Speaker
Description
The 'island of inversion' centered on $^{32}$Mg is characterized by ground state configurations with an inverted ordering of $sd$ and $pf$ (intruder) neutron orbitals due to nuclear deformation and nucleon-nucleon interactions. For neutron rich $sd$ shell nuclei outside of the 'island of inversion', similar configurations incorporating the neutron $pf$ shell occur in levels with high excitation energy and spin. Several recent studies have used fusion-evaporation reactions to preferentially populate and study these intruder states, including a recent experiment at the ISAC-II facility at TRIUMF in which the nuclides $^{25}$Na and $^{28}$Mg were produced following $^{12}$C + $^{18}$O fusion [1, 2].
In this experiment, fusion-evaporation exit channels were separated via time coincident identification of charged particles and gamma rays. Gamma-ray spectroscopy utilized the TIGRESS array at ISAC-II. Charged particles were detected and identified using a recently completed CsI(Tl) `ball' scintillator array, developed at Simon Fraser University and commissioned at TRIUMF [3]. Lifetime measurements of excited states populated in the channels of interest were performed using Doppler shift methods.
Six new excited states in $^{25}$Na and $^{28}$Mg were identified, including candidates for the $I^{\pi}=5^+_1,6^+_1$ levels in $^{28}$Mg. Evidence for negative parity states was also observed, including a candidate for the $I_{\pi} = 13/2^-_1$ level in $^{25}$Na and an unusually long-lived state in $^{28}$Mg thought to decay by an M2 transition ($I^{\pi}=(0,4)^{-}$). The energies of these levels are consistent with predicted intruder states arising from single neutron excitation to the $pf$ shell, using the SDPF-MU and FSU shell model interactions. This data and its interpretation with respect to the `island of inversion' will be discussed, along with future plans to extend this work towards $N=20$ by studying $^{32}$Si and other nearby nuclides populated following $^{12}$C + $^{22}$Ne fusion.
[1] J. Williams et al., PRC 100 014322 (2019).
[2] J. Williams et al., PRC 102 064302 (2020).
[3] J. Williams et al., NIM A 939 1-9 (2019).
