Abstract
It is shown that a Nal (Tl) crystal will act as a detector of photoneutrons by virtue of the production of radioactive 128I within the crystal. This detecting technique has been applied to the comparison of the photo-neutron yields obtained from copper, cadmium, mercury and lead when these elements are irradiated with a bremsstrahlung x-ray beam of maximum energy of 22 mev. The yields from these four elements are found to be in the ratios Cu : Cd : Hg : Pb as 1 : 4· 1; 9·5 : 8·4.

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