Light Output Response of a Barium Fluoride (BaF₂) Inorganic Scintillator Under X-Ray Radiation

Abstract

In this study, the luminescence efficiency of a crystal-form barium fluoride (BaF₂) inorganic scintillator was assessed for medical imaging applications. For the experiments, we used a typical medical X-ray tube (50–140 kVp) for estimating the absolute luminescence efficiency (AE). Furthermore, we examined the spectral matching of the inorganic scintillator with a series of optical detectors. BaF₂ showed a higher AE than cerium fluoride (CeF₃), comparable to that of commercially available bismuth germanate (Bi₄Ge₃O₁₂-BGO), but lower than that of the gadolinium orthosilicate (Gd₂SiO₅:Ce-GSO:Ce) inorganic scintillator. The maximum AE of BaF₂ was 2.36 efficiency units (EU is the S.I. equivalent μWm⁻²/(mR/s) at 140 kVp, which is higher than that of the corresponding fluoride-based CeF₃ (0.8334 EU)) at the same X-ray energy. GSO:Ce and BGO crystals, which are often integrated in commercial positron emission tomography (PET) scanners, had AE values of 7.76 and 3.41, respectively. The emission maximum (~310 nm) of BaF₂ is adequate for coupling with flat-panel position-sensitive (PS) photomultipliers (PMTs) and various photocathodes. The luminescence efficiency results of BaF₂ were comparable to those of BGO; thus, it could possibly be used in medical imaging modalities, considering its significantly lower cost.

1. Introduction

Inorganic scintillators are useful for converting X-rays to light in a variety of applications, including medical imaging [1]. For every application, there are scintillators that can cover some—or all—necessary requirements regarding luminescence performance, speed, resolution, etc. Such an example is barium fluoride (BaF₂), which is an inorganic scintillator that balances the aforementioned properties, especially for applications requiring fast materials (Table 1). Barium fluoride has been studied in the past and continues to attract the attention of the research community, especially with the progress of modern electronics, in areas including nuclear physics, medical imaging, and particle detection, among others [2,3,4,5]. Barium fluoride crystals have a fast decay component, due to cross-luminescence, at around 0.6–0.87 ns, and a slow one, due to self-trapped exciton (STE) emission, at around 620–630 ns (Table 1) [5,6,7,8,9,10,11]. The maximum emission of these two components is within the ultraviolet (UV) range of 310 nm (slow) to 225 nm (fast) [2,4,6,7]; as such, radiation sensors sensitive in the UV range, such as silicon photomultipliers (SiPMs), are required [5,12,13,14,15,16,17,18]. Despite this drawback, the fast decay of BaF₂ improves coincidence timing resolution (CTR), rendering it suitable for applications requiring a fast response, such as time-of-flight (ToF) positron emission tomography (PET) and time-of-flight computed tomography (TOF-CT) systems [5,11,12,19,20]. In medical imaging, and especially in applications such as TOF-PET, conventional BaF₂ is an important material since it provides a rapid temporal resolution [5,12,21]. Barium fluoride has been proposed as a possible alternative to crystals with significantly higher costs, such as lutetium–yttrium oxyorthosilicate doped with cerium (LYSO:Ce) or bismuth germanate (Bi₄Ge₃O₁₂-BGO) and gadolinium orthosilicate (Gd₂SiO₅:Ce-GSO:Ce) [5,11,22,23,24,25,26,27,28].

In addition to its fast decay time, the mechanical properties of BaF₂ (radiation hardness, stability, non-hygroscopicity) make it suitable for environmental and high-energy physics applications [2,4,29,30,31,32,33]. Due to its sub-nanosecond decay time, which is comparable even to emerging metal halide perovskites [34,35,36], and high absorption efficiency for hard X-rays, yttrium-doped barium fluoride and the ultrafast (0.5 ns decay time) gallium-doped zinc oxide (ZnO) were recently selected for state-of-the-art gigahertz (GHz) hard X-ray imaging applications [33,37,38]. Barium fluoride’s crystal structure is cubic (under environmental conditions) with an fm3m (C1) space group [39,40]. The crystal lattice is formed from three intersecting (face-centered) cubic sub-lattices (one for the barium ions and two for the fluorine) [40]. This lattice structure is beneficial for the transparency of BaF₂ across the wavelength range, from UV to infrared (IR), making it suitable for optic components in infrared thermographic monitoring or near-mid-infrared optical coherence (OCT) tomography applied in the non-destructive testing (NDT) of industrial and marine coatings [41,42]. The band gap is up to 10 eV [31,39]. The lattice changes from cubic to orthorhombic phases (space group Pnma (C23)) at pressures above 3 GPa. For pressures above 10 GPa, it transforms to a hexagonal phase (P6₃mmc (B8_b)) [40,43]. The lattice parameters and the consequent luminescence efficiency were found to depend on particle size [31]. The optical and structural characteristics of BaF₂ are useful in high-performance scintillation applications.

