Effect of collimator and depth in a phantom on the neutron spectrum from a 238Pu,Be neutron source.

A study has been made of some factors influencing the fast neutron spectrum emerging from the aperture of a collimated 238Pu,Be source, and influencing the spectrum at a depth in a water phantom, using a miniature liquid scintillator fast neutron spectrometer. The greatest changes in the spectrum emerging from the aperture were produced by changes in the length of the collimator and the field size of the collimator. No difference between the source spectrum above 1.5 MeV of 370 GBq (10 Ci) and 740 GBq (20 Ci) 238Pu,Be sources and a 37 GBq (1 Ci) 241Am,Be source was observed. Graphite was found to be a slightly more efficient reflector than steel or lead, and much better than wax. The mean energy and the proportion of the total neutron fluence with an energy between 1.5 MeV and 3.5 MeV reached a minimum and a maximum value respectively with increase in depth in the phantom. With further increase in depth, the two parameters increased and decreased respectively. The depth at which these minimum and maximum values occurred increased with field size. The overall changes through 15 cm depth in the mean energy and in the low energy fluence proportion were small (less than or equal to +/- 5%). No significant changes were detected in the spectral shape after the neutron beam traversed an air cavity or a bovine tibia in the phantom.