The Kinetics of the Spin-Conversion of Positronium Measured by β+γ∆E Age-Momentum Correlation |
| U. Lauff, K. Maier, J. Major, A. Seeger, H. Stoll, P. Wesołowski, M. Koch Max-Planck-Institut für Metallforschung, Institut für Physik, Postfach 800665, 7000 Stuttgart 80, Germany I. Bilard, J.Ch. Abbé and G. Duplâtre Centre de Recherches Nucléaires, Laboratoire de Chimie Nucléaire, B.P. 20 67037 Strasbourg Cedex, France |
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| Age-momentum correlation (AMOC) measurements provide time-resolved information on the momentum distribution of annihilating electron-positron pairs through correlated determination of Doppler broadening (∆E) of the 511 keV annihilation radiation line and positron age. In the β+γ∆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+γ∆E AMOC measurements thus combine high β+γ∆E triple-coincidence rates with low background. From a practical point of view this constitutes a major progress compared to conventional (source-based) γγ∆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+γ∆E AMOC 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 measurements. It is found that within the concentration range investigated (≤ 0.1 mol/l) the spin-conversion reaction rate at room temperature depends linearly on the HTEMPO concentration with a reaction-rate constant kconv = (22.5 ± 0.5) × 109 l mol-1 s-1. |
| DOI: 10.12693/APhysPolA.83.349 PACS numbers: 78.70.Bj |