Electronic Introscopy

V. I. Gorbunov, V. E. Evstigneev, V. I. Boiko, A. L. Plotnikov

Introduction

The ever-increasing volume and range of industrial products whose quality must be monitored calls not only for improvement of traditional, classical inspection methods, but also for the search for and development of new principles and methods of nondestructive testing.

Comparatively recently, flaw detection of products by means of a beam of fast monoenergetic electrons gained recognition and practical application [1]. However, under modern conditions of large-scale production, single-parameter quality control is becoming insufficient. In this connection, the problem of developing a method of electronic introscopy has become urgent; while preserving the remarkable qualities of electron flaw detection, this method is intended to ensure determination of the shape and character of a defect directly during inspection.

In the present work, on the basis of existing sources of fast electrons and electron-optical and television equipment, the possibility is considered of creating effective electronic introscopes for quality control of materials and products.

Physical foundations of the method of electronic introscopy

The basis of electronic introscopy, as of any radiation method of nondestructive testing, is the interaction of penetrating radiation with matter. The interaction of radiation with matter is characterized both by a change in the coefficient of energy absorption or in the number of particles along the path of its propagation inside the object under study, and by a change in the parameters of the flux associated with scattering, ionization, nuclear reactions, etc.

The information capacity of electronic introscopy means is determined by the physical nature and mechanism of the interaction of electrons with matter, by the intensity, spectrum, and geometry of the distributed flux. In addition, it depends on the structure, shape, and material of the inspected object, the type of defect, and other factors.

The greatest sensitivity of nondestructive testing methods using fast monoenergetic electrons is achieved when the energy of the probing particles is \((0.7 \div 0.9) R_{\mathrm{e}}\) (where \(R_{\mathrm{e}}\) is the extrapolated range of electrons in matter). However, at such thicknesses the spectral and angular distribution of the flux of electrons that have passed through barriers has not been investigated.

For the study of the energy distributions of electrons, an iron-free magnetic beta spectrometer was developed, possessing a homogeneous magnetic field and a scintillation detector operating in counting mode. The source of electrons is a betatron with an inserted electron beam, which was then used in the first version of the electron introscope. The angular distributions were studied by means of a scattering chamber, inside which a remotely controlled scintillation detector was placed.

The angular and energy distributions of electrons with initial energies of 3.0, 5.1, and 8.0 MeV, having passed through aluminum, copper, and lead barriers in the thickness range \((0.1 \div 1.0)R_\mathrm{e}\), were investigated. Analysis of the energy and spatial distributions and of the distributions of absorbed dose with depth made it possible to identify the following features of the interaction of electrons with matter, which play a decisive role in electron introscopy.

1. Energy losses do not depend on the energy of the electrons (in the range studied); therefore particles of different energies, passing through a given barrier, lose the same amount of energy. In this case, as the electrons penetrate into the absorber, their energy decreases linearly with thickness.

2. The dependence of the absorbed-energy distribution on thickness is manifested most strongly at large \((0.7 \div 0.9)R_\mathrm{e}\) depths of the absorber, which ensures high contrast sensitivity.

3. Energy losses in matter depend only weakly on \(z\) (\(z\) is the atomic number of the element). This feature makes it possible to inspect the quality of layered materials and products of different density and chemical composition.

4. The transformation of different energy groups of the spectral distribution of electrons, caused by a change in the thickness of the barrier, is not the same. The high-energy part of the spectrum changes most strongly. At thicknesses of \((0.7 \div 0.9)R_\mathrm{e}\), an additional maximum appears in the low-energy region of the spectrum, caused by the accumulation of electrons with an energy of about 1 MeV. The accumulation is distinctly observed in absorbers with high \(z\).

5. The large cross section for scattering of electrons by the atoms of the medium leads to the fact that, at thicknesses \(>0.5R_\mathrm{e}\), a spatial distribution close to a diffuse one is observed. This feature provides effective inspection of differently oriented extended defects of the microcrack type. Thus, study of the passage of electrons through thick layers of matter not only makes it possible to choose the elements of an introscope and calculate its basic characteristics, but also to outline ways of improving them.

Theoretical Study of the Basic Characteristics of the Method of Electron Introscopy

One of the main problems of introscopy is the conversion of an invisible distributed radiation flux into a visible image. Any “picture” in penetrating radiation can, in principle, be converted into an electrical relief, and the task of conversion in most cases is reduced to its transformation and amplification. Analysis of the functional diagram of introscopy means made it possible to choose the following basic scheme of an electron introscope.

A broad beam of unidirectional and monoenergetic electrons from the source passes through a collimator and falls on the specimen under investigation. The distributed flux of electrons emerging from the specimen, form-

forms in the sensitive layer of the converter a light relief of the internal structure of the object under inspection. From the surface of the converter the image is projected by transfer optics onto the photocathode of an electron-optical amplifier. The brightness-amplified image is transmitted by a television installation to observation points and to stations for documenting the results of nondestructive testing. The following requirements are imposed on an electron source for purposes of introscopy: a relatively high flux density \(\left(\sim 10^{11}\ \frac{\mathrm{el}}{\mathrm{cm}^{2}\,\mathrm{s}}\right)\), uniformity of the flux density over the cross section and in time, monoenergeticity, stability, low \(\gamma\)-background, etc. These requirements are met, to one degree or another, by a linear accelerator, a microtron, and a betatron with beam extraction [2].

