Study on neutron response of criticality accident alarm system detector to quasi-monoenergetic 24 keV neutrons

Tsujimura, Norio   ; Yoshida, Tadayoshi ; Yashima, Hiroshi*

The latest model of the criticality accident alarm system (CAAS), recently developed and installed at the Tokai Reprocessing Plant of the Japan Atomic Energy Agency, consists of a plastic scintillator combined with a cadmium-lined polyethylene moderator and responds both to rays and neutrons. To evaluate the neutron absorbed dose rate response of the CAAS detector, a 24 keV quasi-monoenergetic neutron irradiation experiment was performed at the B-1 facility of Kyoto University Research Reactor. The B-1 facility consists of 45-cm-thick iron and 35-cm-thick aluminum filters built into a radial beam tube of the reactor and provides an intense beam of 24 keV neutrons transmitted through "resonance window" in the cross-sections of iron and aluminum. The CAAS detector was mounted on a wooden platform horizontally scanning across the collimated neutron beam extracted from the beam column. A neutron rem counter and an energy-compensated GM counter were also placed on the platform to measure the reference absorbed dose rates of neutrons and rays, respectively. The neutron response of the CAAS detector was derived by dividing the neutron-induced net reading, deduced by subtraction of the estimated -ray fractional reading based on the GM reading, by the reference neutron dose rate. The evaluated neutron response of the detector was confirmed to be in reasonably good agreement with the prior computer-predicted response.