INSPECTION OF MULTICOMPONENT PRODUCTS BY MEDIUM-ENERGY PROTONS

V. P. Ivakin, B. A. Kononov, A. L. Lukin

Nondestructive quality control of materials and products by the method of proton radiography is finding ever wider practical application owing to a number of advantages that this method has over such a generally known method of radiation flaw detection as X-ray radiography. The principal advantages include the following: 1) obtaining high contrast sensitivity when using radiographic inspection methods based on absorption and slowing down of protons (down to 0.01% under conditions of operation with maximum beam stability) [1,2]; 2) obtaining a sharper image of the boundary regions of products when using a radiographic method based on proton scattering [3].

The physical basis of the method of proton flaw detection consists in the fact that the attenuation of a monoenergetic proton beam occurs mainly in the layer located near the end of the range and comprising approximately 5% of the entire range. The presence of a steep drop on the range-distribution curve makes it possible to obtain exceptionally high contrast sensitivity (provided that the thickness of the inspected product, uniform in density, is matched to the range of protons of the given energy).

Fig. 1. Range distribution of protons with an initial energy of 6.8 MeV in aluminum.

The dependence of the proton range in a substance on the intensity of a monoenergetic proton beam is shown in Fig. 1. The dashed line denotes the mean range of protons of the given energy (the intensity at this point decreases by a factor of two).

At the Scientific Research Institute of Electron Introscopy, an experimental study was carried out of the possibilities of proton radiography using monoenergetic proton beams with initial energies of 6.8 and 10 MeV. A cyclotron was used as the proton source. Schematically, the experiments were conducted as follows. The proton beam was extracted from the ion guide into air through an aluminum foil 100 μm thick. To create a proton beam uniform in density over an area of 80 cm², an additional scattering foil of aluminum 20 μm thick was installed in the ion guide. The extracted proton beam struck the inspected object, placed in the immediate vicinity of the exit window, and was recorded by a detector. The detectors used were RM-1 type X-ray film, photographic film with photosensitivity of 65 and 130 units according to GOST 5554—70, and KCl and NaCl single crystals.

The possibility of using protons with energies of 6.8 and 10 MeV for defectoscopy was initially checked on aluminum specimens, the proton ranges in which have been well studied. The defect was modeled from aluminum foils in the form of a stepped wedge consisting of 10 steps, each 2 mm wide and 10 μm thick. The wedge was placed behind an aluminum absorber selected so that the total thickness corresponded to the mean range of protons of the given energy. For a proton energy of 6.8 MeV this thickness was 320 μm. The dose necessary to obtain a sharp image on RM-1 film had to be on the order of \(10^9\) protons/cm². The dose for KCl and NaCl crystals varied within the range \(2 \cdot 10^{13} \div 6 \cdot 10^{13}\) protons/cm².

The magnitude of the contrast sensitivity in the experiments was estimated by the presence of an image of a terephthalate film placed across the aluminum wedge, and amounted to 0.08%. It should be noted that the contrast sensitivity practically does not depend on the position of the terephthalate film, which was placed both behind the wedge and in front of it.

Of the detectors used by us in the experimental studies, the most suitable proved to be KCl and NaCl single crystals, which, under the action of protons, become colored violet and deep yellow-brown, respectively. The use of single crystals as a detector helps to assess rapidly, from the degree of coloration, the correctness of matching the ranges and thicknesses of the objects, which is especially important when inspecting multicomponent objects, for which an accurate theoretical estimate of proton ranges is impossible. An important advantage of a crystalline detector is the possibility of estimating both the longitudinal and transverse dimensions of a defect relative to the direction of incidence of the proton beam. This occurs because protons that have passed through a defect in the form of a cavity penetrate the single crystal to a greater depth and form a volume image.

Figure 2 gives an example of a proton radiogram of a copper foil inside which there is a titanium-niobium alloy (50% Ti and 50% Nb). The foil thickness was 50 μm. The image was made with a KCl crystal (magnification 5). The detector was in contact with the foil. Inspection was carried out by the above-described method with protons of energy 6.8 MeV. The thickness necessary to obtain an image of the defect was created by additional absorbers made of aluminum foils 10 and 20 μm thick. The defect, as can be seen from the figure, has a wavelike shape and is apparently due to the technology of coating the titanium-niobium alloy with copper and rolling the product. The light bands in the figure correspond to an increase in the thickness of the titanium-nio-

Fig. 2. Proton radiograph of copper foil with a titanium-niobium alloy placed inside it. Foil thickness: 50 µm. Magnification ×5.

of the alloy; dark regions correspond to a decrease. An estimate of the fluctuations of this thickness corresponds to a value of \(2.6 \div 3.0\ \text{mg}/\text{cm}^2\).

Thus, the experiments carried out have shown the possibility of using proton radiography for the inspection of multicomponent articles with a total mass thickness of up to \(86\ \text{mg}/\text{cm}^2\) at a proton energy of 6.8 MeV.

REFERENCES

1. Koehler A. M. Proton radiography. Science. 160, No. 3825, 303—304 (1968).
2. Berger H., Lapinski N. P. and Beyer N. S. Proton radiography: a preliminary report. Proc. 8th Symp. on Nondestructive Evaluation in Aerospace, Weapons Systems, Nuclear Application. Southwest Research Institute, San Antonio, Texas (1971).
3. West D., Sherwood A. C. Proton-scattering radiography. Non-destructive testing. 6, No. 5, 249—257 (1973).