Gamma sensitivity of a ZnS:Ag(6-LiF) wavelength shifting fiber neutron detector in mixed neutron-gamma fields

Gamma sensitivity of an ISIS linear position sensitive wavelength shifting fiber (WLSF) detector was evaluated at the Reactor Institute Delft (RID) using ⁶⁰Co and ¹³⁷CS sources. The caveat of quoting gamma sensitivity of a detector in pure gamma fields is that detectors are never operated in gamma only environments. A concern is that the sensitivity may increase during neutron measurements due to the increase of pulse pile-up which can occur in ZnS:Ag detectors as the afterglow of a neutron event coincides with a gamma scintillation event causing the gamma event to appear brighter than it actually was. For this reason, gamma sensitivity measurements were performed on a time of flight neutron reflectometer at RID. Results presented here detail neutron efficiency and rate capability of the detector as well as demonstrate gamma sensitivity in pure gamma and mixed fields. It is shown that, in a pure gamma field, the detector has gamma sensitivity on the order of 10^-7-10^-6 for ⁶⁰Co and 10^-9-10^-8 for ¹³⁷Cs as a result of detector geometry, fibre coding and signal processing. Gamma sensitivity is unchanged to any statistical significance in a mixed neutron gamma field. A less than 1% gamma-induced deadtiming at neutron rates above 5kHz and high gamma flux on the order of 10⁴ gammas/second/cm² was observed.