·Table of Contents
·Methods and Instrumentation
System of digital radiography for non-destructive testing in the radiation energy range 1-20 MeVChakhlov V. L., Moskalyov Ju. A., Temnik A. C.
Most of SDR employ the classic layout shown in Fig. 1. It consists of the luminescent screen, turning mirror, lens, CCD-camera, controller, and computer with software for image processing.
|Fig 1:Scheme of the SDR|
Since 1-20 MeV X-radiation has high penetrability and produces rather weak radiation contrast, the development of SDR with high contrast sensitivity presents a relatively complicated scientific and technical problem. For successful solution of this problem all the elements of SDR should provide maximum detection efficiency, radiation and light image contrast, and the minimum level of scattered radiation.
Usually the SDR is considered to consist of two principal units. The first is the X-ray image detector and the second is an image processing unit. The former is located in the irradiated region and includes a scintillation screen (converter), mirror, lens, shielded CCD-camera, and controller. The latter is located in a radiation-safe room and includes a digital interface and a computer with software for image processing.
However, according to  fiber screens have no advantages over other types of screens for energies above 1 MeV. Besides, high-energy X-rays cause radiation damage to the fiber scintillation material.
To determine the conversion efficiency of screens having various compositions the dependence of the fraction of X-ray quantum energy F deposited in a luminescent screen versus incident quantum energy E was calculated according to the technique described in a Ref. . All calculations were made for the phosphor screen thickness of 500 m m and CsI× Tl single crystal screen thickness of 5 mm. The results are given in the Table 1.
|Screen material||F (E)´104|
|5 MeV||10 MeV||20 MeV|
|CsI×Tl (single crystal)||46.00||38.00||29.00|
From the table it follows that compositions which are most efficient in terms of deposited energy are CsI× Tl, Gd2O2S× Tb, and PbWO4, with the energy deposited in PbWO4 being twice as large as that in CsI× Tl for the quantum energy range of 5-20 MeV. However, taking into account that the phosphor PbWO4 is still under development and has relatively low luminescence yield, the screens which are in most common use are those based on CsI× Tl and Gd2O2S× Tb. It should be noted that radiation conversion efficiency of a single crystal screen CsI× Tl is larger than that of phosphor screens by a factor of tens even if substrates made of such heavy metals as W and Ta are used. So maximum values of contrast sensitivity and signal/noise ratio can be currently achieved in SDRs with single crystal screens. The advantages of phosphor screens with heavy metal substrates manifest themselves when testing large objects because of the problems with the production of single crystal CsI× Tl screens with diameter larger than 200 mm. In this work the SDR with 5-mm-thick single crystal CsI× Tl screens with diameter of 200 mm and polycrystalline CsI× Tl screens with dimensions of 200´ 400 mm2 and a 1-mm-thick Pb substrate has been developed.
This software was developed using Microsoft Visual C++ 6.0 and Microsoft Foundational Classes Library. It requires Windows 95 or Windows NT. The following options are available in Diada 2.0.
Acquisition of images from CCD-camera SILAR (12 bits) or TWAIN-scanner (8 bits).
Open image files in standard formats: binary arrays, Windows bitmaps, Flexible image transport system (FITS), GIF, PCX, TIFF (8, 12 and 16 bits).
Display of images.
There are 4 different visualization types:
Taking measurements of values in points, along lines or on polygonal regions. For polygons Diada calculates statistical estimations: minimum, maximum and mean. For lines it calculates only mean and standard deviations.
Enhancing and manipulating of images. Diada realizes the standard image processing operations: scaling, rotations, filtering, lookup-table transformation, correlation and autocorrelation, binary arithmetical and logical image operations. There are also some special operations for radiographic images: correction of CCD-errors, line equalization, delete background. All operations are related with the selected rectangular region on the images only.
Documentation of the results. One can use the usual Windows Clipboard functionality to transfer images and scan-lines to a word processor for writing reports. Use also the Save as command to store images in standard formats: BIN, BMP, FITS, GIF, PCX, SET, TIFF. Use Print command to print out the image. Use drawing tools to create overlays (marks like rectangles, ellipses, arrows, texts, pictures and so on) on the images. The internal image format of Diada 2.0 is 4096 gray-levels (12 bits).
Using the SDR with the screen with the size of 200´ 400 mm consisting of the 0.4-mm-thick polycrystalline CsI× Tl on 1-mm-thick Pb substrate yields a contrast sensitivity of 1.0 % for 100-mm-thick steel specimens, i. e. 1-mm-deep grooves and wires of 1 mm in diameter could be detected in the standards.
The reported results on the contrast sensitivity of the SDR are preliminary and additional experiments showed that the sensitivity, resolution, and brightness of the screen can be improved by using new phosphors, increasing the phosphor layer density and applying heterogeneous amplifying screens made of heavy metals.
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