Industrial CT technology is the abbreviation of Industrial Computed Tomography, which was proposed by Randon J in 1917 and was not applied in nondestructive testing until 1970s. As the advancement of computer science and the development of detector technology booming, industrial CT has been improving significantly in recent years, which has widely used in aerospace, nuclear, military, as well as in manufacturing for non-destructive mapping and layered design and manufacturing. Industrial CT, a practical nondestructive testing method, is developing from the low-energy industrial CT that meets the general industrial application to the high-energy field that meets the testing requirements of large and complex structural parts.

Radiation detection server as the foundation of industrial CT. When the collimated beam of energy I0 passes through the object, the attenuation coefficient μi of each volume element in each transmission direction is different. As a result, the transmitted energy I received by the detector is also different. According to a certain image reconstruction algorithm, a thin section of the section of the inspected workpiece without image overlap can be obtained (Fig.1), and a new tomography image can be obtained by repeating the above process. When enough two-dimensional tomography images are measured, three-dimensional images can be reconstructed.

In fact, industrial CT is a ray-detection technology. Compared with conventional ray-detection technology, the main advantages of industrial CT are as follows: (1) The  image target of industrial CT is not blocked by the surrounding details. The feature of the target, such as spatial position, shape and size information, can be obtained directly. (2) Industrial CT has outstanding density resolution ability, and the density resolution of high-quality CT image can reach 0.1% or even higher. (3) industrial CT images are the results of digitization, which are convenient for storage, transmission, analysis and processing.

In industrial CT, X-ray sources are commonly used in X-ray machines and linear accelerators. There are three kinds of detectors commonly used: high resolution CMOS semiconductor chip, flat plate detector and scintillation detector. Flat panel detectors are typically made of amorphous silicon or amorphous selenium covered with scintillation crystals of several hundred microns, such as CsI, with pixel sizes of about 127 microns and image quality close to that of film. The advantage of scintillation detector is high detection efficiency, especially in high energy conditions, it can reach the dynamic range of 16 ~ 20 bits and read speed in the order of microseconds.

Detector is one of the core components of industrial CT system, which has great influence on the quality of reconstructed images. The following performance indexes should be considered in the design of detector: detection efficiency, linearity, dynamic range, uniform consistency, stability, size, response speed, number of channels, etc.

The signal conversion process of industrial CT detector can be clearly seen from the detector structure diagram (Fig.2). First, the ray signal enters the scintillator, where visible light is generated. Photodiodes convert visible light into current signals. Then, the i-v conversion circuit converts the current signal into a voltage signal. Finally, it is converted into digital signal by AD conversion device and transmitted to data transmission system.

From the description of the structure of scintillator detector, it can be seen that its working principle is as follows: when the ray enters the scintillator, secondary electrons are generated at a certain point, which ionize and excite the molecules or atoms of the scintillator and emit a large number of photons in the deexcitation. Due to the effect of the reflective layer around the scintillator, a large part of the fluorescence reaches the photosensitive surface of the photodiode through the photocoupling agent or optical guide. Photodiodes convert optical signals into current signals, and then convert and amplify them through subsequent circuits to output voltage signals that can be measured. To sum up, the working process of scintillator detector can be divided into the following stages:

(1) the ray enters the scintillator and interacts with it. The scintillator absorbs part of the ray energy to excite and ionize molecules, so energy deposition rate exists.

(2) when excited molecular deexcitation occurs, part of the energy is used for fluorescence; The other part of energy is converted into thermal motion, which heats up the scintillator, so the scintillator has absolute scintillation efficiency.

(3) collect as much fluorescence as possible on the photosensitive surface of the photodiode by using reflective materials. In this process, some photons will be absorbed or escape from the scintillator, so there will be light collection efficiency problems.

(4) after the fluorescence enters the p-junction of photodiode, the electron-hole pair is excited. Under the action of internal potential field or external bias voltage, the electrons and holes move to the poles respectively to form photocurrent, and the photocurrent is proportional to the light intensity.

An ideal scintillator crystal should have the following characteristics :

  • it should have high luminous efficiency to convert the energy of radiation into detectable optical signals.
  • the transformation process from ray to fluorescence is linear, that is, the number of photons produced should be proportional to the energy of scintillator deposition as far as possible.
  • there must be good light collection efficiency, and the scintillator should be transparent to the light it emits.
  • the scintillator shall have good optical properties and be made to meet the size required by the actual detector.

