Screens for Thermoluminescent Radiography

Yu. A. Moskalev, D. I. Sviryakin, Yu. A. Egorenko

In works [1–3] it was shown that in a number of practical cases of radiography it is of interest to use, as converters of radiation fluxes into a visible image, luminescent screens based on phosphors exhibiting the specific property of accumulating the energy of radiation excitation and releasing it in the form of light emission upon heating. Such phosphors are called thermoluminophores, and methods of recording radiation using them are called thermoluminescent.

Analysis and investigation of the properties and characteristics of thermoluminophores from the standpoint of the requirements of visual flaw-detection inspection showed that microcrystalline calcium sulfate activated with manganese is the most suitable of the known thermoluminophores. The advantages of CaSO₄·Mn are its moisture resistance and insensitivity to the action of visible and near-ultraviolet light. Heating to obtain thermoluminescence after irradiation is carried out in the region of low temperatures from 50 to 200° C.

The synthesis of the thermoluminophore CaSO₄·Mn is usually carried out by co-calcination of CaSO₄·2H₂O and MnSO₄. The flux is 25% sulfuric acid. It should be noted that the experimental curve reflecting the dependence of the light output of the luminophore on the concentration of the activator has a clearly pronounced maximum. As the concentration of the activator is increased, the light output at first increases, since the density of luminescence centers increases and, consequently, the probability of capture of energy carriers at these centers increases.

At high activator concentrations, the probability of absorption caused by the activator becomes appreciable, coincides with the luminescence spectrum, and the light output decreases [4]. Therefore the technology of luminophore synthesis is the main factor affecting the sensitivity of thermoluminescent radiography, i.e., radiography using thermoluminescent screens.

In manufacturing the screens, a suspension, consisting of a mixture of CaSO₄·Mn luminophore powder with an organosilicon resin, was applied to a polished aluminum substrate by the pouring method. The resulting layer was then dried for 18–20 hours at a temperature of 150–200° C and mechanically ground, after which the thermoluminescent screen was ready for use in radiographic inspection of materials and products. The scheme for irradiating the object under inspection is shown in Fig. 1, where the numbers denote: 1 — radiation source; 2 — object under inspection; 3 — protective cassette, shielding the thermoluminescent screen from contamination and mechanical damage; 4 — layer

Fig. 1. Radiography scheme

of the thermoluminophor \( \mathrm{CaSO_4\cdot Mn} \); 5 — a backing made of polished aluminum.

Radiation that has passed through the object being inspected, upon interacting with the thermoluminophor layer, excites valence electrons, which, entering the conduction band, can be localized at metastable levels caused by the ion-radicals \( \mathrm{SO_3^-} \), \( \mathrm{SO_4^-} \) [5].

The number of electrons localized at metastable levels is determined by the dose of incident radiation and by the properties of the crystallophosphor. The transition of localized electrons to the activator level is accompanied by light emission, the magnitude of whose intensity (i.e., the light sum) serves as a measure of the absorbed energy.

The storage time of the accumulated light sum is determined by the temperature of the thermoluminophor and at \(20^\circ\mathrm{C}\) is 7–10 days, with the initial light sum decreasing by approximately 30%. After the exposure process is completed, the shadow radiation image formed on the thermoluminescent screen is developed by uniformly heating the backing. The form of the brightness distribution function during heating depends on the rate of temperature change. Under a linear heating regime this function has a sharply pronounced maximum in the temperature region of \(100\text{–}110^\circ\mathrm{C}\), whose magnitude, as well as the total integral over the area under the thermoglow curve, is proportional to the absorbed dose, and this proportionality is preserved up to \(10^5\) rad.

Since the thermoluminophor \( \mathrm{CaSO_4\cdot Mn} \) has one trap level, under a linear heating regime the form of the glow curve is explained by two simultaneously occurring competing processes: one is due to the fact that, as the temperature rises, the number of electrons released from the capture levels increases and the luminescence intensity increases; the other is due to the fact that the number of electrons stored in the conduction band decreases and, in connection with this, the luminescence intensity decreases.

