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

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 (E_d) 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.