Speaker
Description
In $sd$ shell nuclides near the N=20 'island of inversion', shell evolution is indicated by the energies of negative parity states which primarily arise due to single neutron excitation to the higher lying $fp$ orbitals. These intruder states often have high spin (due to the participation of the $0f_{7/2}$ orbital) and can therefore be preferentially populated using fusion-evaporation reactions. Nuclear isomerism can also occur when these intruder states are at similar energies to positive parity states of similar spin.
We have investigated the high spin structures of $^{32}$Si (N=18) and $^{29}$Al (N=16) using $^{12}$C + $^{22}$Ne fusion evaporation reactions at TRIUMF/ISAC-II, with the TIGRESS clover array and a spherical CsI(Tl) array used for gamma ray and charged particle detection/identification, respectively. In $^{32}$Si, the properties of both the yrast $4^+$ state and a $5^-$ isomer resulting from neutron cross-shell excitation have been disputed [1, 2]. We investigated these and other high-spin states, placing the $5^-$ isomer at 5504.88(13) keV and identifying the $4^+$ state at higher energy [3]. A comparison to shell model calculations suggests that this inverted ordering of yrast states is influenced by the Z=14 subshell closure. Several newly observed high-spin states and accompanying transitions were also identified. In $^{29}$Al, we have identified several new negative parity states, including a rotor-like band based on the first $7/2^-$ state [4]. Our future plans in this region will also be discussed, including on-going side channel analysis and an upcoming experiment to study intruder states in $^{34}$Si.
[1] B. Fornal et al. Physical Review C 55 762 (1997).
[2] M. Asai et al. JAERI Tandem Annual Report 2001, 23-24.
[3] J. Williams et al. Physical Review C 108 L051305 (2023).
[4] J. Williams et al. Physical Review C 112 014318 (2025).