Stability of neutron beam monitor for High Intensity Total Diffractometer at J-PARC

We have developed a neutron beam monitor with a gas electron multiplier (GEM) for the High Intensity Total Diffractometer (NOVA) at the Japan Proton Accelerator Research Complex (J-PARC). The GEM-based detector was chosen because of its two-dimensional detection and high-counting ability (about 1 MHz). The effective counting rate was 114.11±0.24 events/1012 protons, and the ratio of the standard deviation to the mean was approximately 0.2%. About one-half of this error (0.1%) was estimated to be due to fluctuations in the GEM-based detector and it is therefore comparable with the lower limit of the typical error of scattered neutron counts (0.1%–1%) at NOVA. The stability of the GEM-based detector was also confirmed from the plateau characteristics and high count rate characteristics.