In porous media, the vacuum lifetime of ortho-positronium (o-Ps) of τ = 142 ns can be reduced markedly by pick-off annihilation (interaction with electrons of the host material). So the o-Ps lifetime is determined by the pore size which can be extracted by utilising approved models like the Tao-Eldrup model for pore sizes smaller than 1 nm and the Tokyo model or RTE model for larger pore sizes. The RTE model contains an explicit temperature dependence of the o-Ps lifetime. Experiments on controlled pore glasses (CPG) with different pore sizes (2 -70 nm) at different temperatures (50 -500 K) were performed to verify the RTE model. A general agreement for T = 300 K could be found. The temperature dependence of the lifetime, especially for low temperatures, could not be approved sufficiently. 1 Introduction The growing interest of science and industry in the development of porous glass for various applications (low-dielectric thin films, catalysis, molecular filter) necessitate an adequate characterisation of its properties, especially of the pore size. Positronium annihilation lifetime spectroscopy (PALS) allows its non-destructive measurement. In dielectric amorphous material a part of the positrons is formed to a bound state of a positron and an electron which is called positronium (Ps) [1]. There are two spin states of Ps. The singlet spin state (para-positronium, p-Ps) has a very short self-annihilation lifetime of τ S = 0.125 ns and is not suitable to deliver information about the pore size. The long vacuum lifetime of the triplet spin state (ortho-positronium, o-Ps) of τ T = 142 ns is, however, an adequate sensor for measuring the pore size. The vacuum lifetime of o-Ps can be reduced markedly by pick-off annihilation (interaction with electrons of the host material). So the annihilation lifetime contains information about the pore size which can be extracted by calculating the annihilation rate of o-Ps:

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