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The neutron-deficient nuclei in the vicinity of the Z = 82 shell closure represent one of the most intriguing areas of nuclear chart, where the interplay between single particle and collective degrees of freedom exhibit various nuclear-structure phenomena such as shape coexistence [1], presence of magnetic rotation (MR) bands [2] and isomers [3]. In this region, nuclear shapes gradually evolve from spherical near the neutron shell closure to more deformed configurations near the neutron mid-shell [4]. The studies of odd-mass At (Z = 85) and Bi (Z = 83) isotopes have reported this shape transition with the decreasing neutron numbers [5-7]. Investigations of odd-odd Bi and At isotopes are therefore important to understand the effect of neutron coupling and the effective proton-neutron interaction at the mid-shell. However, exploring the structures of odd-odd neutron-deficient At nuclei remains challenging due to the difficulties in the population and several possible configurations arising from the multiparticle - multihole couplings.
The ground states of odd-odd Bi and At have a spin-parity of 3$ ^+ $, originating from the coupling of proton h$ _{9/2} $ and neutron p$ _{3/2} $ orbitals. In addition, 7$ ^{+} $ and 10 $ ^{-} $ $ \alpha $-decaying isomeric states have also been observed in several odd-odd Bi and At isotopes. The high-spin spectroscopic studies around A $ \approx $ 190-200 have revealed non-collective states, strongly coupled rotational bands and MR band built on either 7$ ^{+} $ or 10 $ ^{-} $ states [8,9]. In contrast, for $ ^{196} $At (N = 111), only one isomeric state (5$ ^{+} $) has been reported above the 3$ ^{+} $ ground state so far. Nothing is known above that level [10]. Moreover, the $ \alpha $-decay studies of $ ^{200} $Fr indicate the existence of single $ \alpha $-decaying 3$ ^{+} $ ground state for $ ^{196} $At [11]. Therefore, investigations of both the high and low spin structures of $ ^{196} $At will be important to understand the evolution of nuclear structure at the neutron number 111.
An experiment using RITU gas-filled separator was performed in the Accelerator Laboratory of the University of Jyväskylä to populate $ ^{196} $At via the reaction - $ ^{165} $Ho ($ ^{36} $Ar, 5n) $ ^{196} $At at a beam energy of 186 MeV. The aim was to investigate the level structures of $ ^{196} $At above the 3$ ^{+} $ ground state and (5$ ^{+} $) isomer using the JUROGAM 3 array and FPGe (Focal Plane Germanium) detectors. The levels above 5$ ^{+} $ isomer have been identified through prompt and delayed spectroscopy and recoil-decay tagging methods which connect to high spin states.
Additionally, in this experiment, the novel detector SIGMA (Segmented Inverted-coaxial GerMAnium) was installed in place of a Broad Energy Germanium Detector at the focal plane [12]. The detector provides the added capability to detect and track the spatial origin of $ \gamma $-rays, which can be further used to correlate and/or veto events. The results demonstrating the performance of SIGMA as a spectroscopic detector as well as a tracking detector in the focal plane will also be presented.
References:
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[3] Andreyev et al. In: Phys. Rev. C 66 014313 (2002)
[4] K. Andgren et al. In: Phys. Rev. C 78 (4 2008), p. 044328
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[6] A. Herzáň et al. In: Phys. Rev. C 96 (1 2017), p. 014301
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[8] A. Herzáň et al. In: Eur. Phys. J. A 56, 165 (2020)
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[12] F.J. Pearce et al. In:NIMA .1027 (2022), 166044