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像素化阵列
线性阵列组件（单行像素）线性阵列最常在行李扫描仪、货物检查和其他无损探伤方面使用。CdW04、LYSO和CsI（Tl）为此类应用的常用闪烁材料（但不限于此）。在用于医学应用时，诸如LYSO等晶体可用于骨密度测定仪器。
2D阵列组件（像素以XY矩阵排列）2D阵列最常用于医学扫描仪（PET、SPECT、CT）和无损探伤。此类应用的常用闪烁材料为LYSO、BGO、和CsI（TL）（但不限于此）。
特点与优势
特点包括极小的像素分离、更少光学串扰，优异的像素对像素均匀性以及阵列（批次对批次）均匀性。圣戈班还可提供包含光电二极管、PSPMT或SiPM等显示设备或任何客户提供设备的全面集成设计。
设计选项
在像素尺寸、像素数量、像素分离材料（反射和/或辐射屏蔽）和晶体表面光洁度方面存在很多设计选项。线性阵列组件通常与像素化光电二极管配合使用。2D阵列通常与像素化光电二极管、位置敏感型光电倍增管（PSPMT）和硅光电倍增器（SiPM）配合使用。
|
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 | 1μs | 14μs | 300μs | 41μs |
| 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 | ||
|---|---|---|---|
|
|
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 | |||||
