Absolute Luminescence Efficiency of Europium-Doped Calcium Fluoride (CaF₂:Eu) Single Crystals under X-ray Excitation

Abstract

The absolute luminescence efficiency (AE) of a calcium fluoride (CaF₂:Eu) single crystal doped with europium was studied using X-ray energies met in general radiography. A CaF₂:Eu single crystal with dimensions of 10 × 10 × 10 mm³ was irradiated by X-rays. The emission light photon intensity of the CaF₂:Eu sample was evaluated by measuring AE within the X-ray range from 50 to 130 kV. The results of this work were compared with data obtained under similar conditions for the commercially employed medical imaging modalities, Bi₄Ge₃O₁₂ and Lu₂SiO₅:Ce single crystals. The compatibility of the light emitted by the CaF₂:Eu crystal, with the sensitivity of optical sensors, was also examined. The AE of the 10 × 10 × 10 mm³ CaF₂:Eu crystal peaked in the range from 70 to 90 kV (22.22 efficiency units; E.U). The light emitted from CaF₂:Eu is compatible with photocathodes, charge coupled devices (CCD), and silicon photomultipliers, which are used as radiation sensors in medical imaging systems. Considering the AE results in the examined energies, as well as the spectral compatibility with various photodetectors, a CaF₂:Eu single crystal could be considered for radiographic applications, including the detection of charged particles and soft gamma rays.

1. Introduction

The use of single crystals as radiation converters is very common in imaging or counting applications, especially when coupled with optical sensors such as photomultipliers, which are frequently employed in radiation detectors [1,2]. Additionally, they are widely used in particle physics, homeland security, and in numerous disciplines of medical imaging, such as X-ray computed and positron emission tomography (CT and PET, respectively), radiotherapy, radiography, and mammography [3,4,5,6,7,8,9,10,11,12,13,14,15,16].

The state-of-the-art electronics that are used in today’s imaging systems require scintillators with exceptional properties, which are tailored for every application [1,17,18,19]. This is also a prerequisite for medical imaging, where the need for exceptional quality of the diagnostic images as well as the lowest possible radiation exposure of patients as directed by the ALARA principle (as low as reasonably achievable) is a priority [20]. In this scope, medical imaging detectors should incorporate scintillators with optimized characteristics in terms of efficiency and imaging performance. An example is the time-of-flight (TOF) technique in PET scanners, requiring inorganic scintillators with primarily extremely short decay time, high density, and light yield (LY) [2,21]. Similar concerns arise in other fast applications such as CT, in which afterglow can cause blur in the final image [7].

During the last century, from the discovery of the very first scintillating materials, up to our era, various research groups have tried to improve and tailor the properties of scintillating materials for particular medical imaging applications [22,23,24,25]. For example, in positron emission tomography, single crystals have been used in order to detect high-energy coincident photons, which are produced after the emission of positrons from radioisotopes [26,27]. The use of scintillators with different properties has been extensively researched and explored by the scientific community, and currently, a number of single crystal materials have been adopted to be used in commercial PET scanners, such as lutetium oxyorthosilicate (LSO), bismuth germinate oxide (BGO), yttrium orthoaluminate perovskite (YAP), gadolinium oxyorthosilicate (GSO), among others [14,23,26,28,29,30]. LSO has been used as detector material, for example in the Biograph™6 PET/CT scanner (Siemens Medical Solutions), whereas BGO was used in the ECAT EXACT HR+ (Siemens Medical Solutions, Erlangen, Germany) and Discovery ST PET/CT (General Electric, Chicago, Illinois, US) PET scanners [31,32].

Europium-activated calcium fluoride (CaF₂:Eu) scintillators have already been incorporated in many different applications such as spectroscopy, charged particle detection, double beta decay investigation, the search for dark matter, low-energy radiation detectors, time-of-flight (TOF) applications, mobile Compton cameras, solar cell application, as well as homeland security [33,34,35,36,37,38,39,40]. Besides, due to its long decay constant, it was used in phoswich detectors [38,39,41,42]. Furthermore, CaF has been used as an efficient dosimeter material, due to the low atomic numbers of calcium (Z = 20) and fluorine (Z = 9), along with its effective atomic number (Z = 16.5), which is close to that of soft tissues.

