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

The purpose of this paper is to find some general rules for the design of robust scintillation electron detectors for a scanning electron microscope (SEM) that possesses an efficient light-guiding (LG) system. The paper offers some general instructions on how to avoid the improper design of highly inefficient LG configurations of the detectors. Attention was paid to the relevant optical properties of the scintillator, light guide, and other components used in the LG part of the scintillation detector. Utilizing the optical properties of the detector components, 3D Monte Carlo (MC) simulations of photon transport efficiency in the simple scintillation detector configurations were performed using the computer application called SCIUNI to assess shapes and dimensions of the LG part of the detector. The results of the simulation of both base-guided signal (BGS) configurations for SE detection and edge-guided signal (EGS) configurations for BSE detection are presented. It is demonstrated that the BGS configuration with a matted disc scintillator exit side connected to the cylindrical light guide without optical cement is almost always a sufficiently efficient system with a mean LG efficiency of about 20%. It is simulated that poorly designed EGS strip configurations have an extremely low mean LG efficiency of only 0.01%, which can significantly reduce detector performance. On the other hand, no simple nonoptimized EGS configuration with a light guide widening to a circular or square profile, with a polished cemented scintillator and with an indispensable hole in it has a mean LG efficiency lower than 6.5%.     

A BSE scintillation detector in the (S)TEM

A single crystal YAG: Ce³⁺ annular scintillator axially placed in a movable light guide forms the essential part of a new BSE detector. Comparison of properties of this detector with those of a semiconductor detector is made. The bandwidth, signal-to-noise ratio, capacitance effects, and relative efficiency are parameters which favour the scintillation detector. Its disadvantage is that it must be equipped with a photomultiplier and a light guide. The position of the scintillator above the specimen permits efficient detection at a large collection angle of BSE. For normal beam incidence, the signal homogeneity from any area of the scintillator ensures that images are obtained without shadow effects due to signal loss in the scintillator or due to detector geometry. The same probe current as for other detection modes can be used. Resolution of details is as high as for an SE image.     

Light collection efficiency and light transport in backscattered electron scintillator detectors in scanning electron microscopy

Experimentally, scintillator detectors used in scanning electron microscopy (SEM) to record backscattered electrons (BSE) show a noticeable difference in detection efficiency in different parts of their active zones due to light losses transport in the optical part of the detector. A model is proposed that calculates the local efficiency of the active parts of scintillator detectors of arbitrary shapes. The results of these calculations for various designs are presented.     

Prospective scintillation electron detectors for S(T)EM based on garnet film scintillators

The performance of a scintillation electron detector for a scanning electron microscope and/or a scanning transmission electron microscope (S(T)EM) based on new epitaxial garnet film scintillators was explored. The LuGAGG:Ce and LuGAGG:Ce,Mg film scintillators with chemical formula (Ce_0.01Lu_0.27Gd_0.74)_3–wMg_w(Ga_2.48Al_2.46)O₁₂ were prepared and their cathodoluminescence (CL) and optical properties were studied and compared with the properties of current standard bulk single crystal YAG:Ce and YAP:Ce scintillators. More specifically, CL decay characteristics, CL emission spectra, CL intensities, optical absorption coefficients, and the refractive indices of the mentioned scintillators were measured. Furthermore, electron interaction volumes with absorbed energy distributions, photomultiplier (PMT) photocathode matchings, modulation transfer functions (MTF), and the photon transport efficiencies of scintillation detectors with the mentioned scintillators were calculated. A CL decay time for the LuGAGG:Ce,Mg film scintillator as low as 28 ns with an afterglow of only 0.02% at 1 μs after the e‐beam excitation was observed. As determined from calculated MTFs, the scintillation detectors with the new film scintillators lose contrast transfer ability above 0.6 lp/pixel, while the currently commonly used YAG:Ce single crystal scintillators already do so above 0.1 lp/pixel. It was also calculated that the new studied film scintillators have an 8% higher photon transfer efficiency, even for a simple disk shape compared with the standard bulk single crystal YAG:Ce scintillator. The studied LuGAGG:Ce,Mg epitaxial garnet film scintillators were evaluated as prospective fast scintillators for electron detectors, not only in S(T)EM but also in other e‐beam devices.     

An efficient single crystal BSE detector in SEM

Using the optical modifications described, the signal of the wide-angle BSE detector can be increased 3.7-fold compared with the detector type illustrated in Figure 1. The increase was achieved by more fully exploiting the optical phenomena of diffusion, reflection at the critical angle and antireflection, and especially that of their combined effects. A higher photon energy transfer was obtained by specific optical modifications to the scintillator and the light guide. Based on the current modifications, it can be estimated that about 75% of the generated light reaches the PMT. It is necessary, therefore, to revise all values previously published of the DQE coefficient of the YAG scintillator. A general value of DQE is of very limited use depending as it does on the specific conditions under which light emerges from the YAG scintillator. The DQE, therefore, does not permit sufficiently accurate comparison with other scintillation materials. The DQE can be evaluated only for a completed detector configuration in which the laws of geometrical optics apply, or it can be used for making comparisons of scintillation materials used in the same detector configuration.     

