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Research supported in part by the Once of Naval Research. )The work presented here forms a portion of the thesis of John J. Quinn, submitted to the faculty of the University of Maryland in partial fulfillment of the requirements for the Ph. D. degree in Physics. The results have been reported at the Washington Meeting of the American Physical Society t J. J. Quinn and R.

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LETTERS TO THE EDITORcontribution to the current from the holes when either the field or frequency is varied. This is permissible since other measurements suggest that the hole mass is much larger than the electron mass. 2 A measurement of the absolute power reflection coefficient at a frequency >COA gives R=0.25. In this region the charge carriers make a negligible contribution to the complex conductivity, and the reflection coefficient is determined simply by the dielectric constant of the lattice, e. From the standard formula for this (R= |e*-l[ 2 /|eH-l| 2 ) we find e = 9. From these data and the above value for e, we find the number of electrons in pure bismuth to be approximately 5X10 16 /cc.

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The long orthopositronium lifetime observed by Paul and Graham and Wackerle and Stump in liquid helium is evidence of a pickoff annihilation rate smaller by more than one order of magnitude than the rate to be expected on the basis of the average electron density. This discrepancy is removed by taking into account the repulsive positronium-helium exchange force already derived in previous work. By repelling the helium atoms and creating a cavity, or bubble, in the liquid, a positronium atom is able to avoid contact with the liquid and thereby greatly increase its lifetime. The small pickoff rate observed is attributed to the saturated vapor in the bubble. The repulsive exchange force further decreases this rate, and good agreement is obtained with experiment. It is predicted that increasing the temperature one degree Kelvin, from the boiling point to the critical point, will decrease the lifetime by a factor of three. Positron lifetime measurements in helium gas at liquid-nitrogen temperature and 0-100 atmospheres pressure would also provide a check on the theory.