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

24keV準単色中性子に対する臨界警報装置の中性子レスポンスに関する研究

辻村 憲雄   ; 吉田 忠義 ; 八島 浩*

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

京都大学原子炉実験所の準単色中性子実験設備(B-1実験孔)において、臨界警報装置の中性子照射実験を行い、24keV準単色中性子に対する感度を実験的に求めた。実験孔から1mの距離にビーム軸に直交した配置で、臨界警報装置,中性子レムカウンタ及びGMカウンタを一列に並べ、遠隔操作によって検出器がビーム軸を交差するよう移動させつつそれぞれの検出器の出力信号を観測した。中性子レムカウンタ及びGMカウンタは、それぞれ中性子, $$gamma$$線の吸収線量率の評価に用いた。$$gamma$$線による信号寄与を差し引いて算出した臨界警報装置の中性子レスポンス(中性子吸収線量率あたりの指示値)は、検出器設計の段階で計算によって算出したレスポンスとほぼ一致することが確認された。

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 $$gamma$$ 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 $$gamma$$ rays, respectively. The neutron response of the CAAS detector was derived by dividing the neutron-induced net reading, deduced by subtraction of the estimated $$gamma$$-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.

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