Luminescence and Scintillation Properties of YAl₃(BO₃)₄ Single Crystal for Thermal Neutron Detection

Abstract

A single crystal of YAl₃(BO₃)₄ was grown using the top-seeded solution growth method. The vacuum ultraviolet (VUV) excitation spectrum, monitored at the emission wavelength of 312 and 372 nm, showed a narrow excitation band at around 162 nm, which is located near the absorption edge of the YAl₃(BO₃)₄ host. Upon VUV excitation at 162 nm, the characteristic self-trapped exciton (STE) emission bands were observed at 312 and 372 nm. The X-ray excited scintillation spectrum shows a broad emission band peaking at 310 nm with a weak shoulder band at around 375 nm, which is consistent with photoluminescence, and can thus be assigned to the STE emission. The scintillation light yield under irradiation at a ²⁵²Cf-thermal neutron reached 2700 photons/thermal neutron.

1. Introduction

A scintillator generates ultraviolet (UV) or visible photons in response to the incident radiation which can be detected by a photo-sensitive detector such as a photomultiplier tube (PMT) and photodiode (PD) to estimate the energy deposited in the scintillation detector by the radiation. Generally, the incident radiation includes high-energy photons and particles such as X-rays, gamma-rays, alpha particles, beta particles, and neutrons. Thus, a variety of scintillators are being designed for radiation detection because different types of ionizing radiation interact with matters. Among other radiation detection techniques expected for the scintillation detector, the type of gaseous ionization detector such as a proportional counter can be mentioned. Helium-3 (³He) has various applications as a gold standard proportional counter for neutron detection due to the ³He(n, p)³H reaction (Q = 0.765 MeV, σ_abs = 5330 barns) for national security purposes, nuclear safeguards measurements, neutron scattering facilities, oil and gas exploration, and Boron Neutron Capture Therapy (BNCT) [1,2,3,4,5,6]. It is also important in cryogenics including low-temperature physics research and quantum computing, and increasingly in medical diagnostics. However, the ³He supply has recently been significantly limited despite the increasing demand for its use in the neutron detector worldwide. For the reasons mentioned above, continuous study of alternative thermal neutron detectors is strongly required. One of the candidate technologies is a scintillation detector consisting of lithium-6 (⁶Li)- and a boron-10 (¹⁰B)-based scintillator that is optically coupled to a photosensitive detector because both ⁶Li and ¹⁰B have large neutron capture cross-sections (⁶Li: 940 barns, ¹⁰B: 3840 barns) [1]. The ⁶Li(n, α)³H reaction releases 4.78 MeV of energy (Q-value), which is distributed as 2.05 MeV to the alpha particle and 2.73 MeV to the triton; meanwhile, the natural abundance of ⁶Li is only 7.5%. Many ⁶Li(n, α)³H reaction-based inorganic scintillators with a good performance including LiI₂:Eu [7], Li-glass [8], LiBaF₃ [9], LiCaAlF₆:Eu [10], LiCaAlF₆:Ce [11], LiF-CaF₂ [12], LiF-CaF₂:Eu [13], LiAlO₂ [14,15], LiGaO₂ [15], Cs₂LiYCl₆:Ce [16], Tl₂LiYCl₆:Ce [17], LiF:W [18], NaI:Tl, Li [19], Li-(PEA)₂PbBr₄ [20], LiI:Ag [21], Ce:Li₆Y(BO₃)₃ [22], and Cs₃Cu₂I₅:Li [23] have been reported thus far. Among the scintillators, LiCaAlF₆:Eu, LiCaAlF₆:Ce, and Cs₂LiYCl₆:Ce are now commercially available. In the ¹⁰B(n, α)⁷Li reaction, 94% of the reactions produce the 1.47 MeV alpha particle and 0.84 MeV ⁷Li, and that of 6% produce the 1.78 MeV alpha particle and 1.01 MeV ⁷Li. Despite the larger neutron capture cross-section and a relatively high natural abundance (~19.9%) when compared to ⁶Li, the number of reports about ¹⁰B(n, α)⁷Li reaction-based scintillators is quite small on a global scale. Although BF₃ gas-filled detectors have also been extensively utilized for thermal neutron detection, the inherent corrosiveness and toxicity of BF₃ present significant safety and operational challenges [24,25]. Hence, there is a need for more research into alternative thermal neutron detectors based on ⁰B-based scintillators. This study focused on yttrium aluminum borate YAl₃(BO₃)₄ as a novel neutron scintillator host because it contains a considerable amount of boron. YAl₃(BO₃)₄ was discovered in the 1960s [26,27] and it belongs to the carbonate huntite-type structure in the space group of R32. Notably, while YAl₃(BO₃)₄ possesses high hardness (Mohs 7.5), favorable physical properties, and chemical stability, its Y³⁺ site is easily replaceable by trivalent rare-earth ions. Therefore, the YAB single crystals activated with rare-earth ions have attracted great interest for application in laser, nonlinear optical, and phosphor material [28,29,30,31,32,33,34,35]. Aloui-Lebbou et al. previously reported that an additional emission band peaking at 300 nm was observed in Ce³⁺-doped YAl₃(BO₃)₄ powder under excitation at X-ray, which is an intrinsic emission due to a self-trapped exciton (STE) [35]. A similar intrinsic emission band under-excited at an ionizing radiation was confirmed in the study of some borate crystals [36,37,38,39,40]. In particular, calcium metaborate CaB₂O₄ showed an intense intrinsic emission band peaking at around 300–400 nm and a scintillation light yield under-irradiated at a thermal neutron reached approximately 3200 photons/thermal neutron [38]. In this study, we examined the photoluminescence and scintillation properties of an undoped YAl₃(BO₃)₄ single crystal with an expectation of efficient intrinsic emission owing to the STE for the neutron detector. Considering the existing industrial availability of the YAB crystals for laser applications, the successful demonstration of their neutron scintillation capabilities in this research could lead to an expedited practical implementation.

