The nature of the interaction between gamma-radiation and the atomic nucleus

A number of independent investigations by Chao, Meitner and Hupfeld, and Tarrant, have shown that the absorption of strongly filtered thorium C" γ-rays, both in magnitude and in the manner of its variation with the atomic number of the absorbing element, was in definite disaccord with what was independently known concerning the absorption of γ-rays by extra-nuclear electronic systems. The additional absorption was attributed to interaction with the nuclei of the atoms concerned. In the case of quantum energies as high as 2½ million electron-volts, interaction with a heavy nucleus is quite important. In lead, for example, it accounts for 20 per cent. of the total absorbing power of the atom. The amount of energy absorbed by a nucleus is roughly proportional to the square of its atomic number, and from the investigations of Jacobson and Tarrant, it appears that the absorption increases regularly from element to element. The object of the present investigation was to discover something of the nature of this interaction between the quantum and the nucleus. It was evident that the whole of the absorbed energy was not re-radiated without change of wave-length, since no certain difference was observed between the intensity of the secondary radiations from heavy and light elements within the angular range 10° to 30°, although the conditions were such that if a third of the energy absorbed by the nucleus had been re-radiated with uniform angular distribution and without change of wave-length it could have been detected with certainty.