Spur reaction between ortho-positronium and radicals in water

Hirade, Tetsuya  ; Lee, J.*; Nakamura, Takemi  

The positronium (Ps), a bond state of an electron and a positron, is formed at the terminal spur of injected positron and annihilates with the lifetimes of 100 pico-seconds to several nano-seconds. The triplet Ps, ortho-Ps, has the longest lifetime that depends on the sub-nano meter hole size where the Ps is captured. The lifetime is longer in a larger hole. In liquids, Ps creates a bubble by itself and the size of the bubble is larger at higher temperature. Therefore the lifetime of ortho-Ps is usually longer at higher temperatures. However, the ortho-Ps lifetime in water showed opposite temperature dependence and the reason has not been clarified. If there are reactions of ortho-Ps and reactants formed by the injected positron, it is possible to explain the temperature dependence of ortho-Ps lifetime in water. We have successfully obtained spin exchange reaction between ortho-Ps and radicals in water by use of the positron annihilation age-momentum correlation measurement (AMOC).