Scintillation Properties of Tb-doped SrLu 2 O 4 Single Crystals

photons/MeV among the prepared crystals


Introduction
Scintillators are a type of luminescent material, and they immediately convert the absorbed energy of ionizing radiation into many low-energy photons.(8) The performance of scintillation detectors depends primarily on the scintillation properties, and the required scintillation properties vary depending on the applications.(34) In the 2000s, with the progress of Sibased photodetectors, two-dimensional X-ray detectors including flat panel detectors (FPDs) (35)(36)(37) and X-ray line scan cameras (38,39) have attracted considerable attention.They consist of scintillators and Si-based photodetectors, and the required properties of scintillators for the above detectors were high effective atomic number (Z eff ), high light yield (LY), high density, low fading, and green-to-red emission. (40)Tl-doped CsI crystals (Z eff : ~54, LY: ~54000 photons/MeV), (41) CdWO 4 crystals (Z eff : ~66, LY: ~20000 photons/MeV), (42,43) and Ce, Pr-doped GOS ceramics (Z eff : ~60, LY: ~30000 photons/MeV) (44) were utilized as scintillators in the FPD and X-ray line scan cameras.However, there are no green-to-red emitting scintillators with large Z eff and high light yield, and thus the development of new scintillators is expected.
(47)(48)(49) In our previous research, Tb-doped SrY 2 O 4 single crystals showed green emission bands, and the estimated light yields were 12000 photons/MeV. (50)However, there are no reports on the scintillation properties of Tbdoped SrLu 2 O 4 .In this study, we investigated the optical and scintillation properties of Tbdoped SrLu 2 O 4 single crystals.

Experimental Methods
SrLu 2 O 4 single crystals doped with various concentrations of Tb were grown by the floating zone technique. (46,51)The raw powders of Tb 4 O 7 (99.99%,Furuuchi Chemical), SrCO 3 (99.99%,Wako Pure Chemical), and Lu 2 O 3 (99.999%,Nippon Yttrium) were mixed in a stoichiometric ratio.Here, the concentrations of Tb were 0.1, 0.5, 1.0, and 2.0 mol% with respect to Lu.The mixture was annealed at 1200 °C for 8 h to desorb carbon dioxide, and then the mixture was formed to a cylindrical shape by cold isostatic pressing at 15 MPa for 15 min.The cylindrical ceramic was sintered at 1400 °C for 8 h using an electric furnace, and the crystal growth was performed using a floating zone (FZ) furnace (Crystal Systems, FZ-T-12000-X-VPO-PC-YH).As growth conditions, the pulling speed and rotation rate were 10 mm/h and 20 rpm, respectively.The surfaces of the obtained crystals were mechanically polished to evaluate the optical and scintillation properties.
For the measurement of the scintillation spectra and decay curves, we used our previously reported original systems. (52,53)To calculate the scintillation light yields, the pulse area spectra of a 137 Cs γ-ray source using the prepared crystals were observed using our customized setup, (54) which can be measured using the scintillators with lifetimes in the ms range.

