BETATRON DEFECTOSCOPY OF HIGH-DENSITY MATERIALS

V. A. Berdonosov, V. I. Gorbunov, G. V. Titov

The wide use of high-density materials (lead, tungsten, uranium) in industry makes it necessary to study methods for nondestructive testing of their quality.

As examination and analysis of a number of works show, the most comprehensive and in-depth studies have been made of questions of betatron defectoscopy of steel [1, 2], polymeric materials [3], and also inhomogeneous multicomponent materials of the concrete and reinforced-concrete type [4].

The restriction of investigations to only the indicated materials does not allow, on the basis of the available data, the development of a methodology for betatron defectoscopy of a broad class of materials and, in particular, high-density materials. In addition, the absence of theoretical and practical data on betatron defectoscopy of the above-mentioned materials is a serious obstacle in evaluating the effectiveness of using betatrons for purposes of radiation defectoscopy. It is therefore of interest, as applied to the indicated materials, to investigate certain questions of the radiographic and radiometric methods of betatron defectoscopy.

The study of the basic parameters of radiographic inspection was preceded by a series of experiments on the selection of intensifying screens.

In order to determine the optimal thickness of the front intensifying screen when radiographing lead with bremsstrahlung with a maximum energy of 30 MeV, lead foils of various thicknesses were used: 0.3; 0.5; 0.6; 0.7; 0.8; 1.0; 1.5; 2.0 mm. Small plates measuring \(5 \times 15\) mm were cut from the foil and, with the aid of rubber cement, were glued onto a backing of vacuum rubber, the size of which was determined by the size of the cassette used. Taking into account the small cross sections of the bremsstrahlung beam at the selected energy values and the dimensions of the lead blocks \((100 \times 100 \times 50\) mm), standard cassettes measuring \(13 \times 18\) cm were used.

Lead foil 3 mm thick was used as the rear screen. Thus, in a single exposure it became possible to obtain a radiograph with several combinations of screens.

Irradiation of the film was carried out with unfiltered betatron radiation behind various thicknesses of a lead absorber (50, 100, 150 mm). The exposure time was determined by the dose necessary to obtain the optimal optical density of blackening of the X-ray emulsion.

films of the RT-1 type. The blackening density value corresponded to 1.5–1.7 [3, 4]. The dose value was selected experimentally, and it was recorded with a DIM-60 dosimeter.

An ionization chamber with a volume of about 5 cm³ was mounted on the side of the collimator facing the target, which formed the betatron radiation beam, in a special holder, thereby ensuring that the position of the chamber remained fixed throughout the entire experiment. The absorber and cassette were positioned so that their centers of symmetry coincided with the center of the beam.

Immediately before the start of the experiment, the exposure dose rate of bremsstrahlung radiation (at the given maximum energy) was measured at a distance of 1 m from the betatron target. To increase the efficiency with which the chamber recorded the hard bremsstrahlung radiation of the betatron, a lead cap with wall thickness of 2.8 mm was placed on its housing. Exposure of the film was stopped by switching off the electron-injection voltage.

A number of measures were adopted for carrying out the experiments, ensuring good reproducibility of the results. First, the parameters of the working radiation beam (its intensity and maximum energy) were stabilized to an accuracy of 2–3%. Maintenance of the constancy of the radiation intensity was monitored with the aid of the ionization chamber of the “Kaktus” instrument, installed in the reverse beam of the betatron radiation. Throughout the entire series of experiments, the “geometry” of the experiment was strictly kept constant by fixing the position of the ionization chamber and the position of the cassette with the film.

The processed and dried films were photometered using an MF-4 microphotometer. The arithmetic mean of four readings of the blackening density near a fixed point was taken as the desired blackening of the radiographs. The accuracy of the measurements in all cases was no worse than 5%.

