Er-Doped Mg₄Ta₂O₉ Single-Crystal Scintillators Emitting Near-Infrared Photons for High-Dose Field Monitoring

Abstract

Mg₄Ta₂O₉ single crystals doped with 0.01, 0.1, and 1% Er were grown using the floating zone method, and their photoluminescence and scintillation properties were studied. X-ray diffraction analysis confirmed that all samples had a hexagonal structure of Mg₄Ta₂O₉. The samples showed that the emission peak at 1550 nm was due to 4f−4f transitions of Er³⁺ ions, with quantum yields of 32.9, 46.6, and 18.0% for 0.01, 0.1, and 1% doping concentrations, respectively. All samples showed scintillation with a broad emission band at 350 nm due to charge transfer from Ta⁵⁺ to O²⁻ ions and emission peak at 1550 nm due to 4f−4f transition of Er³⁺ ions. The dose rate response functions showed that the detection limit for all samples was 0.06 Gy/h for scintillation detector applications.

1. Introduction

Radiation detectors play an important part in various fields, such as space exploration [1,2], environmental radiation dosimetry [3,4], well-logging [5,6], and medical imaging [7,8]. In these applications, scintillators are mostly used due to their high detection efficiency and ease of use. Scintillators are classified as phosphors that immediately emit ultraviolet (UV), visible (Vis), and near-infrared (NIR) photons under irradiations of ionizing radiations [9,10,11]. The required characteristics for scintillators are radiation resistance, high density (ρ), large effective atomic number (Z_eff), and high light yield (LY) [12]. To meet the above characteristics, various scintillators, such as Bi₄Ge₃O₁₂ (BGO) [13], CaWO₄ [14], Tl:NaI [15], and Tl:CsI, have been developed [16]. Because photodetectors based on bi-alkali photocathode have wavelength sensitivity in the range of 300–600 nm, the development of scintillators has mainly been conducted with materials showing emission wavelengths in the UV-Vis range.

In recent years, research and development of NIR scintillators have progressed because of the development of InGaAs-based photodetectors [17,18,19,20]. NIR scintillators have drawn attention for dose monitoring systems in high-dose areas [21,22]. In such applications, scintillation detectors, consisting of a NIR emitting scintillator and an optical fiber, have been proposed [23]. The system using NIR emitting scintillators has two advantages. Firstly, under high dose conditions, Cherenkov light appears in the range of UV-Vis [24]. Therefore, the wavelength of Cherenkov light (noise) overlaps with UV-Vis photons (signal) from the common scintillators, and it is difficult to distinguish these two components. In contrast, since NIR scintillators emit photons in a different wavelength range from Cherenkov light, it is easy to distinguish. Secondly, NIR photons penetrate optical fibers more efficiently than UV-Vis light [25]. The light transmission efficiency of optical fibers degrades in high dose areas, but the extent of degradation is milder in the NIR region than in the UV-Vis region. Therefore, when NIR-emitting scintillators are used, it enables us to efficiently guide emitted scintillation light to a photodetector.

In this study, Mg₄Ta₂O₉ is selected as a host. In recent years, Ta-based compounds have been investigated in a variety of fields, including phosphors [26], microwave dielectrics [27], and photo-catalysis [28]. Among them, Mg₄Ta₂O₉ has been attracting attention as a candidate material for scintillators, owing to its relatively large Z_eff (64.3) and high density (6.20 g/cm³) [29,30,31]. In our previous work, undoped Mg₄(Ta,Nb)₂O₉ single crystals were investigated, and they showed high LY and low afterglow levels comparable with commercial BGO crystal in the UV-Vis range [32]. In terms of luminescence center, we focused on Er³, which is an intriguing luminescent center that can emit NIR photons [33]. Phosphors doped with Er, until now, have been intensively researched as laser materials for optical communication [34,35,36] because transmission loss in a glass fiber is low in the range of 1400–1600 nm [25]. While the optical properties of Mg₄Ta₂O₉ doped with Mn²⁺ and Cr³⁺ have been investigated in previous studies [30,37], the scintillation properties of Mg₄Ta₂O₉ doped with Er³⁺ have not been examined at all. Er-doped Mg₄Ta₂O₉ single crystals were fabricated using the floating zone (FZ) method, and their PL and scintillation characteristics were analyzed.

