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Time-resolved optically stimulated luminescence from α-Al2O3:C
Radiation Measurements, Vol. 24, No. 4, pp. 457--463,1995
Pergamon 1350-4487(94)00119-7
Copyright (~ 1995ElsevierScienceLtd Printed in Great Britain.All rights reserved 1350-4487,/95$9.50+ .00
TIME-RESOLVED OPTICALLY STIMULATED LUMINESCENCE FROM ~-A1203:C B, G. MARKEY, L. E. COLYOTT and S. W. S. MCKEEVER Department of Physics, Oklahoma State University, Stillwater, OK 74078-0444, U.S.A. (Received 27 September 1994; #7final.form 5 December 1994)
Abstract--Results are presented of time-resolved optically stimulated luminescence (OSL) in which the excitation light source is pulsed, and the luminescence during the pulse, and during the subsequent afterglow, are monitored as functions of time. The material under study is ct-A120 3:C. Data are presented on the dependence of the OSL intensity on: (i) the excitation wavelength;(ii) the wavelength of the emitted light; (iii) the sample temperature; and (iv) the absorbed dose. The technique is shown to have potential for applications in dosimetry.
1. INTRODUCTION Optically stimulated luminescence (OSL) is actively used by the archaeological and geological dating community as a method of equivalent dose determination in natural materials. The method was first introduced into this area by Huntley et al. (1985). (For the latest developments and applications of OSL in this field see the Proceedings of the 7th International Seminar on TL and ESR Dating (Bailiff et al., 1994).) The use of OSL in radiation dosimetry has been less extensive, despite several potential advantages. For example, unlike thermoluminescence (TL) one does not have to heat the dosimeter to high temperatures during OSL readout. Thus, one can imagine the incorporation of sensitive OSL phosphors with plastic proton radiators for fast neutron dosimetry applications. Since the dosimeter is not heated the plastic radiator will not be degraded during the readout process. Occasional descriptions of OSL from a variety of dosimetric materials have been published, primarily describing the general OSL properties and potential dosimetry applications (e.g. Pradhan et al., 1983; Miller and Endres, 1990; Allen and McKeever, 1990; Nanto et al., 1993a, b). In each of these previous studies the optical excitation is continuous wave (cw), either from a laser or from a high power arc lamp and monochromator/filter system. Narrow band filters are used in order to discriminate between the excitation light and the emission light, and to prevent scattered excitation light from entering the photomultiplier detector. The OSL emission is monitored as a function of time from the beginning of the excitation period until the emission ceases, i.e. until all the optically-sensitive, radiation-filled traps are emptied. The integrated emission signal is then recorded and used to determine the absorbed dose.
Recently, Sanderson and Clark (1994) discussed the potential of pulsed OSL (from alkali feldspars) in dating applications. Here, the excitation source (a laser) was pulsed, with pulse widths of the order of 10ns, and the OSL emission was followed as a function of time after the excitation pulse. Several emission processes could apparently be resolved. Because of the short time-scale and fast switching speeds, Sanderson and Clark kept the photomultiplier tube (PMT) shutter open during the excitation pulse and used filters to block the excitation light from entering the detector. In radiophotoluminescent (RPL) glass dosimetry systems, where fast (4 ns) laser pulses are also used, the luminescence afterglow is detected as a function of time using a gated PMT in which the detection window is defined by a high voltage pulse applied to the PMT dynode chain (Piesch et al., 1990; Burkghardt et al. 1990; Lommler et al. 1993). For wider pulses and slower switching speeds one can employ fast shutters to interrupt the excitation beam and provide the pulse. In this way, pulse widths of 10-100ms can easily be accommodated. A practical advantage of these slower speeds is that the source itself does not have to be pulsed; thus expensive pulsed laser systems are not required and, since the laser is operated in cw mode, this also removes difficulties associated with power fluctuations from pulse to pulse. Pulse widths of 10-100 ms are still short enough to retain one of the advantages of pulsed OSL over cw OSL, namely that the entire trapped charge population is not depleted in one measurement. Each excitation pulse samples only a fraction of the radiation-induced, trapped charge population. This, in turn, generates the ability to reread the OSL signal many times. For very small doses, signal averaging can be performed to detect weak signals, while for
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larger doses, the samples may be stored for future readings and for multiple checks of questionable results. This multiple reading capability is not available with conventional TL dosimeters. In this paper, we report the use of pulsed OSL for estimating absorbed doses in ~-A1203:C. We have examined the development with time of the OSL intensity during and after the excitation pulse, the time-resolved spectra of the OSL emission on a ms-s time scale, the excitation spectra, the dependence of the OSL signal upon dose, the variation of the OSL signal with temperature, and the feasibility of optically bleaching the signal in order to reuse the dosimeters. 2. EXPERIMENTAL DETAILS A schematic diagram of the OSL system is shown in Fig. 1. An Ar-ion laser (4W all lines), operated in cw mode, is used as an excitation source (Fig. l(a)). Electronic shutters are used to provide excitation pulses to the sample, which is situated in front of a front-silvered concave mirror (Fig. l(b)). The laser pulse is directed through a central hole in the mirror onto the sample. OSL emission from the sample is collected by the mirror and directed onto the photocathode of an EMI 9635QB PMT. The PMT signal
is detected by a gated photon counter and fed to a computer for storage and analysis. In the standard configuration, two narrow band interference filters (center wavelength = 410 nm; bandwidth = 12 nm) are used in front of the PMT to allow passage of the OSL emission, while at the same time absorbing scattered light from the laser. The laser pulse power used in this study was 150 mW. During measurement of the excitation and emission spectra a slightly different arrangement was used. Here the filters were removed and the PMT was shuttered synchronously with the laser so that the PMT shutters were closed when the laser beam was on. Using this arrangement only the afterglow OSL could be recorded. During measurement of the timeresolved emission spectra an EG&G optical multichannel analyzer (OMA III) and an American Holographics (S-100) spectrograph replaced the PMT. The data acquisition was then triggered to record the spectra during periods which were 0.5 s long and 1 s apart from the end of the laser pulse. Thermoluminescence (TL) was monitored using a low temperature cryostat in which the sample could be irradiated with a ~37Cs source at any temperature between -196uC and room temperature. The sample was kept in a vacuum and was heated at 0.Y~C s t during TL measurement. The light was detected using
~
(a)
~Cryostat
y'U
S&F
-ion Laser I
/M
X"~,~ Photodiode
(b) Mirror Sample Shutter
~
Shutter ....::::::iiiiill...................
