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Journal Articles

Neutronic studies on decoupled hydrogen moderator for a short-pulse spallation source

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

Nuclear Instruments and Methods in Physics Research A, 539(1-2), p.345 - 362, 2005/02

 Times Cited Count:18 Percentile:76.37(Instruments & Instrumentation)

Neutronic studies of decoupled hydrogen moderators were performed by calculations taking into account para hydrogen content, decoupling energy, moderator dimensions/shapes and reflector material. Low-energy parts of calculated spectral intensities with different para hydrogen contents were analyzed by a modified Maxwell function to characterize neutron spectra. The result shows that a 100% para hydrogen moderator gives the highest pulse peak intensity together with the narrowest pulse width and the shortest decay times. Pulse broadening with a reflector was explained by time distributions of source neutrons entering into the moderator through a decoupler. Material dependence of time distribution was studied. A decoupling energy higher than 1 eV does not bring about a large improvement in pulse widths and decay times, even at a large penalty in the peak intensity. The optimal moderator thickness was also discussed for a rectangular parallelepiped shape and a canteen shape moderators.

Journal Articles

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

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

Proceedings of ICANS-XVI, Volume 3, p.657 - 666, 2003/07

Neutronic design studies were carried out on the J-PARC coupled H$$_{2}$$ 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$$_{2}$$ 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$$_{2}$$ 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$$^{rm o}$$. Then, we propose a cylindrical shape coupled moderator which brings about a slight intensity decrease for 0$$^{rm o}$$ but a increase in the averaged intensity over the angles of interest.

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