- Email: [email protected]
Analysis of TL emission spectra
Radiation Measuremems, Vol. 23,Nos Z/3.pp. 341-348,1994 &DVrinht
(0
1994
Etsevierscience.Ltd
Printed’6 &aiBritain. All fights rexwai 13504+87/94 57.00 + .oo
13504@7(93)E4lO29-L
ANALYSIS OF TL EMISSION SPECTRA P.D.
TOWNSEND
School of Mathematical and Physical Sciences, University of Sussex, Brighton BNl9QH,
U.K.
Ah&act-Thermoluminescence provides a highly sensitive technique for detection of imperfections in insulating materials and hence is used for fundamental defect studies, radiation dosimetry and TL dating. The standard method has been to record thermo-luminescence signals using broad band filters, which improve signal to noise, but at the expense of spectral information. Where spectral data have been obtained, it is evident that the spectrum is frequently changing with temperature, and analysis of the component emission bands provides considerable information on the types of recombination and defect sites. Modem TL laboratories are routinely introducing spectral measurements and this paper outlines the value in so doing and discusses alternative experimental strategies.
1. INTROWCTION ANY ATTEMPT to provide an overview and a prediction of future direction for a subject which has been active for more than 40 years will inevitably be coloured by one’s own research interest. Consequently, this article will emphasize the benefits of obtaining the emission spectra produced during thermoluminescence and discuss alternative methods of so doing. In the case of thermoluminescence studies, the original experimental problems are to provide a simple linear heating rate to release trapped charges and a detector to record a signal as cleanly as possible. Having obtained data, the next phase is usually to attempt kinetic analysis to describe the form of the glow curves in terms of a set of identifiable trapping peaks with specified activation energies and vibrational frequencies (i.e. an attempt to escape frequency). The final steps are more difficult as these are to positively identify the structure of the charge trapping sites and the recombination centre which generate the light, together with the various defect interactions. Realistically, this identification stage proceeds very slowly and although positive assignment of defect site models can be made from detailed information offered by EPR and ENDOR (electron paramagnetic resonance and electron nuclear double resonance), success is very limited. Luminescence, optical absorption and the majority of the other standard methods of detecting colour centres are not specific, hence most models are based on more circumspect arguments. Such models may be reasonable approximations but should generally be viewed as a guide to discussion rather than as the final and ultimate description for the structure of an imperfection. Fortunately, applications of thermoluminescence can be quite empirical so long as the underlying
radiation dosimetry is not compromised. In the main fields of personnel dosimetry, archaeological and geological dating or mineral prospecting, one can variously tolerate uncertainties of lo-50% since alternative routes to dosimetry and dating may not be available. Further, one may use the technique with a minimal knowledge of the defect species. Empiricism is inevitable since the strength and weakness of TL is that it is an incredibly sensitive technique which can respond to changes in impurity of intrinsic defects at much less than parts per billion. For practical applications of TL, such problems are totally academic, and commercial dosimeters operate by swamping trace impurities with intentional additions of dopant ions. In dating work, one must, nevertheless, be aware of the assumptions that are being made and continue to question the results and dating strategies. In particular, if there is conflict between the ages or estimated doses determined by the familiar altematives, methods of additive dose, partial bleaching or optical bleaching, etc. then a reappraisal of the specific defects is essential. It should not be ignored that some materials may not be datable by all ap proaches.
2. TEMPERATURE CONTROL With 40 years’ development and enormous advances in electronics, one might assume that at least the heating aspects of TL would now be fully under control. Indeed, in terms of control of the sample heater, excellent linearity can be obtained from both laboratory and commercial systems in either the normal heating range above room temperature or in the less commonly used low temperature TL. Nevertheless, it is worth emphasizing that there can be significant temperature gradients from the heater 341
342
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FIG. 1. An example of the peak temperature (T,,,,,) shift with heating rate (B) for a glow peak from a thick (1 mm) sample of feldspar. The original data (solid circles) lie on a curve of the ln(TL,lE ) versus l/T_ plot where the T value is given by the heater strip temperature. After correcting for thermal delays in the sample (open circles), the plot becomes linear. An exchange gas of argon at lOOmbar was used.
