Experimental Studies of Premoderator Effect on Neutronic Characteristics of a Decoupled Hydrogen Moderator at a Pulsed Neutron Source

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We experimentally studied the premoderator effect on the neutronic performance of decoupled hydrogen moderators for a pulsed neutron source. Two reflector systems were examined: graphite as a moderating-type reflector and lead as a non-moderating-type reflector. The premoderator materials studied were H2O and D2O. Decoupler materials were Cd and B4C. We measured the neutron energy spectra and emission time distributions.In the case of the graphite reflector system, neutron intensity increased by at most 7% for a Cd decoupled moderator. On the other hand, in the case of the lead reflector system, neutron intensity increased by about 73% for a Cd decoupler by using a D2O premoderator. It was confirmed that the D2O premoderator is more effective than the H2O premoderator. Concerning the pulse characteristics, the premoderator strongly increased the pulse peak intensity, which is a very preferable characteristic for the pulsed neutron sources since it does not cause any deterioration of the energy resolution in time-of-flight measurement.We compared these results with simulation calculations. Experimental and calculated results showed almost the same trends with respect to premoderator thickness but relative intensity gain with premoderator was overestimated by 10 to 20%.
社団法人日本原子力学会の論文
2004-12-25