Barium fluoride has recently been reported to be useful for various applications, including in nuclear research and real-time dose monitoring in radiation therapy [12,44,45,46]. Furthermore, BaF₂ has been studied in the form of a composite inorganic scintillator for large area imaging [47,48,49] and water-dispersed ultra-small BaF₂ nano-fluorides have been introduced as imaging agents in magnetic resonance imaging (MRI) [50].

In this work, through a series of experiments, we determined the luminescence efficiency and spectral compatibility of a BaF₂ inorganic scintillator with various optical sensors used in medical imaging applications.

Measurements were performed using broad-spectrum X-rays employed in medical diagnostic imaging. The luminescence measurements aimed to determine the absolute luminescence efficiency (AE) under clinical conditions, which is a robust measure for comparing fluorescent materials. In addition, the spectrally useful response was systematically investigated. The results were compared with the inorganic crystal scintillators frequently used in commercial medical imaging systems, i.e., gadolinium orthosilicate (Gd₂SiO₅:Ce-GSO) and bismuth germanate (Bi₄Ge₃O₁₂-BGO), as well as with another fluoride-based scintillator, i.e., cerium fluoride (CeF₃) [51].

Table 1. Properties of BaF₂, CeF₃, BGO, and GSO:Ce single crystals.

	Units	BaF2	CeF3	BGO	GSO:Ce
Wavelength of max emission	nm	310 (slow), 225 (fast) [6,7]	340 (slow), 300–310 (fast) [6,52,53]	480 [6,54,55]	445 [56]
Emission wavelength range	nm	170–460 [8]	250–425 [53]	375–650 [55]	370–560 [56]
Decay times	ns	620–630 (slow), 0.87 (fast) [6,7,8,9,10]	30 (slow), 8 (fast) [52]	300 [6,54,56]	30–60 [9,57,58,59,60]
Timing resolution	ps @ FWHM	80–500 [9,61,62,63,64,65,66,67,68]	522–2000 [6,69]	2500–6000 [6,9]	965 [9,70]
Light yield	photons/MeV	1500 [34]	4.4 × 103 [52]	8.2–8.9 × 103 [9,54]	8–10 × 103 [9,57]
Photoelectron yield	% of NaI:Tl	16 (slow) 3–5 (fast) [6,7,8]	3–10 [6,7,53,55]	15–20 [55]	20 [57,58,59]
Radiation length	cm	2.1 [6,59]	1.654 [6,53,59]	1.12 [6,9,59,71]	1.38 [59]
Refractive index @ max nm		1.57 @ 310 nm [6,7]	1.62–1.68 @ 440 nm [7,52]	2.15 @ 480 nm [6,72]	1.85 [57]
Density	g/cm3	4.87 [6,7,8]	6.16 [6,7,52]	7.13 [6,7,54]	6.7 [57]
Melting point	°C	1386 [8]	1443–1450 [52,73,74]	1050 [55]	1626 [57]
Mechanical hardness	mohs	3 [8]	4.5–5 [73]	5 [75]	5.7 [57]
Radiation hardness	Rad	106–107 [59]	>106 [59,76]	104–107 [59,77]	106–109 [57,58,59]
Hygroscopic		No [6]	No [52]	No [54]	No [59]

Table 1. Properties of BaF₂, CeF₃, BGO, and GSO:Ce single crystals.