A method for decreasing the loading on scintillation channels in facilities with pulsed neutron sources

A method is proposed for decreasing the loading on scintillation channels in nuclear material detection and monitoring facilities comprising pulsed neutron sources, neutron moderators, and scintillators with a system for pulse shape discrimination between neutrons and photons. This method is based on the use of composite scintillators containing cylindrical shells of a thermal-neutron absorbing material. Selecting the sizes of zones in a composite scintillator and the absorber type (cadmium and lithium carbonate with different 6 Li content), it is possible to considerably increase the decay time constant for thermal neutrons in a composite scintillator and thereby reduce its load as compared to a homogeneous scintillator over the same time period after a pulse from the neutron source. The sizes of the scintillator component parts and the absorber material are optimized, which provides a means for decreasing the load on the scintillation channel several-fold while maintaining constant detection efficiency for fast fission neutrons. The capabilities of this method for decreasing the load are demonstrated by the example of the operational prototype of the fissile material detection and monitoring facility with a graphite moderator and LS-13 scintillators. The efficiency of the facility used in this method is compared to that of a facility with a deuterium-containing scintillator, in which no radiative capture photons are produced.     

Application of accelerators for the research and development of scintillators

We introduce experimental systems which use accelerators to evaluate scintillation properties such as scintillation intensity, wavelength, and lifetime. A single crystal of good optical quality is often unavailable during early stages in the research and development (R&D) of new scintillator materials. Because of their beams' high excitation power and/or low penetration depth, accelerators facilitate estimation of the properties of early samples which may only be available as powders, thin films, and very small crystals. We constructed a scintillation spectrum measurement system that uses a Van de Graaff accelerator and an optical multichannel analyzer to estimate the relative scintillation intensity. In addition, we constructed a scintillation time profile measurement system that uses an electron linear accelerator and a femtosecond streak camera or a microchannel plate photomultiplier tube followed by a digital oscilloscope to determine the scintillation lifetimes. The time resolution is approximately 10 ps. The scintillation spectra or time profiles can be obtained in a significantly shorter acquisition time in comparison with that required by conventional measuring systems. The advantages of the systems described in this study can significantly promote the R&D of novel scintillator materials.     

A double detector system for BSE and SE imaging

The double detector system described here is a simple device suitable for any SEM. It permits efficient imaging of specimen surfaces in either the secondary electron (SE) or backscattered electron (BSE) mode. The BSE detector is an annular single-crystal scintillator made of yttrium aluminium garnet (YAG) and the SE detector has a scintillator of the same material. Both detectors have their own light guides which are connected to a single photomultiplier. The choice of signal is made with a mechanical diaphragm mounted on a flange between the light guide and the photomultiplier. The SE detector may be replaced by a second BSE detector to allow the detection of “low” take-off angle BSEs to provide information which differs from that given by the annular BSE detector which operates to detect BSEs with a “high” take-off angle. In this way it is possible to image either material or topographic contrast with high resolution and to take advantage of the choice of detected electrons.     

A method for measuring the detector response function for monochromatic electrons based on Compton scattering

A method for calibrating the energy scale of a scintillation detector using γ rays has been proposed and implemented. The technique is based on Compton scattering in the scintillation detector, followed by photoelectric absorption of a scattered γ-ray photon in a Ge detector. The novelty of the method consists in placing the γ-ray source and the scintillation and Ge detectors tightly to each other. The method is efficient for detectors with a low-Z material for which the ratio of the cross sections for Compton scattering and photoeffect is great in value and the attenuation length of the γ-ray flux is comparable to the detector dimensions. The described technique can be used to precisely investigate the dependence of the light yield in a scintillator on the electron energy.     

Scintillation SE detector for variable pressure scanning electron microscopes

We present results obtained with a new scintillation detector of secondary electrons for the variable pressure scanning electron microscope. A detector design is based on the positioning of a single crystal scintillator within a scintillator chamber separated from the specimen chamber by two apertures. This solution enables us to decrease the pressure to several Pa in the scintillator chamber while the pressure in the specimen chamber reaches values of about 1000 Pa (7.5 Torr). Due to decreased pressure, we can apply a potential of the order of several kV to the scintillator, which is necessary for the detection of secondary electrons. Simultaneously, the two apertures at appropriate potentials of the order of several hundreds of volts create an electrostatic lens that allows electrons to pass from the specimen chamber to the scintillator chamber. Results indicate a promising utilization of this detector for a wide range of specimen observations.     

