Speaker
Description
Cosmogenic background is increasingly recognized as being important for dark matter direct detection experiments, especially as other sources of background have been well-understood and massively reduced, so as to increase the sensitivity for detecting rare dark matter events. This key background in NaI(Tl) arises from the cosmogenic radioisotopes like $^3$H and $^{22}$Na, that are low in the original detector materials, but are generated by exposure to cosmic rays above ground and contribute a background component to the detector signal for years.
A particular issue with cosmogenic backgrounds is their characterisation, due to uncertainties in exposure. The first dedicated experiment to measure cosmic neutron activation in NaI(Tl) was conducted at LANSCE (Los Alamos Neutron Science Center) in November 2019. The LANSCE neutron beam with a cosmic-ray-like spectrum was used to activate NaI crystals with the equivalent of roughly half a million years of sea-level cosmic neutron exposure.
Measurements and simulations have been conducted to analyse the cosmogenic activation in the NaI(Tl) crystal. One measurement was performed shortly after the irradiation at LANSCE so as to evaluate the short-lived cosmogenic isotopes. Another measurement was performed at Yale University to directly count the long-lived cosmogenic isotopes in the NaI(Tl) crystal. A dedicated Geant4 simulation was created to understand the energy deposition of the radioisotopes, which employed a model of the light yield non-linearity to better reproduce the measured signal. The simulated energy spectra were fitted to the observed spectra as a function of time to obtain the cosmogenic radioactivity. The final result of this experiment will be a measurement of how the activities evolve with time and hence a determination of the cosmogenic activation rates of different isotopes in NaI crystals.
This study will inform the development and analysis of NaI(Tl)-based experiments, such as SABRE, ANAIS and COSINE-100, and also improve their sensitivity to probe dark matter.
