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

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 positionsensitive 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/cm 3 density, the QDE, EAE, and spectral correlation results are promising for medical imaging applications.


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].
On the other hand, CeBr 3 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/cm 3 , 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][25][26][27][28][29].
The above-mentioned exceptional properties of CeBr 3 , namely, high light output, fast response, and superior energy resolution, are shared to an extent with LaBr 3 :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 Å 3 [29]. Another report confirms the material's hexagonal structure with the Ce and Br planes bonded together [30]. CeBr 3 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][25][26][30][31][32][33][34][35][36][37][38][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 CeBr 3 include its hygroscopicity, therefore the need for being kept encapsulated, as well as its high production cost [25,32].
In this work, a 10 × 10 × 10 mm 3 CeBr 3 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 CaF 2 :Eu and LuAG:Ce crystals.

Materials and Methods
The crystal sample was purchased from Advatech, in cubical form with all surfaces polished [41]. The optimum combination of CeBr 3 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 CaF 2 :Eu and Lu 3 Al 5 O 12 :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].

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 where m i is the mass fraction (fraction of the element's mass contribution to the total mass) and ðμ/ρÞ i 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].

Quantum Detection Efficiency.
A general measure for describing the detection of X-rays is the quantum detection efficiency [45][46][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][50][51]. The Χ-ray QDE for polyenergetic X-rays was calculated by the equation [48] QDE In Equation (2), Φ 0 ðEÞ denotes the incident X-ray spectrum (X-ray photons/area), μ att ðEÞ/ρ is the X-ray total mass attenuation coefficient, and W is the thickness in g/cm 2 [48,52].

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] where Φ 0 ðEÞ denotes the incident X-ray energy fluence of the incident X-ray spectrum (photons per unit area) at specific X-ray energy (E). μ en ðEÞ/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.

Spectral Matching Factor (SMF
In Equation (4), S p is the optical spectrum produced by the scintillator, S D 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]. Figure 1 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 μ en which is the probability that an X-ray photon's energy is locally transferred to electrons within the crystal lattice and secondary by μ att , which is the probability that an X-ray photon interacts within the crystal by all possible interactions. The value of μ en ðEÞ/ρ is always smaller than μ att ðEÞ/ 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 3 shows the energy absorption values for CeBr 3 along with Lu 3 Al 5 O 12 :Ce and CaF 2 :Eu crystals, against X-ray tube voltage. EAE depends on the behavior of the coefficients shown in Figure 1. CeBr 3 (having a density of 5.1 g/cm 3 ) initially showed lower EAE values than Lu 3 Al 5 O 12 :Ce (density 6.73 g/cm 3 ). However, when the energy increases more than 100 kVp, the energy absorption of CeBr 3 is stronger than that of both Lu 3 Al 5 O 12 :Ce and CaF 2 :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 CaF 2 :Eu (3.18 g/cm 3 ) is not sufficient to maintain high absorption, especially when    3.3. Spectral Matching Factor (SMF). Table 1 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%. CeBr 3 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%.

Conclusions
The X-ray detection and spectral compatibility properties of a 10 × 10 × 10 mm 3 CeBr 3 crystal were examined for energies up to 140 kVp for X-ray imaging applications. The compatibility of CeBr 3 's emitted light with frequently used optoelectronic sensors was determined. The detection and absorption efficiency of the material was compared with corresponding data for CaF 2 :Eu and Lu 3 Al 5 O 12 :Ce crystals. The energy absorption efficiency values of CeBr 3 were found higher than those of both CaF 2 :Eu and Lu 3 Al 5 O 12 :Ce when the energy increased more than 100 kVp. The quantum detection efficiency results show that CeBr 3 detects totally (QDE = 1) the incoming X-ray photons, across the examined energy range. The emitted optical photons of CeBr 3 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 CeBr 3 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.

Data Availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.