It is well known that the absorption of neutrons in their passage through matter is due to nuclear collisions, and not appreciably to interaction with extranuclear electrons. A collision of a neutron with a nucleus may result in the scattering of the neutron, or in the disintegration of the nucleus. The experiments of Feather and of Harkins, Gans, and Newson§ have shown that several light elements, C, N, O, F, Ne are disintegrated, the mechanism probably being absorption of the neutron and emission of an alpha particle. Fermi|| has reported that a variety of elements when bombarded by neutrons show the phenomenon of induced radioactivity, emitting beta rays. He suggests that the disintegration process takes place usually by absorption of a neutron and emission of an alpha particle or proton, the resulting nucleus being an unstable radio element, transforming into a stable body by emission of a beta particle. The experiments here to be described show that when neutrons pass through various substances, gamma rays are produced. The origin of this radiation has not definitely been established; nuclear excitation appears to be the most plausible explanation in most cases. 2—Experimental Method The general method consisted in measuring the ionization current produced by a Po + Be source (usually of about 10-15 millicuries) placed above a high pressure ionization chamber, and observing the increased ionization when a block of scattering material was placed immediately above the source. A correction was applied for the diminution of the natural effect caused by the scatterer. The increase in ionization amounted usually to 2-3%, and thus to obtain even a rough measurement of the effect, accurate measurements of the ionization currents were required. For this reason the high pressure ionization chamber was usually used in preference to the counter, since measurements to one part in a thousand are impracticable with the latter. The ionization method has, however, the disadvantage that both gamma rays and neutrons are detected. To distinguish between the two radiations, two similar ionization chambers were used, one containing argon at a pressure of 90 atmospheres, the other hydrogen at about 60 atmospheres. The former is more sensitive to gamma radiation, the latter to neutrons. The ionization chambers were of steel and had cylindrical walls 1 cm thick; the radiations entered through the roofs of the chambers, which were 2·5 cm thick. The inside dimensions were 16 cm high and 12 cm diameter, with a 2-cm diameter central electrode. Collecting potentials of 250-500 volts were used. Measurements were made by a balance method and followed standard practice. From the measurements of ionization currents in argon and hydrogen estimates may be made of the neutron (
n
) and gamma ray (γ) intensities separately. The method by which this is achieved is described in § 11.
Secondary gamma rays excited by the passage of neutrons through matter