A MOBILE POSITRON-LIFETIME SPECTROMETER FOR FIELD APPLICATIONS BASED ON BETA(-GAMMA COINCIDENCE())

A mobile positron-lifetime spectrometer based on a beta(+)-gamma coinc idence measurement, which has been applied to in-situ examinations of microstructural evolution processes during the fatigue of copper singl e crystals, is described. Since no sandwich-type geometry is required, it is applicable to all specimen geometries commonly used in material s testing and to the non-destructive testing of engineering parts in s ervice. As a radioactive source Se-72 generates the positron emitting As-72, which provides two positron spectra with maximum energies of 2. 5 MeV and 3.3 MeV and a prompt gamma quantum of 835 keV. The positrons emitted in the direction towards the specimen pass through a fast pla stic scintillator and produce a scintillation signal, thereby losing a bout 150 keV of their energy. This signal serves as a start signal for the positron-lifetime measurement and is measured in coincidence with the subsequent 511 keV annihilation quantum. After passage through th e plastic scintillator the remaining positron energy is still high eno ugh to penetrate deep into the material and to allow for real bull; ex aminations. The prompt gamma quantum may serve as an on-line control o f the stability of the electronic system which will be useful under no n-constant service conditions in a proposed field application.