Positron Annihilation in Solid Argon

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Abstract
The angular distribution of the radiation from two-quantum annihilation in polycrystalline argon has been measured. The same distribution was computed theoretically in good quantitative agreement with experiment. The calculation is based on a modification of the method of Berko and Plaskett and ignores electron-positron correlation. Tightly bound electrons are assumed and a model for the positron wave function, based on a Wigner-Seitz calculation, is proposed. The main contribution to the momentum distribution arises from the overlap of the electron wave functions with the positron wave function inside the Wigner-Seitz sphere centered at the origin, but there is also a significant contribution from the overlap of the electron functions with the positron function in neighboring spheres. From the agreement obtained, we may conclude that the angular distribution due to annihilations with core electrons is understandable on the basis of this rather simple picture, and that such annihilations are responsible for the large-angle features typical of the angular distributions for solids in general. However, a calculation of the mean life based on the above model yields a result which overestimates the experimental value by a factor of about 6.

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