Search for Pair Production by Protons

A search was made for pairs produced in the collision of heavy particles by bombarding tantalum with protons of energy up to 1.64 Mev. Only a very small number of positrons was detected, giving an upper limit of 6×${10}^{-13\mathrm{}}$ pair per proton at 1.54 Mev, in contrast to the theoretical value of 38×${10}^{-13\mathrm{}}$ pair per proton obtained by Born approximation.