P.B. Radha

Shell-model Monte Carlo studies of {ital fp}-shell nuclei

We study the gross properties of even-even and {ital N}={ital Z} nuclei with {ital A}=48--64 using shell-model Monte Carlo methods. Our calculations account for all 0{h_bar}{omega} configurations in the {ital fp} shell and employ the modified Kuo-Brown interaction {ital KB}3. We find good agreement with data for masses and total {ital B}({ital E}2) strengths, the latter employing effective charges {ital e}{sub {ital p}}=1.35{ital e} and {ital e}{sub {ital n}}=0.35{ital e}. The calculated total Gamow-Teller strengths agree consistently with the {ital B}({ital GT}{sub +}) values deduced from ({ital n},{ital p}) data if the shell-model results are renormalized by 0.64, as has already been established for {ital sd}-shell nuclei. The present calculations therefore suggest that this renormalization (i.e., {ital g}{sub {ital A}}=1 in the nuclear medium) is universal.

Gamow-Teller strength distributions in fp-shell nuclei

We use the shell model Monte Carlo method to calculate complete 0f1p-shell response functions for Gamow-Teller (GT) operators and obtain the corresponding strength distributions using a maximum entropy technique. The approach is validated against direct diagonalization for 48Ti. Calculated GT strength distributions agree well with data from (n,p) and (p,n) reactions for nuclei with A=48–64. We also calculate the temperature evolution of the GT+ distributions for representative nuclei and find that the GT+ distributions broaden and the centroids shift to lower energies with increasing temperature.

Pairing correlations in N ∼ Z pf-shell nuclei

Abstract We perform Shell Model Monte Carlo calculations to study pair correlations in the ground states of N = Z nuclei with masses A = 48–60. We find that T = 1, Jπ = 0+ proton-neutron correlations play an important, and even dominant role, in the ground states of odd-odd N = Z nuclei, in agreement with experiment. By studying pairing in the ground states of 52–58Fe, we observe that the isovector proton-neutron correlations decrease rapidly with increasing neutron excess. In contrast, both the proton, and trivially the neutron correlations increase as neutrons are added. We also study the thermal properties and the temperature dependence of pair correlations for 50Mn and 52Fe as exemplars of odd-odd and even-even N = Z nuclei. While for 52Fe results are similar to those obtained for other even-even nuclei in this mass range, the properties of 50Mn at low temperatures are strongly influenced by isovector neutron-proton pairing. In coexistence with these isovector pair correlations, our calculations also indicate an excess of isoscalar proton-neutron pairing over the mean-field values. The isovector neutron-proton correlations rapidly decrease with temperatures and vanish for temperatures above T = 700 keV, while the isovector correlations among like-nucleons persist to higher temperatures. Related to the quenching of the isovector proton-neutron correlations, the average isospin decreases from 1, appropriate for the ground state, to 0 as the temperature increases.

Thermal properties of 54Fe.

We study the thermal properties of 54Fe with the Brown-Richter interaction in the complete 1p0f model space. Monte Carlo calculations show a peak in the heat capacity and rapid increases in both the moment of inertia and M1 strength near a temperature of 1.1 MeV that are associated with the vanishing of proton-proton and neutron-neutron monopole pair correlations; neutron-proton correlations persist to higher temperatures. Our results are consistent with a Fermi gas level density whose backshift vanishes with increasing temperature.

Rotational and pairing properties of 74Rb

Abstract We use a cranked shell model Monte Carlo (SMMC) approach to study rotational properties and pair correlationsin the odd-odd N = Z nucleus 74Rb. The calculation is performed in the complete 1p-0ƒ 5 2 -0g 9 2 model space with a residual interaction derived from the Paris potential. The calculated ground state is dominated by isovector J = 0 proton-neutron (pn) pairing. With increasing frequency the J = 0 pn correlations decrease to a constant value at around 〈J〉 = 3 ± 1.5 h while the isoscalar pn correlations (mainly J = 9) increase. Relatedly, the isospin decreases with frequency from its ground state value T = 1 as isoscalar correlations set in. This finding is in agreement with experiement, where at higher rotational frequency a T = 0 band becomes energetically favored over the T = 1 ground state band.

Temperature dependence of pair correlations in nuclei in the iron region

Abstract We use the shell-model Monte Carlo approach to study thermal properties and pair correlations in 54,56,58 Fe and in 56 Cr. The calculations are performed with the modified Kuo-Brown interaction in the complete 1 p 0 f model space. We find generally that the proton-proton and neutron-neutron J = 0 pairing correlations, which dominate the ground-state properties of even-even nuclei, vanish at temperatures around 1 MeV. This pairing phase transition is accompanied by a rapid increase in the moment of inertia and a partial unquenching of the M1 strength. We find that the M1 strength totally unquenches at higher temperatures, related to the vanishing of isoscalar proton-neutron correlations, which persist to higher temperatures than the pairing between like nucleons. The Gamow-Teller strength is also correlated to the isoscalar proton-neutron pairing and hence also unquenches at a temperature larger than that of the pairing phase transition.

Shell model Monte Carlo method for two-neutrino double beta decay

Shell model Monte Carlo techniques are used to calculate two-neutrino double beta decay matrix elements. We test the approach against direct diagonalization for 48Ca in the complete pf shell using the KB3 interaction. The method is then applied to the decay of 76Ge in the (0f5/2,1p,0g9/2) model space using a newly calculated realistic interaction. Our result for the matrix element is 0.12±0.05 MeV-1, in reasonable agreement with the experimental value.

Complete 0ℏgw shell model Monte Carlo calculations of 94Ru, 96Pd, 96,98Cd and 100Sn

Abstract We perform shell model Monte Carlo calculations for nuclei in the 100 Sn region within the complete 0 g -1 d -2 s oscillator shell using an effective interaction derived from the Paris nucleon-nucleon potential. We find good agreement with empirically calculated masses, and reproduce the observed quenching of the total Gamow-Teller strengths in this mass region. The Gamow-Teller strength in 100 Sn is predicted to be nearly a factor of 3 smaller than the single particle estimate.

Shell model Monte Carlo calculations for 170Dy

Abstract We present the first auxiliary field Monte Carlo calculations for a rare earth nucleus, 170 Dy. We calculate various static Hamiltonian is used to demonstrate the physical properties that can be studied in this region. We calculate various static observables for both uncranked and crancked systems and show how the shape distribution evolves with temperature. We also introduce a discretization of the path integral that allows a more efficient Monte Carlo sampling.

Temperature dependence of the nuclear symmetry energy

Abstract We have studied the properties of various isobars with 54 ≤ A ≤ 64 for temperatures T ≤ 1.2 MeV via Monte Carlo shell model calculations with the KB3 interaction. In accord with empirical indications, we find no systematic temperature dependence of the symmetry energy coefficient, b sym , for T ≤ 1 MeV. This contradicts a recent suggestion that b sym increases by 2.5 MeV at this temperature, which would significantly alter the supernova explosion scenario.