Excited state dynamics under high excitation densities in tungstates
Date
2015-09-07
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Abstract
Volframaadi kristallid on laialdaselt kasutuses stsintillaatoritena ioniseeriva kiirguse detekteerimisel. Näiteks omavad nad rakendusi meditsiinilistes uuringutes, turvasüsteemides ja kõrge energia füüsikas.
Selleks, et parandada stsintillaatorite efektiivsust, on oluline omada võimalikult täpset ettekujutust nende stsintillatsiooni mehhanismidest. See puudutab näiteks stsintillaatori mitteproportsionaalset kostet, mis on üks tõsisemaid seni lahendamata probleeme stsintillaatorite jaoks. Mitteproportsionaalsus tähendab, et stsintillatsiooni footonite arv ei ole proportsionaalne neelatava kõrge energiaga osakese energiaga. Tulemuseks on stsintilaatori energeetilise lahutusvõime märgatav halvenemine.
Antud töös uuritakse nii teoreetiliste kui eksperimentaalsete vahenditega ergastatud seisundite dünaamikat, mis on osa stsintillatsiooni protsessist. Arendatakse mitmeid mudeleid kirjeldamaks ergastuse, termilisatsiooni ja luminestsentsi staadiume. Mudeleid rakendatakse edukalt valitud volframaadi kristallide peal. Töö oluliseimaks tulemuseks võib lugeda mitteproportsionaalsuse täpse mudeli väljatöötamist, mis põhineb dipool-dipool interaktsioonil eksitonide vahel ja on rakendatav eksitonkiirgusel põhinevate stsintillaatorite jaoks.
Tungstate crystals are widely used as scintillators for the detection of ionizing radiation, with applications ranging from medical imaging to security systems to high energy physics. In order to improve the efficiency of scintillators, a better understanding of their scintillation mechanisms is required. This includes the phenomenom of scintillator nonproportionality, which is one of the major unsolved problems for scintillators. Nonproportionality means that the total light output of the scintillator is not proportional to the energy of the absorbed high-energy particle and leads to a considerable worsening of the energy resolution. This work includes the theoretical and experimental investigation of the excited state dynamics that constitute the scintillation process. We develop several models describing the excitation, thermalization, and the luminescence stage. The models are successfully tested on experimental data obtained for the selected tungstate crystals. Most importantly, we are able to provide an accurate description of nonproportionality, which is based on the dipole-dipole interaction of excitons and is applicable to excitonic intrinsic scintillators.
Tungstate crystals are widely used as scintillators for the detection of ionizing radiation, with applications ranging from medical imaging to security systems to high energy physics. In order to improve the efficiency of scintillators, a better understanding of their scintillation mechanisms is required. This includes the phenomenom of scintillator nonproportionality, which is one of the major unsolved problems for scintillators. Nonproportionality means that the total light output of the scintillator is not proportional to the energy of the absorbed high-energy particle and leads to a considerable worsening of the energy resolution. This work includes the theoretical and experimental investigation of the excited state dynamics that constitute the scintillation process. We develop several models describing the excitation, thermalization, and the luminescence stage. The models are successfully tested on experimental data obtained for the selected tungstate crystals. Most importantly, we are able to provide an accurate description of nonproportionality, which is based on the dipole-dipole interaction of excitons and is applicable to excitonic intrinsic scintillators.
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Keywords
volframiühendid, ergastatud olekud, tungsten compounds, excited states