Figure 2
From: Vacancy-Hydrogen Interaction in Niobium during Low-Temperature Baking

Positron annihilation photo-peak in a metallic material. The central area A1 is used to quantify the S-parameter, while the areas A2 and A3 are used for the W-parameter. Changes in the density, type and chemical environment of positron traps change the momentum distribution of electrons annihilating positrons and hence lead to different values of the S and W parameters.