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INSTRUMENTATION AND TECHNIQUES

3.1 Relevant Set-Ups at LNL

3.2.1 Introduction

3.2.2 Nuclear physics experiments with traps

3.2.3 RIB studies with a TANDEM/LINAC accelerator complex

Nuclear moments

Nuclear decays

3.2 RIB with Ion and Atomic Traps

Appendix: Acceleration of Radio-Isotopes with the TANDEM-ALPI

Nuclear masses

Foundamental Interactions

Measurement with MOT

Ion traps coupled to a gas-filled separator

Accelerator Complex at the LNL

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Chapter 3

INSTRUMENTATION AND TECHNIQUES

The previous chapter has summarized the exciting research opportunities offered by a newfacility for intense radioactive ion beams. Here we would like to point out the richness and varietyof powerful set-ups already available, or under construction or consideration, at LNL, for theexperiments with neutron-rich exotic beams like those produced with the proposed facility

Taking advantage of those new opportunities requires the capability of selecting rare eventsthrough the use of powerful detection systems like the new generation of γ-ray detector arrays ormodern magnetic spectrometers (PRISMA is under construction at LNL).

Legnaro has acquired a long experience in hosting large γ-ray detectors arrays like GASP andEUROBALL used in combination with ancillary detectors to achieve the necessary reaction channelselection. Several powerful devices for selecting very rare events have been used. Moreover LNLare now involved, in the framework of the program for training and mobility of research in Europe,in developing new γ ray tracking devices (MARS). Many nuclear physics experiments require theability to study final states belonging to reaction channels which involve a relatively large numberof light charged particles detected in 4π hodoscopes (the 8πLp array at LNL is an example of this).The relative population of such states may be very weak, implying that they can be discriminatedonly through the simultaneous detection of all evaporated charged particles in coincidence with γ-rays.

The most powerful method of selecting nuclei far from stability, produced by fusion-evaporation, relies on direct identification by a Recoil Mass Spectrometer (RMS) like CAMEL atLNL (see Fig. 3.1). Coincidence measurements of the γ rays detected in a large Ge array with themass of the recoiling nuclei has indeed made possible to approach the proton drip line.

Such technique, moreover, opens new possibilities already with the present available beams,for example in combination with ion traps, allowing to investigate very short-lived β- decay processeswhose study may represent a strong test of fundamental theories. In this context the possible existenceof new phases of nucleonic matter such as proton-neutron pairing is just an example.

The next paragraph goes into more detail and gives a survey of further instruments andtechniques one will need in the experiments with exotic beams, extrapolating from the presentsituation and expertise at Legnaro.

Further ideas concerning new set-ups using Ion and Atomic Traps (sect. 3.2) and accelerationof radioisotopes (Appendix), will be then presented, some of which already possible with the presentLNL accelerators, with the important purposes of R&D and of getting expertise.

3.1 Relevant Set-Ups at LNL

The advent of advanced facilities for exotic beams brings both enormous opportunities forexciting research on new nuclear phenomena and high technical challenges. The latter stem fromthe fact that exotic beam intensities will be in many cases orders of magnitude lower than beamswe are accustomed to. At energies close to the Coulomb barrier several questions have to be addressedexperimentally. For investigations using neutron-deficient beams one has to face the high γbackground produced from the decay of the scattered beam. This points to the use of ancillarydetector systems to improve the signal to noise ratio and the channel selection.

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For neutron-rich beams such requirements are less severe since the decay of the scatteredbeam will not produce any flux of 511 keV γ radiation [Cat96]. Anyway to investigate the newnuclei that we will have access to, it will be required to develop highly efficient detectors for γ,charged particles and heavy-ions.

