Exposure Duration in Radiographic Inspection of Steel Using 25 MeV Betatron X-Rays

V. I. Gorbunov

(Presented by the scientific seminar of the Faculty of Physics and Technology)

The exposure time in radiographic inspection of a steel absorber depends on the intensity of the betatron radiation, the hardness of the generated radiation, the focal distance, the quality and type of X-ray film and intensifying screen, and also on the thickness of the absorber being tested [1, 2].

All the experiments described below on determining the exposure duration in radiographic inspection of steel were carried out at the maximum radiation energy of 25 MeV, with a known radiation intensity and a preselected focal distance.

It was necessary to obtain the dependence of the exposure time (total dose) on the thickness of the steel absorber for various types of X-ray films and intensifying screens.

Characteristic curves were obtained experimentally for the photosensitive layers of various types of film exposed by radiation of hardness 25 MeV behind a steel absorber of different thicknesses. Taking these curves made it possible to establish a quantitative relationship between the sensitometric parameters of the film photosensitive layers and the magnitude of the radiation dose incident on the film, to construct exposure graphs for various types of films and screens, and also made it possible to compare the properties of different films and select the best type of film for each specific inspection case.

The characteristic curves were obtained for 5 types of X-ray film: “Roentgen XX,” “Roentgen X,” “Roentgen X-5” (experimental), “Agfa-Duro,” and “Ilford,” behind a steel absorber with a thickness from 50 to 400 mm, with 2PZ and 2FPGZ intensifying screens.

In order to facilitate the selection of exposures, we determined the theoretical exposures required to obtain a specified blackening on the film. For this purpose, first of all, the dependence of the absorption coefficient for a broad unfiltered beam of X-rays on the absorber thickness was obtained using an ionization chamber (Fig. 1).

Knowing the absorption coefficients for a steel absorber of different thicknesses, and also having determined from the characteristic curves obtained without an absorber the radiation dose necessary to obtain X-ray images of different blackening density, one can determine the theoretic-

... doses for obtaining the same blackening in the case of radiographing steel plates [1, 2, 3].

As the reference blackenings we took blackenings with densities \(D = 0.5; 1.0; 1.5; 2.0; 2.5; 3.0\).

Fig. 1

Using the law of attenuation of X-rays \(J = J_0 e^{-\mu d}\), one can determine the amount of energy incident on \(1\ \mathrm{cm}^2\) of film placed behind an absorber of thickness \(d\) cm.

\[ P = P_0 e^{-\mu d}, \tag{1} \]

where \(P_0\) is the radiation dose at the surface of the film without an absorber,
\(P\) is the radiation dose at the same point with an absorber of thickness \(d\) cm.

Assuming that the blackening of the film is directly proportional to the amount of energy that has reached the film, one can write the following condition for one and the same density of blackening:

\[ P = k = \mathrm{const}. \tag{2} \]

Then

\[ P_0 e^{-\mu d} = k, \tag{3} \]

whence we find

\[ \ln(P_0) - \mu d = \ln k \tag{4} \]

or

\[ \ln(P_0) = \mu d + \ln k. \tag{4a} \]

Using formula (4a) and knowing \(k\) for the case of exposing the film without an absorber, we determine the radiation doses, knowledge of which made it possible to construct characteristic curves for all types of films for various thicknesses of the steel absorber. After exposure, the films were photometered on an MF-4 microphotometer, and from the results of photometry experimental characteristic curves were constructed. For po-

for constructing each characteristic curve, from 15 to 20 films were exposed in the blackening-density interval from 0 to 3.0 at a distance of 1 m.

The experimental characteristic curves obtained for five types of X-ray film are presented in Figs. 2–5. Along the abscissa axis

[In figure: \(D\); \(D(P)\); “Ilford”; “Agfa-Duro”; “Rentgen X-5”; “Rentgen X”; “Rentgen XX”; \(d = 100\ \text{mm}\); \(l = 1500\ \text{mm}\).]

Fig. 2

[In figure: \(D\); \(D(P)\); \(d = 200\ \text{mm}\); \(l = 1500\ \text{mm}\); “Rentgen XX”; “Rentgen X”; “Rentgen X-5”; “Agfa-Duro”; “Ilford”.]

