Abstract
Industrial X-ray Computed Tomography(XCT), having revolutionized the field of non-destructive testing(NDT), provides a non-destructive evaluation(NDE) to image the cross-sectional view of the interior of the tested object. The detector, however, aiming at transforming x-ray interacted with the tested object into electric signals to be measured and processed, is one of some key techniques in industrial XCT systems. The tendency to use micro-beam x-rays from a micro-focus x-ray source to create x-ray computed microtomography systems in which a high space resolution may be received, lies in the application of x-ray high space resolution detector or detection system. A high space resolution will play an important role in the microstructure non-destructive evaluation of the small sample by NDT. This take place because optoelectronic integrated SSPA(self-scanning photodiode array)with fiber optic face-plate(FOP) window with 6mm(micron) in numerical aperture for optically coupling and transmission is designed, and integrated radiation detectors use CsI(Tl) scintillators screen for x-ray into light conversion. Finally, some beneficial results have been received in this paper.
INTRODUCTION
Industrial computed x-ray tomography(ICT) is a nondestructive testing(NDT) technique whereby a cross-sectional image is obtained. The image is a digital one, whose values correspond to a physical property of the tested object. The physical base of detector in industrial CT systems is based on the principle of the interaction of x-ray and matter (or object), and the measured physical property is the material's linear attenuation coefficients, which abided by Beer's law, so, the detector for x-ray detection is an important technique in industrial CT systems, influencing the system's performance. Now, the type of the detector mainly is scintillation counter in industrial XCT, the forms of which are as follows:
- The scintillator optically coupled to photomutiplier[1-4], the structure is used an array of discrete crystals, aiming at the x-ray turned into photon in the scintillator and then the photon turned into electric signals in photomutiplier easy to be measured and processed.
- The scintillator optically coupled to photodiode[5-6], the structure is used a linear array of 256, 512 or 1024 a-Si photodiodes whereby the x-ray into light conversion is completed.
- Image intensifier optically coupled to CCD(charge coupled device) used for detecting the x-ray[7].
- The crystals optically coupled to line array CCD with a fiber optic face-plate to be used as light entrance window[8].
According to the above in industrial XCT, we have designed a novel design of line array x-ray detector for an experiment so as to be used in industrial XCT systems.
DETECTOR DESCRIPTION
Compared with the line array x-ray detector designed by Smith[8], the scintillator is not a gadolinium oxysulfide(GOS)scintillation phosphor screen optically coupled to a linear photodiode array with a fiber optic entrance window, but a high stopping power and high quantum efficiency of caesium iodine(CsI) with thallium(Tl) scintillation phosphor screen, and the x-ray is perpendicular to incidence on the CsI(Tl) phosphor screen to complete light conversion, as shown in Figure 1.
Fig 1: The designed line scan detector for x-ray detection
|
The CsI(Tl) phosphor screen is 3 mm thick in the direction of x-ray beam, length with 12.5 mm, and height with 2.5mm. Physical properties of CsI(Tl) scintillator are shown in Table 1.
| Light output relative to NaI(Tl)/%
| 45
|
| Peak wavelength/nm
| 550
|
| Decay time/ms
| 1
|
| Afterglow/%
| 0.5
|
| Refractive index
| 1.8(589.3nm)
|
| Density/g.cm-3
| 4.51
|
| Deliquescence
| little
|
| Table 1: Main physical properties of
CsI(Tl) scintillator
|
Fiber optic face-plate (FOP), made in North Industrial, China, is used for light from CsI(Tl) phosphor screen coupled to photodiode array, its main specifications are shown in Table 2.
| Fiber pitch
| £6mm
|
| Numerical aperture
| ³1.0
|
| Average thermal expansion coefficient
| »90×10-7/(20-300)
|
| Transmittance (3mm thick)
| ³65%(collimated white light)
|
| Leakage
| £1×10-10atm.cm3.He/s
|
| Gross distortion & shear distortion
| £0.05mm respectively
|
| Table 2: Main specifications for the fiber optic face-plate |
The photodiode array, for light photons converting to an electric current, is typed for CL1024S, quarter-phase self-scanning linear photodiode array with 1024 photodiode sensor elements with 25 micron center-to-center spacing, manufactured by Department of Optoelectronic Engineering, Chongqing University, China. The photodiode array used optoelectronic integrated technique, is also called self-scanning photodiode array (SSPA). Table 3 is taken from a CL1024S data sheet and gives a few of the electro-optical characteristics of this SSPA.