Concerning the γ detectors, high efficiency, high granularity and high resolution γ arrays willbe needed. The need for high efficiency is clearly due to the low intensity of the beams (at least atthe beginning). The high granularity is required in many cases both to reduce the Doppler effect innuclear reactions and to track γ rays in order to distinguish total energy absorption events fromCompton scattering. The present generation of high efficient and high resolution γ arrays, likeGasp, Euroball and Gammasphere, has opened new possibilities in nuclear structure studies [Ros93].However, the possibility of new technology leading to improvements of orders of magnitude inresolving power for high multiplicity events has recently arisen with the concept of γ ray energytracking devices. Such highly segmented detectors follow each γ ray through its path in the detectormaterial. Efforts in this direction are currently underway both in America (Greta) and in Europe(Mars, Miniball), see Fig.3.2.

A good reaction channel selectivity can be achieved using charged particle detectors, especiallyif combined with neutron detectors. A low energy threshold, moreover, is particularly suited forthe detection of charged particles which may compete, in the decay, with the γ branches, as it isexpected for very proton rich nuclei.

Arrays of Si telescopes using the total energy (E) and the energy loss (∆E) information forthe particle discrimination can have a relatively high geometrical coverage and give a reasonablygood energy measurement of the evaporated light charged particles [Far97].

In order to achieve, at least partially, the discrimination of low energy light particles andheavy fragments that stop into the first stage ∆E detectors, the pulse shape technique has beenrecently applied [Pau94]. The high efficiency 4π Si-ball EUCLIDE, using this pulse shape techniqueand designed as a trigger device for the Euroball γ-ray spectrometer, is shown in Fig. 3.3. EUCLIDEis, composed of segmented Si detectors in a very compact mount, and is presently under developmentat LNL [Dea98].

Large charged particle detection systems with good energy and angular resolutions arenecessary for the studies of reaction mechanisms as well. Interesting fields of investigation are forinstance the competition between break-up and fusion reactions or the inelastic excitation of lightprojectiles impinging on medium mass target nuclei. The presence of a neutron skin/halo caninfluence rather strongly the reaction mechanisms and rates, with severe implications onastrophysical problems.

Direct reactions, performed over a wide range of incident energies, are a fundamental tool toobtain nuclear structure information. Elastic scattering will probe the nuclear density profile andeffective in-medium nucleon-nucleon interactions. Inelastic scattering to collective states yieldsinformation on the nuclear deformation, while transfer reactions will give direct access to nuclearshell structure. These fundamental characteristics are expected to exhibit major modificationswhen moving away from stable nuclear matter.

For unstable nuclei these reactions can be performed by using radioactive beams in inversekinematics where the unstable nucleus of interest impinges on a light target nucleus. The mostpowerful method for determining the scattering angle and excitation energy is the accuratemeasurement of the angle and energy of the recoiling light target nucleus. This can be achieved byusing a large solid angle position sensitive light particle array. First generation arrays dedicated tosuch measurements, and based on Silicon strip technology, have been implemented in severallaboratories for radioactive beam experiments (GANIL, MSU and RIKEN) and have alreadyobtained impressive results [Blu97]. A major goal in the future would be a significant increase ofthe solid angle coverage and performance of such arrays; this could most efficiently be achieved

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through international co-operation. Such detectors can also be used together with γ spectrometersin order to tag different exit channels.

New generation magnetic spectrometers are needed, with efficiencies one order of magnitudelarger than the existing ones. A high efficiency heavy-ion spectrograph (PRISMA) [Ste98] ispresently under construction at LNL. Its optical design is very simple (one quadrupole singlet andone dipole magnet), but PRISMA will exploit fully the possibilities offered by tracking techniquesalong the ion paths. It combines good mass (1/300) and energy (up to 1/1000) resolutions with alarge momentum acceptance (±10%) and a very large solid angle coverage (up to ~80 msr). PRI-SMA will be extremely useful in the studies of reaction dynamics (transfer and deep inelasticprocesses, fusion-fission,…) with RIB. PRISMA is a funded project of INFN and it is planned thatit starts the first test runs with (stable) beams within 2000. A pictorial view of the spectrometer isshown in Fig. 3.4. Further classes of experiments will be possible with PRISMA in conjunctionwith large γ arrays, where the spectrometer will allow to identify and tag exotic neutron rich nucleipopulated via multinucleon transfer and deep-inelastic reactions. These spectroscopic studies canbe complemented by using fission induced by medium or heavy projectiles. An analogousspectrograph of the next generation (VAMOS) is under construction at GANIL, where RIB arealready available. VAMOS will also offer the possibility of beam rejection for the use at 0˚, whileLNL have already CAMEL for this purpose.

