Pressure dependence of the angular correlation of positron-annihilation radiation in sodium

The pressure dependence of the angular correlation of positron-annihilation radiation in sodium was measured in the range from 4 to 68 kbar. Below 55 kbar the measured value of θF was consistent with the predictions of the nearly-free-electron model. Above 55 kbar, θF deviated from the nearly-free-electron model, indicating that above 55 kbar the electronic structure of sodium metal may no longer be free-electron-like. The percentage of core annihilations varied smoothly from 30% at 4 kbar to 42% at 68 kbar. As pointed out by Kubica and Stott, this increase in the core-annihilation percentage with pressure allows the measurements of the pressure dependence of the positron lifetime in sodium made by MacKenzie, LeBlanc, and McKee to be explained. The pressure dependence of the enhancement of the electrons at the positron was also examined. Reasonable absolute agreement with the theories of Kahana and Carbotte was observed, but the variation in the ratios of the enhancement coefficients with pressure was not in agreement with the theoretical predictions.