The Absorption of Neutrons Detected by Boron and Lithium

The neutron-nucleus collision cross sections of various elements have been investigated for neutrons that are detectable by means of (1) the boron and (2) lithium neutron capture reaction and that are strongly absorbed by cadmium. The collision cross section for such slow neutrons detected by boron is for hydrogen about 75 percent of that for neutrons detected by lithium, for HgO about 72 percent, for Sm about 180 percent, and for Al, B, C, Fe, and Li is the same within 10 percent for either boron or lithium detection. With either detector the absorption of cadmium and boron each approximates a single exponential, the absorption coefficient in each case not decreasing more than 25 percent for intensity reduction of 90 percent. The absorption coefficient of lithium for neutrons detected by boron decreases 40 percent, and for neutrons detected by lithium decreases about 30 percent at thicknesses that reduce the intensity 85 percent. Measurements were made with slow neutrons passed through an 8-mm paraffin layer. With lithium detection less than 10 percent increase in the absorption of Ag, HgO, boron glass (Pyrex), and Rh was observed when the temperature of the paraffin layer was changed from 293 to 90°K. The velocity distribution of the neutrons strongly absorbed by cadmium was approximately the Maxwellian for thermal equilibrium at room temperature; but when the paraffin was at the temperature of liquid air the neutron velocity distribution was too broad and flat to be described as Maxwellian for any particular temperature, although the distribution was shifted toward that for the lower temperature. The lithium reaction was found to be a sensitive detector of neutrons whose velocity is much higher than the mean thermal velocity at room temperature and well above the strong absorption range of cadmium. The collision cross section for neutrons, from paraffin at room temperature, that were filtered through 1 gram per ${\mathrm{cm}}^2$ of cadmium was found to be larger in Rh, HgO, Au, and B than in cadmium. After filtering through 11.5 g per ${\mathrm{cm}}^2$ of cadmium, the cadmium absorption was reduced to about a normal value for very high speed neutrons, collision cross section about 5×${10}^{-24\mathrm{}}$ ${\mathrm{cm}}^2$.