TEMPERATURE EFFECT ON 2-GAMMA ANNIHILATION OF POSITRONS IN BISMUTH

We have performed angular-distribution measurements of the two gammas produced by anninilation of positrons in a Bi single crystal and in a Bi polycrystal, at three temperatures: 300 °K, 77 °K, 4.2 °K. Measurements show that the area of the angular distribution curve decreases as the temperature of the single crystal is lowered. The observed decrease is 21% between 300 °K and 77 °K and 28% between 300 °K and 4.2 °K. No temperature effect has been detected with the Bi polycrystal. Our results, obtained with the Bi single crystal, agree with those of Dekhtyar and Mikhalenkov, however observation of the annihilation spectra performed in our measurements excludes the interpretation of the effect given by these authors. We suggest tentatively a model by which the effect could be explained in terms of collective-excitation mode. © 1969 Società Italiana di Fisica.