Advanced facilities for exotic beams open new possibilities also for β decay studies. A recentcollaboration among LNL, University of Surrey, IReS at Strasbourg and the University of Valenciaintends to develop a spectrometer well suited to the study of β decay of exotic nuclei, to exploit itwith the beams of radioactive nuclei of 60 keV energy available now at CERN-ISOLDE, in view ofits eventual use at the LNL radioactive beam facility.

Studies of β decay are an essential part of our attempts to obtain a better understanding of thestructure of atomic nuclei. They are important in themselves for the information they carry aboutnuclear structure, and they are also important for our understanding of other physical processes.For example, β decay plays an important role in nucleosynthesis in stars, particularly in thedetermination of the reaction pathways followed in explosive nucleosynthesis. β decay also playsa particularly important role in studies of nuclei far from stability. First, the Q-values of β decayincrease rapidly as we move away from the line of stability and we have access to a wider range ofstates. Second, at the extremes of production, β decay will often provide the first information onnew nuclei and may be crucial for identifying them.

However, experimental determinations of the Gamow-Teller strength distribution are noteasy. Usually the strength to a particular daughter state is measured by detecting the γ rays emittedsubsequently from this state. In general the state is fed directly in β decay and indirectly byelectromagnetic transitions from higher-lying levels. The β decay strength is deduced from thebalance of feeding and de-excitation by electromagnetic transitions. It is common to use semi-conductor detectors in such measurements. Their efficiency for high energy γ rays is low; often itis as low as a few percent. As a result the experimenters rely on estimates of how much of thefeeding of the levels flows indirectly from the higher-lying states. The resulting GT strengths extractedfrom the data are often unreliable.

The use of the Total Absorption Gamma Ray Spectrometer (TAGS) technique may solve thisproblem by measuring the total population of the states fed in β-decay directly. The TAGS consistsof a large scintillation detector (photopeak efficiency 70% at 5 MeV), of NaI type or with liquidXe, with a hole through the centre to allow the low energy beam of radioactive nuclei to enterdirectly, or be brought to the centre by a tape system. This also allows the insertion of, and connectionsto, ancillary detectors for positrons, electrons, X-rays or β-delayed protons/alphas.

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3.2 RIB with Ion and Atomic Traps

3.2.1 Introduction

Ground state properties of radioactive ion beams (RIB) can be efficiently studied with awealth of experimental methods making use of laser spectroscopic techniques [Bil95] coupled toionic [Klu92] or atomic traps [Spr97, Met44]. The combined use of such techniques to RIB is veryrecent and full of potential applications for a large variety of experiments and technologicaldevelopments. These studies can be performed with very low energy beams, typically ≅ 50 KeV, asthey come out from a conventional ISOL source. However, the applications are much wider andinvolve all those cases in which the beam intensity is too low to make nuclear reaction studies, i.e.when less than 103-104 p/s are available. Since for nuclei very far from stability the intensities areexpected to be anyway of that order, one easily understands that a substantial part of the researchprogram in any new designed RIB facility should consider the development of such techniques.Besides for measurements of ground state properties of nuclei, trap technology has beendemonstrated to be a suitable one for precision measurements of fundamental interactions [Häu95],making the subject very attractive for a wider community of physicists. Peculiar characteristics ofRIB stored in a trap are in fact their very small phase space, the long storage time, the possibilityto perform measurements in a very clean environment and the negligible source thickness. Onceion/atoms have been properly produced, purified and transported into a trap, a suitable detectorarray can be placed very close to the ion/atom cloud and high precision studies of different kindscan be performed. In general, besides RIB, trap devices already found important applications inseveral other areas, microgravity, metallic clusters [Lind91], antimatter [Gabr95], atomic physics(high Z ions) [Wer96], condensed matter physics (Bose-Einstein condensation) [Bra97], andchemistry [Chu98]. The instrumentation for RIB in general requires various technologicaldevelopments, ranging from radiofrequency (RF) devices, cryogenic systems and lasers, but thefeedback in other important parts of RIB equipments has been demonstrated to be very fruitful, inparticular in accelerator related areas [Sav97]. This is the case, for instance, in the bunching systemof the REX-ISOLDE project [Habs97] or in a SPIG device [Vand97]. Activity with traps for RIB isvery recent [Raa87, Gwi94, Sims96, Lu97, Behr97] and some experiments are still in their infancy,with promising applications in a wide range of physics programs, the main ones being outlined inthe following sections.

