[Show abstract][Hide abstract] ABSTRACT: Experimental measurements of evaporation residue (ER) cross sections for the F19 + Pt194,196,198 reactions forming Fr213,215,217 compound nuclei are reported. The cross sections are measured at beam energies in the range of 101–137.3 MeV. The survival probability of the Fr213 compound nucleus with neutron number N=126 is found to be lower than the survival probabilities of Fr215 and Fr217 with neutron numbers N=128 and 130 respectively. Statistical model analysis of the ER cross sections show that an excitation energy dependent scaling of the finite-range rotating liquid drop model fission barrier is necessary to fit the experimental data. The fitted scaling factors for Fr213 are found to be smaller than those of Fr215 and Fr217 for almost the entire range of excitation energies.
PHYSICAL REVIEW C 89, 024609 (2014). 01/2014; C 89,:024609.
[Show abstract][Hide abstract] ABSTRACT: Background: Synthesis of a doubly magic spherical nucleus beyond 82208Pb126 is a key question in contemporary nuclear physics. Such nuclei can exist solely because of shell stabilization. As the formation cross section of super heavy elements is prohibitively low, attempts have been made to understand stabilizing effects of closed proton (Z) and neutron (N) shells in the vicinity of 82208Pb126.
Purpose: The present work attempts to elucidate the stabilizing effect of shell closure, in general, and the same of Z=82, in particular.
Methods: The evaporation residue (ER) excitation function and ER-gated γ-multiplicity distribution have been measured for the reaction 31P + 170Er at a laboratory energy range of 134–172 MeV. The measurements have been carried out using the HYbrid Recoil mass Analyzer (HYRA) in gas-filled mode and a 4π spin spectrometer consisting of 29 NaI(Tl) detectors. Results of the present reaction have been compared with those of the reaction 30Si + 170Er. Statistical model calculation has been performed for both the systems.
Results: The two reactions, induced by 30Si and 31P projectiles, resulted in compound nuclei (CN) 200Pb (Z=82) and 201Bi (Z=83), respectively. To reproduce experimental ER cross sections, the liquid drop fission barrier (Bf) had to be scaled in the statistical model calculation. The scaling factor (Kf) varies from 0.75 to 1.05 and 0.90 to 1.05 for 30Si and 31P induced reactions, respectively. No significant differences have been found between γ-multiplicity distribution and the distribution moments of the two systems.
Conclusions: No clear signature has been observed in favor of extra stability of the ERs with closed proton shell (Z=82) as Kf values of the two systems match within errors. More exclusive measurements and comparison between more systems forming CN/ER around Z=82 are desirable.
[Show abstract][Hide abstract] ABSTRACT: Evaporation Residue (ER) gated gamma-multiplicity was measured for the reaction 30Si + 170Er which formed the compound nucleus (CN) 200Pb. The measured multiplicity distribution was compared with those of two other reactions, viz, 16O + 184W and 19F + 181Ta forming the same CN. Further, experimental results were compared with statistical model calculations. Both comparisons indicate an absence of higher spins in ERs for 30Si + 170Er compared to the other two more asymmetric reactions. This lowering of spin value is attributed to non-compound fission.
Nuclear Physics A 01/2012; 890-891:62-76. · 2.50 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Evaporation residue cross sections for the reaction 16O+194Pt were measured at beam energies in the range 75.4–103.1 MeV using the gas-filled separator Hybrid Recoil Mass Analyzer at the Inter University Accelerator Centre, New Delhi. The transmission efficiency of the separator was obtained using the calibration system 16O+184W and a Monte Carlo simulation code. Measured evaporation residue cross sections were fitted with statistical model calculations using Kramers' formula. The present measurement provides further evidence for the onset of dissipative forces in fission in the mass ∼200 region.
Physical Review C 01/2012; 84(6). · 3.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Indian National Gamma Array (INGA) is a 4π multi-detector gamma-ray spectrometer based on twenty four Compton-suppressed Clover Germanium detectors with a total photo peak efficiency ~ 5 %. INGA was designed to perform high resolution gamma-ray spectroscopy to study nuclear structure at high spins with stable ion beams at Inter University Accelerator Centre (IUAC). Description of the facility and performance of the array are presented in this paper. Since its commissioning, a number of nuclear spectroscopic investigations have been carried out using the array.
Journal of Physics Conference Series 09/2011; 312(5):052015.
[Show abstract][Hide abstract] ABSTRACT: We present γ-ray multiplicity distributions for the formation of evaporation residues in the fusion reaction 19F+184W→20383Bi120 at beam energies in the range of 90–110 MeV. The measurements were carried out using a 14 element BGO detector array and the Heavy Ion Reaction Analyzer at the Inter University Accelerator Centre. The data have been unfolded to obtain angular momentum distributions with inputs from the statistical model calculation. Comparison with another neighboring system, viz. 19F+175Lu→19480Hg114 with nearly similar entrance-channel mass asymmetry, hints at the depletion of higher angular momenta after crossing of the Z=82 shell in the compound nucleus.
