Doppler broadening spectroscopy using the FAST-ComTec two-dimensional coincidence system: a case study

Positron annihilation Doppler broadening spectroscopy using the two-detector coincidence method turned out to be an important tool for the study and identification of defects and their chemical surrounding due to a strong reduction of the background. The lowest background ;10y5. is obtained by using two Ge-detectors measuring the energy of both annihilation g-rays. We used the FAST-ComTec multiparameter system which may be considered as a standard for that purpose. A systematic study of measurement parameters influencing the results in such a system was done. The spectra were recorded in list mode i.e., event by event.allowing off-line stabilization during a replay. The choice of optimum parameters for the stabilization is discussed. We found, however, stabilization in most cases not necessary for the observation of high momentum spectra. The use of 22Na foil sources in this study implies a relatively high background due to 1.28 MeV g-rays from the source. However, no significant influence of the source activity on the spectra was found. It is thus possible to use sources of high activity up to 70 mCi.to obtain a sufficient count rate for coincidence measurements with high statistics. q1999 Elsevier Science B.V. All rights reserved.