We have obtained thermoluminescence glow curves nondestructively from large, solid, ceramic samples by laser spot heating. Although the samples are brittle, laser thermoluminescence glow curves could be obtained with no visible damage to the samples. The experimental glow curves match with theory. By contrast, conventional thermoluminescence measurements require small samples to be removed from a ceramic and placed in a thermoluminescence machine. Laser-induced thermoluminescence glow curves from LiF, silica, and porcelain are presented.
© 2002 Optical Society of America
Thermoluminescence emission from ceramics is very useful for both measuring radiation exposure and analyzing the material’s electronic structure.[1, 2] It is often acceptable for thermoluminescence analysis to be destructive, that is, to require the removal from the ceramic of a small sample which is ground up and placed in a thermoluminescence analysis machine. Thermoluminescence was first observed from ground-up pottery in 1960.. However, thermoluminescence analysis is also a very important tool in situations, such as the dating of ancient vases, for which the removal of a sample, even a small sample, is highly undesirable. In this paper, we demonstrate a technique for nondestructive measurement of thermoluminescence emission by laser heating a spot on an intact in situ ceramic.
Thermoluminescence has been studied extensively with the first models of thermoluminescence emission having appeared over fifty years ago.[4, 5] When a ceramic is irradiated, electrons (or holes) can be excited and subsequently caught in metastable traps. If the ceramic is later heated, typically in an oven, the electrons are released from the traps and some of them will radiatively recombine. Light emitted in this way is called thermoluminescence. Recent work on thermoluminescence has been reviewed.[6–8] Thermoluminescence emission from many materials has been studied. Because of its practical importance in dosimeters, LiF has received the most study. In the form most commonly used for thermoluminescence studies (Harshaw TLD-100), Mg and Ti impurities are added to the LiF to increase the thermoluminescence intensity and many thermoluminescence peaks have been resolved and phenomena such as supra-linearity and sensitization have been studied.[9–16] The thermoluminescence emission from quartz, particularly its 110°C peak, has also been studied[17–22] as well as thermoluminescence emission from feldspars.[23, 24] Porcelain contains quartz and has similar thermoluminescence properties. Thermoluminescence emission from various materials has been reviewed.
Laser-heated thermoluminescence dosimetry was first demonstrated on thin layers of powdered ceramic. Laser heating of the sample is interesting for three reasons. First, because it can be extremely rapid, the peak intensity of the thermoluminescence signal can be much higher than from traditional heating methods.[28–30] Gasiot et al. demonstrated heating rates of 104 K/s. Secondly, it is possible to selectively excite thermoluminescence emission from individual pixels in a two-dimensional array of powdered phosphors for purposes of dose-mapping.[31, 32] Thirdly, laser heating offers the potential of performing thermoluminescence measurements on a sample in situ. This latter feature is what this paper will focus on. This feature is particularly important for dating ancient ceramics: when great value is placed on the item’s aesthetics, drilling holes to remove a sample is highly undesirable. While the historical value of unblemished ceramics is not quantifiable, their commercial value is. As an example, in 1998, a Ming Dynasty white glazed porcelain from the imperial kiln, whose value Christie’s had estimated at $100,000 US, went unsold in a public auction because of the two drilled holes at the base each measured at about 0.5 cm in diameter. Obviously a porcelain in pristine condition is much more attractive. We demonstrate the observation of thermoluminescence emission from ceramics in situ. To our knowledge, this is the first observation of laser-heated thermoluminescence from large, unprepared (bulk), solid materials.
2 Experimental Apparatus
To demonstrate nondestructive thermoluminescence dosimetry, experimental apparatus was constructed as shown in Fig. 1. To heat the target, a CO2 laser (Synrad J48-2-7615) which was water cooled (Polyscience chiller), is directed by a mirror towards the sample which is placed on a holder. The light output from the sample is monitored by a photomultiplier tube (Hamamatsu 1P21). The PMT signal is amplified (Stanford Research Systems SRS-445) and then analyzed by a photon counter (Stanford Research Systems SR-430). The PMT voltage was maintained at 1000 V by a Beltran Associates Model 205B-03R high voltage power supply. The CO2 laser power was measured with a power meter (Molectron PM30). Because some samples react with oxygen when hot, the sample was in a chamber which could be purged with nitrogen before each shot. Samples were irradiated using a Sr-80 (beta) source.
