Positron annihilation in solid and liquid metals

The angular correlation of the gamma rays resulting from the annihilation of positrons in 15 solid and liquid metals and semiconductors has been studied. Experiments have been done on each material at room temperature and at temperatures above and below the melting point.

The elements investigated fall into three categories according to the way the angular correlation distribution changes as the melting point is reached. To within the experimental angular resolution (0.5 milliradians) no change in the angular correlation distribution is observed for Li, Se, Na, and Tl upon heating from room temperature to beyond the melting point. The elements Sb, Bi, Ga, Hg, Sn and Te exhibit changes in their angular correlation distributions only upon being melted, whereas for the metals Al, Cd, In, Pb and Zn changes occur when the specimen is heated from room temperature to temperatures below the melting point.

Changes in the angular correlation distribution upon heating or melting are generally manifested as (a) a narrowing of the central part of the curve, (b) a rounding-off of the parts of the curve near the Fermi cut-off angle and (c) a change in the area of the broad background curve as compared with the area under the central peak.