strip to the emitting surface of an insulating material. In routine repetitive work or in comparisons between similar equipment, any such errors may not be noticed. They must, however, be considered in precision determinations of curve shapes, true peak temperatures as a function of heating rate and subsequent kinetic analysis. The scale of the problems has recently been discussed by Betts et al. (1993). Figure 1 shows an example of the measured peak shift with heating rate. The experimentally recorded curvature of the ln(TL,,/B) versus l/T_ plot does not imply a range of activation energies (T,, is the peak temperature of the glow curve for a heating rate B in “C s-l). Consideration of the thermal conductivity, etc. accurately predicts corrections for the calculated temperature lags in a thick sample, or one in poor contact with the heater strip. These theoretical adjustments lead to a linear plot giving an activation value similar to, but not identical to that for low heating rate data, and further, significantly different frequency factors. 3. KINETIC ANALYSES A plethora of analytical methods have been suggested in order to determine activation energies and defect concentrations (e.g. as reviewed by Kirsh, 1992). Although obvious, it is often forgotten that the quality of the analyses is dependent on the starting data. Significant errors are hidden if the temperature of the sample differs from the recorded heater tem-
perature. Further, many analyses are made with polychromatic light data. If, as will be shown in the next section, there are several overlapping emission bands, or the emission spectra change with tempcrature, then the simple kinetic analyses are suspect. Note, also, that even the use of a narrow band pass filter to record monochromatic light can be miskading, for not only must one correct for overlap from other emission bands, but also the luminescence efficiency can change with temperature and further, the peak position of the band can move with tcmperature. Peak movement is immediately obvious for the lower temperature peaks of the familiar LiF: Mg:Ti dosimeter, as shown in Fig. 2 for an isometric plot and a contour map of the intensity as a function of wavelength and temperature. This effect is quite common and has been noted with many TL emission bands. A final comment on kinetic analysis is that in most models, it is assumed that the trapped charge is released from a defect site and then passes via the conduction band to the recombination centre. There are numerous indications that this idealized de&ption may not be appropriate for many familiar glow peaks (including those in LiF). A recent example revealed by recording the emission spectra of TL in rare earth doped Bi,Ge,O,, shows that nominally the same glow peaks have emission spectra which are determined by transitions within the rare earth ion recombination centres, and the intrinsic blue emission
ANALYSIS
OF TL EMISSION
343
SPECTRA
TLD-100
TLD-100 i
200
I 300
400 Wavelength
I 500 (nm)
600
700
J (b) FIG. 2. The data show (a) an isometric plot of the TL intensity versus wavelength and temperature for a LiF:Mg:Ti dosimctcr TLDIOO sample. The contour map (b) emphasizes that the wavelength peak changes with temperature. The radiation dose was 3 Gy and the heating rate 2.5”C s-‘.
is totally suppressed. Figure 3 shows that the peak temperature depends on the rare earth radius (for ions smaller than the B?+ which they replace). The obvious conclusion is that lattice distortions directly link the trapping site and the recombination centre, without charge transfer through the conduction band.
4. TL EMISSION SPECTRA If the emission spectrum of a sample is constant at all temperatures, and is independent of dose, thermal and optical history, then no matter how complex is the spectrum, it will not introduce any problems for TL analysis. Unfortunately, in the experience of this
P. D. TOWNSEND
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FIG. 3. The change in glow peak temperature for a TL peak in Bi,Ge30,* as a function of rare earth dopant. Note the emission signals are characteristic of the dopant and follow a simple ionic size dependence for ionic radii less than that of the substitutional site of Bi’+. T’he dose was 50 Gy and the heating rate 0.1 K s-l.
author, no sample has ever satisfied these criteria. Many materials exhibit minor changes and, even when. the spectra are a function of temperature, they may not be sensitive to dose. In such systems, kinetics may be suspect, but dating is feasible. In more extreme cases, the spectral changes may result in significant disagreements in dating. A recent example of feldspar dating led to estimates of age which differed by up to 40% for the same material measured in different laboratories (Rendell et al., 1993). Even if the various groups use similar filters and photomultiplier tubes, they will not be identical and so will not equally weight the contributions from overlapping emission bands. Figure 4 shows the spectrally
resolved TL for this material. Quite clearly the standard polychromatic filters used by the TL dating community (such as UGl 1 from Schott or 5-58 from Coming) will weight the component bands differently and predict quite different age values. Kinetic analysis of polychromatic signals in this case will be disastrous. Figure 5 shows an example of a low temperature glow curve for Bi4Gel O,r doped with europium. This example clearly demonstrates that at the lowest temperatures, the emission is controlled by the intrinsic electron-hole recombination of the host lattice, but at higher temperatures, the rare earth recombination centre is totally dominant. This feature has been
Large Feldspar
Grains
FIG. 4. The TL emission spectra from a potassium feldspar sample.
