An integrated model of scintillator-reflector properties for advanced simulations of optical transport

Abstract

Accurately modeling the light transport in scintillation detectors is essential to design new detectors for nuclear medicine or high energy physics. Optical models implemented in software such as Geant4 and GATE suffer from important limitations that we addressed by implementing a new approach in which the crystal reflectance was computed from 3D surface measurements. The reflectance was saved in a look-up-table (LUT) then used in Monte Carlo simulation to determine the fate of optical photons. Our previous work using this approach demonstrated excellent agreement with experimental characterization of crystal light output in a limited configuration, i.e. when using no reflector. As scintillators are generally encapsulated in a reflector, it is essential to include the crystal-reflector interface in the LUT. Here we develop a new LUT computation and apply it to several reflector types. A second LUT that contains transmittance data is also saved to enable modeling of optical crosstalk. LUTs have been computed for rough and polished crystals coupled to a Lambertian (e.g. Teflon tape) or a specular reflector (e.g. ESR) using air or optical grease, and the light output was computed using custom Monte Carlo code. 3 × 3 × 20 mm3 lutetium oxyorthosilicate crystals were prepared using these combinations, and the light output was measured experimentally at different irradiation depths. For all reflector and surface finish combinations, the measured and simulated light output showed very good agreement. The behavior of optical photons at the interface crystal-reflector was studied using these simulations, and results highlighted the large difference in optical properties between rough and polished crystals, and Lambertian and specular reflectors. These simulations also showed how the travel path of individual scintillation photons was affected by the reflector and surface finish. The ultimate goal of this work is to implement this model in Geant4 and GATE, and provide a database of scintillators combined with a variety of reflectors.

Monte Carlo Optical Simulations of a Small FoV Gamma Camera. Effect of Scintillator Thicknesses and Septa Materials

“…Figure 1a presents a photograph of the benchtop system, while a schematic drawing of its detector is illustrated in Figure 1b. and there is need to be studied in detail, using Monte Carlo models, to extend our understanding in the performance of the scintillation detectors [4,5]. Nowadays, GATE Monte Carlo simulation software [6,7] is extensively used in the field of Nuclear Medicine to simulate SPECT and PET medical and preclinical small field of view detectors.…”

“…GATE allows the user to perform optical simulations with accurate modeling of optical photons interactions with the crystal surface. Τwo simulation models are available to describe the optical properties at boundary conditions of the surface: the recently implemented Look-Up- Table (LUT) Davis model [1][2][3][4][5][6][7][8] and the traditional UNIFIED model [9,10].…”

“…Recently, many studies [2,5,11,12] have exploited GATE optical photons simulations, to model the light transportation in scintillators, according to their surface properties. One of the most important parameters that have been used to assess the performance of a scintillation camera is the spatial resolution.…”

Modelling the transport of optical photons in scintillation detectors for diagnostic and radiotherapy imaging

“…This typically results in inaccurate computation of the light transport (Bea et al , 1994b; Janecek and Moses, 2010; Roncali and Cherry, 2013a). More accurate models were developed and have being implemented in GEANT4 (Janecek and Moses) and GATE (Roncali and Cherry, 2014; Roncali et al , 2017). Janecek and Moses have developed a custom apparatus with an array of photodiodes to measure the 3D reflectance of a crystal illuminated with a laser which angle of incidence is varied from 0 to 90°.…”

Performance of Optical Coupling Materials in Scintillation Detectors Post Temperature Exposure

“…The goal of optical coupling materials is to provide a medium for optical photons to pass through the scintillator crystal to the SiPM by maintaining an index of refraction similar to that of the scintillation crystal and by providing flush surface contact [5,6]. Mismatching of the index of refraction or poor surface contact can contribute to elevated scattering at the crystal-SiPM interface, resulting in fewer optical photons detected and reduced detector performance [7]. Thus, optical coupling materials must be chosen based on specific material properties which are assumed to be constant throughout deployment.…”