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Application of modified REFIT code for J-PARC/MLF to evaluation of neutron capture cross section on $$^{155,157}$$Gd

J-PARC/MLF用に改良したREFITの$$^{155,157}$$Gdの中性子捕獲断面積評価への適用

水山 一仁; 岩本 信之; 岩本 修; 長谷美 宏幸*; 木野 幸一*; 木村 敦; 鬼柳 善明*

Mizuyama, Kazuhito; Iwamoto, Nobuyuki; Iwamoto, Osamu; Hasemi, Hiroyuki*; Kino, Koichi*; Kimura, Atsushi; Kiyanagi, Yoshiaki*

ガドリニウムは$$^{155,157}$$Gdが非常に大きな中性子捕獲断面積をもつため原子炉の中性子吸収材料として用いられる。しかしながら、$$^{157}$$GdのRPIデータとJENDL-4.0ではばらつきがあることが知られている。原子炉の臨界性は捕獲断面積に敏感であり、RPIのデータはICSBEPにあるガドミニウム溶液体系の臨界性を大きくしてしまうことがわかっている。近年、$$^{155,157}$$Gdの中性子捕獲断面積がJ-PARC/MLFでANNRIを用いた実験で飛行時間法によって測定された。J-PARCの中性子パルスはダブルバンチ構造を持っており、また、高精度な共鳴解析には中性子パルスの時間分解能関数も適切に考慮して行う必要がある。そこで、本研究では、中性子パルスのダブルバンチ構造やANNRI用の時間分解能関数を組み込んだ改良REFITを用いて中性子捕獲断面積の実験データをフィットし、共鳴パラメータを導出する。

Gadolinium has been used as neutron-absorbing material in a thermal reactor since $$^{155,157}$$ have large thermal neutron capture cross sections. Nevertheless, there is a discrepancy between RPI data and JENDL-4.0 data for $$^{157}$$Gd. The criticality in the reactor is very sensitive to the capture cross section. The RPI data made the criticality of Gd-loaded thermal systems in ICSBEP overestimated. Recently, the neutron capture cross sections of $$^{155,157}$$Gd were measured by the neutron time-of-flight (TOF) method using the Accurate Neutron-Nucleus Reaction measurement Instrument (ANNRI) in the J-PARC/MLF. The pulsed neutron beam from the Japan Spallation Neutron Source (JSNS) was used with a double-bunch structure in this measurement, since the incident proton beam is normally delivered in a double-bunch scheme in the J-PARC. In addition to this, it is necessary to take into account the energy resolution of the pulsed neutron beam at the JSNS for the accurate derivation of resolved resonance parameters. In this study, using the least-squares multilevel R-matrix code REFIT modified to include the double bunch structure and the resolution function for the ANNRI, we fitted the calculated capture cross sections of $$^{155,157}$$Gd to the experimental data at the ANNRI. We derived the resonance parameters for some low-lying resonances of the two Gd isotopes.

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