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 cyclotron at Kolkata: Scopes and possibilities SAILAJANANDA BHATTACHARYA Variable Energy Cyclotron Centre, 1/AF, Bidhan Nagar, Kolkata 700 064, India E-mail: saila@veccal.ernet.in Abstract. The K500 superconducting cyclotron at the Variable Energy Cyclotron Centre, 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 development activities are taking shape. A general review of the nuclear physics research opportunities with the superconducting cyclotron and the present status of the development 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 Centre (VECC), Kolkata, India has recently accelerated ion beams up to the extraction radius and external beams of energetic ions (typically, 10 60 MeV/nucleon for A P 100 and 5 20 MeV/nucleon for A P 200; A P is the projectile mass number) are soon expected to be delivered to the experimental area for nuclear physics research. On the experimental side, activities are now focussed on the completion of the major experimental facilities which are being developed over the past few years as a part of the SCC utilization programme [1 3]. Current status of these activities, along with a brief review of the scope of nuclear physics research using these facilities will be highlighted in the following paragraphs. Energetic ion beams from SCC may be used as a powerful tool for the production and study of hot nuclear matter [4]. Several interesting details about the hot nuclear matter are yet to be understood, such as the themalization process on a small timescale (10 21 10 22 s), the mechanism of the nuclear disintegration process (thermal multifragmentation and vis-à-vis liquid gas phase transition, dynamical multifragmentation, etc.), stability limit of the hot nucleus, to mention a few. Similarly, the study of binary dissipative collisions provides information of nuclear relaxation processes (energy, N/Z, shape equilibration) in greater details. Observed features near Fermi energy, such as the emission of a significant fraction of intermediate mass 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 the reaction mechanism (from statistical to dynamical regime). Nucleus nucleus collision in and around the Fermi energy domain is also used to study 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. The study of hard and soft photon emissions in n n bremsstrahlung process provides important clue about the dynamics of the system at the beginning and at the later thermalization stages of the reaction, respectively. The study of fragment isotopic distributions and isoscaling in heavy-ion collisions provides information on the equation of state of nuclear matter (symmetry energy). The medium heavy ion beams from SCC can be utilized to produce and study the properties of many exotic nuclei close to the drip lines using projectile fragmentation as well as deep inelastic reactions. 2. Nuclear physics with SCC: Scopes and possibilities The SCC experimental area is schematically shown in figure 1. There are three beam halls (H-1, H-2, H-3) at present, and there is plan for future extension of the experimental 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 research activities. Nuclear physics experimental facilities will be presently set up around the 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 experimental hall. the major experimental facilities, a large, horizontal segmented reaction chamber will be placed in beamline-1, and 4π charged particle detector array will be stationed in beamline-2. The beamline-3 in the beam hall H-2 will be the site for placing the large modular BaF 2 γ-ray detector array, the neutron time-of-flight (TOF) array and the 4π neutron multiplicity detector. In addition, a superconducting solenoid channel 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 designed for general purpose reaction studies using a variety of detector systems in various experimental configurations (figure 2). The segments, placed on external rails, can be 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 to facilitate rotational and up down movements of the ladder, is mounted on a pair of internal rails so that the target can be positioned at any point along the axis of the chamber to adjust the flight path (max. 1.5 m) of the ejectiles. The movements of the target ladder can be controlled locally as well as remotely. Detectors may be 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 are fully automated (programmed on PLC) with manual override options. Pramana J. Phys., Vol. 75, No. 2, August 2010 307

Sailajananda Bhattacharya Figure 3. Scheme of the charge particle detector array (left) and forward array 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., multifragmentation, phase transition etc., it is essential to have complete kinematical information of ion ion collisions on event-by-event basis. The 4π charged particle detector array, with its high granularity and resolution, will be a nearly ideal detector system for this purpose. A schematic view of the charged particle detector array is displayed in figure 3. The array consists of (a) the forward array, (b) the backward array and (c) the extreme forward array. The forward array will consist of 24 telescopes, each having (i) 50 µm ( E) single-sided Si strip detector (SSSD), (ii) 500 µm/1 mm ( E/E) double-sided Si strip detector (DSSD) and (iii) four 6 cm (E) CsI(Tl) detectors. Both SSSD and DSSD are of 5 5 cm 2 active area with 16 strips per side (oriented perpendicular for DSSD). The angular coverage of the forward array would be 7 45. The backward array, covering the angular range of 45 175, will be made up of nearly 350 CsI(Tl) detectors of varying dimensions (thickness 2 4 cm) [3]. The extreme forward array for the angular range 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, θ pixeli+1 θ pixeli 0.8 ) and high energy resolution (typically 1% for 5 MeV α- particles), will be exploited to systematically investigate the decay of highly excited nuclear systems using multiparticle correlation technique. Here, one measures twoparticle correlation function, R(q), which is expressed as 1 + R(q) = 1 C 12 Y 12 ( p 1, p 2 ) Y 1 ( p 1 )Y 2 ( p 2 ), (1) where Y 12 and Y 1, Y 2 are coincidence and singles yields of particles 1, 2 with momenta p 1, p 2 such that q (= p 1 p 2 ), and C 12 is a normalization constant. The measured correlation function is then used to extract the excited state populations of primary fragments, and subsequently, the temperature of the emitting system, as well as the spatial and temporal extents of the emitting source. The detector system is also ideal for resonance decay spectroscopic study of the nuclei at high excitations (above the particle emission threshold). The particle particle correlation 308 Pramana J. Phys., Vol. 75, No. 2, August 2010

