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Positronium is a metastable hydrogen-like bound state of an electron and a positron which can exist in two spin states. Para-positonium, p-Ps, ( the positon and electron spins are anti-parallel) is a singlet state with a characteristic self-annihilation lifetime of 125 ps in vacuum ^[1]. Ortho-positronium, o-Ps, is a triplet state (the positron and electron spins are parallel) with a characteristic self-annihilation lifetime of 142 ns in vacuum. ^[1] In molecular materials, the lifetime of o-Ps is environment dependent and it delivers information pertaining to the size of the void in which it resides. Ps can pick up a molecular electron with an opposite spin to that of the positron, leading to a reduction of the o-Ps lifetime from 142 ns to 1-4 ns (depending on the size of the free volume in which it resides).^[1] The the size of the molecular free volume can be derived from the o-Ps lifetime via the semi-empirical Tao-Eldrip model.^[2]

PALS spectrometer

References[edit]

^ ^a ^b ^c Jean, Y. C.; Schrader, D. M.; Mallon, P. E. (2002). Principles and Applications of Positron and Positronium Chemistry. World Scientific Publishing Co Pte Ltd.
^ Eldrup, M.; Lightbody, D.; Sherwood, J. N. (1981). "The temperature dependence of positron lifetimes in solid pivalic acid". Chemical Physics. 63: 51.

[schader02-1] Jean, Y. C.; Schrader, D. M.; Mallon, P. E. (2002). Principles and Applications of Positron and Positronium Chemistry. World Scientific Publishing Co Pte Ltd.

[eld_1-2] Eldrup, M.; Lightbody, D.; Sherwood, J. N. (1981). "The temperature dependence of positron lifetimes in solid pivalic acid". Chemical Physics. 63: 51.

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