Ortho-positronium in liquid alkanes

Positron lifetime spectra were measured for a series of odd-numbered liquid n-alkanes (C(7)H(16), C(9)H(20), C(13)H(28) and C(19)H(40)) in a broad range of temperature. According to the "bubble model" the ortho-positronium (o-Ps) lifetime is determined by surface tension. Bubble radius was calculated using the Tao-Eldrup model and compared to the radius expected from the surface tension of liquid. The radii are not identical, their difference rises with temperature. If one assumes the difference of radii as constant, the o-Ps lifetime calculated from the Tao-Eldrup model rises faster than the experimental one. The distinct rise of the difference of radii has no physical explanation in the accepted o-Ps annihilation model. This effect can be the result of radiation chemistry processes not taken into account so far in the bubble model.