Bency John

Threshold anomaly in 16O + 209Bi system

Abstract Elastic-scattering angular distributions for the 16O + 209Bi system have been measured at laboratory energies of 80, 83, 90,95, 98 and 100 MeV. The present data along with the data available from the literature for this system, have been analysed using the optical model, employing both the phenomenological and the microscopic potentials. Evidence is found for a marked energy dependence of the real part of the optical potential — threshold anomaly — around the Coulomb barrier. The application of the dispersion relation proposed by Mahaux et al. to the energy variation of the imaginary part of the potential, reproduces the observed energy dependence of the real potential. Further it is demonstrated by a calculation that the “threshold anomaly” determined from elastic-scattering analysis is consistent with the enhancement of the fusion cross section observed at sub-barrier energies. It is also observed that the dispersive correction to the real potential already reported for 16O + 208Pb is similar to that found here for 16O + 209Bi which has an extra proton.

Angular momentum dependence of the nuclear level-density parameter around Z ~ 50

{alpha}-particle evaporation spectra and {gamma}-ray multiplicities were measured for various target-projectile systems corresponding to residual nuclei in the range of Z{sub R}=48-55 and with excitation energy in the range of 30-40 MeV. The high-energy part of the evaporation spectra were analyzed using the statistical model code PACE2 to derive values of the inverse level-density parameter (K). The K values were found to be in the range of 9.0-10.5 for all systems. Angular momentum dependence of the inverse level-density parameter was investigated using the {gamma}-ray multiplicity data. It is seen that there are strong variations in K as a function of angular momentum for many systems. Present results provide important input information for a systematic understanding of the statistical properties of nuclei at moderate excitation energies and angular momenta.

Determination of the {sup 233}Pa(n,f) reaction cross section from 11.5 to 16.5 MeV neutron energy by the hybrid surrogate ratio approach

A new hybrid surrogate ratio approach has been employed to determine neutron-induced fission cross sections of {sup 233}Pa in the energy range of 11.5 to 16.5 MeV for the first time. The fission probability of {sup 234}Pa and {sup 236}U compound nuclei produced in {sup 232}Th({sup 6}Li, {alpha}){sup 234}Pa and {sup 232}Th({sup 6}Li, d){sup 236}U transfer reaction channels has been measured at E{sub lab}=38.0 MeV in the excitation energy range of 17.0 to 22.0 MeV within the framework of the absolute surrogate method. The {sup 233}Pa(n,f) cross sections are then deduced from the measured fission decay probability ratios of {sup 234}Pa and {sup 236}U compound nuclei using the surrogate ratio method. The {sup 233}Pa(n,f) cross section data from the present experiment along with the data from the literature, covering the neutron energy range of 1.0 to 16.5 MeV have been compared with the predictions of statistical model code EMPIRE-2.19. While the present data are consistent with the model predictions, there is a discrepancy between the earlier experimental data and EMPIRE-2.19 predictions in the neutron energy range of 7.0 to 10.0 MeV.

Mass Distribution in 12C Induced Fission of 232Th

Formation cross sections of several fission products have been determined using recoil catcher technique followed by gamma-ray spectrometry in 12C induced fission of 232Th at Elab = 72 MeV, corresponding to Ecm just above the Coulomb barrier. The measured formation cross sections were used to get the mass distribution by using known charge distribution systematic. Critical data analysis was carried out to look for the signatures of transfer induced fission. However, within the experimental uncertainty of about 10%, no clear indication of transfer induced fission could be seen at this energy level. The mass distribution shows a single peaked broad Gaussian distribution with the most probable mass of 119.5±1.1 and FWHM of 40.6 mass units. The total fission cross section computed from the mass distribution curve is 771±50 mb.

A Bi-Dimensional Multi-Wire Cathode Strip Detector for Fission Fragments

A large area bi-dimensional position sensitive multi-wire cathode strip detector (MCSD) has been developed for the detection of fission fragments (FFs). The performance of the detector has been characterized with FFs from 252Cf and α particles from 241Am-239Pu sources. The position information is obtained from the measurement of time delay signals of the cathode strips (X-direction) and split-cathode wires (Y-direction) with respect to the timing signal from the anode wires. The position resolution is found to be about 1.0 and 1.5 mm in X-and Y-directions respectively.

