Cadmium tungstate (CdWO4 or CWO), the cadmium salt of tungstic acid, is a dense, chemically inert solid which is used as a scintillation crystal to detect gamma rays. It has density of 7.9 g/cm3 and melting point of 1325 °C. It is toxic if inhaled or swallowed. Its crystals are transparent, colorless, with slight yellow tint. It is odorless. Its CAS number is . It is not hygroscopic.
| Names | |
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| IUPAC name
Cadmium(II) tungstate
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| Identifiers | |
3D model (JSmol)
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| ChemSpider | |
| ECHA InfoCard | 100.029.297 |
| EC Number |
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PubChem CID
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| UNII | |
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| Properties | |
| CdWO4 | |
| Molar mass | 360.25 g·mol−1 |
| Appearance | colorless crystals with a yellow tint |
| Density | 7.9 g/cm3, solid |
| Melting point | 1,325 °C (2,417 °F; 1,598 K) |
| 0.04642 g/100 mL (20 °C) | |
| Hazards | |
| GHS labelling: | |
 
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| Warning | |
| H302, H312, H332, H410 | |
| P261, P264, P270, P271, P273, P280, P301+P312, P302+P352, P304+P312, P304+P340, P312, P322, P330, P363, P391, P501 | |
| NIOSH (US health exposure limits): | |
PEL (Permissible)
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[1910.1027] TWA 0.005 mg/m3 (as Cd)[1] |
REL (Recommended)
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Ca[1] |
IDLH (Immediate danger)
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Ca [9 mg/m3 (as Cd)][1] |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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The crystal is transparent and emits light when it is hit by gamma rays and x-rays, making it useful as a detector of ionizing radiation. Its peak scintillation wavelength is 480 nm (with emission range between 380 and 660 nm),[2] and efficiency of 13000 photons/MeV. It has a relatively high light yield, its light output is about 40% of NaI(Tl), but the time of scintillation is quite long (12−15 μs).[2] It is often used in computed tomography. Combining the scintillator crystal with externally applied piece of boron carbide allows[citation needed] construction of compact detectors of gamma rays and neutron radiation.
Cadmium tungstate was used as a replacement of calcium tungstate in some fluoroscopes since the 1940s.[3][4] Very high radiopurity allows use of this scintillator as a detector of rare nuclear processes (double beta decay, other rare alpha and beta decays) in low-background applications.[5] For example, the first indication of the natural alpha activity of tungsten (alpha decay of 180W) was found in 2003 with CWO detectors.[6] Due to different time of light emission for different types of ionizing particles, the alpha-beta discrimination technique has been developed for CWO scintillators.[7]
Cadmium tungstate films can be deposited by sol-gel technology. Cadmium tungstate nanorods can be synthesized by a hydrothermal process.[8]
Similar materials are calcium tungstate (scheelite) and zinc tungstate.
It is toxic, as are all cadmium compounds.
References
edit- ^ a b c NIOSH Pocket Guide to Chemical Hazards. "#0087". National Institute for Occupational Safety and Health (NIOSH).
- ^ a b Burachas S. F.; et al. (1996). "Large volume CdWO4 crystal scintillators". Nucl. Instrum. Methods Phys. Res. A. 369 (1): 164–168. Bibcode:1996NIMPA.369..164B. doi:10.1016/0168-9002(95)00675-3.
- ^ "Patterson Hand-Held Fluoroscope (ca. 1940s)". Oak Ridge Associated Universities. 2021. Retrieved 2021-10-12.
- ^ Kroeger, F. A. (1948). Some Aspects of the Luminescence of Solids. Elsevier.
- ^ Bardelli L.; et al. (2006). "Further study of CdWO4 crystal scintillators as detectors for high sensitivity 2β experiments: Scintillation properties and pulse-shape discrimination". Nucl. Instrum. Methods Phys. Res. A. 569 (3): 743–753. arXiv:nucl-ex/0608004. Bibcode:2006NIMPA.569..743B. doi:10.1016/j.nima.2006.09.094. S2CID 7311888.
- ^ Danevich F. A.; et al. (2003). "α activity of natural tungsten isotopes". Phys. Rev. C. 67 (1): 014310. arXiv:nucl-ex/0211013. Bibcode:2003PhRvC..67a4310D. doi:10.1103/PhysRevC.67.014310. S2CID 6733875.
- ^ Fazzini T.; et al. (1998). "Pulse-shape discrimination with CdWO4 crystal scintillators". Nucl. Instrum. Methods Phys. Res. A. 410 (2): 213–219. Bibcode:1998NIMPA.410..213F. doi:10.1016/S0168-9002(98)00179-X.
- ^ Wang Y, Ma J, Tao J, Zhu X, Zhou J, Zhao Z, Xie L, Tian H (September 2006). "Hydrothermal synthesis and characterization of CdWO4 nanorods". Journal of the American Ceramic Society. 89 (9): 2980–2982. doi:10.1111/j.1551-2916.2006.01171.x.