	Units	BaF2	CeF3	BGO	GSO:Ce
Wavelength of max emission	nm	310 (slow), 225 (fast) [6,7]	340 (slow), 300–310 (fast) [6,52,53]	480 [6,54,55]	445 [56]
Emission wavelength range	nm	170–460 [8]	250–425 [53]	375–650 [55]	370–560 [56]
Decay times	ns	620–630 (slow), 0.87 (fast) [6,7,8,9,10]	30 (slow), 8 (fast) [52]	300 [6,54,56]	30–60 [9,57,58,59,60]
Timing resolution	ps @ FWHM	80–500 [9,61,62,63,64,65,66,67,68]	522–2000 [6,69]	2500–6000 [6,9]	965 [9,70]
Light yield	photons/MeV	1500 [34]	4.4 × 103 [52]	8.2–8.9 × 103 [9,54]	8–10 × 103 [9,57]
Photoelectron yield	% of NaI:Tl	16 (slow) 3–5 (fast) [6,7,8]	3–10 [6,7,53,55]	15–20 [55]	20 [57,58,59]
Radiation length	cm	2.1 [6,59]	1.654 [6,53,59]	1.12 [6,9,59,71]	1.38 [59]
Refractive index @ max nm		1.57 @ 310 nm [6,7]	1.62–1.68 @ 440 nm [7,52]	2.15 @ 480 nm [6,72]	1.85 [57]
Density	g/cm3	4.87 [6,7,8]	6.16 [6,7,52]	7.13 [6,7,54]	6.7 [57]
Melting point	°C	1386 [8]	1443–1450 [52,73,74]	1050 [55]	1626 [57]
Mechanical hardness	mohs	3 [8]	4.5–5 [73]	5 [75]	5.7 [57]
Radiation hardness	Rad	106–107 [59]	>106 [59,76]	104–107 [59,77]	106–109 [57,58,59]
Hygroscopic		No [6]	No [52]	No [54]	No [59]

2. Results

Figure 1 shows indicative diagnostic X-ray spectra, measured with a portable cadmium telluride (CdTe) solid-state detector Amptek XR-100T X-ray spectrometer (Bedford, MA, USA). The detector was calibrated for energy scales, linearity, and energy resolution using ¹²⁵I and ^99mTc gamma ray calibration sources [78]. Pile-up distortions were minimized using a Tungsten spacer collimation system (EXVC-W-Spacer) with a thickness of 36 mm and a diameter of 300 $\mathsf{\mu }\mathrm{m}$ for X-rays greater than 100 keV. The measured X-ray spectra were corrected for CdTe detector efficiency.

Figure 2 associates the signal (μW/m²) of a BaF₂ inorganic scintillator with the X-ray exposure levels that were used for excitation. The signal data are compared with materials commonly used in PET scanners, such as Gd₂SiO₅:Ce and Bi₄Ge₃O₁₂ inorganic scintillators. Furthermore, results are presented for another fluoride inorganic scintillator, the CeF₃, under similar irradiation conditions [79]. As can be seen in Figure 2, output signal and exposure rate show an almost linear relation from 0 to 372 mR/s. The output signal of BaF₂ is considerably higher than that of CeF₃. However, in the examined energy range, the output signals of BGO and GSO:Ce, which are commercially available inorganic crystals in various medical imaging modalities, are greater than both the fluoride scintillators.

The experimentally assessed AE results are shown in Figure 3. Results are shown for BaF_2, Gd₂SiO₅:Ce, and Bi₄Ge₃O₁₂, as well as for the CeF₃ fluoride inorganic scintillator. Luminescence results are shown with increments of 10 kVp in the range of 50 to 140 kVp.

In the examined X-ray energy range, the efficiency of all crystal samples increases with energy, especially after the X-ray energy exceeds the K-edges of all the materials. When the X-ray photon energy exceeds the K-edge, a characteristic saw-tooth-shaped discontinuity is observed in the total mass attenuation coefficient. These discontinuities correspond to the absorption edges K, L, M, or N, in which the energies of the photons become equal to the binding energies of the atom’s electrons within the crystals in the K, L, M, or N shells; therefore, due to resonance, the probability of photon absorption increases sharply. Part of the absorbed energy is released as characteristic radiation, which might deposit some of its energy in the scintillator mass. In addition, the energy not carried away by the characteristic photon contributes to the energy deposition process in the scintillator. This energy fraction can be quantified using the ratio (μen/ρ)/(μatt/ρ). In any case, the extra deposited energy contributes cumulatively to the optical photon generation process, resulting in higher signal values to the output. This can be directly observed by the variation in the absolute efficiency with X-ray tube voltage, which follows the corresponding variation in the energy absorption efficiency. The latter seems to increase after 60 kilovolts. This may be explained by the fact that, as kilovolts increase, a larger part of the X-ray spectrum exceeds the K-absorption edge; that is, the number of X-photons with energy slightly higher or higher than the K-absorption edge increases (these are more easily absorbed). In addition, we must consider that the absolute efficiency is defined in terms of X-ray exposure. Thus, it is also affected by the variation in the exposure to fluence conversion factor with energy. This is not the case for the X-ray luminescence efficiency, defined as X-ray over light energy flux. This efficiency shows a very slight variation at higher tube voltages.

