Nuclear physics with superconducting cyclotron at Kolkata: Scopes and possibilities

PRAMANA c© Indian Academy of Sciences Vol. 75, No. 2— journal of August 2010

physics pp. 305–316

Nuclear physics with superconducting cyclotronat Kolkata: Scopes and possibilities

SAILAJANANDA BHATTACHARYAVariable Energy Cyclotron Centre, 1/AF, Bidhan Nagar, Kolkata 700 064, IndiaE-mail: [email protected]

Abstract. The K500 superconducting cyclotron at the Variable Energy Cyclotron Cen-tre, Kolkata, India is getting ready to deliver its first accelerated ion beam for experiment.At the same time, the nuclear physics programme and related experimental facility de-velopment activities are taking shape. A general review of the nuclear physics researchopportunities with the superconducting cyclotron and the present status of the develop-ment of different detector arrays and other experimental facilities will be presented.

Keywords. Nuclear radiation detector; nuclear reaction.

PACS Nos 29.40.Wk; 29.40.Mc; 29.40.Cs

1. Introduction

The K500 superconducting cyclotron (SCC) at the Variable Energy Cyclotron Cen-tre (VECC), Kolkata, India has recently accelerated ion beams up to the extractionradius and external beams of energetic ions (typically, ∼10–60 MeV/nucleon forAP � 100 and ∼5–20 MeV/nucleon for AP ∼ 200; AP is the projectile mass num-ber) are soon expected to be delivered to the experimental area for nuclear physicsresearch. On the experimental side, activities are now focussed on the completionof the major experimental facilities which are being developed over the past fewyears as a part of the SCC utilization programme [1–3]. Current status of theseactivities, along with a brief review of the scope of nuclear physics research usingthese facilities will be highlighted in the following paragraphs.

Energetic ion beams from SCC may be used as a powerful tool for the productionand study of hot nuclear matter [4]. Several interesting details about the hot nuclearmatter are yet to be understood, such as the themalization process on a small time-scale (10−21–10−22 s), the mechanism of the nuclear disintegration process (thermalmultifragmentation and vis-a-vis liquid–gas phase transition, dynamical multifrag-mentation, etc.), stability limit of the hot nucleus, to mention a few. Similarly,the study of binary dissipative collisions provides information of nuclear relaxationprocesses (energy, N/Z, shape equilibration) in greater details. Observed featuresnear Fermi energy, such as the emission of a significant fraction of intermediatemass fragments (IMF; 3 ≤ Z � 20) from the mid-rapidity region, the presence of

305

Sailajananda Bhattacharya

neutron-rich matter in the neck region, etc., point to the onset of transition in thereaction mechanism (from statistical to dynamical regime).

Nucleus–nucleus collision in and around the Fermi energy domain is also used tostudy the collective dynamics of hot nuclear systems, i.e., the evolution of fusion–fission and nuclear viscosity as well as giant resonances built on excited states. Thestudy of hard and soft photon emissions in n–n bremsstrahlung process providesimportant clue about the dynamics of the system at the beginning and at thelater thermalization stages of the reaction, respectively. The study of fragmentisotopic distributions and isoscaling in heavy-ion collisions provides information onthe equation of state of nuclear matter (symmetry energy). The medium heavyion beams from SCC can be utilized to produce and study the properties of manyexotic nuclei close to the drip lines using projectile fragmentation as well as deepinelastic reactions.

2. Nuclear physics with SCC: Scopes and possibilities

The SCC experimental area is schematically shown in figure 1. There are threebeam halls (H-1, H-2, H-3) at present, and there is plan for future extension of theexperimental area at the left-hand side of the present experimental area.

Two of the beam halls (H-1, H-2) will be used for nuclear physics experiments,whereas, the other beam hall (H-3) is earmarked for multidisciplinary researchactivities. Nuclear physics experimental facilities will be presently set up aroundthe three available beamlines (1, 2 and 3 in figure 1) in these two beam halls. Among

Figure 1. The plan of the experimental area with the lay-out of beam lines.

306 Pramana – J. Phys., Vol. 75, No. 2, August 2010

Nuclear physics with superconducting cyclotron at Kolkata

Figure 2. The horizontal segmented reaction chamber in the SCC experi-mental hall.

the major experimental facilities, a large, horizontal segmented reaction chamberwill be placed in beamline-1, and 4π charged particle detector array will be stationedin beamline-2. The beamline-3 in the beam hall H-2 will be the site for placing thelarge modular BaF2 γ-ray detector array, the neutron time-of-flight (TOF) arrayand the 4π neutron multiplicity detector. In addition, a superconducting solenoidchannel selector will also be installed in beamline-3 in near future.

