Do you know the classification and introduction of industrial CT radiation detectors?
(1) Industrial CT discrete detector
There are two main types of detectors used in industrial CT—discrete detectors and area detectors. There are two types of X-ray detectors commonly used in discrete detectors: gas and scintillation.
Gas detectors have natural collimation characteristics, which limit the influence of scattered rays; there is almost no crosstalk; and the device has good consistency. The disadvantage is that the detection efficiency is not easy to improve, and high-energy applications have certain limitations; secondly, the detection unit interval is several millimeters, which is too large for some applications.
More widely used are scintillation detectors. The photoelectric conversion part of the scintillation detector can be a photomultiplier tube or a photodiode. The former has an excellent signal-to-noise ratio, but because of the large device size, it is difficult to achieve a high degree of integration, and the cost is also high. The most widely used in industrial CT is the scintillator-photodiode combination.
The main advantage of the discrete detector using the scintillator is that the depth of the scintillator in the ray direction can be unlimited, so that most of the incoming X photons are captured and the detection efficiency is improved. Especially under high-energy conditions, the acquisition time can be shortened; because the scintillators are independent, there is almost no optical interference; at the same time, there are tungsten or other heavy metal spacers between the scintillators, which reduces the X-ray interference. The readout speed of discrete detectors is very fast, on the order of microseconds. At the same time, the accelerator output pulse can be used to gate data acquisition, minimizing the noise superimposed on the signal. Discrete detectors are also the least sensitive to radiation damage.
The main disadvantage of discrete detectors is that the pixel size cannot be made too small, and its adjacent interval (pitch) is generally greater than 0.1mm; in addition, the price is also more expensive.
(2) Industrial CT surface detector
There are three main types of area detectors: high-resolution semiconductor chips, flat panel detectors, and image intensifiers. Semiconductor chips are further divided into CCD and CMOS. The CCD is not sensitive to X-rays, and the surface is covered with a layer of scintillator to convert X-rays into visible light that CCD is sensitive to.
The semiconductor chip has the smallest pixel size and the largest number of detection units. The pixel size can be as small as about 10 microns. The number of detection units depends on the maximum size of silicon single crystal, and the diameter is generally more than 50mm. Because the detection unit is small and the signal amplitude is small, several detection units can be combined in order to increase the measurement signal.
In order to enlarge the effective detector area, they can be optically coupled to a large-area scintillator with lenses or optical fibers. Theoretically, the effective area of the detector can be extended to any required length in one direction by using the fiber coupling method. The technology of using optical coupling can also keep these semiconductor devices away from the direct radiation of X-ray beams to avoid radiation damage.
Flat panel detectors are usually made of amorphous silicon or amorphous selenium covered with scintillation crystals (such as CsI) of hundreds of microns. The pixel size is 127 or 200μm, and the panel size* is about 45cm (18in). The readout speed is about 3-7.5 frames/s. The advantage is that it is relatively simple to use and there is no image distortion. The image quality is close to that of film photography, and it can basically be used as an upgrade product of the image intensifier. The main disadvantage is that the scintillation crystal covered on the surface cannot be too thick, and the detection efficiency of high-energy X-rays is low; it is difficult to solve the problem of scattering and interference, which reduces the dynamic range. Radiation shielding of electronic circuits for higher energy applications. Generally speaking, the effect of using low energy below 150kV is better.
The image intensifier is a traditional area detector, which is a vacuum device. Nominal pixel size <100μm, diameter 152-457mm (6-18in). The readout speed can reach 15-30 frames/s, which is the fastest surface detector. Due to the inherent noise generated by statistical fluctuations in the image enhancement process, the image quality is relatively poor, and the general radiographic sensitivity is only 7-8%. In the case of data superposition using a computer, the radiographic sensitivity can be increased to more than 2%. Another disadvantage is that it is fragile and has image distortion. The basic advantage of area detectors is self-evident - it has a much higher ray utilization than line detectors. Surface detectors are also more suitable for 3D direct imaging. All surface detectors have common disadvantages due to structural reasons, that is, low ray detection efficiency; inability to limit scattering and interference; small dynamic range, etc. Higher energy ranges are less effective.
