|Solubility in H2O, g/100g@25oC||85.5||0.5||–||–|
|Relative Light Output [photons/keV]||54||13||9||32|
|Relative Rad Hardness||Medium||High||High||High|
|Wavelength of Maximum Emission [nm]||550||475||480||420|
|Primary Decay Time (ns)||1000||14000||300||36|
|Afterglow||0.5-5% @6ms||0.1% @3ms||0.005% @3ms||<0.1% @6ms|
CdWO4 high light output and low afterglow make it ideal for use with silicon photodiodes in detectors for medical and industrial CT scanners. CdWO4 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 container and vehicle scanning.
CsI(Tl) has the highest light output of these scintillators, and its emission matches well with silicon photodiodes. However, its long afterglow limits its use to applications for which intervals between sampling are long or some residual signal can be tolerated. CsI(Tl) is a rugged, malleable material that can be easily fabricated into a variety of geometries. It is slightly hygroscopic but is packaged in a manner to minimize exposure to moisture. CsI(Tl) is regularly fabricated into both linear and 2-dimensional (2D) arrays with pixel sizes as small as 500 microns square.
LYSO is a Cerium doped lutetium based scintillation crystal that offers high density and a short decay time. It has an improved light output and energy resolution compared to BGO, which has a similar density. Applications that require higher throughput, better timing and better energy resolution will benefit from using LYSO material.
Saint-Gobain Crystals fabricates arrays from a variety of other scintillation materials including BGO and ceramics.
There are choices of scintillator materials and separator / reflectors to optimize performance to a specific application. The listing of parameters addresses the elements that must be considered in the design of a linear or 2D array.
The table below shows the materials and the associated pixel sizes that are regularly produced today. The pixel sizes are controlled primarily by mechanical properties of the crystals, e.g. hardness, cleavage, ease of machining. For example, CdWO4 has a cleavage plane in one crystallographic direction. For that reason, it is not possible, with current techniques, to achieve 0.3 x 0.3 mm2 pixels because of fractures along the cleavage planes that occur during cutting and grinding in manufacture. However, 0.3 x 1.0 mm2 pixels can be produced.
|Minimum Discrete Pixel Sizes Available
in Crystal Scintillators
|Material||Minimum Pixel Sizes *||Comments|
|Linear (mm)||2D (mm)|
|* Guidelines, not hard numbers|
|Separator Types and Thicknesses in Order of Decreasing Reflectivity|
|Material||Thickness Range||Approximate Relative
|White Powder (e.g. TiO2, MgO) **||0.25 mm and up||100%|
|Teflon Sheet **||0.15 mm - 0.50 mm||98%|
|White Reflector Paint||0.04 mm - 0.10 mm||96%|
|White Plastic||0.05 mm and up||95%|
|White Epoxy||0.10 mm - 0.75 mm||94%|
|Composites ***||0.10 mm and up||94%|
|Aluminum/Epoxy||0.05 mm - 0-.1 mm||75%|
|Metals (Pb, Ta) / Epoxy||0.05 mm and up||65%|
|* These are guidelines only and are based on optimum, not minimum, thickness.
Values will vary with pixel geometry, surface finish and other specific design parameters.
** These are used as reflector materials in large scintillation crystal packaging.
*** Composite separators are clear epoxy-paint-clear epoxy, white epoxy- metal-white epoxy.
|1D Array||2D Array|
|Example Model Numbers||82.58X4.2A30/16/5.2CsI(Tl)||82.58X4.2A30/16x4/5.2x4CsI(Tl)|
|Active area length||82.58||82.58|
|2||Active area height||4.175||4.175|
|3||X-ray crystal depth (Z)||30||30|
|4||Number of pixels
If the array is 2D, this is in the format [Pixels X] x [Pixels Y]
|5||Pitch [X + Gap X(A)]
If the array is 2D AND the pitch is different in X and Y, this is in the format [X+GapX(A)]x[Y+GapY(B)].
Recent advances in CsI(Tl) array manufacturing have resulted in afterglow reduction, improved light output, and afterglow uniformity. Single energy, dual-energy, fixed frame, rotating gantry, CsI(Tl) based arrays can be used in almost any high-quality X-Ray imaging application in a multitude of industries (Security Baggage Scanning, Cargo Scanning, Medical, Non-Destructive Industrial Inspection). To download the product performance sheet, click here.
Array Performance (Typical)
X-Ray Test Parameter
|Light output uniformity||±10% within an array
(requires matching photodiode)
|X-Ray Power||120KV @ 1mA|
|Light output array to array||±10%||Irradiation Time||2.5 Seconds|
|Afterglow||5000ppm @100ms (initial test)
≤2500ppm (after burn in)
|Afterglow uniformity||±10% within an array||Distance Crystal to X-Ray Source||60 cm|
Array Design Capabilities
|Typical Sampling Rate||1 ms|
|Number of channels (typical)||8-64||
Afterglow in PPM (parts-per-million) is calculated by dividing the output at 100 ms by the reference output with the X-Ray beam on, times 1 million to convert it to PPM:
|Minimum pitch||0.5mm||Light output calculation:
-% uniformity = (Min-Avg)/Avg
+% uniformity = (Max-Avg)/Avg
|X-Ray thickness||50mm max|