2.1 Segmented reaction chamber

This is a horizontal, three-segment cylindrical chamber (dimension: 1 m diameter,2.2 m long) with its axis coinciding with the beam axis, which has been designedfor general purpose reaction studies using a variety of detector systems in variousexperimental configurations (figure 2). The segments, placed on external rails, canbe moved apart to insert detectors, targets inside the chamber. The target assembly,consisting of the target ladder and a pair of high vacuum compatible motors tofacilitate rotational and up–down movements of the ladder, is mounted on a pair ofinternal rails so that the target can be positioned at any point along the axis of thechamber to adjust the flight path (max. ∼1.5 m) of the ejectiles. The movementsof the target ladder can be controlled locally as well as remotely. Detectors maybe mounted on another pair of internal rails. Clean vacuum (typically ∼1 × 10−7

mbar in ∼8 h) is achieved by means of two turbomolecular and two cryopumps.Both vacuum (all pumps and valves) and target ladder movement operations arefully automated (programmed on PLC) with manual override options.

Pramana – J. Phys., Vol. 75, No. 2, August 2010 307


Figure 3. Scheme of the charge particle detector array (left) and forwardarray telescope (right) components.

2.2 Light charged particle and complex fragment measurement

In order to pursue our goal to study hot nuclear matter and related issues, i.e., mul-tifragmentation, phase transition etc., it is essential to have complete kinematicalinformation of ion–ion collisions on event-by-event basis. The 4π charged particledetector array, with its high granularity and resolution, will be a nearly ideal de-tector system for this purpose. A schematic view of the charged particle detectorarray is displayed in figure 3. The array consists of (a) the forward array, (b) thebackward array and (c) the extreme forward array. The forward array will consistof 24 telescopes, each having (i) 50 μm (ΔE) single-sided Si strip detector (SSSD),(ii) 500 μm/1 mm (ΔE/E) double-sided Si strip detector (DSSD) and (iii) four6 cm (E) CsI(Tl) detectors. Both SSSD and DSSD are of 5 × 5 cm2 active areawith 16 strips per side (oriented perpendicular for DSSD). The angular coverageof the forward array would be ∼7◦–45◦. The backward array, covering the angularrange of ∼45◦–175◦, will be made up of nearly 350 CsI(Tl) detectors of varyingdimensions (thickness ∼2–4 cm) [3]. The extreme forward array for the angularrange of ∼3◦–7◦ will be made up of 32 plastic slow–fast phoswitch detectors (fast:2 ns, 200 μm, slow: 280 ns, 100 mm) [3].

Unique features of the forward array, i.e., high granularity (24 × 256 pixels,θpixeli+1

− θpixeli � 0.8◦) and high energy resolution (typically ∼1% for 5 MeV α-particles), will be exploited to systematically investigate the decay of highly excitednuclear systems using multiparticle correlation technique. Here, one measures two-particle correlation function, R(q), which is expressed as

1 + R(q) =1

C12

Y12(�p1, �p2)Y1(�p1)Y2(�p2)

, (1)

where Y12 and Y1, Y2 are coincidence and singles yields of particles 1, 2 with mo-menta �p1, �p2 such that q (= |�p1 − �p2|), and C12 is a normalization constant. Themeasured correlation function is then used to extract the excited state populationsof primary fragments, and subsequently, the temperature of the emitting system,as well as the spatial and temporal extents of the emitting source. The detectorsystem is also ideal for resonance decay spectroscopic study of the nuclei at high ex-citations (above the particle emission threshold). The particle–particle correlation



Figure 4. α–α correlation function (left), nuclear temperatures (middle) andisoscaling at low energy (right).

technique is also very useful for investigating the structures of highly exotic nu-clei (typical example being the case of 10C∗, conjectured to be a super-Borromean(ααpp) nucleus) [5].