Slow neutron reactions in inverse kinematics for isotope production: a primer

Slow neutron inverse-kinematic reactions offer unique advantages since their kinematical conditions can be tuned to utilize very high resonance cross-sections and to separate transmuted-isotopes online. New advances in plasma sciences and thermal neutron facilities can usher practical applications of this type of reactions in some special cases. Production and implantation of isotopically-pure short-lived and spin-polarized radiotracers has been identified as a probable first application. The production rates for 110m,gAg radio-nuclides in 109Ag(n, γ) capture, that occur when a beam of 109Ag ions traverses a thermal neutron column, have been calculated and the results are presented. To develop the idea of inverse reactions in physical grounds, further analyses and advances on the non-trivial technological issues related to the generation and control of high density and high volume particle beams, isotope separation methods, and high flux thermal neutron channels of matching parameters are required and a brief discussion on these aspects are also included.

Design and construction of a target chamber and associated equipments for the BARC Charged Particle Detector Array

A 60 cm diameter spherical high-vacuum target-chamber with side-opening hemispherical-lids, two ancillary-chambers, beam-line-tubes, tees and other high-vacuum components, and chamber-lid handling systems have been designed, constructed and installed for the Charged Particle Detector Array in BARC-TIFR Pelletron-LINAC Facility, Mumbai. This array of several tens of Si-CsI detector modules and other ancillary-detectors will be used for investigations in fusion-fission dynamics, nuclear structure at elevated temperatures and angular momenta, exotic nuclear clusters and related fields. This paper describes the unique features of the system that aid different coincidence experiments, the chamber fabrication experience and the pump-down characteristics with a turbo molecular pump. Unlike many other target chambers in use, this chamber allows multiple overall geometrical configurations to be set to reach experimental goals. For instance, by replacing a hemispherical-lid from one side with a flat-lid, the overall configuration becomes hemispherical. This way, high geometrical efficiency can be provided to an ancillary gamma detector array by allowing it to move close to target from the flat-lid side, although with some sacrifice of geometrical efficiency for charged particles. In experiments where a further improvement of geometrical efficiency for a gamma array is desired, a third compact-cylinder configuration can also be arrived at. Thinned portion of the lids of the chamber also allow neutron coincidence measurements with charged particles and gamma rays.

Heavy Cluster Knockout Reaction {sup 16}O({sup 12}C,2{sup 12}C){sup 4}He and the Nature of the {sup 12}C-{sup 12}C Interaction Potential

The heavy cluster knockout reaction (16)O((12)C,2(12)C)(4)He performed for the first time, reveals the true nature of the (12)C-(12)C interaction. The observed cross section is enhanced by almost 2 orders of magnitude over the conventional zero range distorted wave impulse approximation (DWIA) predictions. An attractive (12)C-(12)C optical potential, as obtained in the folding model, does not explain the enhanced cross section in the finite range (FR) DWIA framework. The inclusion of a hard core of fairly long range ∼3.65  fm explains the data. The present investigation of (16)O((12)C,2(12)C)(4)He along with the (12)C-(12)C elastic scattering also proves beyond doubt that the folding models deep attractive heavy ion potentials are unsuitable to describe the highly overlapping heavy ions. The application of FR-DWIA opens up new avenues to use the heavy core knockout for the detailed investigation of heavy as well as Borromean halo nuclei.

Fission fragment angular distributions in the {sup 9}Be + {sup 232}Th reaction

Fission fragment angular distributions have been measured for a {sup 9}Be + {sup 232}Th system at four different beam energies around the Coulomb barrier. The experimental results on fission fragment anisotropies have been compared with predictions of the standard statistical saddle-point model (SSPM) and the preequilibrium fission (PEQ) model including projectile ground-state spin. It is observed that both SSPM and the PEQ model fail to reproduce the experimental results, indicating that projectile breakup may affect the fission fragment anisotropies.

Nuclear level-density parameters of nuclei in the Z{approx}70 and A{approx}180 mid-shell regions

{alpha}-particle evaporation energy spectra have been measured as a function of {gamma}-ray fold for various target-projectile systems leading to residual nuclei in the range of Z{approx}70 and A{approx}180 with excitation energy of 30-40 MeV. The inverse level-density parameter K was determined for various nuclei by comparing the high-energy part of the {alpha}-particle evaporation spectra with PACE2 predictions. It is observed that the inverse level-density parameter remains constant for all systems studied in this work within statistical errors in the angular momentum range of 15-30 ({Dirac_h}/2{pi}). The fold-gated {alpha}-particle energy spectra in these systems are well reproduced by the PACE2 code with a level-density parameter value of A/(8.2{+-}1.1) MeV{sup -1}.