Detector
Laser Beam
Fig. I. (a) Schematic diagram of the experimental arrangement for recording the OSL signal, M: mirror; F: filter; BS: beam splitter; S: shutter, (b) Detail of the sample cryostat showing the laser beam passing through the center of a front-silvered curved mirror which directs the luminescence onto the detector.
T I M E - R E S O L V E D OSL F R O M ~-A1203:C an uncooled 9635QB P M T and the P M T signal was recorded in the integrated current mode. N o filters were used between the sample and the PMT. ~-AI203:C single crystals from the Urals Polytechnical Institute (Russia) were obtained through Harshaw/Bicron, U.S.A, in the form of TLD-500 dosimeters. This material has a reported TL sensitivity 40 60 times that of LiF:Mg,Ti (TLD-100: Akselrod et aL, 1993). The luminescence centers in this material are oxygen vacancy centers (F and F ~ centers; Summers, 1984; Akselrod et al., 1993). The TL from this material is known to be very sensitive to light (e.g. Musk, 1993; Moscovitch et al., 1993; Walker, 1994) and shows considerable light-induced fading of the TL signal when exposed to visible light. In using this material as an OSL detector, we wish to exploit the light sensitivity by using light, rather than heat, to empty the radiation-filled traps. 6"Co, t~VCs and 9°Srfl°Y were used as radiation sources. All irradiations for the OSL measurements were carried out at room temperature.
3. R E S U L T S Figure 2 shows a typical time-resolved OSL emission signal from ~-AI~O3:C following irradiation with 4 × 10 2Gy beta particles from the 9°Sr/9°Y source. The laser pulse width was 100 ms and the end of the laser pulse is marked in the figure by the vertical line. Both the irradiation and the measurement were carried out at 2 5 C . The OSL signal consists of a rise, followed by a decay of the luminescence in response to the excitation pulse. The signal from one pulse only is shown in the figure. At the power levels used in these studies we observed that the signal showed no detectable reduction (i.e. there was no sign of significant depletion of trapped charge carriers) until approximately 100 pulses had been applied to the sample. Experiments in which the laser
600
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E400 ~300 ~o 2O0
O
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0.0
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I
i
i
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Fig. 2. Time-resolved OSL from ~-A1203:C following irradiation at room temperature with a dose of 0.04Gy 9"Sr/9°Y. A single laser pulse (150mW, lOOms wide) was used to excite the OSL. The measurement temperature was 25"C. NT 24,4~K
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.~ 150 ..J
0
100
50
0 300
350
400
450
500
Wavelength(rim) Fig. 3. Time-resolved emission spectrum following a dose of 15 Gy from a 6°Co source. The emission is detected with the OMA-lll system and the spectra are recorded over periods of 0.5 s, at 1 s intervals, thus: (a) 0~0.5 s; (b) 1.5 2.0 s; (c) 3.0 3.5 s. The timing began at the end of the laser pulse.
pulse width was varied showed that the OSL signal reached a maximum (i.e. charge carrier equilibrium was established) within 0.15 s of the start of the laser pulse. Figure 3 shows the emission spectrum of the OSL signal. The data in this figure were obtained from a more heavily irradiated sample (15 Gy, ~°Co) and were recorded using the diode array O M A and the procedures described in Section 2. The emission for each spectral measurement was integrated over the time intervals shown in the figure following the end of the laser pulse and, when measured in this configuration, the emission is observed to decrease with time. The spectrum has not been corrected for the wavelength response of the detection system, since the emission wavelength range extends only over 350-450 nm and the detection system response is approximately flat in this spectral region. We see from this figure that the emission peaks at ~ 4 1 0 nm, and that the emission peak remains fixed at this wavelength during the decay process. This suggests that the OSL process is not a donor acceptor pair type of recombination, since it would be expected that a wavelength shift with time would occur in this case. In contrast, this result provides support for the notion that OSL in this material involves electron and hole transitions via the delocalized bands. Photoconductivity measurements would test this conclusion. Emission at 410 nm is characteristic of F-center relaxation in Al~O 3 from the 3 P excited state to the 1 S ground state (Summers, 1984) and this is the same emission process as is observed in TL from this material (Akselrod et al., 1993). The high TL sensitivity that this material exhibits is a consequence of both the highly reducing growth conditions and the C doping. These give rise to the formation of large numbers of oxygen vacancy centers (i.e. F and F* centers), and the observation that OSL uses the
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B . G . MARKEY et al. 1600
1400 1300
o o
1200
25 °C 40 °C 100 °C
.~1100
.e
800
#, O
~ 900
400
•,~ 800
o
7o0
I
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0.0
"0
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T i m e (s)
50O 450
I 460
I 470
I 480
I 490
f 500
I 510
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Wavelength (nm)
Fig. 4. Excitation spectra for OSL from ct-Al203:C. The power from the laser at each of the wavelengths shown was adjusted to give the same number of photons per unit time per unit area incident on the sample.
same emission centers as TL (i.e. F centers) implies that this material may also be an extremely sensitive OSL detector. The excitation spectrum for the OSL is shown in Fig. 4. To obtain this the different lines from the Ar-ion laser were used, the power from each being adjusted to give the same number of photons s - l c m 2 at the sample surface. In this way the efficacy with which each wavelength induces OSL can be easily compared. The wavelength range is, of course, restricted by the availability of emission lines from the Ar-ion laser. The most intense emission was observed in the UV region of the spectrum around the line at 457.9 nm. However, the line at this wavelength from the Ar-ion laser has a maximum power of only 300 mW, and thus this line may prove to be impractical for low dose dosimetry. The 514.4 nm line (maximum power 1.7 W) was chosen to be the standard excitation wavelength in the present study,
Fig. 5. Dependence of the OSL signal on temperature for a 100 ms 150 mW laser pulse. The absorbed dose was 0.04 Gy from the ~Sr/9°Y beta source.