BaF₂ reaches a maximum value of 2.36 EU at 140 kVp, which is higher compared to CeF₃’s AE maximum (0.8334 EU) at the same X-ray energy. The corresponding values for Gd₂SiO₅:Ce (GSO:Ce density 6.7 g/cm³) and Bi₄Ge₃O₁₂ (BGO density 7.13 g/cm³), which are commercially used in nuclear medicine imaging modalities, are 7.76 EU and 3.41 EU, respectively [56,79].

The spectral matching of BaF₂ with a series of optical sensors is shown in Figure 4. The emitted spectrum of BaF₂, with maximum emission at 310 nm and an emission range from 170 to 460 nm [6,8], is incompatible with sensors used in digital radiography, such as charge-coupled devices (CCDs), which have been used in older systems, and complementary metal oxide semiconductors (CMOS) [80].

Figure 4 shows that the spectral matching of BaF₂ to CCDs and CMOS is almost zero; therefore, this material could not be used in conjunction with such optical sensors. However, the light shift in the BaF₂ fluoride scintillator towards lower wavelengths makes it suitable for detectors frequently used in nuclear medicine modalities, such as photocathodes and flat-panel position-sensitive and silicon photomultipliers (PMTs) [81,82]. The light emitted from BaF₂ shows 87% optical matching with the optical sensitivity of flat-panel position-sensitive (PS) photomultipliers (flat-panel PS-PMT-H8500D-03, Hamamatsu photonics, Shizuoka, Japan), 76% with multi-alkali photocathodes, and 73% with the extended photocathode (E-S20).

In the following figures (Figure 5a and Figure 6b), a collection of various sensors’ quantum efficiencies (QEs) is shown, along with the emitted light from the examined BaF₂ inorganic scintillator. Figure 5a shows the spectral responses between BaF₂ and various photocathodes. Figure 5b shows corresponding grouped quantum efficiencies for various silicon photomultipliers. Figure 6a shows a group of CCD sensors and Figure 6b shows complementary metal oxide semiconductors. These data were used to calculate the spectral matching between BaF₂ and the various sensors.

In a complete imaging system, the luminescence efficiency of scintillator materials is weighted by the percentage of light produced that is not detected due to the optical sensor’s sensitivity to specific wavelengths. For this reason, the effective luminescence efficiency (ELE) is used to estimate the percentage of light produced by the scintillator that is actually detected. The effective efficiency of BaF₂ is shown in Figure 7. These results were calculated from the combination of the spectral matching factors and the luminescence efficiency. Figure 7 shows indicative results for photocathodes, silicon photomultipliers, CCDs, and CMOS sensors. The effective efficiency of BaF₂ was the maximum for flat-panel position-sensitive (PS) photomultipliers and various photocathodes. The corresponding effective efficiency values of the commercial available materials, which were used for comparison purposes in Figure 2 and Figure 3, were 3 for BGO, which has been used in Discovery IQ scanner (GE Healthcare, Milwaukee, WI, USA) [83], and 5 for GSO:Ce, which can be found in Gemini GXL PET/CT (Philips Medical Systems, Eindhoven, The Netherlands) [16,84,85]. By considering the much higher price of BGO [86] in comparison to BaF₂ [87], and considering their close efficiency values, the latter could be used as an alternative to BGO in various applications. For example, the GE Discovery-ST (PET/CT) positron emission tomography/computed tomography system integrates a total of 10080 (6.3 × 6.3 × 30 mm) BGO crystals, integrated into blocks of 6 × 6 crystals, connected to a PMT. Blocks are arranged into modules of 2 × 4 (eight blocks each), resulting in a PET detector ring of 35 modules with 280 crystal blocks [88]. Considering the difference in the current market value of a 10 × 10 × 1 mm BaF₂ (USD 83.95) [87] and a 10 × 10 × 1 mm BGO (USD 179.95) [86] scintillation crystal, the total cost for more than ten thousand single crystals is a very important factor that should be considered when designing a complete system.

