# Cerium Bromide Single-Crystal X-Ray Detection and Spectral Compatibility Assessment with Various Optical Sensors

Authors: Dionysios Linardatos; Konstantinos Velissarakos; Ioannis Valais; George Fountos; Nektarios Kalyvas; Christos Michail
Journal: Material Design & Processing Communications
 (2022)

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## Abstract

Scintillators with high light yield (LY) values are of interest for medical imaging applications, in harsh environments, nondestructive testing (NDT), etc. CeBr3 has a LY of 60000 photons per MeV, a value much higher than other efficient materials, such as Lu3Al5O12:Ce (25000 photons/MeV); thus, its X-ray detection properties would be of interest to be examined for medical imaging applications. The X-ray detection and absorption properties of a single crystal CeBr3 sample along with the compatibility of its produced light with various optoelectronic sensors were examined. In this study, the quantum detection (QDE) and the energy absorption efficiency (EAE) of CeBr3 were calculated. The findings were compared with data for 10×10×10mm3 Lu3Al5O12:Ce and CaF2:Eu single crystals. The measured optical spectrum produced by CeBr3 was well correlated with the spectral response of commercial optical sensors, yielding spectral matching higher than 93% for various photocathodes, e.g., GaAs (94%), E-S20 (95%), and bialkali and multialkali (95-97%), as well as with flat panel position-sensitive photomultipliers (95-99%). The energy absorption properties of CeBr3 were found higher than those of Lu3Al5O12:Ce and CaF2:Eu for X-ray tube voltages greater than 100 kVp. The quantum detection efficiency was 100% across the examined energy range. Even though CeBr3 is hygroscopic and has a mediocre 5.1 g/cm3 density, the QDE, EAE, and spectral correlation results are promising for medical imaging applications.


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## Body

## 1. Introduction

During the last decades, the X-ray imaging chain of medical detectors has dramatically evolved [1]. The scintillator material is a crucial part in this chain, since it converts the Χ-ray flux to optical photons which in turn are captured by appropriate sensors. Research upon scintillator material processing and characterization has always been in the spotlight of the scientific community [2].Scintillators are incorporated in high-energy particle detectors, security applications, and medical modalities (tomographic or planar imaging) [1, 3–9]. In demanding applications, such as time of flight positron emission tomography (TOF-PET), detectors of rapid decay, increased density, and high light yield (LY) are necessary [10–13]. Various scintillating materials have been used so far, such as bismuth germanate (Bi4Ge3O12–BGO), lutetium oxyorthosilicate (Lu2SiO5:Ce–LSO:Ce), calcium fluoride activated with europium (CaF2:Eu), and lutetium aluminium garnet (Lu3Al5O12:Ce–LuAG:Ce) among others [14–18], with internal properties that make them compatible for various medical detector modalities. Such properties include the increased light production, the nonhygroscopic crystal lattice, and the mechanical and thermal stability [19, 20].CaF2:Eu and LuAG:Ce scintillators have been used in applications such as medical physics (TOF-PET), spectroscopy, optoelectronic solar devices, high-energy physics, astronomy, security, and low-energy radiation detectors [4, 17, 19–23].On the other hand, CeBr3 is a relatively recent scintillator that has been previously reported with a LY of 60000 photons per MeV, being much greater than the corresponding value of LuAG:Ce (25000 photons/MeV). Furthermore, it has a very short decay time (~19 ns), a density of 5.18 g/cm3, and wavelength of maximum emission at 380 nm. These properties render this material attractive, and thus, it would be worth examining its X-ray detection properties for medical applications [24–29].The above-mentioned exceptional properties of CeBr3, namely, high light output, fast response, and superior energy resolution, are shared to an extent with LaBr3:Ce, another counterpart of the same rare-earth trihalide group. As members of this group, the two materials exhibit significant orientation-dependent differences in their thermal and mechanical properties and as a result are mechanically fragile and susceptible to cleaving and cracking [27]. Characterization studies of the material have been performed by Wei et al. using the X-ray powder diffraction (XRD) method, and the crystal pattern appeared to have the hexagonal P31/c structure. From the same study, the lattice parameters resulted to be a=b=7.96Å, c=4.45Å, and the unit cell volume was 244.05 Å3 [29]. Another report confirms the material’s hexagonal structure with the Ce and Br planes bonded together [30].CeBr3 has been used in TOF-PET, in dual nuclear and fluoroscopic detectors, in gamma-ray spectrometers, in Compton scattering tomography, in extreme environments applications, in astronomy, etc. [24–26, 30–39]. Regarding its application in the field of materials processing, it has also been proposed as an alternative radiation detector for screening purposes in the steel production industry. The absence of cryogenic cooling makes the detecting system easier to maintain and reduces the associated costs [40].Drawbacks of CeBr3 include its hygroscopicity, therefore the need for being kept encapsulated, as well as its high production cost [25, 32].In this work, a10×10×10mm3 CeBr3 single crystal was examined in terms of the absorption of X-rays and the compatibility of the produced light with frequently used light sensors. The quantum detection (QDE) and the energy absorption efficiency (EAE) were determined, and the results were compared with data for CaF2:Eu and LuAG:Ce crystals.