Nuclear physics with superconducting cyclotron at Kolkata Figure 4. α α correlation function (left), nuclear temperatures (middle) and isoscaling at low energy (right). technique is also very useful for investigating the structures of highly exotic nuclei (typical example being the case of 10 C, conjectured to be a super-borromean (ααpp) nucleus) [5]. The capability of the detector array to measure isotopic composition of various reaction products (up to Z 10) will be used to study the isospin physics in intermediate 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 nuclear equation of state (EOS) and its density dependence, which is important from astrophysical context. There are several observables which are sensitive to symmetry part of the EOS; some of them are (a) fragment isotopic distribution, isotopic and isobaric 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 excited nuclear systems, it has been found that the ratio, R 12 (N, Z) = Y 1 (N, Z)/Y 2 (N, Z), between the yields of a given fragment in two different reactions 1 and 2, differing by the N/Z ratio of the composite system, satisfies a scaling behaviour R 12 (N, Z) exp(αn + βz). This phenomenon is termed as isoscaling and its characteristic parameters have been found to be sensitive to the density dependence of the symmetry energy (see, for example, the review by Colonna and Tsang in [4], p. 165). The efficacy of the detector system is apparent from the experiments performed with individual elements of the array. In one experiment [6], light charged particles and fragments emitted in the reaction 20 Ne (145 MeV) + 12 C were measured, two particle correlation functions were extracted, and the nuclear temperature was estimated in three different ways, i.e., from the slope of energy distribution, from the excited state population ratio, and from the double isotope ratios; it was found that unlike at higher energies, they were consistent (figure 4, left, middle). In another experiment [7], isoscaling phenomenon was investigated in low-energy nuclear reaction ( 12 C + 12 C and 13 C + 12 C 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 and thermalization. Pramana J. Phys., Vol. 75, No. 2, August 2010 309

Sailajananda Bhattacharya 2.3 High energy γ-ray measurement This programme involves the development of the large area modular BaF 2 detector array (LAMBDA), a granular high energy photon (E γ 10 100 MeV) detector array, which consists of 162 BaF 2 detectors (each having a dimension of 3.5 3.5 35 cm 3 ) [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 up events due to large granularity. In addition, there is one 50-element BaF 2 γ-ray multiplicity filter array (each with dimensions of 3.5 3.5 5 cm 3 ), which may be used along with the LAMBDA spectrometer for event-by-event angular momentum estimation. 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 electronics consists of CAMAC front-end electronics and VME-based data acquisition system. The detectors were fabricated in-house from bare BaF 2 crystals (figure 5, right). The non-uniformity was less than 5%. The percentage energy resolution is typically 16/ E γ. Typical time resolution, between two BaF 2 detectors, measured with the 60 Co 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 detectors) was recently used to study high-energy γ-rays emitted in 20 Ne (145, 160 MeV) + 93 Nb, 12 C, 27 Al reactions using the K-130 cyclotron at VECC. The main motivations were to study (i) the variation of GDR width with angular momentum and temperature in near-sn nuclei ( 113 Sb) and (ii) to search for very large deformation in light mass nuclei ( 32 S and 47 V) due to rapid rotation using GDR as a probe. The experimental γ-ray energy spectra measured for the 20 Ne + 93 Nb reaction and the extracted linearized GDR spectra are shown in figure 6 (top). They are found to match well with a modified version of statistical model code CASCADE [9]. The set up was also used to study GDR in deformed systems 32 S, 47 V produced in the reactions 20 Ne + 12 C, 27 Al, respectively. As expected, splitting of the GDR was observed in these cases (figure 6, bottom). In the case of GDR decay of 47 V, clear signature of Jacobi transition is evident. On the other hand, the GDR decay of 32 S clearly indicates large prolate deformation [10]. The observation of such large deformation in 32 S 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). 310 Pramana J. Phys., Vol. 75, No. 2, August 2010

Nuclear physics with superconducting cyclotron at Kolkata Figure 6. Experimental and extracted linearized GDR γ-decay spectra of 113 Sb (top), 47 V and 32 S (bottom). orbiting dinuclear structure in 20 Ne + 12 C reaction at such high excitation, which was first conjectured on the basis of our previous charged particle measurements [11,12]. The temperature dependence of GDR width is not completely understood and still remains an open field to be reinvestigated. At a certain critical temperature, the thermal energy is high enough that nucleus may vapourize, leading to the disappearance of the GDR. However, it was found that the collective strength of the GDR vanishes well before this temperature is reached. This unexpected Pramana J. Phys., Vol. 75, No. 2, August 2010 311

