title: The three-photon yield from e+ annihilation in various fluids.
abstract: Positronium in the triplet state decays by the emission of three photons and it has been proposed that their simultaneous detection can be used for medical imaging. The three-photon yield has been observed to be enhanced in low O(2) levels in some fluids but has never been measured in biologically relevant liquids. In this study, the delayed three-photon decay yield, at both high and low O(2) levels, has been extracted by fitting the time dependence of the two-photon yield to a set of coupled differential equations. The differential equations, in a simple yet seemingly satisfactory fashion, account for the e(+) capture to form positronium, its decay and the interconversion of the two spin configurations. Our results indicate that the delayed three-photon fraction is 0.25% in water (or blood-like) samples and exhibits no (or exceedingly small) dependence on the dissolved oxygen content. If one assumes that the direct component contributes a fraction expected by annihilation on free electrons (1/372), then the total three-photon fraction is 0.52% in the samples of biological relevance.
authors.affiliation: Department of Physics, Washington University, St Louis, MO 63130, USA.
authors.forename: K
authors.initials: K
authors.lastname: Mercurio
authors.forename: P
authors.initials: P
authors.lastname: Zerkel
authors.forename: R
authors.initials: R
authors.lastname: Laforest
authors.forename: L G
authors.initials: LG
authors.lastname: Sobotka
authors.forename: R J
authors.initials: RJ
authors.lastname: Charity
mesh_terms: D000465:Algorithms; D001826:Body Fluids / Q000000981:diagnostic imaging / Q000528:radiation effects*; D003198:Computer Simulation; D004583:Electrons; D006801:Humans; D010100:Oxygen / Q000737:chemistry; D017785:Photons; D011859:Radiography; D020266:Radiotherapy, Conformal; D013997:Time Factors
chemical_list: D010100:Oxygen