2. Materials and Methods

Mg₄Ta₂O₉ single crystals were fabricated using a FZ furnace (FZD0192) from Canon Machinery. The selected Er concentrations were 0.01, 0.1, and 1%. The starting materials were MgO (High Purity Chemicals, 99.99%), Ta₂O₅ (Rare Metallic, 99.99%), and Er₂O₃ (Furuuchi Chemical, 99.99%). The powders were mixed homogeneously using an agate mortar and then formed into cylindrical rods by hydrostatic pressure of 10 MPa for 10 min. The cylindrical rods were sintered in air at 1400 °C for 8 h for crystal growth. In crystal growth, the pull-down speed was 5 mm/h, and the rotational speed was 3 rpm. After the growth was finished, the polycrystalline part was removed from the resulting crystalline rod by using pliers, and the part (5–7 mm) forming single crystals was taken out. The polisher (MetaServ 250) from Buehler was used to mechanically grind the large upper and lower surfaces so that they were parallel. To identify crystal phases of the synthesized sample, powder X-ray diffraction (PXRD) patterns were assessed in the 10–90° range with a diffractometer (MiniFlex 600) from Rigaku.

The Quantaurus-QY Plus device (C13534) from Hamamatsu Photonics was used to measure the PL excitation and emission spectrum, as well as the quantum yield (QY). The measurements were conducted using a Xenon lamp as the source of excitation light and with two different types of linear image sensors for photodetection: a silicon sensor capable of detecting UV-Vis light and an Indium Gallium Arsenide (InGaAs) sensor capable of detecting NIR light. Additionally, the PL decay time profile was measured using the Quantaurus-τ (C11367) from Hamamatsu Photonics.

Our custom-built setup was used to measure the X-ray-induced scintillation spectra [38]. For this, a X-ray generator (XRB80N100/CB) from Spellman was set to 80 kV for bias voltage and 1.2 mA for tube current as the X-ray source. The scintillation light was then transmitted to a spectrometer consisting of a monochromator (163) from Shamrock and a Si-based line camera (DU-420-BU2) from Andor for monitoring wavelength at 200–700 nm and an InGaAs-based line camera (DU492A) from Andor for monitoring wavelength at 700–1600 nm. Our own setup was used to measure the X-ray-induced scintillation decay time profiles and the afterglow profiles [39]. Additionally, to evaluate the dose rate characteristics as a NIR scintillation detector, the emission intensity under different X-ray dose rates were measured by using our own setup [40]. For this measurement, the tube voltage supplied to the generator (XRB80P&N200X4550) from Spellman was fixed at 40 kV, while the tube current was varied from 5.2 to 0.052 mA. The NIR photon was detected with an InGaAs PIN photodiode (G12180–250A) from Hamamatsu Photonics through an optical fiber (FP600ERT) from Thorlabs.

3. Results

3.1. Sample Conduction

Figure 1 shows photographs of the 0.01, 0.1, and 1% Er-doped Mg₄Ta₂O₉ used for the subsequent characterizations. The samples had a size of approximately ~13 × 4 × 1.5 mm. Since near-infrared scintillators have not been practically implemented yet, the optimal sample size for measurements has not been determined. Hence, the samples were processed into sufficient size to measure the scintillation properties. The 0.1 and 0.01% Er-doped samples were colorless and transparent, whereas the 1% Er-doped sample exhibited a pale pink color, which is typical for Er-doped materials. Because of the significant presence of cracks in the fabricated samples, transmission spectra could not be measured. However, the 0.1% sample was more transparent than the 1% sample, and no opaque cores due to impurity were observed.

