Neutrons of thermal energies

Abstract

If fast neutrons, such as those emitted by beryllium under alpha-particle bombardment, are allowed to pass through a considerable bulk of material rich in hydrogen, they rapidly lose energy by collisions with protons, so that the neutrons present in the medium have energies which are, on the average, very much less than those with which they left the source. Fermi and his collaborators have shown that these slow neutrons are strongly absorbed by the nuclei of a large number of elements; frequently this absorption results in the formation of a radioactive nucleus. They suggested that when the hydrogen-containing medium through which the neutrons pass is free from such strongly absorbing elements, the elastic scattering of the neutrons may preponderate so much over absorption that a considerable fraction of the original neutrons may become reduced to thermal velocities before becoming absorbed. In the hope of verifying this suggestion, they irradiated specimens of rhodium and of silver within a large volume of a mixture of liquid hydrocarbons at room temperature, and again at 200°C in order to and whether the intensity of artificial radioactivity showed and dependence upon the "temperature" of the neutrons, possible effects of thermal expansion were eliminated by the use of different mixtures at the two temperatures, the density and atomic composition being kept constant, Although the measurements had an accuracy of ± 2%, no such temperature-dependence was found. We, too, bad made similar experiments, also with negative results. Our measurements were inferior in accuracy, mainly because we attempted to correct rather than to eliminate the effects of expansion; but our conditions of irradiation were perhaps more favourable than those employed in Rome: the distance of the specimen from the source was greater (5 cm instead of 2 cm), and we worked up to 250°C.