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

Study of nuclear structure in 32Al and 33Al by β-delayed-neutron decay of spin-polarized 33Mg

5 Mar 2025, 15:31

1m

Main Lecture Hall (University of Aizu)

Poster session

Speaker

M. Matsuda (Dept. of Phys., Osaka Univ.)

Description

Shape coexistence has been observed in nuclei located close to the “Island of Inversion”, where the disappearance of the magic number N = 20 is well known. This nuclear structure is one of the important topics that has been extensively studied both experimentally and theoretically. Our group has systematically investigated nuclear structure in this mass region, focusing on the isotopes of Mg and Al. We have used our original method to clearly confirm spin and parity of the states in daughter nuclei using $\beta$-ray spatial asymmetry. In 2023, experiment on $\beta$- and $\beta$-delayed-neutron decays of ${}^{33}\text{Mg}$ to ${}^{33}\text{Al}$ (N=20) and to ${}^{32}\text{Al}$, respectively, was performed using a spin-polarized beam at TRIUMF in Canada.
As neutron-rich nucleus ${}^{33}\text{Al}$ has large $\beta$-decay Q value and low neutron-separation energy, it is important to measure not only $\gamma$-ray, but also neutron in the energy of several tens of keV to several MeV. Spin polarized ${}^{33}\text{Mg}$ beam with energy of 30 keV and intensity of 8 kpps was implanted into MgO stopper which was surrounded by magnets with magnetic field of 0.34 T to keep the spin polarization. The polarization of this beam was around 8.2%. Gamma ray was detected by seven Ge detectors. $\beta$-ray was measured by ten plastic scintillators. The $\beta$-decay spatial asymmetry was obtained by using data from two of $\beta$-ray detectors, which were positioned along the polarization axis. $\beta$-delayed neutron was detected by ten plastic scintillators and one Li glass scintillator for high- and low-energy neutron, respectively.
In order to construct the decay scheme of $\beta$-delayed neutron of ${}^{33}\text{Mg}$ to ${}^{32}\text{Al}$, $\beta$-$\gamma$-n coincidence data has been analyzed. The energy of the $\beta$-delayed neutron is deduced using time-of-flight (TOF) method. Using this neutron information, the level scheme of the neutron unbound states in ${}^{33}\text{Al}$ has been also constructed. These states are compared to theoretical calculation using large scale shell model to understand nuclear structure such as shape coexistence in these nuclei.