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Application of KCl:Eu$$^{2+}$$ storage phosphors to wide-range neutron detection

KCl:Eu$$^{2+}$$輝尽性蛍光体の広領域中性子検出への応用

坂佐井 馨  ; 藤 健太郎  ; 中村 龍也  ; 高倉 耕祐; 今野 力  ; 岩元 洋介   

Sakasai, Kaoru; To, Kentaro; Nakamura, Tatsuya; Takakura, Kosuke; Konno, Chikara; Iwamoto, Yosuke

KCl:Eu$$^{2+}$$輝尽性蛍光体を広領域の中性子検出に応用するため、本輝尽性蛍光体に$$^{6}$$LiFとポリエチレンを混合したサンプルを製作し、FNSにて14MeV中性子照射実験を行った。その結果、ポリエチレン濃度が約5%で最大の輝尽性蛍光強度が得られることがわかった。これはモンテカルロ計算及び理論式によるエネルギー付与計算結果と一致した。また、低速・中速エネルギー範囲においては、$$^{6}$$Li(n,$$alpha$$)反応によるものが支配的であることもわかった。さらに、$$^{6}$$Li(n,$$alpha$$)反応において、相対論を用いて$$alpha$$粒子のエネルギーを中性子エネルギーの関数として計算したところ、中性子エネルギーが高い場合、実験室系では$$alpha$$粒子は一定のエネルギーを有するのではなく、ある幅を持った一様分布になることがわかった。これを用いて$$alpha$$粒子の平均の付与エネルギーを計算したところ、5MeV以上の中性子エネルギーでは$$^{6}$$Li(n,$$alpha$$)反応だけでなく、K(n,$$alpha$$)反応のような核破砕反応がエネルギー付与に寄与することがわかった。

Storage characteristics of KCl:Eu$$^{2+}$$ phosphor were investigated in order to apply the phosphor to wide-range neutron detection. For detection of both slow and fast neutrons, $$^{6}$$LiF powders and polyethylene powders were added to the phosphor at the same time. The optimization of the phosphor for maximum photostimulated luminescence yields have been performed and obtained at a polyethylene weight ratio of about 0.05 when the mixtures were irradiated by fast neutron with energy of 14.8 MeV, where the weight ratio of KCl:Eu$$^{2+}$$ phosphor to $$^{6}$$LiF powders was fixed to be two to three. These experimental results well agreed with theoretical predictions by Monte Carlo simulation using PHITS code. The energy deposition by slow or intermediate neutrons was mainly due to charged particles as a result of nuclear reaction of $$^{6}$$Li(n,$$alpha$$)T. According to relativistic calculation, the energy of alpha particles or tritons cannot be uniquely determined in a laboratory system but has a certain distribution even if incident neutron energy is fixed when neutron energy is high. The calculation showed that charged particles produced by nuclear reactions such as $$^{39}$$K(n,$$alpha$$) or $$^{35}$$Cl(n,$$alpha$$), in addition to protons by $$^{1}$$H(n,n) $$^{1}$$H reaction, will contribute the total energy deposition at high energy of incident neutrons.

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