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Potential of 1 micro-eV energy resolution of the Si-analyzer backscattering spectrometer DNA at the spallation neutron source J-PARC

J-PARCパルス中性子源のSiアナライザー背面反射型分光器DNAにおけるエネルギー分解能1micro-eVの可能性

高橋 伸明; 柴田 薫  ; 佐藤 卓*; 川北 至信*; 筑紫 格*; 中島 健次  ; 新井 正敏

Takahashi, Nobuaki; Shibata, Kaoru; Sato, Taku*; Kawakita, Yukinobu*; Tsukushi, Itaru*; Nakajima, Kenji; Arai, Masatoshi

飛行時間型のSiアナライザー背面反射分光器DNAは、J-PARC物質生命科学実験施設(MLF)に設置が計画されている。本装置には、Si(111), Si(311)アナライザーが、線源から43mに位置する真空散乱槽内に設置される。この分光器は、数meV程度の低エネルギー(Siアナライザーの反射エネルギー)の中性子を最大強度で享受するため、結合型減速材を線源に選択している。一方、結合型減速材のパルス時間幅は、3種類の減速材中最も広いため、ビームライン上の線源からできるだけ近い位置にパルス整形デバイスとして高速ディスクチョッパーを備える予定である。本発表では、本実験装置の概念設計並びに、そこから期待されるエネルギー分解能を示すとともに、定常炉線源に設置されている同型の実験装置やパルス中性子源に設置される飛行距離が長く、パルス時間幅が狭いが強度の弱い減速材を線源としている分光器と比較議論する。

A time-of-flight Si-analyzer backscattering spectrometer (BSS) DNA is proposed for construction in the Materials and Life Science Facility (MLF) of Japan Proton Accelerator Research Complex (J-PARC) located in Tokai, Ibaraki, Japan. Four crystal analyzer sets, namely Si(111), Si(311), PG(002) and Ge(311) had been planned to be mounted on it in near-backscattering configurations in two evacuated tanks in order to cover wide energy and momentum transfer ranges that suit research on the dynamics of bio-molecules and proteins. After extensive discussion with instrument scientists of BSSs as well as biophysicists in the committee the highest energy resolution option provided by the Si analyzer came to the highest priority. Therefore the design of the DNA has been rather simplified. Si(111) and Si(311) analyzers will be mounted in a vacuum vessel putting at 43 m away from a coupled moderator to enjoy the highest peak intensity around several meV, which is the elastic energies of the Si analyzers, among three moderators of JSNS, namely the coupled, a decoupled and a decoupled-poison. Because the pulse time-width of the coupled moderator is the widest of the three, it is designed to utilize a pulse-shaping device, which is high speed counter rotating disc choppers, on the beamline staying as far away from the sample position as possible to narrow the time-width. In this paper we will present the conceptual design and the expected performance of the pulse-shape short-flight-length BSS DNA. The obtainable performance will be compared with BSSs at reactor sources as well as a decoupled-poisoned-moderator-source long-flight-length BSS at a spallation neutron source.

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