Neutronics of a poisoned para-hydrogen moderator for a pulsed spallation neutron source

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

Neutronic performance of a poisoned hydrogen moderator was studied in details. Using a simple rectangular-shape moderator-model, its basic characteristics were studied as a function of the poison position. We, for the first time, turned up that the pulse width was rather decreasing with increasing the moderator thickness of the back part from the poison. This is due to the fact that source neutron pulses entering through the back part of poison are narrow while those from the target and the reflector through the decoupler and liner are broad. Next, we studied the pulse deterioration in regard to a finite beam-extraction-angle by using a canteen shape and a concave shape. It turns out that the pulse deterioration is large in both cases even at a small extraction angle. The concave-shape moderator indicates a finite improvement compared to the canteen shape one. Finally, merits and demerits of two poison materials, Cd and Gd, were discussed considering the burn-up issue of poison.