Up until now, CaF has been used in a limited number of biomedical applications [22,23,24,25,43,44,45,46,47,48,49,50,51,52]. For example, CaF₂ has been applied in conjunction with SiPMs in gamma spectrometry, and due to its biocompatibility and the stability of the material, it has proven to be significant for fluorescent labeling in biological applications. Furthermore, the very good transparency in a wide wavelength range, the low refractive index, and the low phonon energy, make CaF₂ suitable to host luminescent ions, and thus a good choice for multimodal imaging [22,43,52,53,54,55,56,57,58,59,60].

CaF₂:Eu has been used and studied for decades, in single crystal and nanocrystal forms [43,52,53,54,55,56,57,58,59,60]. This material is cheap and can be prepared easily in large quantities. Europium-activated calcium fluoride is easily machined; it is also non-hygroscopic, inert, and insoluble (solubility 0.0017 g/100 g H₂O). The properties of this material are also compatible for particle and low-energy sensors, which are well suited to use within vacuums since it has very low-vapor pressure point. Furthermore, it is robust to thermal and mechanical shock, with a high melting point (1418 °C). It has low inherent background radiation, but due to the density value of 3.18 g/cm³ and the low Z, the light yield is also relatively moderate (19,000 photons/MeV) and the output of the emitted light is approaching 60% of sodium iodide (Nal) doped with thallium. Thus, this material is not adequate for high-energy experiments. CaF₂:Eu has sharp maximum emission at around 435 nm and it can be used with titanium dioxide (TiO₂) as an efficient reflector, but has also partially overlapping absorption and emission around 400 nm. The index of refraction is 1.443, which is very close to that of most photomultiplier’s glasses and silicon P-I-N photodiodes with shallow junctions. The band gap energy is large (12 eV), and the decay time is around 940 ns. The energy resolution is excellent (5.7–6.07% at 662 keV), taking into consideration the atomic value of this material [34,35,36,39,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75].

In this study, the efficiency of a 10 × 10 × 10 mm CaF₂:Eu single crystal was measured in order to examine the performance of this material as a possible candidate for medical imaging applications against the established single crystals, taking into consideration distinct advantages, such as: (i) the light photons emitted by CaF₂ are approaching 60% of that of Nal:Tl, and (ii) contrary to LSO, it has no afterglow [24]. The light energy flux for exposure rates within a range of 80 kVps, covering X-ray radiography examinations, was measured. Additionally, the light emitted by CaF₂:Eu was recorded and correlated with various optical sensors that are used in radiation detectors. The results of this study were compared with previously published results for Lu₂SiO₅:Ce and Bi₄Ge₃O₁₂ single crystals that are frequently used in commercial imaging systems. Finally, the stopping power of the crystals, described via the energy absorption efficiency (EAE), was calculated and compared to previously published data.

2. Materials and Methods

A CaF₂:Eu single crystal, with polished surfaces, was purchased from Advatech UK Limited [76]. The crystal had dimensions of 10 × 10 × 10 mm. The crystal was irradiated by X-rays on a ΒMI Merate X-ray tube with inherent filtration equivalent to 2 mm of Al and added filtration of 20 mm of Al. The examined tube voltages span in a range of 80 kVps (50–130) [77].

