Spectrum measurement of fast neutron through air ducts in water, concrete, and lead

Fast neutron spectral through straight air ducts in water, concrete, and lead have been measured from 1 to 12 MeV by using an NE-213 spectrometer. Wall scattered component has been abstracted from the measured data. It was found that Simon and Clifford's equation can not be applied to describe the measured flux perhaps mainly due to the inadequate assumption of angular distribution of reflected neutrons. The ratio of wall-scattered component to line-of-sight component was found to be dependent on the duct length more or less with an inverse square proportionality for ducts in water and concrete and with an inverse cubic or even more strong proportionality for ducts in lead. (author)