactive IonBeams Facility in Brazil (RIBRAS)was also given. News was given onthe successful commissioning of thelarge acceptance spectrometer MAG-NEX and the forthcoming commis-sioning of the high resolutionspectrometer SHARAQ. We had apresentation of the ICHOR project,which will be of great help for thestudy of spin-isospin excitations inunstable nuclei. There was an over-view of the REX-ISOLDE physicsprogram and an outlook for theupgrade of energy, beam intensity,and quality of the ISOLDE complex(HIE-ISOLDE). An interesting pre-sentation of the experimental set-upfor the cosmic ray shower detection inHanoi was given by a member of theVATLY laboratory at INST, which isan associated member of the interna-tional Pierre Auger Project in studiesof ultra-high energy cosmic rays.
On the experimental side, the mainemphasis is of course on the studies ofnuclei far from stability. Because ofthis, the topic of giant resonances isless actively explored whereas theinterest is now on low-lying states,pygmy states, and resonances at lowerenergies. Results from invariant massspectroscopy of halo nuclei—11Li and
14Be—were shown. Recent measure-ments of 18–21N isotopes and excitedstates of 18–21O by beta-delayedneutron and gamma emission werepresented. Other issues of currentinterest were discussed in severaltalks: the changes in subshell closuresaround N = 28 and Z = 50, N = 82; coreplus particle structure in Carbon iso-topes; transitions in equilibriumshapes of nuclei. A presentation wasgiven on the subject of superheavynuclei produced in 48Ca induced reac-tions, and the evolution of fissiontimes in the region Z = 114–124 wasalso discussed in another talk. Anotherimportant issue is the symmetryenergy term of the nuclear Equation ofState, and there were two talks dis-cussing it based on the latest heavy ionfragmentation measurements. In ashort but very interesting section ofnuclear astrophysics, stellar nucleo-synthesis has been discussed in sev-eral talks: a study of the 7Be(p,γ)Bstellar reaction, a measurement of thealpha spectroscopic factor of the 6.356MeV state in 17O, a preliminary mea-surement of the 21Na(a,p)24Mg reac-tion for investigating the abundance of22Na in novae. In the nuclear reactionstudies with unstable beams, therewere interesting presentations on theresults obtained at GSI from the frag-mentation of 208Pb beam, analyzingpower measurements at RIKEN ofelastic proton scattering off 6He, reac-tion mechanisms in Be + 64Zn systemsaround the Coulomb barrier. It wasalso stressed that no enhancement isobserved in the 6He + 64Zn sub-barrierfusion.
On the theory side, several talkswere devoted to clustering phenomenain stable and unstable nuclei usingAMD and AMD + GCM approaches,and to the 4-alpha condensate struc-ture of the 4th 0+ state in 16O. The
issues of magnetic properties of lightneutron-rich nuclei, and the structureof medium mass exotic nuclei werediscussed in the framework of thesnhell model. A comprehensive over-view of the developing V-low kapproach was given. Several presenta-tions dealt with the self-consistentmean field approach in non-relativisticand relativistic framework, applica-tions to the description of superheavynuclei and deformed exotic nuclei, thescreening of pairing interactioninduced by low-lying surface vibra-tions. The pairing correlations near theneutron drip line, or at finite tempera-ture, were also discussed. Nuclearexcitations treated by RPA or QRPAwere the subject of a series of presenta-tions, with emphasis either on continuumeffects, deformation effects, or damp-ing effects due to particle-vibrationcoupling. Nuclear reactions were wellrepresented with talks on the Multi-Channel Algebraic Scattering theory,studies of three-body resonances,spectroscopy of exotic nuclei byknock out reactions, and the presenta-tion of a classical stochastic model ofbreak up. Nucleon-nucleus and nucleus-nucleus optical potentials calculatedfrom Dirac-Brueckner-Hartree-Fockapproach using the folding modeltechnique are shown to describe ratherwell nucleon– and nucleus–nucleuselastic scattering data. Transport mod-els were shown to be quite success-fully in studying the isospin dynamicsin nuclear collisions. Finally, theeffects of three-body forces on theEquation of State of nuclear matter,and on the structure of hybrid (had-rons and quarks) neutron stars werereported.
The ISPUN07 symposium willgive a long-lasting memory to all itsparticipants. Especially, for the youngAsian students it has been surely a
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precious experience of having theopportunity to meet and discuss phys-ics with many world experts of thefield, a benefit that certainly overridesthe hardships of traveling. For others,it was not only just a scientific confer-ence but also a unique occasion to dis-cover the beauty of a far away countryas well as the hospitality of its friendlypeople.
