Speaker
Description
Shell evolution around magic and double-magic numbers, such as $^{68}$Ni ($N=40$) and $^{78}$Ni ($N=50$), has always attracted many theoretical and experimental investigations. For this purpose, the experimentalists have been studying a set of observables, as i.e. the first excited 2$^{+}$ state energy and the transition probability between the first 2$^{+}$ states and the ground state, $B$($E$2; 2$^{+}_{1} \rightarrow$ 0$^{+}_{1}$). Moreover, adding valence nucleons to N = 40 open shell leads to a rapid increase of collectivity, with an interplay of both collective and single-particle degrees of freedom. Such rapid changes indicate underlying complex effects and make this region ideal for testing theoretical calculations. However, conducting this research towards the most exotic nuclei, around N = 50, was impossible up until recently due to the difficulty of producing these neutron-rich Ni beams. In this region, zinc isotopes, which are two protons above the Z magic number 28, between N = 40 to N = 50, are ideal for investigating the evolution of the nuclear structure. Up-to-date, all the known transition probabilities have been depicted via in-beam experiments [1-10], and in some cases the results are controversial.
Recent studies indicated the presence of a ns isomeric state at high excitation energies in $^{76}$Zn [11]. The isomeric state was interpreted as a negative-parity level arising from neutron excitation of the pf shell to the $g_{9/2}$ orbital [11]. Its isomeric character was explained as a result of a very small E1 transition rate, with a value of 10$^{-8}$ e$^2$fm$^2$. This transition is much more hindered than any other E1 transition measured in this region. Hence, this value might indicate the influence of some structure effects that cannot be reproduced by any shell-model calculation so far.
With the main goal to verify and expand the in-beam results, a new beta-decay study of Cu isotopes was carried out at the ISOLDE facility, to investigate the excited states of zinc isotopes between N = 40 and N = 50. This measurement took place at the new ISOLDE Decay Station (IDS), equipped with four highly efficient clover-type Ge detectors, along with a compact fast-timing setup consisting of two LaBr$_3$(Ce) detectors and a fast $\beta$-plastic detector. The employment of this setup allows us to extract lifetimes down to the ps range. This contribution will be focused on the lifetimes measured in $^{74,76,78}$Zn, in particular, we will present the comparison with the previous in-beam experiments and new isomer states, including the new observed by Chester et al.[11]. As a conclusion, we will discuss the new results and their influence on the understanding of the systematic of the excited states in the even-even Zn isotopes.
[1] S. Leenhardt et al., The European Physical Journal A 14, 1 (2002).
[2] O. Perru et al., Physical Review Letters 96, 232501 (2006).
[3] D. M ̈ucher et al., Phys. Rev. C 79, 054310 (2009).
[4] I. Celikovi ́c et al., Phys. Rev. C 91, 044311 (2015).
[5] J. Van de Walle et al., Physical Review Letters 99, 142501 (2007).
[6] J. Van de Walle et al., Physical Review C 79, 014309 (2009).
[7] C. Louchart et al., Phys. Rev. C 87, 054302 (2013).
[8] M. Niikura et al., Physical Review C 85, 054321 (2012).
[9] Y. Shiga et al., Phys. Rev. C 93, 024320 (2016).
[10] A. Illana et al., Phys. Rev. C 108, 044305 (2023).
[11] A. Chester et al., Phys. Rev. C 104, 054314 (2021).