Conversion of the electron image into a visible one is most promising with cathodoluminophores of the ZnS(Cu) type in the form of thin \(\sim 15\ \mathrm{mg}/\mathrm{cm}^{2}\) screens, which have maximum conversion efficiency for electrons with minimum sensitivity to \(\gamma\)-radiation. To obtain the highest resolution, the use of organic and plastic converters is also preferable.

A large gain coefficient, the ability to operate at low illuminances, and the possibility of observing the image directly on the output screen determine the choice of multi-chamber electron-optical amplifiers, for example, of the U-72M type.

A fast photographic lens is used as image-transfer optics; for transmitting images to the observation station, a television installation based on a superorthicon is used; for documenting the inspection results, the photographic method is used.

In the general case of inspection by the change in the electron energy flux \(\Phi(t)\), the sensitivity of the method is conveniently expressed through the parameter [3]

\[ A=\frac{\Delta\Phi(t+\Delta t)}{\Delta t}\cdot\frac{t}{\Phi(t)}=\frac{\partial\Phi(t)}{\delta t}, \]

equal to the ratio of the change in the function \(\Phi(t)\) to the relative change in mass thickness \(t\) that caused it. In the static mode, the contrast sensitivity can be determined by taking into account the reliability coefficient \(\xi\), which characterizes the useful-signal-to-noise ratio, and the relative noise level \(\omega\), caused by the action of all destabilizing factors,

\[ |\delta t|_{\min}=\left|\frac{\Delta t}{t}\right|_{\min}=\frac{\xi\omega}{A}. \]

Relating \(\Phi(t)\) to the energy absorbed by the scintillator \(E_{\mathrm{abs}}(t)\) and the energy of the light radiation \(E_{\mathrm{l}}(t)\), and neglecting the nonlinearity of the dependence of \(E_{\mathrm{l}}(t)\) over a small interval in the vicinity of \(t=t_{0}=0.8R_{э}\), we obtain

\[ |\delta t|_{\min}=\frac{\xi\cdot\omega\cdot E(t_{0})\cdot dt}{dE_{\mathrm{abs}}(t_{0})\cdot t_{0}}. \]

Thus, the problem is reduced to determining the energy absorbed by the sensitive volume of the converter and determining the relative noise level.

The determination of \(dE_{\mathrm{abs}}(t)\) can be carried out using the spectral distribution of electrons, the distribution of mean losses, or the distribution of absorbed energy \(D(t)\) over the depth of the specimen. Thus, if it is assumed that the converter with the substrate does not significantly distort

distribution \(D(t)\), then after transformations for a linear section of \(D(t)\) we obtain

\[ |\delta t|_{\min}=\frac{\xi_{\omega}\cdot t}{\left(1-\frac{t}{R_{\mathrm{e}}}\right)\cdot R_{\mathrm{e}}}. \]

Numerical calculation shows that under real conditions the contrast sensitivity is equal to \(0.7\%\).

In calculating \(|\delta t|_{\min}\) it is assumed that the transverse size of the defect \(l\) is arbitrarily large. The calculation of detail sensitivity must take into account the transverse dimensions of the defect, its shape, and fluctuations of the electron flux. Therefore let us assume that the change in specimen thickness by the amount \(|\delta t|_{\min}\) occurred in a region having the form of a square with side \(l\). Then the average number of electrons participating in the formation of the defect image is determined by the expression

\[ N=\eta_N\cdot N_0\cdot l^2\cdot \tau . \]

As a result of fluctuation oscillations of the number of electrons, caused by the probabilistic nature of transmission and registration, the brightness \(B\) of an element \(l^2\) will be perceived by the receiver with accumulation time \(\tau\) as a quantity varying randomly, with mean relative oscillation amplitude

\[ \left|\frac{\Delta B}{B}\right|_{\mathrm{fl}}\approx \frac{\Delta N}{N}\approx \frac{1}{\sqrt{N}} . \]

Then, taking into account the reliability coefficient, the electron transmission coefficient \(\eta_N\), the sensitivity of the receiver, and the electron flux density

\[ N_0\sim 10^{11}\ \frac{\mathrm{el}}{\mathrm{cm}^2\,\mathrm{sec}}, \]

we determine that, for \(|\delta t|_{\min}\geq 0.7\%\), a defect with \(l\geq 0.03\ \mathrm{mm}\) is detected.