基于蒙特卡罗模拟的NaI，BGO，LSO，GSO，YSO闪烁晶体对比研究

发射断层成像系统中大都采用闪烁晶体与光电倍增管相耦合组成的γ射线探测器。闪烁晶体的光收集效率直接影响着探测器的能量、时间和空间分辨力。为了设计更好的探测器,利用M onte Carlo方法对N aI,BGO,LSO,GSO,YSO闪烁晶体进行了对比研究,分析比较了不同表面处理、晶体尺寸,外界反射材料对光收集效率的影响。模拟结果表明:入射面为粗糙面,其余为抛光面同时外层包装上高反射率的材料可得到最大的光输出,其中YSO表现出最优的光输出性能。晶体的几何尺寸对得到高的光输出也起着非常重要的作用。     

Light transport in single-crystal scintillation detectors in SEM

A Monte Carlo simulation method was developed to determine the efficiency of photon transport through a modified rotationally symmetric Everhart-Thornley detector. The method makes use of the random generation of photon emission from a luminescent centre and describes the trajectory of photons and the efficiency of their transport toward the photocathode of the photomultiplier tube. The model includes photon generation in a point source, mirror reflection by a metal-coated surface, Fresnel reflection by a metal-uncoated surface, Fresnel passage through the boundary of different materials, diffusion reflection, and passage through a matted surface and optical absorption in material. For the simulation, an IBM-PC-compatible program was written and applied to detection systems with disc, conical, and hemispherical YAG:Ce single-crystal scintillators with cylindrical or tapered light guides or without any light guide. The model was verified by measuring the efficiency of detection systems excited by the primary electron beam in the line-scan SEM mode.     

Detectors for low voltage scanning electron microscopy

Two simple electron detectors (low and high take-off angles) for low-voltage scanning electron microscopy were built and tested. They contain large area scintillators with an applied high voltage and are able to detect backscattered electrons with high efficiency. These detectors also can record the sum of backscattered and secondary electrons. The primary beam of the microscope is screened from the scintillator high voltage by grids, which also permit switching from the BSE to the (SE + BSE) mode. The circular symmetry of the grids minimizes the influence of applied potentials on the primary beam. The use of the low and high take-off detectors permits the detection of back-scattered electrons emitted from the specimen surface into different ranges of take-off angles.     

Quantitative analysis of backscattered-electron contrast in scanning electron microscopy

Backscattered-electron scanning electron microscopy (BSE-SEM) imaging is a valuable technique for materials characterisation because it provides information about the homogeneity of the material in the analysed specimen and is therefore an important technique in modern electron microscopy. However, the information contained in BSE-SEM images is up to now rarely quantitatively evaluated. The main challenge of quantitative BSE-SEM imaging is to relate the measured BSE intensity to the backscattering coefficient η and the (average) atomic number Z to derive chemical information from the BSE-SEM image. We propose a quantitative BSE-SEM method, which is based on the comparison of Monte–Carlo (MC) simulated and measured BSE intensities acquired from wedge-shaped electron-transparent specimens with known thickness profile. The new method also includes measures to improve and validate the agreement of the MC simulations with experimental data. Two different challenging samples (ZnS/Zn(O_xS_1–x)/ZnO/Si-multilayer and PTB7/PC₇₁BM-multilayer systems) are quantitatively analysed, which demonstrates the validity of the proposed method and emphasises the importance of realistic MC simulations for quantitative BSE-SEM analysis. Moreover, MC simulations can be used to optimise the imaging parameters (electron energy, detection-angle range) in advance to avoid tedious experimental trial and error optimisation. Under optimised imaging conditions pre-determined by MC simulations, the BSE-SEM technique is capable of distinguishing materials with small composition differences.     

Topographic and material contrast in low-voltage scanning electron microscopy

Two scintillation backscattered electron (BSE) detectors with a high voltage applied to scintillators were built and tested in a field emission scanning electron microscope (SEM) at low primary beam energies. One detector collects BSE emitted at low take-off angles, the second at high takeoff angles. The low take-off detector gives good topographic tilt contrast, stronger than in the case of the secondary electron (SE) detection and less sensitive to the presence of contamination layers on the surface. The high take-off detector is less sensitive to the topography and can be used for detection of material contrast, but the contrast becomes equivocal at the beam energy of 1 keV or lower.     

A scintillation γ-ray detector based on a solid-state photomultiplier

A scintillation detector based on a silicon photodetector of a new type—a solid-state photomultiplier—and a single-crystal CsI(Tl) scintillator was investigated. The effect of temperature on the low-energy detection threshold and the energy resolution of the detector and the influence of the radiation level on the detector efficiency (i.e., the influence of the counting rate of the distortion in the recorded spectrum shape) were estimated. A mathematical model of the detector was developed in order to assess its parameters.     

A phoswich detector for β-ray spectrometry

The spectrometric characteristics of a scintillation phoswich detector for β-ray spectrometry are described. The phoswich detector is composed of two detectors, one of which is an inorganic scintillator (calcium chlorborate) and the other is a scintillating plastic. The background of this phoswich detector is a factor of 9.3 lower than that of a single detector based on a plastic scintillator. At the same time, the dependence of its pulse heights on the β-particle energy is shown to be linear.     

A Digital Scintillation Detector for the MEDIANA Medical Diagnostic Station

A digital position-sensitive X-ray imaging scintillation detector has been designed for the MEDIANA medical diagnostic station of the Kurchatov Synchrotron Radiation Center (the Kurchatov Institute). A single-crystal CsI(Tl) scintillator 8 mm thick is used as a screen with approximately 100% detection efficiency for X rays with energies as high as 100 keV. A CCD matrix of dimensions 1024 × 1024 pixels is the photodetector. The spatial resolution is five line pairs per mm in a field of vision of 35 × 35 mm for 100-keV X rays.