Luminescence E ﬃ ciency of Cadmium Tungstate (CdWO 4 ) Single Crystal for Medical Imaging Applications

: Background: In this study, the light output of a cadmium tungstate (CdWO 4 ) single crystal was measured under various X-ray radiographic energies. Methods: A CdWO 4 single crystal (10 × 10 × 10 mm 3 ) was exposed to X-rays in the 50–130 kVp range. Measurements were evaluated against published data for single crystals of equal dimensions (CaF 2 :Eu and Lu 3 Al 5 O 12 :Ce). Since the crystal was examined for application in medical imaging detectors, the emitted optical spectrum was classiﬁed with respect to the spectral compatibility of numerous commercial optical sensors. Results: The luminescence e ﬃ ciency (LE) was found to constantly increase with X-ray energy and was higher than that of CaF 2 :Eu for energies above 90 kVp. However, the e ﬃ ciency of the previously published Lu 3 Al 5 O 12 :Ce was found to be constantly higher than that of CdWO 4 . The light emitted from CdWO 4 can be optimally detected by certain charge-coupled devices (CCDs), amorphous silicon photodiodes, and photocathodes. Conclusions: The high density (7.9 g / cm 3 ) of CdWO 4 and the luminescence signal of this material make it suitable for medical imaging (such as dual energy), high-energy physics or for applications of scintillators in harsh environments.


Introduction
Cadmium tungstate (CdWO 4 or CWO) is a scintillator with a long history as an energy converting medium. CdWO 4 attracted scientists' attention due to its unique internal properties (Table 1) [1]. The first scientist who investigated properties regarding luminescence for this material was Kroger [2]. CdWO 4 is one of the denser scintillating materials (ρ = 7.9 g/cm 3 ), with one of the shortest radiation lengths of 10.6 mm and exceeds about 30% to 50% of the light output of sodium iodide doped with thallium (NaI:Tl) [2][3][4][5][6][7][8][9][10][11][12]. The attenuation coefficients of this material are very high, and the light yield has been reported to range from 6200 to 28,000 photons/MeV depending upon the crystal defects [6,8,[13][14][15][16][17][18]. The emission of CdWO 4 covers a wide band with the main peak at 490 nm, which is ideal for coupling with a variety of sensors, such as photomultiplier tubes (PMTs) and silicon photodiodes (Si) [4,7,[19][20][21][22]. The reported energy resolution varies from 7.5% to 8.5% using To this aim, the output signal of CdWO 4 under X-ray excitation was recorded and compared with data for other materials (CaF 2 :Eu and Lu 3 Al 5 O 12 :Ce), of the same dimensions and thickness (10 mm × 10 mm × 10 mm). CdWO 4 was selected (i) because it had a light yield (LY) value of 28 photons/keV, higher than that of Lu 3 Al 5 O 12 :Ce (25 photons/keV) which was recently examined by our group [47] and was found to have high luminescence efficiency values, and (ii) due to the fact that it is a cheap material [44,45]. The spectrum of emitted light was also studied for its compatibility with the optical sensitivities of various optical sensors.

Materials and Methods
For the experiments, a CdWO 4 single crystal was used (Advatech) [45] with dimensions of 10 mm × 10 mm × 10 mm. The surfaces of the crystal were polished. The X-ray tube (BMI, Merate, Curno, Italy) that was used for the X-ray measurement was operated in the voltage range from 50 to 130 kVp. Crystals 2020, 10, 429 3 of 16 In the output of the X-ray tube, an aluminum filter was added (thickness 20 mm) in order to consider the attenuation of the incident X-ray spectrum by a typical human chest [48].

