A Study of the Slowing Down Distribution ofSb124-Be Photo-Neutrons in Graphite, and of the Use of Indium Foils

The spatial density distribution of indium resonance neutrons (1.44 ev resonance) has been measured around a point ${\mathrm{Sb}}^{124}$-Be photo-neutron source in a graphite slowing down medium. This distribution was found to be in agreement with appropriate theory. The source energy of the photo-neutrons was calculated from the measured neutron distribution and a value of 20±4 kev was obtained in fair agreement with values obtained by other methods. It was found that there is a quite appreciable capture cross section in ${\mathrm{In}}^{115}$ above the 1.44-ev resonance extending in energy to at least several kev. It is shown that this capture of neutrons with energies above 1.44 ev can introduce an error into neutron slowing down measurements which can be important under some circumstances, and it is shown how this error can be corrected.