A Gd Based Gaseous Electron Multiplier Detector for Neutron Scattering Applications

Abstract

The Gaseous Electron Multiplier (GEM) neutron detector is one of several new concepts that are being designed for the next generation neutron sources. The detector contains multiple modules where each module contains a central neutron converter, several cascaded GEM foils, and readout plates on both sides of the cathode. The device employs a Gd/CsI converter sandwich to convert the neutron to an electric signal. Upon neutron absorption in the Gd layer, conversion electrons are emitted with a probability of 86.5%. Primary electrons travel into the CsI and multiply, inducing the emission of low energy electrons from the converter surface. Several cascaded GEM foils placed on both sides of the converter amplify the signal. The position of the signal is then time stamped and detected by position sensitive anodes and localization electronics.

Extensive Monte Carlo simulations, using the PENELOPE code package, have been completed in order to calculate integral and differential characteristics (i. e. the average number of secondary electrons emitted from the converter and the secondary electron escape probability distribution) of various converter thicknesses. The performance of the detector can be assessed by estimating the secondary electron (SE) leakage from the converter foil. Simulations to calculate the SE leakage have been performed for both the single and several multiple module designs. The multiple module design allows for a greater SE leakage while maintaining high detector efficiency and divides the total count rate among several sets of decoding electronics.

Tests of several different detector prototypes were completed at the NCSU PULSTAR reactor. Results of the tests of the prototype detectors revealed the essential elements of detector operation. Detector efficiencies, pulse height spectra, and 2-D position spectra have been measured with each of the detector prototypes.