Cadmium tungstate

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Cadmium tungstate
Names
IUPAC name
Cadmium(II) tungstate
Identifiers
ECHA InfoCard 100.029.297
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
Harmful (Xn)
US health exposure limits (NIOSH):
PEL (Permissible)
[1910.1027] TWA 0.005 mg/m3 (as Cd)[1]
REL (Recommended)
Ca[1]
IDLH (Immediate danger)
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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Infobox references

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 7790-85-4. It is not hygroscopic.

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 380-660 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 1940's.[3][4] Very high radiopurity allows to use 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) had been 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]

  1. ^ a b c "NIOSH Pocket Guide to Chemical Hazards #0087". National Institute for Occupational Safety and Health (NIOSH). 
  2. ^ a b Burachas S. F.; et al. (1996). "Large volume CdWO4 crystal scintillators". Nucl. Instrum. and Methods in Phys. Research A. 369 (1): 164–168. doi:10.1016/0168-9002(95)00675-3. 
  3. ^ "Patterson Hand-Held Fluoroscope (ca. 1940s)". Oak Ridge Associated Universities. 1999. Retrieved 2008-04-26. 
  4. ^ Kroeger, F. A. (1948). Some Aspects of the Luminescence of Solids. Elsevier. 
  5. ^ 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. and Methods in Phys. Research A. 569 (3): 743–753. doi:10.1016/j.nima.2006.09.094. 
  6. ^ Danevich F. A.; et al. (2003). "α activity of natural tungsten isotopes". Phys. Rev. C. 67 (1): 014310. doi:10.1103/PhysRevC.67.014310. 
  7. ^ Fazzini T.; et al. (1998). "Pulse-shape discrimination with CdWO4 crystal scintillators". Nucl. Instrum. and Methods in Phys. Research A. 410 (2): 213–219. doi:10.1016/S0168-9002(98)00179-X. 
  8. ^ 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. Retrieved 2008-04-26. 

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