3.2.2 Nuclear Physics experiments with traps

Nuclear masses

Theoretical models for nuclei far from stability predict nuclear masses differing strongly andmeasurements with a precision of the order of 1 MeV or less are needed. Such a precision has beenreached in storage rings [Sto97] and in Penning traps [Klu92, Klu97], where measurements can beperformed on nuclei with lifetimes as short as ≅ 1 sec. In the Penning trap the mass is determined bythe cyclotron frequency of the stored ions in the magnetic field of the trap and precisions of theorder of 10-6-10-7 for medium-heavy nuclei have been reached at ISOLDE (see Fig. 3.5).

For lifetimes shorter than ≅ 1 sec, another technique employs a radiofrequency transmissionspectrometer to determine the cyclotron frequency of the ion during a short trajectory in ahomogeneous magnetic field (MISTRAL) [Lun96]. In general, the interest in performing precisionmass measurements in a trap goes well beyond nuclear physics, being of utmost importance forfundamental interaction studies, as e.g., neutrino mass, QED tests and CPT invariance [Sto97].

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Nuclear moments

The nucleus affects the electronic levels via the hyperfine interactions, and it is from theprecision measurements of the energy levels of the atom that one can deduce nuclear properties.One important observable is the mean square radius r2 of the nucleus, which depends on its size(volume effect), deformation, and vibrational nature. r2 is derived from the measurement of theisotope shifts (IS), which are typically less than 10-5 of the central frequency, but which can bedetermined to 1% or better by laser spectroscopy. If the nucleus has a spin, the atomic levels dependon the orientation of the nuclear magnetic dipole moment µ and of the electric quadrupole momentQ with respect to the atomic fields, producing a multiplet of levels for each state. It is then usuallypossible to determine µ and Q from the position and intensities of the hyperfine components (HFS).The successful application of laser spectroscopic techniques to short lived radioisotopes derivesfrom the very large photon absorption cross section at resonance, and the sensitivity reachednowadays is impressive, being possible to deal with nuclear production rates as low as ten atomsper second [Bil95]. Even the poor energy resolution due to the inhomogeneous Doppler broadeningcaused by the thermal motion of the atoms, has been elegantly solved by a variety of techniques,most of which were borrowed by atomic spectroscopy. Two main methods have been developed,the collinear beams methods (CB) and the use of ion/atom traps. In the CB method, the beamextracted from the ion source is accelerated to some tenths of KeV and then superimposed to acollinear laser beam. The laser frequency, or the ion energy, is then tuned to get the resonance withthe atomic level of interest. The acceleration of the ion from the eV to KeV range reduces theDoppler broadening by a factor 10-3 in the direction of the acceleration, and the residual value is ofthe order of 50 MHz, i.e. comparable to the natural linewidth of the transition. Measurements arecarried out by detecting the fluorescence photons in coincidence with ions, by optical pumpingwith state-selective charge exchange, by resonance ionization, or by laser-induced nuclear orientationcombined with NMR [Bil95].

Ion/atom trap represent an alternative approach to CB methods, attaining similar sensitivities.With this method atoms/ions are repeatedly interacting with a laser after they have been stored intoa trap, and since the holding time for ions may be several hours, one can perform laser spectroscopyon a very small number of ions.

