The Kinetics of the Spin-Conversion of Positronium Measured by β+γ∆E Age-Momentum Correlation

Abstract

Age-momentum correlation (AMOC) measurements provide time-resolved information on the momentum distribution of annihilating electron— positron pairs through correlated determination of Doppler broadening (SE) of the 511 keV annihilation radiation line and positron age. In the β+ ∆E AMOC technique implemented at the Stuttgart pelletron accelerator the start signals for the age measurements are generated when the relativistic positrons pass through a fast plastic scintillator, which has a detection efficiency close to unity. Beam-based β+ ∆E AMOC measurements thus combine high fl+ γ6,E triple-coincidence rates with low background. From a practical point of view this constitutes a major progress compared to conventional (source-based) ∆E AMOC measurements. The AMOC technique permits direct access to a wide variety of time-dependent processes, among them the reaction kinetics in positronium chemistry. As an example, we present a β+g ∆EAMOC investigation of the kinetics of a spin-conversion reaction of positronium in methanol induced by the presence of a nitrosyl free-radical paramagnetic solute (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl , HTEMPO). The AMOC measurements allowed the time-resolved observation of the spin-conversion process, which had been investigated before by uncorrelated positron lifetime and Doppler-broadening nieasurements. It is found that within the concentration range investigated (< 0.1 moll) the spin-conversion reaction rate at room temperature depends linearly on the HTEMPO concentration with a reaction-rate constant kconv = (22.5 ± 0.5) x 10 9 1 moI -1