The capability of the detector array to measure isotopic composition of variousreaction products (up to Z ∼ 10) will be used to study the isospin physics in in-termediate energy heavy-ion collisions, which is a research area of current interest.Here, the research plan is broadly to study the symmetry energy part of the nuclearequation of state (EOS) and its density dependence, which is important from as-trophysical context. There are several observables which are sensitive to symmetrypart of the EOS; some of them are (a) fragment isotopic distribution, isotopic andisobaric yield ratios, (b) pre-equilibrium n/p ratio, (c) mid-rapidity emission etc.In the case of isotopic composition of clusters produced in the decay of excitednuclear systems, it has been found that the ratio, R12(N,Z) = Y1(N,Z)/Y2(N,Z),between the yields of a given fragment in two different reactions 1 and 2, dif-fering by the N/Z ratio of the composite system, satisfies a scaling behaviourR12(N,Z) ∝ exp(αN + βZ). This phenomenon is termed as ‘isoscaling’ and itscharacteristic parameters have been found to be sensitive to the density depen-dence of the symmetry energy (see, for example, the review by Colonna and Tsangin [4], p. 165).

The efficacy of the detector system is apparent from the experiments performedwith individual elements of the array. In one experiment [6], light charged parti-cles and fragments emitted in the reaction 20Ne (145 MeV) + 12C were measured,two particle correlation functions were extracted, and the nuclear temperature wasestimated in three different ways, i.e., from the slope of energy distribution, fromthe excited state population ratio, and from the double isotope ratios; it was foundthat unlike at higher energies, they were consistent (figure 4, left, middle). In an-other experiment [7], isoscaling phenomenon was investigated in low-energy nuclearreaction (12C + 12C and 13C + 12C systems, ∼6.5 MeV/nucleon incident energy).Isoscaling of light fragments is demonstrated at these low energies (figure 4, right),indicating that it is a general feature for any reaction involving equilibration andthermalization.



2.3 High energy γ-ray measurement

This programme involves the development of the large area modular BaF2 detectorarray (LAMBDA), a granular high energy photon (Eγ ∼ 10–100 MeV) detectorarray, which consists of 162 BaF2 detectors (each having a dimension of 3.5 ×3.5 × 35 cm3) [8]. The detector array has large γ-detection efficiency (typically∼50% at 15 MeV), fast timing response and very low rate of γ–γ and γ–n pile upevents due to large granularity. In addition, there is one 50-element BaF2 γ-raymultiplicity filter array (each with dimensions of 3.5 × 3.5 × 5 cm3), which may beused along with the LAMBDA spectrometer for event-by-event angular momentumestimation. Figure 5, left) shows the schematic view of the detector set-up for a 7 ×7 matrix arrangement along with the γ-multiplicity filter. The read-out electronicsconsists of CAMAC front-end electronics and VME-based data acquisition system.

The detectors were fabricated in-house from bare BaF2 crystals (figure 5, right).The non-uniformity was less than 5%. The percentage energy resolution is typically∼16/

√Eγ . Typical time resolution, between two BaF2 detectors, measured with

the 60Co source, is ∼960 ps.The array, due to high efficiency and low pile up, enables precision measurement

of high energy γ-rays much above GDR energies. One part of the array (49 detec-tors) was recently used to study high-energy γ-rays emitted in 20Ne (145, 160 MeV)+ 93Nb, 12C, 27Al reactions using the K-130 cyclotron at VECC. The main moti-vations were to study (i) the variation of GDR width with angular momentum andtemperature in near-Sn nuclei (113Sb) and (ii) to search for very large deformationin light mass nuclei (32S and 47V) due to rapid rotation using GDR as a probe. Theexperimental γ-ray energy spectra measured for the 20Ne + 93Nb reaction and theextracted linearized GDR spectra are shown in figure 6 (top). They are found tomatch well with a modified version of statistical model code CASCADE [9]. Theset up was also used to study GDR in deformed systems 32S∗, 47V∗ produced in thereactions 20Ne + 12C, 27Al, respectively. As expected, splitting of the GDR wasobserved in these cases (figure 6, bottom). In the case of GDR decay of 47V∗, clearsignature of Jacobi transition is evident. On the other hand, the GDR decay of32S∗ clearly indicates large prolate deformation [10]. The observation of such largedeformation in 32S∗ nucleus confirmed the phenomenon of survival of long-lived

Figure 5. Schematic view of the LAMBDA γ-ray detector array set-up (left)and detectors after fabrication (right).



Figure 6. Experimental and extracted linearized GDR γ-decay spectra of113Sb∗ (top), 47V∗ and 32S∗ (bottom).

orbiting dinuclear structure in 20Ne + 12C reaction at such high excitation, whichwas first conjectured on the basis of our previous charged particle measurements[11,12].