Nuclear decays

Having in mind with a trap high precision measurements like those outlined in the previousSections, one must take special cares, as e.g. to uniformities in the magnetic fields for an ion trap,or alignment in the optical system for a magnetooptical trap (MOT). Alternatively, a trap can bedesigned as a “simple” containment device for nuclear decay studies, whose most attractivecharacteristic is the possibility to detect nuclear radiation in a clean environment, free of distortionsdue to source thickness. Charged particles and γ detectors can be placed extremely close to the RIBcloud in the trap, opening the possibility to perform high efficiency and high resolution experiments.One can think for instance to install a small bolometer, with sub-KeV energy resolution, for β-decay studies or to place small Si-detector arrays for proton decay studies. Nuclei with unknowndecay schemes can be also studied by performing β-γ coincidences with a proper geometry of thecorresponding detectors [Shi98].

3.2.3 RIB studies with a Tandem/LINAC accelerator complex

In various laboratories a growing activity is devoted in implementing new techniques to trapa RIB produced via a suitable nuclear reaction with primary beams delivered by a conventional

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Tandem/LINAC accelerator complex. This kind of research is extremely interesting for those ionswhose production rate is higher for particular low energy heavy ion reactions or for those nucleiwhich, for chemical reasons, cannot easily be extracted by a conventional ISOL source and arebetter produced with in-flight methods. Essentially two main techniques are being developed, oneusing a MOT and another one using an ion trap coupled to a gas-filled separator.

Fundamental interactions

Ion/atom trap represents a competitive tool for precision studies of fundamental interactions[Häu95], like, for instance, parity non-conservation (PNC) in atoms and nuclear β decay. In fact, thestrong confinement in six-dimensional phase space and the negligible source volume make themvery attractive to perform e-/particle spectroscopy in a clean environment, or to perform ν - ecorrelation measurements by detecting the electrons and the recoils in coincidence. The expectedperformance of a well designed trap opens the possibility to search for physics beyond the standardmodel (SM) of electroweak and strong interactions. For instance, while in nuclear weak processesthere is strong evidence for the V-A form of the charged weak current, the constraints on scalar (S)(and also tensor (T)) couplings are very poor. S interactions give rise to a β decay process which isforbidden in the SM, but which would occur if a charged scalar boson or a leptoquark were exchangedinstead of the W±. Signatures of a scalar or a leptoquark exchange can be inferred in the e± elicitiesσ or in the ν - e angular correlation coefficient a of pure 0+-0- Fermi transitions. The presentexperimental upper limit (95% CL) for an S interaction is 17% (the one of the T interaction being9%). This is so mainly because the scalar exchange amplitudes affect a only to second order, andhigh precision results have been obtained in only a few cases, namely 6He and n-decay. A trap wouldcertainly be an appropriate and new tool to approach these measurements. First tests for detectingnuclear recoils inside a MOT have been made very recently at TRIUMF [Behr97] and LOS ALAMOSlaboratories. At TRIUMF a double MOT system was coupled to the TISOL on-line separator forstudies of ν - e correlations in the β-decay of 38Km (T1/2=0.925 s) and 37K (T1/2=1.266 s). At STONYBROOK [Spr97, Gwi94, Sims96] and BERKELEY [Lu97], efficient trapping of Na-Rb-Fr alkaliradioactive neutral atoms has been demonstrated. Particular efforts are put in improving the productionand trapping rates of Fr-isotopes, for PNC studies.

Measurements with MOT

Activity with a MOT is focused at the moment in studies on alkali atoms, with first attemptson noble gas atoms. The choice of alkali depends on the fact that they have a simple electronic levelscheme, suitable both for the trapping mechanism itself (based on Zeeman splitting in aninhomogeneous magnetic field), and for precision atomic theory calculations. Although in principleany kind of atom can be trapped, each one requires a specific development of the source/transportsystem and a dedicated laser set-up, whose frequency must match the proper atomic transitions. AMOT is then mainly useful for specific high accuracy measurements, and reaches nowadays trappingefficiencies of the order of 10-3-10-4 from production to trapping. As a representative example, atSTONY BROOK (see Fig.3.6), trapping of 79Rb (T1/2= 22m) [Gwi94] and of 210Fr (T1/2=3.2m)[Sims96], this last for PNC studies, have been successfully demonstrated.

