DEVELOPMENT OF INTROSCOPE SYSTEMS WITH INCREASED SENSITIVITY

V. I. GORBUNOV, D. I. SVIRYAKIN, A. N. KOLUPAEV, V. F. PETUSHENKO,
V. I. CHANTSEV

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

An analysis of the signal-to-noise ratio \((\psi)\) in radiation introscope systems shows that, in order to increase their sensitivity, it is first of all necessary to strive to increase the transmission coefficients of the first stages of the system, since a decrease in the signal-to-noise ratio occurring in the first stages cannot be compensated in subsequent stages. Therefore, the choice of the type and parameters of the radiation transducer of an introscope has a substantial influence on the sensitivity of the introscope as a whole. Contrast sensitivity, expressed by the value reciprocal to the minimum contrast registered by the operator, the resolving power, and the number of discernible brightness gradations are limited by noise, or by the signal-to-noise ratio \([1, 2, 3]\) of the entire system.

The greatest number of distinguishable gradations \(m\) of image brightness at the output of the system in the brightness interval \(B_{\min} \div B_{\max}\), in the presence of a threshold contrast \(K_{\text{th}}\), is expressed by the following formula:

\[ m = \frac{1}{\ln(1 + K_{\text{th}})} \cdot \ln \frac{B_{\max}}{B_{\min}} . \tag{1} \]

The ratio

\[ \frac{B_{\max}}{B_{\min}} \]

is related to the optical contrast as follows:

\[ \frac{B_{\max}}{B_{\min}} = \frac{1 + K_{\text{opt}}}{1 - K_{\text{opt}}}, \]

therefore formula [1] can be written in the form

\[ m = \frac{ \ln \dfrac{1 + K_{\text{opt}}}{1 - K_{\text{opt}}} }{ \ln(1 - K_{\text{th}}) }. \tag{2} \]

At the same time, the relationship between the number of brightness gradations transmitted by the system and the value of the signal-to-noise ratio \(\psi\), for the case of complete independence of noise from the signal, has the form \([2, 5]\)

\[ m \simeq \frac{1}{p\sqrt{2}}\, \psi , \tag{3} \]

where \(p\) is a coefficient characterizing the excess of the signal over the noise that is necessary for reliable detection of the signal in noise, and op-

determined by the properties of vision, the nature of fluctuation noise, and the brightness of the image.

Equating the right-hand sides of equalities (2) and (3), we obtain

\[ K_{\mathrm{opt}}= \frac{\left(1+K_{\mathrm{th}}\right)^{\psi/p\sqrt{2}}-1} {\left(1+K_{\mathrm{th}}\right)^{\psi/p\sqrt{2}}+1}. \tag{4} \]

This formula is valid for superorthicons and other transmitting tubes in which the introduced external noise is greater than the noise directly associated with the signal. At a detection probability of 85%, \(p=\sqrt{2}\), then

\[ K_{\mathrm{opt}}= \frac{\sqrt{\left(1+K_{\mathrm{th}}\right)^{\psi}-1}} {\sqrt{\left(1+K_{\mathrm{th}}\right)^{\psi}+1}}. \tag{5} \]

In the general case, the optical contrast is not equal to the radiation contrast \(K_{\mathrm{r}}\), and the relation between them is determined by the ratio [6]

\[ K_{\mathrm{opt}}=\gamma K_{\mathrm{r}}, \tag{6} \]

where \(\gamma\) is the gradient of the system’s light characteristic. Hence

\[ K_{\mathrm{r}}= \frac{\sqrt{\left(1+K_{\mathrm{th}}\right)^{\psi}-1}} {\gamma\sqrt{\left(1+K_{\mathrm{th}}\right)^{\psi}+1}}. \tag{7} \]

Formulas (2) and (5) show that the higher the value of \(\psi\) of the system at a known threshold contrast of the operator’s vision, the greater the number of brightness gradations the radiation introscope can reproduce and the smaller the optical contrast of the image permissible on the output screen.

It follows from formula (7): a) the greater the value of \(\gamma\), the smaller the radiation contrast that can be distinguished on the output screen of the introscope; b) for correct transmission of halftones when radiographing objects that provide high radiation contrast, the system must have a high signal-to-noise ratio.

The value \(\psi\) of the system is determined mainly by the first stages. Therefore, in order to increase the sensitivity of introscopes, it is necessary to strive to increase the value \(\psi\) of the degree of conversion of spatially modulated radiation fluxes into a visible image or electrical signals. In addition, when constructing introscope circuits, it is necessary to use the minimum possible number of stages of conversion and information transmission, since every link newly introduced into the circuit contributes to increasing noise and reducing the value \(\psi\) of the system as a whole.

