Measurements of Thermal Neutron Response in Cherenkov Glasses Designed for MeV Photon Detection

In order to obtain a meaningful gamma ray measurement in mixed neutron and high-energy photon environments, a quantitative understanding of neutron-induced backgrounds is crucial. Neutrons that can produce Cherenkov photons indirectly through (n, γ) activation reactions are found in radiation backgrounds in both passive and active-source search environments. The objectives of this research are to establish the properties and fabrication techniques for materials suitable for the production of photon-sensitive Cherenkov detectors (including low neutron sensitivity), or, for certain samples, to understand how the response to neutrons can be electronically discriminated from the signal caused by the interaction of MeV photons. Over a hundred glass samples have been fabricated, and radiation characterization results from selected samples are reported. The thermal neutron responses of these glasses were determined using neutron activation followed by time dependent radiation measurement and gamma ray spectroscopy. The results showed that the probability of thermal neutron capture, the thermal neutron activation time, the half-life of the activation products, the abundance of the target materials, and the magnitude of the energy of gamma and beta radiations that result from activation play significant roles in understanding the observed count rate due to Cherenkov radiation. Implications for Cherenkov detectors to be used for MeV photon detection are discussed.