Study of beta-delayed multi-neutron emission of the neutron-rich Ga isotopes

Beta-delayed neutron emission probabilities ($P_{xn}$ ) are one of the essential inputs for the $r$-process nuclei abundance calculation since they can modify the isotopic population of the ${\rm \beta}$-decay path back to stability [1]. Although most of the exotic nuclei along the $r$-process path are expected to be multi-neutron emitters, experimental data of two or more neutron emission properties are almost non-existent.

One-neutron and two-neutron branching ratios ($P_{1n}$ and $P_{2n}$) were measured in the decay of $^{84-87}$Ga at the RI-beam Factory at the RIKEN Nishina Center using a high-efficiency array of He neutron counters, BRIKEN. We obtained the result suggesting that the competition between multi-neutron emission channels reduces the average number of neutrons compared to conventional predictions. (See Ref. [3] for detail.)

To explain the competition between multi-neutron emission channels, we combined shell-model calculations with Hauser-Feshbach statistical model calculation [4].  We found that the ($P_{1n}$, $P_{2n}$) and ${\rm \gamma}$-ray branching ratios are very sensitive to the level densities of the daughter nuclei. The statistical model reproduced the experimental branching ratios much more consistently when using the shell-model level densities than those based on the Gilbert-Cameron formula [5]. The result shows the importance of the gamma-neutron studies to benchmark the statistical model calculation.

[1]          R. Surman {\it et al.}, JPS Conf. Proc. , 010010 (2015)

[2]          P. Möller, B. Pfeiffer, and K.-L. Kratz, Phys. Rev. C 67, 055802 (2003).

[3]          R. Yokoyama {\it et al.}, Phys. Rev. C 100, 031302(R) (2019)

[4]         T. Kawano {\it et al.}, Nucl. Phys. A 913, 51 (2013).

[5]          A. Gilbert and A. G. W. Cameron, Can. J. Phys. 43, 1446 (1965)