ANALYSIS
OF TL EMISSION
345
SPECTRA
BGO:Eu
FIG. 5. A glow curve obtained from Eu-doped Bi,Ge30,, after X-ray irradiation at 20 K. Note the intrinsic emission is seen at low temperature and Eu recombination sites dominate at higher temperature.
observed in many Bi,Ge~Oir crystals doped with rare earth ions. The observed luminescence spectrum often differs from the expected version, for example among the Bi,Ge,O,, samples measured was one codoped with Nd and Er with 0.43 wt% of each oxide in the melt. The TL spectrum is entirely from Eu which is included at the 3 ppm level to aid crystal growth. In hindsight, one can perhaps justify such effects as the Eu is a closer fit to the Bi’+ ion, so replacing it in the lattice, whereas the Nd and Er introduce larger distortions and may even be incorporated in pairs to minimize strain. The appearance of many emission bands is not confined to doped samples and for example, much greater complexity is observed for the spectra obtained from alkali halides. Figure 6 offers an example of an isometric plot for a KBr sample X-ray irradiated at 20 K. Even at the low resolution shown on this figure, it is apparent that there are at least seven glow peaks, each with a different emission spectrum. Higher sensitivity plots reveal further peaks, particularly between 20 and 100 K. One can immediately see that to attempt TL measurements without spectral information would be misleading and rejecting considerable amounts of information. In these figures, the spectra have been corrected for the wavelength dependence of the system and it
should be noted that these correction factors can vary by a factor of 100 or more across a wide spectral range. It is unfortunate that most systems use wavelength dispersion rather than the physically more meaningful energy dispersion. Conversion from wavelength to energy scales additionally requires correction for fixed bandwidth spectrometer slits. Hence, in the wavelength axis graphs shown here, and in most examples of TL spectra, the true importance of the red bands relative to the shorter wavelengths has been minimized. The examples shown here demonstrate the value of spectral information in TL studies and further arguments and examples were made in an earlier review (Townsend and Kirsh, 1989). 5. DESIGNING
A TL SPECTROMETER
The basic problem is that one must record a transient weak signal. Early systems using scanning monochromators or sets of broad band filters (e.g. Harris and Jackson, 1970; Mattem et al.. 1971; Bailiff er al., 1977). These provided valuable data but, intrinsically, are not desirable as they lack spectral resolution (filters) or only sample the signal periodically if scanned. A 100 point scanning system only records data at any one wavelength for 1% of the time. The obvious advance is to use wavelength
346
P. D. TOWNSEND
KBr
KBr
FIG. 6. TL emission spectra obtained after an X-ray irradiation dose of 800 Gy in KBr at 20 K. The heating rate was 0.1 K s-l. The data are shown in two parts to accommodate the change in scale between the lower and higher temperatures.
ANALYSIS
OF TL EMISSION
multiplexed recording in which the entire spectrum is taken in parallel for 100% of the time. This has the added advantage that one may use a high heating rate without having to allow time for scanning. The problem then becomes one of designing a spectrometer and detector system with optimum light gathering power. It is sobering to realize that even the simple filter TL systems (i.e. polychromatic) typically gather less than 2% of the emitted light. The light collection optics used recently in commercial or laboratory spectrometers for TL has ranged fromf/6 up tofll.5, that is one is collecting between 0.2 and 3% of the emitted light. Indeed, there is no advantage in using more complex optics as the efficiency will be set by the highest f number component in the system, which is probably the spectrometer. One must note that, whereas with filter TL equipment, a 1 cm diameter sample can be imaged entirely on the PM tube, spectrometers normally have slits, and so the 1 cm diameter sample becomes effectively 5 mm by lOOpm, which is a 99% loss in signal. Addition of cylindrical lenses may be advantageous. Anti-reflection coatings are useful if many lenses or windows are included in the light path as, even for silica, there is a 4% retIection loss at each interface. Hence, a design with two lenses and a window (six surfaces) reduces the signal by 20%. The choice of dispersive element can be a diffraction grating, a prism or moving mirrors in a Fourier transform spectrometer. Prescott and his coworkers (Prescott ef of., 1988; Jensen and Prescott, 1982) built a Fourier transform instrument with an excellent f value for TL studies that, over its operating wavelength range, is the most sensitive TL spectrometer yet constructed. Despite this superb demonstration of experimental skill, it is not the route most people will follow. Such spectrometers are mechanically very demanding as they require parallelism of the mirrors and mirror step sizes which are less than half the shortest wavelength to be monitored. Prescott accepted a minimum wavelength near 350 run, and although shorter wavelength instruments have been built, the time required for design, construction and maintenance exclude them for those who are primarily interested in TL rather than advanced instrument technology. Another variant on this multiplex route was given by Ckxkowski (1992). Most other recent spectrometers have used diffraction gratings rather than prisms, but for weak transient signals, this may be undesirable as grating efficiencies are often as low as 60% for the optimum polarization and worse for the perpendicular plane. Prism spectrometers might offer lower transmission loss, but in commercial instruments, diffraction gratings prevail. Diffraction gratings have a limited working range and as a guide-line they only cover a range from one half to twice the blaze wavelength. So, a grating blazed for 400 nm is useful from 200 to 8OOnm. However, this disguises the fact that the
