Neutron fluence monitoring system in mono-energetic neutron fields at FRS / JAEA

Mono-energetic neutron calibration fields were developed from 8 keV to 19 MeV using an accelerator at the Facility of Radiation Standards (FRS). Two kinds of neutron detectors are used for monitoring the neutron fluence at a calibration point. One is the Long Counter placed at the 60 degree angle with respect to the beam line except for the 14.8 and 19 MeV fields where the angle is 90 degree. The other is the 110deg-monitor installed in a collimator at 110 degree angle with respect to the beam line. These two monitors were designed to have flat response to the neutrons from a few keV to a few MeV. The neutron fluence at a calibration point can be evaluated from the counts of the Long Counter with corrections of scattered neutrons. The neutron energy is sensitive to the beam energy especially for the neutrons below a few hundreds of keV. It was found that the beam energy was able to be monitored using the count ratio of two neutron monitors.