Fig. 3

Fig. 4

Fig. 5

...the total doses in roentgens are plotted; along the ordinate axis—the blackening density.

The experimental characteristic curves obtained make it possible to construct exposure graphs.

The comparative exposure graphs obtained for various grades of film make it possible to compare the sensitivity of different films behind a steel absorber up to 500 mm thick (Figs. 6, 7). All curves are valid for a focal distance equal to 1 m \((l = 1\ \text{m})\). Along the axis

Fig. 6

Fig. 7

on the abscissa is plotted the thickness of the steel absorber in cm, while on the ordinate are plotted the total radiation dose in roentgens and also the irradiation time in minutes, taking the radiation intensity to be \(60\ \text{R/min}\) at a distance of one meter. The exposure graphs for all types of film, except “Rentgen XX,” are valid with the use of composite intensifying screens of the type (2FPF3). For “Rentgen XX” film, a (2P3) screen is used.

In practical flaw detection one has to deal with radiography of various parts, which differ considerably in their dimensions. In addition, it is sometimes necessary to radiograph individual parts with a large surface area in a single exposure, and also to obtain enlarged radiographs of especially critical assemblies. All these requirements can be met if the focal distance is chosen correctly for each specific radiographic case. Hence there arises the need to know the radiation doses, as well as the exposure times, when radiographing steel plates of various thicknesses at different focal distances.

Since all exposure graphs were obtained in radiographing a steel absorber with a broad divergent beam of X-rays, using the inverse-square law for radiation propagating from a point source [1], one can construct exposure graphs for different focal distances.

Figures 8–12 present exposure graphs constructed for different types of X-ray films and screens for six values of focal distances from 75 to 300 cm. The graphs were constructed for an optical density of blackening \(D = 2.0\).

Fig. 8

In experimental radiography of various steel and cast-iron parts, the exposures were determined from the graphs given above. As the results of photometry of the obtained radiographs showed, the difference between the blackening densities taken from the exposure graphs and the blackening densities obtained by photometry does not exceed \(\pm 0.2\), which is practically quite acceptable.

Figure 9

Figure 10

Fig. 11. Exposure time \(t\) [min] as a function of thickness \(d\) [cm] for “Roentgen X” (2FPF3), \(D=2\). Curves are labeled for distances \(0.75\ \text{m}\), \(1.00\), \(1.50\), \(2.00\), \(2.50\), and \(3.00\).

Fig. 12. Exposure time \(t\) [min] as a function of thickness \(d\) [cm] for “Roentgen X” (2G3), \(D=2\). Curves are labeled for distances \(0.75\ \text{m}\), \(1.00\), \(1.50\), \(2.00\), \(2.50\), and \(3.00\).

Conclusions

1. As a result of the experimental work carried out, working exposure charts were obtained for radiographic inspection of a steel absorber up to 500 mm thick by means of hard X-radiation with a maximum accelerated-electron energy of 25 MeV.

2. The exposure charts were constructed for 5 types of X-ray film: “Ilford,” “Agfa-Duro,” “Rentgen X-5” (experimental), “Rentgen X,” and “Rentgen XX,” using two combinations of intensifying screens: 2FPFZ and 2PZ.

3. The exposure charts were constructed for a blackening density \(D = 2.0\) and 6 values of the focal distance, from 75 to 300 cm.

4. It follows from the charts that, in the case of using 2FPFZ screens, the film “Ilford” has the greatest sensitivity, followed by “Agfa-Duro,” “Rentgen X-5,” and, finally, “Rentgen X.” The film “Rentgen XX” with 2PZ intensifying screens, which provide increased sharpness in images of defects, is best used, because of its low sensitivity, for radiographing steel no more than 200 mm thick.

Literature

1. Rumyantsev S. V., Grigorevich Yu. A. Quality Control of Metals by Gamma Rays. Metallurgizdat, 1954.

2. Tatbek L. K., Medvedev S. V. Industrial Gamma Defectoscopy. Metallurgizdat, 1955.

3. Trapeznikov A. K. X-ray Defectoscopy. Mashgiz, 1948.