Figure 2 shows the PXRD patterns of three different concentrations of Er-doped Mg₄Ta₂O₉ and the reference patterns of Mg₄Ta₂O₉. The reference data are from the Crystallography Open Database (COD) 2002422. The diffraction peaks observed in the Er-doped samples in Figure 2a were found to be identical to those of the reference pattern, indicating that the samples consisted of a single phase of Mg₄Ta₂O₉ with a hexagonal structure. No additional peaks, corresponding to impurity phases, were detected in the samples. These results suggest that all the Er-doped Mg₄Ta₂O₉ single crystals possessed a well-defined crystal structure [41,42]. Figure 2b shows an enlarged view of the strongest diffraction peaks. Peak shifts due to Er-doping were not clearly observed in the PXRD patterns. This is because Er concentrations were too low to detect the peak shifts in our XRD measurements.

3.2. PL Properties

Figure 3 shows the PL emission and excitation contour graph of the 0.1% Er-doped Mg₄Ta₂O₉ monitoring wavelengths at (a) 220–950 nm and (b) 950–1685 nm. The excitation wavelength of the intrinsic luminescence of Mg₄Ta₂O₉ is 220 nm [32], which is outside the measurable range of the instrument used in this study, so no emission was observed in Figure 3a. In Figure 3b, an emission peak at 1550 nm under excitation light at 380 nm and 520 nm was observed, which was attributed to the 4f−4f transitions of Er³⁺ [43,44]. The spectral characteristics of the other samples were consistent with that of the Mg₄Ta₂O₉ doped with 0.1% Er. Therefore, only the 0.1% sample was selected as the representative one for this measurement. The QYs monitored between 1450–1650nm for samples doped with 0.01%, 0.1%, and 1% Er under excitation at 520 nm were 32.9%, 46.6%, and 18.0%, respectively. The highest QY values were observed at 0.1% Er-doped sample, while the QY of 1% Er-doped sample was lower than that of 0.1% Er-doped sample. It is likely due to concentration quenching.

3.3. Scintillation Properties

Figure 4 shows the X-ray-induced scintillation spectra of the 0.01, 0.1, and 1% Er-doped Mg₄Ta₂O₉ in the range of (a) 250–700 nm and (b) 700–1625 nm. The maximum signal intensity at ~1540 nm was 1,855 for the 0.01% sample, 21,153 for the 0.1% sample, and 21,056 for the 1% sample. Therefore, it is suggested that the emission intensity of the 0.01% sample is significantly lower than those of the 0.1% and 1% samples. In the range of 250–700 nm, a broad emission band due to the charge transfer (CT) from Ta⁵⁺ to O²⁻ was observed [29]. Furthermore, a partial decrease in the emission spectrum with increasing Er concentration was observed at 300–500 nm, which coincided with the excitation wavelengths shown in Figure 3 and was found to be due to the absorption of the 4f−4f transitions of Er³⁺. Furthermore, emission peaks originated from the 4f−4f transitions of Er³⁺ were observed at 550 and 560 nm [45,46]. However, the emission peaks were hardly observed owing to the weak emission intensity. In the 700–1625 nm range, sharp emission peaks were observed at 980 nm and 1550 nm, which were attributed to the 4f−4f transitions of Er³⁺ [43,47]. The broad peak at 1200 nm in the 0.01% sample is noise. This is observed due to the weak intensity of the Er-derived emission.

X-ray-induced scintillation decay time profiles of the 0.01, 0.1, and 1% Er-doped Mg₄Ta₂O₉ single crystals are shown in Figure 5. All the decay curves were approximated by a single exponential function. The decay time constants of the 0.01, 0.1, and 1% Er-doped Mg₄Ta₂O single crystals were 6.11, 5.66, and 5.65 μs, respectively. The obtained decay time constants were nearly the same as those due to the CT from Ta⁵⁺ to O²⁻ in Mg₄Ta₂O₉ [29]. According to previous studies [33], emission from CT at 300–500 nm overlaps with the excitation band of Er³⁺ ions (Figure 3), suggesting that the energy transfer from CT to Er³⁺ ions caused a decrease in the decay time constant.

