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

Journal Article · Wed Nov 13 00:00:00 EST 2024 · Physical Review. C

DOI:https://doi.org/10.1103/physrevc.110.054313· OSTI ID:2538533

Mercenne, A. ^[1]; Fanto, P. ^[2]; Ryssens, W. ^[3]; Alhassid, Y. ^[2]

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1. Yale University, New Haven, CT (United Staes); Louisiana State University, Baton Rouge, LA (United States); Yale University
2. Yale University, New Haven, CT (United Staes)
3. Université Libre de Bruxelles, Brussels (Belgium)

We calculate the magnetic dipole $$\gamma$$-ray strength functions in a chain of even-mass neodymium isotopes $$^{144-152}$$Nd 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 $$\gamma$$-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 which 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. Here, we also observe a finite-temperature spin-flip mode.

• Research Organization: Yale University, New Haven, CT (United Staes)
• Sponsoring Organization: National Science Foundation (NSF); USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities (SUF); USDOE Office of Science (SC), Nuclear Physics (NP)
• Grant/Contract Number: AC02-05CH11231; SC0019521
• OSTI ID: 2538533
• Journal Information: Physical Review. C, Journal Name: Physical Review. C Journal Issue: 5 Vol. 110; ISSN 2469-9985
• Publisher: American Physical Society (APS)
• Country of Publication: United States
• Language: English

Related Subjects

73 NUCLEAR PHYSICS AND RADIATION PHYSICS
Electromagnetic transitions
gamma-ray strength functions
nuclear many-body theory
nuclear structure
nuclear structure & decays
quantum Monte Carlo shell model
shell model
thermal and statistical models