SPECTRA
347
efiiciency may have fallen by up to a factor of 10 at the limits of the range. Diffraction gratings suffer from the presence of higher order beams so second- and third-order fltering is needed. If a single spectrometer and detector is used, this can be inconvenient. This problem has recently been highlighted in a high sensitivity TL spectrometer described by Piters et al. (1993). One expensive solution is to use more than one spectrometer to cover a wide range in which there is optimum grating blaze for each region (Luff and Townsend, 1993). The final stage is the choice of detector. Reference to the simple quantum efficiency is an insu8icient indicator of the 6nal performance. Photomultiplier tubes perform well and can accommodate large area slit images from the spectrometer. Two versions of PM detection are to either image an array of waveguiding plates on to an array of small PM tubes or to use a continuous large face PM tube with a fourlead anode so that the signal pulses can be processed to give position information. The former example is simple to construct and offers a very large dynamic range but needs careful design in order not to have dead spaces between the recording sections. The latter is termed an IPD (imaging photomultiplier detector). In the UV/bhre region, quantum efficiencies are up to 25%, but in some of the forms packaged for position sensitive detection, it is customary to include a protective film to inhibit self destruction if there is a bright burst of light. For an expensive detector used by many people for a wide range of samples, this is a sensible precaution, but it reduces the peak efficiency down to about 10%. The figures shown in this paper were obtained by the IPD route on the equipment constructed by Luff and Townsend (1993) using tubes made by Photek in Hastings (U.K.). Charge coupled devices (CCD) and diode arrays are alternatives to the PM tubes but for the W/blue end of the spectrum they are delicate and expensive specialist items at present. Examples include instruments by Bakas (1984). Homyak and Franklin (1988) or Rasheedy et al. (1991). The semiconductor detectors may have better quantum efficiency in the red end of the spectrum compared with the PM tubes. However, high quantum efficiency is necessary but it is not the only consideration. In CCD or diode arrays, there is a dead space between detector regions which limits the spectrometer resolution. For example, in an extreme case, line spectra might fall partially across the dead space if the spectrometer entrance slits are comparable with the repeat and dead space of the detector. Signal levels are low for the majority of materials studied by TL, and so no matter which route is used, the detection will be down to the photon counting level in all the systems described so far. PM tubes, CCD and diode arrays all benefit by having reduced dark current noise if they are cooled. However in the CCD and diode systems, there is readout noise which
P. D. TC tWNSEND
348
can be a few electrons per channel per read outcycle. In spectral measurements for astronomy, this is a minimal problem for a CCD system as the signal can be accumulated for 20 min and readout noise is then a small percentage of the total. So, for astronomy a CCD may be preferable to a PM tube even if the sensitivity is lower. For TL with high sample heating rates, spectra must be read at least once a second which exacerbates the noise problem, and one must consider the choice of detector in terms of the state of the art. There is not an absolute answer, and even within the last few years, detector performance, area and costs have improved considerably. Having recorded the signal, a good data processing package is essential to correct the observed spectra for the wavelength dependent sensitivity of the entire system. Commercial packages are often excellent but one must be cautious that high quality software is matched by high quality optics. This is not the case on all instruments. A further factor in the spectrometer design will be cost. In fact, all the types of spectrometer discussed so far come to rather similar cost for equivalent performance. Finally, if one wishes to build or purchase a TL spectrometer, then a reappraisal of the instruments produced so far in terms of modem detectors and spectrometers could result in spectacular advances in TL spectral sensitivity.
6. SUMMARY In the future, it is certain that every major TL laboratory will routinely run studies of emission spectra, even for material that is being dated by existing methods. The examples of information revealed by the spectral studies suggest that the effort in introducing such sophistication is worthwhile and essential. There is no unique solution but the present article has indicated some of the problems that must be addressed. Acknowledgements-1 would particularly like to acknowledge my debt to Dr Paul Levy who introduced me to the study of TL and whose initial examples of TL spectral analysis offered a challenging standard and inspiration for all subsequent work.
REFERENCES Bailiff 1. K., Morris D. A. and Aitken M. J. (1977) A rapid scanning interference spectrometer: application to low level TL emission. J. Whys. E. IO, 1156-l 160. Bakas G. V. (1984) A new optical muhichannel analyser using a charge coupled device as detector for thermoluminescence emission measurements. Rad. Prorecr. Dosim. 9, 301-305.
Bctts D. S., Couturier L., Khyarat A. H., Luff B. J. and Townsend P. D. (1993) Temperature distribution in thermoluminescence experiments I: experimental results. J. Phys. D X,843-848; Temperature distribution in thermoluminescence experiments II: some cakulation models. J. Phys. D 26, 849-857. Harris A. M. and Jackson J. H. (1970) A rapid scanning spectrometer for the region ZOO-850 nm: application to thermoluminescent emission spectra. J. Phys. E 3, 374-376. Homyak W. F. and Franklin A. D. (1988) A second generation 3d-spectrophotometer for TL analysis. J. Lumin. 42, 89-96.