Output Luminescence Signal
During X-ray excitation, the energy flux (power per unit of area) of luminescence light emitted by the crystal was measured by placing the crystal sample in the input port of a sphere that integrates the incident light, in order to correct for crystal light output spatial and angular distribution irregularities (Oriel 70451) [49]. The light at the output port was collected by a PMT connected to an electric current meter (Model 6430 Keithley Instruments Inc., Cleveland, OH, USA) [50]. Light energy flux data were divided by the corresponding incident X-ray exposure rate ( . X) data (at each tube voltage) directly measured via an RTI Piranha P100B wireless digital multimeter. By combining the energy flux of light ( . Ψ λ ) (emitted by the crystal) with the exposure rate incident on the detector, the output signal luminescence efficiency (LE) of the sample can be obtained using the following Equation (1): where η A (kVp) is LE at a particular X-ray tube voltage, expressed in efficiency units (EU) (µW × m −2 /(mR × s −1 ).

Scintillator/Sensor Spectral Matching and Effective Luminescence Efficiency
Every optical sensor can detect to a different degree the optical photons produced by a scintillator. The degree of overlapping between the spectral response SD(λ) of the sensor and the optical spectrum distribution of photon fluence φ L (λ) produced by the scintillator can be quantified by a factor expressing the spectral matching, as follows (2) [51,52]: A high-resolution HR2000 spectrometer (Ocean Optics Inc., Largo, FL, USA) was used to measure crystals' light, excited at 312 nm by a Vilber Lourmat (VL-215M, Paris, France) ultraviolet (UV) lamp. The spectral response of various optical detectors that are used in a large number of medical applications was obtained from manufacturers' data [53][54][55][56]. The effective luminescence efficiency (ηe f f ) between the output signal of the scintillator and the spectral response of various sensors was estimated by multiplying the luminescence efficiency with the spectral matching factor [51].

Energy-Absorption Efficiency (EAE)
The scintillator's general detection and absorption characteristics can be quantified using the quantum detection efficiency (QDE) and the energy absorption efficiency (EAE). QDE is the fraction of incident photons attenuated by the detector material and is traditionally used to evaluate detection properties of all detector types; however, it is more suitable for photon-counting devices (e.g., those used in nuclear medicine). On the other hand, in most applications of X-ray detection, the output signal is proportional to the total energy absorbed in the detector material. Thus, EAE, being equal to the fraction of energy deposited within the scintillator mass [51,52], is of particular interest. To calculate EAE, the ratio of the mass energy absorption coefficient over the total mass attenuation coefficient is required: Φ 0 (E)E is the X-ray photon fluence (photons per unit of area) incident on the scintillator, multiplied by the corresponding energy (E), giving the incident X-ray energy fluence. µ tot,t (E)/ρ is the X-ray total mass attenuation and µtot, en(E)/ρ is the total mass energy absorption coefficient. W denotes crystal thickness.
The coefficient of energy absorption represents the average fraction of the kinetic energy of secondary charged particles that is locally deposited in the detector mass [57]. The coefficients of total attenuation and energy absorption of CdWO 4 were obtained from the National Institute of Standards and Technology (NIST) data regarding cadmium (Cd), tungsten (W) and oxygen (O) using the XmuDat photon attenuation database software [57][58][59][60].

Results
The variation of the CdWO 4 's LE with X-ray energy is shown in Figure 1, for X-rays from 50 to 130 kVp. Results were compared with data for CaF 2 :Eu and Lu 3 Al 5 O 12 :Ce crystals (10 × 10 × 10 mm 3 ) [46,47].
The LE values showed a tendency to increase, with both CdWO 4 and Lu 3 Al 5 O 12 :Ce crystals within the examined energy range following a similar trend. However, for the examined samples (preparation method, impurity levels, etc.), CdWO 4 values were in all cases lower than those of Lu 3 Al 5 O 12 :Ce, from 9.65% to 16.95%. On the other hand, CaF 2 :Eu showed a plateau at 80 kVp (luminescence efficiency equal to 22.22 Efficiency Units-E.U.) and decreased thereafter. This may be explained by taking into account the effect of significant and continuous decrease of the absorption efficiency of this material with increasing peak voltage. This in turn is due to the low density and low effective atomic number of CaF 2 :Eu resulting in low light yield, when interacting with higher energy photons [46,61]. To calculate EAE, the ratio of the mass energy absorption coefficient over the total mass attenuation coefficient is required: Ε is the X-ray photon fluence (photons per unit of area) incident on the scintillator, multiplied by the corresponding energy (E), giving the incident X-ray energy fluence.
is the X-ray total mass attenuation and , ( )/ tot en E μ ρ is the total mass energy absorption coefficient. W denotes crystal thickness.
The coefficient of energy absorption represents the average fraction of the kinetic energy of secondary charged particles that is locally deposited in the detector mass [57]. The coefficients of total attenuation and energy absorption of CdWO4 were obtained from the National Institute of Standards and Technology (NIST) data regarding cadmium (Cd), tungsten (W) and oxygen (O) using the XmuDat photon attenuation database software [57][58][59][60].

