Positron Annihilation in Alkali Halides

Abstract

The time annihilation spectrum of positrons in most alkali halides is measured. Positrons appear to decay through at least two different processes which lead to a complex time annihilation spectrum. The first component is conceivably due to positrons which annihilate when free; the second, which occurs with an intensity $I_2$ between 25 and 85%, is characterized by a mean life ${\tau }_2$ whose amount ranges between 4×${10}^{-10\mathrm{}}$ and 8×${10}^{-10\mathrm{}}$ sec. From the whole set of the collected data, empirical regularities can be extracted; the annihilation rate ${\lambda }_2$ appears simply correlated with the molecular density $n$ within each halides series, according to the relation: ${\lambda }_2={\lambda }_0+\alpha n$. ${\lambda }_0$ decreases, passing from fluorides to bromides, and $\alpha$ turns out to be simply proportional to the square of the radius of negative ion. The ${\tau }_2$ component is sensitive to the application of a static magnetic field; the magnetic quenching, however, is appreciably lower than what would be expected if one attributes this component to the decay of a free positronium atom.