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Transparent ceramic scintillators of La2Hf2O7:Ti4+ were developed by a novel combustion synthesis method. The optical transmittance for a 1.0-mm-thick specimen is about 60% of the incident light, and the x-ray stopping power is also quiet high. The broad emission band centered at 475 nm originates from the oxide-Ti4+ charge-transfer transitions, which renders fast decay time on the order of 10 ␮s. The highest relative light output has reached about 1.5 times that of Bi4Ge3O12 (BGO) single crystal when excited by 120 kV x-rays.

X-ray computed tomography (CT) creates a crosssectional image by digitally imaging the x-ray absorption coefficient in a particular part of the human body. The performance of an x-ray CT is strongly influenced by its x-ray detectors. At present, most x-ray CTs have used solid-state detectors constructed using scintillators coupled to Si photodiode arrays. X-ray energy is first converted to light energy, and then the light energy is converted to electric current by the Si photodiode. The desirable properties required for scintillator materials in x-ray CT detectors are (i) high light output, (ii) low afterglow, (iii) high x-ray absorption efficiency, (iv) low radiation damage, (v) high uniformity, (vi) high machinability, (vii) chemical stability, and (viii) a spectral match to the sensitivity of Si photodiodes.1,2 Scintillators in single-crystal form have excellent optical transparency and were once used for x-ray CT detectors. In recent years, many single-crystal scintillators with fast decay time and high light output have been reported.3–6 However, it is rather difficult, if not impossible, to grow single-crystal scintillator materials with uniform chemistry to control the critical properties of afterglow, radiation damage, and x-ray hysteresis. The ceramic route provides a practical option of evaluating multicomponent chemical compositions and dopants to tailor the scintillator properties of the host material. Recently, ceramic scintillators, such as (Y,Gd)2O3:Eu,2 Gd2O2S:Pr,Ce,F,7 have been developed for x-ray CT detectors. Transparent (Y,Gd)2O3:Eu is the first ceramic scintillator used for medical x-ray CT detectors. Its high light output combined with excellent transparency and the low values of afterglow and radiation damage allows

(Y,Gd)2O3:Eu ceramic to be widely used in commercial x-ray CT scanners.2 However, its slow decay time of ∼1 ms cannot meet the requirements of current fast-speed x-ray CT detectors. The newly developed Pr3+-activated Gd2O2S ceramic scintillator is the second ceramic scintillator used in CT scanners.7 The Pr3+ in the lattice of Gd2O2S exhibits a blue-green transition between states of the same multiplicity (3Pj–3Hk), which renders high light output superior to that of CdWO4 and fast decay time on the order of 4 ␮s. Unfortunately, the hexagonal oxysulfide cannot be sintered into fully transparent ceramics. Ti4+-activated La2Hf2O7 powder has an efficient emission and high x-ray stopping power. It was found that the output of La2Hf2O7:2%Ti reaches 1.6 times tha