Characterization of a GEM-based fast neutron detector

The neutron efficiency of a Gas Electron Multiplier (GEM)-based detector designed for fast neutron measurements in fusion devices was determined through the combined use of Monte Carlo (MCNPX) calculations and analysis of deuterium-deuterium and deuterium-tritium neutron irradiation experiments. The detector, characterized by a triple GEM structure flushed with a Ar/CO2/CF4 - 45/15/40 gas mixture, features a digital read-out system and has two sub-units for the detection of 2.5 + 14 MeV neutrons and 14 MeV neutrons (U-DD and U-DT, respectively). The pulse height spectra (PHS) determined from the curves of experimental efficiency as a function of the detector's high voltage (HV) and the MCNPX-simulated PHS were compared using a fitting routine that finds the best match between the experimental and simulated PHS by assuming a parametric model for the relation between HV (that determines the detector's gain) and the energy deposited in the gas. This led to express the experimental neutron efficiency as a function of the discrimination level set on the deposited energy (energy threshold). The detector sensitivity to gamma-rays was also analyzed and the operational range in which the gamma-ray contribution to the signal is not negligible was determined It is found that this detector can reach a maximum neutron efficiency of similar to 1 x 10(-3) counts/it at 2.5 MeV (U-DD sub unit) and of similar to 4 x 10(-3) counts/n at 14 MeV (U-DT and U-DD sub units) (C) 2014 EURATOM ENEA. Published by Elsevier B.V. All rights reserved