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.

• Received 8 July 1957

DOI:https://doi.org/10.1103/PhysRev.108.167