Results
The variation of the CdWO4′s LE with X-ray energy is shown in Figure 1, for X-rays from 50 to 130 kVp. Results were compared with data for CaF2:Eu and Lu3Al5O12:Ce crystals (10 × 10 × 10 mm 3 ) [46,47]. The LE values showed a tendency to increase, with both CdWO4 and Lu3Al5O12:Ce crystals within the examined energy range following a similar trend. However, for the examined samples (preparation method, impurity levels, etc.), CdWO4 values were in all cases lower than those of Lu3Al5O12:Ce, from 9.65% to 16.95%. On the other hand, CaF2:Eu showed a plateau at 80 kVp (luminescence efficiency equal to 22.22 Efficiency Units-E.U.) and decreased thereafter. This may be explained by taking into account the effect of significant and continuous decrease of the absorption efficiency of this material with increasing peak voltage. This in turn is due to the low density and low effective atomic number of CaF2:Eu resulting in low light yield, when interacting with higher energy photons [46,61].   Figure 2 shows the light output curves of CdWO 4 , CaF 2 :Eu, and Lu 3 Al 5 O 12 :Ce single crystals, relating the light output signal with the incident X-ray exposure rate. A linear response can be depicted between the output LE and incident exposure in the examined range of exposures, with R 2 values higher than 0.9974 in all cases. The three curves followed the same trend with the luminescence efficiency results, in which CdWO 4 showed high light output values across the examined range, close to those of Lu 3 Al 5 O 12 :Ce.
Crystals 2020, 10, 429 5 of 16 Figure 2 shows the light output curves of CdWO4, CaF2:Eu, and Lu3Al5O12:Ce single crystals, relating the light output signal with the incident X-ray exposure rate. Α linear response can be depicted between the output LE and incident exposure in the examined range of exposures, with R 2 values higher than 0.9974 in all cases. The three curves followed the same trend with the luminescence efficiency results, in which CdWO4 showed high light output values across the examined range, close to those of Lu3Al5O12:Ce.   [53][54][55][56]. The CdWO4 spectrum shows the main luminescence peak at 490 nm (2.54 eV), associated with transitions in the tungstate group (WO6) and a low intensity peak at about 595 nm (2.09 eV) [14,36,62].    [53][54][55][56]. The CdWO 4 spectrum shows the main luminescence peak at 490 nm (2.54 eV), associated with transitions in the tungstate group (WO 6 ) and a low intensity peak at about 595 nm (2.09 eV) [14,36,62].
