M. Saha Sarkar

Fusion ofLi6withTb159at near-barrier energies

Complete and incomplete fusion cross sections for {sup 6}Li + {sup 159}Tb have been measured at energies around the Coulomb barrier by the {gamma}-ray method. The measurements show that the complete fusion cross sections at above-barrier energies are suppressed by {approx}34% compared to coupled-channel calculations. A comparison of the complete fusion cross sections at above-barrier energies with the existing data for {sup 11,10}B + {sup 159}Tb and {sup 7}Li + {sup 159}Tb shows that the extent of suppression is correlated with the {alpha} separation energies of the projectiles. It has been argued that the Dy isotopes produced in the reaction {sup 6}Li + {sup 159}Tb at below-barrier energies are primarily due to the d transfer to unbound states of {sup 159}Tb, while both transfer and incomplete fusion processes contribute at above-barrier energies.

7Li+12C and 7Li+13C fusion reactions at subbarrier energies

Abstract The 7Li+12C and 7Li+13C reactions have been studied at incident energies below and around the Coulomb barrier, by the measurement of cross sections for the characteristic γ-rays emitted from the residual nuclei. Statistical model calculations of the decay of the compound nucleus have been used to deduce the absolute cross sections for different channels and to obtain the total fusion cross sections. There is no evidence for fusion cross section limitations for these systems at energies near the Coulomb barrier as observed in an earlier experiment.

Influence of projectile α-breakup threshold on complete fusion

Abstract Complete fusion excitation functions for B 11 , 10 + Tb 159 have been measured at energies around the respective Coulomb barriers, and the existing complete fusion measurements for Li 7 + Tb 159 have been extended to higher energies. The measurements show significant reduction of complete fusion cross sections at above-barrier energies for both the reactions, B 10 + Tb 159 and Li 7 + Tb 159 , when compared to those for B 11 + Tb 159 . The comparison shows that the extent of suppression of complete fusion cross sections is correlated with the α-separation energies of the projectiles. Also, the two reactions, B 10 + Tb 159 and Li 7 + Tb 159 were found to produce incomplete fusion products at energies near the respective Coulomb barriers, with the α-particle emitting channel being the favoured incomplete fusion process in both the cases.

Inverstigation of 6Li+16O and 7Li+16O reactions at low energies

Abstract The cross sections for the characteristic γ-rays of the residual nuclei following 6 Li+ 16 O and 7 Li+ 16 O reactions have been measured at low energies with HPGe detectors. The fusion cross sections obtained from these γ-ray cross sections for both the systems are found to be in excellent agreement with the total reaction cross sections and this indicates that there is no inhibition of fusion cross sections for these systems at low energies, which is in contradiction to the results obtained from the measurement of evaporation residues and light particles. These cross sections also show good agreement with the IWBC (Incoming Wave Boundary Condition) model and the 1-D BPM (One-Dimensional Barrier Penetration Model) calculations. The critical angular momenta (lcr) dediced from the fusion cross sections are also found to have an energy dependence similar to other Li-induced reactions.

Fusion cross sections for 6Li+12C and 6Li+13C reactions at low energies

Abstract The partial cross sections for the 6Li+12C and 6Li+13C reactions below and above the Coulomb barrier energy have been determined from the cross sections of the characteristic γ-rays measured with HPGe detectors and using the branching factor σ γ σ ch obtained from the statistical model calculations. The fusion cross sections obtained from the sum of these partial cross sections are found to be equal to the total reaction cross sections upto an energy well above the Coulomb barrier energy and there appears to be no evidence for limitation of fusion cross sections for these systems at such energies contrary to the evaporation residue measurements. The measured cross sections are also found to agree nicely with the IWBC (Incoming Wave Boundary Condition) and one-dimensional BPM (Barrier Penetration Model) calculation.