DAO TIEN KHOA
INST Hanoi
NGUYEN VAN GIAI
IPN Orsay DAO TIEN KHOA NGUYEN VAN GIAI
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New Promises for the Determination of the Neutrino Mass? (A Brainstorming Meeting at GSI, Darmstadt)
Neutrino physics has followed along way, full of striking discoveries,since Wolfgang Pauli postulated theexistence of the ghostly particle in hisfamous letter to “Liebe radioaktiveDamen und Herren.” The last fascinat-ing discovery concerns the observationthat neutrinos oscillate, that is, changeflavor, which illustrates that the neutrinomust have mass. However, oscillationexperiments themselves are not able toobtain the neutrino rest mass. They candetermine only the mass differencessquared. The absolute mass must bedetermined by different measurements.
Over the last 60 years heroicattempts have been undertaken to finda neutrino mass different from zero. Adramatic decrease in the upper masslimit from 1000 eV down to 2 eV wasobtained over this long period of timefor the electron antineutrino in the tri-tium decay experiments, which aremost sensitive. The advances in theneutrino mass measurements were notso impressive and the knowledge ofthe neutrino mass presently is stuck atthe level of 225 eV obtained in mea-surements of the internal bremsstrahl-ung spectra from the decay of 163Hoby electron capture. This nuclide hasthe smallest decay Q-value known sofar and could be considered as themostly favorable for the neutrino massdetermination.
What is the reason for such a largeneutrino–antineutrino difference inthe mass limit? Do we need toimprove the neutrino mass value? Andif, how to achieve a dramatic increasein precision of mass determination inthe electron-capture sector? All thesecornerstone questions have been dis-
cussed in an exchange of ideas takingplace at GSI on March 8–9, 2007.
The idea of neutrino mass determi-nation by electron capture was put for-ward a long time ago. One of the firstattempts was made at ISOLDE/CERNat the beginning of the 1980s. Theupper limit of 1300 eV for the neu-trino mass was determined by observ-ing the M-shell x-rays and Augerelectrons in the decay 163Ho ⇒ 163Dyand by additional measuring a decayQ value in a single-nucleon transferreaction. Attempts performed duringthe subsequent decade by differentexperimental groups improved thislimit only by a small factor, twoorders of magnitude less stringent thanin the case of the antineutrino massvalue. In that time the technical possi-bilities did not keep pace with require-ments for more precise measurements.
However, over the last decade theprogress achieved in both the tech-niques of mass measurements in Pen-ning traps as well as ofmicrocalorimetry of atomic de-excita-tions, allows one to resurrect attemptsfor neutrino mass determination at anew level of accuracy.
The difference between the massesof parent and daughter atoms in theelectron capture process is sharedbetween the captured-electron bindingenergy and the total neutrino energy,which includes also the neutrino restmass. In order to determine the latter,the atomic masses of the parent anddaughter atoms and the electron bind-ing energies should be measured asaccurately as possible.
There exist different scenarios howto proceed experimentally as dis-
cussed by Yu. Novikov (PNPI andGSI) in his contribution. Mass differ-ences can be measured by state-of-the-art Penning traps with an accuracy ofthe order of 10−11 as it was pointed outby K. Blaum (Mainz and GSI) in hisreview talk about ion traps and theirimpact on physics research. Measure-ments of atomic de-excitation spectra,occurring after filling the vacancy inthe atomic shell, can be carried out bymicrocalorimeters, which detect thetotal energy release from electrons andphotons but neutrinos. As noted by F.Gatti (Genoa) and L. Gastaldo(Heidelberg) in their presentationsabout the unique potential of micro-calorimetry, the energy position ofpeaks in these spectra, that is, the elec-tron binding energies, can be deter-mined with an accuracy better than1 eV for a sum energy of about 1 keV.This corresponds to M-electron bind-ing energies in the region of holmiumand dysprosium.
Two different approaches exist, inboth of which the determinations ofcalorimetric spectra and of Q valuesare essential; One involves the deter-mination of branching ratios for elec-tron capture from different atomicorbits, the other method the shapeanalysis of the calorimetric sum peak.
In the first approach, the branch-ing ratios must be determined with anaccuracy of about 10−4 and the Q val-ues should reach an accuracy ofapproximately 0.1 eV. Both require-ments can not be met presently. Asdiscussed by W. Quint (GSI), eventhe very auspicious Penning trapfacility for highly charged ions,HITRAP, presently being installed at
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the Experiment Storage Ring ESR atGSI, will not allow Q value measure-ments with such precision in the fore-seeable future.
