Nondestructive Testing of Materials Using Xerography

A. B. Andreev, V. A. Berdonosov

(Presented by the scientific seminar of the Scientific Research Institute of Electronic Introscopy)

The development of research in the field of semiconductors has opened up broad possibilities for creating new methods of obtaining photographic images, fundamentally different from ordinary photographic methods based on the use of silver-halide gelatin layers. Among the new photographic processes, the electrophotographic method of obtaining images is very promising [1, 3]. At present, electrophotography has become a new field of scientific and technical photography. In the literature this process is called “xerography,” which means a dry process for obtaining an image (“xeros” is Greek for “dry”).

The electrophotographic process replaces the complex chemical processing of silver-halide gelatin layers with the simple electrostatic interaction of a developing powder and a semiconductor layer. In addition, xerography requires neither such an expensive material as silver halide nor a complicated technology for producing the layers. At the same time, the same electrophotographic layers can be used repeatedly, with the image obtained after its transfer to ordinary paper [3]. Along with this, electrophotography opens up great possibilities for automating the processes of obtaining photographs, as well as for extending the limits of spectral sensitivity through the use of various semiconductors.

At present, xerography is gradually acquiring ever broader significance in various fields of science and technology: in documentation technology, in printing—for the production of printing plates, in the output of computer data, in communications technology, in technological processes for the manufacture of electronic equipment (printed-circuit assembly), in nondestructive testing (X-ray defectoscopy), in medical X-ray diagnostics, etc. [2, 4, 6].

When xerography is used in X-ray defectoscopy, in terms of sensitivity it is not inferior to the radiographic method of inspection, and under certain conditions, at energies on the order of 50–1000 kV, it surpasses it [4]. Moreover, if one takes into account that producing a single image by radiography requires 20–30 min, whereas a xeroradiograph requires from 1.5 to 2 min (not including exposure time), the indisputable advantage of xerography becomes very obvious [2]. In addition, one should note the increased contrast sensitivity, when small thicknesses are transilluminated, that xerographic images possess in comparison with radiographic ones. This is explained by the presence of the inherent xero-

...graphy effect, which has been called the edge effect [5]. The essence of this effect is that the lines of force of the electrostatic field bulge out along the edges of the latent image, which leads to the deposition in these places of an increased number of particles of the developer composition. As a result, at the boundary of even a slight difference in thicknesses there appears a boundary clearly visible to the eye, which cannot be determined from an image obtained by the radiographic method.

The edge effect, so necessary for expanding the limits of contrast sensitivity, can also give a negative result when obtaining halftone images over relatively large areas of equal thickness. To eliminate the density distortions that arise in this case, an additional developing electrode is used, or else rasterization of the xerographic plate is carried out, i.e., transformation of the image (large uniformly charged areas of the layer) into systems of points or lines. In this case the electrostatic image is divided into a number of points or lines, around which, above the surface of the layer, an electric-field gradient arises. As a result of these measures, one can practically completely eliminate the undesirable phenomenon of density-transfer distortion [1]. It should be noted that the exposure values for xerographic plates in the energy interval from 30 to 200 kV are 3–6 times smaller than the exposures for X-ray film. A graph of exposures when xerography is used as the recording method is given in Fig. 1. When the energy is increased to

Fig. 1. Dependence of exposure time on the thickness of a steel absorber. Selenium-layer thickness 200 μm.

1000 kV these exposures become approximately equal. In the region of very high radiation energies (10–20 MeV), the use of film becomes preferable [2]. It should be noted that at present investigations are being conducted to increase the sensitivity of plates to harder radiation (above 5–10 MeV). In this connection, the dependence of sensitivity on the presence of a sublayer, on the temperature of deposition of the layer, on the presence of impurities in the selenium layers, etc., is to be investigated.

Speaking of the advantages of xerography (high productivity, contrast sensitivity, and also high resolving power—according to foreign authors, up to 1000 lines/mm), one cannot fail to mention the shortcomings of this method, among which first of all must be included insufficient sensitivity in the region of high energies, ...

Fig. 2. Curve of the change in the dark potential of the layer over time

a relatively rapid drop in the potential (Fig. 2) on the layer; in addition, xerographic plates are very sensitive to mechanical effects, which lead to the appearance of cracks and scratches and, as a consequence, to the occurrence of false defects, which makes interpretation of xeroradiographs difficult.

Tracing the path of development of xerography and, in particular, xeroradiography, one may say that it is gaining an ever larger place in science and technology, beginning quite legitimately to compete with X-ray film. With the appearance of cheaper semiconductor layers, such as CdS, ZnS, etc., with increased sensitivity and the possibility of use up to 50,000 times, the role and importance of the xerographic method in radiation flaw detection of materials and products will increase still further.

REFERENCES

1. S. G. Grenishin. The electrophotographic process. Moscow, “Nauka,” 1970.

2. S. V. Rumyantsev, A. S. Shtan, Yu. P. Popov. Handbook of the X-ray and gamma-ray flaw detector. Moscow, Atomizdat, 1969.

3. Electrophotography and magnetography. Vilnius, “Mintis” Publishing House, 1972.

4. A. P. Semenov, V. I. Gorbachev. “Defektoskopiya,” 1972, 3, 122.

5. A. I. Kaminskas, V. G. Chepenko, R. A. Kavaliauskas, Yu. K. Rakauskas. “Defektoskopiya,” 1967, 3, 64.

6. B. A. Tazenkov, V. G. Boitsov, G. N. Sandalov, I. B. Shneidman. Processes and apparatuses of electrophotography. Moscow, “Mashinostroenie,” 1972.