The experiment on determining the optimum thickness of the front screen showed that the greatest blackening density is observed behind screens with a thickness of 0.4–0.7 mm. However, the amplification factor is very small and close to unity. The amplifying action of the screens is almost independent of the thickness of the absorber. Thus, the use of front intensifying screens in radiographic inspection of lead does not provide such amplification as, for example, in the case of radiographic inspection of steel.

The productivity of inspection in the radiographic method of betatron defectoscopy is usually determined from preliminary preparation of exposure charts, which are a graphical expression of the dependence of exposure time on the thickness of the material being inspected for some fixed value of the focal distance, intensity and maximum radiation energy, detector sensitivity, and radiograph blackening density. The reliability of data on inspection productivity obtained with the aid of exposure charts naturally depends to a considerable degree on the conditions under which the radiographs are processed, or more precisely on the standardization of these conditions.

Figure 1 gives an exposure chart representing the dependence of the radiation dose (exposure time) required to obtain a blackening density of 1.7 in the images on the thickness of the absorber when lead is radiographed using betatrons at maximum radiation energies of 30 and 25 MeV.

The exposure dose rate of the radiation at a distance of 1 m from the target behind 2.8 mm of lead, for the indicated values of the maximum energy, was respectively 40 and 150 R/min; in the experiments

RT-1 film was used, and intensifying screens of optimum thickness were employed. The focal distance was 2 m.

To determine the feasibility of the radiographic method of betatron defectoscopy of high-density materials and to verify the correctness of the choice of the maximum energy of the bremsstrahlung spectrum, we carried out a study of defect detectability on lead specimens of various thicknesses.

Graph labels: vertical axis—dose on film in roentgens; horizontal axis—lead thickness, mm; curves—25 MeV and 30 MeV.

Fig. 1. Exposure chart for lead

Such studies were carried out in accordance with the accepted standard, using the image on the radiograph of reference defects. The reference defects were made of the same material as the absorber under investigation. The ratio of the smallest dimension of the reference defect in the direction of radiography, detectable on the radiograph, to the total thickness of the absorber determines the reference detectability of defects, or simply the detectability of defects.

In all cases of radiography, metallic intensifying screens were used which, despite the insignificant intensification effect, increased image sharpness by absorbing the secondary radiation arising in the absorber under investigation.

Figure 2 presents the results of a study of the detectability of standard defects when radiographing lead. The ordinate axis gives the size of the smallest detectable defect in the direction of radiography; the abscissa axis gives the thickness of the item being inspected. The curves characterizing defect detectability (Fig. 2) were constructed for maximum radiation-spectrum energies of 25 and 30 MeV.

As follows from Fig. 2, defect detectability varies depending on the thickness of the absorber and on the maximum energy of the radiation spectrum.

Fig. 2. Dependence of the relative sensitivity of the radiographic method on the thickness of lead for betatron radiation with maximum radiation energies of 25 and 30 MeV

Thus, at a lead thickness of 150 mm, the smallest size of a standard defect distinguishable on the radiograph is, respectively, 0.6% and 0.8% at maximum energy values of 30 and 25 MeV. The observed deterioration in defect detectability with increasing absorber thickness at a fixed value of the radiation-spectrum energy, and with decreasing maximum energy at a fixed thickness, can be explained by the increasing influence of secondary radiation.

To assess the possibility of inspecting high-density materials by the radiometric method, we developed a betatron flaw detector using scintillation counters as radiation detectors. Such a flaw detector consists of a remote unit with a set of interchangeable input collimators, a control console including a circuit for processing and recording information, a power-supply and control circuit, and a traversing mechanism [5].

The purpose of the experiments carried out with the scintillation flaw detector was to obtain data on the sensitivity and productivity of inspection of lead products, necessary for assessing

effectiveness of the use of betatrons with the radiometric method of recording, the choice of the betatron operating mode when inspecting lead products, and also a check of the operability of the flaw detector.

