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大型加速器を用いた核破砕中性子源

Spallation neutron source using high power accelerator

羽賀 勝洋

Haga, Katsuhiro

高出力加速器を用いたパルス核破砕中性子源は、物質科学や生命科学の分野において最先端の研究を行うために適した高強度で幅の狭い良質な中性子を供給できる有用な装置である。このシステムでは、高出力加速器で作り出すkWからMWオーダーの陽子ビームを重金属の標的に入射し、標的原子核との核破砕反応により大量の中性子を生成し、この中性子を減速材や反射体で熱・冷中性子に減速し、先端的な中性子実験装置群に供給する。本稿では主にJ-PARCのパルス核破砕中性子源を例に、中性子ビームを生成するしくみと中性子源の構造の概要、水銀標的等の標的の工学的設計手法、パルス陽子ビームの入射により水銀中で発生する最大40MPaにも達する圧力波による標的容器の損傷を低減する方法などの技術課題について、水銀標的システムを例に取りながら解説する。

The pulsed spallation neutron source driven by a high-power accelerator is one of the most powerful apparatus to provide high intensity and high quality neutrons with narrow pulse width for conducting cutting-edge researches in several domains of materials and life science. In this system, proton beams of several kW to MW order extracted from the high power accelerator is injected into a target, which is heavy metal, to generate vast amount of neutrons via the spallation reactions with the target nuclei, and slows down these neutrons to thermal to cold neutrons with a moderator and a reflector. Resultant neutron beams are then supplied to a suit of the state-of-the-art experimental devices. In this paper, mechanism to produce neutron beams and outline of the spallation neutron source, engineering design of a target system such as a mercury target, and technical topics to solve the pitting damage problem of the target vessel which is caused by the pressure wave of up to 40MPa at maximum generated in the mercury by the pulsed proton beam injection are reviewed by referring mainly to the mercury target system of the pulsed spallation neutron source at J-PARC.

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