Results and Discussion
Figure 1 shows the XRD patterns of Tb-doped SrLu 2 O 4 single crystals and the standard card of SrLu 2 O 4 (JCPDS: 00-032-1242).All the diffraction patterns matched the reference data for an orthorhombic structure of SrLu 2 O 4 .Since other diffraction peaks were not confirmed, we have successfully prepared the single phase of SrLu 2 O 4 by the FZ method.Figure 2 shows the diffuse-transmittance spectra of Tb-doped SrLu 2 O 4 single crystals.All the crystals have a high transmittance of ~80% in the wavelength range from 450 to 850 nm.The decrease in transmittance was confirmed at wavelengths below 450 nm, and absorption bands were shifted to longer wavelengths with increasing Tb concentration.(57)(58) PL excitation/emission maps and quantum yield (QY ) values of the Tb-doped SrLu 2 O 4 single crystals are shown in Fig. 3.All the Tb-doped crystals exhibited several sharp emission peaks in the wavelength range of 350-650 nm under excitation at 290 nm.The excitation spectral peak around 290 nm corresponds to the absorption band of the Tb 3+ 4f-5d transition (as shown in Fig. 2).Since the same spectral positions of emission peaks were confirmed in previous reports on Tb-doped phosphors, the emission peaks were attributed to Tb 3+ 4f-4f transitions. (49,59,60)With increasing Tb concentration, the emission intensity decreased below 500 nm, which would be due to the effect of the self-absorptions of Tb 4+ and Tb 3+ .The QY of the 0.1% Tb-doped crystal (29.4%) was the highest among the prepared Tb-doped crystals, and this value was higher than that of Pr-doped SrLu 2 O 4 crystals (~18.4%), which were the same host material. (46)nder an excitation of 265 nm, the PL decay curves of Tb-doped SrLu 2 O 4 single crystals monitored at 550 nm are shown in Fig. 4. A single exponential decay function was used to approximate the decay curves except for the instrumental response functions (IRFs) and the rise part of each decay curve.The derived lifetimes were 1.8-2.3)(61) In SrLu 2 O 4 with low Tb concentration, the increase in rise time was observed.64)   Figure 5 shows the X-ray-induced scintillation properties of Tb-doped SrLu 2 O 4 single crystals.All the crystals showed several sharp peaks originating from Tb 3+ 4f-4f transitions, and the spectral features were similar to the results in PL.In a previous report, the nondoped SrLu 2 O 4 crystal showed a broad emission peak around 300-500 nm due to self-trapped excitons, (46) whereas this was not observed in Tb-doped crystals.The 0.1% Tb-doped crystal showed several sharp emission peaks in the wide range of 350-650 nm.In the 0.5-2.0%Tbdoped SrLu 2 O 4 single crystals, the green emission was mainly observed because of the effect of the self-absorptions of Tb 4+ and Tb 3+ , which was suitable for the wavelength sensitivity of Si-based photodetectors.Figure 6 shows the X-ray-induced scintillation decay curves of Tb-doped SrLu 2 O 4 single crystals.The decay curves can be fitted with a single exponential decay function except for the IRF.The decay time constants were 1.0-1.6 ms, which were typical values for Tb 3+ 4f-4f transitions under X-ray irradiation. (65,66)he pulse area spectra of Tb-doped SrLu 2 O 4 single crystals and the BGO scintillator under γ-ray irradiation from a 137 Cs source are shown in Fig. 7.The commercial BGO scintillator, which has a LY of ~8200 photons/MeV, (67) was used as the reference.For the 0.1, 0.5, 1.0, and 2.0% Tb-doped crystals, a full-energy absorption peak appeared at the channels of 745, 277, 175, and 108, respectively.Considering a full-absorption peak position of the BGO (263 ch), the calculated LYs of the 0.1, 0.5, 1.0, and 2.0% Tb-doped crystals were 16100, 19600, 12400, and 7600 photons/MeV, respectively.Here, the quantum efficiencies (QEs) of the PMT to emission peak wavelengths at 415 nm (QE = 36% for 0.1% Tb-doped crystals), 480 nm (QE = 25% for BGO), and 545 nm (QE = 11% for 0.5, 1.0, and 2.0% Tb-doped crystals) were taken into consideration.The LY of the 1.0% Tb-doped SrLu 2 O 4 single crystal was higher than that of Tb-doped SrY 2 O 4 single crystals (~12000 photons/MeV) in our previous report. (50)In contrast, the LY of Tb-doped SrLu 2 O 4 crystals was lower than that of the commercial Tl-doped CsI scintillators (LY: ~54000 photons/MeV) for the FPD. (41)

Conclusions
The structure and optical and scintillation properties of Tb-doped SrLu 2 O 4 single crystals were investigated.From XRD results, all the prepared crystals had the single phase of SrLu 2 O 4 .Under 290 nm excitation and X-ray irradiation, several emission lines appeared in the range of 350-650 nm, and emission origins were Tb 3+ 4f-4f transitions.With increasing Tb concentration, spectral shapes were changed by the self-absorptions of Tb 4+ and Tb 3+ .Since the 0.5, 1.0, and 2.0% Tb-doped SrLu 2 O 4 single crystals exhibited strong green emission peaks around 540 nm, they were suitable for the wavelength sensitivity of the Si-photodiode.Among the prepared crystals, the light yield of the 1.0% Tb-doped SrLu 2 O 4 crystal was the highest (~19600 photons/MeV), which is higher than that of a commercial BGO scintillator (~8000 photons/MeV).

Fig. 7 .
Fig. 7. (Color online) Pulse area spectra of Tb-doped SrLu 2 O 4 single crystals under γ-ray irradiation from a 137 Cs source.The inset shows the pulse area spectrum of a commercial Bi 4 Ge 3 O 12 (BGO) scintillator as the reference.