Figure 8 shows the X-ray luminescence efficiency (XLE) of the BaF₂ crystal with additional data on commonly used crystals in PET/CT scanners, such as Gd₂SiO₅:Ce and Bi₄Ge₃O₁₂, as well as CeF₃. These sensors are employed in medical imaging systems, where the signal output corresponds to the energy absorbed by the crystal. The XLE values of BaF₂ are higher than those of the CeF₃ fluoride scintillator used for comparison purposes. The XLE of BaF₂ maximizes at 8.0 × 10⁻⁴ at 50 kVp and decreases thereafter. Similar behavior can be seen for CeF₃, with a maximum XLE value of 3.2 × 10⁻⁴ at 50 kVp, and with a decrease thereafter to 2.3 × 10⁻⁴ at 140 kVp.

GSO:Ce and BGO had higher XLE values, following the previously shown findings of Figure 2 and Figure 3. The corresponding values were 2.2 × 10⁻³ for GSO:Ce and 7.3 × 10⁻⁴ for BGO, at 50 kVp, with maximum values of 2.3 × 10⁻³ for GSO:Ce and 1.1 × 10⁻³ for BGO, both at 80 kVp. Thereafter, their values decreased across the rest of the examined range.

The attenuation (μ_att) and the energy absorption (μ_en) coefficients of BaF₂, Gd₂SiO₅:Ce, Bi₄Ge₃O₁₂, and CeF₃ are shown in Figure 9. All coefficients were calculated in the range of 8 to 140 keV. XmuDat was used to obtain the required coefficients for the following elements: barium (Ba, gram atomic mass = 137.33 g/mol), fluorine (F, gram atomic mass = 18.99 g/mol), cerium (Ce, gram atomic mass = 140.12 g/mol), bismuth (Bi, gram atomic mass = 208.98 g/mol), germanium (Ge, gram atomic mass = 72.64 g/mol), gadolinium (Gd, gram atomic mass = 157.25 g/mol), silicon (si, gram atomic mass = 28.08 g/mol), and oxygen (O, gram atomic mass = 15.99 g/mol). This was in order to calculate the mixtures in this study: BaF₂ (gram molecular mass 175.32 g/mol), CeF₃ (gram molecular mass 197.11 g/mol)_, Gd₂SiO₅:Ce (gram molecular mass 422.58 g/mol), and Bi₄Ge₃O₁₂ (gram molecular mass 1245.83 g/mol) [89,90]. Both attenuation and absorption coefficients decreased upon energy increase, with the exception of the energy range of the characteristic K-absorption, which was unique for every chemical element. The K-edge of BaF₂ appeared first, at about 38 keV, and then came that of CeF₃ at 41 keV, Gd₂SiO₅:Ce at 51 keV, and Bi₄Ge₃O₁₂ much higher at 90.5 keV. The early appearance of the K-edge of BaF₂ at only 38 keV contributed to the higher absolute efficiency values of BaF₂ compared to Bi₄Ge₃O₁₂ at 50 kVp, as shown in Figure 3 and Figure 8. On the other hand, the K-edge values of BaF₂, Gd₂SiO₅:Ce, and CeF₃ were in a very close energy range (38 to 51 keV); as such, Gd₂SiO₅:Ce, with its much higher density (6.7 g/cm³ compared to the 4.87 g/cm³ of BaF₂), and the cerium activator, displayed the highest efficiency values compared to the other three examined materials.

The energy absorption efficiency (EAE) values of BaF₂ compared to Gd₂SiO₅:Ce, Bi₄Ge₃O₁₂, and CeF₃ are shown in Figure 10. The energy absorption efficiency is influenced by the density of the crystal and the attenuation and absorption coefficients shown in Figure 9. Despite the fact that the EAE of BaF₂ appeared lower at its K-edge energy, after 90 kVp, it showed higher values than both Gd₂SiO₅:Ce and CeF₃. It is clear that the high density of BGO (7.13 g/cm³), combined with the higher μ_en/μ_att ratio (from 39 keV to 90 keV), gives it an edge over the other three materials.

3. Materials and Methods

Figure 11 shows the experimental setup for the irradiation of the 1 cm³ crystal samples with a square face S = 100 mm² (Advatech, London, UK) [8], with X-rays and produced light energy signal recordings $\left({\Psi }_{\Lambda }\right)$ (μW m⁻²) in the prescribed range.

