EXPERIMENTAL STUDY OF AN X-RAY VIDICON

D. I. Sviryakin, A. I. Manets, A. N. Kolupaev

An estimate of the contrast sensitivity of an X-ray vidicon in the first approximation can be made from the “radiation–signal” characteristic, which is the dependence of the magnitude of the signal generated by the X-ray vidicon at the “air–absorber” boundary on the intensity of the radiation flux incident on the target. By the “air–absorber” boundary we mean here the plane of separation between the region where the radiation flux is not absorbed by anything and the region of complete absorption.

Figure 1 presents the block diagram and geometry of the experiment for determining the dependence of the signal magnitude on the radiation intensity of an X-ray tube for various absorber thicknesses. The X-ray tube of the RUP-120-5-1 apparatus is designated in the figure by number 1; the absorber of thickness \(d\) with a slit is designated by number 2; the X-ray vidicon by number 3; and the video amplifier and EO-58 oscilloscope, operating in the “line selection” mode, by numbers 4 and 5, respectively. To carry out the experiment, an industrial television installation PTU-26M, modified for X-ray vidicons, was used. The video signal generated by the X-ray vidicon is small; therefore, before being fed to the oscilloscope input, it was amplified by means of the preliminary video amplifier of the transmitting television camera. A complete television raster was provided on the X-ray vidicon target by the scanning generators, but in the measurements only the signal of one central line crossing the slit in the absorber was used. Selection of the required raster line was provided by the oscilloscope.

Fig. 1. Block diagram and geometry of the experiment.

The distance from the target of the X-ray tube to the target of the X-ray vidicon (\(L\)) was taken to be 150 mm. The voltage on the anode of the X-ray tube was maintained at 120 kV; the intensity of the X-ray-

The X-ray radiation was regulated by changing the anode current of the tube, and the dose rate, for an exposure time measured with a stopwatch, was measured with a “KD”-type dosimeter. The dosimeter was mounted on the surface of the absorber facing the X-ray tube. Aluminum and lead plates were used as the absorber. To make it convenient to measure the signal, each absorber plate had a slit 1 mm wide, much larger than the aperture of the scanning beam of the X-ray vidicon. The axis of the slit was arranged perpendicular to the direction of the line scan of the LI-417-1 X-ray vidicon, which operated in the fast-electron mode.

With all other conditions equal, the magnitude of the signal from the target of the X-ray vidicon depends on the attenuation factor of the radiation in the absorber material; therefore, measurements were carried out for different thicknesses. The aluminum absorber plates had thicknesses of 1.0, 1.5, 2.0, and 5.0 mm, and the lead absorber was 0.8 mm thick (respectively, curves 1, 2, 3, 4, and 5 in Fig. 2).

Curves 1, 2, 3, 4, and 5 in Fig. 2 show that the signal of the X-ray vidicon from the image of the slit increases as the attenuation factor of the radiation in the absorber increases, while curves 1, 2, 3, and 4 show that, as the radiation intensity increases, the signal first increases and reaches a maximum at an intensity corresponding in this case to an exposure-dose rate of 2.2–2.4 r/sec, and then decreases. This is explained by the fact that aluminum plates of the above-indicated thicknesses do not provide complete absorption of the radiation flux, and discharge of the X-ray-vidicon target occurs not only in the region located directly behind the slit, but over the entire area. Therefore, the potential difference between the closed and open regions of the target decreases, and the signal magnitude from the slit also decreases as the absorber thickness decreases.

Fig. 2. Dependence of the signal magnitude on dose rate at different radiation attenuation factors.

Fig. 3. Shadow X-ray image of a watch
on the screen of a video-monitor device: a) with
standard scans;

b) with exposure accumulation for 2 sec
and pulse readout.

The horizontal portion of curve 5 in Fig. 2 at a high radiation-attenuation ratio is obtained as a result of the limiting discharge of the exposed areas of the target to a potential below which discharge of the target is impossible. Therefore a further increase in dose rate does not change the signal magnitude. The absence of a descending section on the graph is associated with the practically complete absorption of the radiation flux by the lead plate and the very small intercommutation discharge of the closed areas of the target.

Determination of the dependence of the x-ray vidicon signal on the magnitude of a defect at different thicknesses was carried out according to the same scheme as in the preceding experiment. But instead of one absorber plate, two plates of the same material were now placed in front of the x-ray vidicon; moreover, the plate with a slit the same as in the first case was placed closer to the target of the x-ray vidicon, while the second (solid) plate was placed tightly against the first on the side of the x-ray tube. Only aluminum was used as the absorber. Several series of experiments were carried out, in which the plate with the slit had thicknesses of 0.07; 0.1; 0.2; 0.3; 0.5 mm. During each series the thickness of the solid absorber plate was varied from measurement to measurement. On the basis of the experimental results, graphs were plotted that made it possible to determine the contrast sensitivity of the LI-417-1 x-ray vidicon for different total absorber thicknesses. The sensitivity of the introscope obtained in the experiments for an aluminum thickness of 3–5 mm, at an x-ray radiation hardness corresponding to a voltage on the x-ray-tube anode of 100–120 kV, with a well-adjusted video channel and a specially selected x-ray vidicon, lay within the range of 2.5–3%.

The experimental results showed that improving the detectability of defects and increasing the image contrast of small details on the screen of the video-monitoring device by increasing only the intensity of the x-ray radiation flux is not always possible, since in a number of cases this leads to an increase in background radiation in the areas of the target shielded by the absorber. A certain reserve for increasing the contrast sensitivity of the introscope is contained in the method of exposure accumulation over several tens of television frames, followed by pulsed readout from the x-ray vidicon target over three to five frames. The effect provided by this method of increasing contrast sensitivity is illustrated by the photographs presented in Figs. 3a and 3b. Each of them gives an x-ray image of the internal structure of a wristwatch. The radiography was carried out through both covers of the case. During exposure, the watch mechanism was not stopped, which caused blurring of the image of the balance wheel in both photographs. Fig. 3a shows an image in the operating mode of the introscope without accumulation (more precisely, with accumulation for \(1/25\) s), and Fig. 3b shows the image with exposure accumulation for 2 s.

To eliminate image flicker in the accumulation mode, it is necessary to use an intermediate recording unit with an information capacity of one television frame. The difference in the quality of these television images indicates the promise of the exposure-accumulation method. A quantitative assessment of the effectiveness of this method showed that the contrast sensitivity of the introscope when it is used can be improved to 1.0–1.5%.