Neutron Transmission Properties of Cd and B Filters in Moderating Media and Its Application in the Determination of 1/ E -Spectrum

The energy dependent neutron transmission function of the Cd-filter covering a cylindrical 1/ν-detector was calculated with consideration given to the perturbation effect in the surrounding medium. The results were found to agree well with experiments using lead slowing-down time spectrometer.The effective Cd cut-off energy for 1mm thick Cd was calculated from the above transmission function, and found to be about 50% larger than the reported values.The validity of the assumption of 1/E1+β spectrum in a graphite moderator is discussed, and the β-value determined with an accuracy of 0.01 from the reaction rate ratio of BF3-counters covered with the different thicknesses of Cd- and B-filters. The region of the 1/E-spectrum neutron field in the graphite standard pile was determined by using this technique.