Jensen H. E. and Prescott J. R. (1982) A Fourier transform spectrometer for the measurement of thermoluminescence emission spectra. PACT 6, 542-549. Kirsh Y. (1992) Kinetic analysis of thermoluminescence. Phys. Slut. Sol. (a) 129, 15-48. Luff B. J. and Townsend P. D. (1993) High sensitivity thermoluminescence spectrometer. Meas. Sci. Technof. 4, 65-71.
Mattem P. L., Lengweiler K. and Levy P. W. (1971) Apparatus for the simultaneous determination of thermoluminescent intensity of spectral distribution. Mod. Geol. 2, 293-294; Three dimensional thermoluminescence analysis of minerals. Mod. Geol. 2, 295-297. Oczkowski H. L. (1992) Hadamard spectrometry for thermoluminescence investigations. Acru phys. Pol. A 82, 367-375. Piters T. M., Met&mans W. H. and Bos A. J. (1993) An automated research facility for measuring thermoluminescence emission spectra using an optical multichannel analyzer. Rev. Sci. Instrum. 64, 109-117. Prescott J. R., Fox P. J., Akber R. A. and Jensen H. E. (1988) Thermoluminescence emission spectrometer. Appl. Optics 27, 3496-3501.
Rasheedy M. S., Nishimura F. and Ichimori T. (1991) On the thermoluminescence emission spectra of CaF, : Tm. Nucl. Instrum. Meth. B61, 67-71. Rendell H. M., Townsend P. D., Luff B. J., Wintle A. G. and Balescu S. (1993) Spectral analysis of thermoluminescence in the dating of potassium feldspars. Phys. Stut. Sol. (II) 138, 335-341.
Townsend P. D. and Kirsh Y. (1989) Spectral measurement during thermoluminescenccan essential requirement. Contemp. Phys. 30, 337-354.
(0
1994
Etsevierscience.Ltd
Printed’6 &aiBritain. All fights rexwai 13504+87/94 57.00 + .oo
13504@7(93)E4lO29-L
ANALYSIS OF TL EMISSION SPECTRA P.D.
TOWNSEND
School of Mathematical and Physical Sciences, University of Sussex, Brighton BNl9QH,
U.K.
Ah&act-Thermoluminescence provides a highly sensitive technique for detection of imperfections in insulating materials and hence is used for fundamental defect studies, radiation dosimetry and TL dating. The standard method has been to record thermo-luminescence signals using broad band filters, which improve signal to noise, but at the expense of spectral information. Where spectral data have been obtained, it is evident that the spectrum is frequently changing with temperature, and analysis of the component emission bands provides considerable information on the types of recombination and defect sites. Modem TL laboratories are routinely introducing spectral measurements and this paper outlines the value in so doing and discusses alternative experimental strategies.
1. INTROWCTION ANY ATTEMPT to provide an overview and a prediction of future direction for a subject which has been active for more than 40 years will inevitably be coloured by one’s own research interest. Consequently, this article will emphasize the benefits of obtaining the emission spectra produced during thermoluminescence and discuss alternative methods of so doing. In the case of thermoluminescence studies, the original experimental problems are to provide a simple linear heating rate to release trapped charges and a detector to record a signal as cleanly as possible. Having obtained data, the next phase is usually to attempt kinetic analysis to describe the form of the glow curves in terms of a set of identifiable trapping peaks with specified activation energies and vibrational frequencies (i.e. an attempt to escape frequency). The final steps are more difficult as these are to positively identify the structure of the charge trapping sites and the recombination centre which generate the light, together with the various defect interactions. Realistically, this identification stage proceeds very slowly and although positive assignment of defect site models can be made from detailed information offered by EPR and ENDOR (electron paramagnetic resonance and electron nuclear double resonance), success is very limited. Luminescence, optical absorption and the majority of the other standard methods of detecting colour centres are not specific, hence most models are based on more circumspect arguments. Such models may be reasonable approximations but should generally be viewed as a guide to discussion rather than as the final and ultimate description for the structure of an imperfection. Fortunately, applications of thermoluminescence can be quite empirical so long as the underlying
radiation dosimetry is not compromised. In the main fields of personnel dosimetry, archaeological and geological dating or mineral prospecting, one can variously tolerate uncertainties of lo-50% since alternative routes to dosimetry and dating may not be available. Further, one may use the technique with a minimal knowledge of the defect species. Empiricism is inevitable since the strength and weakness of TL is that it is an incredibly sensitive technique which can respond to changes in impurity of intrinsic defects at much less than parts per billion. For practical applications of TL, such problems are totally academic, and commercial dosimeters operate by swamping trace impurities with intentional additions of dopant ions. In dating work, one must, nevertheless, be aware of the assumptions that are being made and continue to question the results and dating strategies. In particular, if there is conflict between the ages or estimated doses determined by the familiar altematives, methods of additive dose, partial bleaching or optical bleaching, etc. then a reappraisal of the specific defects is essential. It should not be ignored that some materials may not be datable by all ap proaches.
