The cytogenetic effect of slow neutrons.

HE study of the biological effect of different radiations has progressed T as sources of the various radiations have been developed. The effects of X, gamma, beta and alpha rays, of protons and fast neutrons, and recently of fission fragments have been studied in numerous organisms, both plant and animal. Before the development of the nuclear reactor, however, there was no source of slow neutrons in the amounts necessary for a study of biological effect. The fast neutrons from uranium fission can be moderated by elastic collision with graphite or heavy water in a nuclear reactor until their velocity is reduced to that of thermal energies (average 2200 meters/sec= 0.025 electron volts); such very slow neutrons are called thermal neutrons. Unlike fast neutrons, whose principal reaction with matter is one of scattering, thermal neutrons because of their very low energy and velocity are more likely to be captured than scattered by the atoms they encounter. Most of the reactions of biological elements with thermal neutrons are capture reactions. After an atom captures a neutron, it forms a new compound nucleus with an excess of energy. This new compound nucleus may then: emit a gamma ray immediately to form a stable isotope, e.g.,’ H1 + n + EH2] + H2 + y;