Crystals 2020, 10, 429 5 of 16 Figure 2 shows the light output curves of CdWO4, CaF2:Eu, and Lu3Al5O12:Ce single crystals, relating the light output signal with the incident X-ray exposure rate. Α linear response can be depicted between the output LE and incident exposure in the examined range of exposures, with R 2 values higher than 0.9974 in all cases. The three curves followed the same trend with the luminescence efficiency results, in which CdWO4 showed high light output values across the examined range, close to those of Lu3Al5O12:Ce.   [53][54][55][56]. The CdWO4 spectrum shows the main luminescence peak at 490 nm (2.54 eV), associated with transitions in the tungstate group (WO6) and a low intensity peak at about 595 nm (2.09 eV) [14,36,62].          Table 2 shows spectral matching factor (SMF) values of the CdWO 4 with light sensors, such as PMT photocathodes, with position sensitive photomultipliers (PS-PMTs), silicon PMT (SiPMs), etc. The light emitted by the CdWO 4 scintillator shows an 85% overlap with the (E-S20) photocathode (SMF = 0.85) that was used in our experimental setup. This value was considered for correcting the luminescence efficiency measurements. CdWO 4 light makes a perfect match with CCD sensors (SMF = 0.97) and also with hydrogenated amorphous silicon (a-Si:H) photodiodes (SMF = 0.97). Furthermore, the SMF with photomultiplier photocathodes was also excellent, showing an SMF value of 0.96 with a gallium arsenide photocathode (GaAs).  Table 2 shows spectral matching factor (SMF) values of the CdWO4 with light sensors, such as PMT photocathodes, with position sensitive photomultipliers (PS-PMTs), silicon PMT (SiPMs), etc. The light emitted by the CdWO4 scintillator shows an 85% overlap with the (E-S20) photocathode (SMF = 0.85) that was used in our experimental setup. This value was considered for correcting the luminescence efficiency measurements. CdWO4 light makes a perfect match with CCD sensors (SMF = 0.97) and also with hydrogenated amorphous silicon (a-Si:H) photodiodes (SMF = 0.97). Furthermore, the SMF with photomultiplier photocathodes was also excellent, showing an SMF value of 0.96 with a gallium arsenide photocathode (GaAs).
CdWO4 also showed good compatibility with a monolithic CMOS sensor (SMF = 0.84) and a high resolution RadEye CMOS (SMF = 0.82), used in medical and industrial radiography systems. With silicon photomultipliers, it showed SMF values in the range from 0.66 to 0.88 (MicroFM-10035 SMF = 0.88).    used, having maximum detection efficiency in lower wavelengths (Figures 3, 4, 7 and 8), CdWO 4 shows higher SMF values, (ranging from 10.13% to 48.28%) compared to Lu 3 Al 5 O 12 :Ce. This in turn results in higher effective efficiency (EE), showing the importance of this factor when designing an optical detection system. In detail, the increase (with kVp) in the detected luminescence signal ranged from 26.47% to 34.91% when CdWO 4 was coupled with the H10966A PS-PMT flat panel position sensitive photomultiplier, from 23.53% to 31.78% when CdWO 4 was coupled with the bi-alkali photocathode, and from 8.52% to 15.55% when CdWO 4 was coupled with the MicroFB-30035-SMT silicon photomultiplier, with respect to the corresponding values when these optical sensors were coupled to Lu 3 Al 5 O 12 :Ce [47].

