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The Cr isotopes with N≥28 are a good testing ground for rapid shape evolution from a spherical to a well deformed region close to N=40 [1]. Among the Cr isotopic chain, the $^{56}$Cr (N=32) shows a very particular interest. Shell-model calculations, using various interactions and/or effective charges, are able to reproduce well the trend of the energy of the 2$^{+}_1$ state along the Cr isotopic chain but fail to reproduce the staggering of the B(E2:2$^+_1$ →0$^+_1$) values with a minimum at $^{56}$Cr (N=32) [1]. Beyond mean-field calculations using Gogny interaction reproduce the experimental zigzag behaviour in the Ti isotopes without any need to invoke effective charges but again this is not the case for the Cr isotopes [2]. Calculations performed with the AMD+HFB framework [3] aiming to investigate the triaxial deformation of the states and shape coexistence in this region reproduce the ‘staggering’ of B(E2) values at N=32 but the theoretical values of B(E2) remain much higher than the experimental values [4].
To get an insight into the structure $^{56}$Cr, shape coexistence and triaxial deformation were studied in a recent experiment via lifetime measurements of the 0$_2^+$ and 2$_2^+$ states. The states of interest were populated using a two-neutron transfer reaction: $^{54}$Cr($^{18}$O,$^{16}$O)$^{56}$Cr. Gamma rays were measured using the state of the art of gamma-ray spectroscopy, the AGATA array [5], coupled with the SPIDER silicon detector [6] to reach the needed channel selectivity. Experimental results will be discussed and compared to theoretical calculations.
[1] M. Seidlitz et al., Phys. Rev C 84, 034318 (2011).
[2] T.R. Rodriguez and J. Luis Edigo, PRL 99, 062501 (2007).
[3] Y. Kanada-En’yo et al., C.R. Physique 4 (2003) 497-520
[4] M. Kimura, Presentation at TNP meeting.
[5] J. J. Valiente Dobón et al., Nuc. Instr. and Meth. A 1049 (2023) 168040
[6] M. Rocchini et al., Nucl. Instr. and Meth. A 971 (2020) 164030