despite its low efficiency, simply because of the extra power available. Figure 5 shows the variation of the OSL signal with sample temperature. Increased signal is observed as the temperature is increased and variations in the decay rate also become apparent. This latter feature is seen in detail in Figs 6(a) and (b). In Fig. 6(a) the decay of the OSL signal is fitted to two exponentials. The faster component--with a temperatureindependent lifetime of approximately 35 m s - corresponds to the F-center lifetime (Summers, 1984). The lifetime of the slower component varies with temperature, as shown in the Arrhenius plot of Fig. 6(b), and quenches completely by 50°C. Two decay processes are observed in Fig. 6(b), and from the slopes activation energies of 0.65 eV and 0.77 eV are obtained. Figure 7 shows the variation of the OSL signal (measured at 100°C) with absorbed dose (from the ~37Cs source). These data have been obtained after summing the OSL from 400, 100ms/150mW laser pulses, and integrating the OSL sum from 0 to 0.5 s.
o
(b)
(a)
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E 400
~
-ff
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Time (s)
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-3 3.05
i 3.15
3.25
3,35
1000/'1" (K"1)
Fig. 6. (a) Decay of the OSL signal, at 25°C, at the end of the laser pulse. The decay has been fitted to two exponentials. The faster component is temperature independent and the lifetime (~35 ms) corresponds to the F-center lifetime. The lifetime of the slower component (~ 545 ms at 25°C) is temperature dependent. (b) Arrhenius plot of the variation of the lifetime of the slow component as a function of temperature. Two processes are discerned, with activation energies of 0.65 eV and 0.77 eV.
TIME-RESOLVED OSL FROM ~-AI20 3:C 10O--
O
103 10-2
10-1
10o
Dose (mGy)
Fig. 7. OSL versus absorbed dose. The OSL signal is obtained by summing the OSL from 400 laser pulses (150 mW, 100 ms wide) and integrating the sum from 0 to 0.5 s. The irradiation source was a ~37Cssource. The sample was irradiated at 25°C and the OSL measurement temperature was 100°C.
The data show a slightly sublinear increase in the OSL signal with absorbed dose. Minimum detectable dose levels have not yet been investigated, but using the current arrangement doses as small as 10 pGy were easily measured. If the OSL were either summed over more laser pulses, or recorded using a laser operated in cw mode, it is expected that the measurement of absorbed doses as low as 1 #Gy would be possible. Each datum point in Fig. 7 was obtained from the same sample. After each usage the sample was annealed by heating it to a temperature of 800°C for 15 minutes; this treatment was found to completely remove any remaining OSL signal. Since the eventual goal of the research is to develop an all-optical dosimetry system, the possibility of 'optical annealing', or 'bleaching', of the residual signal was also investigated. This was done using a continuous laser exposure (514 nm) for 30 min at a maximum power of 1.7 W, and under these conditions it was found
46l
that the OSL signal could be reduced to background levels. (A rise in sample temperature of ~ 10°C was observed during this procedure.) Additional exposure to the laser for times greater than 30 min did not reduce the residual signal further. Experiments with white light exposure from a Xe arc lamp, and with exposure to different wavelengths from the laser, were also performed in order to establish the optimum wavelength and exposure times for bleaching the OSL signal. It was found that the optimum wavelength to use during optical annealing is the same as that used to excite the OSL (514nm in the present case). Although more studies are required in this area, these measurements indicate that 'optical annealing' is a viable possibility for this material.
4. DISCUSSION The OSL signal observed in Fig. 2 has the expected rise and decay of luminescence in response to the laser pulse. The rise and decay time are dictated by two processes. The first is the relatively fast ( ~ 3 5 ms) lifetime associated with the F-center luminescence (Summers, 1984). The second, slower component is due to phosphorescence from shallow states. The phosphorescence is caused both by the thermal decay of those electrons trapped in these states during the initial radiation exposure, and also (mainly) by those electrons phototransferred to these states during laser excitation. The dependence of the OSL emission upon temperature (Figs 5 and 6) reveals that two shallow states are involved, with trap depths of 0.65 eV and 0.77 eV. Figure 8 shows the TL signal from TLD-500 obtained by irradiating the sample to a dose of 0.35mGy (137Cs) at a temperature of -190°C. In addition to the main dosimetry peak at 177°C (450 K: not shown in the figure) there are two peaks due to shallow states at lower temperatures. In the figure, we show the result of a fit of the low temperature part of the glow curve (200K < T < 350 K) to the usual Randall-Wilkins, first order equation (Randall and Wilkins, 1945a, b), namely
100
l(T) = rlnos(T)exp{ -- E/kT }
80 c 60
exp(-l/~fs(O)exp{-E/kOldO ).
(1)
40 20
0
200
"
225
250
275
300
325
350
Temperature(K)
Fig. 8. Thermoluminescence from ~-A1203:C irradiated with 0.35 mGy (UTCs) at -190°C, and heated at a rate of 0.3°C s-~. The dotted lines are the fits to the experimental data points assuming first-order kinetics and using equation (1). The fitted trapping parameters for the two low temperature peaks are: E = 0.77 eV and s = 8.5 x 10j2 s-~ for the -3°C (270 K) peak; and E = 0.65eV and s = 1.0 x 109s-L for the 35°C (308 K) peak.
