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Description
Cs isotopes (Z = 55, N < 82) occupy the transitional region of the nuclear chart between deformed and near-spherical nuclei. Nuclei in this region exhibit a variety of interesting structural phenomena such as $\gamma$-softness, triaxiality and octupole correlations. Lifetime measurements serve as valuable probes of nuclear structure as they provide direct access to reduced transition probabilities and hence to the evolution of nuclear collectivity in the A ≈ 130 mass region. In particular, the odd-A lighter-mass Cs isotopes, having an even number of neutrons and only a few valence protons outside the Z = 50 shell closure, are expected to have low-lying states predominantly generated by valence-proton excitations. These nuclei therefore provide an ideal testing ground for the validity of Large Scale Shell Model (LSSM) calculations in this mass region.
Despite extensive spectroscopic investigations of ¹³¹Cs [1–2], information on the electromagnetic properties of its excited states remains very limited and therefore requires precise lifetime measurements. In the present work, low-lying states of ¹³¹Cs were populated from the electron-capture decay of ¹³¹Ba produced from the decay of ¹³¹La. The ¹³¹La activity was obtained via the ¹²⁴Sn(¹⁴N,7n)¹³¹La reaction at a beam energy of 115 MeV delivered by the K-130 cyclotron at VECC, Kolkata.
Lifetimes of excited states in ¹³¹Cs in the nanosecond and picosecond regime were measured using the Generalized Centroid Difference (GCD) fast-timing method [3] with a setup consisting of 1.5″ × 1.5″ CeBr₃ fast scintillators coupled to a Hamamatsu R13089-100 PMT [4–5] and a single-crystal HPGe detector of 25% efficiency. A VME-based data acquisition system recorded the events in list-mode format with the master trigger condition [(CeBr₃.AND.CeBr₃).OR.HPGe].
Large Scale Shell Model calculations were performed using the NuShellX@MSU code [6], with the effective interaction sn100pn [7]. The model space included the $g_{7/2}$, $d_{5/2}$, $d_{3/2}$, $s_{1/2}$ and $h_{11/2}$ orbitals for both protons and neutrons. The calculated lifetimes show reasonable agreement with the experimental results. Furthermore, a systematic study of B(E2) values for the Z = 55 isotopes and N = 76 isotones was carried out and compared with the available experimental data.
References:
[1] R. Kumar et al., Eur. Phys. J. A 24, 13-22 (2005)
[2] S. Sihotra et al., Phys. Rev. C 78, 034313 (2008)
[3] J. M. Regis et al., Nucl. Instrum. Meth. A 726 ,191 (2013)
[4] S. Bhattacharya et al., Nucl. Instrum. Meth. A 1014, 165737 (2021)
[5] S. Das et al., JINST 17 09012 (2022)
[6] B. A. Brown et al., Nucl. Data Sheets, 120, 115 (2014).
[7] B. A. Brown et al., Phys. Rev. C. 71, 044317 (2005).