Optimization of scintillator–reflector optical interfaces for the LUT Davis model

Abstract

Purpose Designing and optimizing scintillator‐based gamma detector using Monte Carlo simulation is of great importance in nuclear medicine and high energy physics. In scintillation detectors, understanding the light transport in the scintillator and the light collection by the photodetector plays a crucial role in achieving high performance. Thus, accurately modeling them is critical. Methods In previous works, we developed a model to compute crystal reflectance from the crystal 3D surface measurement and store it in look‐up tables to be used in the Monte Carlo simulation software GATE. The relative light output comparison showed excellent agreement between simulations and experiments for both polished and rough surfaces in several configurations, that is, without and with reflector. However, when comparing them at the irradiation depth closest to the photodetector face, rough crystals with a reflector overestimated the predicted light output. Investigating the cause of this overestimation, we optimized the LUT algorithm to improve the reflectance computation accuracy, especially for rough surfaces. However, optical Monte Carlo simulations carried out with these newly generated LUTs still overestimate the light output. Based on previous observations, one probable cause is the erroneous assumption of perfect couplings between the reflector and crystal and between the crystal and photodetector, which likely results in an important overestimation of the light output compared to experimental values. In practice, several factors could degrade it. Here, we investigated possible suboptimal optical experimental configurations that could lead to a degraded light collection when using Teflon or ESR reflectors coupled to the crystal with air or grease. We generated look‐up tables with a mixture of air and grease and showed the effect of three possible sources of light loss: the presence of a small gap between the crystal and the reflector edges close to the photodetector face, the infiltration of grease in the crystal–reflector coupling, and the presence of inhomogeneities in the photodetector–crystal interface. Results The strongest effect is linked to the presence of a small gap of grease between the edges of the reflector material and the crystal (light loss of 10%–12% for 0.2 mm gap). The optical grease infiltrating the crystal–reflector air coupling decreases the light output, depending on the infiltration's extent and the amount of grease infiltrated. Five percent of air in the crystal–photodetector coupling can cause a light output decrease of 2% to 4%. The individual and combined effect of these advanced models can explain the discrepancy of the relative light output obtained with ESR in simulations and experiments. With Teflon, the study indicates that the light output loss strongly depends on the reflectance deterioration caused by grease absorption. Conclusions Our results indicate that when studying scintillation detector performance with different finishes, performing simulations in ideal coupling cond...

The Accuracy of Cerenkov Photons Simulation in Geant4/Gate Depends on the Parameterization of Primary Electron Propagation

“…The optical photons produced by scintillation and Cerenkov emission from energetic electrons were simultaneously generated and tracked using the Livermore Model [28]. The transport of optical photons was conducted using the LUT Davis model [29][30][31][32][33] with polished surfaces LUTs wrapped in Teflon, except the face in contact with the photodetector, which was modeled coupled to optical grease (index of refraction 1.5) for all simulations.…”

Full modulation transfer functions of thick parallel‐ and focused‐element scintillator arrays obtained by a Monte Carlo optical transport model

“…Previous work has shown that although QE increases as the detector element thickness increases, it does not always lead to a higher light output 28 . Light output depends on a variety of optical properties that should be carefully chosen in order to gain optimal performance 32,46 . For the parameters used in the present study, the increase in CsI:Tl crystal thickness from 10 to 60 mm did not increase the number of optical photons exiting from a typical element, although the total number of optical photons generated in that element increased (Figure 3).…”

“…28 Light output depends on a variety of optical properties that should be carefully chosen in order to gain optimal performance. 32,46 For the parameters used in the present study, the increase in CsI:Tl crystal thickness from 10 to 60 mm did not increase the number of optical photons exiting from a typical element, although the total number of optical photons generated in that element increased (Figure 3). The light loss occurs due to longer pathways that the majority of optical photons have to traverse, and the corresponding increase in the number of incidences on the detector element wall in thicker elements, which causes a reduction in light collection efficiency and an increase in the optical Swank noise.…”

“…24 The crystal surface plays an important role in the amount of light output. 46,47 Absorption in the crystal coating, rather than in the crystal itself, is the main contributor to light loss for typical thick segmented scintillator arrays, especially those with walls coated with a diffuse reflector. 28,48 For individual elements, the bell-shaped distribution of the incident optical photons on the underlying screen depended on the thickness of the elements (Figure 2).…”

“…The light loss occurs due to longer pathways that the majority of optical photons have to traverse, and the corresponding increase in the number of incidences on the detector element wall in thicker elements, which causes a reduction in light collection efficiency and an increase in the optical Swank noise 24 . The crystal surface plays an important role in the amount of light output 46,47 . Absorption in the crystal coating, rather than in the crystal itself, is the main contributor to light loss for typical thick segmented scintillator arrays, especially those with walls coated with a diffuse reflector 28,48 …”