In situ positron annihilation analysis produced by high energy photon-induced neutron deficient positron emitting nuclei

Thick samples of materials have been investigated by the in situ production of positrons arising from β+ decay of neutron deficient positron emitting nuclei photo-induced by high energy γ rays. The Idaho State University–Idaho Accelerator Center (ISU–IAC) possesses an electron LINAC that produces 10–20 MeV bremsstrahlung photons when a tungsten target is bombarded with the electron beam. These γ photons induce positron active radio-nuclei in the sample by means of photonuclear reactions. The radio-nuclei subsequently β+ decay via a positron within the sample which can be up to several inches thick. Materials that undergo such positron emitting excitation include most of the metals and semi-conducting elements. Once the positrons are produced within the material they diffuse and thermalize in the normal manner seeking out defects and vacancies deep within the material and further produce the characteristic Doppler broadened annihilation spectra dependent on defect type and environment. As examples of this technique, data are presented for a set of fatigued stainless steels and an aluminum alloy under heat treated aging. Possible applications as a field deployable diagnostic technique will be discussed.