Cadmium tungstate (CdWO4) is a high density, high atomic number scintillator with a relatively high light yield. The emission maximum is at 475nm and the total light output is 12 to 15 photons/keV. The light yield relative to NaI(Tl) on a bialkali PMT is 30 to 50%.
The high light output and low afterglow make it ideal for use with silicon photodiodes in detectors for medical and industrial Computed Tomography (CT) scanners. It has very good radiation resistance, and its temperature dependence is small in the 0 to 60oC range. Its high density makes it a good choice for 300+ keV imaging for security luggage, container and vehicle scanning for security.
The intensity of the scintillation emission of CdWO4 (irradiation with γ-rays) at room temperature (25oC), the curve is relatively flat (-0.1%/oC) which can be important for some applications. For further technical information, see IEEE NS, Volume 41, No. 4, August 1994.
The intensity of the scintillation emission of CdWO4 as a function of wavelength distribution is a good match to photodiodes. CT scanners for high and low energies have been built with CdWO4. A long decay time of 14000 nanoseconds is not a drawback for CT scanners or other systems that operate with a current-mode readout.
Furthermore, CdWO4 has a low afterglow. This is important in preventing glare and streaking in the generated images and allows for faster processing.
| Material | K-edge keV | 40keV mm | 60keV mm | 80keV mm | 100keV mm | 300keV mm | 600keV mm |
|---|---|---|---|---|---|---|---|
| NaI | 33.2 | 0.43 | 1.3 | 2.7 | 4.9 | 49.2 | 99.2 |
| CsI | 33.2 | 0.29 | 0.8 | 1.8 | 3.3 | 36.5 | 79.3 |
| CdOW4 | 69.5 | 0.33 | 0.9 | 0.5 | 0.9 | 12.6 | 35.9 |
| BGO | 90.5 | 0.38 | 1.1 | 2.2 | 1.1 | 13.3 | 37.8 |
| LSO | 63.3 | 0.55 | 1.6 | 0.7 | 1.3 | 16.0 | 40.9 |
| LYSO | 63.3 | 0.58 | 1.65 | 0.8 | 1.4 | 17.3 | 43.2 |
CdWO4 has very little 228Th and 226Ra contamination and is suitable for low activity counting applications. (Reference: Nuclear Instruments and Methods, A369(1996) 164-168.)
| Density [g/cm3] | 7.9 |
| Melting point [K] | 1598 |
| Thermal expansion coefficient [C-1] | 10.2x10-6 |
| Cleavage plane | <010> |
| Hardness (Mho) | 4 - 4.5 |
| Hygroscopic | No |
| Wavelength of emission max. [nm] | 475 |
| Lower wavelength cutoff [nm] | 330 |
| Refractive index @ emission max | 2.2 - 2.3 |
| Primary decay time [ns] | 14000 |
| Light yield [photons/keVγ | 12 - 15 |
| Photoelectron yield [% of NaI(Tl)] (for γ-rays) | 30 - 50 |
Arrays with pixels as small as 0.3mm x 1mm have been provided. In other forms, round crystals range from 50mm diameter x .5mm minimum; for rectangular sizes, (depending on geometry), crystals can typically range in cross-sections as large as 130mm x 50mm or as thin as 10 microns (size dependant). Contact Saint-Gobain for specific applications and alternate sizes.
CdWO4 cleaves readily along one plane. This is an asset that is used in manufacturing to produce polished surfaces. Cleaved CdWO4 has been fabricated in lengths up to 7 inches.
CdWO4 is non-hygroscopic and does not require a hermetic package. Crystals are typically provided in block or array form and is a good match to photodiodes.
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