Figure 1

Schematic diagram of the $\gamma$-Auger coincidence measurement system. A low energy positron beam is used to place positrons at the sample surface. The $\gamma$ energy was measured using a HPGe detector connected to a multichannel analyzer (MCA). The Auger electrons were energy selected using an $\mathbf{E}\times \mathbf{B}$ filter and detected using a microchannel plate. The Doppler-broadened $\gamma$ spectrum for a selected core level was acquired by opening a gate to the MCA only after the simultaneous detection of a $\gamma$ ray and an electron from an Auger transition resulting from the annihilation of the selected level. Conventional Doppler-broadened spectra were acquired by holding the MCA gate open.Reuse & Permissions

Figure 2

Comparison of the “core $+$ valence” and “core” annihilation $\gamma$-ray energy spectra resulting from the bombardment of polycrystalline Cu (a) and Ag (b) foils with a 12 eV positron beam. The core $+$ valence spectra (open symbols) were acquired without a coincidence requirement. The core spectra (solid symbols) were acquired in time coincidence with the detection of an electron in the range of the peaks of the energy distribution of Auger electrons emitted as a result of the annihilation of a positron with the Cu $3p$ ($M_{23}$) and Ag $4p$ ($N_{23}$) electrons for (a) and (b), respectively.Reuse & Permissions

Figure 3

Momentum distribution of positron-electron pairs for the Cu $3p$ and the Ag $4p$ core states. The experimental distributions (solid squares) were extracted from Doppler-broadened $\gamma$ spectra (shown in Fig. 2b for Cu) by using Eq. (2) to convert the annihilation $\gamma$ energy to electron-positron pair momentum (given in dimensionless units, $q{a}_0$, where $q$ is the wave vector and $a_0$ is the Bohr radius). The measured distributions (solid squares) are compared to theoretical calculations (see text) shown as solid lines.Reuse & Permissions