Cadmium tungstate (CdWO
4 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/cm
3 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.
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
Ionizing radiation (or ionising radiation), including nuclear radiation, consists of subatomic particles or electromagnetic waves that have sufficient energy to ionize atoms or molecules by detaching electrons from them. Some particles can travel ...
. Its peak scintillation wavelength is 480
nm (with emission range between 380-660 nm),
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).
It is often used in
computed tomography
A computed tomography scan (CT scan; formerly called computed axial tomography scan or CAT scan) is a medical imaging technique used to obtain detailed internal images of the body. The personnel that perform CT scans are called radiographers ...
. Combining the scintillator crystal with externally applied piece of
boron carbide
Boron carbide (chemical formula approximately B4C) is an extremely hard boron–carbon ceramic, a covalent material used in tank armor, bulletproof vests, engine sabotage powders,
as well as numerous industrial applications. With a Vickers hard ...
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. 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. For example, the first indication of the natural alpha activity of tungsten (alpha decay of
180W) had been found in 2003 with CWO detectors. Due to different time of light emission for different types of ionizing particles, the alpha-beta discrimination technique has been developed for CWO scintillators.
Cadmium tungstate films can be deposited by
sol-gel technology. Cadmium tungstate
nanorods can be synthesized by a
hydrothermal process.
Similar materials are
calcium tungstate (
scheelite) and
zinc tungstate
Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic ta ...
.
It is toxic, as are all cadmium compounds.
References
External links
Scintillator materials
{{Cadmium compounds
Cadmium compounds
Tungstates
Phosphors and scintillators