Radiography Using Thermoluminescent Converters
Abstract Generated abstract
This paper examines the use of thermoluminescent screens and films as reusable converters for industrial radiographic flaw detection, addressing the labor and processing costs associated with conventional X-ray film. It describes the physical basis of image storage in crystalline phosphors, outlines the inspection cycle, compares suitable phosphor compositions, and develops expressions for exposure time and total inspection time as functions of dose rate, brightness, and absorber thickness. Using calcium sulfate activated with manganese as an example, the authors report visually distinguishable images with short exposure and preparation times, penetration sensitivities meeting stated radiographic inspection requirements for thin aluminum samples, and reduced total inspection time for aluminum and magnesium sections up to 20 mm compared with RT-1 X-ray film.
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RADIOGRAPHY USING THERMOLUMINESCENT CONVERTERS
Yu. A. Egorenko, Yu. A. Moskalev, D. I. Sviryakin
In recent years, along with the widespread introduction of television-based and radiation-based inspection systems for materials and products, the methods and apparatus of film radiography have continued to be improved. Interest in film radiography is connected with a number of advantages of film converters of radiation. These advantages amount to the following:
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Film converters make it possible to obtain documentary visual information about internal inhomogeneities of the object being inspected; to determine their shape and dimensions.
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Owing to the small dimensions of film converters, as well as the possibility of changing their area and their ability to deform along the contour of the inspected object, film converters often become indispensable when inspecting hard-to-reach places in products of complex configuration.
A drawback of radiography using X-ray film is the need to carry out additional operations of development, fixing, and drying. The use, for the purpose of increasing inspection productivity, of an automatic system for photographic processing of film is expedient when the volume of work is large (more than 5 m² of film per shift); moreover, in this case it is necessary to use special heat-resistant films. The large volume of work associated with the preparation and processing of X-ray film, and the considerable consumption of photographic-processing materials and of the film itself, make this inspection method labor-intensive and expensive.
In some cases the technological disadvantages of X-ray film can be eliminated by using, for purposes of industrial defectoscopy, screens and films based on luminescent light compositions that have the ability to accumulate the light sum upon excitation and to emit it after the action of radiation has ceased, under the influence of an optimizing factor, for example heating.
The physical basis of this method of converting a shadow radiation image into an optically visible one is the manifestation of the specific properties of a number of crystalline phosphors to accumulate the absorbed energy of ionizing radiation by localizing the charge carriers formed—electrons and holes—in capture centers, i.e., traps.
The stored energy, upon additional excitation, is released in the form of light energy.
Additional excitation may be caused either by heating (thermoluminescence) or by illuminating the phosphor (photoluminescence).
In recent years such crystalline phosphors have been used successfully in luminescent methods of dosimetry. These include calcium fluoride, lithium fluoride, calcium sulfate activated with manganese, and calcium sulfate activated with samarium.
Radiation converters made from these phosphors in the form of screens and films can be used for the purposes of radiation flaw-detection inspection.
The procedure for the inspection process is as follows.
The thermoluminescent converter is placed at the site where the object under inspection is to be transilluminated, analogously to X-ray film. After the exposure is completed, stimulated emission of the stored light sum is induced by heating. The heating regime is chosen so as to provide the best conditions for observation on the thermoluminescent converter.
After the operator has finished viewing the image, the converter is heated until the luminescence is completely quenched, then cooled, after which it is again ready for exposure. The preparatory time for the entire inspection process takes 8–10 min, which is 4–5 times less than the preparatory time for inspection with X-ray film [1].
The choice of phosphor for thermoluminescent radiography is determined by fulfillment of the following basic requirements:
a) maximum sensitivity of the phosphor to ionizing radiation;
b) high luminescence yield;
c) absence of luminescence fading over a broad temperature range;
d) coincidence of the luminescence spectrum with the spectral sensitivity of the eye or of another recording instrument;
e) low cost of inspection.
If the technological inspection cycle is such that the time interval from the end of exposure to the start of image viewing is short, then, in addition to the phosphors listed above, a broad class of phosphors of temporary action can be used for radiation flaw detection.
Because of the presence of “shallow traps,” such phosphors do not preserve the accumulated light sum, but instead emit it over a prolonged period of time after the end of exposure. The duration of their afterglow ranges from several minutes to 8–10 hours [2].
These should include, first of all, the phosphors: Sr · Bi, Cu; ZnS · Cu, Co; ZnS · Cu.
In these phosphors the luminescence yield is also a function of temperature; therefore, the duration of preservation of the accumulated light sum can be increased by preliminary cooling of the converter, and, by regulating the heating regime, rapid quenching of the luminescence can be carried out after completion of viewing the image formed on the converter, thereby preparing it for the next exposure.
