Scattering of slow neutrons

The adsorption and scattering of slow neutrons have been studied by various methods. In their first survey, Amaldi, D'Agostino, Fermi, Pontecorvo, Rasetti and Segré (1935) investigated the absorption of slow neutrons by different elements inside a paraffin block. The number of slow neutrons capture in an indicator (e. g. silver). The values for the absorption coefficients which they obtained with this arrangement can be regarded as a measure of the ''true'' absorption of slow neutrons. Later, Dunning, Pegram, Fink and Mitchell (1935) measured the "total" cross-sections, i. e. the sum of the well-defined beam of slow neutrons, and a lithium-coated ionization chamber as indicator. Recently, Griffiths and Szilard (1937) have determined the cross-section of some strongly absorbing elements using the captured γ-rays from cadmium as indicator. The scattering of slow neutrons was studied for some elements by MItchell and Murphy (1935), Mitchell, Murphy and Whitaker (1936), Budnitzky and Kurtschatow (1935) Pontecorvo and Wick (1936), and others. In these experiments, the slow neutrons issuing from a paraffin block passed through an indicator (e. g. silver), and were scattered backwards from the substance under investigation. When thin scattered are used, the percentage increase of the radioactivity produced in the indicator gives a measure of the scattering cross-section. This method has the disadvantage, even in the case of good scatters, that the increase in the radioactivity of the indicator is usually small compared with the effect due to the primary neutrons. A quantitative interpretation of the results may be further complicated by the fact that the neutrons leave the surface of the paraffin block at angles from 0 to 90°. Also, multiple scattering is not avoided.