U. Datta Pramanik

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.

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.

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.

Oblate bands in doubly odd 134La

Abstract The band structures of the doubly odd 134La nucleus have been investigated using the 133Cs(α, 3n) 134La reaction at a beam energy of 40 MeV. Altogether six bands have been established, of which details of the yrast band were only known from earlier works. A negative parity oblate band has also been observed in this nucleus. The observed level properties of the members of the yrast band have been compared with theoretical calculations performed within the Particle Rotor Model (PRM) with axially symmetric core. The experimental branching ratios, B (M1) B (E2) values and mixing ratios of the transitions in the yrast band are well reproduced by PRM, assuming an axial oblate core. A cranked shell model calculation has been carried out for the proposed bands.

Search for resonances in 4n, 7H and 9He via transfer reactions

Investigation of unbound nuclear systems 9He, 7H and 4n was performed at GANIL‐SPIRAL using the 8He beam at 15.3 A MeV and a CD2 target. The missing mass spectra were deduced from kinetic energies and emission angles of light ejectiles detected by the Silicon array MUST. In addition to previously known low‐lying narrow resonant states in 9He, the d(8He,p) reaction displays a structure just above neutron emission threshold, identified with the “true” ground state of 9He. The analysis of angular distributions shows that the inversion of s1/2 and p1/2 neutron shells previously observed in 11Be and 10Li also exists in the lightest N=7 isotone 9He. The d(8He,3He) and d(8He,6Li) reactions were used to search for resonant states in the t+4n (7H) and 4n systems, respectively. The missing mass spectrum of the 4n system does not give evidence for the existence of a bound “tetraneutron”. However the comparison with the results of 5‐body phase‐space calculations emphasizes the existence of correlations in the 4n syst...

Coulomb excitation of exotic nuclei at the R3B-LAND setup

Exotic Ni isotopes have been measured at the R3B-LAND setup at GSI in Darmstadt, using Coulomb excitation in inverse kinematics at beam energies around 500 MeV/u. As the experimental setup allows kinematically complete measurements, the excitation energy was reconstructed using the invariant mass method. The GDR and additional low-lying strength have been observed in 68Ni, the latter exhausting 4.1(1.9)% of the E1 energy-weighted sum rule. Also, the branching ratio for the non-statistical decay of the excited 68Ni nuclei was measured and amounts to 24(4)%.

Structure of neutron-rich oxygen isotopes

One-nucleon removal reactions at relativistic energies have been used as a spectroscopic tool for obtaining information about the ground state of neutron-rich oxygen isotopes. Using the FRS at GSI, the longitudinal momentum distributions of the emerging fragments after breakup were measured along with the one-nucleon removal cross-sections. The relative contributions of the remaining fragments in their ground and excited states have been determined for a few cases from measurements of γ rays in coincidence with the longitudinal momentum distribution. The controversial case of 23O has been directly addressed. Comparison of our exclusive momentum distributions for the one-neutron removal channel to theoretical momentum distributions calculated with an Eikonal model for the knockout process rendered a spin and parity of Iπ = 1/2+ for the 23O ground state. This result resolves the existent experimental discrepancy and supports sub-shell closure at N = 14 with a fairly pure 2s1/2 neutron in 23O.

Coulomb breakup of psd-shell neutron-rich nuclei

Inelastic scattering of loosely bound nuclei by Coulomb interaction at intermediate energies (400?600 MeV/nucleon) has been utilized as a spectroscopic tool for exotic nuclei. The observed electromagnetic dipole (E1) strength above the one neutron threshold of neutron-rich C, Be, B and O isotopes can be explained by a non-resonant transition of a neutron into the continuum. The shape of these strength distributions reflects properties of the wavefunction of the released neutron in the nucleus and hence ground-state properties of these isotopes. Neutron capture cross-sections such as for the 14C(n,?) 15C reaction which are of astrophysical relevance can be deduced indirectly.

Band structure in104Ag

The level structure of104Ag has been studied through the103Rh (α, 3nγ) reaction at Eα=40 and 45 MeV. The principal features of the proposed level scheme are in agreement with those obtained earlier through heavy ion reaction. Two quasiparttcle-plus-rotor model calculation has been performed, and the results are compared with experimental data.