The table gives the principal radiation and photometric characteristics of phosphors with accumulation of the light sum. As can be seen from the table, the maximum of the emission spectrum of most phosphors is well-
Table
| No. | Phosphor | Wavelength of the maximum of the luminescence spectrum, nm | Quantum yield of thermoluminescence, quanta/coulomb | Duration of information retention at \(t = 20^\circ \mathrm{C}\) |
|---|---|---|---|---|
| 1 | \(\mathrm{ZnS}\cdot\mathrm{Cu}\) | 520 | — | 30–60 min |
| 2 | \(\mathrm{ZnS}\cdot\mathrm{Bi}, \mathrm{Cu}\) | 520 | — | 2.5–5 h |
| 3 | \(\mathrm{ZnS}\cdot\mathrm{Cu}, \mathrm{Co}\) | 500 | — | 8–10 h |
| 4 | \(\mathrm{CaSO_4}\cdot\mathrm{Mn}\) | 500 | \(1.2\times10^{18}\) | 10–12 h |
| 5 | \(\mathrm{CaSO_4}\cdot\mathrm{Sm}\) | \(\lambda_1 = 560\) \(\lambda_2 = 590\) \(\lambda_3 = 620\) |
\(0.8\times10^{18}\) | 1.5–2.5 months |
| 6 | \(\mathrm{CaF_2}\cdot\mathrm{Mn}\) | 470–550 | \(0.4\times10^{18}\) |
* quanta/coulomb—the quantum yield of thermoluminescence at an exposure dose producing a charge of 1 coulomb in 1 kg of air under normal conditions.
which agrees with the visibility curve for normal vision, whose maximum spectral sensitivity lies in the region of 555 nm.
Radiation inspection of products using thermoluminescent converters combines the advantages of the film radiography method—for example, the effect of information accumulation and the separation in time of the processes of exposure and analysis of the results, which eliminates radiation hazards for the operator—and of the fluoroscopy method: rapid acquisition of visual information after completion of exposure, repeated use of a single converter, and operational reliability.
According to the data of works [3, 4], the method under consideration makes it possible, through accumulation, to operate at a lower input-flux density. The contrast sensitivity when images are obtained with thermal neutrons reaches 10%. The resolving power of converters based on thermoluminophors depends on the grain size of the phosphor composition and on the thickness of the luminescent layer, and may be no worse than the resolving power of fluoroscopic inspection systems, i.e., 2–4 lines/mm.
Although the quality of a fluoroscopic image is lower than that of images obtained by film radiography, the quality of inspection can nevertheless be sufficiently satisfactory for many cases of flaw detection.
The field of application of thermoluminescent converters should be evaluated in terms of inspection productivity, i.e., in terms of the time required to obtain information on the flaw condition of a product.
The total time expenditure \(T_{\Sigma}\) can be written in the form
\[ T_{\Sigma}=t_{\mathrm{prep}}+t_{\mathrm{exp}}, \tag{1} \]
where
\(t_{\mathrm{prep}}\)—preparatory time, including the time for the operator to view the image;
\(t_{\mathrm{exp}}\)—the exposure time of the converter.
To create the background brightness of screen luminescence \(B_{\phi}\), it is necessary that the converter be irradiated with a dose \(D_{0}\), the magnitude of which is determined by the relation
\[ D_{0}=P_{0}t_{\mathrm{э}k}F\cdot \exp(-\mu d). \tag{2} \]
Here
\(P_{0}\) is the dose rate on the surface of the article on the source side;
\(F\) is the dose accumulation factor;
\(\mu\) is the dose attenuation coefficient;
\(d\) is the thickness of the article being inspected.
Since the filling of traps in the crystal phosphor by electrons is a statistical process, the number of traps filled per unit time is proportional to the number of free traps and to the irradiation dose rate and, as shown in [5],
\[ n_{\mathrm{з}}=n_{0}\left[1-\exp(-\beta Pt_{\mathrm{э}k})\right], \tag{3} \]
where
\(n_{\mathrm{з}}\) is the number of filled traps at the instant of irradiation time \(t_{\mathrm{э}k}\);
\(n_{0}\) is the maximum number of traps for the given type of phosphor;
\(P\) is the irradiation dose rate;
\(\beta\) is a constant equal to the relative number of empty traps filled under the action of a unit dose.
The brightness of luminescence \(B\) is a function of the temperature regime of heating of the converter, but its absolute value at the corresponding instants of time under a constant heating regime is proportional to the number of filled traps.
