Single-particle and collective motions from nuclear many-body correlation (PCM2025)

Study of multi-neutron emission process through beta-neutron-gamma spectroscopy of N>50 Gallium isotopes

5 Mar 2025, 14:45

20m

Main Lecture Hall (University of Aizu)

session #7

Speaker

Rin Yokoyama

Description

Beta-delayed neutron emission occurs in neutron-rich nuclei where the decay energy window is large enough to populate states above the neutron separation energy in the daughter nucleus. Multi-neutron emission is expected to be the dominant decay mode for the nuclides far from stability, along the astrophysical r-process path. The number of neutrons emitted after 𝛽-decays affects the final isobaric abundance pattern after the r-process by providing neutrons for the late-time capture process and changing the decay path back to stability. Therefore, understanding the neutron emission process is crucial for astrophysical r-process abundance calculations.
A comprehensive 𝛽-delayed neutron-𝛾 spectroscopy on the decay of gallium isotopes ($A=84$ to $87$) was conducted at RI beam Factory, RIKEN Nishina Center. The isotopes were examined using a high efficiency array of ${}^3$He neutron counters (BRIKEN) [1] and two clover-type HPGe detectors, enabling 𝛽-2n-𝛾 coincidence measurements of the excited states of two-neutron daughter nuclei. Previously, we found large one-neutron emission probability ($P_{1n}$) values and unexpectedly small $P_{2n}$ values, even the major part of the $B(GT)$ is expected to be concentrated above two-neutron separation energy for those Ga isotopes. This was interpreted as a signature of one-neutron emission from two-neutron unbound states. This result raised the necessity of modeling the competition between multi-neutron emission channels [2].
Hauser-Feshbach statistical model calculations [3] showed that the $P_{1n}$ and $P_{2n}$ ratios and the 𝛾 branching ratios are sensitive to the nuclear level density of one-neutron daughter nuclei. The statistical model calculation was optimized using experimental and shell-model level densities in the daughter nuclei, thereby improving the reproduction of the $P_{2n}/P_{1n}$ ratio. Given that neutron emissions significantly influence the final abundance pattern of r-process nucleosynthesis, these results underscore the importance of understanding level densities and the detailed decay scheme, which could be achieved by neutron spectroscopy data for multi-neutron emitters.
This work was published as Ref. [4].
This work is supported in part by the Office of Nuclear Physics, U.S. Department of Energy under Award No. DE-FG02-96ER40983 (UTK) and DE-AC05-000R22725 (ORNL).
References
[1] A. Tolosa-Delgado et al., NIM A 925,133 (2019)
[2] R. Yokoyama et al., Phys. Rev. C 100, 031302(R) (2019)
[3] T. Kawano et al., Phys. Rev. C 78, 054601 (2008)
[4] R. Yokoyama et al., Phys. Rev. C 108 064307 (2023)