Photoluminescence and Scintillation Properties of Eu:CaHfO 3 Single Crystals

Eu:CaHfO 3 single crystals with various Eu concentrations were synthesized by the floating zone method, and their photoluminescence (PL) and scintillation properties were investigated. All the samples showed PL peaks at 590, 620, and 650 nm, which were due to the 4f-4f transitions of Eu 3+ . The PL quantum yield of 1.0% Eu:CaHfO 3 was the highest at 42.2% among the samples. The X-ray-induced scintillation spectra showed several emission peaks in the range of 400–690 nm, and the emissions were attributed to the 4f-4f transitions of Eu 3+ . Among the samples, 0.05% Eu:CaHfO 3 showed the highest intensity. In the afterglow curves, the levels were within the range of 1,000–10,000 ppm and those of previous studies.


Introduction
(3) Taking advantage of this feature, the scintillators have been applied to radiation detectors by combining them with photodetectors that can convert the photons into electrical signals. (4,5)The scintillation detectors are classified into current and photon counting mode measurements depending on their application. (6,7)In particular, the detectors for the current mode type integrate the signal for a period of milliseconds and have been used in applications such as X-ray computed tomography (CT) and flat panel detectors for X-ray radiography. (8)The scintillators for the current mode type require properties such as high emission intensity, a large effective atomic number (Z eff ), high density (ρ), and low afterglow level (AL).11)(12)(13)(14) HfO 2 -based compounds such as RE 2 Hf 2 O 7 (RE = La, Gd, Lu) and AEHfO 3 (AE = Ca, Sr, Ba) have attracted attention as candidates for scintillators owing to their large Z eff and high ρ.(17)(18)(19)(20)(21) Moreover, our research group has investigated the scintillation properties of CaHfO 3 doped with Ti, Ce, Pr, Tb, and Tm, (18,(21)(22)(23)(24)(25)(26) and CaHfO 3 doped with various rare-earth elements was found to show high luminescence properties.
Eu 3+ is used as a luminescence center for red scintillators, and the emission wavelength is well matched with a Si-based photodetector.(32)(33)(34)(35)(36)(37)(38) Therefore, it is considered that Eu 3+ is useful for scintillators.However, the scintillation properties of Eu:CaHfO 3 have not been reported.In this study, Eu:CaHfO 3 single crystals were synthesized by the floating zone (FZ) method, and their photoluminescence (PL) and scintillation properties were investigated.

Experimental Setup
An FZ furnace with four Xe lamps (Crystal Systems Corporation, FZ-T-12000-X-VPO-PC-YH) was used to synthesize Eu:CaHfO 3 single crystals. (16)The Eu concentrations were 0.01, 0.05, 0.1, 0.5, and 1.0 at% with respect to Ca. CaO (99.99%,Furuuchi Chemical), HfO 2 (99.95%,Rare Metallic), and Eu 2 O 3 (99.99%,Furuuchi Chemical) were used as starting materials and uniformly blended using a mortar and pestle.To account for Ca evaporation during the crystal growth, CaO was raised by 10% in relation to the stoichiometric composition. (18)The mixtures were placed inside balloons to form cylindrical rods by applying hydrostatic pressure.The rods with different Eu concentrations were subsequently calcined at 1400 °C for 8 h in air using an electric furnace (Crystal Systems, BF-1800-IV-II), and the acquired rods were used for crystallization.During the crystallization, the pull-down speed was 10 mm/h, and the upper and lower rotation rates were 23 rpm to the right and 20 rpm to the left, respectively.The synthesized samples were split, and a grinder polisher (Buehler, MetaServ 250) was used to mechanically polish the surfaces.
A diffractometer (Rigaku, MiniFlex600) equipped with a Cu Kα micro-X-ray tube containing a Be window was used to measure the powder X-ray diffraction (XRD) patterns of the samples in the range of 2θ = 10-70°.Diffuse transmission spectra were measured using a spectrophotometer (Shimadzu, Solidspec-3700).
Three-dimensional PL excitation and emission spectra and quantum yield (QY) were measured using Quantaurus-QY (Hamamatsu Photonics, C11347).The measurement ranges of emission and excitation wavelengths were 300-800 nm and 250-400 nm, respectively.Quantaurus-τ (Hamamatsu Photonics, C11367) was used to evaluate PL decay time profiles.X-ray-induced scintillation spectra were measured using our original setup. (39)The X-rayinduced scintillation decay and afterglow curves were evaluated using our original measurement system. (40)

