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

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

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.