| Numbers of photodiode array/piece
| 1024
|
| Center-to-center spacing/mm
| 25
|
| Aperture width/mm
| 2.5
|
| Non-uniformity of response/±%
| £10
|
| Sensitivity of response/mV.lx-1.s-1
| 30-200
|
| Saturation exposure/lx.s
| 0.1-0.5
|
| Dark signal/%
| £1.5
|
| Spectral response range/nm
| 400-1000 (Typ. 750 nm wavelength)
|
| Table 3: Electro-optical characteristics for CL1024S |
The working principle for the designed detector is that the CsI(Tl) scintillating screen converts the x-ray photons of penetrating the tested object into light photons, which are optically coupled to the photodiode arrays through fiber optic face-plate window and subsequently converted to an electric current in the photodiodes, and finally integrated on the photodiode storage capacitor. For the charge distribution pattern across the diode array is proportional to the integrated x-ray flux distribution on the screen, while the designed detector used in industrial XCT, that corresponding to a single line projection through the tested object being scanned.
EXPERIMENTS AND RESULTS
In order to test a novel design of line array x-ray detector in industrial XCT, we have used such a method as shown in Figure 2. In fig.2, the radiation energy from x-ray tube is 160kV, 1mA for current, 0.8mm´0.8mm for source focus size. In measuring circuits, there is a 12 bit, successive approximation sampling analog-to-digital converter(ADC) preceded by a programmable gain amplifier, the digital signals subsequently are transmitted to the image processing computer through I/O parallel interface in the computer's slots, for the acquisition line projection data through the object being scanned, CT image can be reconstructed by a so-called filter convolution back-projection reconstruction algorithm based on Radon transform, those CT images are shown in Figure 3-4.
Fig 2: A industrial XCT experimental system for the design of x-ray line array detector
|
Fig 3: This XCT image of a plastic/aluminum at 120kV
| 
Fig 4: This XCT image of a steel needle points on plastic circle at 120kV
|
CONCLUSIONS
The designed detector has been experimented and the results show that the space resolution for the novel detector is up to 3 pairs of lines per mm., this is better than the common detector used in industrial XCT. If a micro-focus x-ray source were used, the space resolution for this detector would be much better, this further research still is underway. Furthermore, it also is beneficial to the performances with high integrated, high efficiency, miniaturized and low cost in industrial XCT systems.
ACKNOWLEDGMENTS
This research was supported under Commission for Postdoctoral and Chongqing Science & Technology Commission. The author wishes to thank S.L.Huang and X.H.Yuan for helpful discussions throughout the course.
REFERENCE
- M.Srapaport, A.Gayer, E.Iszak, et al., "A dual-mode industrial CT", Nucl. Instr. And Meth. In Phys. Res., Vol. A352, PP.652-658, May 1995.
- W.B.Gilboy, "X- and g-ray tomography in NDE applications", Nucl. Instr. And Meth. In Phys. Res., Vol. A221, PP.193-200, Jan. 1984.
- M.E.Coles, E.L.Muegge and E.S.Sprunt, "Applications of CAT scanning for oil and gas production research", IEEE Trans. On Nucl. Sci., Vol.38, No.2, PP.510-515, Apr. 1991.
- T.luthi, A.flisch and P.Wyss, "Industrial computed x-ray tomography, INSIGHT Vol.40, No.3, PP.196-197, Mar. 1998.
- Testsuhiko Takahashi, Haruo Itoh, Toshikazu Shimada, et al., "Design of integrated radiation detectors with a-Si photodiodes on ceramic scintillators for use in x-ray computed tomography", IEEE Trans. On Nucl. Sci., Vol.37, No.3, PP.1478-1482, June 1990.
- E.Cendre, G.Peix, V.Kaftandjian, et al., "High resolution x-ray computed tomography applied to bone structure characterization", Proc. Of 14th WCNDT, PP.1211-1214, New Delhi, India, Dec.1996.
- R.Clarke, "CCD x-ray detectors: opportunities and challenges", Nucl. Instr. And Meth. In Phys. Res., Vol. A347, PP.529-533, Feb. 1994.
- C.R.Smith and J.W.Erker, "Low cost, high resolution x-ray detector system for digital radiography and computed tomography", Proc. Of SPIE, Vol.2009, X-ray detector physics and applicationsêê, PP.31-35,1993.