2. TEMPERATURE CONTROL With 40 years’ development and enormous advances in electronics, one might assume that at least the heating aspects of TL would now be fully under control. Indeed, in terms of control of the sample heater, excellent linearity can be obtained from both laboratory and commercial systems in either the normal heating range above room temperature or in the less commonly used low temperature TL. Nevertheless, it is worth emphasizing that there can be significant temperature gradients from the heater 341
342
P. D. TOWNSEND
* 14.0 ii w
13.5 -
ii? -0 c ._
13.0 -
pl 6 ._ L
12.5 -
d
12.0 -
g v c *_
11.5 -
._ G >
0 10.5 t 190
! tto.0 K
'
0.0021
i
t c
t
180 c
”
170
”
0.0022
t c
160
t c
130
”
”
” 0.0023
l/Lax
t
t c
140
” 0.0024
in
130
c
”
degrees
t c
” 1’ 0.0025
120
c
t
t
110 c
100 c
” 0.0026
“1
0.0027
Kelvin.
FIG. 1. An example of the peak temperature (T,,,,,) shift with heating rate (B) for a glow peak from a thick (1 mm) sample of feldspar. The original data (solid circles) lie on a curve of the ln(TL,lE ) versus l/T_ plot where the T value is given by the heater strip temperature. After correcting for thermal delays in the sample (open circles), the plot becomes linear. An exchange gas of argon at lOOmbar was used.
strip to the emitting surface of an insulating material. In routine repetitive work or in comparisons between similar equipment, any such errors may not be noticed. They must, however, be considered in precision determinations of curve shapes, true peak temperatures as a function of heating rate and subsequent kinetic analysis. The scale of the problems has recently been discussed by Betts et al. (1993). Figure 1 shows an example of the measured peak shift with heating rate. The experimentally recorded curvature of the ln(TL,,/B) versus l/T_ plot does not imply a range of activation energies (T,, is the peak temperature of the glow curve for a heating rate B in “C s-l). Consideration of the thermal conductivity, etc. accurately predicts corrections for the calculated temperature lags in a thick sample, or one in poor contact with the heater strip. These theoretical adjustments lead to a linear plot giving an activation value similar to, but not identical to that for low heating rate data, and further, significantly different frequency factors. 3. KINETIC ANALYSES A plethora of analytical methods have been suggested in order to determine activation energies and defect concentrations (e.g. as reviewed by Kirsh, 1992). Although obvious, it is often forgotten that the quality of the analyses is dependent on the starting data. Significant errors are hidden if the temperature of the sample differs from the recorded heater tem-
perature. Further, many analyses are made with polychromatic light data. If, as will be shown in the next section, there are several overlapping emission bands, or the emission spectra change with tempcrature, then the simple kinetic analyses are suspect. Note, also, that even the use of a narrow band pass filter to record monochromatic light can be miskading, for not only must one correct for overlap from other emission bands, but also the luminescence efficiency can change with temperature and further, the peak position of the band can move with tcmperature. Peak movement is immediately obvious for the lower temperature peaks of the familiar LiF: Mg:Ti dosimeter, as shown in Fig. 2 for an isometric plot and a contour map of the intensity as a function of wavelength and temperature. This effect is quite common and has been noted with many TL emission bands. A final comment on kinetic analysis is that in most models, it is assumed that the trapped charge is released from a defect site and then passes via the conduction band to the recombination centre. There are numerous indications that this idealized de&ption may not be appropriate for many familiar glow peaks (including those in LiF). A recent example revealed by recording the emission spectra of TL in rare earth doped Bi,Ge,O,, shows that nominally the same glow peaks have emission spectra which are determined by transitions within the rare earth ion recombination centres, and the intrinsic blue emission
ANALYSIS
OF TL EMISSION
343
SPECTRA
TLD-100
TLD-100 i
200
I 300
400 Wavelength
I 500 (nm)
600
700
J (b) FIG. 2. The data show (a) an isometric plot of the TL intensity versus wavelength and temperature for a LiF:Mg:Ti dosimctcr TLDIOO sample. The contour map (b) emphasizes that the wavelength peak changes with temperature. The radiation dose was 3 Gy and the heating rate 2.5”C s-‘.
is totally suppressed. Figure 3 shows that the peak temperature depends on the rare earth radius (for ions smaller than the B?+ which they replace). The obvious conclusion is that lattice distortions directly link the trapping site and the recombination centre, without charge transfer through the conduction band.
