Record high counting rate of a non-beam-based positron annihilation age–momentum correlation (AMOC) spectrometer achieved by geometrical optimization of detectors

Abstract

Positron annihilation age–momentum correlation (AMOC) spectroscopy is not only a critical approach to investigate positronium formation and annihilation, but also a powerful technique for materials science to characterize the chemical environment of pores. However, due to the long measurement time (several days to collect 10 million counts for one sample, at a low coincidence counting rate of approximately 20–40 cps) of a conventional non-beam-based AMOC spectrometer, AMOC technique has been rarely applied. In this work, based on the geometrical simulations of detectors using Geant4 program, we developed a new semi-digital AMOC spectrometer with two start detectors. After systematic optimizations of geometrical configurations of detectors, timebase of digital oscilloscope, and the energy windows of start and stop detectors, this instrument achieved the highest coincidence counting rate for a non-beam-based AMOC spectrometer (∼ 180 cps for a narrow stop energy window of 469–589 keV and the shaping time of HPGe detector at THPGeshaping = 6μs, and ∼ 500 cps for a wide stop energy window of 104–589 keV and THPGeshaping = 2μs). The drastic rise of coincidence counting rate will broaden the applications of AMOC spectrometers in positronium physics and materials characterization in future.