2.1. Absolute and X-ray Luminescence Efficiency

In order to estimate the absolute and the X-ray luminescence efficiencies of the crystal sample, the light flux was measured using a light integration sphere (Oriel 70451), connected to a photomultiplier tube (PMT) (EMI 9798B). The signal of the PMT was fed to a Cary 401 vibrating reed electrometer and the exposure rate was measured with a calibrated dosimeter (RTI Piranha P100B) [78]. The measured emitted light from the single crystal, divided by the X-ray exposure, provided the absolute luminescence efficiency (AE), i.e., the energy flux of the emitted light ${\dot{\mathrm{\Psi }}}_{\lambda }$, per unit of incident exposure rate $\dot{X}$:

$${\eta }_{\mathrm{A}}={\dot{\mathrm{\Psi }}}_{\lambda }/\dot{X}$$

(1)

The units of AE are given in $\mu W\times m^{-2}/(mR\times s^{-1})$ (efficiency units: E.U.).

Afterwards, the X-ray luminescence efficiency (XLE) was defined as the fraction of incident energy converted into emitted light; that is, (${\eta }_{\psi }={\mathrm{\Psi }}_{\mathrm{\Lambda }}/{\mathrm{\Psi }}_0$). XLE was determined by converting the X-ray exposure data that was used to calculate AE into X-ray energy flux (${\mathrm{\Psi }}_0$), as follows: ${\mathrm{\Psi }}_0=X\widehat{\mathrm{\Psi }}$ where $\widehat{\mathrm{\Psi }}$ is defined as the X-ray energy flux per exposure rate, estimated according to Equation (2):

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

(2)

${({\mu }_{en}/\rho )}_{air}$ is the X-ray mass energy absorption coefficient of air, and $(W_A/e)$ is the average energy per unit of charge required to produce an electron–ion pair in air, as obtained from tabulated data [79].

2.2. Spectral Matching

The percentage of the emitted light from a single crystal that can be detected by a sensor can be quantified using the factor of the spectral matching $\alpha s$, which is given by Equation (3):

$$\alpha s={\displaystyle \int \varphi \lambda }(\lambda )SD(\lambda )d\lambda /{\displaystyle \int \varphi \lambda (\lambda )d\lambda }$$

(3)

In Equation (3), $\varphi \lambda (\lambda )$ is the emitted light, and $SD(\lambda )$ is the sensor’s detection efficiency in particular wavelengths [78].

The emitted light was measured by a grating optical spectrometer (Ocean Optics Inc., HR2000). The spectral sensitivities of the optical detectors were obtained from tabulated data [80,81,82,83,84]. The effective efficiency ($\eta eff$) was defined as the product of AE with the Spectral Matching Factor [84].

2.3. Energy Absorption Efficiency

The radiation detection properties of a scintillator can by quantified by the energy absorption efficiency (EAE) [85,86,87]. EAE expresses the energy locally deposited at the sites of primary photon interactions and is a useful parameter for characterizing the efficiency of planar scintillators. The energy absorption efficiency was calculated by Equation (4):

$${\langle {\eta }_{\epsilon }\rangle }_E=\frac{{\displaystyle \underset{0}{\overset{E_0}{\int }}\Psi (E)\left(\frac{{\mu }_{tot,en}(E)}{{\mu }_{tot,t}(E)}\right)\eta q(E)dE}}{{\displaystyle \underset{0}{\overset{E_0}{\int }}\Psi (E)dE}}$$

(4)

$\eta q(E)$ is the monoenergetic quantum efficiency. ${\mu }_{tot,t}(E)/\rho$ is the X-ray total mass attenuation coefficient of the scintillator, and ${\mu }_{tot,en}(E)/\rho$ is the total mass energy absorption coefficient of the scintillator, which includes all the mechanisms of energy deposition locally at the point of X-ray interaction within the crystal [79,88].

