Magnetic quenching of positronium in thorium phosphate: an experimental approach to p-Ps pick-off lifetime ?

The effects of magnetic field (B) quenching on positronium (Ps) are studied in solid thorium phosphate by using both the lifetime spectroscopy (LS) and Doppler broadening (DBARL) techniques, in view of gaining information on the pick-off decay rate constant of positronium singlet state (p-Ps) and compare it to that of the triplet state (o-Ps). At B=0, the longest lifetime is τ 3 o ≃700 ps, with an intensity of some 2.1%. The effects of B on LS parameter R (normalized integral of counts in a time window) show that τ 3 o is ascribable to o-Ps. Fitting the variations with B of both R and τ 4 (lifetime of the m=-0 o-Ps substate) requires two phenomenological parameters: (i) the contact density parameter, η≃0.22 and (ii) either an additional contact density parameter, η'=0.36, or a p-Ps pick-off decay constant, λ po1 ≃2.59 ns -1 , different from that of o-Ps, λ po3 =1/τ 3 o . Resolving the deconvoluted DBARL spectra to derive the intensity of the intrinsic decay component at B=0 gives a better agreement with the first (η,η') than with the second (η,λ po1 ) of these possibilities