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Magnetic dipole γ -ray strength functions in the crossover from spherical to deformed neodymium isotopes

par Mercenne, Alexis;Fanto, P.;Ryssens, Wouter ;Alhassid, Yoram
Référence Physical Review C, 110, 5
Publication Publié, 2024-11

Article révisé par les pairs

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Résumé :

We calculate the magnetic dipole γ-ray strength functions in a chain of even-mass neodymium isotopes Nd144-152 in the framework of the configuration-interaction (CI) shell model. We infer the strength function by applying the maximum entropy method (MEM) to the exact imaginary-time response function calculated with the shell-model Monte Carlo (SMMC) method. The success of the MEM depends on the choice of a good strength function as a prior distribution. We investigate two choices for the prior strength function: the static path approximation (SPA) and the quasiparticle random-phase approximation (QRPA). We find that the QRPA is a better approximation at low temperatures (i.e., near the ground state), while the SPA is a better choice at finite temperatures. We identify a low-energy enhancement (LEE) in the MEM deexcitation M1 strength functions of the even-mass neodymium isotopes and compare with recent experimental results for the total deexcitation γ-ray strength functions. The LEE is already seen in the SPA strength function but not in the QRPA strength function, indicating the importance of large-amplitude static fluctuations around the mean field in reproducing the LEE. Our method is currently the only one that can reproduce LEE in heavy open-shell nuclei where conventional CI shell-model calculations are prohibited. With the onset of deformation as number of neutrons increases along the chain of neodymium isotopes, we observe that some of the LEE strength transfers to a low-energy excitation, which we interpret as a finite-temperature scissors mode. We also observe a finite-temperature spin-flip mode.