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Description
The disappearance of the N=20 shell closure in the so-called “island of inversion” around $^{32}$Mg is one of the most striking examples of the strength of nucleon-nucleon correlations. In this region, the quadrupole-deformed intruder configuration (based on a multi-particle multi-hole configuration) becomes the ground state, subverting the expected shell ordering predicted by a harmonic oscillator plus spin-orbit term. The odd N=21 isotonic chain provides the possibility to study the single-particle and intruder states as a function of decreasing Z. Available spectroscopic evidence points out the appearance of strong branching ratios among the single-particle and collective intruder configurations in $^{37}$S (Chapman et al, Phys. Rev. C, 93 044318 (2016)), suggesting that they mix significantly, contrary to the notion of $^{37}$S being well out the island of inversion. However, a precise quantification of this phenomenon in terms of transition strength is still lacking. The first excited state (3/2$^{-}$ state at 646 keV) is the only one with a measured lifetime (Wang et al., Phys. Rev. C, 94 044316 (2016).), but no transition probability has been firmly determined for the intruder states, in particular those decaying to the \textit{a priori} spherical single-particle states.
A combined DSAM+RDDS measurement has been performed to measure such transition probabilities, in particular for the 2p-1h 3/2$^{+}$ state at 1397 keV and the 3p-2h 7/2$^{-}$ at 2023 keV, exploiting the performance of the AGATA spectrometer in terms of energy and angular resolutions. The $^{37}$S nucleus has been produced via the $^{36}$S(d,p) reaction in inverse kinematics, detecting the recoiling protons in the silicon array SPIDER to obtain an accurate reconstruction of the excitation energy of $^{37}$S. The short lifetimes measured point to large M1 and/or E2 strengths connecting the intruder and spherical states. This would imply a significant mixing between the configurations, arising questions about the determination of the neutron p$_{3/2}$-p$_{1/2}$ single-particle strength distribution in $^{37}$S.
