Energy Relations of Positron-Electron Pairs Emitted from Surfaces

The impact of a primary positron onto a surface may lead to the emission of a correlated positron-electron pair. By means of a lab-based positron beam we studied this pair emission from various surfaces. We analyzed the energy spectra in a symmetric emission geometry. We found that the available energy is shared in an unequal manner among the partners. On average the positron carries a larger fraction of the available energy. The unequal energy sharing is a consequence of positron and electron being distinguishable particles. We provide a model which explains the experimental findings.

Figure 1

The two transfer lenses of the spectrometer (with $\pm 15°$ acceptance) are symmetrically aligned with respect to the surface normal. The positron beam is aligned along the surface normal.

Figure 2

2D-energy distribution of positron-electron pairs from Ag(100) surface. The primary energy was 42 eV. The insets indicate which of the particles is detected by the respective spectrometer. The $x$ axis in both plots refers to the energy measured by the “right” spectrometer.

Figure 3

Energy sharing curves obtained by fixing the sum energy to $30\pm 2\mathrm{eV}$. The Ag, Co, and NiO samples were excited by 42 eV positrons. The insets indicate which of the particles is detected by the respective spectrometer. Additionally, we present the curve from the corresponding ($e$, $2e$) experiments.

Figure 4

The collision between positron (red) and electron (blue) in the laboratory and center-of mass frame. In the laboratory frame, the electron is initially at rest. The angles $\vartheta$ and $\phi$ characterize the propagation direction after the scattering. For equal mass particles, it is known that $\vartheta +\phi =90°$ and $\mathrm{\Theta }=2\vartheta$.

Figure 5

We present the sharing distribution $E^{+}-E^{-}$ using Eq. (3) together with the data of Ag shown in Fig. 3.