SELECTING ELECTROMAGNETIC RADIATION ENERGY FOR RADIOGRAPHIC FLAW DETECTION OF LAYERED BODIES: THE CASE OF REINFORCED CONCRETE STRUCTURES

V. A. VOROBYEV

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

For optimal conditions in the radiographic flaw detection of layered bodies, it is necessary to use an energy range that provides the desired combination of productivity and sensitivity. In radiographic flaw detection, detection of a defect is based on a change in the density of the radiation flux that has passed through the defect, in comparison with the density of the radiation flux that has passed through a sound portion of the material. The change in flux density is determined by the magnitude of the radiation attenuation coefficient in the material. The size of the smallest detectable defect can be expressed through the attenuation coefficient [1, 2]

\[ \Delta x = 0.23 \frac{\Delta S_{min}}{\gamma} \cdot \frac{B + K_{\mathrm{v}}}{\mu}, \tag{1} \]

where \(\Delta S'\) is the sensitivity of the eye to the difference in blackening density of the film;

\(\gamma\) is the contrast of the radiograph;

\(B\) is the build-up factor;

\(K_{\mathrm{v}}\) is additional fogging of the film due to scattered radiation that has not passed through the investigated section of the object being inspected;

\(\mu\) is the radiation attenuation coefficient at the location of the defect.

According to (1), in order to obtain the best detectability, the attenuation coefficient must be maximal. Since, in addition to the layer under investigation, the radiation also passes through other layers of material lying in its path and having a different chemical composition, in them, according to (1), the attenuation coefficient must be minimal. Thus, the condition for selecting the radiation-energy range for inspecting a material layer with a large effective atomic number and density reduces to the use of radiation for which the difference between the attenuation coefficients in the investigated layer and in the remaining layers will be greatest [2].

To inspect the position and condition of reinforcement in reinforced concrete, the energy must be selected on the basis of the conditions for radiographing a denser layer against the background of less dense ones.

For this purpose, studies were carried out of the attenuation coefficients of bremsstrahlung radiation in concrete and steel.

The attenuation coefficients were calculated from transmission curves obtained by measuring the radiation intensity while successively increasing the thickness of the material under study [3]. The measurements were performed using thin-walled ionization chambers [2]. The choice of a thin-walled ionization chamber as the radiation detector was determined by the fact that, both in X-ray film and in a thin-walled ionization chamber, ionization is produced mainly by a flux of secondary electrons. The ionization chamber had front and rear walls 0.05 mm thick made of aluminum and was placed directly against the absorber under study. Ionization in the chamber was caused by secondary electrons knocked out of the body under study. Thus, radiography without intensifying screens was simulated. The thickness of the front wall and the depth of the air cavity of the chamber’s sensitive volume were chosen from the condition that the ranges of secondary electrons, knocked out by bremsstrahlung radiation with maximum energy of 10–30 MeV, should be equivalent in RT-type X-ray film emulsion and in the chamber.

The measurements were made using a “Kaktus”-type roentgenometer. Specimens for studying the transmission curves were made of construction concrete with a bulk density of 2.4 t/m³; the total thickness of the composite specimen in the experiments ranged from 5 to 150 cm.

In civil engineering, steel reinforcement with a diameter greater than 20 mm is rarely used [4]; therefore, for steel we measured the attenuation coefficient for a layer thickness of 20 mm. From the obtained values of the attenuation coefficients of betatron bremsstrahlung radiation with maximum energies of 15, 20, 25, and 30 MeV, the attenuation coefficients obtained for the same energies for concrete specimens of different thicknesses were subtracted. The difference thus obtained represented the difference in the attenuation coefficients of bremsstrahlung radiation in steel and in concrete at different depths in a reinforced-concrete structure.

Fig. 1. Dependence of the difference between the attenuation coefficients of bremsstrahlung radiation with maximum energies of 15, 20, 25, and 30 MeV in steel and concrete on the thickness of the concrete layer.

In Fig. 1, the abscissa gives the thickness of the concrete layer in t/m², and the ordinate gives the difference between the mass attenuation coefficients of bremsstrahlung radiation in steel and concrete in g/cm². In the range of maximum energies of betatron bremsstrahlung radiation of 15–30 MeV, the difference in the attenuation coefficients for steel and concrete increases with increasing energy (Fig. 1). The largest value of the difference is obtained for a maximum bremsstrahlung energy of 30 MeV. This situation is maintained also when the magnitude of the attenuation coefficient changes with concrete thickness up to 100 cm, despite the decrease in the absolute value of the difference.

Thus, for monitoring the position and condition of steel reinforcement in reinforced-concrete structures by transillumination with betatron bremsstrahlung radiation, in the range of maximum energies 15–30 MeV, the best detectability is achieved at an energy of 30 MeV.

In Fig. 2, the abscissa gives the maximum energy of bremsstrahlung radiation in MeV, and the ordinate gives the diameter of the smallest

of the detected reinforcing bar, in mm. Beginning with a thickness of reinforced-concrete structures greater than 60 cm, the detectability of reinforcement increases as the radiation energy increases (Fig. 2).

Labels in the figure: \(d\) [mm]; diameter of reinforcement; maximum energy of bremsstrahlung [MeV]; RT-2 film, lead screens; \(1\ \mathrm{T/m^2}\), \(2\ \mathrm{T/m^2}\), \(3\ \mathrm{T/m^2}\).

Fig. 2. Dependence of the smallest detectable diameter of a reinforcing bar in concrete during radiography with bremsstrahlung on the maximum radiation energy, in MeV.

The method of selecting radiation energy for radiographic flaw detection by the difference in attenuation coefficients can also be applied to the inspection of other multilayer bodies.

REFERENCES

1. A. A. Vorob’ev, V. I. Gorbunov, V. A. Vorob’ev, G. V. Titov. Betatron flaw detection of materials and products. Gosatomizdat, Moscow, 1965.
2. V. A. Vorob’ev. Candidate’s dissertation, Tomsk, TPI, 1965.
3. S. P. Kruglov. ZhTF, 31, 1092, 1961.
4. G. F. Dolzhenko. Reinforcement work. Stroiizdat, Moscow, 1951.