4. TL EMISSION SPECTRA If the emission spectrum of a sample is constant at all temperatures, and is independent of dose, thermal and optical history, then no matter how complex is the spectrum, it will not introduce any problems for TL analysis. Unfortunately, in the experience of this
P. D. TOWNSEND
344
200%
i 0 I ii 120f? 10
8.b
.Tb$ b+s
’
hl*a
8003
0.60
L+s II
’ 0.44 +* 0.98 Ionic Radius (A)
%?a
Nd+S
’
1.62
FIG. 3. The change in glow peak temperature for a TL peak in Bi,Ge30,* as a function of rare earth dopant. Note the emission signals are characteristic of the dopant and follow a simple ionic size dependence for ionic radii less than that of the substitutional site of Bi’+. T’he dose was 50 Gy and the heating rate 0.1 K s-l.
author, no sample has ever satisfied these criteria. Many materials exhibit minor changes and, even when. the spectra are a function of temperature, they may not be sensitive to dose. In such systems, kinetics may be suspect, but dating is feasible. In more extreme cases, the spectral changes may result in significant disagreements in dating. A recent example of feldspar dating led to estimates of age which differed by up to 40% for the same material measured in different laboratories (Rendell et al., 1993). Even if the various groups use similar filters and photomultiplier tubes, they will not be identical and so will not equally weight the contributions from overlapping emission bands. Figure 4 shows the spectrally
resolved TL for this material. Quite clearly the standard polychromatic filters used by the TL dating community (such as UGl 1 from Schott or 5-58 from Coming) will weight the component bands differently and predict quite different age values. Kinetic analysis of polychromatic signals in this case will be disastrous. Figure 5 shows an example of a low temperature glow curve for Bi4Gel O,r doped with europium. This example clearly demonstrates that at the lowest temperatures, the emission is controlled by the intrinsic electron-hole recombination of the host lattice, but at higher temperatures, the rare earth recombination centre is totally dominant. This feature has been
Large Feldspar
Grains
FIG. 4. The TL emission spectra from a potassium feldspar sample.
ANALYSIS
OF TL EMISSION
345
SPECTRA
BGO:Eu
FIG. 5. A glow curve obtained from Eu-doped Bi,Ge30,, after X-ray irradiation at 20 K. Note the intrinsic emission is seen at low temperature and Eu recombination sites dominate at higher temperature.
observed in many Bi,Ge~Oir crystals doped with rare earth ions. The observed luminescence spectrum often differs from the expected version, for example among the Bi,Ge,O,, samples measured was one codoped with Nd and Er with 0.43 wt% of each oxide in the melt. The TL spectrum is entirely from Eu which is included at the 3 ppm level to aid crystal growth. In hindsight, one can perhaps justify such effects as the Eu is a closer fit to the Bi’+ ion, so replacing it in the lattice, whereas the Nd and Er introduce larger distortions and may even be incorporated in pairs to minimize strain. The appearance of many emission bands is not confined to doped samples and for example, much greater complexity is observed for the spectra obtained from alkali halides. Figure 6 offers an example of an isometric plot for a KBr sample X-ray irradiated at 20 K. Even at the low resolution shown on this figure, it is apparent that there are at least seven glow peaks, each with a different emission spectrum. Higher sensitivity plots reveal further peaks, particularly between 20 and 100 K. One can immediately see that to attempt TL measurements without spectral information would be misleading and rejecting considerable amounts of information. In these figures, the spectra have been corrected for the wavelength dependence of the system and it
should be noted that these correction factors can vary by a factor of 100 or more across a wide spectral range. It is unfortunate that most systems use wavelength dispersion rather than the physically more meaningful energy dispersion. Conversion from wavelength to energy scales additionally requires correction for fixed bandwidth spectrometer slits. Hence, in the wavelength axis graphs shown here, and in most examples of TL spectra, the true importance of the red bands relative to the shorter wavelengths has been minimized. The examples shown here demonstrate the value of spectral information in TL studies and further arguments and examples were made in an earlier review (Townsend and Kirsh, 1989). 5. DESIGNING
A TL SPECTROMETER
The basic problem is that one must record a transient weak signal. Early systems using scanning monochromators or sets of broad band filters (e.g. Harris and Jackson, 1970; Mattem et al.. 1971; Bailiff er al., 1977). These provided valuable data but, intrinsically, are not desirable as they lack spectral resolution (filters) or only sample the signal periodically if scanned. A 100 point scanning system only records data at any one wavelength for 1% of the time. The obvious advance is to use wavelength
346
P. D. TOWNSEND
KBr
KBr
FIG. 6. TL emission spectra obtained after an X-ray irradiation dose of 800 Gy in KBr at 20 K. The heating rate was 0.1 K s-l. The data are shown in two parts to accommodate the change in scale between the lower and higher temperatures.