Figure 1 presents averaged curves of the relative sensitivity (\(\Delta d/d\) %, where \(\Delta d\) is the detectable difference in thickness, \(d\) is the thickness) of certain types of introscopes as a function of the thickness of the steel products being radiographed. The number 1 denotes the curve characterizing an introscope built according to the scheme: X-ray radiation source—object being radiographed—X-ray electron-optical converter (EOC)—optics—vidicon—video channel—kinescope. In this and other introscopes whose sensitivity curves (2, 3) are shown in the figure, an RUP 150/300-10 X-ray unit was used as the radiation source. The anode voltage on the X-ray tube is set optimally, based on the thickness of the steel absorber. To determine sensitivity, standard No. 2 with grooves according to GOST 7512-55 was used. In the introscope, the sensiti-

Fig. 1. Curves of relative sensitivity of certain types of introscopes: 1 — an introscope built according to the scheme: X-ray radiation source — object being radiographed — optics — vidicon — video channel — kinescope; 2 — an introscope in which the RU-135 device with special high-aperture binocular optics was used as the converter and information-transmission channel; 3 — an introscope in which the penetrating-radiation conversion unit contains CsJ(Tl) single crystals, an “Electron-1” lens, and a supericonoscope; 4 — a betatron introscope using the compact PMB-6 betatron.

…the sensitivity of which is represented by curve 1, the RU-135 X-ray EOP was placed close against the object being radiographed, and the image from its output screen was transmitted, by means of an optical system (two coupled Yu-3 lenses), to the photosensitive target of the LI-422-1 vidicon, where it was converted into electrical signals and transmitted via the video channel to the kinescope. On the kinescope screen the operator observed the resulting shadow X-ray image in an unshaded room. This introscope has comparatively low sensitivity, but is convenient in operation because of the simplicity and reliability of the X-ray EOP and the PTU-27M industrial television installation, and because no special radiation shielding is required, since the transmitting television camera with the X-ray EOP and the installation control console are separated by a considerable distance.

The curve marked with the number 2 was obtained using an introscope in which the RU-135 device with high-aperture binocular optics specially developed for this introscope was used as the converter and information-transmission channel; this increased the sensitivity and ensured transmission of the image from the output screen of the RU-135 to the operator’s eyes. The optical system is made in the form of an elbow and, together with a booth lined with lead sheets, provides…

reliable protection of the operator. The drawback is that such an introscope cannot be used in hard-to-reach and dangerous places or for inspecting large thicknesses of steel. These drawbacks are eliminated in the X-ray television introscope, in which the unit for converting penetrating radiation into electrical signals contains a CsI(Te) scintillation single crystal 5 mm thick and 150 mm in diameter, a fast image-transfer lens “Elektron-1,” and a super-orthicon transmitting television tube, which, at an illumination on the photocathode of \(10^{-3}\) lx, has a signal-to-noise ratio of at least 30. The entire television part of this introscope is built on the industrial television installation PTU-101. The focal distance is 300 mm; standard No. 2 is placed in front of the object being radiographed, and the single crystal is placed flush against the rear wall of the object. The comparatively high light sensitivity and large value of \(\psi\) of the super-orthicon, and the good utilization of the luminous flux emitted by the CsI(Te) single crystal, made it possible to attain the sensitivity of radiographic films. Curve 3 shows that the range of optimum steel thicknesses for this introscope lies within 15–35 mm, and satisfactory X-ray inspection of steel products can be carried out up to 60 mm. A further increase in thickness leads to such a decrease in the brightness of the scintillation-crystal glow that the sensitivity of the super-orthicon proves insufficient, and the image contrast on the kinescope screen of the PTU-101 installation becomes lower than the threshold value.

Steel products of large thicknesses can be inspected with the aid of betatrons. The shadow image is obtained according to the scheme of the preceding introscope, but on condition that the X-ray apparatus is replaced by a betatron and the thin CsI(Te) single crystal by a thicker one. Curve 4 was obtained with a betatron introscope using a small-size GiMB-6 betatron (maximum energy 6 MeV, dose rate 0.3 r/min at a distance of 1 m), a CsI(Te) single crystal 10 mm thick and 150 mm in diameter, transfer optics— a “Yu-9” lens with a \(+2\) attachment lens—and a PTU-101 installation with a super-orthicon in the transmitting camera. The distance from the target to the object being radiographed was 250 mm; the single crystal was placed flush against the rear wall, and standard No. 4, GOST 7512-55, was placed on the side of the front wall of the object.

The course of curve 4 indicates that the optimum steel thicknesses for the hard bremsstrahlung radiation of the betatron are thicknesses greater than 60 mm; however, at thicknesses greater than 60 mm the radiation dose rate behind the object falls so much that the brightness of the single-crystal glow becomes insufficient for normal operation of the super-orthicon.

The results of these investigations make it possible to hope that the use in such an introscope of super-orthicons having high sensitivity and signal-to-noise ratio, more luminous optics, and units for storage and rerecording of television information will make it possible to inspect steel products with wall thicknesses up to 100–120 mm. Increasing the dose rate of betatron radiation by one to two orders of magnitude will lead to an increase of the inspectable thicknesses to 250–300 mm.

REFERENCES

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