3. Results and Discussion

Figure 1 shows the AE results for the CaF₂:Eu single crystal in the examined energy range. The results were compared with published data, for LSO and BGO crystals of equal dimensions (10 × 10 × 10 mm³) [28]. The AE of the CaF₂:Eu increased up to 80 kilovolts. From 80 to 130 kilovolts, the AE values progressively decreased. The maximum value at 80 kVp was 22.22 E.U. The 10 × 10 × 10 mm³ CaF₂:Eu crystal had increased AE compared to both LSO and BGO, across the examined kVp range [28]. However, for values higher than 90 kVp, the AE of the CaF₂:Eu crystal showed a tendency to decrease due to the lower density and Z value of CaF₂:Eu, which results also in low light yield (19000 photons/MeV) when interacting with higher energies [57]. In this energy range, the AE values of the 10 × 10 × 10 mm³ LSO crystal showed a constant increase (light yield ≥ 26,000 photons/MeV). BGO, due to the low light yield (8200 photons/MeV), showed lower AE values. Figure 1 also shows XLE results (right Y axis) for the 10 × 10 × 10 mm³ CaF₂:Eu and LSO crystals in the examined energy range. The BGO data were excluded, since their values were hardly visible in the graph. The X-ray luminescence efficiency of CaF₂:Eu constantly decreases within the radiographic energies, showing a maximum at 50 kVp. The XLE of LSO increases up to 70 kVp and remains almost constant thereafter. This behavior is influenced by the X-ray mass energy absorption coefficients of air ${({\mu }_{en}/\rho )}_{air}$ in this energy range and the average energy per unit of charge that is required to produce an electron ion pair in air $(W_A/e)$; the latter is used upon the conversion of X-ray exposure data into X-ray energy flux, in order to determine the fraction of incident energy converted into emitted light.

The luminescence efficiency results indicate that the AE values of CaF₂ were higher than both the commercial employed LSO and BGO single crystals, across the examined energy range, thus supporting the main hypothesis of the current study regarding the possibility of application of CaF₂ in medical imaging applications. This finding is significant for medical imaging modalities lying in this energy range. For higher energies, CaF₂ efficiency shows a tendency to decrease, thus suggesting that it would not be efficient for nuclear medicine applications.

Figure 2a–d show the normalized emitted optical spectrum of CaF₂:Eu crystal upon excitation with UV light. The spectral data are shown along with the sensitivity values of a variety of optical sensors. Figure 2a,b show mostly the spectral sensitivities of various PMTs and photocathodes used in nuclear medicine techniques. Figure 2c,d show the spectral sensitivities of various charge-coupled devices (CCD) and complementary metal–oxide semiconductors (CMOS) used in imaging modalities. The transition of the Eu ions with oxidation state of +2, in CaF₂, from the 4f⁶5d¹ levels configuration to the 4f⁷ ground state configuration, result in a near ultraviolet, bluish emission maximum at 424 nm, with a full-width-at-half-maximum (FWHM) of about 29 nm, which is close to previously published values [89,90,91].

The spectral matching factor data for CaF₂:Eu in conjunction with various optical sensors used in medical imaging detectors are shown in

Table 1. Spectral matching factor data.

Light Sensors	CaF2:Eu	Light Sensors	CaF2:Eu
CCD broadband AR coating	0.94	GaAsP phosphor photocathode	0.52
CCD infrared (IR) anti-reflection (AR) coating	0.54	Extended photocathode (E-S20)	0.94
CMOS hybrid with blue anti-reflection (AR) coating	0.60	Si PM MicroFC-30035-SMT	0.94
Hybrid CMOS blue	0.79	Si PM MicroFB-30035-SMT	0.92
CMOS (monolithic 0.25 μm)	0.64	Si PM MicroFM-10035	0.61
a-Si:H passivated	0.63	Si PM S10985-050C	0.95
a-Si:H_non-passivated	0.92	Si PM S10362-11-025U	0.96
CCD with indium tin oxide (ITO) gates with microlenses	0.68	Si PM S10362-11-050U	0.95
CCD with indium tin oxide (ITO) gates	0.51	Si PM S10362-11-100U	0.96
CCD with poly gates	0.18	Flat panel PS-PMT H8500C-03	0.91
CCD no poly gates LoD	0.34	Flat panel PS-PMT H8500D-03	0.78
CCD with traditional poly gates	0.34	Flat panel PS-PMT H10966A	0.79
CMOS (photogate array 0.5 μm)	0.26	Flat panel PS-PMT H8500C	0.86
CMOS RadEye HR	0.68	Bialkali Photocathode	0.78
GaAs Photocathode	0.95	Multialkali Photocathode	0.81

