Dense and optical transparent CdWO 4 films by sol-gel processing for scintillation applications
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M. Bliss and S. Heald Pacific Northwest National Laboratory, Richland, Washington 99352
T.K. Sham and F. Heigl Department of Chemistry, University of Western Ontario, Ontario, N6A 5B7, Canada
G.Z. Caoa) Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98105 (Received 5 October 2006; accepted 27 December 2006)
In this paper, we report the first successful fabrication of dense and optically transparent cadmium tungstate (CWO) films by sol-gel processing and the study of their optical and x-ray scintillation properties. A new sol-gel processing method was developed using tungstic acid and cadmium nitrate as precursors and hydrogen peroxide as solvent; homogeneous and stable CWO sols were aged at room temperature and used for the preparation of CWO films. A rapid sintering process was investigated and found to be necessary to make dense and optically transparent nanocrystalline CWO films. CWO films were uniform, fully dense, and crack-free, with CWO as the only detectable crystalline phase, as determined by x-ray diffraction. The thickness, density, grain size, and crystallinity of CWO films are all found to be strongly dependent on the sintering conditions and in turn impact the optical and x-ray scintillation properties. Sol-gel-derived dense CWO films demonstrated intense photoluminescence and x-ray excited optical luminescence intensity. The relationships between sol-gel processing, nanostructures, and optical and x-ray scintillation properties are discussed in detail.
I. INTRODUCTION
Cadmium tungstate (CdWO4, CWO) is a luminescent material that absorbs the energy of high-energy radiation such as x-rays, ␥-rays, and neutrons and then converts the energy to visible or near visible light within a short time of less than 10−8 s,1 making it a valuable scintillation material. Due to its high emission efficiency (∼40% that of sodium iodide),2 high radiation stability, little afterglow, and high density, CWO has been widely used in high-energy radiation imaging and detection, spectrometry and radiometry of radionuclides in extra-low activities, and especially for computer tomography (CT).3,4 Moreover, based on radiation capture reactions, CWO has been investigated for application in neutron detection.5
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0215 J. Mater. Res., Vol. 22, No. 6, Jun 2007
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Although CWO has been widely used, most studies and applications of CWO were focused on single crystals synthesized by the Czocharalski method at high temperatures.6 Single-crystal CWO is difficult to grow at a high temperature since stoichiometric control is difficult to maintain due to incompatible vapor pressures of the constituents during crystal growth. The maintenance of chemistry stoichiometry is critical because the scintillation properties of CWO are highly dependent on its chemical composition. Conventional ceramic processing is difficult to apply to the fabricati
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