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Pixellated Arrays
Offering pixellated array assemblies that include crystals such as CdW04, CsI(Tl), BGO, and LYSO or other materials such as scintillating plastic. Saint-Gobain can also produce crystal arrays made from other materials not grown by Saint-Gobain. We offer a variety of design options and reflector materials to optimize array performance for your application. Our manufacturing process ensures high light output as well as excellent pixel to pixel uniformity with minimal crosstalk.
The type of material depends on the application, and the information being sought.
| Applications Well-suited for Scintillation Array Materials | ||||
|---|---|---|---|---|
| Material | CsI(TL) | CdWO4 | BGO | LYSO |
| Gamma Cameras | X | |||
| CT Imaging | X | X | ||
| Positron Emission Tomography | X | X | X | |
| Line Scanning | X | X | ||
| Industrial Imaging | X | X | X | |
| Flash Radiography | X | X | ||
| Digital Radiography | X | X | ||
The optimal material choice involves a combination of factors including the application, the photo readout device to be used, etc. For example, if the application calls for rapid detection of radiation pulses, then the decay time and afterglow drive the choice (BGO, LYSO, CdWO4). If high efficiency and low cost are paramount, then NaI(Tl) with a PMT readout or CsI(Tl) with a photodiode readout are the first detectors to consider. The physical properties of each material must be considered as well. For example, the natural cleavage plane in CdWO4 restricts the minimum size of 2D pixels. Saint-Gobain’s technical experts can help guide your decision. Talk to us about your application or request a quotation.
Features/Benefits
- Minimal pixel separation
- Reduced optical crosstalk
- Excellent pixel to pixel uniformity
- Array to array (batch to batch) uniformity.
- Offers a fully integrated design with several choices of readout devices including photodiodes, PSPMTs, or SiPMs, or any customer-supplied devices.
Design Options
There are many design options in terms of pixel size, the number of pixels, pixel separator materials (reflective and/or radiation barrier) and crystal surface finishes.
Linear Array Assemblies (single row of pixels) – Linear array assemblies are typically used with pixelated photodiodes and used quite often in security type applications, most common are baggage scanners, cargo inspection, and other non-destructive testing. When used in a medical application, crystals such as LYSO are used in Bone Mineral Densitometry machines.
2D Array Assemblies (pixels arranged in an X-Y matrix) – 2D arrays are typically used with pixelated photodiodes, position-sensitive photomultiplier tubes (PSPMT) and silicon photomultipliers (SiPM) and most common use is in medical imaging scanners (PET, SPECT, CT), and non-destructive testing and inspection.
|
Material |
CsI(Tl) | BGO | ||
|---|---|---|---|---|
| Density, g/cm3 | 4.51 | 8.0 | 7.13 | 7.10 |
| Solubility in H2O, g/100g@25oC | 85.5 | 0.5 | – | – |
| Hygroscopic | slightly | no | no | no |
| 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.
Design Parameters
- Material: Type of scintillation crystal or material desired.
- Pixel or Element Size: The “X” and “Y” dimensions of each scintillator pixel.
- Separator Type and Thickness: The type of reflector between the crystal pixels and its overall thickness, “Gap X(A)” or “Gap Y(B)”. Note: this may be a composite or laminate of white reflector and metal materials. The geometry of the pixel, the thickness of the reflector, the scintillator material used and other factors influence the reflectivity obtained in each array design. Array reflector materials are listed in the order of decreasing reflectivity.
- Pitch: This is the distance between the center of one element to the center of an adjacent element, “X” + “Gap X(A)” or “Y” + “Gap Y(B)” Note: In 2D arrays with rectangular pixels, the pitches in the “X” and “Y” directions will be different.
- Radiation Thickness: This is the “Z” dimension and specifies the thickness of the array in the direction of the incoming radiation.
- Back Reflector Thickness: Usually a white reflector is applied to the radiation entrance side of the array to reflect the light back into the pixel so it can be directed to the light sensor.
- Material adjacent to the end pixels or elements: The end crystals may need a special reflector thickness or other treatment, e.g., to keep a constant pitch from array to array if they will be joined together in use.
1D and 2D Configurations
| Minimum Discrete Pixel Sizes Available in Crystal Scintillators |
|||
|---|---|---|---|
| Material | Minimum Pixel Sizes * | Comments | |
| Linear (mm) | 2D (mm) | ||
| CsI(Tl) | 0.3 | 0.5 | |
| CdWO4 | 0.3 | 1.0 | Cleavage Plane |
| BGO | 0.3 | 0.3 | |
| LYSO | 0.8 | 0.8 | Min. Untested |
| * Guidelines, not hard numbers | |||
| Separator Types and Thicknesses in Order of Decreasing Reflectivity | ||
|---|---|---|
| Material | Thickness Range | Approximate Relative Reflectivity * |
| 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 | ||
|---|---|---|---|
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|
Example Model Numbers | 82.58X4.2A30/16/5.2CsI(Tl) | 82.58X4.2A30/16x4/5.2x4CsI(Tl) |
|
1 |
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] |
16 | 16x4 |
| 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)]. |
5.2 | 5.2x4 |
| 6 | Scintillator | CsI(Tl) | CsI(Tl) |
Linear CsI(Tl) Segmented Arrays
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 Measurements | 100ms | |||
| 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:
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| Minimum pitch | 0.5mm | Light output calculation: -% uniformity = (Min-Avg)/Avg +% uniformity = (Max-Avg)/Avg |
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| X-Ray thickness | 50mm max | |||||
