Photoluminescence and Scintillation Properties of Ce-doped BaHfO 3 Crystals

The photoluminescence (PL) and scintillation properties of Ce-doped BaHfO 3 crystals were investigated. The Ce-doped BaHfO 3 crystal is transparent and colorless after annealing treatment under a reduction condition. The broad luminescence band at ~370 nm was observed under ~280 nm excitation with the decay time of ~17 ns. The X-ray-induced scintillation spectra also showed the broad luminescence band at ~380 nm with a fast decay. The origin of the broad luminescence band at ~380 nm is attributed to the 5d–4f transition of Ce 3+ . The pulse height distribution under 137 Cs gamma ray irradiation using Ce-doped BaHfO 3 exhibited a distinguishable photoabsorption peak. The light yield of Ce-doped BaHfO 3 is ~1600 photons/ MeV.

MHfO 3 (M = Ca, Sr, or Ba) have high density and effective atomic number.(40)(41)(42) The light yield of a Cedoped CaHfO 3 single crystal, which has a density of 6.9 g/cm 3 , is ~7800 photons/MeV with a fast decay of ~22 ns. (43)The light yield of Ce-doped CaHfO 3 is increased by Mg substitution. (44)he density of a SrHfO 3 single crystal is 7.6 g/cm 3 , and the light yield is ~400 photons/MeV. (45)aHfO 3 has the highest density of 8.3 g/cm 3 among the MHfO 3 series.The scintillation properties of Ce-doped BaHfO 3 transparent ceramics have been investigated. (46)In this work, the integrated light yield was investigated; however, there is no report on the light yield of Cedoped BaHfO 3 determined from the pulse height distribution.
In this study, Ce-doped BaHfO 3 crystals were synthesized by the floating zone (FZ) method, and the photoluminescence (PL) and X-ray-induced scintillation spectra and decay curves were measured.Finally, the pulse height distribution under 137 Cs gamma-ray irradiation was investigated to estimate the quantitative light yield of Ce-doped BaHfO 3 .

Materials and Methods
Ce-doped BaHfO 3 crystals were synthesized by the FZ method.Four xenon lamps as a heating source were installed in the FZ instrument.The details of the equipment are found in Ref. 47.The feed rods were synthesized by a simple sintering method.The starting materials, namely, BaCO 3 (99.99%,Rera Metallic), HfO 2 (99.95%,Furuuchi Chemical), and CeO 2 (99.99%,Furuuchi Chemical), were mixed using a mortar and pestle.The mixed powder was sintered at 1200 °C for 8 h.The sintered powder was remixed and further sintered at 1400 °C for 8 h.The resintered powder was hydrostatically pressed into a cylindrical rod and again sintered at 1500 °C for 8 h.The amount of BaCO 3 in this powder was increased by 30% from the stoichiometric composition of BaHfO 3 to compensate for the volatilization of Ba during the crystal growth.The pulling-down rate was 50 mm/h, and the rotation rates of the upper and lower shafts were 28 and 25 rpm, respectively.The synthesized crystals were treated under a reduction atmosphere to suppress the formation of Ce 4+ ions.The crystals were put into graphite powder under vacuum.The heating temperature was 800 °C, and the treatment time was 96-144 h.The treatment time was determined on the basis of the appearance of the samples.
The measurement range of the X-ray diffraction (XRD) patterns was 2θ = 10 to 70 degrees (Rigaku, Miniflex600), and the X-ray source was CuKα.The PL excitation and emission map and quantum yield (QY) were measured using the Hamamatsu Photonics C11347 spectrometer.The PL decay curves were also measured using the Hamamatsu Photonics C11367 spectrometer.The X-ray-induced scintillation spectra, decay curves, and afterglow curves were measured with original setups. (48,49)The voltage and current applied to the X-ray tube for the scintillation spectra were 40 kV and 1.2 mA, respectively.The pulse height distribution under 137 Cs gammaray irradiation was measured using a laboratory-made setup. (48)The PMT used was of the ultrabi-alkali type (Hamamatsu Photonics, R7600-200).

