Speaker
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,
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, but no transition probability has been firmly
determined for the intruder states, in particular those decaying to the 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 37S 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 p3/2-p1/2 single-particle strength
distribution in $^{37}$S.
