Is neutron emission statistical following β decays near ⁵⁴Ca?

To date, state-of-the-art theories treat β-delayed neutron emission (β-n) as a two-step process. Following β decay, neutron-unbound states undergo statistical neutron emission according to the compound-nucleus (CN) hypothesis. However, a large discrepancy was recently observed in the β-n branching ratios of ¹³⁴In between experimental data and model prediction [1], bringing a challenge to the validity of the approach near shell closures where nuclear level density is generally low. In this contribution, we will present an experimental work studying the β-n branching ratios in the vicinity of ⁵⁴Ca at the ISOLDE Decay Station at CERN. Combining high-resolution γ and neutron TOF spectroscopy, we measured the neutron branching ratios from the states between 5 and 10 MeV in ^51,52,53Ca, respectively, following the β decays of ^51,52,53K. The results were compared with the Hauser-Feshbach model [2]. Despite the proximity to the shell closures (Z=20, N=32, 34), we found surprisingly good agreement with only a few exceptions, which were attributed to the nuclear structure effect from the nearby doorway states, the states having large spectroscopic overlaps with the β-n daughter, interacting with the states populated in β decay and affecting the neutron-emission branching ratios. The results provide substantially new insight into understanding the β-n process in medium-heavy and heavy nuclei.

[1] J. Heideman et al., accepted by Phys. Rev. C.

[2] W. Hauser and H. Feshbach, Phys. Rev. 87, 366 (1952).