Here E is the trap depth (eV), s is the frequency factor (s-~), no (cm 3) is the initial trapped charge concentration at temperature To (K), /~ is the heating rate (K s -~), k is Boltzmann's constant (eV K ~), T is temperature (K), and 0 (also temperature) is a dummy variable. The parameter rt takes account of the units, the luminescence efficiency of the recombination process, and the detection efficiency of the PMT. The F-center luminescence efficiency in A1203:C is known to be temperature-dependent and strong thermal quenching of the emission occurs at higher temperatures (Kortov et al., 1994). However,
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B . G . MARKEY et aL
recent analyses of the luminescence efficiency as a function of temperature (Kitis et al., 1994; Kortov et al., 1994) reveal that thermal quenching does not occur until temperatures in excess of ~ 100°C are reached. Therefore, /I is assumed to be temperature independent (i.e. a constant) for the purposes of fitting. The fitting parameters are, therefore, E, s and the product qno. The fitting is achieved using the software package 'Peakfit' by JandePm. The results of this fitting procedure show that the two lower temperature, glow peaks can be described by single, first-order TL peaks, with trap depths of 0.77 eV and 0.65 eV, respectively, in excellent agreement with the OSL temperature-dependence analysis of Fig. 6(b). Phototransferred TL has been examined in this material by Akselrod and colleagues (e.g. Akselrod et aL, 1990a, b; Akselrod et al., 1993). These authors showed that there are several deep states which act as the source traps for phototransfer during illumination. Walker (1994) has also examined the light-induced fading of the main dosimetry TL peak in this material and has discussed how this may be caused by the release of charge from deep states, or from the dosimetric traps, or both. This optically stimulated charge is then available for recombination (yielding OSL) and for transfer to less stable (shallow) traps. The phototransferred charge will reside in the shallow states for times that will depend upon the temperature. It will then be thermally released back into the delocalized bands, where it will become available for recombination. Thus, by increasing the sample temperature, an increase in the OSL intensity is observed due to the fact that, at elevated temperatures, the shallow states are not effective at localizing the charge and more charge takes part in the recombination (OSL) process. Note that the temperatures used in this study (up to 100°C) are not high enough to induce quenching of the luminescence (Kitis et al., 1994; Kortov et al., 1994). The emission spectra of Fig. 3 illustrate F-center luminescence due to 3 P-to-1 S transitions. This, in turn, indicates that the released charge carriers are electrons and that the recombination process may be represented by a reaction of the type e + F + ~ F* ~ F+hv410nm.
(2)
Here an electron recombines with an F ÷ center, producing an F center. F* represents the F center in an excited 3 P state, and F represents an F center in a relaxed i S state. The relaxation is accompanied by the emission of a photon of wavelength 410 nm. Thus, a reasonable model for OSL emission in c¢-A1203:C involves the optical release of electrons from deep traps into the conduction band, followed by recombination at F ÷ centers and emission at 410 nm. At the same time, electrons are transferred through the conduction band to shallow states, predominantly the electron traps at ~ E c - 0.65 eV and E c - 0.77 eV, where Ec is energy at the bottom of
the conduction band. Thermal excitation of electrons from these traps leads to phosphorescence and a long tail to the OSL following the end of the excitation pulse.
5. CONCLUSIONS In this paper OSL from a-Al~ 03 :C has been shown to be sensitive to absorbed dose. It has been demonstrated that the OSL results from the recombination of optically stimulated electrons with F ÷ centers. Since these centers are also the main cause of the high TL sensitivity of this material it can be expected that ~-A1203:C will also be a sensitive OSL dosimeter. Although minimum measurable dose levels have not yet been investigated, the dose response shown in this paper indicates that doses of the order of 10/~ Gy may be detected using the present system. Only pulsed OSL has been examined in this work; cw OSL may be used for the very low doses in which case it may prove possible to detect doses of the order of 1/zGy. The thermal and optical stability of the OSL signal has yet to be fully investigated. In a recent study (Walker, 1994) the optical fading of the TL from this material was examined and it is reasonable to suggest that the OSL will be similarly affected by light. Thus, handling of the materials in a low light environment is clearly desirable. The OSL signal observed in, for example, Fig. 2 may contain a component due to pure phosphorescence from those shallow traps that were partially filled during radiation exposure. One might expect, therefore, that such a component would be unstable at room temperature. However, our preliminary measurements on thermal stability of the OSL signal have not revealed any detectable, unstable component. This is supported by TL measurements from samples irradiated at room temperature for which we observe only very weak, low temperature TL peaks (i.e. the TL peak at 35°C (308 K) seen in Fig. 8). Although further study is clearly necessary, we conclude from the present preliminary measurements that OSL of ~-AI203:C has a strong potential for use in dosimetry. Acknowledgement--This research is supported by the Na-
tional Science Foundation under grant EHR 9108771.
REFERENCES
Akselrod M. S., Kortov V. S. and Gorelova E. A. (1993) Preparation and properties of a-A120~:C. Radiat. Prot. Dosim, 47, 159 164. Akselrod M. S., Kortov V. S., Kravetsky D. J. and Gotlib V. I. (1990a) Highly sensitive thermoluminescent anion-defective ~-Al203:C single crystal detector. Radiat. Prot. Dosim. 32, 15 20. Akselrod M. S., Kortov V. S, Kravetsky D. J. and Gotlib V. I. (1990b) Highly sensitive thermoluminescent anion-defectivea-A1203:C single crystal detector. Radiat. Prot. Dosim. 33, 119-122.
T I M E - R E S O L V E D O S L F R O M ~-A120 3:C Alien P, and McKeever S. W. S. (1990) Studies of PTTL and OSL in TLD-400. Radiat. Prot. Dosim. 33, 19-22. Bailiff I. K., F/iin J., Prescott J. R., Townsend P. D., Vana N, and Visocukas R. (eds) (1994) Proceedings of the 7th International Specialist Seminar on TL and ESR Dating, Krems, Austria, 5 9 July 1993. Radiat. Meas. 23. Burkghardt B., Piesch E., Vilgis M., Ishidoya T. and Ikegami T. (1990) Modern automatic readout system for phosphate glass dosemeters. Radiat. Prot. Dosim. 34, 369 372. Huntley D. J., Godfrey-Smith D. L, and Thewalt M. L. W. (1985) Optical dating of sediments. Nature 313, 105 107. Kitis G., Papadopoulos J. G., Charalambous S. and Tuyn J. W. (1994) The influence of heating rate on the response and trapping parameters of a-A1203:C. Radiat. Prof. Dosim. 55, 183 190. Kortov V. S., Milman I. I., Kirpa V. 1. and Lesz J. (1994) Somc features of ~t-AI203:C dosimetric thermoluminescent crystals. Radiat. Prot. Dosim. 55, 279-283. Lommler B., Pitt E., Scharmann A. and Guldbakke S. (1993). Fast neutron dosimetry by photoluminescence detection of proton recoil tracks in laser scanned RPL glasses. Radiat. Prot. Dosim. 47, 285-288. Millcr S. D. and Endres G. W. R. (1990) Laser-induced optically stimulated M-center luminescence in LiF. Radiat. Prot. Dosim. 33, 59-62. Moscovitch M., Tawil R. A. and Svinkin M., (1993) Lightinduced fading in 7-A1203:C. Radiat. Prot. Dosim. 47, 251 253. Musk J. H, (I993) Time-dependent and light-induced fading in Victoreen model 2600 80 aluminum oxide thermo-
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luminescence dosimeters. Radiat Prot. Dosim. 47, 247-249. Nanto H., Murayama K., Usada T., Taniguchi S. and Takuschi N. (1993a) Optically stimulated luminescence in KCI:Eu single crystals. Radiat. Prot. Dosim. 47, 281 284. Nanto H., Usada T., Murayama K., Nakamura S., lnabe K. and Takeuchi N. (1993b) Emission mechanism of optically stimulated luminescence in copper-doped sodium chloride single crystals. Radiat. Prot. Dosirn. 47, 293 296. Piesch E., Burkghardt B. and Vilgis M. (1990) Photoluminescence dosimetry: progress and present state of the art. Radiat. Prot. Dosim. 33, 215 226. Pradhan A. S., Bhuwan Chandra and Bhatt R. C. (1983) Phosphorescence and photostimulated luminescence of CaSO4:Dy embedded polyethylene discs at elevated temperatures for fast neutron dosimetry. Radiat. Prot. Dosim. 5, 159 162. Randall J. T. and Wilkins M. H. F. (1945a) Phosphorescence and electron traps I. The study of trap distributions. Proc. R. Soc. London 184, 366-389. Randall J. T. and Wilkins M. H. F. (1945b) Phosphorescence and electron traps II. The interpretation of long-period phosphorescence. Proc. R. Soc. London 184, 390~J,07. Sanderson D. C. W. and Clark R. J. (1994) Pulsed photostimulated luminescence in alkali feldspars. Radiat. Meas. 23, 633-639. Summers G. P. (1984) Thermoluminescence in single crystal ~t-A1203:C. Radiat. Prof. Dosim. 8, 69 80. Walker F. D. (1994) MS thesis. Oklahoma State University, OK.
