Speaker
Description
The isotopic mass region defined by the convergence of the Z = 20 and N = 28 magic numbers sits atop multiple areas of active study, offering a unique opportunity to constrain various distinct structural effects in a singular experiment. Working out towards the neutron-rich exotic Ca isotopes from the last stable isotope at $^{48}$Ca, closures of the $N$ = 32 and $N$ = 34 neutron sub-shells are expected to emerge based on the recent mass measurements [1] and high $2^+_1$ energies, while a monotonic increase in the nuclear radii from $^{48}$Ca to $^{52}$Ca suggests otherwise [2,3]. Looking proton deficient of this region, the evolution of $B(E2)$ values can be used to understand deformation and core breaking across the $N$ = 28 shell gap [4], approaching the 2nd island of inversion surrounding the collective $^{44}$Si [5]. Lifetime measurements of low-lying states in the yrast bands of $^{50,51,52}$Ca and $^{46,47,48}$Ar provides a mechanism of probing the largely unconstrained $B(E2)$ values for these states, providing a stringent test for the shell-model interactions in this region.
In 2024 at INFN LNL, a 305 MeV beam of $^{48}$Ca beam was delivered to a $^{238}$U target at the combined AGATA/PRISMA experimental station, populating $^{50,51,52}$Ca and $^{46,47,48}$Ar among other nearby isotopes in a multi-nucleon transfer reaction. Surrounding the $^{238}$U target, the high-purity germanium Advanced GAmma Tracking Array (AGATA) [6,7] was used to measure high-resolution $\gamma$-ray lineshapes from the states in the excited isotopes. These emitted recoil isotopes were then collected in the PRISMA large acceptance magnetic spectrometer [8], providing event-by-event resolution of the mass and atomic number. Two target configurations were used to study two distinct lifetime ranges using the Doppler-shift attenuation method (DSAM) and recoil distance Doppler-shift (RDDS) technique, utilizing a $^{238}$U target with a thick $^{93}$Nb backing and a $^{238}$U target separated from a $^{93}$Nb degrader in the Cologne Compact Plunger [9], respectively.
In this contribution, we will discuss the lifetime analysis of states in the Ca and Ar isotopes beyond $N$ = 28, using shell-model calculations to provide further context to these observations.
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[2] G. Ruiz et al. (2016) Nature Physics, 12, 594
[3] M. Tanaka et al. (2020) Phys. Rev. Lett., 124, 102501
[4] A. Gade et al. (2003) Phys. Rev. C, 68, 014302
[5] M. Mougeot et al. (2020) Phys. Rev. C, 102, 014301
[6] S. Akkoyun et al. (2012) NIM A, 668, 26
[7] J.J. Valiente-Dobón et al. (2023) NIM A, 1049, 168040
[8] S. Szilner et al. (2007), Phys. Rev. C, 76, 024604
[9] M. Beckers et al. (2022) NIM A, 1042, 167418