The resolving power of the electron-introscopy method was calculated from the contrast of the image of a defect of known size, using the experimental dependence of the resolution of multichamber image intensifier tubes on the contrast of the input image. It is assumed here that the defect is located on the side of the specimen facing the converter. The energy absorbed by the screen behind defective and defect-free regions of the specimen is determined from the results of measuring the mean energy losses by electrons in thick absorbers. When the image is transmitted through the elements of the electron-optical path, losses of image quality and brightness are inevitable. In calculating the illumination of the photocathode, the spectral matching of the characteristics of the elements and the losses in the transfer optics are taken into account. The calculation results show that the resolving power of an electron introscope manufactured in accordance with the basic scheme described above is about 10 line pairs per millimeter in the optimum case.

Using the method of contrast-frequency characteristics, an analysis of the system as a whole was carried out on the basis of scattering and unsharpness functions. It is assumed here that the scattering functions of all elements obey the normal law or are close to it.

Experimental study of the characteristics of the electron introscope and its application

The television electron-introscopic installation consists of a betatron with an extracted electron beam, designed by the Tomsk Polytechnic Institute, an electron-optical unit, and a television-

installation. Structurally, the electron-optical unit combines the U-72M electron-optical amplifier, the high-voltage power-supply unit, the image-transfer optics, and the device for mounting the objects of inspection and the radiation converters in a single metal housing.

An experimental study was carried out of the metrological characteristics of the manufactured betatron electronic introscope BEI-1. The following results were obtained. The limiting contrast sensitivity, determined using step defectometers, is 1.0%; the detail sensitivity, investigated with wire standards, is 2.1%; the maximum resolving power is 5 line pairs per millimeter. All values are given for the case in which the defect was located on the side of the specimen facing the converter (i.e., at zero depth of occurrence of the defect).

As was to be expected from the features of the interaction of electrons with matter in general and from the experimental study of angular distributions in particular, the sensitivity of electronic introscopy worsens by approximately a factor of 10 when the defect is moved to the side of the specimen facing the electron source. The dependence of detail sensitivity on the depth of occurrence of the defect was investigated. The results show that a stronger dependence is observed when the defect is located at a depth of \((0 \div 0.08)t\) and \((0.4 \div 0.5)t\), where \(t\) is the thickness of the article. This is explained by the strong scattering of electrons carrying information in the layers of matter lying after the defect. The depth range \((0.4 \div 0.5)t\) is also explained by the fact that the process of electron passage through matter in this region is characterized by the greatest disorder (the maximum number of secondary electrons, the angular distribution approaches a diffuse one, etc.). A study of the guaranteed sensitivity at different initial energies showed that, within the limits of the experimental errors, in the investigated range it does not depend on energy. It should be noted, however, that with increasing initial energy the maximum sensitivity is achieved at greater specimen thicknesses. Thus, at \(E_0 = 2\) MeV the optimum thickness is \(0.8R_e\), while at \(E_0 = 8\) MeV it tends to \(1.0R_e\).

In the experimental investigation of the possibilities for inspecting homogeneous and layered articles by the method of electronic introscopy, the passage of electrons through extended inhomogeneities in matter of the crack type was studied. In accordance with the conclusions we made in analyzing the features of the interaction of electrons with matter, it was shown experimentally that, in the process of passage through a specimen having an extended inhomogeneity, some of the electrons are captured into the cavity of the inhomogeneity and are effectively transported along its walls. This phenomenon, which we have called macrochanneling (the name is suggested by the qualitative similarity of this effect to the effect of classical channeling), is preserved even in the case of a significant (up to \(60^\circ\)) mismatch between the orientation of the crack and the direction of the incident electron beam. The spatial and energy distribution of electrons that have passed through inclined plane channels was studied. It was found that the deformation of the characteristics of the electron beam in this case is incomparably smaller than when it overcomes a continuous barrier. Thus, when passing through a specimen of thickness \(1.56R_e\) with a channel whose opening width is 100 μm and whose orientation angle is \(30^\circ\), the probable energy of a two-MeV electron decreases by 576 keV for Al and 520 keV for Cu. Studies with the BEI-1 have shown that through cracks with orientation angles are reliably detected at opening widths of 10 μm and greater.

At orientation angles of 45–60°, it is possible to record through defects with an opening of about 100 μm.

An assessment of the parameters of the method for inspecting the light layer of laminated products was carried out with the aid of standards consisting of Plexiglas flaw detectors shielded on both sides by copper plates. It was established that the contrast sensitivity of the method in detecting a change in the thickness of the light layer, whose mass thickness amounts to 30% of the total mass thickness of the standard, is 8–9%; the resolving power is no more than 1 line pair per millimeter. Extended defects in the light filler at orientation angles of 0–30° are recorded if their opening widths are 100 μm or more. BEI-1 detects nonuniformity of the light filler equal to 10–12%.

REFERENCES

1. B. A. Kononov, V. V. Evstigneev et al., Defectoscopy, 1971, No. 2.
2. B. A. Kononov, V. V. Evstigneev, V. I. Boiko et al., Electron Accelerators, Issue 2. Moscow, Atomizdat, 1970.
3. V. I. Boiko, V. V. Evstigneev et al., Defectoscopy, 1973, No. 1.
4. V. I. Boiko, V. V. Evstigneev et al., Defectoscopy, 1973, No. 6.