Pergamon 1350-4487(94)00119-7
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TIME-RESOLVED OPTICALLY STIMULATED LUMINESCENCE FROM ~-A1203:C B, G. MARKEY, L. E. COLYOTT and S. W. S. MCKEEVER Department of Physics, Oklahoma State University, Stillwater, OK 74078-0444, U.S.A. (Received 27 September 1994; #7final.form 5 December 1994)
Abstract--Results are presented of time-resolved optically stimulated luminescence (OSL) in which the excitation light source is pulsed, and the luminescence during the pulse, and during the subsequent afterglow, are monitored as functions of time. The material under study is ct-A120 3:C. Data are presented on the dependence of the OSL intensity on: (i) the excitation wavelength;(ii) the wavelength of the emitted light; (iii) the sample temperature; and (iv) the absorbed dose. The technique is shown to have potential for applications in dosimetry.
1. INTRODUCTION Optically stimulated luminescence (OSL) is actively used by the archaeological and geological dating community as a method of equivalent dose determination in natural materials. The method was first introduced into this area by Huntley et al. (1985). (For the latest developments and applications of OSL in this field see the Proceedings of the 7th International Seminar on TL and ESR Dating (Bailiff et al., 1994).) The use of OSL in radiation dosimetry has been less extensive, despite several potential advantages. For example, unlike thermoluminescence (TL) one does not have to heat the dosimeter to high temperatures during OSL readout. Thus, one can imagine the incorporation of sensitive OSL phosphors with plastic proton radiators for fast neutron dosimetry applications. Since the dosimeter is not heated the plastic radiator will not be degraded during the readout process. Occasional descriptions of OSL from a variety of dosimetric materials have been published, primarily describing the general OSL properties and potential dosimetry applications (e.g. Pradhan et al., 1983; Miller and Endres, 1990; Allen and McKeever, 1990; Nanto et al., 1993a, b). In each of these previous studies the optical excitation is continuous wave (cw), either from a laser or from a high power arc lamp and monochromator/filter system. Narrow band filters are used in order to discriminate between the excitation light and the emission light, and to prevent scattered excitation light from entering the photomultiplier detector. The OSL emission is monitored as a function of time from the beginning of the excitation period until the emission ceases, i.e. until all the optically-sensitive, radiation-filled traps are emptied. The integrated emission signal is then recorded and used to determine the absorbed dose.
Recently, Sanderson and Clark (1994) discussed the potential of pulsed OSL (from alkali feldspars) in dating applications. Here, the excitation source (a laser) was pulsed, with pulse widths of the order of 10ns, and the OSL emission was followed as a function of time after the excitation pulse. Several emission processes could apparently be resolved. Because of the short time-scale and fast switching speeds, Sanderson and Clark kept the photomultiplier tube (PMT) shutter open during the excitation pulse and used filters to block the excitation light from entering the detector. In radiophotoluminescent (RPL) glass dosimetry systems, where fast (4 ns) laser pulses are also used, the luminescence afterglow is detected as a function of time using a gated PMT in which the detection window is defined by a high voltage pulse applied to the PMT dynode chain (Piesch et al., 1990; Burkghardt et al. 1990; Lommler et al. 1993). For wider pulses and slower switching speeds one can employ fast shutters to interrupt the excitation beam and provide the pulse. In this way, pulse widths of 10-100ms can easily be accommodated. A practical advantage of these slower speeds is that the source itself does not have to be pulsed; thus expensive pulsed laser systems are not required and, since the laser is operated in cw mode, this also removes difficulties associated with power fluctuations from pulse to pulse. Pulse widths of 10-100 ms are still short enough to retain one of the advantages of pulsed OSL over cw OSL, namely that the entire trapped charge population is not depleted in one measurement. Each excitation pulse samples only a fraction of the radiation-induced, trapped charge population. This, in turn, generates the ability to reread the OSL signal many times. For very small doses, signal averaging can be performed to detect weak signals, while for
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larger doses, the samples may be stored for future readings and for multiple checks of questionable results. This multiple reading capability is not available with conventional TL dosimeters. In this paper, we report the use of pulsed OSL for estimating absorbed doses in ~-A1203:C. We have examined the development with time of the OSL intensity during and after the excitation pulse, the time-resolved spectra of the OSL emission on a ms-s time scale, the excitation spectra, the dependence of the OSL signal upon dose, the variation of the OSL signal with temperature, and the feasibility of optically bleaching the signal in order to reuse the dosimeters. 2. EXPERIMENTAL DETAILS A schematic diagram of the OSL system is shown in Fig. 1. An Ar-ion laser (4W all lines), operated in cw mode, is used as an excitation source (Fig. l(a)). Electronic shutters are used to provide excitation pulses to the sample, which is situated in front of a front-silvered concave mirror (Fig. l(b)). The laser pulse is directed through a central hole in the mirror onto the sample. OSL emission from the sample is collected by the mirror and directed onto the photocathode of an EMI 9635QB PMT. The PMT signal
is detected by a gated photon counter and fed to a computer for storage and analysis. In the standard configuration, two narrow band interference filters (center wavelength = 410 nm; bandwidth = 12 nm) are used in front of the PMT to allow passage of the OSL emission, while at the same time absorbing scattered light from the laser. The laser pulse power used in this study was 150 mW. During measurement of the excitation and emission spectra a slightly different arrangement was used. Here the filters were removed and the PMT was shuttered synchronously with the laser so that the PMT shutters were closed when the laser beam was on. Using this arrangement only the afterglow OSL could be recorded. During measurement of the timeresolved emission spectra an EG&G optical multichannel analyzer (OMA III) and an American Holographics (S-100) spectrograph replaced the PMT. The data acquisition was then triggered to record the spectra during periods which were 0.5 s long and 1 s apart from the end of the laser pulse. Thermoluminescence (TL) was monitored using a low temperature cryostat in which the sample could be irradiated with a ~37Cs source at any temperature between -196uC and room temperature. The sample was kept in a vacuum and was heated at 0.Y~C s t during TL measurement. The light was detected using
~
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~Cryostat
y'U
S&F
-ion Laser I
/M
X"~,~ Photodiode
(b) Mirror Sample Shutter
~
Shutter ....::::::iiiiill...................
