Temperature dependence of the longest positron annihilation lifetime in water

Hirade, Tetsuya  ; Lee, J.; Oka, Toshitaka   

The longest positron annihilation lifetimes, i.e. ortho-positronium lifetimes, in almost all liquids show larger lifetime at higher temperatures. However, only water shows opposite temperature dependence. It is caused by larger possibility of reactions between ortho-positronium and radical species formed in the terminal spur, i.e. the end of the positron track. We have successfully obtained the temperature dependence of the reactions by use of the positron annihilation age-momentum correlation measurement that is the best method to observe ortho-positronium reactions.