GaAs Photocathode
0.96 Photocathode Multi-alkali 0.64 Figures 7-10 show LE values of the CdWO4 as was effectively degraded by various optical detectors. These values are always less than the nominal light efficiency, since the matching of the light emitted by the examined crystals is registered differently by the various photodetectors due to their inherent light photon sensitivity. The optimum values for this study were obtained when CdWO4 was coupled with a CCD with broadband AR coating and with an a-Si:H (non-passivated) photodiode. When silicon, or flat panel position sensitive photomultipliers, bi-or multi-alkali photocathodes are used, having maximum detection efficiency in lower wavelengths (Figures 3, 4, 7, and 8), CdWO4 shows higher SMF values, (ranging from 10.13% to 48.28%) compared to Lu3Al5O12:Ce. This in turn results in higher effective efficiency (EE), showing the importance of this factor when designing an optical detection system. In detail, the increase (with kVp) in the detected luminescence signal ranged from 26.47% to 34.91% when CdWO4 was coupled with the H10966A PS-PMT flat panel position sensitive photomultiplier, from 23.53% to 31.78% when CdWO4 was coupled with the bi-alkali photocathode, and from 8.52% to 15.55% when CdWO4 was coupled with the MicroFB-30035-SMT silicon photomultiplier, with respect to the corresponding values when these optical sensors were coupled to Lu3Al5O12:Ce [47].      shows a tendency to increase up to 70 kVp (EAE = 0.73), and after 80 kVp, it shows higher EAE values than both CaF2:Eu and Lu3Al5O12:Ce crystals. The CaF2:Eu crystal, having the lowest density value of 3.18 g/cm 3 , shows a clear tendency to decrease in the whole examined energy range. The higher density of CdWO4 (CdWO4 d = 7.9 g/cm 3 , Lu3Al5O12:Ce d = 6.73 g/cm 3 ) and the influence of the K-edge of the material (Figure 11 inset) contribute to the higher overall energy absorption efficiency values after 80 kVp.
Taking into consideration the absorption properties of CdWO4, it would be of interest to mention previous relevant studies and possible applications. When high X-ray energies are considered, it has been previously demonstrated that for dual-energy applications, such as in material recognition imaging, the CsI-CdWO4 dual detector combination performed sufficiently well when CdWO4 acted as the high-energy component of the detector [63]. In diagnostic radiology X-rays, CdWO4 has been  crystals are used, thickness separators, placed between the crystals, may be utilized to prevent the effect of internal light scattering and crosstalk. These separators should be thin and possess good reflectivity and low transparency [15]. A thin aluminum foil of 0.01 mm thickness has been reported to reduce light scatter [78]. In addition, 2D back illuminated photodiode arrays have been reported to adequately meet CT performance characteristics, including the effect of crosstalk, when large area CT detection systems are considered [79].  Taking into consideration the absorption properties of CdWO 4 , it would be of interest to mention previous relevant studies and possible applications. When high X-ray energies are considered, it has been previously demonstrated that for dual-energy applications, such as in material recognition imaging, the CsI-CdWO 4 dual detector combination performed sufficiently well when CdWO 4 acted as the high-energy component of the detector [63]. In diagnostic radiology X-rays, CdWO 4 has been studied as part of a Gd 2 O 2 S-CdWO 4 combination detector by Juste et al. [64] in the energy range from 10 to 160 keV and has been reported to exhibit adequate contrast between the low and the high-energy dual-energy spectral component. In addition, the ZnSe/CdWO combination has also been theoretically studied by Grinyov et al. [65] in 80-140 kV tube voltages. Finally, the two X-ray absorption edges of CdWO 4 at approximately 30 and 75 keV make it a suitable candidate for a dual-energy radiation detector either for single or double shot technique [66][67][68]. For single shot applications where the incident X-ray spectrum could be an 80 kVp filtered by 900 µm cerium (Ce) or a 90 kVp spectrum filtered with 2500 µm barium (Ba), which are presenting exposure peaks at the aforementioned energies, CdWO 4 would perform rather well [66][67][68][69]. Dual-energy applications are currently used in computed tomography (CT) systems in clinical practice. These can be regarded as either dual-energy or dual-source irradiation [70][71][72]. The X-ray tube voltages used are 80 kV (low-energy component) and 140 kV (high-energy component) [70][71][72], to satisfy the range of CT tube voltages [73]. As we can observe from Figure 1, at 80 kV, CdWO 4 is comparable to the CaF 2 scintillator, while for the higher kV energies, its efficiency characteristics are superior and present an increasing tendency. This may lead to improved signal to noise ratio (SNR) for the dual-energy CT high X-ray tube voltages, contributing to dose savings. The improved efficiency in high kV of CdWO 4 can be further exploited in dual-energy CT with a contrast agent [70,72], where in the low-energy region, the contrast-to-noise ratio is mainly affected by the contrast agent X-ray absorption characteristics, while in the high-energy part, a sensitive image detector can contribute to noise minimization through adequate dose management.
