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Simulation of $$^{235}$$U(n,fission) gamma-ray energy spectrum by FIFRELIN fission event generator

核分裂モデルFIFRELINを使った$$^{235}$$U(n,fission)$$gamma$$線スペクトルのシミュレーション

小川 達彦 ; Litaize, O.*; Mancusi, D.*; Chebboubi, A.*; Serot, O.*

Ogawa, Tatsuhiko; Litaize, O.*; Mancusi, D.*; Chebboubi, A.*; Serot, O.*

近年$$^{235}$$U(n,fission)等の核分裂反応から生じるガンマ線スペクトルの計測を通して、中性子過剰核の核構造やガンマ線放出メカニズムについての研究が進んでいる。本研究では、CEAの核分裂モデルFIFRELINを開発し、これを用いて、核分裂ガンマ線スペクトルの決定因子を究明した。ガンマ線スペクトルの形状は、4MeV付近のピークと、6MeV付近の肩、10MeV以上のテールで特徴づけられる。従来のFIFRELINを使って前者二つは再現することができた一方、10MeV以上のガンマ線は、多くの励起エネルギーを持つ軽い分裂片から生じていると考えられるが、FIFRELINの計算値は実験値より低いことが確認されていた。また、90以下の質量数の核から生じるガンマ線のマルチプリシティは、過去のFIFRELINの計算結果では過大評価されていた。そこで、質量数90以下の核分裂片に対してスピンカットオフパラメータを5倍増やすことで、ガンマ線のマルチプリシティを減らすことを試した。これにより、マルチプリシティを正確に再現するだけでなく、ガンマ線一個当たりのエネルギーが上昇することにより、ガンマ線エネルギースペクトルのより正確な再現も可能になった。

The gamma-ray spectrum of $$^{235}$$U(n,fission) reactions measured by Makii et al., at PF1B cold-neutron beam facility at the Institut Laue Langevin (ILL) in Grenoble (France) was compared with the data calculated by the fission event generator FIFRELIN to get the insight into the mechanisms which determine the gamma-ray energy spectrum. The features structures of the spectrum, a bump at 4 MeV, a shoulder at 6 MeV agreed between experiment and calculation while the shape of the tail beyond 10 MeV did not agree well. The bump around 4 MeV is mostly attributed to the deexcitation of closed shell nuclei such as Sn-132 from their excited state around 4 MeV to their ground state. The shoulder near 6 MeV is explained by the fact that the neutron separation energy of most of fission fragments are around 6 MeV. The excitation energy of fragments is randomly distributed, therefore the fragments with excitation energy higher than the neutron separation energy favor neutron emission, whereas those with low excitation energy emit gamma-rays. Consequently, gamma-ray spectrum drops around 6 MeV. According to FIFRELIN, gamma-rays above 10 MeV are mainly from fragments with mass below 90. Meanwhile, the multiplicity of gamma-rays from the fragments with mass lower than 90 was overestimated according to the FIFRELIN previous results. Therefore reducing the multiplicity of gamma-rays from fragments with mass below 90 can explain both discrepancies. We show in this work that by increasing the spin cut-off parameter of fragments with mass lower than 90 by a factor of 5, FIFRELIN reproduced the measured gamma multiplicity mass dependence as well as the measured gamma-ray energy spectrum.

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