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 is a chemical compound consisting of cadmium (Cd), tungsten (W), and oxygen (O). It is primarily known for its use in scintillation detectors, particularly in the field of radiation detection.
Cadmium tungstate exhibits excellent scintillation properties, meaning it can emit flashes of light when exposed to ionizing radiation (such as X-rays, gamma rays, or charged particles). 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.
Properties
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.
- Chemical formula: 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)
- Solubility in water: 0.04642 g/100 mL (20 °C)
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), 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).
Applications
- Scintillation Detectors: One of the main uses of CdWO₄ is in scintillation detectors, where it serves as a scintillator material. When ionizing radiation strikes the crystal, it produces visible light, which can be detected by photodetectors (like photomultiplier tubes). This makes it useful in a variety of radiation detection devices, including medical imaging, nuclear physics experiments, and environmental monitoring.
- Medical Imaging: CdWO₄ is used in some types of positron emission tomography (PET) scanners and gamma cameras. The material’s ability to detect gamma radiation with high efficiency makes it a valuable component in medical diagnostics.
- High-Energy Physics: CdWO₄ is used in experiments and detectors that measure high-energy particles, such as in particle accelerators or in astrophysics for detecting cosmic rays or gamma rays.
- Environmental Radiation Monitoring: CdWO₄ is sometimes used in monitoring devices that detect radiation in the environment, especially in areas of potential contamination from nuclear sources.
- X-ray Detectors: In industrial applications, cadmium tungstate can also be used for X-ray detection, as it is sensitive to high-energy radiation.
Advantages
- High Atomic Number: The high atomic number of tungsten (W) contributes to better detection of high-energy radiation (like gamma rays and X-rays).
- Good Scintillation Efficiency: CdWO₄ offers a relatively high light yield, meaning it can efficiently convert absorbed radiation into visible light for detection.
- Thermal and Radiation Stability: It is relatively stable under thermal and radiation exposure, which is important for long-term use in detecting high-energy particles.
Disadvantages
- Toxicity: Cadmium is toxic, and special precautions must be taken during the handling and disposal of cadmium tungstate crystals.
- Cost: The synthesis of cadmium tungstate can be relatively expensive, which might limit its use in some applications.