2. Materials and Methods

The single crystal sample of YAl₃(BO₃)₄ was prepared in an 80 mm diameter platinum crucible provided by the OXIDE Corporation via the conventional top-seeded solution growth method with a Li₂O-Al₂O₃-B₂O₃ flux under an atmosphere of N₂ + 3% H₂. High-purity powders of Y₂O₃ (5N), Al₂O₃ (4N), B₂O₃ (5N), and Li₂CO₃ (4N) were used as raw materials for the crystal growth. The transmittance spectrum in the wavelength ranges from 190 to 800 nm was measured using a UV-2700 spectrometer (SHIMADZU, Kyoto, Japan). The intrinsic emission properties were investigated through the use of vacuum ultraviolet (VUV) light as an excitation source. The excitation and emission spectrum measurements were performed at beamline 7B of the Universal Virtual Spectroscope for Optoelectronics Research (UVSOR) facility at the Institute for Molecular Science, Okazaki, Japan [41,42]. The sample was irradiated with a monochromated VUV light, and the fluorescence photons from the sample were detected using a spectroscopic system consisting of a charged-coupled device (CCD) detector and a monochromator in a vacuum chamber. The excitation wavelength is in the VUV region from 50 to 200 nm. The scintillation spectrum was measured under continuous irradiation from a SA-HFM3 X-ray generator (Rigaku Holdings Corp., Tokyo, Japan) operating at 40 kV and 40 mA. The scintillation photons from the sample were recorded from 200 nm to 700 nm by a QE Pro spectrometer (Ocean Insight, Inc., Orlando, FL, USA) that was cooled using a thermoelectric cooler (TEC). The scintillation pulse height spectra under-irradiated at 5.5 MeV alpha-rays from a ²⁴¹Am, 662 keV gamma-rays from a ¹³⁷Cs, and thermal neutron from a ²⁵²Cf sealed sources were measured using the sample optically coupled to an R7600U-200 photomultiplier tube (PMT, Hamamatsu Photonics K.K.). Details of the experimental setup were reported previously in Ref. [43]. The scintillation photons were recorded with shaping times of 10 μs for YAl₃(BO₃)_4, 2 μs for Bi₄Ge₃O₁₂ (BGO, CASTECH Inc., light yield = ~8500 ph/MeV [44]), and 0.5 μs for Li-glass (GS20, Saint-Gobain, light yield = ~6000 ph/nth [45]), where the latter were used as a reference sample to estimate the light yield.