Nuclear Physics A 01/2011; 850(1):22-33. · 2.50 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Hybrid Recoil mass Analyzer (HYRA) operated in gas-filled mode has recently been coupled with the TIFR 4pi spin spectrometer at IUAC, New Delhi to carry out spin (or angular momentum) distribution of heavy residues from fusion-evaporation reactions and spin gated GDR measurements. The details of the combined facility, initial experiments on evaporation residue tagged gamma-multiplicity and the extraction of average transmission efficiency of HYRA are elaborated.
[Show abstract][Hide abstract] ABSTRACT: Fusion excitation functions and angular distributions of evaporation residues (ERs) have been measured for 28Si+90,94Zr systems around the Coulomb barrier using the recoil mass spectrometer, Heavy Ion Reaction Analyzer (HIRA). For both systems, the experimental fusion cross sections are strongly enhanced compared to the predictions of the one-dimensional barrier penetration model (1-d BPM) below the barrier. Coupled channels formalism has been employed to theoretically explain the observed sub-barrier fusion cross section enhancement. The enhancement could be explained by considering the coupling of the low-lying inelastic states of the projectile and target in the 28Si+90Zr system. In the sub-barrier region, the measured fusion cross sections for 28Si+94Zr turned out to be about an order of magnitude higher than the ones for the 28Si+90Zr system, which could not be explained by coupling to inelastic states alone. This observation indicates the importance of multinucleon transfer reaction channels with positive Q values in the sub-barrier fusion cross section enhancement, because 90,94Zr are believed to have similar collective strengths. This implies that no strong isotopic dependence of fusion cross sections is expected as far as the couplings to collective inelastic states are concerned. In addition, the role of projectile and multiphonon couplings in the enhancement has been explored.
[Show abstract][Hide abstract] ABSTRACT: A 4π multi-detector gamma-ray spectrometer named the Indian National Gamma Array (INGA) has been set up at the Inter University Accelerator Centre, New Delhi, for nuclear structure studies. The array is designed to incorporate twenty four Compton-suppressed Clover germanium detectors with a total photopeak efficiency ∼5%. The spectrometer along with sub-systems developed in-house like, mechanical support structure, high voltage power supplies, automatic liquid nitrogen filling system, front-end electronics and data acquisition system are described. The mechanical support structure facilitates the use of the Clover Germanium array with a recoil mass separator. The array has been used in a number of nuclear spectroscopic investigations. The in-beam and off-beam performance of the array are reported.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2010; 622(1):281-287. · 1.14 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Hybrid recoil mass analyzer (HYRA) is a unique, dual-mode spectrometer designed to carry out nuclear reaction and structure
studies in heavy and medium-mass nuclei using gas-filled and vacuum modes, respectively and has the potential to address newer
domains in nuclear physics accessible using high energy, heavy-ion beams from superconducting LINAC accelerator (being commissioned)
and ECR-based high current injector system (planned) at IUAC. The first stage of HYRA is operational and initial experiments
have been carried out using gas-filled mode for the detection of heavy evaporation residues and heavy quasielastic recoils
in the direction of primary beam. Excellent primary beam rejection and transmission efficiency (comparable with other gas-filled
separators) have been achieved using a smaller focal plane detection system. There are plans to couple HYRA to other detector
arrays such as Indian national gamma array (INGA) and 4π spin spectrometer for ER tagged spectroscopic/spin distribution studies and for focal plane decay measurements.
KeywordsFusion-heavy evaporation residues-gas-filled separator-recoil mass spectrometer-momentum achromat
[Show abstract][Hide abstract] ABSTRACT: Two large area multistep position sensitive (two dimensional) multiwire proportional counters have been developed for experiments involving study of fission dynamics using general purpose scattering chamber facility at IUAC. Both detectors have an active area of 20x10 cm(2) and provide position signals in horizontal (X) and vertical (Y) planes, timing signal for time of flight measurements and energy signal giving the differential energy loss in the active volume. The design features are optimized for the detection of low energy heavy ions at very low gas pressures. Special care was taken in setting up the readout electronics, constant fraction discriminators for position signals in particular, to get optimum position and timing resolutions along with high count rate handling capability of low energy heavy ions. A custom made charge sensitive preamplifier, having lower gain and shorter decay time, has been developed for extracting the differential energy loss signal. The position and time resolutions of the detectors were determined to be 1.1 mm full width at half maximum (FWHM) and 1.7 ns FWHM, respectively. The detector could handle heavy ion count rates exceeding 20 kHz without any breakdown. Time of flight signal in combination with differential energy loss signal gives a clean separation of fission fragments from projectile and target like particles. The timing and position signals of the detectors are used for fission coincidence measurements and subsequent extraction of their mass, angular, and total kinetic energy distributions. This article describes systematic study of these fission counters in terms of efficiency, time resolution, count rate handling capability, position resolution, and the readout electronics. The detector has been operated with both five electrode geometry and four electrode geometry, and a comparison has been made in their performances.
