Evidence is presented here which indicates that large fragments (much larger than alpha-particles) are emitted among the competitive products of transmutation throughout the entire range of atomic numbers of the elements. Threshold considerations for the observed nuclear reactions show that the reactions are observed with small cross sections well below the threshold for spallation reactions in which the maximum number of alpha-particles are considered as being emitted from the excited nucleus. The calculated thresholds include the mass difference between the reactants and the products and the excitation energy which the product particles or fragments must have in order to pass over the coulombic barrier. Preliminary experiments on the ranges of recoil fragments from copper irradiated with 340-Mev protons give additional evidence for the emission of heavy fragments. It is suggested that the term "fission" is proper for such reactions, throughout the entire range of atomic numbers, in which the nucleus is split essentially into pieces of comparable weight.

• Received 19 February 1951

DOI:https://doi.org/10.1103/PhysRev.82.607