The use of e+-e- pair production in the imaging of land mines

Thermal neutron activation is based on the well-known ¹⁴N(n,γ)¹⁵N reactions, one of which has a cross-section of 0.010 barns and produces γs of 10.8 MeV. Fast neutrons from a convenient source are thermalised, after which they induce γ emission from elements in the soil, including the nitrogen which makes up 20% or more of the weight of high explosive, but which is present at only the few % level in plants and other materials that themselves make up fairly small proportions of soil. Nitrogen, like most nuclides, has its own characteristic energy spectrum, allowing it and therefore the mine to be detected. The main problem with this technique is background arising from competing capture reactions in other materials in soil. One reason for the high background is that the detectors used so far are not sensitive to the direction of the γs. A detector that could measure both the energy and the direction of the incident γs would be very useful. The purpose of the present work is to investigate the use in this effort of another phenomenon of particle physics: pair production. The combination of e ⁺-e^- conversion with good spatial resolution would enable many background signals to be rejected. Because both the incident fast neutrons and the emerging γs penetrate fairly effectively, it ought to be possible to see some distance into the ground