## 2. Materials and Methods

The crystal sample was purchased from Advatech, in cubical form with all surfaces polished [41]. The optimum combination of CeBr3 crystal with various sensors, along with its efficiency to detect X-rays, was determined by calculating a series of X-ray detection properties and comparing with published data for CaF2:Eu and Lu3Al5O12:Ce crystals [42]. The calculations were performed considering simulated polyenergetic X-ray spectra, up to energies of 140 kVp and crystal thickness of 10 mm [43, 44].


### 2.1. Attenuation Coefficients for Compounds

The attenuation coefficients for compounds (materials comprised of 2 or more elements) can be determined as the weighted average (by mass) of the individual mass attenuation coefficients of the compound’s constituent elements as(1)μρcompound=∑i=1Nmiμρi,where mi is the mass fraction (fraction of the element’s mass contribution to the total mass) and μ/ρi is the mass attenuation coefficient of element i in the compound. This is important for estimating attenuation probabilities of compounds and materials that cannot be easily measured and particularly for computer simulations [42, 44].


### 2.2. Quantum Detection Efficiency

A general measure for describing the detection of X-rays is the quantum detection efficiency [45–47]. In addition, QDE is particularly useful in characterizing photon counting detectors. QDE is defined as the percentage of X-rays interacting within the scintillator body [48]. The required coefficients (total attenuation and total energy absorption) for the crystal under investigation were calculated by using data on the energy absorption and attenuation coefficients of cerium and bromine that can be found in the XmuDat photon attenuation database [49–51]. The Χ-ray QDE for polyenergetic X-rays was calculated by the equation [48]
(2)QDEE=∫0E0Ф0E1−e−μattE/ρwdE∫0E0Ф0EdE.In Equation (2), Φ0E denotes the incident X-ray spectrum (X-ray photons/area), μattE/ρ is the X-ray total mass attenuation coefficient, and W is the thickness in g/cm2 [48, 52].


### 2.3. Energy Absorption Efficiency (EAE)

This metric quantifies the amount of the X-ray energy incident on the detector and absorbed at the points of interaction, contributing to the output signal formation. EAE is considered most suitable for characterizing detectors used in energy integrating projection imaging, e.g., projection radiography. EAE can be calculated by [42, 46, 47]
(3)EAEE=∫0E0Ф0EEμenE/ρ/μattE/ρ1−e−μattE/ρwdE∫0E0Ф0EEdE,where Φ0E denotes the incident X-ray energy fluence of the incident X-ray spectrum (photons per unit area) at specific X-ray energy (E). μenE/ρ is the total energy absorption mass attenuation coefficient, representing the energy transferred by photons to secondary electrons, which is absorbed locally, i.e., very close to the interaction point.


### 2.4. Spectral Matching Factor (SMF)

During the development of an imaging detector, it is crucial to incorporate optical sensors that would optimally capture the produced optical photons. The spectral matching of the optical sensor’s response with the light emitted by crystals can be quantified by the spectral matching factor [48]
(4)SMF=∫SpλSDλdλ∫Spλdλ.In Equation (4), Sp is the optical spectrum produced by the scintillator, SD is the optical sensor’s response, and λ denotes the photon wavelength.The optical spectrum was measured with an Ocean Optics Inc. (HR2000) spectrometer, under UV excitation. A number of light detectors were considered, and the spectral data were extracted from their datasheets [53].


