Nickel bombarded with 5 MV deuterons exhibits an intense positron activity of half-life 3.4±0.1 hours. Chemical analysis shows this activity to be isotopic with copper. The energy excitation curve for this reaction (proton capture) between 3 and 5 MV has been obtained, and is found to be very different from that found for copper (neutron capture). The variation with energy of the disintegration cross section agrees satisfactorily with theoretical formulae for the penetration of a deuteron into a nucleus of radius 4.5×${10}^{-13\mathrm{}}$ cm. This result does not support the larger nuclear radii recently suggested by Bethe. The upper limit of the positron spectrum is approximately at $H\rho 4400$ gauss-cm.

• Received 29 March 1937

DOI:https://doi.org/10.1103/PhysRev.51.893