Investigation of Thickness-Dependent Scintillator-Photosensor Interface Reflection Coefficients for Improved Light Yield Calculations in Inorganic Scintillators
Open Access
- Author:
- Logoglu, Faruk
- Graduate Program:
- Nuclear Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 09, 2023
- Committee Members:
- Jon Schwantes, Program Head/Chair
Marek Flaska, Chair & Dissertation Advisor
Kenan Unlu, Major Field Member
Douglas Wolfe, Outside Unit & Field Member
Arthur Motta, Major Field Member - Keywords:
- Scintillators
Light Yield
Light Transport
Monte Carlo Simulations
Interface Reflection Coefficient - Abstract:
- Radiation scintillation detectors are used in numerous fields including low and high energy physics (HEP) experiments, medical imaging (MI), space exploration, well-logging, and homeland security. Since they can be manufactured with vastly different properties for virtually any type of radiation detection application, they are ubiquitous in scientific research and industry. For a given application, scintillation detectors are chosen based on their properties such as their efficiency, energy resolution (ER), and time resolution. Light yield (LY) is one of the most important properties of scintillators in characterizing their performance under irradiation. It has a direct correlation with the energy and time resolutions of scintillators since a larger number of light photons produced in scintillators translates to a larger number of light photons detected by photosensors, which improves both energy and time resolutions. Excellent energy and time resolutions of scintillators are critical in various applications such as radioisotope identification and positron emission tomography - time of flight (PET-TOF). LY is also an important parameter in simulation studies where the goal is to either characterize already existing detectors and compare simulation results to experimental observations or investigate novel configurations involving scintillators. In the latter case, LY plays a critical role in the optimization of new detection systems. Although the importance of LY is well known in the field of radiation detection, its accurate measurement is not straightforward. Any measurement of LY is bound to have large uncertainties due to various physical processes involved in its estimation. Accurate measurement of scintillator LYs necessitates accurate knowledge of numerous properties of both the scintillators and the photosensors that are used to convert the scintillation light into electronic signal. These properties include but not limited to the surface conditions and bulk properties of scintillators, the photodetection efficiency of photosensors, the single-photoelectron response (SPER) of photosensors and the light reflection properties between scintillators and photosensors. Consequently, there are large differences in reported LY values even for widely used scintillators such as thallium-activated sodium-iodide (NaI(Tl)) and cerium-activated lutetium-yttrium oxyorthosilicate (LYSO:Ce3+), as any major change in the aforementioned quantities has an impact on the estimated LY. One of the approaches to determine the scintillator LYs is to measure their light outputs (LO) as a function of scintillator thickness and fit an analytical model function to the data set to extrapolate the LO for a point scintillator. Since this method does not require relatively sophisticated equipment such as integrating spheres, electron monochromators or electron collimators, it can be carried out in most radiation detection laboratories. However, the model functions need to be as realistic as possible, and they need to contain all the relevant physics to make LY estimations accurate. In this thesis, two new analytical light transport model (LTM) functions for cuboid scintillators are derived, and it is demonstrated that they make LY estimations more accurate. These two analytical LTM functions are called the extended 2D-model and the 3D-model. The 3D-model is shown to estimate the LO of scintillators as accurately as Monte Carlo (MC) simulations, which makes full-scale MC simulations in cuboid scintillators needless in certain applications. One of the most important quantities that affect LY estimations is the scintillator-photosensor interface reflection coefficient (SPIRC). SPIRC is the observed reflection coefficient of the scintillator-photosensor boundary averaged over photon angles and energies and is typically considered a constant. Since scintillation emission contains photons of various wavelengths, and scintillators have wavelength dependent surface and bulk properties, it is not straightforward to reliably estimate SPIRCs. Moreover, SPIRC is dependent on the optical coupling methods employed in radiation measurements, which are typically dry (air) coupling and grease coupling. In this thesis, two new experimental methods (the single-PMT setup and the dual-PMT setup) are suggested and tested to estimate the SPIRCs in different optical coupling configurations. Furthermore, a new hypothesis is put forward regarding the SPIRC dependency on scintillator thickness. It is hypothesized through MC simulations that SPIRC decreases with increasing scintillator thickness as a result of change in the scintillation photons’ angular distribution on the scintillator-photosensor interface (SPI). This hypothesis is tested with three different inorganic scintillation crystals by employing the two new experimental methods, and the SPIRC dependency on scintillator thickness is experimentally observed by evaluating the extended 2D-model and the 3D-model for LYSO:Ce3+ and cerium-activated gadolinium-aluminum-gallium garnet (GAGG:Ce3+) inorganic scintillators. The SPIRC dependency on scintillator thickness is observed by the 3D-model with more than 2σ confidence for both LYSO:Ce3+ and GAGG:Ce3+ scintillators in the single-PMT setup. The SPIRC dependency on scintillator thickness is observed with more than 3σ confidence for GAGG:Ce3+ scintillators in the dual-PMT setup. Consequently, it is suggested that the measurements should consider thickness-dependent SPIRCs (TD-SPIRC) to make accurate LY estimations.