Research Of Neutron Survey Meter With Two-dimension Moderator

A measurement system which consists of a spherical moderator and two types of thermal-neutron-sensitive proportional counters has been designed. The spherical 3He proportional counter called as an "inner count" is imbedded at the center of the spherical moderator. Six cylindrial BF3 proportional counters are located close to the moderator surface, and their total counts are called as an "outer count". The dual structure detector of the "inner and outer counter" in the device is composed. In order to detect the high energy neutron, a lead layer is embedded between inner and outer detector. Based on the Pb(n, xn) nuclear reactions of high energy neutrons, the detector sensitivity is improved in the high energy range and the upper neutron limit is up to GeV-level.Position and thickness of lead layer, counters position, and moderator diameter was calculated and compared by MCNP3B and FLUKA codes. The ideal design was determined. Considering the influence factors, the influence of response function such as the air pressure of counters, the density of moderator and the angle response has been analyzed. The correction factor and few-channel unfolding methods were adopted for dose evaluation.In order to verify the reasonability of the detector design and the feasibility of the two dose assessment models, the detector was tested in the 241Am-Be source reference radiation field, and the results are compared. The ambient dose equivalent of the correction factor method is 1% lower, and the few-channel unfolding method's is 17% higher. Taking all kinds of uncertainty into account, the total measuring uncertainty of the correction factor method is wihin 37%, and the few-channel unfolding method's is wihin 30%.The experimental results in the paper show that the design of detector can improve the neutron dose response of the instrument, and wide the evaluated neutron energy range.