Detector
Laser Beam
Fig. I. (a) Schematic diagram of the experimental arrangement for recording the OSL signal, M: mirror; F: filter; BS: beam splitter; S: shutter, (b) Detail of the sample cryostat showing the laser beam passing through the center of a front-silvered curved mirror which directs the luminescence onto the detector.
T I M E - R E S O L V E D OSL F R O M ~-A1203:C an uncooled 9635QB P M T and the P M T signal was recorded in the integrated current mode. N o filters were used between the sample and the PMT. ~-AI203:C single crystals from the Urals Polytechnical Institute (Russia) were obtained through Harshaw/Bicron, U.S.A, in the form of TLD-500 dosimeters. This material has a reported TL sensitivity 40 60 times that of LiF:Mg,Ti (TLD-100: Akselrod et aL, 1993). The luminescence centers in this material are oxygen vacancy centers (F and F ~ centers; Summers, 1984; Akselrod et al., 1993). The TL from this material is known to be very sensitive to light (e.g. Musk, 1993; Moscovitch et al., 1993; Walker, 1994) and shows considerable light-induced fading of the TL signal when exposed to visible light. In using this material as an OSL detector, we wish to exploit the light sensitivity by using light, rather than heat, to empty the radiation-filled traps. 6"Co, t~VCs and 9°Srfl°Y were used as radiation sources. All irradiations for the OSL measurements were carried out at room temperature.
3. R E S U L T S Figure 2 shows a typical time-resolved OSL emission signal from ~-AI~O3:C following irradiation with 4 × 10 2Gy beta particles from the 9°Sr/9°Y source. The laser pulse width was 100 ms and the end of the laser pulse is marked in the figure by the vertical line. Both the irradiation and the measurement were carried out at 2 5 C . The OSL signal consists of a rise, followed by a decay of the luminescence in response to the excitation pulse. The signal from one pulse only is shown in the figure. At the power levels used in these studies we observed that the signal showed no detectable reduction (i.e. there was no sign of significant depletion of trapped charge carriers) until approximately 100 pulses had been applied to the sample. Experiments in which the laser
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E400 ~300 ~o 2O0
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Fig. 2. Time-resolved OSL from ~-A1203:C following irradiation at room temperature with a dose of 0.04Gy 9"Sr/9°Y. A single laser pulse (150mW, lOOms wide) was used to excite the OSL. The measurement temperature was 25"C. NT 24,4~K
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Wavelength(rim) Fig. 3. Time-resolved emission spectrum following a dose of 15 Gy from a 6°Co source. The emission is detected with the OMA-lll system and the spectra are recorded over periods of 0.5 s, at 1 s intervals, thus: (a) 0~0.5 s; (b) 1.5 2.0 s; (c) 3.0 3.5 s. The timing began at the end of the laser pulse.
pulse width was varied showed that the OSL signal reached a maximum (i.e. charge carrier equilibrium was established) within 0.15 s of the start of the laser pulse. Figure 3 shows the emission spectrum of the OSL signal. The data in this figure were obtained from a more heavily irradiated sample (15 Gy, ~°Co) and were recorded using the diode array O M A and the procedures described in Section 2. The emission for each spectral measurement was integrated over the time intervals shown in the figure following the end of the laser pulse and, when measured in this configuration, the emission is observed to decrease with time. The spectrum has not been corrected for the wavelength response of the detection system, since the emission wavelength range extends only over 350-450 nm and the detection system response is approximately flat in this spectral region. We see from this figure that the emission peaks at ~ 4 1 0 nm, and that the emission peak remains fixed at this wavelength during the decay process. This suggests that the OSL process is not a donor acceptor pair type of recombination, since it would be expected that a wavelength shift with time would occur in this case. In contrast, this result provides support for the notion that OSL in this material involves electron and hole transitions via the delocalized bands. Photoconductivity measurements would test this conclusion. Emission at 410 nm is characteristic of F-center relaxation in Al~O 3 from the 3 P excited state to the 1 S ground state (Summers, 1984) and this is the same emission process as is observed in TL from this material (Akselrod et al., 1993). The high TL sensitivity that this material exhibits is a consequence of both the highly reducing growth conditions and the C doping. These give rise to the formation of large numbers of oxygen vacancy centers (i.e. F and F* centers), and the observation that OSL uses the
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Fig. 4. Excitation spectra for OSL from ct-Al203:C. The power from the laser at each of the wavelengths shown was adjusted to give the same number of photons per unit time per unit area incident on the sample.