Figure 2 shows a block diagram of the apparatus for recording thermoglow characteristics. The thermoluminescent screen 2 irradiated with X-rays was placed on the heating device 1, with an adjustable rate of temperature change. A hollow cylinder was slid onto the heating device from above, providing thermostating and light insulation of the thermoluminescent screen. In the cylinder cover there is an inlet for the light guide 3, which is a fiberglass bundle 40 cm long and \(5\times5\ \mathrm{mm^2}\) in cross section. The light transmission of the light guide is 60%. The light emission of the thermoluminescent screen through the light—

Fig. 2. Block diagram of the apparatus for recording the kinetic characteristics of thermoluminescence

the light guide was transmitted to the input of photomultiplier 6; the electrical signal from the PMT passed through direct-current amplifier 7 to recorder 8 and was recorded on chart paper. Temperature control was carried out with thermocouple 4, introduced together with the light guide into the cylinder and in contact with the surface of the thermoluminescent layer. The thermocouple emf was recorded by microvoltmeter 5 and converted, using the calibration curve, into a temperature scale.

The curves of the dependence of the brightness \(B\) of the screen thermoglow on heating time, at a constant heating rate of 18 deg/min, for different irradiation doses are presented in Fig. 3.

Fig. 3. Dependence of the screen brightness on heating time at a constant heating rate for different irradiation doses. Curves 1, 2, 3 correspond respectively to exposure doses of X-ray radiation: \(1.5 \cdot 10^{-3}\), \(3.8 \cdot 10^{-3}\), and \(6 \cdot 10^{-3}\) C/kg

In addition to the dose and heating regime, the brightness of thermoglow depends on the thickness of the thermoluminescent layer. Since the light composition \(\mathrm{CaSO_4 \cdot Mn}\) is opaque to its own light radiation, and observation of the image was carried out from the surface of the screen that, during exposure, faced the radiation source, the curve of the dependence of the relative light output on the thickness of the thermoluminescent layer should have a “saturation” region. This is confirmed by the experimentally obtained graph of the dependence of the light sum per unit absorbed dose \(\left(\frac{S}{D}\right)\) on the screen thickness \(d\), presented

in Fig. 4. As can be seen from the graph, the light output increases linearly as the screen thickness is increased up to 0.5 mm and enters the saturation region at \(d > 0.6\) mm. The effect of specular reflection from the backing (the dashed curve in Fig. 4) was not recorded in the experiment, because the thermoluminophor \(\mathrm{CaSO_4 \cdot Mn}\) has its own coefficient of diffuse reflection of visible light close to unity.

Fig. 4. Dependence of the thermoluminescence light output of \(\mathrm{CaSO_4 \cdot Mn}\) on screen thickness

Thus, with a screen thickness made of the thermoluminophor \(\mathrm{CaSO_4 \cdot Mn}\) of 0.5–0.7 mm, the surface of the backing does not affect the magnitude of the light output; at the same time, this screen thickness is optimal in terms of light output in the X-ray energy range of 20–90 keV.

Tests of screens made from the powdered thermoluminophor \(\mathrm{CaSO_4 \cdot Mn}\) showed that, with a linear increase in temperature, the maximum difference in the brightnesses of two regions that received different irradiation doses occurs in the temperature range of maximum light output. The duration for which the brightness remains at the level of 0.7 of the maximum is about 2 minutes and is sufficient for analysis of a radiation image measuring \(15 \times 25\ \mathrm{cm^2}\). Increasing the screen thickness beyond 0.6 mm does not significantly affect the thermoluminescence light output.

Thermoluminescent radiography of materials is at present a little-studied area of application of thermoluminophors and, in its development, will become an additional method to the known systems of X-ray and film radiographic inspection of industrial products. The ability of screens based on thermoluminophors to accumulate information will make it possible to inspect objects over a wide range of thicknesses using a radiation source with a comparatively low dose rate.

REFERENCES

1. V. A. Dobromyslov, S. V. Rumyantsev. Radiation Introscopy. Moscow, Atomizdat, 1972.
2. Methods of Nondestructive Testing. Edited by R. Sharp. Moscow, Mir, 1972.
3. V. I. Gorbunov et al. X-ray radiography using thermoluminescent converters. (This collection).
4. K. K. Shvarts et al. Thermoluminescent Dosimetry. Riga, “Zinātne,” 1968.
5. L. N. Ivanov. Abstract of a Candidate dissertation. Moscow State University, 1972.