Cadmium tungstate

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.

Cadmium tungstate
Names
IUPAC name
Cadmium(II) tungstate
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.029.297
EC Number
  • 232-226-2
UNII
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 pictograms
GHS Signal word Warning
H302, H312, H332, H400, H410
P261, P264, P270, P271, P273, P280, P301+312, P302+352, P304+312, P304+340, P312, P322, P330, P363, P391, P501
NIOSH (US health exposure limits):
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

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 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) 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
  1. NIOSH Pocket Guide to Chemical Hazards. "#0087". National Institute for Occupational Safety and Health (NIOSH).
  2. Burachas S. F.; et al. (1996). "Large volume CdWO4 crystal scintillators". Nucl. Instrum. Methods Phys. Res. 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. Methods Phys. Res. A. 569 (3): 743–753. arXiv:nucl-ex/0608004. doi:10.1016/j.nima.2006.09.094. S2CID 7311888.
  6. Danevich F. A.; et al. (2003). "α activity of natural tungsten isotopes". Phys. Rev. C. 67 (1): 014310. arXiv:nucl-ex/0211013. doi:10.1103/PhysRevC.67.014310. S2CID 6733875.
  7. Fazzini T.; et al. (1998). "Pulse-shape discrimination with CdWO4 crystal scintillators". Nucl. Instrum. Methods Phys. Res. 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.
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