Optimization of poisoned and unpoisoned decoupled moderators in JSNS

Harada, Masahide   ; Teshigawara, Makoto   ; Watanabe, Noboru; Kai, Tetsuya   ; Ikeda, Yujiro

For two decoupled moderators in JSNS, optimization studies were performed by model calculations using NMTC/JAM and MCNP-4C codes. The model was based on a realistic Target-Moderator-Reflector Assembly. We assumed a para-hydrogen ratio of 100%. The shape of poisoned and unpoisoned moderators is a canteen type with dimensions of 13126.2 cm. A decoupling energy of about 1 eV was adopted to meet the user's requests. As a decoupler material we selected silver-indium-cadmium alloy. It was found that for the decoupled moderators, especially the poisoned moderator, pulse broadening due to a finite beam-extraction angle () was very serious. Therefore, for the poisoned and the unpoisoned moderators were limited to be 7.5 and 17.5, respectively. Cadmium (Cd) was selected as a poison material due to higher cut-off energy than gadolinium and higher peak intensity with narrower pulse width. The poison plate will be placed at 25 mm from the viewed surface which meets the user's requirements.