Nuclear Disintegration by Positron-KElectron Annihilation

A new process for the annihilation of fast positrons is discussed, in which a positron with insufficient energy to excite or disintegrate a nucleus by collision, annihilates a $K$ electron of an atom with subsequent excitation or disintegration of its nucleus. If the positron energy is close to threshold for the process, competition from two-quanta annihilation does not occur. The process is of first order and, apart from the occurrence of negative energy states, is the reverse of internal conversion. The cross section can be factored into a cross section for annihilation with emission of a photon converging on the nucleus times a probability for nuclear disintegration. In the electric dipole case, the latter is just the ratio of the photodisintegration cross section to the $P$-wave blackbody absorption cross section of the nucleus. The photo-disintegration cross section is taken from experiment.