  Here we describe an advanced multi functional, variable-energy positron beam
system capable of measuring the energies of multiple positron-induced electrons
in coincidence with the Doppler-shifted gamma photon resulting from the
annihilation of the correlated positron. The measurements were carried out
using the unique characteristics of the digital time-of-flight spectrometer and
the gamma spectrometer available with the advanced positron beam system. These
measurements have resulted in (i) the first digital time of flight spectrum of
positron annihilation-induced Auger electrons generated using coincident
signals from a high-purity Ge detector and a micro-channel plate; (ii) a
two-dimensional array of the energy of Doppler-broadened annihilation gamma and
the time of flight of positron-annihilation induced Auger electrons or
secondary electrons measured in coincidence with the annihilation gamma photon;
and (iii) the time of flight spectra of multiple secondary electrons ejected
from a bilayer graphene surface as a result of the impact and(or) annihilation
of positrons. The novelty of the gamma electron coincidence spectroscopy has
been demonstrated by extracting the Doppler-broadened spectrum of gamma photons
emitted due to the annihilation of positrons exclusively with 1s electrons of
carbon. The width of the extracted Doppler-broadened gamma spectrum has been
found to be consistent with the expected broadening of the annihilation gamma
spectrum due to the momentum of the 1s electrons in carbon.
