A study of electron-field pair production by photons near the threshold with a streamer chamber technique

Abstract

Recent papers (2.~) show the fundamental importance of an experimental study of triplet production in the threshold energy region from 2 to 6 MeV. The validity of Mork's (z) corrections to the Borsellino (4) cross-section could be tested; in particular, experimental data on the total cross-section would give an evaluation of the u interaction and the exchange effects involving the two electrons. Now, to our knowledge, no experimental data are available below 6 MeV, as shown in Fig. 1. We have performed a preliminary experiment with the 2.62 MeV y-rays from 2~STh. Pairs and triplets were produced in the gas of a streamer chamber filled with helium; we must indeed work with a low-atomic-number target to increase the ratio of the triplet production rate to the pair production rate. Moreover, at the energy considered here, the momentum of the recoil electron is much larger than each momentum of the helium atomic electrons; therefore, these ones can be considered as free electrons and Mork's (2) theoretical calculation is valid.