same emission centers as TL (i.e. F centers) implies that this material may also be an extremely sensitive OSL detector. The excitation spectrum for the OSL is shown in Fig. 4. To obtain this the different lines from the Ar-ion laser were used, the power from each being adjusted to give the same number of photons s - l c m 2 at the sample surface. In this way the efficacy with which each wavelength induces OSL can be easily compared. The wavelength range is, of course, restricted by the availability of emission lines from the Ar-ion laser. The most intense emission was observed in the UV region of the spectrum around the line at 457.9 nm. However, the line at this wavelength from the Ar-ion laser has a maximum power of only 300 mW, and thus this line may prove to be impractical for low dose dosimetry. The 514.4 nm line (maximum power 1.7 W) was chosen to be the standard excitation wavelength in the present study,
Fig. 5. Dependence of the OSL signal on temperature for a 100 ms 150 mW laser pulse. The absorbed dose was 0.04 Gy from the ~Sr/9°Y beta source.
despite its low efficiency, simply because of the extra power available. Figure 5 shows the variation of the OSL signal with sample temperature. Increased signal is observed as the temperature is increased and variations in the decay rate also become apparent. This latter feature is seen in detail in Figs 6(a) and (b). In Fig. 6(a) the decay of the OSL signal is fitted to two exponentials. The faster component--with a temperatureindependent lifetime of approximately 35 m s - corresponds to the F-center lifetime (Summers, 1984). The lifetime of the slower component varies with temperature, as shown in the Arrhenius plot of Fig. 6(b), and quenches completely by 50°C. Two decay processes are observed in Fig. 6(b), and from the slopes activation energies of 0.65 eV and 0.77 eV are obtained. Figure 7 shows the variation of the OSL signal (measured at 100°C) with absorbed dose (from the ~37Cs source). These data have been obtained after summing the OSL from 400, 100ms/150mW laser pulses, and integrating the OSL sum from 0 to 0.5 s.
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Fig. 6. (a) Decay of the OSL signal, at 25°C, at the end of the laser pulse. The decay has been fitted to two exponentials. The faster component is temperature independent and the lifetime (~35 ms) corresponds to the F-center lifetime. The lifetime of the slower component (~ 545 ms at 25°C) is temperature dependent. (b) Arrhenius plot of the variation of the lifetime of the slow component as a function of temperature. Two processes are discerned, with activation energies of 0.65 eV and 0.77 eV.
TIME-RESOLVED OSL FROM ~-AI20 3:C 10O--
O
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Dose (mGy)
Fig. 7. OSL versus absorbed dose. The OSL signal is obtained by summing the OSL from 400 laser pulses (150 mW, 100 ms wide) and integrating the sum from 0 to 0.5 s. The irradiation source was a ~37Cssource. The sample was irradiated at 25°C and the OSL measurement temperature was 100°C.
The data show a slightly sublinear increase in the OSL signal with absorbed dose. Minimum detectable dose levels have not yet been investigated, but using the current arrangement doses as small as 10 pGy were easily measured. If the OSL were either summed over more laser pulses, or recorded using a laser operated in cw mode, it is expected that the measurement of absorbed doses as low as 1 #Gy would be possible. Each datum point in Fig. 7 was obtained from the same sample. After each usage the sample was annealed by heating it to a temperature of 800°C for 15 minutes; this treatment was found to completely remove any remaining OSL signal. Since the eventual goal of the research is to develop an all-optical dosimetry system, the possibility of 'optical annealing', or 'bleaching', of the residual signal was also investigated. This was done using a continuous laser exposure (514 nm) for 30 min at a maximum power of 1.7 W, and under these conditions it was found
46l
that the OSL signal could be reduced to background levels. (A rise in sample temperature of ~ 10°C was observed during this procedure.) Additional exposure to the laser for times greater than 30 min did not reduce the residual signal further. Experiments with white light exposure from a Xe arc lamp, and with exposure to different wavelengths from the laser, were also performed in order to establish the optimum wavelength and exposure times for bleaching the OSL signal. It was found that the optimum wavelength to use during optical annealing is the same as that used to excite the OSL (514nm in the present case). Although more studies are required in this area, these measurements indicate that 'optical annealing' is a viable possibility for this material.
4. DISCUSSION The OSL signal observed in Fig. 2 has the expected rise and decay of luminescence in response to the laser pulse. The rise and decay time are dictated by two processes. The first is the relatively fast ( ~ 3 5 ms) lifetime associated with the F-center luminescence (Summers, 1984). The second, slower component is due to phosphorescence from shallow states. The phosphorescence is caused both by the thermal decay of those electrons trapped in these states during the initial radiation exposure, and also (mainly) by those electrons phototransferred to these states during laser excitation. The dependence of the OSL emission upon temperature (Figs 5 and 6) reveals that two shallow states are involved, with trap depths of 0.65 eV and 0.77 eV. Figure 8 shows the TL signal from TLD-500 obtained by irradiating the sample to a dose of 0.35mGy (137Cs) at a temperature of -190°C. In addition to the main dosimetry peak at 177°C (450 K: not shown in the figure) there are two peaks due to shallow states at lower temperatures. In the figure, we show the result of a fit of the low temperature part of the glow curve (200K < T < 350 K) to the usual Randall-Wilkins, first order equation (Randall and Wilkins, 1945a, b), namely
100
l(T) = rlnos(T)exp{ -- E/kT }
80 c 60
exp(-l/~fs(O)exp{-E/kOldO ).
(1)
40 20
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Temperature(K)
Fig. 8. Thermoluminescence from ~-A1203:C irradiated with 0.35 mGy (UTCs) at -190°C, and heated at a rate of 0.3°C s-~. The dotted lines are the fits to the experimental data points assuming first-order kinetics and using equation (1). The fitted trapping parameters for the two low temperature peaks are: E = 0.77 eV and s = 8.5 x 10j2 s-~ for the -3°C (270 K) peak; and E = 0.65eV and s = 1.0 x 109s-L for the 35°C (308 K) peak.
