Edgar J. Murphy

Relative efficiencies of radioactive neutron sources

Abstract A number of radium-beryllium and radon-beryllium neutron sources are compared. It is found that the radon sources may vary inefficiency by more than a factor of two, while the radium sources seldom vary by more than 15 per cent. These efficiencies are based on the customary assumed ratio of gamma ray activity to neutron-production. Sources manufactured under similar conditions are found to compare more favorably; i.e. with less variation among themselves. The reasons for the variations are investigated, and also the factors influencing the efficiency. It is found that, for the radium sources, the mixing is especially important; the size of the radium grains is important; the purity of the beryllium is important. The geometrical shape of the source and the absolute amount of beryllium are not found to be especially important.For the radon sources, it is in addition especially important that the container be as full as possible, with as little space as possible occupied by gas alone. The contribution of photoneutrons to the total emission is investigated, and found to be of the order of two tenths.