The temperature dependence of GDR width is not completely understood andstill remains an open field to be reinvestigated. At a certain critical tempera-ture, the thermal energy is high enough that nucleus may vapourize, leading tothe disappearance of the GDR. However, it was found that the collective strengthof the GDR vanishes well before this temperature is reached. This unexpected



phenomenon may be associated with the loss of collectivity in the nucleus at highexcitation energy or it can be due to the fact that the GDR width increases verymuch and the line shape spreads. It is planned to systematically investigate thephenomenon using LAMBDA array along with other ancillary systems. It is furtherplanned to use the array for the study of the nuclei near superheavy region throughGDR decay using SCC beams (for example, 40Ar (Elab ∼ 20–50 MeV/nucleon) onheavy targets). High-energy γ-rays (thermal photons, Eγ � 25 MeV) emitted ina reaction may also be used as a tool to study temporal evolution of hot nuclearmatter (time-scale of multifragmentation).

2.4 Neutron measurement

Neutron measurement programme involves the development of two types of neutrondetector systems; (i) time-of-flight (TOF) neutron detector array and (ii) 4π neu-tron multiplicity detector. The TOF neutron array will be made up of 100 detectors,first phase of which consists of the development of a 50-detector array; fabricationof these detectors are underway. The detectors are liquid scintillator (BC501A)based, cylindrical in shape, 12.7 cm (5′′) in diameter, either 12.7 cm (5′′) or 18 cm(7′′) long (a combination of 5′′ and 7′′ detectors will be used for the TOF array),and coupled to XP4512B photomultiplier tubes. The detectors have excellent n-γdiscrimination, time resolution (∼1.5 ns) characteristics; typical neutron detectionefficiency for a (5′′) 7′′ detector is (64%) 72% for 2 MeV neutrons, and (37%) 49%for 10 MeV neutrons [13]. The performances of the detectors have been tested indetails and compared with simulations done using GEANT4 [13]. Typical responsefunction simulation using GEANT4 for 10 MeV neutrons and a sample neutronspectrum measured in online experiment (16O (90.5 MeV) + 197Au) are shown inthe bottom left and bottom right panels of figure 7, respectively. Detailed GEANT4simulation for optimization of the array configuration is in progress.

Measurement of neutron energy and angular distribution is an essential ingredientto study the reaction dynamics at intermediate energies. Fragment–neutron and/orneutron–neutron correlation measurements of the decay of unstable neutron-richejectiles will facilitate better understanding of the structure of these nuclei. Neu-tron measurement provides important clues about time-scales of low-energy fusion–fission processes. It is further conjectured that neutron emission studies may helpin distinguishing fusion–fission from quasifission. Understanding the competitionbetween the two processes is important for the synthesis of superheavy elements.To facilitate the detection of heavy fragments, large area hybrid gas-Si detectors arebeing developed, which consist of Si-Pad E-detectors and a gas multiwire propor-tional counter (MWPC) ΔE/position/timing detectors. A flexible detector standis being developed which will allow the detectors to be arranged in various con-figurations as required by experimental situation. Artists impression of a typicalconfiguration (section of TOF wall) is shown in figure 7 (top). Using the samestands, other configurations (i.e., well-like) may also be achieved.

Apart from neutron TOF array, a 4π neutron multiplicity detector has also beenfabricated, which will be used in conjunction with other charged particle detectorsfor intermediate energy reaction studies. As neutron multiplicity is sensitive to



Figure 7. Neutron TOF wall design (top), GEANT4 simulation of response(bottom left) measured energy spectrum (bottom right).

the thermal state (temperature) of the hot nuclear matter, it may be used as aneffective tool to segregate events with various degrees of thermal excitation. Thedetector consists of two stainless steel hemispherical containers (figure 8), eachwith a capacity of 250 l, which are filled with liquid scintillator BC521 (0.5% Gdloaded). The reaction chamber (13 cm diameter, 13 cm long) is placed between thetwo hemispheres.

Neutrons, after entering the detector volume, quickly get thermalized by colli-sion with (mainly) hydrogen atoms of the scintillator liquid; once thermalized, theneutrons diffuse through the detector volume till they are captured by Gd atomsto emit multiple γ-rays of summed energy ∼8 MeV and subsequently, scintilla-tion light. Each hemisphere is coupled with five 12.7 cm (5′′) photomultiplier tubes(Thorn EMI 9823B) to collect the scintillation light. Efficiency of this detector,simulated using code DENIS, will be around 90% for 2 MeV neutrons and 30%for 20 MeV neutrons. Neutron capture time distributions for the scintillator usedat two different Gd loading (0.2%, 0.5%) has been experimentally determined [14].It is seen that, typical capture time is ∼35 μs for 0.5% Gd loading, and increasesto ∼50 μs for 0.2% loading; this may be due to less number of available capturesite (Gd) at less concentration, leading to increase in the average diffusion time ofneutrons.