The reactions were 51V(31P,2np)79Rb (100 nA 31P beam at Elab=90 MeV) and 197Au(18O,5n)210Fr(100 nA 18O beam at Elab= 100 MeV) for the two cases, and the average intensity of the alkali nucleientering the experimental area were 105-106/s. In general, the trap lifetime is limited by collisionswith background gas, which has to be kept at a pressure below 10-9 Torr. Therefore separation of theproduction and of the trapping regions is critical in order to operate the trap in a high vacuumenvironment.

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Ion traps coupled to a gas-filled separator

A promising technique has been recently developed at ARGONNE [Sav97] which uses acombination of a magnetic spectrograph and a radiofrequency quadrupole trap to collect and bunchions, to be subsequently delivered to a high precision Penning trap (CPT spectrometer). The RIBions are created by low energy heavy-ion fusion reactions and are injected in an Enge split-polespectrograph used in a gas-filled mode at near 00. The RIB with different atomic charge states isthen focused in a small spot at the focal plane of the spectrograph, decelerated in high purity heliumgas, cooled and bunched in a RF quadrupole trap structure through which the helium gas is evacuated.The linear RF structure is pumped by large roots blowers which provide differential pumpingwhile ions are guided to a linear ion trap where they are accumulated. The interest of this techniquelies in the possibility to decelerate, cool and accumulate ions of any kind in less than 100 ms,avoiding problems related to chemical extraction from conventional ISOL sources. The beam canbe then directed towards other experimental areas, as in the ARGONNE case, where a Penningtrap has been installed for precision mass measurements. Another recognized advantage is that thequality of the bunched beam after the RFQ structure is in principle good enough to allow collinearlaser spectroscopy to be performed for HFS studies. This kind of experimental approach is nowproposed at GSI [Shi98], within a large collaboration, for studies on superheavy elements producedby fusion reactions and separated by SHIP. The concept of the SHIPTRAP set-up (see Fig. 3.7) issimilar to the CPT spectrometer, i.e. the superheavy RIB extracted from SHIP is first decelerated ina He/Ar filled stopping chamber, transferred to a cooling/bunching RF quadrupole trap and finallydelivered to a collection/purification system based on a long cylindrical Penning trap operated inhelium as a buffer gas at about 10-5 mbar. The Penning trap can be efficiently loaded at anyincoming ion energy up to about 100 eV, and global efficiencies (from production) are quoted tobe ≅ 1-15%. After the Penning trap, ions can be delivered to different experimental areas for a largevariety of experiments.

The main physics aims are 1) precision mass measurements of trans-uranium and N≅Zisotopes, 2) nuclear decay spectroscopy, with experiments on proton and cluster radioactivity, andβ-decays, 3) nuclear fission, 4) chemistry of heavy elements and 5) optical spectroscopy usinglaser techniques. As said before, in principle, a substantial physics program can be carried out alsoafter the RF quadrupole trap, if the quality of the beam is good enough. For instance, with the CBmethod, IS and HFS of RIB can be approached in all those cases where intensities are ≤103 p/s.Since much less data is available on the neutron-rich side of the nuclear chart than on the proton-rich side, one might focus on neutron-rich ions produced via suitable reactions, for instance deep-inelastic or multinucleon transfer. The new large solid angle spectrometers (≤100 msr) like PRI-SMA or VAMOS processes, especially designed for heavy ions, might be suitable for a gas-filledmode operation to be coupled to a trap system.

