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Multi-photon decay mode spectroscopy of positronium
An approximation for the branching ratio of the four-photon decay of parapositronium (BR4γ) was measured using a multi-gamma-ray spectrometer. For the first time in such measurements, the spectrometer consisted of an array of eight identical LaBr3:Ce scintillator detectors, each of which combines go...
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| Format: | Thesis |
| Language: | English |
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Department of Physics
2022
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| _version_ | 1867614092324241408 |
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| access_status_str | Open Access |
| author | Johnson, Storm |
| author2 | Leadbeater, Thomas |
| author_browse | Johnson, Storm Leadbeater, Thomas |
| author_facet | Leadbeater, Thomas Johnson, Storm |
| author_sort | Johnson, Storm |
| collection | Thesis |
| description | An approximation for the branching ratio of the four-photon decay of parapositronium (BR4γ) was measured using a multi-gamma-ray spectrometer. For the first time in such measurements, the spectrometer consisted of an array of eight identical LaBr3:Ce scintillator detectors, each of which combines good energy resolutions (5% and 10% at 511 keV for the signals from the eighth dynode and anode of the photomultiplier tube, respectively) with an excellent timing resolution (∼ 300 ps). These energy resolutions were minimised through an optimal selection of the digital signal processing parameter settings. The detectors were situated in a planar geometry, where the source-to-detector distance of the detector system was selected such that the effect of peak pulse pile-up was minimised (to less than 3%), while maximising the full-energy peak detection efficiency at 511 keV (to 3%). For this work, locally-produced 22Na radioactive sources were used as positron emitters, which enabled the formation of positronium and subsequent gamma decays. Energy calibration measurements were performed using a 152Eu source, where the prominent energy peaks of (121.8, 244.7, 344.3, 778.9, 964.1, 1408.0) keV were used for calibration. For the BR4γ measurement, 5×1011 events were accumulated over a measurement period of 60 days, which resulted in low statistical uncertainties for the coincident counting between detector pairs (less than 1%). Through simplifying assumptions that neglected the background corrections and efficiency normalisations for each of the 2γ and 4γ decays, a first order approximation of BR4γ was determined as the ratio between measured 4γ events (N4γ) and measured 2γ events (N2γ), such that BR4γ ∼ N4γ N2γ = 4.8 (19) × 10−7. This measured value of BR4γ differs from previous measurements and accepted literature values by a factor of 3. |
| format | Thesis |
| id | oai:open.uct.ac.za:11427/36466 |
| institution | University of Cape Town (South Africa) |
| language | eng |
| last_indexed | 2026-06-10T12:46:32.808Z |
| license_str | Not specified — see source repository |
| provenance_str_mv | Harvested via OAI-PMH from UCTD — University of Cape Town Open Access Repository |
| publishDate | 2022 |
| publishDateRange | 2022 |
| publishDateSort | 2022 |
| publisher | Department of Physics |
| publisherStr | Department of Physics |
| record_format | dspace |
| source_str | UCTD — University of Cape Town Open Access Repository |
| spelling | oai:open.uct.ac.za:11427/36466 Multi-photon decay mode spectroscopy of positronium Johnson, Storm Leadbeater, Thomas Jones, Pete Physics An approximation for the branching ratio of the four-photon decay of parapositronium (BR4γ) was measured using a multi-gamma-ray spectrometer. For the first time in such measurements, the spectrometer consisted of an array of eight identical LaBr3:Ce scintillator detectors, each of which combines good energy resolutions (5% and 10% at 511 keV for the signals from the eighth dynode and anode of the photomultiplier tube, respectively) with an excellent timing resolution (∼ 300 ps). These energy resolutions were minimised through an optimal selection of the digital signal processing parameter settings. The detectors were situated in a planar geometry, where the source-to-detector distance of the detector system was selected such that the effect of peak pulse pile-up was minimised (to less than 3%), while maximising the full-energy peak detection efficiency at 511 keV (to 3%). For this work, locally-produced 22Na radioactive sources were used as positron emitters, which enabled the formation of positronium and subsequent gamma decays. Energy calibration measurements were performed using a 152Eu source, where the prominent energy peaks of (121.8, 244.7, 344.3, 778.9, 964.1, 1408.0) keV were used for calibration. For the BR4γ measurement, 5×1011 events were accumulated over a measurement period of 60 days, which resulted in low statistical uncertainties for the coincident counting between detector pairs (less than 1%). Through simplifying assumptions that neglected the background corrections and efficiency normalisations for each of the 2γ and 4γ decays, a first order approximation of BR4γ was determined as the ratio between measured 4γ events (N4γ) and measured 2γ events (N2γ), such that BR4γ ∼ N4γ N2γ = 4.8 (19) × 10−7. This measured value of BR4γ differs from previous measurements and accepted literature values by a factor of 3. 2022-06-10T10:11:07Z 2022-06-10T10:11:07Z 2022 2022-06-10T10:10:13Z Master Thesis Masters MSc http://hdl.handle.net/11427/36466 eng application/pdf Department of Physics Faculty of Science |
| spellingShingle | Physics Johnson, Storm Multi-photon decay mode spectroscopy of positronium |
| thesis_degree_str | Master's |
| title | Multi-photon decay mode spectroscopy of positronium |
| title_full | Multi-photon decay mode spectroscopy of positronium |
| title_fullStr | Multi-photon decay mode spectroscopy of positronium |
| title_full_unstemmed | Multi-photon decay mode spectroscopy of positronium |
| title_short | Multi-photon decay mode spectroscopy of positronium |
| title_sort | multi photon decay mode spectroscopy of positronium |
| topic | Physics |
| url | http://hdl.handle.net/11427/36466 |
| work_keys_str_mv | AT johnsonstorm multiphotondecaymodespectroscopyofpositronium |