Experiments with iso-energetic neutrons

It has been recognized for some time that the degree of interaction of neutrons with matter varies rather rapidly with the energy of the neutrons. In the higher energy ranges this effect has been difficult to investigate be cause the more common radioactive sources yield neutrons with a wide distribution of velocity. It is very desirable, then, that experiments should be performed with neutrons of known energy. The reaction D2 1 + D2 1 = He3 2 +n' o provides a very convenient source of such iso-energetic neutrons, and the experiments to be described have mainly been carried out with the neutrons obtained by bombarding deuterium with heavy hydrogen ions in a high-voltage discharge tube. Neutrons obtained in this manner are thought to have an energy of about 2·5 × 106 e-volts (Bonner and Brubaker 1936). One of the most fundamental and interesting cases of interaction is that between neutrons and protons. The simplicity of the phenomenon is such as to allow fairly rigid mathematical analysis. Measurement of the neutron-proton cross-section gives a most valuable check on certain basic assumptions made in the theoretical treatment of this problem. We have effected such a measurement with the D + D neutrons, and the result, viz. a cross-section of 1·8 × 10-24 cm.2 , is in agreement with the theoretical predictions. Experiments have also been carried out to investigate the production of induced radioactivity with 2·5 × 106 e-volt neutrons in those elements which emit either a proton or anα -particle at the moment of capture. Only in two cases were measurable effects observed—with the 2·6 hr. (proton) period of phosphorus, which was already known, and with the 14-day (proton) period obtained from sulphur. This latter reaction has not hitherto been reported to take place with neutrons of such low energy.