Thermal neutron activation of a CeBr[formula omitted] gamma-ray sensor

Abstract

We observed significant activation of a CeBr3 gamma-ray sensor following exposure to thermal neutrons with a fluence of 105 cm−2 over a 40-minute-long period. Following irradiation, the intrinsic background rate increased from 30 to 18,100 counts s−1. Pulse height spectra of the activated detector were collected over a 45-minute-long period following neutron irradiation, and the spectra showed evidence for radioactive decay products produced via both β decay and gamma decay of meta-stable excited states (isomeric transitions). The decay energies and half lives revealed the presence of 80Br (t1/2=17 min), 80Br* (t1/2=4.4 h), 82Br (t1/2=6.1 min), and 139Ce (t1/2=55 s). The presence of additional, longer-lived radioisotopes cannot be ruled out by our measurements. All of the observed activation products were formed via neutron capture on primary detector elements, and thus would be expected for any CeBr3 sensor exposed to similar (or higher) thermal neutron flux environments. Given the dramatic increase in intrinsic count rates that we observed following the relatively modest neutron exposure, we propose that CeBr3-based gamma-ray sensors may not be suitable for use in environments with high thermal neutron fluxes without the inclusion of a thermal neutron shield surrounding the sensor.