The Review of scientific instruments 12/2009; 80(12):123502. · 1.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Elastic scattering angular distributions have been measured for 7Be + 9Be system at Elab = 17, 19 and 21 MeV in the angular range θcm=26○–58°, and for 7Li + 9Be system at Elab= 15.75, 24 and 30 MeV.
An optical model (OM) analysis of these data have been carried out.
For the 7Li + 9Be system fusion cross sections were obtained at Elab = 15.75, 24 and 30 MeV by measuring the α-evaporation spectra from the compound nucleus at backward angles. The measured
α-evaporation spectra were reproduced by the statistical model calculations and fusion cross sections were extracted therefrom.
The ratios of the experimental fusion cross sections to the total reaction cross sections (obtained form OM analysis) were
found to be rather small. This result suggests that break-up process has a strong influence on fusion process leading to a
reduction in fusion cross section.
The European Physical Journal Special Topics 01/2007; 150(1):75-78. · 1.80 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Quasi-elastic-scattering and transfer reaction cross-section measurements were made for the 7Be+27Al system at Elab=17, 19, and 21 MeV in the angular range θc.m.=12°-43°. An optical model (OM) analysis of the quasi-elastic scattering data was carried out. The fusion cross sections were derived at these energies by subtraction of the integrated transfer cross sections from the reaction cross sections obtained from the fits to quasi-elastic-scattering data. These fusion cross sections were found to be consistent with those obtained from the coupled-channels calculations. Elastic scattering and fusion cross sections were measured for the 7Li+27Al system at Elab=10, 13, 16, 19, and 24 MeV. For elastic scattering the angular coverages were in the θlab=12°-72° range and for fusion the α-evaporation spectra from the compound nucleus were measured in the angular range θlab=52°-132° (142° at 10 MeV). The elastic-scattering angular distributions were subjected to OM analysis. The α-evaporation spectra were reproduced with the statistical model calculations, and the fusion cross sections were extracted from them. The fusion cross sections were also extracted by subtraction of the integrated inelastic-scattering cross sections from the reaction cross sections obtained from the OM fits to the elastic-scattering data, and these fusion data were found to be consistent. The CCDEF calculations describe these data quite well. A comparison of the fusion data for the 7Be+27Al and 7Li+27Al systems shows a similar and consistent behavior.
Physical Review C 02/2006; 73(2). · 3.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Angular distribution for 7Be+7Li elastic scattering has been measured using a 7Be radioactive beam at the Nuclear Science Center, New Delhi, at Ec.m.=9.87 and 8.87 MeV. A compact and highly efficient detector system in kinematic coincidence mode and an in-vacuum target transfer system have been developed to minimize contributions from unwanted channels. The angular ranges covered were θc.m.=42°–66° and θc.m.=114°–138°. At the backward angles at both energies, the experimental angular distribution shows higher cross sections than the theoretical predictions for the direct elastic channel alone. The experimental data could be fitted with an isospin-dependent complex potential that is analogous to the Lane potential. A coupled-channel calculation with ground-state reorientation could also fit the data.
Physical Review C 10/2005; 72(4). · 3.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: At Nuclear Science Centre (NSC), New Delhi, we have implemented a facility to produce low energy light radioactive ion beams (RIBs) using (p,n) type of reactions in inverse kinematics. For this purpose primary beams from the 15-UD Pelletron accelerator impinged on a thin polypropylene foil mounted on a rotating/linearly moving target assembly. For efficiently separating the secondary beam from primary beam, the existing recoil mass spectrometer (RMS) HIRA was operated with new ion optics. Suitable hardware modifications were also made. Using this facility, we have extracted a 7Be beam of purity better than 99% and spot-size ∼4mm in diameter. This 7Be beam has been utilized in a variety of experiments in the energy range of 15–22MeV. Typical beam parameters are: intensity 104 pps, angular spread ±30mrad and energy spread ±0.5MeV. Development of appropriate detector setup/target arrangement were also made to perform these experiments. In this paper, we describe the implementation of this project.
Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 01/2005; 241(1):953-958. · 1.19 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 7Be radioactive ion beam (RIB) has been successfully extracted by the in-flight separation technique (J. Phys. G. Nucl. Part. Phys. 24 (1998) 1371)  using the recoil mass spectrometer (RMS) HIRA (Nucl. Instr. and Meth. A 339 (1994) 543)  at NSC. Many experiments measuring elastic scattering and transfer reactions using low intensity 7Be beam have been performed. To compensate for low beam intensity, a new, compact geometry, large-area, high-efficiency detector system has been developed. The detector setup consists of two position-sensitive silicon annular strip detectors, a large-area two-dimensional position-sensitive silicon detector and a transmission-type gas ionization detector. The silicon detectors give the energy as well as the position of the implanted particles and the gas ionization chamber gives the differential energy loss for particle identification.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2005; 539(1):269-277. · 1.14 Impact Factor