3. Results

A photograph of the as-grown YAl₃(BO₃)₄ crystal is shown in Figure 1a. The crystal seems to be highly transparent and colorless; this is followed by a discussion of the transmittance spectrum measurement. Part of the as-grown crystal was cut and polished optically to dimensions of 5.0 × 3.0 × 1.0 mm³ for the optical and scintillation measurements as described below. The optical transmittance spectrum is shown in Figure 1b. The spectrum exhibited more than 75% transmittance, and no absorption bands in this wavelength were detected. This suggests that the grown crystal is of high purity and good quality. The cutoff wavelength in the crystal cannot be confirmed in this wavelength because of its large bandgap energy (~7.32 eV) [31]. Yu et al. previously reported that the cutoff wavelength for YAl₃(BO₃)₄ is about 170 nm by using a VUV spectrophotometer [46], which is in agreement with our result.

The excitation and emission spectra of YAl₃(BO₃)₄ are shown in Figure 2. The excitation spectra with an emission wavelength of 312 and 372 nm showed a narrow excitation band at about 162 nm. Considering the bandgap energy of ~7.32 eV for YAl₃(BO₃)₄, the narrow excitation band is located around the bandgap. A similar excitation band from the YAl₃(BO₃)₄:Dy³⁺, YAl₃(BO₃)₄:Ce³⁺, and YAl₃(BO₃)₄ was reported [30,33,35], which is due to the absorption of the host crystal, resulting in the formation of electron-hole pairs like excitons. Yoshida et al. discussed the electronic structure of YAl₃(BO₃)₄ with a first-principles molecular orbital method using the refined crystal structure [31]. The electronic structure calculations show that the valence band of YAl₃(BO₃)₄ is primarily formed by the O 2p state, whereas the conduction band edge is typically a complex mixture of B 2p and Y 4d states. Therefore, the excitation band near the bandgap energy is attributed to optical transitions from the O 2p state to the complex B 2p-Y 4d states. A similar excitation band has been confirmed by previously reports about YAl₃(BO₃)₄ powder samples synthesized by a solid-state reaction [31]. The emission spectrum excited at 162 nm showed a major emission band at 312 nm with a sub band at 372 nm. The Stokes shift can be calculated to be about 29,680 cm⁻¹ for 312 nm and 34,851 cm⁻¹ for 372 nm. Yokosawa et al. and Yoshida et al. reported these intrinsic emission bands from some borate compounds such as La(BO₂)₃, LaMgB₅O₁₀, YBO₃, and YAl₃(BO₃)₄ and suggested that the origins of these emission bands are due to different BO₃ units in the crystal host [31,47]. In the case of the YAl₃(BO₃)₄, there are two kinds of BO₃ units (B(1)O₃ and B(2)O₃) within a unit cell. The B(1)O₃ units exhibit an equilateral triangular geometry, whereas the B(2)O₃ units adopt an isosceles triangular configuration. Therefore, considering the emission and structural characteristics, previous reports concluded that the origin of the emission band peaking at 312 nm and 372 nm is due to the STE related to the B(1)O₃ and B(2)O₃ units.

