Surface and Near-Surface Positron Annihilation Spectroscopy at Very Low Positron Energy

Abstract

We present a monoenergetic positron beam specifically tailored to the needs of (near-) surface positron annihilation spectroscopy. The Setup for LOw-energy Positron Experiments (SLOPE) comprises a high-activity 22Na source, a tungsten moderator, electrostatic extraction and acceleration, magnetic beam guidance, as well as an analysis chamber with a movable sample holder and a γ-ray detection system. The tungsten moderator foil, biased between 0 and 30 V, in combination with the HV-biasable sample holder, enables positron implantation energies between 3 eV and 40 keV. At low energies (<20 eV), the count rate typically amounts to 4400 counts per second, and the beam diameter is smaller than 12 +/- 3 mm. We conduct phase space simulations of the positron beam using COMSOL Multiphysics (c) to characterize the beam properties and compare the findings with the experimentally determined energy-dependent beam diameter. To showcase the capabilities of SLOPE, we perform studies of positronium (Ps) formation on boehmite and depth-resolved coincidence Doppler-broadening spectroscopy (CDBS) of copper. In particular, the Ps formation at the hydrogen-terminated surface of boehmite is found to be maximum at a positron implantation energy of 10 eV. The range of positron energies for which we observe Ps formation agrees with the hydrogen ionization energy.