Abstract
The results of the experiment were previously reported in which the moisture-content of a thin slab could be detected by using the geometrical arrangement with a neutron source (Ra+Be) placed in a moderator.
In this paper it is described that some investigations have been made to pursue the effects of energy-dependent neutrons on the moisture-detection in the same arrangement as previous.
In the first place, the present experiment is performed with the arrangement that the surface of the moderator is covered with the cadmic plate, and its result shows that the epicadmium neutrons contribute little to moisture-detection. Therefore, an object of this analysis is limited to those neutrons with less energies than them, which are assumptively divided into two groups of thermal neutrons (0.025-0.07 eV) and slow neutrons (0.07-0.5 eV) . Then, two-group diffusion equations apply to calculate the rate of the thermalization of slow neutrons in the slab, and on the assumption that thermal neutrons reciprocate repeatedly through the air gap where detectors are placed between the moderator and the slab, the total number of neutron corresponding to the counting rate is calculated with the aid of the rate of the thermalization obtained above, the albedo of thermal neutrons and the decrement-rate of neutron current density in the air gap.
The results of these calculations are in good agreement with the experimental results at various moisture-contents, and it is quantitatively shown that the thermalization of slow neutrons and the thermal neutrons repeatedly reflected in both the moderator and the slab contribute remarkably to moisture-detection. This approximate analysis will lead to the means by which the position of the source and the quantity of the moderator to make the efficiency of detection the highest can be decided.