The scintillation spectrum of YAl₃(BO₃)₄ at 300 K along with the fitted function is shown in Figure 3a. The spectrum can be roughly fitted with sums of two Gaussian functions centered at 317 and 370 nm. This result is consistent with the spectrum under-excited at VUV lights that the dominant emission bands are assigned to the STEs which is strongly dependent on the two kinds of BO₃ units (B(1)O₃ and B(2)O₃). Figure 3b shows the scintillation spectra at different temperatures in the ranges from 11 to 300 K. There was no clear change in the spectral peak position; meanwhile, the scintillation intensity of the bands at around 317 nm (STE-1) and 370 nm (STE-2) seems to depend on the temperature. The integral intensities of the two STE bands were derived from a two-Gaussian fit to the scintillation spectra as a function of photon energy. Figure 3c shows the temperature dependence between 11 and 300 K of the STE-1 band intensity (pink circle), the STE-2 band intensity (green triangle), and the total emission intensity (blue square). The total intensity is almost constant at temperatures below 100 K, and increased monotonically at temperatures at higher than 100 K. As the temperature increased from 100 to 300 K, the total intensity eventually reached 1.8 times the value compared to the value measured at 11 K. In contrast to the case of the total intensity, the STE-1 and STE-2 band intensity showed an inverse correlation in the temperature ranges from 11 K to 125 K. According to the trends in the data, we speculate that it is be due to a thermally activated energy transfer from STE-2 to STE-1. In the temperature between 150 and 300 K, the band intensity of both STE-1 and STE-2 increased gradually, which is similar to the total intensity. Although the mechanism of the monotonic increase in scintillation intensity remains unclear, our analysis indicates that a two-level model is insufficient to account for the temperature dependency observed in YAl₃(BO₃)₄. Koshimizu et al. previously observed a similar monotonic increase in scintillation in Cs₂HfCl₆ and attributed its temperature dependence to a three-level model with emissions from two excited states [48]. Hence, at least two or three excited levels are required to explain the temperature dependence of the scintillation in the YAl₃(BO₃)₄. In such a model, STE-1 and STE-2 are attributed to two kinds of BO₃ units (B(1)O₃ and B(2)O₃) which possibly play an important role in the excited levels (see Figure 3d).

The 5.5 MeV alpha-rays from a ²⁴¹Am irradiated scintillation pulse height spectrum of YAl₃(BO₃)₄ were evaluated, as shown in Figure 4a, which simulated neutron irradiation because the deposited secondary charged particles such as alpha and ⁷Li particles excited the host scintillator according to the ¹⁰B(n, α)⁷Li reaction. The spectrum of the GS20 measured by using a similar experimental set-up is also shown for comparison. The dominant peaks in the spectra correspond to the total absorption of alpha-particles with an energy of 5.5 MeV. The position of the peak channels was calculated by fitting with a single Gaussian function. From the calculation, the peak channels for the YAl₃(BO₃)₄ and GS20 were found to be about 421 and 296 channels, respectively. Thus, the scintillation light yield under alpha-rays excitation for the YAl₃(BO₃)₄ can be estimated to be about 140% of that of GS20 considering the number of relative peak channels and the quantum efficiency in the 300–400 nm wavelength ranges of the PMT used as a detector. Figure 4b shows the scintillation pulse height spectra of YAl₃(BO₃)₄ compared with the BGO under irradiation at 662 keV gamma-rays from a ¹³⁷Cs. In the spectra, the ¹³⁷Cs-662 keV gamma-ray photopeak for the YAl₃(BO₃)₄ is located at the 260 channel, whereas that of the BGO is observed at the 641 channel. From the result, the scintillation light yield for the YAl₃(BO₃)₄ is found to be approximately 2380 photons/MeV. Taking into account the pulse height spectrum measurements, the α/β ratio is defined as follows [49].

