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Radioluminescence: A simple model for fluorescent layers
Linköping University, Department of Medical and Health Sciences, Radiation Physics. Linköping University, Faculty of Health Sciences.
2011 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The aim of this thesis is to present a simple model for the radiation to light conversion processes in fluorescent layers as an aid in future developments and applications. Optimisation between sensitivity and spatial resolution for fluorescent layers in digital radiology is a delicate task where the extrinsic efficiency for various phosphors needs to be established for varying parameters. The extrinsic efficiency of a fluorescent layer can be expressed as the ratio of the light energy per unit area at the screen surface to the incident xray energy fluence. Particle size is a critical factor in determining the value of the extrinsic efficiency, but in most models it is not treated as an independent variable. Based on the definition of a light extinction factor (ξ), a model is proposed such that, knowing the intrinsic efficiency η, the particle size and the thickness of a certain make of screen, the extrinsic efficiency can be calculated for an extended range of particle sizes and / or screen thicknesses. The light extinction factor ξ is an optical parameter determined from experimental data on extrinsic efficiency. The proposed model is compared to established methods. Further experiments have confirmed the validity of the model. Monte-Carlo simulations have been utilised to refine the calculations of energy imparted to the phosphor by taking into account the escape of scattered and K-radiation generated in the screen and interface effects at the surfaces. The luminance was measured for a set of in-house manufactured screens of varying thicknesses and particle sizes. Utilising the proposed model, the corresponding calculated values deviated ± 14 %.within the studied range.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press , 2011. , 36 p.
Series
Linköping Studies in Health Sciences. Thesis, ISSN 1100-6013 ; 117
Keyword [en]
Extrinsic efficiency, extinction factor, radioluminescence, modelling, Monte Carlo, optical attenuation, optimisation, particle size, phosphor, scintillator
National Category
Medical and Health Sciences
Identifiers
URN: urn:nbn:se:liu:diva-70539ISBN: 978-91-7393-098-7 (print)OAI: oai:DiVA.org:liu-70539DiVA: diva2:440163
Supervisors
Available from: 2011-09-12 Created: 2011-09-12 Last updated: 2013-09-03Bibliographically approved
List of papers
1. A simple model for estimating the particle size dependence of absolute efficiency of fluorescent screens
Open this publication in new window or tab >>A simple model for estimating the particle size dependence of absolute efficiency of fluorescent screens
1999 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 44, no 5, 1353-67 p.Article in journal (Refereed) Published
Abstract [en]

The absolute efficiency of a phosphor screen is the ratio of the light energy per unit area at the screen surface to the incident x-ray energy fluence. Particle size is a critical factor in determining the absolute efficiency, but in most models its influence is not accounted for. To allow derivation of the particle size dependence, a model is proposed that describes the optical properties of the screen by means of a single parameter, the light extinction factor, xi, and assumes that the intrinsic efficiency (light energy/energy imparted to the phosphor material) is independent of particle size. The value of xi depends on the type of screen (phosphor, reflective backing, coating and binder) and has to be determined from measurements on at least two screens with known particle size and thickness. The absolute efficiency can then be calculated for an extended range of particle sizes and/or screen thicknesses. To test the model, experimental data from the literature were used to derive values of xi for screens of La2O2S:Tb, LaOBr:Tm and ZnCdS:Ag. The extinction factor was found to vary between -6 and +20%. The non-physical negative value for xi, found from one set of experiments on La2O2S:Tb screens, may be explained as resulting from a lack of accurate knowledge of the actual tube potential, influencing calculated values of the energy imparted to the screen. The results are promising but further well-controlled experiments (including improved dosimetric calculations to account forescape of K-radiation from the screen) are needed to confirm the model.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-70537 (URN)doi:10.1088/0031-9155/44/5/319 (DOI)10368024 (PubMedID)
Available from: 2011-09-12 Created: 2011-09-12 Last updated: 2017-12-08Bibliographically approved
2. Experimental verification of a model for estimating the particle size dependence of extrinsic efficiency of radioluminescent layers
Open this publication in new window or tab >>Experimental verification of a model for estimating the particle size dependence of extrinsic efficiency of radioluminescent layers
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The extrinsic (absolute) efficiency of a phosphor is expressed as the ratio of light energy emitted per unit area at the phosphor surface to incident x-ray energy fluence. Several studies on polycrystalline phosphor materials show that the optical parameters of a specific phosphor may vary within a wide range of values. The aim of this work was to verify a previously published model where all optical parameters were replaced with a single parameter, the light extinction factor ξ. The varying extrinsic efficiency for an extended range of particle sizes and layer thicknesses are calculated from the input parameters: the intrinsic efficiency η, the mean particle size of the phosphor, the thickness of the layer, the light extinction factor and the calculated energy imparted to the layer. The X-ray spectrum was simulated utilising the SpekCalc software (Poludniowski et al 2009). In this work, calculations of the energy imparted to in-house manufactured Gd2O2S:Tb screens, were also compared to calculations improved making use of Monte Carlo simulations (software PENELOPE (Baro et al 1995)). KAP (kerma area product) -rate-values were noted and the corresponding luminance measured and compared to calculated values. The deviations were ±14 % within the studied range.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-70538 (URN)
Available from: 2011-09-12 Created: 2011-09-12 Last updated: 2013-09-03Bibliographically approved

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