Optimization of Coupled and Decoupled Hydrogen Moderators for a Short-pulse Spallation Source

Article type: Research Article

Authors: Watanabe, Noboru | Harada, Masahide | Kai, Tetsuya | Teshigawara, Makoto | Ikeda, Yujiro

Affiliations: Center for Proton Accelerator Facility, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki-ken 319-1195, Japan

Note: [] Corresponding author. E-mail: [email protected]

Abstract: Neutronic performance of coupled and decoupled hydrogen moderators for a short pulse spallation source was extensively studied by calculations to seek the optimal parameters, especially the para/ortho hydrogen ratio. A decoupled 100% para-hydrogen moderator gives excellent pulse characteristics at lower energies. Gadolinium (Gd) poisoning is useful to obtain narrower pulses up to about 70 meV at a reasonable intensity penalty. At higher energies, where the pulse tail decay characteristics become most important, the choice of the reflector material and the decoupling energy (Ed) play an important role. One important technical issue is the radiation damage of a Gd-poison and a boron carbide decoupler. We discuss the possibility of using a composite decoupler material such as Cd + In (indium) + Eu (europium). In a coupled moderator a 100% para-hydrogen moderator of a larger volume gives an additional intensity gain over a normal one but the gain is not as large as in a decoupled one.

Keywords: Short-pulse spallation source, Moderator neutronics, Hydrogen moderator, para/ortho hydrogen ratio, Reflector, Decoupling energy

DOI: 10.1080/1023816031000100879

Journal: Journal of Neutron Research, vol. 11, no. 1, 2, pp. 13-23, 2003