Monitoring 135Xe after the shutdown of a research reactor using gamma-ray spectroscopy

135 Xe, a high-yield fission product with a large neutron absorption cross section, can greatly influence the reactivity of a reactor, posing challenges to reactor operation and safety if the transient 135 Xe concentration is not taken into account. To date, 135 Xe has only been monitored indirectly with neutron flux monitors, studied from off-gas, or studied in simulation. In this study, we show that 135 Xe in a zero-power research reactor can be directly monitored after shutdown, using gamma spectroscopy. We use an HPGe gamma-ray spectrometer in the beam port of the CROCUS research reactor and measure the 135 Xe and 135 I signature gamma-ray emissions (250 keV and 1260 keV, respectively) after shutting down the reactor. We directly measure the transient behavior of 135 Xe and 135 I, the commonly known “Xenon Pit” corresponding to maximum neutron absorption, following reactor shutdown. The expected 135 Xe peak time with known half-life values is 11.13 h, and we calculate 11.15 ± 0.22 h when fitting the measured data with free half-life parameters. We provide results for code validation, especially useful for reactor shutdown and accident scenarios, as well as reactivity margin predictors.