Cloud-Chamber Energy Measurement of Photo-Neutron Sources

The energy distributions of the neutrons from several photo-neutron sources have been measured by means of the range distribution of recoil protons produced in a hydrogen-filled cloud chamber. It is shown that slowing-down in the source itself produces a broad spread in energy instead of the almost monoenergetic spectrum to be expected from a single $\gamma$-energy. The values of the mean neutron energy, $E$, and the maximum energy, $E_M$, for the sources measured are as follows: As the $E_M$ values should be directly related to the energies of each of the $\gamma$-sources, they are compared to the $\gamma$-energies as reported by different observers for each source.