Figure 8. 4π neutron multiplicity detector (top), capture site distribution(bottom left) and capture time distribution (bottom right).

2.5 Ion trap development

In recent years, there has been a strong interest in precision measurement of themass and Q-values of nuclei. Precise mass values are important for a variety ofapplications, ranging from nuclear structure studies such as the investigation ofshell closures and the onset of deformation, test of nuclear mass models and massformula, the tests of the weak interaction and of the Standard Model. The trapcan also be used to study the change in the electron capture decay rate of 7Be dueto hyperfine interaction, which is of astrophysical interest [15].

The cryogenic Penning ion trap being developed at VECC is shown in figure 9.The magnet for the trap is a liquid helium cooled superconducting magnet which canprovide 5 Tesla field. The magnet is run in persistent mode, with field uniformity∼0.1 ppm over 1 cm DSV, and the temporal stability is ∼1 ppb/h. The cryostatwith magnet, the schematic design of which is shown in figure 9 (bottom left), hasalready been fabricated (figure 9, top). The design of the trap has been finalized(figure 9, bottom right).



Figure 9. Magnet with cryostat after fabrication (top), schematic view(bottom left) and trap schematic design (bottom right).

3. Summary

As the superconducting cyclotron is getting prepared to deliver accelerated beamsfor nuclear physics experiments, initial research plans are already in place and asper the plan, several new experimental facilities are either already in operationor in advanced stages of completion. The research plans mentioned here are onlyindicative and not exhaustive.

Acknowledgements

This is a joint presentation of the activities of the SCC utilization programmeimplementation team of VECC; core members of the team are, S R Banerjee, ARay, C Bhattacharya, P Das, S Kundu, K Banerjee, S Mukhopadhyay, T K Rana,G Mukherjee, D Pandit, T K Ghosh, P Mukhopadhyay, J K Meena and R Saha.



The cooperation, help and contributions of A Roy, P Dhara, D L Bandopadhyay,M Ahmed, T Bhattacharjee, P Bhaskar (all of VECC) at various stages of planningand implementation are thankfully acknowledged. The contributions of the doctoraland post-doctoral fellows, namely, D Gupta, Aparajita Dey, Srijit Bhattacharya andM Gohil are also thankfully appreciated.

References

[1] S Bhattacharya, Proc. DAE-BRNS Symp. Nucl. Phys. A44, 229 (2001)[2] S Bhattacharya, et al, Proc. Int. Workshop on Multifragmentation IMW2007 (Caen,

France, 2007) Vol. 95, p. 175[3] C Bhattacharya et al, Proc. DAE-BRNS Symp. Nucl. Phys. 53, 121 (2008)[4] For a detailed review, see Dynamics and thermodynamics with nuclear degree of free-

dom edited by Ph Chomaz, F Gulminelli, W Trautmann, S J Yennello, Eur. Phys. J.A30 (2006)

[5] R J Charity et al, Phys. Rev. C80, 024306 (2009)[6] T K Rana et al, Phys. Rev. C78, 027602 (2008)[7] T K Rana et al, Proc. DAE-BRNS International Symp. Nucl. Phys. 54, 388 (2009)[8] S Mukhopadhyay et al, Nucl. Instrum. Methods A582, 603 (2007)[9] Srijit Bhattacharya et al, Phys. Rev. C77, 024318 (2008)

[10] D Pandit et al, Phys. Rev. C81, 061302R (2010)[11] C Bhattacharya et al, Phys. Rev. C72, 021601R (2005)[12] Aparajita Dey et al, Phys. Rev. C74, 044605 (2006)[13] K Banerjee et al, Nucl. Instrum. Methods A608, 440 (2009)[14] K Banerjee et al, Nucl. Instrum. Methods A580, 1383 (2007)[15] A Ray et al, Phys. Lett. B455, 69 (1999)


Date post:	03-Dec-2023
Category:	Documents
Upload:	vecc
View:	0 times
Download:	0 times

Nuclear physics with superconducting cyclotron at Kolkata: Scopes and possibilities

Documents