## 2.1. Attenuation Coefficients for Compounds

The attenuation coefficients for compounds (materials comprised of 2 or more elements) can be determined as the weighted average (by mass) of the individual mass attenuation coefficients of the compound’s constituent elements as(1)μρcompound=∑i=1Nmiμρi,where mi is the mass fraction (fraction of the element’s mass contribution to the total mass) and μ/ρi is the mass attenuation coefficient of element i in the compound. This is important for estimating attenuation probabilities of compounds and materials that cannot be easily measured and particularly for computer simulations [42, 44].


## 2.2. Quantum Detection Efficiency

A general measure for describing the detection of X-rays is the quantum detection efficiency [45–47]. In addition, QDE is particularly useful in characterizing photon counting detectors. QDE is defined as the percentage of X-rays interacting within the scintillator body [48]. The required coefficients (total attenuation and total energy absorption) for the crystal under investigation were calculated by using data on the energy absorption and attenuation coefficients of cerium and bromine that can be found in the XmuDat photon attenuation database [49–51]. The Χ-ray QDE for polyenergetic X-rays was calculated by the equation [48]
(2)QDEE=∫0E0Ф0E1−e−μattE/ρwdE∫0E0Ф0EdE.In Equation (2), Φ0E denotes the incident X-ray spectrum (X-ray photons/area), μattE/ρ is the X-ray total mass attenuation coefficient, and W is the thickness in g/cm2 [48, 52].


## 2.3. Energy Absorption Efficiency (EAE)

This metric quantifies the amount of the X-ray energy incident on the detector and absorbed at the points of interaction, contributing to the output signal formation. EAE is considered most suitable for characterizing detectors used in energy integrating projection imaging, e.g., projection radiography. EAE can be calculated by [42, 46, 47]
(3)EAEE=∫0E0Ф0EEμenE/ρ/μattE/ρ1−e−μattE/ρwdE∫0E0Ф0EEdE,where Φ0E denotes the incident X-ray energy fluence of the incident X-ray spectrum (photons per unit area) at specific X-ray energy (E). μenE/ρ is the total energy absorption mass attenuation coefficient, representing the energy transferred by photons to secondary electrons, which is absorbed locally, i.e., very close to the interaction point.


## 2.4. Spectral Matching Factor (SMF)

During the development of an imaging detector, it is crucial to incorporate optical sensors that would optimally capture the produced optical photons. The spectral matching of the optical sensor’s response with the light emitted by crystals can be quantified by the spectral matching factor [48]
(4)SMF=∫SpλSDλdλ∫Spλdλ.In Equation (4), Sp is the optical spectrum produced by the scintillator, SD is the optical sensor’s response, and λ denotes the photon wavelength.The optical spectrum was measured with an Ocean Optics Inc. (HR2000) spectrometer, under UV excitation. A number of light detectors were considered, and the spectral data were extracted from their datasheets [53].


## 3. Results and Discussion

Figure1 shows the calculated attenuation coefficients for all the examined materials, calculated following Equation (1) [49, 50]. The incident X-ray energy that is absorbed by a crystal can be described primarily by the coefficient μen which is the probability that an X-ray photon’s energy is locally transferred to electrons within the crystal lattice and secondary by μatt, which is the probability that an X-ray photon interacts within the crystal by all possible interactions. The value of μenE/ρ is always smaller than μattE/ρ, since the former excludes the energy of all secondary photons emitted as a result of an interaction by an incident photon. These secondary photons may interact further from the initial interaction point; hence, they are not useful for projection imaging. With increasing X-ray energy, the attenuation coefficients decrease, except from specific energies in which characteristic X-rays are produced (K-edge, L-edge, etc.). At these points, the probability of photoelectric interaction shows a local maximum, as it can be seen in Figure 1.Figure 1
Attenuation coefficients of CeBr3, Lu3Al5O12:Ce, and CaF2:Eu.


### 3.1. Quantum Detection Efficiency

Figure2 shows the calculated QDE values for the CeBr3 crystal and corresponding data for Lu3Al5O12:Ce and CaF2:Eu crystals. QDE values of CeBr3 were constantly equal to 1, up to 140 kVp. This was also the case for Lu3Al5O12:Ce. QDE values of CaF2:Eu decreased, starting from 40 kVp (QDE=0.991) up to 140 kVp (QDE=0.655).Figure 2
Quantum detection efficiency of CeBr3, Lu3Al5O12:Ce, and CaF2:Eu.