The block diagram of the experiment is shown in Fig. 3. The beam of hard bremsstrahlung radiation 14, generated by target 2 of betatron 1, is formed by collimator 3. In the bremsstrahlung beam the inspected article 4, assembled from separate lead blocks, is installed. In the course of the experiments it was assumed that radiographic examination of a comparatively small area of the cross section of the blocks, in comparison

Fig. 3. Block diagram of the experiment for determining the principal parameters of inspection by means of the radiometric method of betatron flaw detection

with the area of the article in full size, would not introduce a significant error into the measurement results. A “defect” in the form of lead plate 9 was moved at a constant speed in the horizontal direction perpendicular to the axis of the radiation beam by means of motion mechanism 5. The position of the “defect” through the absorber thickness and its speed of motion could be varied. The gamma radiation that had passed through the inspected article and the holes of collimator 6 of the remote unit 10 of the flaw detector was recorded by scintillation counters 7, the signals from which were transmitted by cable to the information-processing unit located in the control console of the flaw detector. Betatrons with maximum gamma-quantum energies of 25 and 30 MeV and radiation dose rates at a distance of 1 m from the target of, respectively, 40 and 150 R/min were used as sources of hard bremsstrahlung radiation.

A large number of experiments were carried out to determine the sensitivity of the flaw detector under various operating modes of the betatron installation and at various speeds of motion of the “defect.” Detectability was evaluated from records on the chart tape of an EPP-09 electronic automatic potentiometer; it was considered that a defect had been detected if the useful signal on the chart exceeded the noise level by a factor of 2. If the noise level was higher, the defect was considered undetected.

Figure 4 shows graphs of the dependence of the static sensitivity of the flaw detector on the thickness of lead. In this case the time constant of the integrating circuit was \(\tau = 0.5\) sec, and the area of the collimator entrance window was \(S_k = 1\ \text{cm}^2\), with the beam projection of the “defect” completely covering the collimator window. Similarly, i.e., from the record on the chart tape of the self-recording instrument, the sensitivity of the flaw detector was evaluated for various article thicknesses as a function of the speed of motion of the “defect” for a specified collimator area (Fig. 5).

Fig. 4. Dependence of the static sensitivity of a betatron defectoscope on lead thickness for betatrons with maximum radiation energies of 25 and 30 MeV.

Fig. 5. Dependence of the dynamic sensitivity of a betatron defectoscope on lead thickness for betatrons with maximum radiation energies of 25 and 30 MeV.

The study of the principal parameters of radiation inspection of high-density materials ultimately permits a number of conclusions to be drawn:

1. The use of front intensifying screens in radiographic inspection of high-density materials carried out with betatrons at various maximum bremsstrahlung energies is not effective from the standpoint of reducing exposure time. The use of rear intensifying screens is advisable.

2. Increasing the thickness of high-density materials leads to poorer detectability of defects; moreover, this change in detectability is nonlinear and increases with increasing absorber thickness. Defect detectability depends on the maximum energy of the radiation spectrum and worsens as it decreases.

3. With radiometric systems for betatron defectoscopy using scintillation counters as radiation detectors, high-density materials can be inspected reliably. In this case, inspection sensitivity in the lead-thickness range of \(50 \div 250\) mm can be obtained no worse than 1.5%. The inspection productivity, for a collimator entrance-window area of \(1.0\ \mathrm{cm}^2\), is \(1.4 \div 1.6\ \mathrm{m}^2/\mathrm{h}\).

References

1. A. A. Vorob’ev, V. I. Gorbunov, V. A. Vorob’ev, G. V. Titov. Betatron Defectoscopy of Materials and Products, Atomizdat, Moscow, 1966.

2. Betatron Gamma-Defectoscopy of Steel. Collection of articles. Publishing House of the USSR Academy of Sciences, Moscow, 1961.

3. G. V. Titov. Candidate’s dissertation, Tomsk, 1966.

4. V. A. Vorob’ev. Candidate’s dissertation, Tomsk, 1965.

5. V. A. Berdonosov. Candidate’s dissertation, Tomsk, 1968.