Speaker
Description
The isospin mirror asymmetry parameter in mirror systems is evaluated based on the $\beta$-decay data of the mirror partners. In particular, $\beta$ decay intensities are traditionally obtained from $\gamma$-ray spectroscopy experiments with HPGe detectors. However, due to the limited efficiency of such detectors, this approach is known to be potentially affected by the Pandemonium effect [1], specially when high excitation-energy levels are fed and de-excitation cascades are very fragmented. A well consolidated tool to avoid this bias and determine the complete $\beta$-intensity distributions is the total absorption $\gamma$-ray spectroscopy technique [2].
In this contribution we will focus on a recent experiment with the Lucrecia total absorption spectrometer at ISOLDE [3] aimed at investigating the $\beta$ decay of $^{27}$Na, studied in the past with germanium detectors [4]. Our preliminary results point to previously unseen $\beta$ intensity populating excited states in $^{27}$Mg, including the observation of competition between neutron emission and $\gamma$ de-excitation above the neutron separation energy. This would be the first confirmation of Pandemonium effect for such a light system and it would reduce the isospin mirror asymmetry of the $^{27}$Na - $^{27}$S pair, suggested to be an evidence of a proton halo structure in $^{27}$S [5]. The $\gamma$ emission from neutron-unbound states, observed so far in heavier nuclei, can be interpreted as a nuclear structure effect by means of Hauser-Feshbach statistical model calculations [6].
[1] J. Hardy et al., Phys. Lett. B 71, 307 (1977)
[2] B. Rubio et al., J. Phys. G: Nucl. Part. Phys. 31, S1477 (2005)
[3] B. Rubio et al., J. Phys. G: Nucl. Part. Phys. 44, 084004 (2017)
[4] D. Guillemaud-Mueller et al., Nucl. Phys. A 426, 37 (1984)
[5] L. J. Sun et al., Phys. Rev. C 99, 064312 (2019)
[6] J.L. Tain et al., Phys. Rev. Lett. 115, 062502 (2015)