(5) the refractive index of the scintillator material should be similar to that of silicon, so that the fluorescence can be effectively coupled to the photosensitive surface of the photodiode array.

In fact, all existing scintillator materials cannot meet all the above characteristics at the same time. Therefore, the most suitable scintillator materials should be selected according to the practical application requirements. Currently, CsI(T1), BGO and CdWO4 are the most widely used inorganic scintillators in industrial CT and the scintillation properties are shown in Table 1. These kinds of scintillators are introduced respectively below.

propertiesCsI(Tl)BGOCdWO4
Light yield (photos/MeV)52000850013000
Peak wavelength (nm)550480515
Density (g/cm3)4.57.137.9
Radiation length (cm)1.861.121
Melting point (℃)62110501123
Refractive index ( peak wavelength)1.782.252.15
Decay time (ns)100030020000
Light yield coefficient (%/℃)0.32-1.60.1

CsI

Wavelength(Max. emission) (nm)410
Decay time (ns)40
Light yield (photons/keV)25
Light output relative to Nal(Tl)  (%)75
Refractive index1.82@410nm

With the development of nuclear technology and high-energy physics, the application fields of inorganic scintillation crystals have been broadened, and more and higher requirements have been placed on inorganic scintillation crystals. As early as the 1980s, people noticed the advantages of CsI(Tl) crystals: the emission spectrum can be matched with silicon photodiodes, the light yield is high, the irradiation length is shorter than NaI(Tl) crystal, and the mechanical properties are good. An excellent and practical scintillation crystal material. In recent years, the crystal has been favored for its improved anti-irradiation ability.

The light yield of CsI(Tl) scintillator reaches 52,000 photons/MeV, and the peak wavelength of emission spectrum is 550nm, which matches the response spectrum of silicon photodiode. Its decay time is composed of fast component 0.6us and slow component 3.5us. In addition, CsI(Tl) scintillator has high mechanical strength, strong impact and vibration resistance, and strong plasticity, which makes it easy to be processed into thin sheets. Compared with NaI(T1) scintillator, CsI(Tl) scintillator has better performance than NaI(Tl) except that its luminescence efficiency is lower than NaI(Tl): it is not easy to be dissoluble, has a large ray absorption coefficient, and the volume can be relatively small. The peak luminescence wavelength is 565nm, and it matches well with silicon photodiode. Therefore, CsI(Tl) scintillator is generally used in low-energy industrial CT.

illustrates the module of a discrete scintillation camera. The prototype  has a 3×4 array of pixels, each composed of a 3×3~5 mm3 CsI(TI) crystal coupled to a 3×3 mm2 PIN photodiode. The readout circuitry consists of two 3×3 mm2 ICs. A camera of useful imaging size can be constructed from an array of individual modules.

Structure diagram of a discrete scintillation camera

References
[1] Csl(Tl)-Calorimeter Calibration with Positive-Kaon Decay Products
[2] Study of the growth atmosphere effect on optical and scintillation characteristics of large CsI(TI) crystals
[3] A Discrete scintillation Camera Module Using Silicon Photodiode Readout of CsI(T1) Crystals for Breast Cancer Imaging
[4] Light response and particle identification with large CsI(T1) crystals coupled to photodiodes
[5] New Limits on Interactions between Weakly Interacting Massive Particles and Nucleons Obtained with CsI(Tl) Crystal Detectors
[6] Limits on Interactions between Weakly Interacting Massive Particles and Nucleons Obtained with CsI(Tl) Crystal Detectors

BGO

Wavelength(Max. Emission) (nm)480
Decay time (ns)300
Light yield (photons/keV)8~10
Refractive index2.15
Energy resolution (%)12

