Xe136

Xenon-136 (136Xe) is an isotope of the noble gas xenon with 54 protons and a mass number of 136, corresponding to 82 neutrons. It occurs naturally in xenon gas at roughly 8–9% abundance and can be enriched for experimental use. Its long-lived nature and nuclear properties make it a key isotope in fundamental physics research as well as in detector media for rare-event searches.

Nuclear properties and decay modes: 136Xe is a candidate for double beta decay to barium-136 (136Ba). In the two-neutrino double beta decay mode, two neutrons convert to two protons with the emission of two electrons and two antineutrinos. This process has been observed, with a half-life of about 2×10^21 years, making 136Xe effectively stable on human timescales. The decay is energetically allowed, with a total energy release (Q-value) of approximately 2.458 MeV.

Neutrinoless double beta decay searches: The potential observation of neutrinoless double beta decay (0νββ) would indicate that neutrinos are Majorana particles. No evidence for 0νββ has been found to date in 136Xe, and experiments have set lower limits on the 0νββ half-life that typically exceed 10^26 years, translating into constraints on the effective Majorana neutrino mass depend­ing on nuclear-model calculations.

Applications in physics research: 136Xe is widely used in experiments searching for neutrinoless double beta decay and in large-scale detectors for rare-event physics. Enriched 136Xe is chosen as a target material in several detectors, including KamLAND-Zen, EXO-200, NEXT, and planned large-scale facilities such as nEXO, due to its favorable combination of high atomic mass, scintillation and ionization properties, and and well-understood detector techniques.