Speaker
Description
CUPID will use cryogenic bolometers to search for neutrinoless double beta decay of (^100)Mo with a projected 3σ discovery sensitivity corresponding to a half-life of 1.0×10^27 years, probing well into the inverted neutrino mass hierarchy. Achieving this sensitivity requires both ultra-low background and excellent energy resolution.
Germanium NTD thermistors will be used to read out the bolometric thermal signals. The detector performance critically depends on the dopant concentration in the Ge thermistors. These devices are created by neutron transmutation doping (NTD) of high-purity Ge wafers in a nuclear reactor, followed by cooldown and processing into the thermistor devices. Accurate characterization of the thermal neutron fluence during irradiation is therefore essential to ensure optimal doping concentration.
We report on an initial test irradiation performed at the MIT reactor and the determination of the thermal neutron fluence using neutron activation analysis (NAA) of monitor foils irradiated alongside the Ge wafers. The induced activities of the (^59)Fe, (^95)Zr, and (^60)Co were measured via gamma-ray spectroscopy with a high-purity germanium (HPGe) detector at Virginia Tech.
Absolute activities were extracted using GEANT4 simulation models of the detector validated with calibrated reference sources. Corrections for the decay irradiation, cooldown, and counting were used for the fluence calculation.
We present the detector characterization, efficiency modeling, and extracted neutron fluences from the test irradiation. The methodology presented will be used for monitoring production irradiations of Ge wafers for future CUPID NTD fabrication.