Speaker
Description
Actinides are of great interest in astrophysics and in technology applications since
they can fission. However, the microscopic calculation of their statistical properties
presents a major theoretical challenge. The configuration-interaction (CI) shell-
model is a suitable framework to calculate these properties but the required model
spaces are much too large for conventional diagonalization methods. The shell-
model Monte Carlo (SMMC) method enables calculations in very large model spaces
and was applied to nuclei as heavy as the lanthanides [1]. Using the SMMC, we have
calculated level densities for the heaviest nuclei ever thus modeled, the actinides,
which require many-particle space dimensions as large as $10^{32}$ [2]. We find the
SMMC level densities to be in good agreement with Oslo method experiments,
neutron resonance data and level counting at low excitation energies.
We have also used the SMMC to calculate the magnetic dipole (M1) γ-ray strength
function (γSF) in a selected set of actinides [3]. We identify a low-energy enhance-
ment (LEE), the first such observation either theoretically or experimentally in
actinides. A LEE has been observed γSF of mid-mass nuclei, and conventional CI
shell model calculations suggest that this enhancement originates in the M1 γSF [4].
However, conventional CI shell model calculations are intractable in heavy nuclei,
and the standard approach to calculate γSFs – the quasiparticle random-phase ap-
proximation (QRPA) – does not reproduce the LEE. Using the SMMC, a LEE was
observed in chains of even-even [5, 6] and odd-mass [7] lanthanide isotopes.
We also observed in the M1 γSF of actinides a scissors mode that is built on top
of excited states. We compare our results with experiments by the Oslo group.
This work was supported in part by the U.S. DOE grant No. DE-SC0019521.
[1] For a recent review, see Y. Alhassid, in Emergent Phenomena in Atomic Nuclei
from Large-Scale Modeling: a Symmetry-Guided Perspective, edited by K. D.
Launey (World Scientific, Singapore, 2017), pp. 267-298.
[2] D. DeMartini and Y. Alhassid, arXiv:2509.26571.
[3] C. Rodgers, D. DeMartini and Y. Alhassid, arXiv:2511.11565.
[4] J. E. Mitdbø, A. C. Larsen, T. Renstrøm, F. L. Bello Garrote, and E. Lime,
Phys. Rev. C 98, 064321 (2018), and references therein.
[5] P. Fanto and Y. Alhassid, Phys. Rev. C 109, L031302 (2024).
[6] A. Mercenne, P. Fanto, W. Ryssens, and Y. Alhassid, Phys. Rev. C 110, 054313
(2024).
[7] D. DeMartini and Y. Alhassid, Phys. Rev. C 111, 034315 (2025).