Speaker
Description
A low-energy enhancement (LEE) has been observed in the $\gamma$-ray strength function ($\gamma$SF) of mid-mass nuclei and several lanthanides, and configuration-interaction (CI) shell model calculations suggest that this enhancement originates in the magnetic dipole ($M1$) $\gamma$SF [1]. However, conventional CI shell model calculations are intractable in heavy nuclei, and the standard approach to calculate $\gamma$SFs -- the quasiparticle random-phase approximation (QRPA) -- does not reproduce the LEE. Thus the theoretical identification of a LEE in heavy open-shell nuclei has been an open problem. If a LEE persists in neutron-rich heavy nuclei, it would likely have profound effects on $r$-process nucleosynthesis by significantly enhancing the radiative neutron-capture rates of nuclei near the neutron drip line [2].
The shell model Monte Carlo (SMMC) method [3] is a powerful method to calculate thermal observables in model spaces that are many orders of magnitude larger than those that can be addressed in conventional methods. We used a combination of SMMC and other many-body methods in chains of even-mass samarium [4] and neodymium [5] isotopes and identified a LEE in their $M1$ $\gamma$SF. We also identified a LEE in the odd-mass samarium and neodymium isotopes despite a Monte Carlo sign problem that originates from the projection on an odd number of particles [6].
Recently we have extended SMMC to the heaviest nuclei ever thus modeled, the actinides, which requires many-particle space dimensions as large as 10$^{32}$ [7]. We identified a LEE in the $M1$ strength function for a selected set of actinides, the first such observation either theoretically or experimentally in this mass region [8].
This work was supported in part by the U.S. DOE grant No.~DE-SC0019521.
[1] J.E. Mitdbo, A. C. Larsen, T. Renstrom, F. L. Bello Garrote, and E. Lime, Phys. Rev. C 98, 064321 (2018), and references therein.
[2] A.C. Larsen and S. Goriely, Phys. Rev. C 82, 014318 (2010).
[3] 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.
[4] P. Fanto and Y. Alhassid, Phys. Rev. C Letters 109, L031302 (2024).
[5] A. Mercenne, P. Fanto, W. Ryssens, and Y. Alhassid, Phys. Rev. C 110, 054313 (2024) [Editors' Suggestion].
[6] D. DeMartini and Y. Alhassid, Phys. Rev. C 111, 034315 (2025).
[7] D. DeMartini and Y. Alhassid, arXiv:2509.26571.
[8] C. Rodgers, D. DeMartini and Y. Alhassid, arXiv:2511.11565.
| Contribution category | Theory |
|---|---|
| Presenter status | Faculty/Staff |