Neutronic study on coupled hydrogen moderator for J-PARC spallation neutron source

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

Neutronic design studies were carried out on the J-PARC coupled H moderator. The highest time- and energy- integrated intensity below 15 meV, pulse peak intensity at 2 meV and 10 meV, were given by 100% para-H moderators, but the optimal moderator thickness was different for each: more than 220 mm, about 120 mm and 80 mm, respectively. Finally, we concluded 140 mm was the optimal thickness for the 100% para-H coupled moderator. Cold neutron distributions on the moderator viewed surface were found to exhibit an intensity-enhanced region at a picture frame part near premoderator. This rather peculiar distribution suggested that the moderator and the viewed surface must be designed so as to take the brighter region near premoderator in use. The intensity decreases along with beam-extraction angle to the normal direction down to about 70% at 25.4. Then, we propose a cylindrical shape coupled moderator which brings about a slight intensity decrease for 0 but a increase in the averaged intensity over the angles of interest.