$${\displaystyle \frac{\alpha }{\beta }}={\displaystyle \frac{E_{\beta }}{E_{\alpha }}}{\displaystyle \frac{ch{\left(E_{\alpha }\right)}_{\alpha }}{ch{\left(E_{\beta }\right)}_{\beta }}}$$

where ch(E_α)_α and ch(E_β)_β are the channel numbers of full absorption peaks on pulse height spectra under-irradiated at α- and β (γ)- rays, respectively. The increased α/β ratio results in a more favorable signal-to-noise ratio for neutron detection. Some neutron scintillators with a high or moderate α/β ratio have been reported previously such as LiI:Eu (α/β = 0.87), Cs₂LiYCl₆:Ce (α/β = 0.66), Li₆Y_0.5Gd_0.5(BO₃)₃:Ce (α/β = 0.32) [50], and GS20 (α/β = 0.3) [1,49]. Based on the calculations, the α/β ratio for YAl₃(BO₃)₄ is estimated to be around 0.42, which is a superior value compared to both Li₆Y_0.5Gd_0.5(BO₃)₃:Ce and GS20.

In the concluding test as a neutron scintillator, the neutron response characteristics of YAl₃(BO₃)₄ were examined. Figure 5 compares the pulse height spectra of the YAl₃(BO₃)₄ with the GS20 under the thermal neutron exposure. In the spectra, the thermal neutron peak channels for YAl₃(BO₃)₄ and GS20 were the 304 channel and 675 channel, respectively. Thus, it is estimated that the scintillation light yield of the YAl₃(BO₃)₄ reaches 2700 photons/thermal neutron. The thermal neutron peak detected in the YAl₃(BO₃)₄ was associated with the Q-value of 2.31 and 2.79 MeV resulting from the ¹⁰B(n, α)⁷Li reaction. In contrast, the neutron peak observed in the GS20 was attributed to the Q-value of 4.78 MeV for the ⁶Li(n, α)T reaction. For this reason, the relative light yield of the YAl₃(BO₃)₄ compared to GS20 differs from that estimated under 5.5 MeV alpha-ray irradiation. Compared with borate crystalline scintillators based on a ¹⁰B(n, α)⁷Li reaction previously reported such as CaB₂O₄, SrB₂O₄, SrB₂O₄:Ce, and CaB₂O₄:Ce [37,38,51], the YAl₃(BO₃)₄ is found to have moderate light yield with a suitable scintillation wavelength for commercial PMT (see

Table 1. Characteristics of YAl₃(BO₃)₄ compared with CaB₂O₄, SrB₂O₄, SrB₂O₄:Ce, and CaB₂O₄:Ce scintillators previously reported.

Borate Scintillator	Scintillation Wavelength [nm]	Light Yield [ph/nth]
CaB2O4	300–400	3200
SrB2O4	300, 375	1500
CaB2O4:Ce (0.5%)	300, 375	2200
SrB2O4:Ce (0.5%)	300–400	1000
YAl3(BO3)4	317, 370	2700

). Potentially, YAl₃(BO₃)₄ activated with rare-earth ions such as Ce³⁺ and Pr³⁺ should be appropriate for the development of bright neutron scintillators based on a ¹⁰B(n, α)⁷Li.

4. Conclusions

In our research, we investigated the optical and scintillation properties of a YAl₃(BO₃)₄ single crystal prepared by the top-seeded solution growth method for a thermal neutron detector. Upon both VUV lights and X-ray excitation, the characteristic STE emission bands owing to the two kinds of BO₃ units (B(1)O₃ and B(2)O₃) were observed at 300–400 nm, which is a suitable emission wavelength for a commercial PMT. The scintillation light yield of 2700 photons/thermal neutron is smaller than that of the well-known GS20 commercial scintillator based on the ⁶Li(n, α)³H; meanwhile, the α/β ratio of 0.42 is a superior value compared with the GS20 that offers the possibility of excellent neutron/gamma-ray discrimination. The light yield achieved by making use of the YAl₃(BO₃)₄ is comparable to some borate scintillators based on the ¹⁰B(n, α)⁷Li previously reported. In conclusion, the properties and performance presented in this research for YAl₃(BO₃)₄ demonstrate that this material is a good thermal neutron scintillator host based on the ¹⁰B(n, α)⁷Li.