The absorption of neutrons of medium energy

The process of absorption of slow neutrons by many elements has been extensively studied, but similar experiments with fast neutrons (Dunning 1935) are more difficult to interpret since the absorption cross-section is very small and the neutrons from many of the available sources are not homogeneous. In the case of fast neutrons also, scattering and inelastic collisions are usually as probable as absorption. This difficulty does not arise if the absorption of a neutron leads to the formation of a radioactive isotope. In such cases the relative values at any rate, of the absorption cross-section for different elements may be obtained by comparing the activities produced in these elements under standard conditions. This paper contains the results of measurements of this type (Booth and Hurst 1937; von Halban and Kowarski 1938) giving the relative values of the absorption cross-section for several elements for the neutrons produced by the action of radium γ-rays on Be. These neutrons are pro­duced by the two γ-rays of 1.8 and 2.2 × 106 meV and should therefore consist of two groups of neutrons of energy 0.04 and 0.45 × 106 meV. Similar results have recently been published by von Halban and Kowarski (1938) who used the neutrons formed by the action of the γ-rays from Th C" on deuterium.