Overview of S(T)EMelectron detectors with garnet scintillators: Some potentials and limits

Abstract

The paper is focused on a complete configuration and design of a scintillation electron detector in scanning electron and/or scanning transmission electron microscopes (S(T)EM) with garnet scintillators. All processes related to the scintillator and light guide were analyzed. In more detail, excitation electron trajectories and absorbed energy distributions, efficiencies and kinetics of scintillators, as well as the influence of their anti-charging coatings and their substrates, assigned optical properties, and light guide efficiencies of different configurations were presented and discussed. The results indicate problems with low-energy detection below 1 keV when the scandium conductive coating with a thickness of only 3 nm must be used to allow electron penetration without significant losses. It was shown that the short rise and decay time and low afterglow of LuGdGaAG:Ce liquid-phase epitaxy garnet film scintillators guarantee a strong modulation transfer function of the entire imaging system resulting in a contrast transfer ability up to 0.6 lp/pixel. Small film scintillator thicknesses were found to be an advantage due to the low signal self-absorption. The optical absorption coefficients, refractive indices, and the mirror optical reflectance of materials involved in the light transport to the photomultiplier tube photocathode were investigated. The computer-optimized design SCIUNI application was used to configure the optimized light guide system. It was shown that nonoptimized edgeguided systems possess very poor light guiding efficiency as low as 1%, while even very complex optimized ones can achieve more than 20%.

Comparison of photon transport efficiency in simple scintillation electron detector configurations for scanning electron microscope

“…However, the measured increase in the LG efficiency is less distinctive compared to the simulation because ideal polished material is unlike the real free of diffuse surfaces. As is clear from the results of our previous paper (Schauer et al, 2021), using the BGS configuration with the conical scintillator cemented to the light guide is more advantageous only in the case of predominant signal electron collection at the axis of the detection system. In any case, designing the BGS configuration for SE detection is much simpler than designing the EGS one for BSE detection.…”

“…One way to break the regularity of the parallelepiped EGS configurations and thus change the disadvantageous angles of photon incidence on the internal light guide walls is to use the widening light guides. In other words, the widening systems are better because they can straighten the photons' trajectories towards the photocathode, but not from all locations of the scintillator surface, as was presented in our previous paper (Schauer et al, 2021). It is not important whether the light guide widening to the square profile or the circular one is used.…”

“…The effect of inclination of the EGS widening systems was not simulated to compare the simple configurations in this paper. However, based on our previous studies (Schauer, 2007;Schauer et al, 2021), it can be deduced that optimized inclinations will several times increase efficiency differences between the EGS strip and widening configurations in favor of the widening ones. A major and common problem of the simple nonoptimized EGS configurations is the poor homogeneity of photon transport efficiency over the electron excited scintillator surface.…”

“…The MC simulations of light collection from the scintillators are more commonly used to design the detectors in the field of high-energy physics, especially in medical scintillography techniques. There are various commercial or freely available programs such as ZEMAX (Bauer et al, 2009;Lorincz et al, 2010;Salomoni et al, 2018) or GEANT (Badiei et al, 2019;Knyazev et al, 2021;van der Laan et al, 2010). However, the common disadvantage of these programs is that they are not suitable for an overly complex geometry of the scintillator or light guide, which cannot be avoided in the SEM detectors.…”

Composite Detectors Based on Single-Crystalline Films and Single Crystals of Garnet Compounds

“…The technology of the liquid-phase epitaxy (LPE) developed over the last 30 years offers the possibility of creating luminescent materials on the base of single-crystalline films (SCF) of complex oxides [ 1 , 2 , 3 ] for application in cathodoluminescent screens [ 4 , 5 , 6 ], laser media [ 7 , 8 ], scintillators for registration of α- and β-particles, X-rays, or γ-quanta [ 1 , 9 , 10 ], and scintillating screens for microtomography detectors employing X-ray sources and synchrotron radiation [ 11 , 12 ].…”