Speaker
Description
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Excited States Lifetime measurements in neutron-rich Ca, Ar isotopes: impact on the shell evolution along N=28 and Z=20
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A. Gottardo$^1$, G. Andreetta$^1$, C. Fransen$^2$, D. Mengoni$^3$, J.J. Valiente-Dobon$^1$
$^1$ INFN, Laboratori Nazionali di Legnaro, Legnaro, Italy
$^2$ University of Cologne, Cologne, Germany
$^3$ INFN and University of Padova, Padova, Italy
and the AGATA collaboration
The region around doubly magic $^{48}$Ca is a cornerstone of our understanding of nuclear structure, characterized by the appearance of subshell closures at N=32 and 34 [1], and by the gradual disappearance of N=28 shell closure below Z=20 [2]. However, several experimental findings have challenged this understanding. Large charge and matter radii were found in $^{51-52}$Ca [3,4], showing insensitivity to the N=32,34 subshell closures. The long-standing conundrum of the small B(E2) value in $^{46}$Ar [5,6] has recently been attributed to a new proton sub-shell closure at Z=18 in $^{46}$Ar [7].
We measured lifetimes of excited levels in $^{50-(52)}$Ca and $^{46-(48)}$Ar isotopes (and other nearby nuclei) produced in a multi-nucleon transfer reaction at the Legnaro Laboratories. A beam of 48Ca at 300 MeV impinged on a $^{238}$U target and the target-like recoils were identified event-by-event in mass and atomic number by the mass spectrometer PRISMA [8]. Gamma rays were measured in coincidence with reaction recoil with the AGATA $\gamma$-ray tracking array [9]. Lifetimes were measured using the recoil differential Doppler shift method by mounting the uranium target on a plunger device. Shorter lifetimes were probed using the differential Doppler shift attenuation technique, using a $^{238}$U target with a thick niobium backing. Thanks to the cutting-edge AGATA resolving power, the lifetimes of several yrast and yrare states with spin ≤ 4 were measured.
We will present preliminary results concerning the lifetimes of excited states in $^{46}$Ar and ${50}$Ca, comparing them with shell-model calculations with the interactions available in this region, also including three-body effects. We will discuss the measurement impact on the neutron $f_{5/2}$ shell evolution towards N=34 as well as on the proposed proton subshell closure in $^{46}$Ar [7]. We will also discuss possible fingerprints of the large neutron $p_{3/2}$ shell radius [10] in $\gamma$-ray spectroscopy.
[1] D. Steppenbeck et al., Nat. 502, 207–210 (2013)
[2] T. Glasmacher et al. Phys. Lett. B, 395, 163 (1997)
[3] R. F. Garcia Ruiz et al., Nat. Phys. 12, 594 (2016)
[4] M. Tanaka et al., Phys. Rev. Lett. 124, 102501 (2020)
[5] A. Gade et al., Phys. Rev C 68, 014302 (2003)
[6] S. Calinescu et al., Phys. Rev. C 93, 044333 (2016)
[7] D. Brugnara et al., submitted
[8] A. Stefanini, et al. Nuc. Phys. A, 701, 217 (2002)
[9] S. Akkoyun et al, NIM A 668, 26 (2012)
[10] M. Enciu, Phys. Rev. Lett. 129, 262501 (2022)