Appendix: Acceleration of Radio-Isotopes with the TANDEM-ALPI AcceleratorComplex at the LNL

The acquisition of the technique of re-acceleration of radio-isotopes is one of the first milestonesfor the development and implementation of radioactive beams at the TANDEM/ALPI acceleratorcomplex of the LNL. This tool would provide the laboratories with an “off shelf” radioactive beam,making use of species with medium/short lifetimes (few hours to days), thus allowing for competitiveexperiments in fields like nuclear astrophysics in the proton-rich as well as in the neutron rich areaof the chart of nuclides or the studies of nuclei at the proton drip-line (vicinity of 100Sn, protonemitters, etc.). The technique would also allow to develop important expertise for the acceleration,

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transport and focusing of weak beams.The unstable species will be produced elsewhere by light ion (p, α) induced reactions and

will then be shipped to Legnaro (within a period permitted by their lifetime). The material will beintroduced in a dedicated ion-source set-up at the TANDEM accelerator and used for experimentationuntil exhaustion. Fig. 3.8 shows a scheme of this source set-up.

Those beams are generally weak and request guide beams as a necessary tool for the acceleratorsetup. A beam of stable ions of a different mass, but of the same mass-to-charge ratio as the desiredone is used to tune the machine. The same settings are then used for the weak exotic beam.

Experiments using this technique have been made e.g. at the ATLAS facility of the ArgonneNational Laboratory [Reh98] and have yielded forefront results in nuclear astrophysics. The dataproduced in these experiments have given valuable glues for the understanding of the puzzle ofstellar nucleo-synthesis employing the proton-rich species 18F and 56Ni. The latter nucleus with ahalf-life of 6.1 days is of special interest, also beyond astrophysical applications. Being two neutronscloser to the proton drip-line, it could serve for studies of nuclei at the very border of stability in themedium mass range.

The number of particles accelerated per second at ATLAS suffered from the limitation of themaximum TANDEM voltage of 9 MV and the need of linac-post-acceleration. Synchronizationproblems with the weak beam led to a poor transmission through ATLAS. As a consequence theoverall acceleration efficiency was a factor of 50 lower as compared with the stable isotope 58Ni.As the XTU-TANDEM is a 15.5 MV machine and is capable of producing the needed energieswithout additional linac-acceleration, the some 104 particles/s of beam current achieved at ANLcould in principle be multiplied by this loss factor for similar experiments at LNL. 106 particles/scould already be interesting for the study of the lowest excited states of exotic nuclei, having athand an efficient γ-ray array like GASP or EUROBALL and a powerful recoil identification systemlike RMS or PRISMA. Applying the recoil decay triggering (RDT) technique nuclear structurestudies close to the drip-line could already become interesting at this stage.

Some more medium/short lived candidates of some interest in various fields have alreadybeen identified by the SPES working group, including short lived isotopes to be produced in-beamin a gas target in reversed kinematics and mentioned in a letter addressed to the TANDEM-ALPIusers. In the following table, with the only purpose of giving some examples, some species arelisted together with their half-lives and possible guide-beam.

Chapter 3

Isotope T1/2 possible guide beam

1. proton rich18F 110 min 18O

48Cr 21.6 h 24Mg

56Ni 6.1 d 28Si, 28Si2

56Co 77.3 d 28Si, 28Si2

72Se 8.5 d 24Mg2. neutron rich

66Ni 6.4 h 33S

72Zn 46.5 h 24Mg3. in-beam

17F 65 sec 17O+p or 16O+d *

* production reactions

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REFERENCES

[Behr97] J.A.Behr et al., Phys. Rev. Lett. 79 (1997) 375[Bil95] J.Billowes and P.Campbell, J. Phys. G21 (1995) 707[Blu97] Y. Blumenfeld et al., MUST proposal, IN2P3-CNRS, Orsay (1997)[Bra97] C.C.Bradley et al., Phys. Rev. Lett. 78 (1997) 985[Cat96] W.N. Catford et al., Nucl. Inst. and Meth. A371 (1996) 449[Chu98] I. Chu, C.N Cohen - Tannoudji, W.D. Phillips, Rev. Mod. Phys. 70 (1998) 1685[Dea98] G. de Angelis et al., Ancillary detectors and devices for Euroball, edited by H. Grawe,

