Energy dissipation of para-positronium in polymers and silica glass

In this letter we discuss the energy dissipation of short-lived para-positronium (p-Ps) in polymers and silica glass. The S parameter characterizing the Doppler broadening of p-Ps annihilation is determined from the previously reported systematic data of positron annihilation age momentum correlation for various polymers and silica glass. A comparison of the S parameter with that expected for thermalized p-Ps trapped in a free volume reveals that p-Ps is not thermalized and possesses excess energy in fluorinated polymers and silica glass, indicating that it is difficult for Ps to lose energy in substances containing heavy elements such as fluorine and silicon.