Here E is the trap depth (eV), s is the frequency factor (s-~), no (cm 3) is the initial trapped charge concentration at temperature To (K), /~ is the heating rate (K s -~), k is Boltzmann's constant (eV K ~), T is temperature (K), and 0 (also temperature) is a dummy variable. The parameter rt takes account of the units, the luminescence efficiency of the recombination process, and the detection efficiency of the PMT. The F-center luminescence efficiency in A1203:C is known to be temperature-dependent and strong thermal quenching of the emission occurs at higher temperatures (Kortov et al., 1994). However,
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recent analyses of the luminescence efficiency as a function of temperature (Kitis et al., 1994; Kortov et al., 1994) reveal that thermal quenching does not occur until temperatures in excess of ~ 100°C are reached. Therefore, /I is assumed to be temperature independent (i.e. a constant) for the purposes of fitting. The fitting parameters are, therefore, E, s and the product qno. The fitting is achieved using the software package 'Peakfit' by JandePm. The results of this fitting procedure show that the two lower temperature, glow peaks can be described by single, first-order TL peaks, with trap depths of 0.77 eV and 0.65 eV, respectively, in excellent agreement with the OSL temperature-dependence analysis of Fig. 6(b). Phototransferred TL has been examined in this material by Akselrod and colleagues (e.g. Akselrod et aL, 1990a, b; Akselrod et al., 1993). These authors showed that there are several deep states which act as the source traps for phototransfer during illumination. Walker (1994) has also examined the light-induced fading of the main dosimetry TL peak in this material and has discussed how this may be caused by the release of charge from deep states, or from the dosimetric traps, or both. This optically stimulated charge is then available for recombination (yielding OSL) and for transfer to less stable (shallow) traps. The phototransferred charge will reside in the shallow states for times that will depend upon the temperature. It will then be thermally released back into the delocalized bands, where it will become available for recombination. Thus, by increasing the sample temperature, an increase in the OSL intensity is observed due to the fact that, at elevated temperatures, the shallow states are not effective at localizing the charge and more charge takes part in the recombination (OSL) process. Note that the temperatures used in this study (up to 100°C) are not high enough to induce quenching of the luminescence (Kitis et al., 1994; Kortov et al., 1994). The emission spectra of Fig. 3 illustrate F-center luminescence due to 3 P-to-1 S transitions. This, in turn, indicates that the released charge carriers are electrons and that the recombination process may be represented by a reaction of the type e + F + ~ F* ~ F+hv410nm.
(2)
Here an electron recombines with an F ÷ center, producing an F center. F* represents the F center in an excited 3 P state, and F represents an F center in a relaxed i S state. The relaxation is accompanied by the emission of a photon of wavelength 410 nm. Thus, a reasonable model for OSL emission in c¢-A1203:C involves the optical release of electrons from deep traps into the conduction band, followed by recombination at F ÷ centers and emission at 410 nm. At the same time, electrons are transferred through the conduction band to shallow states, predominantly the electron traps at ~ E c - 0.65 eV and E c - 0.77 eV, where Ec is energy at the bottom of
the conduction band. Thermal excitation of electrons from these traps leads to phosphorescence and a long tail to the OSL following the end of the excitation pulse.
5. CONCLUSIONS In this paper OSL from a-Al~ 03 :C has been shown to be sensitive to absorbed dose. It has been demonstrated that the OSL results from the recombination of optically stimulated electrons with F ÷ centers. Since these centers are also the main cause of the high TL sensitivity of this material it can be expected that ~-A1203:C will also be a sensitive OSL dosimeter. Although minimum measurable dose levels have not yet been investigated, the dose response shown in this paper indicates that doses of the order of 10/~ Gy may be detected using the present system. Only pulsed OSL has been examined in this work; cw OSL may be used for the very low doses in which case it may prove possible to detect doses of the order of 1/zGy. The thermal and optical stability of the OSL signal has yet to be fully investigated. In a recent study (Walker, 1994) the optical fading of the TL from this material was examined and it is reasonable to suggest that the OSL will be similarly affected by light. Thus, handling of the materials in a low light environment is clearly desirable. The OSL signal observed in, for example, Fig. 2 may contain a component due to pure phosphorescence from those shallow traps that were partially filled during radiation exposure. One might expect, therefore, that such a component would be unstable at room temperature. However, our preliminary measurements on thermal stability of the OSL signal have not revealed any detectable, unstable component. This is supported by TL measurements from samples irradiated at room temperature for which we observe only very weak, low temperature TL peaks (i.e. the TL peak at 35°C (308 K) seen in Fig. 8). Although further study is clearly necessary, we conclude from the present preliminary measurements that OSL of ~-AI203:C has a strong potential for use in dosimetry. Acknowledgement--This research is supported by the Na-
tional Science Foundation under grant EHR 9108771.
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luminescence dosimeters. Radiat Prot. Dosim. 47, 247-249. Nanto H., Murayama K., Usada T., Taniguchi S. and Takuschi N. (1993a) Optically stimulated luminescence in KCI:Eu single crystals. Radiat. Prot. Dosim. 47, 281 284. Nanto H., Usada T., Murayama K., Nakamura S., lnabe K. and Takeuchi N. (1993b) Emission mechanism of optically stimulated luminescence in copper-doped sodium chloride single crystals. Radiat. Prot. Dosirn. 47, 293 296. Piesch E., Burkghardt B. and Vilgis M. (1990) Photoluminescence dosimetry: progress and present state of the art. Radiat. Prot. Dosim. 33, 215 226. Pradhan A. S., Bhuwan Chandra and Bhatt R. C. (1983) Phosphorescence and photostimulated luminescence of CaSO4:Dy embedded polyethylene discs at elevated temperatures for fast neutron dosimetry. Radiat. Prot. Dosim. 5, 159 162. Randall J. T. and Wilkins M. H. F. (1945a) Phosphorescence and electron traps I. The study of trap distributions. Proc. R. Soc. London 184, 366-389. Randall J. T. and Wilkins M. H. F. (1945b) Phosphorescence and electron traps II. The interpretation of long-period phosphorescence. Proc. R. Soc. London 184, 390~J,07. Sanderson D. C. W. and Clark R. J. (1994) Pulsed photostimulated luminescence in alkali feldspars. Radiat. Meas. 23, 633-639. Summers G. P. (1984) Thermoluminescence in single crystal ~t-A1203:C. Radiat. Prof. Dosim. 8, 69 80. Walker F. D. (1994) MS thesis. Oklahoma State University, OK.