PROCEEDINGS
OF THE TOMSK ORDER OF THE RED BANNER OF LABOR POLYTECHNIC
INSTITUTE named after S. M. KIROV

Vol. 149　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1966

BETATRON FLAW DETECTION OF POLYMER ELECTRICAL INSULATING MATERIALS

V. A. VOROB'EV, G. V. TITOV

(Presented by the scientific seminar of the Research Institute of High Voltages)

High voltages are widely used in various fields of science and technology, as well as in industry. To ensure reliable and safe operation of high-voltage equipment, it is necessary to use high-quality electrical insulating materials possessing the required electrical strength. Local deviations from the calculated electrical strength, caused by macro-inhomogeneities in the structure of the material, must not exceed the permissible limit, which should be established on the basis of the operating conditions of the installation. One of the methods that makes it possible to determine inhomogeneities in a material opaque to visible light is radiation flaw detection, which is now widely used for quality control of products made of metals [1, 2].

Inspection of parts of high-voltage equipment by radiation from radioactive isotopes and X-ray installations, owing to their low penetrating power, is limited to thicknesses for polymer electrical insulating materials not exceeding 25–35 cm. Therefore, only parts of small thickness can be subjected to inspection. Inspection of large structural elements of high-voltage installations having great thickness by radiation from these sources is practically impossible. One of the ways to reduce rejects of parts made from electrical insulating materials and to reduce accident rates is preliminary inspection of blanks or of the material before machining, in order to reveal defects in the material and remove the defective portion of the material outside the part during manufacture.

The large thicknesses of blanks and starting materials do not make it possible to carry out inspection by radiography and gamma radiography. The use of betatrons as radiation sources makes it possible to increase the thicknesses of polymer electrical insulating materials accessible to radiation flaw detection to 1.5 meters and more [3].

The great penetrating power of the bremsstrahlung radiation of betatrons and the high intensity of the radiation, as well as the large exposure doses on the surface of products when radiographing large parts, require careful studies of the effect of radiation on the macrostructure of polymer electrical insulating materials.

Prolonged exposure to high-energy radiation, which occurs in the inspection of thick-walled products, may cause cracks and ruptures to appear in the material of the article being inspected [4]. The appearance of such damage is highly characteristic of polymer materials.

Along with disruption of the macrostructure under irradiation, changes are observed in many other properties of polymers, for example, electrical properties [5]. Thus, for polymer electrical-insulating materials whose quality is monitored by transillumination with high-energy radiation, it is also necessary to determine the degree of danger posed by changes in physical properties during irradiation.

We have investigated disruption of the macrostructure of polymer materials—Teflon, polyethylene, and polystyrene—when they are irradiated by betatron radiation with a maximum quantum energy of up to 30 MeV. Samples of Teflon, polyethylene, and polystyrene, having dimensions from 3 to 10 cm, were irradiated with various radiation doses; the radiation damage was studied in layers corresponding to the maximum of the absorbed energy. Numerous investigations showed that radiation damage with disruption of the continuity of the structure of the article occurs only upon irradiation with doses greater than 10 roentgens.

To reduce radiation damage to the macrostructure of polymers during irradiation, we investigated possible ways of shortening exposure times. In our opinion, scintillation intensifying screens [6] are highly promising for these purposes. The quality of the image obtained with scintillation screens is, of course, poorer than with lead or fluorescent screens; however, the reduction in exposure time obtained was about 100-fold.

Another way of shortening exposure times is the simultaneous exposure of two or three films, with their subsequent examination by superposition. The dose required to obtain the necessary blackening of such a composite radiograph is reduced in proportion to the number of films exposed.

To determine exposure times, we obtained a formula relating the measurement parameters and the material being inspected to the geometry of the experiment [7]. If the intensity of the radiation source at a distance of one meter is \(I_0\) roentgens/sec·cm\(^2\), \(x\) is the thickness of the absorber, and \(\mu\) is the linear attenuation coefficient of the radiation in the material being inspected, then the exposure time \(t\) is determined as

\[ t=\frac{D_0 \cdot k \cdot e^{\mu x}\cdot r^2}{I_0}\cdot m, \]

where \(D_0\) is the radiation dose required to obtain the necessary blackening of the film without an absorber;

\(k\) is a coefficient taking into account the change in film sensitivity with increasing absorber thickness (always \(k>1\));

\(m\) is the screen intensification factor;

\(r\) is the focal distance.

The detectability of defects, which determines the size of the smallest detectable defect, depends on the thickness of the article, the radiation energy, the sensitivity of the detector, etc. Most complexly, detectability depends on the radiation energy: on the one hand, an increase in the radiation energy leads to a decrease in the value of the attenuation coefficient and, consequently, to poorer detectability of defects; on the other hand, an increase in energy lowers the level of secondary radiation,

worsening the detectability of defects. We investigated the dependence of defect detectability at various radiation energies in the range 10–30 MeV. The results obtained are summarized in the graph in Fig. 1.

The optimum conditions for radiography were determined from the image on the film of reference defects, consisting of stepped plates with holes of various diameters. The thickness of the step visible on the radiograph determines the detectability of defects in the direction of the central beam, and the diameter of the smallest distinguishable hole determines the dimensions of the defect in the direction perpendicular to the radiography.

Fig. 1. Dependence of defect detectability in polyethylene under radiography with bremsstrahlung radiation on the radiation energy.

The studies carried out of the quality of electrical-insulating materials by radiography with betatron radiation showed the applicability of this method for inspection of large thicknesses (up to 1000 mm).

REFERENCES

1. V. S. Sokolov, Defectoscopy of Materials. Gosenergoizdat, Moscow, 1961.

2. S. K. Rumyantsev, Yu. A. Grigorovich. Quality Control of Metals by Gamma Rays. Metallurgizdat, 1954.

3. A. A. Vorob’ev, V. A. Vorob’ev, V. I. Gorbunov, G. V. Titov. Betatron Defectoscopy of Materials and Products. Gosatomizdat, 1965.

4. J. Dins and J. Vinnard. Radiation Effects in Solids. Moscow, Foreign Literature Publishing House, 1960.

5. Charlesby. Nuclear Radiations and Polymers. Moscow, Foreign Literature Publishing House, 1960.

6. V. A. Vorob’ev, V. I. Gorbunov, A. A. Silinskii, G. V. Titov. Scintillation intensifying screens in betatron radiography. Abstracts of reports of the Fifth Interuniversity Scientific Conference on Electron Accelerators, Tomsk State University Press, Tomsk, 1964.

7. V. A. Vorob’ev, A. A. Silinskii, G. V. Titov. Analytical determination of exposure time in betatron radiography. Abstracts of reports of the Fifth Interuniversity Scientific Conference on Electron Accelerators, Tomsk State University Press, Tomsk, 1964.