GEM-based thermal neutron beam monitors for spallation sources

The development of new large area and high flux thermal neutron detectors for future neutron spallation sources, like the European Spallation Source (ESS) is motivated by the problem of He-3 shortage. In the framework of the development of ESS, GEM (Gas Electron Multiplier) is one of the detector technologies that are being explored as thermal neutron sensors. A first prototype of GEM based thermal neutron beam monitor (bGEM) has been built during 2012. The bGEM is a triple GEM gaseous detector equipped with an aluminum cathode coated by 1 mu m thick B4C layer used to convert thermal neutrons to charged particles through the B-10(n,Li-7)alpha nuclear reaction. This paper describes the results obtained by testing a bGEM detector at the ISIS spallation source on the VESUVIO beamline. Beam profiles (FWHMx = 31 mm and EWHM gamma = 36 mm), bGEM thermal neutron counting efficiency (approximate to 1%), detector stability (3.45%) and the time spectrum of the beam were successfully measured. This prototype represents the first step towards the development of thermal neutrons detectors with efficiency larger than 50% as alternatives to He-3-based gaseous detectors. 2013 The Authors. (C) Published by Elsevier B.V. All rights reserved.