. CaF₂:Eu is compatible with Multi-Pixel Photon Counters (MPPC) silicon (Si) PMTs (SMF = 0.96 with Hamamatsu Si PM S10362-11-025U and S10362-11-100U, SMF = 0.95 with S10985 and S10362-11-050U). Furthermore CaF₂:Eu was compatible with the following photocathodes: the gallium arsenide GaAs (0.95) and the extended (E-S20) photocathodes (0.94). The same value was found for spectral matching when coupled with charge-coupled devices (0.94) and the Sensl’s MicroFC-30035 (0.94) silicon PMT. The spectral matching value for a non-passivated amorphous hydrogenated silicon photodiode (a-Si:H), was 0.92, but only 0.63 with a passivated a-Si:H. It was also found to be compatible with Hamamatsu PS-PMTs, such as the H8500C-03 (0.91). When coupled with certain types of complementary metal–oxide semiconductors, the spectral matching value was found to be 0.79.

CaF₂:Eu also showed good compatibility with a monolithic (0.25 $\mathsf{\mu }\mathrm{m}$) (0.64) and high-resolution RadEye complementary metal–oxide semiconductor (0.68), and with a CCD with indium tin oxide gates with microlenses (0.68).

It was found to have moderate compatibility with Sensl’s silicon photomultiplier MicroFM-10035 (0.61). CaF₂:Eu was found to be incompatible with a CCD having polygates (0.18) and a CMOS with photogate array (0.26).

Figure 3 shows the luminescence efficiency of the 10 × 10 × 10 mm³ CaF₂:Eu crystal, degraded by the spectral matching values when coupled with various optical sensors. Following the results of Table 1, the highest effective efficiency value was attributed to CaF₂:Eu/Hamamatsu MPPC Si-PMT. The lowest values are shown when matched with certain CCD types with optimum sensitivity toward the red part of the light spectrum.

Figure 4 shows values for the EAE of the 10-mm thick CaF₂:Eu crystal along with a comparison with LSO and BGO crystals incorporated in commercial imaging units. The difference of all these values from unity is explained considering that EAE neglects mechanisms of interaction inside the crystal, such as scattered, characteristic, and bremsstrahlung radiation. The EAE values of CaF₂:Eu crystal (0.82 at 40 kV) were lower than both BGO (0.834 at 40 kV) and LSO (0.875 at 40 kV) due to the considerably higher density of these materials (7.13 and 7.4 g/cm³, respectively) in respect to the 3.18 g/cm³ of CaF₂:Eu. The difference between the absorption efficiency values of CaF₂, in respect to those of LSO and BGO, increases with the increase in X-ray energies. This result is in accordance with the luminescence efficiency behavior that is shown in Figure 1 for higher energies; thus, this material would not be efficient for nuclear medicine applications.

4. Conclusions

The absolute luminescence efficiency and the spectral matching of a CaF₂:Eu crystal were investigated in conditions usually met in general radiography. The results were compared with data of crystals with equal dimensions, which are frequently used in commercial imaging modalities, such as LSO and BGO. The examined sample showed optimum efficiency at an X-ray tube voltage of 80 kV. The luminescence efficiency of the 10 × 10 × 10 mm³ CaF₂:Eu sample showed increased values compared to both LSO and BGO across the examined energy range. The light emitted from the CaF₂:Eu was found to be compatible with commercial photocathodes, charge-coupled devices (CCD), and Si MTs. CaF₂:Eu could be potentially used in radiographic applications due to the promising absolute efficiency values in the radiographic energy range, as well as the matching of the emitted light with commercial optical sensors, besides the detection of charged particles and soft gamma rays.