3 Results and Discussion
We began our experiments with the standard Harshaw LiF (TLD-100) pellets. A lens was placed in front of the laser beam to produce a spot size of 8 mm on the target. After exposure to beta radiation, a strong thermoluminescence signal was observed. A sample of thermoluminescence emissions is shown in Fig. 2. Two thermoluminescence peaks were observed. No damage was observed after eight pulses of 10 seconds duration at the maximum laser power of 38 W. To confirm that this signal was due to thermoluminescence, we ran the experiment again with a LiF pellet that was not exposed to beta radiation. This resulted in the data, labeled “0 Rads,” for which, as expected, no peak was observed.
At smaller spot sizes, it was possible to damage the LiF pellet. With a beam size of about 3 mm, damage was observed for powers as low as 15 W after a few seconds of exposure. A notable feature was that, even at powers for which only slight damage to the pellet was apparent to the eye, significant changes to the thermoluminescence curve were observed. As an example, Fig. 3 shows thermoluminescence emission resulting from exposure to 11.2 W of CO2 radiation. Compared to Fig. 2, strong distortion of the thermoluminescence peak is observed.
Next, we exposed a 0.5 mm thick quartz slide to 172 Rads and then placed it on the sample holder. Because quartz exhibits weaker thermoluminescence than LiF, we exposed this sample to a higher dose of radiation and increased the laser power to obtain more signal. The slide was exposed to 38 W of CO2 laser radiation for 10 seconds. A single peak of thermoluminescence emission was observed as shown in Fig. 4.
Lastly, we tested the usefulness of this technique on a 3-cm diameter and 5-mm thick piece of a porcelain ceramic. This ceramic was taken from a “blue and white” soup can which is commonly available in Asian supermarkets. This sample was used to demonstrate the ability of laser-induced thermoluminescence to make measurements on uncontrolled materials in situ. After irradiation, a clear thermoluminescence signal in the form of a broad single peak was observed as shown in Fig. 5. No damage to the sample was observed. The peak is unusually broad because the temperature in the sample is nonuniform: we are looking at the emission integrated over the sample surface and different parts of the surface reach the peak emitting temperature at different times.
The theoretical curves shown in the plots were derived from a numerical solution of the unsteady three-dimensional heat conduction equation with adiabatic boundary conditions combined with first-order thermoluminescence kinetics. This was an extension of the unsteady 2-D numerical calculations that we described previously. The calculations show that we have achieved heating rates as high as 200° C/s in our samples. For conventional thermoluminescence instruments, the temperature, T, rises linearly with time, t, and the peak thermoluminescence intensity is proportional to the heating rate, Ṫ = dT/dt. Under laser heating the temperature profiles are often nonlinear. In this case, for well-behaved temperature profiles, the peak thermoluminescence intensity scales as:
where E is the trap energy, k is Boltzmann’s constant, and T̈ = d 2 T/dt 2 where the temperature and its derivatives are all evaluated at the time of the peak intensity. For laser heating, T̈ is typically negative and Eq. (1) shows that this reduces the peak intensity although the Ṫ scaling still dominates. Laser heating is capable of producing very large Ṫ, and the resulting high intensity indicates a potential for laser-induced thermoluminescence to be not only nondestructive but also more sensitive.
In conclusion, nondestructive thermoluminescence testing has been demonstrated using CO2 laser heating. The technique permits the absorbed dose to be measured in selected locations on large objects without physically removing any samples. We particularly anticipate applications of this approach to thermoluminescence tests on antiquities.
This work is supported, in part, by Earmarked Research Grant No. CUHK4009/99H of the Government of Hong Kong SAR.
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