The trapping of positrons by dislocations produced in single crystal bending

Abstract

The deformation by bending of high purity copper and zinc single crystals was studied by detection and measurement of the Doppler-broadened gamma ray spectrum due to the annihilation of positrons in the crystals. The measurements were performed with a Ge(Li) spectrometer. The crystals were oriented and bent so as to introduce primarily edge dislocations into the material. The range of dislocation densities studied was from about 1.6 × 10⁹ m^-2 to 1.3 × 10¹² m^-2.

Upon bending the copper crystals showed essentially a linear increase in shape factor (a parameter which describes the positron trapping) above a calculated dislocation density of 5 × 10¹¹ m⁻². The positron trapping rate per unit dislocation density was calculated from the experimental data and certain assumptions in the trapping model and found to bebetween 1.6 × 10¹⁶ and 3.5 × 10¹⁶ s⁻¹

Annealing of the bent crystals is also discussed from the standpoint of the effect it has on the trapping of positrons. Polygonization produced an increase in shape factor which is attributed to trapping by low angle subgrain boundaries.