BACKGROUND IN A TEST OF DETECTING “COOPERATIVE” PARAPOSITRONIUM ANNIHILATION BY THE 32-CRYSTAL SPECTROMETER ARGUS
Nevenka M. Antović, Sergey K. Andrukhovich, Alexandr V. Berestov
Received: 14 FEB 2015, Received revised: 11 MAR 2015, Accepted: 15 MAR 2015, Published Online: 28 APR 2016
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Parapositronium – a singlet positronium ground state, has the total angular momentum of electron and positron forming the atom Js = 0, magnetic quantum moment m = 0, and its annihilation spectrum is dominantly created by the 511 keV discrete photons. A process of cooperative emission of annihilation photons (cooperative annihilation) by a system of positronium atoms, i.e., annihilation superradiance, had been considered by other researchers, and the theory of annihilation superradiance in a system of parapositronium atoms for two-photon annihilation was constructed (two interacting parapositronium atoms; emission of the 1022 keV photons flying apart at an angle of 180°). The 32-crystal spectrometer ARGUS, with 16 detector pairs at an angle of 180° capable of registering double gamma coincidences, with lead collimators (80 mm in diameter) mounted on each detector – was used to test the phenomenon. Parapositronium annihilation spectra were acquired using 22Na (A = 4×105 Bq) as a positron source, and SiO2 (as „positronium forming“ medium; probability: 32 %), as well as Al (as „positronium not forming“ target) used as a blank – for estimation of the background events. In the case when after emission of two starting positrons from 22Na (i.e., the 1275 keV nuclear photons) coincident registration (respecting the spectrometer time resolution) of four annihilation photons should be considered as a six-fold coincidence event, experimentally obtained counting rates were – 0.25 s-1 (SiO2) and 0.23 s-1 (Al), while theoretically predicted – 0.34 s-1. The main background process competitive to a registration of the parapositronium cooperative annihilation is four-fold coincidence event (the two 180° detectors register 1275 keV photons, and two – summing of the 511 keV annihilation photons), with experimental counting rates – 0.026 s-1 (SiO2) and 0.023 s-1 (Al), as theoretically predicted – 0.023 s-1. On the other hand, cooperative annihilation should be a four-fold coincidence event (the two 180° detectors register 1275 keV photons, and two – 1022 keV photons), which has not been registered by the ARGUS spectrometer (theoretically predicted counting rate – 1.7×10-5 s-1). The analyses showed that probability of detecting the parapositronium cooperative annihilation will increase significantly with increasing positron source activity, but also with decreasing diameter of lead collimators.
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