### Speaker

### Description

Beta-delayed neutron emission ($\beta$-n) plays a vital role in shaping the elemental abundance distribution in the $r$-process via modifying the decay path back to stability and by contributing significantly to the neutron flux after freeze-out [1]. Modeling the $\beta$-n process requires a good model of the beta-strength functions and of the neutron emission mechanism. Statistical neutron-emission models assume gamma and neutron decay from a compound nucleus following beta decay and have successfully predicted gross properties of $\beta$-n probabilities ($P_{1n}$, $P_{2n}$, etc) in some medium- and heavy-mass nuclei. However, recent experimental work found evidence of non-statistical neutron emission after the beta decay in the vicinity of doubly magic $^{132}$Sn [2]. Therefore, it is of great importance to study $\beta$-n spectroscopy in a broader area of the nuclear chart to provide stringent experimental constraints to the theories, which in astrophysical applications predict those properties for many more $r$-process nuclei currently out of experimental reach.

We expanded our study onto the nuclei near $^{54}$Ca, which is thought to be doubly magic. An experiment studying $\beta$-n spectroscopy of $^{52,53}$K was carried out at the ISOLDE Decay Station (IDS). These isotopes have large Q$_{\beta}$ values (energy window for $\beta$ decay) and low neutron-separation energies in their daughters ($^{52,53}$Ca respectively), making them ideal for the studies of the $\beta$-n process. In coincidence with the beta decay of $^{52,53}$K, gamma-ray and neutron-time-of-flight (TOF) spectra were measured using HPGe clover detectors and VANDLE [3]. In this contribution, I will present the latest results from the experiment, including the reconstructed excitation energies (Εx) and apparent beta feeding (I$_{\beta}$) of the neutron unbound states in $^{52,53}$Ca, together with their exclusive neutron-emission branching ratios to the states in $^{51,52}$Ca, respectively. The experimental findings were compared with the shell-model calculations and Hauser-Feshbach statistical model. These comparisons provide valuable insights into the $\beta$-n process and its

connection with the nuclear structure far from the stability line.

[1] R. Yokoyama et al, Phys. Rev. C 100, 031302(R) (2019).

[2] J. Heideman et al, being reviewed by Phys. Rev. Lett.

[3] M. Madurga et al, Phys. Rev. Lett 117, 092502 (2016).