Therefore expression (3) may be written in the form
\[ B=B_{\max}\left[1-\exp(-\beta Pt_{\mathrm{э}k})\right]. \tag{4} \]
\(B_{\max}\) is a function of the maximum luminescence brightness when all phosphor traps are filled and of the selected heating regime.
Assuming that \(Pt_{\mathrm{э}k}=D_{0}\), equality (4), taking (2) into account, is written in the following form:
\[ B_{\phi}=B_{\max}\{1-\exp[-\beta P_{0}t_{\mathrm{э}k}\cdot F\cdot \exp(-\mu d)]\} \tag{5} \]
whence we determine
\[ t_{\mathrm{э}k}= \frac{\ln \dfrac{B_{\max}}{B_{\max}-B_{\phi}}}{\beta\cdot P_{0}\cdot F}\exp(\mu d). \tag{6} \]
The value of the accumulation factor \(F\) for X-radiation with maximum spectrum energy up to 100 keV may, with accuracy sufficient for practical purposes, be taken equal to 1.
Thus, the ratio
\[ \frac{\ln \dfrac{B_{\max}}{B_{\max}-B_{\phi}}}{\beta P_{0}} \]
is a quantity numerically equal to the exposure time required to achieve the background brightness \(B_{\phi}\) of the thermoluminescent converter in the absence of an absorber, i.e., at \(d=0\), for the selected regime of action of the stimulating factor. The above considerations are valid for phosphors with deep trap levels, for example, \(\mathrm{CaSO}_{4}\cdot\mathrm{Mn}\), LiF.
As the brightness \(B_{\phi}\) one takes a brightness reliably distinguishable, for example, by the operator’s eye, throughout the entire time of image analysis.
When an X-ray apparatus RUP-120-5-1 is used as the radiation source with tube voltage \(U_{a}=100\) kV, anode current 1 mA, and focal distance 0.75 m, the average exposure time required to create a visually distinguishable brightness of a thermoluminescent converter made of a \(\mathrm{CaSO}_{4}\cdot\mathrm{Mn}\) phosphor during a time...
viewing the image, \(\sim 3\) min, and heating rate 15 deg/min, was 1.2 min.
Thus, the exposure time, expressed in minutes, for the CaSO\(_4\):Mn converter as a function of the thickness of the article being radiographed can be determined from the expression
\[ t_{\text{ex}} = 1.2 \cdot \exp(\mu d) \tag{7} \]
and the total time expenditures for inspection can be written in the form
\[ T_{\Sigma} = t_{\text{prep}} + 1.2 \cdot \exp(\mu d). \tag{8} \]
The results of determining the total time expenditures for the selected radiographic regime as a function of the thickness of the inspected article at constant brightness \(B_{\phi}\) of the thermoluminescent converter are presented in Fig. 1, where curve 1 is calculated for aluminum, and curve 2 for magnesium alloys.
Fig. 1. Dependence of time expenditures on absorber thickness \((1, 1' —\) aluminum, \(2, 2' —\) magnesium) in radiography using a thermoluminescent converter \((1, 2)\) and RT-1 X-ray film \((1', 2')\)
For comparison, dependences of the total time expenditures using RT-1 X-ray film [1] have been plotted: curves \(1'\) and \(2'\), respectively, for aluminum and magnesium alloys.
As can be seen from the graphs in Fig. 1, because the preparatory time expenditures for X-ray film are significantly greater than for a thermoluminescent converter, there is a range of thicknesses for various materials in which the total time expenditures for thermoluminescent radiography are less than the time expenditures when using X-ray film.
The penetration sensitivity of converters based on the phosphor composition CaSO\(_4\):Mn for aluminum thicknesses of 5 and 10 mm is, respectively, 4.2 and 3.4%, which agrees with the GOST requirements for radiographic inspection systems.
It should be noted that no special measures to increase sensitivity were used in this case. Therefore, it should be expected that the selection of optimal radiographic regimes and the choice of phosphor compositions will make it possible to increase flaw-detection sensitivity.
Conclusions
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Radiography using thermoluminescent screens and films, in a number of cases, differs advantageously from radiography on X-ray film in the simplicity of processing, observation, and repeated use.
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When inspecting thicknesses up to 20 mm in aluminum and magnesium, the time required to obtain information on defects in a product is reduced by a factor of 4–5 in comparison with X-ray film.
References
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Kostner J., Berger H., Kraska J. R., A. Thermoluminescent Image Detection Method for Neutron Radiagraphy, Nucl. Applic., 2, 252, 1966.
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Berger H., Recent Progress in Neutron Imaging, Br. J. Non-Dest. Test., 10, 26, 1968.
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K. K. Shvarts et al. Thermoluminescent Dosimetry. Riga, “Zinante,” 1968.