Interaction of Neutrons with Matter

Abstract

The interaction of fast neutrons with matter has been studied through their transmission out of spheres of various materials. Of the order of 90 percent or more of the collisions with atomic nuclei were found to be approximately elastic, probably around 10 percent of the neutrons which make collisions with nuclei being captured or suffering energy losses which make them undetectable through projected protons. The interaction of "slow" neutrons with matter has been studied largely through the ionizing particles ejected with high efficiency from Li (or B) by slow neutrons, with very sensitive ionization chambers whose inner surfaces are entirely of Li metal, the ionization being detected through amplifier systems using thyratron or photographic oscillograph recorders. The anomalous effects characteristic of slow neutrons, such as high absorption in Cd, are not observable with the lowest energy neutrons detectable through projected protons. The use of a Cd shutter with the Li ion chamber permits separation of fast and slow neutrons effects. Cd is used as shielding against slow neutrons, and serves as effective material for slits to define beams of slow neutrons. The production of slow neutrons through collisions of fast neutrons with H nuclei has been studied by using paraffin, ${\mathrm{H}}_2$O and ${\mathrm{D}}_2$O. The slow neutron production curve with paraffin and ${\mathrm{H}}_2$O shows a maximum with spheres of about 10 cm radius and the decrease in the number of slow neutrons emerging from larger spheres is attributed largely to absorption processes such as combination with H to form deuterium. The presence of large numbers of neutrons with energies already below 100,000 e.v. in Rn-Be neutron sources appears necessary to account for this and other effects. Li also detects neutrons having energies intermediate between fast and slow. ${\mathrm{H}}_2$O is about 5.5 times as effective as ${\mathrm{D}}_2$O in producing slow neutrons, as determined through the relative enhancement of the artificial radioactivity produced in Ag. The question of thermal equilibrium of slow neutrons was investigated, and in the range from about 95° to 373°K, the change in the number of disintegrations of Li was found to be very small and the change in the absorption of Cd was found to be of the order of 5 percent, the results probably indicating that many of the neutrons approach thermal equilibrium but that these effects are not sensitive functions of the neutron energy in this region. The absorption of Cd for slow neutrons has been found to be very nearly exponential when measured with a reasonably parallel beam of slow neutrons. The neutron-nucleus collision cross sections for fast neutrons lie on a fairly smooth curve, indicating increasing nuclear radii with atomic weight in fair accord with other data, although all the lower atomic weights have about the same cross section for fast neutrons. The neutron-nucleus collision cross section for slow neutrons has been investigated over a wide range of elements with an approximately parallel slow neutron beam, and varies from values which may be slightly smaller than the comparison values for fast neutrons in such cases as Al and S, to values several thousand times larger, in the case of Gd, Sm and Cd, the first two having the largest absorption coefficients for slow neutrons yet measured. Studies of the amount of elastic scattering of slow neutrons from Cd show that, compared to Cu, less than 1 percent of the slow neutrons are elastically scattered, indicating that the large neutron-nucleus cross sections for slow neutrons are due to neutron capture and not elastic scattering. The same appears to be true for other elements of high absorbing power.