Neutron detection with threshold reactions

Detection of fast neutrons by threshold activation reactions (or threshold detectors) is based on the existence of an energy threshold for certain reactions of neutrons with nuclei. Thus, if one activates a foil made of such nuclei, the activity of the foil will give a measure of the neutron flux above the threshold. 

The angular distribution of the neutrons obtained by bombardment with 23 Mev protons has been studied by Cohen 2 using threshold detectors. The activities detected were Ag caused by the Agi< (n, 2n) reaction (threshold 9.6 Mev) and Mg from the [n, p) reaction in AP (threshold 2.0 Mev). If we assume that the statistical theory is valid, we should expect that the neutron angular distribution to be isotropic, or at least symmetrical about the 90 position. These measurements, show, on the contrary, that the angular distribution, like that found by Rosen and Stewart for Bi (Sect. 27) is far from isotropic, the forward direction being favoured. Thus direct interactions must contribute to the cross... 

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