Optimization of Grooved Thermal Moderator for Pulsed Neutron Source, and Its Characteristics

抄録

A grooved moderator having grooves on the neutron emanating surface of the moderator has been proposed for the purpose of increasing the neutron intensity of the pulsed neutron source. Experimental studies on polyethylene grooved moderators are performed, and the effects of the groove, the optimum dimensions and the influence of the reflector on the optimum condition are examined with a view to enhancing the emanating thermal neutron intensity. The pulse shapes of emanating neutrons are also measured and the figures of merit of the grooved moderator are discussed.
The experimental results reveal that the optimum condition is determined by the balance of increase of intensity from the grooves and the decrease from the fins. The optimum dimensions for the moderator with a graphite reflector are as follows. The fin width is less than 1 cm, the fin length 810 cm and the fin area about 60% of the emanating surface. These values does not depend strongly on whether the moderator is covered by the reflector or not. This size of grooved moderator increases the emanating neutron intensity by about twice at the neutron energy of 25 meV, as compared with the rectangular shaped moderator of 5 cm thick, although it broadens the pulse width considerably. Analysis of the figures of merit suggests that although the grooved moderator is not useful for the usual neutron scattering experiments, it may be of value in experiments having a resolution which is not very sensitive to the pulse width.