Scintillators can efficiently convert high-energy particles and radiation into light  with a wavelength in or around the visible spectral regions, and they are widely used in nuclear medical imaging, industrial CT , and high-energy physics applications. Bismuth germanate (Bi4Ge3O12 or BGO) is a kind of excellent scintillator developed in 1975 and has been extensively studied because its interesting luminescent properties, like short decay time, photo, radioluminescence and the two-photon absorption property under the high-power laser at the wavelength of visible waveband.. Its biggest advantages are high atomic number and density (7.138/ cm3), so it has a large absorption coefficient for both low energy and high energy rays, and high detection efficiency. Its emission spectrum ranges from 350nm-650nm, and its peak wavelength is around 480nm. In addition, it also has the advantages of good transparency, short luminescence decay time and no delifaction. However, its disadvantage is low luminous efficiency, only 14% of the luminous efficiency of NaI(Tl), which is generally used in high-energy industrial CT.

 illustrates a detector consisting of three or more optically isolated BGO crystals attached to a single10 mm square photomultiplier tube, which provides a fast timing pulse for the group. Each crystal is also individually coupled to a position-sensitive photodiode that identifies the crystal that stopped the annihilation photon and determines the depth of interaction.

Structure diagram of a detector consisting of three or more optically isolated BGO crystals

Reference [1] Synthesis and characterization of BGO with different chelating compounds by the polymeric precursor method, and their effect on luminescence properties
[2] INITIAL CHARACTERIZATION OF A POSITION-SENSITIVE PHOTODIODEBGO DETECTOR FOR PET
[3] Calibration and irradiation study of the BGO background monitor for the BEAST II experiment
[4]High Light Response Uniformity in Industrial Growth of 600-mm-Long BGO Crystals for DArk Matter Particle Explorer
[5] Study of the gamma spectrum of 16N with a BGO detector, for the purpose of calibration and of determining the fluorine grade of mineral samples
[6] Radiation tolerance studies on the VA32 ASIC for DAMPE BGO calorimeter
[7] Readout system for groundbased tests of BGO calorimeter of DAMPE satellite
[8] Nonlinear Absorption Response Correlated to Embedded Ag Nanoparticles in BGO Single Crystal From Two-Photon to ThreePhoton Absorption
[9] Simulation study of BGO array for characteristic gamma rays from neutronstimulated elements
[10] A system for low-level the cosmogenic 22Na radionuclide measurement by gamma–gamma coincidence method using BGO 

CdWO4

Wavelength(Max. emission) (nm)490
Decay time (ns)14000
Light yield (photons/keV)12~15
Light output relative to Nal(Tl) (%)50
Refractive index2.2-2.3

Chromium tungstate scintillator (CdWO4) has the advantages of non-deliquency, high density, large absorption coefficient of X ray, short radiation length and strong resistance to radiation damage. In addition, the emission spectrum of CdWOscintillator ranges from 400nm-600nm, and the emission peak wavelength is about 515nm, which can well match the sensitive wavelength of photodiode. CdWO4 scintillator has excellent scintillation performance and is the best in tungstate series. Because of these advantages, CdWO4 scintillator is very suitable for high energy industrial CT detectors, and CdWO4 crystals can make the detectors very dense, improving the spatial resolution of the CT system. CdWO4 crystal’s superior scintillation and optical properties make it widely used in high energy industrial CT detectors.

With the continuous improvement of the reliability and safety requirements of large components, the monitoring of assembly quality changes of components has become a key technology to ensure safety. The solution to the above problems must rely on industrial CT, especially the high-energy industrial C T side can be achieved very well. Therefore, the use of industrial C T, especially high-energy industrial C T technology, is expected to solve the following technical problems: (1) precision testing of welding quality of special components. (2) Monitoring of the assembly quality of large components (such as component attitude, assembly clearance and position changes) and precision inspection of internal structures. (3) Structural simulation and industrial CT inspection of other materials and components.

References
[1] Size effects on the properties of high z scintillator materials
[2] Low thermal gradient Czochralski growth of large CdWO4 crystals and electronic properties of (010) cleaved surface
[3] Growth Defect W3+-W2+ of CdWO4 Crystals
[4] Preparation, structural and optical properties of ZnWO4 and CdWO4 nanofilms
[5] Multi-mode photocatalytic performances of CdS QDs modified CdIn2S4CdWO4 nanocomposites with high electron transfer ability
[6] Photoluminescence Studies and Core–Shell Model Approach for Rare Earthdoped CdWO4 Nano Phosphor
[7] Systematic Control of Monoclinic CdWO4 Nanophase for Optimum Photocatalytic Activity