### 3.2. Energy Absorption Efficiency (EAE)

Figure3 shows the energy absorption values for CeBr3 along with Lu3Al5O12:Ce and CaF2:Eu crystals, against X-ray tube voltage. EAE depends on the behavior of the coefficients shown in Figure 1. CeBr3 (having a density of 5.1 g/cm3) initially showed lower EAE values than Lu3Al5O12:Ce (density 6.73 g/cm3). However, when the energy increases more than 100 kVp, the energy absorption of CeBr3 is stronger than that of both Lu3Al5O12:Ce and CaF2:Eu crystals, due to the combined effects of density and the increase in absorption that can be attributed to the K-edge. The low density of CaF2:Eu (3.18 g/cm3) is not sufficient to maintain high absorption, especially when the X-ray energy increases more than 60 kVp. From this point, CaF2:Eu shows clearly lower EAE values than CeBr3 and Lu3Al5O12:Ce. As it can be seen from Figures 2 and 3, the quantum detection values are higher than the energy absorption, since QDE considers all possible interactions, whereas EAE does not take into consideration scattered photons, bremsstrahlung, and K,L fluorescence.Figure 3
Energy absorption efficiency of CeBr3, Lu3Al5O12:Ce, and CaF2:Eu.


### 3.3. Spectral Matching Factor (SMF)

Table1 summarizes the spectral matching factor with various detectors and clearly shows that the optimum spectral match is mostly attained with flat panel position sensitive photomultipliers (PS-PMT). For example, with the H8500C-03 photomultiplier, there is a spectral matching of 99% and with the PS-PMT H8500C 97%. The compatibility is also high with multialkali (97%) and bialkali photocathodes (95%). The gallium arsenide (GaAs) photocathode provides an SMF of 94%, whereas the extended photocathode E-S20 95%. CeBr3 turns to be almost incompatible with digital sensors such as charge-coupled devices and complementary metal-oxide semiconductors, where the resulting SMF values range from 3% to 76%.Table 1
CeBr3 spectral matching with various available sensors.
Optical sensorsCeBr3Optical sensorsCeBr3CCD broadband AR coating0.76GaAsP phosphor photocathode0.35CCD infrared (IR) antireflection (AR) coating0.42Extended photocathode (E-S20)0.95CMOS hybrid blue antireflection (AR) coating0.51Si PM MicroFC-30035-SMT0.86Hybrid CMOS blue0.63Si PM MicroFB-30035-SMT0.78CMOS (monolithic 0.25μm)0.32Si PM MicroFM-100350.34a-Si:H passivated0.55Si PM S10985-050C0.86a-Si:H nonpassivated0.84Si PM S10362-11-025U0.86CCD indium tin oxide (ITO) gates, microlenses0.58Si PM S10362-11-050U0.82CCD with indium tin oxide (ITO) gates0.39Si PM S10362-11-100U0.88CCD with polygates0.03Flat panel PS-PMT H8500C-030.99CCD no polygate LoD0.19Flat panel PS-PMT H8500D-030.95CCD with traditional polygates0.20Flat panel PS-PMT H10966A0.96CMOS (photogate array 0.5)0.14Flat panel PS-PMT H8500C0.97CMOS RadEye HR0.05Bialkali photocathode0.95GaAs photocathode0.94Multialkali photocathode0.97


## 3.1. Quantum Detection Efficiency

Figure2 shows the calculated QDE values for the CeBr3 crystal and corresponding data for Lu3Al5O12:Ce and CaF2:Eu crystals. QDE values of CeBr3 were constantly equal to 1, up to 140 kVp. This was also the case for Lu3Al5O12:Ce. QDE values of CaF2:Eu decreased, starting from 40 kVp (QDE=0.991) up to 140 kVp (QDE=0.655).Figure 2
Quantum detection efficiency of CeBr3, Lu3Al5O12:Ce, and CaF2:Eu.