Results and Discussion
A photograph of the synthesized samples is shown in Fig. 1.The as-synthesized samples were brown.The 1 and 5% samples were transparent, and the 0.3 and 3% samples were opaque.The 0.3 and 1% samples were black because some graphite powders were inside the sample.The 3% sample was also partially black for the same reason.The 5% sample was orange.The assynthesized sample was brown; thus, the treatment time or temperature of the 5% sample was insufficient compared with those of the other samples.To investigate the absorption wavelength, diffuse transmission spectra were measured, and the results are shown in Fig. 2. Absorption bands were observed at ~250 and 300 nm in all the samples.In contrast, the 5% sample showed a deep absorption band at ~450-550 nm compared with the 1% sample.The 5% sample was orange, as shown in Fig. 1, because the absorption band was at ~450-550 nm.The transmittance of the 0.3% sample was the lowest among the samples, and the black color contributes to the low transmittance.
Figure 3 shows the XRD patterns of the synthesized samples and references.All the samples included BaHfO 3 (ICDD.24-0102) and monoclinic HfO 2 (COD.9013470).The melting point of BaHfO 3 is quite high, and the vapor pressure of Ba is also high; thus, Ba was volatilized during crystal synthesis, and HfO 2 remained in the samples.The XRD peak position did not seem to shift.The ionic radii of Ba 2+ (1.61 Å in twelve-coordination), Hf 4+ (0.71 Å in six-coordination), Ce 3+ (1.34 Å in twelve-coordination and 1.01 Å in six-coordination), and Ce 4+ (1.14 Å in twelvecoordination and 0.87 Å in six-coordination) are not the same in the same coordination number. (50)The doped Ce ions were considered to be incorporated into the Ba or Hf sites; thus, the peak shift was quite small.
Figure 4 shows the PL excitation and emission map of the 1% sample as the representative sample because the other samples also exhibit similar spectra but with a different intensity, and  the as-synthesized samples showed a rather low luminescence intensity.The horizontal and vertical axes are emission and excitation wavelengths, respectively.All the samples exhibited the broad luminescence band at ~370 nm under the excitation wavelength of ~300 nm.These emission and excitation wavelengths were similar to those in previously reported results of Cedoped BaHfO 3 . (41,51)The PL QYs were 11.6% for the 0.3% sample, 30.5% for the 1% sample, 18.0% for the 3% sample, and 11.8% for the 5% sample.The highest PL QY was observed in the 1% sample, which was colorless compared with the same sample before treatment.The brownish color is attributed to Ce 4+ . (52)The sample treatment was conducted to suppress the formation of Ce 4+ ions because Ce 4+ ions do not contribute to the luminescence.Thus, the colorless or white sample involved less Ce 4+ ions and emitted luminescence due to the 5d-4f transition of Ce 3+ , leading to an increase in the PL QY.
Figure 5 shows the PL decay curves of Ce-doped BaHfO 3 .The excitation and monitoring wavelengths were 280 and 370 nm, respectively.The decay curves were fitted by one exponential function.The obtained PL decay time is listed in Table 1.The PL decay time of ~17 ns was faster than the typical decay time due to the 5d-4f transition of Ce 3+ .For example, the decay times were ~60 ns (Ce-doped Gd 2 SiO 5 ), (53) ~40 ns (Ce-doped Lu 2 SiO 5 ), (54) and ~100 ns (Ce-doped Y 3 Al 5 O 12 ). (55)In contrast, the decay time of Ce-doped YAlO 3 and LuAlO 3 was ~17 ns. (4)Shorter wavelengths and higher refractive indices are associated with faster decay times.The emission wavelength of Ce-doped BaHfO 3 was relatively short, and the refractive index was greater than 2. (56) Therefore, the decay time of Ce-doped BaHfO 3 was fast.
Figure 6 shows the X-ray-induced scintillation spectra of Ce-doped BaHfO 3 .The peak wavelength was ~380 nm, which was slightly shifted to a longer wavelength compared with the PL emission peak.The measurement geometry of X-ray-induced scintillation spectra was of the transmission type.In contrast, the PL spectrum measurement was conducted in the reflectiontype geometry.Thus, the scintillation light was absorbed by the sample, and the peak wavelength was shifted to a longer wavelength.
Figure 7 shows the X-ray-induced scintillation decay curves of Ce-doped BaHfO 3 .The observed decay curves contained fast and slow decay components.The obtained decay time is    listed in Table 1.The fast decay time was ~14-18 ns, which was close to those in PL decay.The origin of the fast decay was attributed to the 5d-4f transition of Ce 3+ owing to the decay time, the broad emission band, and the emission wavelength, as shown in Fig. 5.The slow decay was typically observed in the Ce-doped MHfO 3 (M = Ca and Sr) series, (38,43,44,57) and the origin was attributed to some types of defect.
Figure 8 shows the afterglow curves of Ce-doped BaHfO 3 .The afterglow level is defined as the ratio of the intensity at 20 ms after stopping X-ray exposure to the intensity during X-ray exposure.The afterglow levels of the 0.3, 1, 3, and 5% samples were 1550, 420, 1040, and 860 ppm, respectively.These afterglow levels were lower than those in the Ce-doped MHfO 3 series. (38,43,45)igure 9 shows the pulse height distribution of the 137 Cs gamma ray using Ce-doped BaHfO 3 and Bi 4 Ge 3 O 12 (BGO).The 1 and 3% samples exhibited a discriminable peak due to photoabsorption.The light yields of the 1 and 3% samples were 1600 and 1300 photons/MeV, respectively, compared with the peak position of BGO (6800 photons/MeV) and the quantum efficiency of PMT.Although the 3% sample had a black region on the sample surface, the peak was observed.Thus, the black region did not affect the results of the pulse height distribution.The reported integrated light yield of Ce-doped BaHfO 3 was ~40000 photons/MeV. (46)The integrated light yield is estimated on the basis of the integrated intensity of the scintillation spectrum compared with a reference and a sample.The integrated light yield depends on not only the light yield, but also the stopping power of materials. (58)Thus, we must evaluate the light yield using the pulse height distribution.The light yield of the present Ce-doped BaHfO 3 was more than 10 times lower than the previous result. (46)

Conclusions
The PL and scintillation properties of Ce-doped BaHfO 3 were investigated.The synthesized samples contained not only the BaHfO 3 phase but also the monoclinic HfO 2 phase.The assynthesized BaHfO 3 showed low emission intensity.After the annealing treatment under the reduction condition, the emission intensity and the PL QY increased.The 1% sample exhibited the highest PL QY of 30.5% among the samples.The broad luminescence band at ~370-380 nm was observed in PL and scintillation.The PL and scintillation decay curves showed a fast decay of ~17 ns and a slow decay of ~241-375 ns.Ce-doped BaHfO 3 crystals exhibited a photoabsorption peak under 137 Cs gamma-ray irradiation, and the maximum light yield was 1600 photons/MeV.The Ce-doped MHfO 3 (M = Ca, Sr, and Ba) showed blue luminescence and fast decay under X-ray irradiation.The light yield of Ce-doped BaHfO 3 is lower than that of Cedoped CaHfO 3 , (43) but higher than that of Ce-doped SrHfO 3 . (45)

Table 1
PL and scintillation decay times of Ce-doped BaHfO 3 .