Results and Discussion
Figure 1 shows a photograph of the 0.01, 0.05, 0.1, 0.5, and 1.0% Eu:CaHfO 3 single crystals.All the samples looked colorless and transparent.Additionally, the length, width, and thickness of the 0.01, 0.05, 0.1, 0.5, and 1.0% Eu:CaHfO 3 were approximately 4 × 4 × 0.75 mm 3 .Figure 2 shows the powder XRD patterns of the Eu:CaHfO 3 single crystals and the reference data from International Crystallographic Diffraction Data (ICDD) No. 36-1473.The diffraction peaks of the samples were observed at the same positions as those of the reference data, and no impurity phases were detected.Hence, the synthesized samples had a single-phase structure of CaHfO 3 . (41,42)igure 3 shows the diffuse transmission spectra of Eu:CaHfO 3 in the wavelength range of 200 to 850 nm.In the 300-850 nm range, Eu:CaHfO 3 showed the maximum transmittance at approximately 70-80%, and the transmittances of the 0.05 and 0.5% doped samples were slightly lower than those of the other samples.The decrease in the transmittance arose from the presence of cracks, as shown in Fig. 1.Absorption attributed to the charge transfer of Eu 3+ -O 2− was observed in all the samples at wavelengths shorter than 300 nm. (43)igure 4 shows the three-dimensional PL excitation and emission spectrum of the 1.0% Eu:CaHfO 3 as a typical sample.This Eu:CaHfO 3 showed the three emission peaks at 590, 620, and 690 nm originating from the 4f-4f transitions of Eu 3+ . (44,45)A similar trend was observed in other samples.The PL QYs in the range of 550-750 nm under the 300 nm excitation wavelength of 0.01, 0.05, 0.1, 0.5, and 1.0% Eu:CaHfO 3 were 1.5, 4.6, 12.2, 32.6, and 42.2%, respectively.The increase in PL QY is attributed to an increase in Eu concentration, and no concentration quenching was observed in 0.01, 0.05, 0.1, 0.5, and 1.0% Eu:CaHfO 3 .
Figure 5 shows the PL decay time profiles of 0.01, 0.05, 0.1, 0.5, and 1.0% Eu:CaHfO 3 single crystals, where the dashed lines indicate the fitted curves.All the decay curves of the samples were well approximated by a single-exponential function.Each obtained PL decay time constant was 0.96-0.99ms, which was the range of typical values of the Eu 3+ -doped materials. (46,47)ence, the emission of the samples originated from the 4f-4f transitions of Eu 3+ .
Figure 6 shows the X-ray-induced scintillation spectra of Eu:CaHfO 3 .All the Eu:CaHfO 3 samples showed several emission peaks at 420 nm ( 5 D 3 -7 F 1 ), 430 nm ( 5 D 3 -7 F 2 ), 450 nm ( 5 D 3 -      Figure 7 shows the X-ray-induced scintillation decay time profiles of Eu:CaHfO 3 , and the dashed lines show the fitted curves.The obtained decay curves of Eu:CaHfO 3 were approximated by a sum of two exponential components except for an instrumental response function (IRF).The first and second components, which were 0.80-1.1 ms and 2.8-3.0 ms, would be attributed to the host emission and the 4f-4f transitions of Eu 3+ , (21,43,50,51) respectively.
Figure 8 shows the afterglow curves of Eu:CaHfO 3 samples.AL is defined as follows: AL [ppm] = 10 6 × (I 2 -I bg )/(I 1 -I bg ), where I bg , I 1 , and I 2 are the mean signal intensity in the background, with X-ray irradiation, and at t = 20 ms after irradiation, respectively.The AL values of 0.01, 0.05, 0.1, 0.5, and 1.0% Eu:CaHfO 3 were 10000, 3800, 4100, 1100, and 1000 ppm with a typical error of 8%, respectively.4)26) Since general commercial scintillators such as CdWO 4 and Bi 4 Ge 3 O 12 have an AL value of around 10 ppm, (40) the AL value of Eu:CaHfO 3 was quite high.

Conclusions
Eu:CaHfO 3 single crystals were synthesized by the FZ method, and their PL and scintillation properties were investigated.PL QY was the highest at 42.2% in 1.0% Eu:CaHfO 3 under the excitation and emission wavelengths of 300 and 550-750 nm, respectively.The value was higher than those of Ce-and Pr-doped CaHfO 3 (32.6%and 6.8%, (18,24) respectively).The scintillation spectra showed several emission peaks from 400 to 690 nm originating from the 4f-4f transitions of Eu 3+ .0.05% Eu:CaHfO 3 showed the highest emission intensity among the present samples.0.01% Eu:CaHfO 3 showed the lowest AL value of 1,000 ppm among the samples, and this value was higher than those of Ce-and Pr-doped CaHfO 3 (666 and 228 ppm, (18,24) respectively).The results obtained in this study showed that the material properties of the Eu:CaHfO 3 single crystal were inadequate for practical use.On the other hand, previous studies showed that CaHfO 3 has excellent scintillation properties. (21,26)Hence, the scintillation properties when using dopants other than Eu and the rare-earth elements considered in previous studies should be evaluated.

Fig. 5 .
Fig. 5. (Color on li ne) PL decay cu r ves of Eu:Ca Hf O 3 .T he mon itored a nd excit at ion wavelengths were 620 and 300 nm, respectively.

Fig. 8 .
Fig. 8. (Color on l i ne) A f te rglow cu r ves of Eu:CaHfO 3 with 2 ms X-ray irradiation.