Teflon was used to cover all crystal faces except from the exit surface. BaF₂ was placed within the entrance circular port of the integrating sphere. Upon X-ray irradiation, the light of the crystal travels through the sphere and imps on the photocathode of the photomultiplier tube. Prior to sample irradiation, quality control was performed on the X-ray tube using a calibrated dosimeter. The signal (current) of the PMT was fed to an electrometer (Figure 11).

The recorded data, regarding the light emission of the sample and the dose as a result of the exposure, were used to calculate the absolute luminescence efficiency (units EU = (μW m⁻²)/(mR s⁻¹); following Equation (1), the international system of units (SI) equivalent is μWm⁻²/(mGy/s):

$$AE={\eta }_A={\displaystyle \frac{{\Psi }_{\Lambda }}{X}}$$

(1)

The quantities in Equation (1) are shown in Figure 11. ${\Psi }_{\Lambda }$ is the light energy flux (output signal) in units of μW m⁻², while $X$ is the exposure rate (mR s⁻¹).

Since the luminescence efficiency depends upon the sensitivity of the optical sensor, the effective luminescence efficiency is also estimated using Equation (2) [79] for a range of optical sensors:

$$EE={\eta }_{eff}={\eta }_A{\alpha }_s$$

(2)

In Equation (2), the luminescence efficiency is multiplied by the spectral matching factor (α_s). Optical emission spectrum data for BaF₂ were obtained from the provider site [87].

The X-ray exposure (Equation (1)) is converted to X-ray energy flux ${(\Psi }_o$) by the product ${\Psi }_o=X\widehat{\Psi }$, where $\widehat{\Psi }$ is the X-ray energy flux per exposure rate, calculated according to Equation (3):

$$\widehat{\Psi }={\displaystyle \frac{\int {\Psi }_0\left(E\right)dE}{\int {\Psi }_0\left(E\right)\left[\frac{{\rm X}}{{\Psi }_0\left(E\right)}\right]dE}}={\displaystyle \frac{\int {\Psi }_0\left(E\right)dE}{\int {\Psi }_0\left(E\right)\left[{\left(\frac{{\mu }_{en}\left(E\right)}{\rho }\right)}_{air}{·\left(\frac{W_A}{e}\right)}^{-1}\right]dE}}$$

(3)

In this equation, we used the X-ray mass energy absorption ${({(\mu }_{en}\left(E\right)/\rho )}_{air}$) coefficient of the air and the average energy/unit of charge ($W_A/e)$ to produce an electron/ion pair in air. This calculation is used to estimate the X-ray luminescence efficiency (XLE), which is the ratio of the crystal’s emitted light energy flux over the X-ray energy flux (${\eta }_{\Psi }={\Psi }_{\Lambda }/{\Psi }_0$) [87].

The energy that is locally absorbed at the points of X-ray interaction within the crystal is calculated through the energy absorption efficiency [87]:

$$EAE\left(E\right)={\displaystyle \frac{{\int }_0^{E_0}{\Phi }_0\left(E\right)E\left(\frac{{\mu }_{en}\left(E\right)/\rho }{{\mu }_{att}\left(E\right)/\rho }\right)\left(1-e^{-\left({\mu }_{att}\left(E\right)/\rho \right)\rho W}\right)dE}{{\int }_0^{E_0}{\Phi }_0\left(E\right)EdE}}$$

(4)

In Equation (4), (E) is photon energy and Φ₀(E) is the X-ray photon fluence; together, they make up the X-ray energy fluence. The BaF₂ mixtures’ attenuation and absorption coefficients (μ_att(E)/ρ, μ_en(E)/ρ) were calculated using elemental data from XmuDat software Version 1.0.1 [89,90]. Furthermore, W is the crystal’s thickness and $\rho$ is the density (g/cm³) [91].

4. Conclusions

This study examined the luminescence efficiency of a barium fluoride (BaF₂) inorganic scintillator in crystal form against CeF₃, as well as with the scintillators already used in medical imaging modalities, in order to assess suitability for various applications. All the experiments were carried out with diagnostic X-rays. In addition to the luminescence efficiency (AE) response of the crystal upon X-ray irradiation, spectral matching with various optical sensors—and especially with sensors used in nuclear medicine—was examined. BaF₂ showed a higher AE than CeF₃, comparable to that of the commercially available bismuth germanate, but lower than that of the gadolinium orthosilicate inorganic scintillator. Additionally, the maximum emission of BaF₂ is adequate for coupling with flat-panel position-sensitive (PS) photomultipliers and various photocathodes, and its significantly lower cost supports its implementation in medical imaging modalities.