100 Years of Nuclear Isomers

Nuclear isomers often possess a unique configuration or shape that allows testing nuclear models and advancing the understanding of nuclear structure. In addition, in many cases isomers allow for an easier experimental identification and/or correlated and background-reduced data analysis. Our recent nuclear structure studies of neutron-rich Kr isotopes using prompt and delayed gamma spectroscopy discovered new gamma-transitions feeding, depopulating and bypassing a new short-lived nanosecond isomer in ${}^{94}$Kr[1] and new transitions feeding and bypassing the known microsecond isomer in ${}^{95}$Kr[2,3].
These Kr isotopes were studied during the second SEASTAR campaign [4] at the RI Beam Factory[5] at the RIKEN Nishina Center and during the NuBall campaign [6] at the ALTO facility at the IPN Orsay. While the former experiment populated the isotopes of interest via nucleon knockout reactions of a relativistic radioactive beam on a liquid-hydrogen target [7], the latter used a pulsed ${}^7$Li beam together with the fast-neutron source LICORNE [8] to induce pulsed fission of a ${}^{238}$U stacked-target. In the measurement at RIKEN, prompt gamma-rays after the knockout were detected by the DALI2 NaI array [9] and, after the subsequent flight of the exotic ions through the ZeroDegree spectrometer, delayed gamma-rays were detected by the EURICA HPGe array [10]. At the IPN Orsay, the NuBall hybrid array [11] consisting of HPGe and LaBr gamma-ray detectors surrounded the fission target and a triggerless data acquisition collected all data.
The experimental results will be presented and compared to known data in neighbouring isotones and theoretical models. Aspects of nuclear structure like single-particle and quasiparticle states, onset of deformation and shape-coexistence in the neutron-rich Kr isotopes approaching N=60 will be discussed. *Supported by the DFG under Grant No. BL 1513/1-1 .

[1] R.-B. Gerst et al., Phys. Rev. C 102, 064323 (2020).
[2] R.-B. Gerst et al., Phys. Rev. C 105, 024302 (2022).
[3] J. Genevey et al., Phys. Rev. C 73, 037308 (2006).
[4] P. Doornenbal et al., RIKEN Accel. Prog. Rep. 49, 35 (2016).
[5] T. Kubo et al., Prog. Theor. Exp. Phys. 2012, 03C003 (2012).
[6] N. Jovancevic et al., Acta Phys. Pol., B 50, 297 (2019).
[7] A. Obertelli et al., Eur. Phys. J. A 50, 8 (2014).
[8] M. Lebois et al., Nucl. Instrum. Methods Phys. Res. A 735, 145 (2014).
[9] S. Takeuchi et al., Nucl. Instrum. Methods Phys. Res. A 763, 596 (2014).
[10] P.-A. Söderström et al., Nucl. Instrum. Methods Phys. Res. B 317, 649 (2013).
[11] M. Lebois et al., Nucl. Instrum. Methods Phys. Res. A 960, 163580 (2020).