Analyzing the high-momentum part of positron annihilation Doppler spectra with a single germanium detector

Abstract

We present a novel way for recording and mathematically analyzing the Doppler-broadening of the 511 keV annihilation line to obtain the momentum distribution of the annihilating electron–positron pair. A necessary precondition is employing either background free positron beams or e.g. a 68Ge-source, instead of the widely used positron emitter 22Na. The 68Ge-source emits positrons with an end-point energy of about 1.9 MeV, while the contribution of gamma quanta having higher energies than the annihilation radiation at 511 keV is negligible low. Compared to 22Na, this results in a significant lower background in the photo peak. Employing such a 68Ge-source allows an analysis of the high-momentum part of the annihilation radiation. However, this part contains valuable chemical information about the annihilation site of the positron, originating from high momentum electrons. Providing an adequate mathematical analysis in determining the remaining background and its subsequent subtraction, we are able to obtain the contribution of these high electron momenta to the annihilation peak using only a single Ge-detector. After background subtraction, the same information contained in the momentum range up to 35×10 −3 m 0 c can be extracted by analyzing spectra from metals and semiconductors as if measured employing a coincidence setup with two Ge-detectors. With the method presented here it is possible to obtain the whole information of a positron annihilation Doppler spectrum employing just a single Ge-detector.