12 (1998)[Far97] E. Farnea et al., Nucl. Inst. and Meth. A400 (1997) 87[Gabr95] G.Gabrielse et al., Phys. Rev. Lett. 74 (1995) 3544[Gwi94] G.Gwinner et al., Phys. Rev. Lett. 72 (1994) 3795[Habs97] D.Habs, Nucl. Instr. and Methods B 126 (1997) 218[Häu95] O.Häusser, Nucl. Phys. A585 (1995) 133c[Klu92] H.J.Kluge and G.Bollen, Nucl.Instr. and Methods B70 (1992) 473[Klu97] H.J.Kluge, Hyp. Int. 108 (1997) 207[Lind91] M. Lindinger et al., Z. Phys. D 20 (1991) 441[Lu97] Z-T Lu et al., Phys. Rev. Lett. 79 (1997) 994[Lun96] M.D.Lunney et al., Hyp. Int. 99 (1996) 105[Met44] H.Metcalf and P. van der Straten, Phys. Rep. 244 (1944) 203[Pau94] G. Pausch et al, Nucl. Inst. and Meth. A349, 281 (1994)[Raa87] E.L.Raab et al., Phys. Rev. Lett. 59 (1987) 2631[Reh98] K.E. Rehm et al., Phys. Rev. Lett. 80 (1998) 676[Ros93] C. Rossi-Alvarez, Nucl. Phys. News Europe 3 (3), p. 10 (1993)[Sav97] G.Savard, Nucl. Instr. and Methods B 126 (1997) 361[Shi98] J. Äystö et al., proposal for SHIPTRAP, GSI 1998[Sims96] J.E.Simsarian et al., Phys. Rev. Lett. 76 (1996) 3522[Spo95] P. Spolaore et al., Nucl. Inst. and Meth. A359, 500 (1995)[Spr97] G.D.Sprouse and L.A.Orozoco, Annu. Rev. Nucl. Part. Sci. 47 (1997) 429[Ste98] A. Stefanini et al., Report INFN 120/97[Sto97] Proc. 3rd Int. Conf. on “Nuclear physics at storage rings”, Bernkastel-Kues (Germany),

September 30-October 4, 1996, F.Bosch and P.Egelhof eds., Nucl. Phys. A 626 (1997)[Vand97] P. Van den Berg et al., Nucl. Instr. and Methods B 126 (1997) 194[Wer96] G.Werth, Hyp. Int. 99 (1996) 3

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Fig. 3.1 - A picture of the spectrometer CAMEL at LNL.

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Fig. 3.2 - Present design of the high efficiency, high resolution γ detector system MARS. It uses 36 Gedetectors, each one 24-fold electrically segmented. The total efficiency calculated at Eγ=1.33MeV is ε≈30%. The performance of such an array can be further improved by following theinteraction story of the individual γ-rays, namely by using the tracking technique which allowsto identify the effective interaction point through a pulse shape analysis.

Fig. 3.3 - Present design of the EUCLIDE Si-ball. It is composed by 52 ∆E-E telescopes, the first ringconsisting of segmented Si detectors. A total efficiency of ≈80% for protons and ≈70% for αparticles is estimated.

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Fig. 3.4 - Layout of the PRISMA Spectrometer now under development at LNL.

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Fig. 3.6 - Stony Brook apparatus for creating, transporting, and trapping radioactive atoms.

Fig. 3.5 - The ISOLTRAP spectrometer installed at ISOLDE/CERN (left). The inserts on the right (seetext) demonstrate the functions of the two traps. The lower trap acts as an ion beam buncher,cooler and isobar separator. The lower picture to the right shows the transmitted ion species withA = 138 as a function of applied radio frequency. The upper trap operates as a high-accuracymass spectrometer. The upper picture to the right shows the cyclotron resonance of 138Sm (T1/2= 3.1 min) detected by a time-of-flight technique (see text). In this case only

138Sm was transferred

from the first to the second trap. The solid line is a fit of the data points by the theoretical lineshape.

46

SPES Project Study Chapter 3

Fig. 3.7 - A schematic of the planned SHIPTRAP facility. The beam pulse characteristics are for ions extractedfrom the collection and purification system.

Fig. 3.8 - Schematic view of an additional source for acceleration of medium/short lived radio-isotopes,using the guide beam method.