Speaker
Description
The first spectroscopy of $^{78}$Ni [1] together with indications of shape coexistence just below the N=50 shell closure for $^{79}$Zn [2, 3] suggest that deformed intruder configurations could play a crucial role in low-energy structure properties in this region and towards the limits of the nuclear chart [4]. Such configurations are predicted to originate from multiparticle-multihole excitations [5] above the N=50 and Z=28 shell gaps pushed down in energy due to neutron-proton correlations which enhance quadrupole collectivity.
Because these intruder states involve many-particle excitations more difficult to describe theoretically, their predicted energies vary more drastically between models than for yrast states originating from ``normal'' configurations on which they tend to agree. Characterizing fully the properties of those states hence provides a good asset to benchmark microscopic models [1], or constrain effective shell model interactions [6].
This topic is the main objective of an experiment performed at the RIBF facility (RIKEN, Japan) [7] in order to identify and characterize, for the first time, 2p-1h intruder states in $^{83}$Ge. Neutron hole states in this N=51 nucleus were populated via neutron knockout reactions from the N=52 nucleus $^{84}$Ge which has about two neutrons in the $s_{1/2}d_{5/2}$ valence space above N=50. This direct reaction allows one to remove one of the neutrons from the quasi-full $g_{9/2}$ orbital below N=50 and selectively populate the $9/2^+$ intruder states in $^{83}$Ge based on a $\nu(g_{9/2})^{-1}(s_{1/2}d_{5/2})^{+2}$ configuration. In order to identify the populated states, gamma-rays from their in-flight decay were measured using the HiCARI Germanium array comprising six MINIBALL triple clusters, four Clovers, and two GRETINA-type detectors. In order to confirm the intruder nature of the states observed, parallel momentum distributions of reaction products were measured using the ZeroDegree spectrometer.
We propose to present here the identification of the main intruder candidate in $^{83}$Ge based on measurements including for the first time lifetime, spectroscopic factors, and parallel momentum distributions. These measurements will be compared with state-of-the-art Shell-Model calculations.
[1] R. Taniuchi et al., Nature 569, 53 (2019).
[2] X. Yang et al., Phys. Rev. Lett. 116 (2016).
[3] L. Nies et al., Phys. Rev. Lett. 131, 222503 (2023).
[4] F. Nowacki, A. Obertelli, and A. Poves, Prog. Part. Nuc. Phys. 120, 103866 (2021).
[5] K. Heyde and J. L. Wood, Rev. Mod. Phys. 83, 1467–1521 (2011).
[6] F. Nowacki, A. Poves, E. Caurier, and B. Bounthong, Phys. Rev. Lett. 117, 272501 (2016).
[7] H. Okuno, N. Fukunishi, and O. Kamigaito, PTEP 2012, 03C002 (2012).