Fission of Medium Weight Elements

The fission reaction has been observed with high energy accelerator projectiles for elements as light as tantalum but has not been reported for medium weight elements. The present note presents evidence for the occurrence of reactions which are probably most properly described by the term 'fission' and which seem to occur with very small yield throughout the region where this type of reaction is only slightly exoergic or even endoergic with respect to mass balance. In the course of detailed investigation of the spallation of copper and the variation of the product yields with energy of the bombarding particle the threshold for formation of radioactive Cl{sup 38} (38-minute half-life) from elemental studied. The energetically most economical way in which Cl{sup 38} might be spallation reactions is by emission from the bombarded copper nucleus of nucleons in groups such as alpha-particles instead of single nucleons 0 The energetic requirements for the reaction Cu{sup 63}(p,pn6a)Cl{sup 38}, in which the maximum number of alpha-particles are emitted, include (1) the mass difference between the reactants and the products and (2) the excitation energy which the alpha-particles must have in order to pass over the coulombic barrier, Since the reaction is endoergic with respect to atomic masses, about 50 Mev must be supplied by the impinging proton to make up the mass difference. If the alpha-particles are considered as coming out consecutively, a value of about 50 Mev can be obtained for the coulombic requirement and thus the threshold for this spallation reaction is roughly 100 Mev