More promising looks to be thesecond approach whose idea was putforward many years ago by A. DeRujula: The tails of the peaks of theatomic de-excitation spectra dependon the neutrino mass and the peaknearest to the Q value is most sensitiveto its value. In principle, analysis ofthe spectral peaks in the capture chan-nel has a big advantage in comparisonto observation of the spectrum of theβ− decay near the endpoint: For theexample of the 163Ho decay, oneexpects for the 2 eV part at the edge ofthe 2.047 keV peak normalized to thefull peak area a ratio ranging from 10−10
to 10−11 if the Q value varies between2.3 and 2.8 keV. This corresponds to again in sensitivity by about two ordersof magnitude as compared to the tri-tium decay with the decay Q value of18.59 keV.
In the second approach, the neu-trino mass or mass limit deduced fromelectron capture in 163Ho also stronglydepends on both the accuracy of decayQ value as well as on the absolutevalue of Q. The required data acquisi-tion time is estimated to rangebetween 60 and 600 days for derivinga 2 eV neutrino mass limit from mea-surements with a multi-pixel micro-calorimeter.
Another crucial question discussedduring the brainstorming at GSI waswhether or not other candidates existappropriate for capture measurementsif Nature will not be so kind to pro-vide a favorable Q value for 163Ho.The search for new candidates shouldbe supplemented by precise spectro-scopic measurements of atomic andnuclear excitation energies by bolom-eters whose features were presented
by P. Egelhof (GSI). A search of exist-ing data reveals that there are about adozen of relevant pairs of nuclides forwhom the decay Q values could bevery small if also nuclei with decaysfrom ground states of the mothernuclides to excited states of the daugh-ters are included. However, an assess-ment of how small the Q-values areand if those nuclides are at all suitablefor neutrino mass determination ispresently not feasible because of thelarge uncertainties, in many casesexceeding even 10 keV.
In order to search for additionalcandidates for a determination of theneutrino mass one does not need tostart with ultra-precise mass measure-ments from the very beginning. Forthese purposes the existing Penningtrap systems, which are on-line withthe various radioactive beam facili-ties, can initially be used. In presenta-tions given by F. Herfurth (GSI) andA. Herlert (CERN) the possibilitiesfor atomic mass measurements ofsuch candidates at SHIPTRAP/GSI,ISOLTRAP/CERN and other facili-ties have been discussed. Measure-ments at these installations can beconsidered as mutually complemen-tary and as a prerequisite for laterultra-accurate mass measurements atthe HITRAP facility at GSI. Plans fora new Penning trap mass spectrome-ter were presented by S. George(Mainz and GSI). It consists of fourcryogenic traps in the very samesuperconducting coil and is presentlydesigned at Mainz University by K.Blaum et al. The mass uncertainty isexpected to reach a level of 1 eV forheavy nuclides if ions in high chargestates are used. As discussed by O.Kester (GSI), those can be producedby stripping of relativistic projectilesby a massive target or by impact ofhigh-energy electron.
The domain of antineutrino massdetermination in continuous β− decaywas not omitted at the brainstorm-ing. The flagship project KATRINwas presented by Ch. Weinheimer(Münster) who expects a mass limitor mass value at the level of 0.2 eVto be reached in the measurementcampaign planned for 2010 to 2015.A. Fässler (Tübingen) announcedsimilarly low (but model-dependent)limits from the analysis of theHeidelberg-Moscow measurementsof the half-life of neutrinoless dou-ble β-decay of 76Ge. The WITCHexperiment installed at ISOLDE/CERN was discussed by N. Sever-ijns (Leuven). It allows the searchfor a heavy electron neutrino with amass value reaching up to a fewMeV.
A review of the impressive poten-tial (but also of conceptual problems)of neutrino physics was given by M.Lindner (Heidelberg). Accurate neu-trino mass measurements will lead tonew physics beyond the present Stan-dard Model of elementary particlesand the eventual deviations of theneutrino and antineutrino parame-ters would result in rather radical the-oretical changes such as CPTviolation.
Among the wide variety of studiesof neutrino properties, mass measure-ments are very ambitious ventures thatare of paramount importance forlarge-scale physics and cosmology.Therefore dissimilar approaches usingnovel scenarios and completely differ-ent techniques (and, thus, sufferingfrom completely different systematicuncertainties) should be stronglyencouraged.
