Positron Annihilation in $\gamma$- and $\alpha$-Cerium

The $\gamma \to \alpha$ phase transition of cerium has been investigated by measurement of the positron lifetime and angular correlation of positron-annihilation radiation in the two phases. The lifetime in the $\alpha$ phase was observed to be 8% shorter than the lifetime in the $\gamma$ phase when the $\alpha$ phase was produced by the application of pressure; and it was observed to be 5% longer when the $\alpha$ phase was produced by lowering the temperature. The angular correlation from $\alpha$-cerium produced by pressure was observed to be slightly broader than that from $\gamma$-cerium, in contrast to the narrowing previously observed when $\alpha$-cerium was produced by lowering the temperature. Since the observed changes are much smaller than predicted by models which explain the $\gamma \to \alpha$ transition by assuming a transfer, or partial transfer, of the $4f$ electron to the conduction band, it is concluded that such electronic promotion does not occur.