The Slowing Down of Low Energy Neutrons in Water. II. Determination of Photo-Neutron Energies

A new method for determining neutron energies which should be particularly useful below 500 kev has been developed and applied. The method consists in slowing down the neutrons from the source in water and measuring the distribution of D group neutrons. The arrangement is calibrated with neutron sources of known energy. The energies of the photo-neutrons emitted from $\mathrm{Y}\mathrm{}\left(100d\right)+\mathrm{Be}$ and $\mathrm{Sb}\mathrm{}\left(60d\right)+\mathrm{Be}$ have been determined and found to be 220±20 kev and 100±20 kev respectively. This is in agreement with previous measurements by other methods. This method has the advantage of great simplicity and should be applicable to neutron energies below 100 kev, which cannot be studied by other known methods.