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Increase of sensitivity of X-Ray TV Inspection N.G.Bely, V.A.Troitsky E.O.Paton Electric Welding Institute, Kiev S.V.Debnovetsky, A.V.Leshchishin, S.R.Mikhailov, KPI National Technical University of Ukraine, Kiev Contact

The method of X-ray TV inspection of the quality of materials and items using various X-ray television systems (RTVS) is becoming widely accepted by industry. This is promoted by the known advantages of RTVS over film roentgenography such as high efficiency and low cost of control, ability to conduct dynamic control of plants, as well as the ability of fast processing of X-ray images with the aim of their quality improvement. Currently available RTVS allow images filtering with the aim of noise suppression and correction of the spatial and time distortions, performance of colour coding of images, addition and subtraction and documenting of images, image integration to increase the signal/noise ratio, automatic measurement of the plant parameters in the image, etc.
RTVS differing in their structure and parameters are applied in industry for non-destructive testing [1] (Fig. 1).

Fig 1: Block-diagrams of X-ray TV systems: 1 - X-ray radiation source; 2 - plant being examined ; 3 - fluoroscopic screen, luminescent screen or single-crystal transducer; 4 - X-ray image intensifier; 5 - optical system; 6 - light amplifier (EOP); 7 - light-signal transducer; 8 - X-ray vidicon; 9 - television system; 10 - picture monitor (TV monitor).


Various kinds of image pickup tubes, namely vidicons, plumbicons, satikons, newvicons, harpicons, SIT-vidicons, as well as sensors of charge-coupled devices (CCD) of different format are used as light-signal converters. Collimator-type lens tandem objects, fiber-optics connectors and faceplates, which allow minimizing the light losses in image transfer, are used as optical systems of RTVS, as well as anamorphotic optics. Digital television systems incorporating digital memory and video processor for computerized image processing, have lately become applied in RTVS as television systems. In the latter case, the X-ray image is represented on the computer display.
RTVS shown in Fig. 1, a, Fig. 1, b and Fig. 1, c are used for control of a broad range of thicknesses of controlled plants both in the statics and dynamics, and provide the relative sensitivity of control of 2 - 3% with the resolution of up to 3 line pairs/mm. Such systems are inferior to film roentgenography in terms of control sensitivity and resolution.
Better parameters are offered by RTVS incorporating X-ray vidicons (Fig. 1 d), which do not use the intermediate transformation of X-ray image into the optical image.
In X-ray vidicons incorporating a target of amorphous selenium or lead oxide, the X-ray image is directly transformed into an electric signal. The currently produced X-ray vidicons can have the input window diameter of more than 150 mm [2]. RTVS based on X-ray vidicons have a high resolution (up to 30 line pairs/mm) and relative sensitivity of control of 1.5% in the range of steel thicknesses up to 10 mm. As regards control sensitivity and the range of object thicknesses being controlled, such RTVS are still inferior to film roentgenography.
An effective method of improvement of relative sensitivity of control and widening the range of the plant thicknesses controllable by RTVS, is switching the image pickup tubes (IPT) into the mode of adjustable duration of signal accumulation on the target. This operational mode is also possible in CCD sensors. In this mode, the input image is accumulated on the target with the image pickup tube electron beam cut off for a certain time, then the accumulated image is read in one television frame, and entered into the storage device, from which it is read out for displaying or processing. Many image-pickup tubes of both the optical range and X-ray vidicons, can operate effectively in the adjustable accumulation duration mode. A many time increase of the image pickup tube sensitivity is possible in this mode, and, hence, of relative sensitivity of control by RTVS. The degree of sensitivity increase depends on the possible duration of signals accumulation in the image pickup tube, which is determined by the dark conduction of its target material. The smaller the dark current of the image pickup tube target, the longer the possible duration of accumulation in the image pickup tube, and the higher the relative sensitivity of RTVS control.
Switching IPTs used in RTVS (Fig. 1 a and Fig. 1 b), into the mode of adjustable duration of signal accumulation on the target, allows increase of the relative sensitivity of control, reduction of the required power of X-ray radiation exposure dose in RTVS input window and requires incorporation of a storage device into RTVS.
Improvement of the relative sensitivity of control by RTVS, Fig. 1 a, was studied for the case of RTVS based on Li-702 SIT-vidicon and EK-1 single-crystal transducer of CsI (T1) material of 200 mm diameter. Steel samples up to 25 mm thick were examined in RAP-150/300 X-ray unit with 1.2-3-BPM-5-300 X-ray tube. The distance from the X-ray tube to the single-crystal transducer and the examined samples was 350 mm. RTVS examination showed that when LI-702-3 SIT-vidicon was switched into the mode of an adjustable accumulation duration, RTVS sensitivity in the above range of steel thicknesses was improved by 2.5% to 2% on the average, by groove references, the optimal accumulation duration in the SIT-vidicon being up to 20 TV frames (up to 0.8 s).
Improvement of the relative sensitivity of control with switching of the image pickup tube into the mode of adjustable duration of accumulation was also found in RTVS, Fig. 1, c incorporating an X-ray image intensifier (XII) of ZOX-193VK type and TV camera with Li-469 vidicon.
Improvement of sensitivity in the range of steel thicknesses up to 40 mm was 0.5 to 1% by wire gauges, when RAP-150/300 X-ray unit with 1.2-3-BMP-5-300 X-ray tube was used.
Application of the mode of adjustable duration of accumulation is the most effective in RTVS incorporating X-ray vidicons (Fig. 1, d) whose target is made of amorphous selenium with low dark conductivity. These are LI-444, LI-473 and LI-447 X-ray vidicons with the working field diameter of 18, 90 and 150 mm, respectively. LI-504 X-ray vidicon based on amorphous selenium with working field diameter of 90 mm was developed specially for this mode, allowing X-ray images to be accumulated in the range from 40 ms up to 10 min.
Application of the mode of adjustable duration of accumulation in XTVS also allows small-sized pulsed X-ray units to be used for examination. Examination with XTVS incorporating LI-504 X-ray vidicon with pulsed X-ray units "MIRA-2D" and "ARINA-02" showed that XTVS provide a relative sensitivity of control of 1 - 1.5% in the range of steel thicknesses up to 30 mm.
XTVS can operate in one of the three accumulation modes:
• adjustable duration of accumulation of X-ray images on IPT target, one-frame reading in IPT and image entering into the storage device (SD).
• Multiframe accumulation in a digital SD of the TV signal generated by X-ray vidicon in the continuous reading mode;
• Cyclic accumulation in which the SD accumulates TV frames, each of which is generated in the X-ray vidicon in the mode of an adjustable duration of accumulation.
X-ray TV systems incorporating image pickup tubes of both the optical and X-ray vidicons when operating in the mode of adjustable duration of accumulation, allow an essential increase in the control efficiency, compared to film radiography, widening the range of material thicknesses being controlled and improvement of the relative sensitivity of control.

References

1. Denbnovetsky S.V., Troitsky V.A., Bely N.G., Leshchishin A.V., Mikhailov S.R. Defectoscopic X-ray TV systems of a high sensitivity // Techn. Diagnost. i Nerazr. Kontr. - 1997, #4. - p.13-22.
2. R.Luhta and J.A.Rowlands, 1997. - Feasibility of a large area X-ray sensitive vidicon for medical fluoroscopy: Signal and noise factors. Medical Physics 24: 521-631.

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