Figure 6 displays the afterglow profiles of single crystals of Mg₄Ta₂O₉ doped with 0.01, 0.1, and 1% Er after being exposed to X-ray irradiation for 2 ms. The afterglow level at 20 ms after X-ray irradiation (AGL₂₀) can be expressed as the ratio of the difference between the signal intensity at 20 ms after X-ray irradiation (I₂₀) and the background signal intensity (I_bg), to the signal intensity during X-ray irradiation (I_max). In other words, the formula for AGL₂₀ can be rewritten as: AGL₂₀ = (I₂₀ − I_bg) / (I_max − I_bg). The afterglow levels of Mg₄Ta₂O₉ doped with 0.01, 0.1, and 1% Er were found to be 267, 247, and 138 ppm, respectively. These values were lower than those of Tl:CsI scintillator, which were reported to have an afterglow level of around 300 ppm in the UV-Vis range when evaluated using the same evaluation system [48].

To evaluate the performance of the Er-doped Mg₄Ta₂O₉ as a scintillation detector, the correlation between NIR emission intensity and X-ray irradiation dose rate was measured for samples doped with 0.01, 0.1, and 1% Er in Mg₄Ta₂O₉, as shown in Figure 7. The y-axis is defined as the difference between the signal detected during X-ray irradiation and the background signal. All the samples showed a good linearity in the dose rate range of 0.06–6 Gy/h. For comparison, the Z_eff, density, dose–response characteristics, and photodetector are summarized in

Table 1. Z_eff, density, dose–response properties, and type of photodetector for the reported NIR-emitting scintillators.

Sample	Zeff	Density [g/cm3]	lowest Detection Limit [Gy/h]	Type of Photodetector	Reference
Gd2O2S:Pr	61.1	7.33	0.8	Si photodiode	[23]
BSO:Er	77.3	6.82	0.006	InGaAs PIN photodiode	[44]
BGO:Er	75.2	7.14	0.006	InGaAs PIN photodiode	[49]
SrY2O4:Nd	36.6	5.34	0.06	InGaAs PIN photodiode	[50]
BaTi4O9:Pr	41.5	4.47	0.3	InGaAs PIN photodiode	[51]
Mg4Ta2O9:Er	64.2	6.20	0.06	InGaAs PIN photodiode	This work

. The dose rate response functions of Er-doped BGO and Bi₄Si₃O₁₂ (BSO) NIR scintillators evaluated in the same measurement system have been reported to show a lower detection limit of 0.006 Gy/h [44,49]. Previous studies reported a lower detection limit of 0.8 Gy/h in measurement systems combining commercially available Pr-doped Gd₂O₂S and Si-based photodetectors, 0.3 Gy/h for a system combining a Pr-doped BaTi₄O₉ and an InGaAs-based photodetector, and 0.06 Gy/h for a Nd-doped SrY₂O₄ and InGaAs-based system. Hence, the lower detection limit of measurement of 0.06 Gy/h is still sufficient for radiation monitoring in nuclear facilities [23]. Therefore, Er-doped Mg₄Ta₂O₉ is considered to be a candidate for NIR emitting scintillators.

4. Conclusions

The 0.01%, 0.1%, and 1% Er-doped Mg₄Ta₂O₉ single crystals were synthesized by the FZ method. The PL characteristics of all the Er-doped Mg₄Ta₂O₉ exhibited luminescence attributed to the 4f−4f transition of Er³⁺ ions. In addition, the PL QY in the NIR region of the 0.1% Er-doped Mg₄Ta₂O₉ single crystal was the highest value of 46.6% among the samples presented. under X-ray irradiation, all the Er-doped samples exhibited similar luminescence characteristics to PL, and additional luminescence due to the CT from Ta⁵⁺ to O²⁻ in Mg₄Ta₂O₉ was observed. The afterglow levels of 0.01%, 0.1%, and 1% Er-doped Mg₄Ta₂O were 267.0, 247.0, and 137.9 ppm, respectively. The relationship between NIR luminescence intensity and X-ray dose rate showed good linearity from 0.06–6 Gy/h, confirming sufficient coverage for process radiation monitoring in nuclear facilities. These results suggest that Er-doped Mg₄Ta₂O₉ single crystals are promising candidates for NIR emission scintillators for high-dose rate monitoring applications. The 0.1% samples were more transparent compared with the 1% sample, and no visible impurity cores were observed. In addition, they showed higher QY values and scintillation light intensities in the NIR range compared with the 0.01% sample. These results suggest that the 0.1% concentration offers the most favorable results in terms of optical response and quality.