Measurements of gamma-ray sensitivity of a GEM based detector using a coincidence technique

Gas Electron Multiplier based detectors have been recently used for neutron measurement at spallation sources and for fusion relevant applications. The present work is meant to characterize the detector sensitivity to gamma-ray background which often can represent the main background source associated to neutron measurements. The gamma-ray sensitivity has been measured as a function of detector gain and it has been found to be lower than 10(-7) at the typical gain used for neutrons detection. Coincidence measurements, giving the possibility to select mono-energetic photons and to suppress background events, allowed to measure the absolute sensitivity at a well defined gamma-ray energy and at low interaction rates. The mechanism of interaction and energy deposition of gamma-rays in the detector has been studied using MCNPX Monte Carlo simulations.