Sailajananda Bhattacharya phenomenon may be associated with the loss of collectivity in the nucleus at high excitation energy or it can be due to the fact that the GDR width increases very much and the line shape spreads. It is planned to systematically investigate the phenomenon using LAMBDA array along with other ancillary systems. It is further planned to use the array for the study of the nuclei near superheavy region through GDR decay using SCC beams (for example, 40 Ar (E lab 20 50 MeV/nucleon) on heavy targets). High-energy γ-rays (thermal photons, E γ 25 MeV) emitted in a reaction may also be used as a tool to study temporal evolution of hot nuclear matter (time-scale of multifragmentation). 2.4 Neutron measurement Neutron measurement programme involves the development of two types of neutron detector systems; (i) time-of-flight (TOF) neutron detector array and (ii) 4π neutron 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; fabrication of 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-γ discrimination, time resolution ( 1.5 ns) characteristics; typical neutron detection efficiency 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 in details and compared with simulations done using GEANT4 [13]. Typical response function simulation using GEANT4 for 10 MeV neutrons and a sample neutron spectrum measured in online experiment ( 16 O (90.5 MeV) + 197 Au) are shown in the bottom left and bottom right panels of figure 7, respectively. Detailed GEANT4 simulation for optimization of the array configuration is in progress. Measurement of neutron energy and angular distribution is an essential ingredient to study the reaction dynamics at intermediate energies. Fragment neutron and/or neutron neutron correlation measurements of the decay of unstable neutron-rich ejectiles will facilitate better understanding of the structure of these nuclei. Neutron measurement provides important clues about time-scales of low-energy fusion fission processes. It is further conjectured that neutron emission studies may help in distinguishing fusion fission from quasifission. Understanding the competition between the two processes is important for the synthesis of superheavy elements. To facilitate the detection of heavy fragments, large area hybrid gas-si detectors are being developed, which consist of Si-Pad E-detectors and a gas multiwire proportional counter (MWPC) E/position/timing detectors. A flexible detector stand is being developed which will allow the detectors to be arranged in various configurations as required by experimental situation. Artists impression of a typical configuration (section of TOF wall) is shown in figure 7 (top). Using the same stands, other configurations (i.e., well-like) may also be achieved. Apart from neutron TOF array, a 4π neutron multiplicity detector has also been fabricated, which will be used in conjunction with other charged particle detectors for intermediate energy reaction studies. As neutron multiplicity is sensitive to 312 Pramana J. Phys., Vol. 75, No. 2, August 2010

Nuclear physics with superconducting cyclotron at Kolkata 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 an effective tool to segregate events with various degrees of thermal excitation. The detector consists of two stainless steel hemispherical containers (figure 8), each with a capacity of 250 l, which are filled with liquid scintillator BC521 (0.5% Gd loaded). The reaction chamber (13 cm diameter, 13 cm long) is placed between the two hemispheres. Neutrons, after entering the detector volume, quickly get thermalized by collision with (mainly) hydrogen atoms of the scintillator liquid; once thermalized, the neutrons diffuse through the detector volume till they are captured by Gd atoms to emit multiple γ-rays of summed energy 8 MeV and subsequently, scintillation 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 used at 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 increases to 50 µs for 0.2% loading; this may be due to less number of available capture site (Gd) at less concentration, leading to increase in the average diffusion time of neutrons. Pramana J. Phys., Vol. 75, No. 2, August 2010 313

Sailajananda Bhattacharya 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 the mass and Q-values of nuclei. Precise mass values are important for a variety of applications, ranging from nuclear structure studies such as the investigation of shell closures and the onset of deformation, test of nuclear mass models and mass formula, the tests of the weak interaction and of the Standard Model. The trap can also be used to study the change in the electron capture decay rate of 7 Be due to 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 can provide 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 cryostat with magnet, the schematic design of which is shown in figure 9 (bottom left), has already been fabricated (figure 9, top). The design of the trap has been finalized (figure 9, bottom right). 314 Pramana J. Phys., Vol. 75, No. 2, August 2010

Nuclear physics with superconducting cyclotron at Kolkata 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 beams for nuclear physics experiments, initial research plans are already in place and as per the plan, several new experimental facilities are either already in operation or in advanced stages of completion. The research plans mentioned here are only indicative and not exhaustive. Acknowledgements This is a joint presentation of the activities of the SCC utilization programme implementation team of VECC; core members of the team are, S R Banerjee, A Ray, 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. Pramana J. Phys., Vol. 75, No. 2, August 2010 315

Sailajananda Bhattacharya 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 planning and implementation are thankfully acknowledged. The contributions of the doctoral and post-doctoral fellows, namely, D Gupta, Aparajita Dey, Srijit Bhattacharya and M 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 freedom 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) 316 Pramana J. Phys., Vol. 75, No. 2, August 2010