Dual-energy chest radiography improves the diagnostic value of an X-ray by separating soft tissue from bones, producing two different images using either a single or a double exposure technique. In the single exposure technique, two phosphor plates are exposed to X-rays, separated by a copper filter. The front plate receives the unfractionated beam and produces a standard chest X-ray [74]. The high-energy photons are received by the back plate, while the lower energy photons are removed due to the front plate and copper filter. In the dual exposure technique [74][75][76], two images are obtained at different kVs, mainly at 60 and 120 kV [74,75], for the low and high energy, respectively. The resulting images are subtracted to produce the tissue-selective and bone-selective images. Although dual energy requires higher radiation doses, higher diagnostic accuracy can be achieved [74]. The higher X-ray energy absorption characteristics of CdWO 4 above 80 kV, as shown in Figure 11, make it a promising candidate for DE X-ray imaging that can contribute to dose saving when the high X-ray energy component is considered.
The use of crystalline scintillators in large detector arrays should address the problem of internal light scattering and crosstalk at the crystal edges [15,77]. The effect should be considered in positron emission tomography (PET) imaging and is affected by the photon energy [77]. When separated crystals are used, thickness separators, placed between the crystals, may be utilized to prevent the effect of internal light scattering and crosstalk. These separators should be thin and possess good reflectivity and low transparency [15]. A thin aluminum foil of 0.01 mm thickness has been reported to reduce light scatter [78]. In addition, 2D back illuminated photodiode arrays have been reported to adequately meet CT performance characteristics, including the effect of crosstalk, when large area CT detection systems are considered [79].
The differences between CdWO 4 and Lu 3 Al 5 O 12 :Ce in the tube voltage range of 40-70 kVp ( Figure 11) can be explained by taking into account the ratios of the mass energy absorption coefficients over the total mass attenuation coefficients that are used for the EAE calculation, see Equation (3) and Figure 12. This range corresponds to energies of about 34-51 keV. From Figure 12, it can be seen that in the 34-51 keV energy range, the ratio of the mass energy absorption over the mass attenuation coefficients for Lu 3 Al 5 O 12 :Ce is higher that the corresponding ratio of CdWO 4 , despite the fact that the absolute values of mass energy absorption and the mass attenuation coefficients of CdWO 4 are higher in this range. Thereafter, the ratio of the coefficients for CdWO 4 is higher than that of Lu 3 Al 5 O 12 :Ce, and this is reflected in EAE results of Figure 11. By contrast, CaF 2 :Eu starts with competitive values, in the low-energy range, but thereafter, the ratio of the coefficients is lower than for the other two materials, also resulting in lower EAE properties. The differences between CdWO4 and Lu3Al5O12:Ce in the tube voltage range of 40-70 kVp ( Figure 11) can be explained by taking into account the ratios of the mass energy absorption coefficients over the total mass attenuation coefficients that are used for the EAE calculation, see Equation (3) and Figure 12. This range corresponds to energies of about 34-51 keV. From Figure 12, it can be seen that in the 34-51 keV energy range, the ratio of the mass energy absorption over the mass attenuation coefficients for Lu3Al5O12:Ce is higher that the corresponding ratio of CdWO4, despite the fact that the absolute values of mass energy absorption and the mass attenuation coefficients of CdWO4 are higher in this range. Thereafter, the ratio of the coefficients for CdWO4 is higher than that of Lu3Al5O12:Ce, and this is reflected in EAE results of Figure 11. By contrast, CaF2:Eu starts with competitive values, in the low-energy range, but thereafter, the ratio of the coefficients is lower than for the other two materials, also resulting in lower EAE properties.

Conclusions
The output luminescence signal of a CdWO4 single crystal was examined within the radiographic energy region for possible applications in novel radiographic applications (such as dual

Conclusions
The output luminescence signal of a CdWO 4 single crystal was examined within the radiographic energy region for possible applications in novel radiographic applications (such as dual energy). With this aim, the spectral compatibility with various commercial sensors was also considered. CaF 2 :Eu and Lu 3 Al 5 O 12 :Ce crystals of equal dimensions, were used in order to be compared with CdWO 4 . The maximum luminescence efficiency was obtained at the maximum examined X-ray energy (130 kVp) since the output of the crystal constantly increased in the examined energy range. The emitted optical photons of CdWO 4 were found to be optimally detected by charge-coupled devices and amorphous hydrogenated silicon photodiodes employed in medical flat panel detectors. These values are perfectly matched for various imaging and non-imaging applications, such as dual-energy imaging, high-energy physics detectors, or applications of scintillators in harsh environments.

Conflicts of Interest:
The authors declare no conflict of interest.