As a result of extensive discus-sions at the meeting, the followingmilestones in the neutrino mass pro-gram can be outlined:
facilities and methods
50 Nuclear Physics News, Vol. 17, No. 4, 2007
• Masses for the pair 163Ho – 163Dyshould be measured with utmostaccuracy at the new-generationfour-trap Penning mass spectrome-ter, which is under design for theHITRAP facility at GSI. The long-lived radioactive nuclide 163Ho canbe produced either at ISOLDE/CERN and then moderately ion-ized in the MAXEBIS (electronbeam ion source) of HITRAP oron-line produced in highest chargestates at GSI. Similarly, the massof moderately or highly chargedstable 163Dy can be measured. Theexpected uncertainties for massvalues are about 1 eV, which isadequate to provide a neutrinomass determination on a level of afew eV. An exact Qε value for thepair 163Ho-163Dy is a very crucialstarting point for subsequent anal-ysis of microcalorimetric atomicde-excitation spectra.
• Independent of the mass measure-ments the atomic de-excitationspectra after electron capture by163Ho should be measured off-lineby multi-pixel microcalorimetricdetectors with an uncertainty in thepeak position of about 1 eV. This ispresently achievable, for example,at the Universities of Genoa and
Heidelberg. The 10−6 s time resolu-tion of the detector is sufficient toreach the required statistics withina few weeks to a few months ofmeasuring time as determined bythe exact Qε value for 163Ho providedby Penning trap measurements.
• In parallel to these investigations,new candidates for the neutrinomass determination by electroncapture should be searched for.As the mass uncertainties for theabout ten candidates with small Qvalues are still too large precur-sory measurements at the differ-ent existing traps (ISOLTRAP,SHIPTRAP, JYFLTRAP, etc.)should be performed with subse-quent ultra-precise mass measure-ments of the most promisingcandidates at the HITRAP facil-ity. In addition, trap-assistednuclear spectroscopy of selectednuclides can be performed forvery accurate determination ofdecay channels.
• The analysis of the experimentaldata should be accompanied byvery accurate theoretical calcula-tions of the atomic structure of theatomic systems under investiga-tion. Here, the electron wavefunctions of the innermost atomic
shells of neutral atoms and those ofhighly charged ions are of supremeimportance and can be scrutinizedby atomic theorists from St.Petersburg.
From this list of required work it isobvious that a campaign for neutrinomass measurements with an accuracyof a few eV (two orders of magnitudebetter than known now) requires thecollaboration of multidisciplinaryexperts and demands well-coordinatedcommon efforts. A very attractiveaspect of this innovative activity isthat the experiments can be performedat existing facilities or new instru-ments coming soon into operation. Itcan be expected that the tools andapproaches of atomic physics (atomicmasses, atomic de-excitation spectra,atomic electron binding energies, etc.)will enable in the near feature a newaccess to a fundamental property of afundamental particle.
H.-JÜRGEN KLUGE
GSI, Darmstadt
YURI NOVIKOV
PNPI, St. Petersburg
news from EPS/NPB
Vol. 17, No. 4, 2007, Nuclear Physics News 51
Call for Nominations for the Lise Meitner Prize for Nuclear Science of the European Physical Society, 2008
The Nuclear Physics Board of theEPS invites nominations for the “LiseMeitner Prize” for the year 2008. Theaward will be given to one or severalindividuals for outstanding work inthe fields of experimental, theoretical,or applied nuclear science. The Boardwelcomes proposals that represent thebreadth and strength of Europeannuclear sciences.
Nominations need to be accompa-nied by a filled-in nomination form, abrief curriculum vitae of the nominee(s)and a list of major publications. Lettersof support from authorities in the fieldthat outline the importance of the workof the nominee(s) are also helpful.
Nominations will be treated as strictlyconfidential and although they will beacknowledged there will be no furthercommunication from the selection com-mittee. Nominations should be sent to:
Selection Committee Lise MeitnerPrize
c/o Chairman Prof. HartwigFreiesleben
Institut für Kern- und Teilchenphysik Technische Universität Dresden 01069 Dresden, Germany Phone: +49 (0)351 46335461; Fax:
For the nomination form and moredetailed information go to the websiteof the EPS, Nuclear Physics Division:http://ific.uv.es/epsnpb/ or the web-site of the EPS: www.eps.org (EPSPrizes, Lise Meitner Prize)
The deadline for the submission ofnominations has been set for January11, 2008.
