Speaker
Description
Neutrinoless double beta decay (0νββ) is a rare nuclear process predicted by beyond-Standard Model theories, offering crucial insights into the nature of neutrinos and lepton number violation. A confirmed observation of 0νββ would establish the Majorana nature of neutrinos and provide constraints on their absolute mass scale. Among candidate isotopes, the decay of $^{136}$Xe to $^{136}$Ba is extensively studied in large-scale experiments such as EXO, KamLAND-Zen, nEXO, and PandaX. However, to date, experiments have only set lower limits on the decay lifetimes [1].
A significant challenge remains in the precise determination of nuclear matrix elements (NMEs), which introduce uncertainties in extracting neutrino properties from measured decay rates. Theoretical predictions of NMEs vary considerably [2], highlighting the need for improved nuclear structure data. This study investigates the nuclear structure of $^{136}$Ba, the daughter nucleus of $^{136}$Xe, through high-resolution gamma-ray spectroscopy using the FIPPS array at ILL. The focus is on low-spin states in $^{136}$Ba populated via the $^{135}$Ba(n,γ)$^{136}$Ba reaction, with particular emphasis on the characterization of low-spin 0$^{+}$ states. These states play a fundamental role in 0νββ decay transitions but remain incompletely understood.
The level scheme of $^{136}$Ba has been studied through $^{136}$Cs β decay and $^{136}$Ba(n,γ) reaction experiments. Although several (n,γ) studies have been conducted, the only published data dates back to 1969 [3]. More recently, a study of the $^{138}$Ba(p,t)$^{136}$Ba reaction [4] identified several previously unknown 0$^{+}$ states in $^{136}$Ba. The high statistics of this experiment will allow for a significant expansion of the existing data set.
The experimental setup consisted of 16 HPGe Clover detectors with anti-Compton shields, achieving an efficiency of 3.5 % at 1.4 MeV and an energy resolution of ~2 keV at 1.3 MeV. The experiment employed a thermal neutron beam from the ILL reactor with an intensity of ~10$^{7}$ n/s/cm² [5]. The results will highlight newly identified transitions and spin assignments for states up to 5 MeV in excitation energy. The coincidence method was used to assign new decay paths by analyzing gamma-gamma matrices, while spin assignments were determined through angular correlation analysis of coincident gamma rays, referencing existing literature on tentative spin values and mixing ratios.
Additionally, the findings will be compared with theoretical calculations to provide further insights into the nuclear structure of $^{136}$Ba. Lifetime measurements will be conducted to reduce uncertainties and provide new data. The vibrational and mixed-symmetry properties of $^{136}$Ba (N = 80) will also be explored to enhance the understanding of its collective dynamics. These results aim to reduce NME uncertainties, advance knowledge of 0νββ, and contribute to broader nuclear structure studies.
References:
[1] A. Gando et al., Phys. Rev. Lett. 117, 082503 (2016).
[2] J. Engel and J. Menéndez, Rep. Prog. Phys. 80, 046301 (2017).
[3] W. Gelletly et al., Phys. Rev. 181, 1682 (1969).
[4] B. M. Ribeiro et al., Phys. Lett. B 809, 135702 (2020).
[5] C. Michelagnoli et al., EPJ Web Conf. 193, 01018 (2018).