## 3.2. Energy Absorption Efficiency (EAE)

Figure3 shows the energy absorption values for CeBr3 along with Lu3Al5O12:Ce and CaF2:Eu crystals, against X-ray tube voltage. EAE depends on the behavior of the coefficients shown in Figure 1. CeBr3 (having a density of 5.1 g/cm3) initially showed lower EAE values than Lu3Al5O12:Ce (density 6.73 g/cm3). However, when the energy increases more than 100 kVp, the energy absorption of CeBr3 is stronger than that of both Lu3Al5O12:Ce and CaF2:Eu crystals, due to the combined effects of density and the increase in absorption that can be attributed to the K-edge. The low density of CaF2:Eu (3.18 g/cm3) is not sufficient to maintain high absorption, especially when the X-ray energy increases more than 60 kVp. From this point, CaF2:Eu shows clearly lower EAE values than CeBr3 and Lu3Al5O12:Ce. As it can be seen from Figures 2 and 3, the quantum detection values are higher than the energy absorption, since QDE considers all possible interactions, whereas EAE does not take into consideration scattered photons, bremsstrahlung, and K,L fluorescence.Figure 3
Energy absorption efficiency of CeBr3, Lu3Al5O12:Ce, and CaF2:Eu.


## 3.3. Spectral Matching Factor (SMF)

Table1 summarizes the spectral matching factor with various detectors and clearly shows that the optimum spectral match is mostly attained with flat panel position sensitive photomultipliers (PS-PMT). For example, with the H8500C-03 photomultiplier, there is a spectral matching of 99% and with the PS-PMT H8500C 97%. The compatibility is also high with multialkali (97%) and bialkali photocathodes (95%). The gallium arsenide (GaAs) photocathode provides an SMF of 94%, whereas the extended photocathode E-S20 95%. CeBr3 turns to be almost incompatible with digital sensors such as charge-coupled devices and complementary metal-oxide semiconductors, where the resulting SMF values range from 3% to 76%.Table 1
CeBr3 spectral matching with various available sensors.
Optical sensorsCeBr3Optical sensorsCeBr3CCD broadband AR coating0.76GaAsP phosphor photocathode0.35CCD infrared (IR) antireflection (AR) coating0.42Extended photocathode (E-S20)0.95CMOS hybrid blue antireflection (AR) coating0.51Si PM MicroFC-30035-SMT0.86Hybrid CMOS blue0.63Si PM MicroFB-30035-SMT0.78CMOS (monolithic 0.25μm)0.32Si PM MicroFM-100350.34a-Si:H passivated0.55Si PM S10985-050C0.86a-Si:H nonpassivated0.84Si PM S10362-11-025U0.86CCD indium tin oxide (ITO) gates, microlenses0.58Si PM S10362-11-050U0.82CCD with indium tin oxide (ITO) gates0.39Si PM S10362-11-100U0.88CCD with polygates0.03Flat panel PS-PMT H8500C-030.99CCD no polygate LoD0.19Flat panel PS-PMT H8500D-030.95CCD with traditional polygates0.20Flat panel PS-PMT H10966A0.96CMOS (photogate array 0.5)0.14Flat panel PS-PMT H8500C0.97CMOS RadEye HR0.05Bialkali photocathode0.95GaAs photocathode0.94Multialkali photocathode0.97


## 4. Conclusions

The X-ray detection and spectral compatibility properties of a10×10×10mm3 CeBr3 crystal were examined for energies up to 140 kVp for X-ray imaging applications. The compatibility of CeBr3’s emitted light with frequently used optoelectronic sensors was determined. The detection and absorption efficiency of the material was compared with corresponding data for CaF2:Eu and Lu3Al5O12:Ce crystals. The energy absorption efficiency values of CeBr3 were found higher than those of both CaF2:Eu and Lu3Al5O12:Ce when the energy increased more than 100 kVp. The quantum detection efficiency results show that CeBr3 detects totally (QDE=1) the incoming X-ray photons, across the examined energy range. The emitted optical photons of CeBr3 were found to be optimally detected by position-sensitive photomultipliers and photocathodes (maximum spectral matching of 99% and 97%, respectively). These properties, together with the high light yield (60000 photons/MeV), support the use of CeBr3 in modern medical imaging modalities, such as ultrafast computed tomography (CT) or the CT part of hybrid nuclear medicine systems, i.e., PET/CT or single photon emission tomography (SPECT)/CT.



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*Source: 7008940-2022-03-16.xml*