Characteristics of a Compton suppressed Clover detector up to 5 MeV

The Clover detectors in their addback mode are excellent tools for detecting high-energy gamma rays ðX2 MeVÞ: The characteristics ofthese detectors, at energies above 2 MeV ; are usually determined from simulation data or from extrapolation ofthe empirical data. This is the first time that the characteristics ofa Compton suppressed Clover germanium detector have been studied up to 5 MeV using a radioactive 66 Ga ðT1=2 ¼ 9:41 hÞ source. r 2002 Elsevier

Fusion cross sections for Li 6 , 7 + Mg 24 reactions at energies below and above the barrier

Measurement of fusion cross sections for the {sup 6,7}Li + {sup 24}Mg reactions by the characteristic {gamma}-ray method has been done at energies from below to well above the respective Coulomb barriers. The fusion cross sections obtained from these {gamma}-ray cross sections for the two systems are found to agree well with the total reaction cross sections at low energies. The relatively large difference between total cross sections and measured fusion cross sections at higher energies is consistent with the fact that other channels, in particular breakup, open up with an increase of bombarding energy. The breakup channel, however, appears not to have any influence on fusion cross sections. The critical angular momenta (l{sub cr}) deduced from the fusion cross sections are found to have an energy dependence similar to other Li-induced reactions.

New shell closure for neutron-rich Sn isotopes

The evolution of shell structure away from stability [1]has been a topic of intense theoretical [2, 3] and experi-mental [4–6] studies since last decade. Theoretical stud-ies have identified different reasons for the phenomenonof shell evolution with neutron or proton excess. Amongthe various components of the nucleon - nucleon interac-tion, the spin-orbit, tensor part and three - body effectplay important roles in the shell evolutions [1]. Due totensor interactions, nuclear mean field undergoes varia-tions with neutron excess. This leads to monopole mi-gration [2]. It is observed for both proton-rich as well asneutron-rich nuclei. While approaching the neutron dripline, the neutron density becomes very diffused [3] whichcan also lead to shell quenching. For exotic light nucleithe well established magic numbers for the stable nu-clei are found to be modified or new magic numbers haveevolved. At least four doubly magic oxygen isotopes havebeen observed [6–8]. They are

Structure of even-even A = 138 isobars and the yrast spectra of semi-magic Sn isotopes above the Sn 132 core

Abstract Large basis untruncated shell model calculations have been done for the A=138 neutron -rich nuclei in the π(gdsh) ⊕ ν(hfpi) valence space above the Sn core. Two (1+2) -body nuclear Hamiltonians, viz., realistic CWG and empirical SMPN in this model space have been used. Calculated ground state binding energies, level spectra and other spectroscopic properties have been compared with the available experimental data. Importance of untruncated shell model calculations in this model space has been pointed out. Shell model results for the very neutron rich Sn isotope (Sn, N/Z=1.76) of astrophysical interest for which no spectroscopic information except β -decay half life is available, have been presented. Shell structure and evolution of collectivity in the even-even A=138 isobars have been studied as a function of valence neutron and /or proton numbers. Calculations done for the first time, reproduce remarkably well the collective vibrational states in Te and Xe. Comparison of some of the important two-body matrix elements of the empirical SMPN, CW5082 and the realistic CWG interactions has been done. These matrix elements are important for ground state binding energies and low-lying spectra of nuclei in this region. Consideration of the predictability of the two interactions seems to suggest that, in order to incorporate the special features of the N-N interaction in such exotic n-rich environment above the Sn core, the use of local spectroscopic information from the region might be essential.

Rotational bands in the doubly odd 138Pm

Abstract The band structures of the doubly- odd 138Pm nucleus have been investigated using the 115In(28Si, 2p3n)138Pm reaction at a beam energy of 145 MeV. The three previously known rotational bands viz., (i) the yrast one based on the πh 11 2 ⊗ νh 11 2 configuration, (ii) a ΔI = 2 band with πh 11 2 ⊗ ν[400] 1 2 + configuration at lower frequency but with a change in the neutron configuration to ν[660] 1 2 + at higher frequency, and (iii) one consisting of stretched E2 cascades at lower frequency but of dipole transitions after backbend and with suggested configuration of π[413] 5 2 + ⊗ νh 11 2 have been modified and extended to higher spins. Two new bands have been identified. Of these, one consists of only quadrupole transitions, similar to that observed in band (iii), mentioned above, while the other consists of dipole transitions. The observed level properties have been compared to theoretical calculations performed within the Particle Rotor Model (PRM) with axial core and cranked shell model. The experimental branching ratios and B (M1) , B (E2) ratios of the transitions in the yrast band are well reproduced by PRM, assuming an axial prolate core.