ANALYSIS
OF TL EMISSION
multiplexed recording in which the entire spectrum is taken in parallel for 100% of the time. This has the added advantage that one may use a high heating rate without having to allow time for scanning. The problem then becomes one of designing a spectrometer and detector system with optimum light gathering power. It is sobering to realize that even the simple filter TL systems (i.e. polychromatic) typically gather less than 2% of the emitted light. The light collection optics used recently in commercial or laboratory spectrometers for TL has ranged fromf/6 up tofll.5, that is one is collecting between 0.2 and 3% of the emitted light. Indeed, there is no advantage in using more complex optics as the efficiency will be set by the highest f number component in the system, which is probably the spectrometer. One must note that, whereas with filter TL equipment, a 1 cm diameter sample can be imaged entirely on the PM tube, spectrometers normally have slits, and so the 1 cm diameter sample becomes effectively 5 mm by lOOpm, which is a 99% loss in signal. Addition of cylindrical lenses may be advantageous. Anti-reflection coatings are useful if many lenses or windows are included in the light path as, even for silica, there is a 4% retIection loss at each interface. Hence, a design with two lenses and a window (six surfaces) reduces the signal by 20%. The choice of dispersive element can be a diffraction grating, a prism or moving mirrors in a Fourier transform spectrometer. Prescott and his coworkers (Prescott ef of., 1988; Jensen and Prescott, 1982) built a Fourier transform instrument with an excellent f value for TL studies that, over its operating wavelength range, is the most sensitive TL spectrometer yet constructed. Despite this superb demonstration of experimental skill, it is not the route most people will follow. Such spectrometers are mechanically very demanding as they require parallelism of the mirrors and mirror step sizes which are less than half the shortest wavelength to be monitored. Prescott accepted a minimum wavelength near 350 run, and although shorter wavelength instruments have been built, the time required for design, construction and maintenance exclude them for those who are primarily interested in TL rather than advanced instrument technology. Another variant on this multiplex route was given by Ckxkowski (1992). Most other recent spectrometers have used diffraction gratings rather than prisms, but for weak transient signals, this may be undesirable as grating efficiencies are often as low as 60% for the optimum polarization and worse for the perpendicular plane. Prism spectrometers might offer lower transmission loss, but in commercial instruments, diffraction gratings prevail. Diffraction gratings have a limited working range and as a guide-line they only cover a range from one half to twice the blaze wavelength. So, a grating blazed for 400 nm is useful from 200 to 8OOnm. However, this disguises the fact that the
SPECTRA
347
efiiciency may have fallen by up to a factor of 10 at the limits of the range. Diffraction gratings suffer from the presence of higher order beams so second- and third-order fltering is needed. If a single spectrometer and detector is used, this can be inconvenient. This problem has recently been highlighted in a high sensitivity TL spectrometer described by Piters et al. (1993). One expensive solution is to use more than one spectrometer to cover a wide range in which there is optimum grating blaze for each region (Luff and Townsend, 1993). The final stage is the choice of detector. Reference to the simple quantum efficiency is an insu8icient indicator of the 6nal performance. Photomultiplier tubes perform well and can accommodate large area slit images from the spectrometer. Two versions of PM detection are to either image an array of waveguiding plates on to an array of small PM tubes or to use a continuous large face PM tube with a fourlead anode so that the signal pulses can be processed to give position information. The former example is simple to construct and offers a very large dynamic range but needs careful design in order not to have dead spaces between the recording sections. The latter is termed an IPD (imaging photomultiplier detector). In the UV/bhre region, quantum efficiencies are up to 25%, but in some of the forms packaged for position sensitive detection, it is customary to include a protective film to inhibit self destruction if there is a bright burst of light. For an expensive detector used by many people for a wide range of samples, this is a sensible precaution, but it reduces the peak efficiency down to about 10%. The figures shown in this paper were obtained by the IPD route on the equipment constructed by Luff and Townsend (1993) using tubes made by Photek in Hastings (U.K.). Charge coupled devices (CCD) and diode arrays are alternatives to the PM tubes but for the W/blue end of the spectrum they are delicate and expensive specialist items at present. Examples include instruments by Bakas (1984). Homyak and Franklin (1988) or Rasheedy et al. (1991). The semiconductor detectors may have better quantum efficiency in the red end of the spectrum compared with the PM tubes. However, high quantum efficiency is necessary but it is not the only consideration. In CCD or diode arrays, there is a dead space between detector regions which limits the spectrometer resolution. For example, in an extreme case, line spectra might fall partially across the dead space if the spectrometer entrance slits are comparable with the repeat and dead space of the detector. Signal levels are low for the majority of materials studied by TL, and so no matter which route is used, the detection will be down to the photon counting level in all the systems described so far. PM tubes, CCD and diode arrays all benefit by having reduced dark current noise if they are cooled. However in the CCD and diode systems, there is readout noise which
P. D. TC tWNSEND
348
can be a few electrons per channel per read outcycle. In spectral measurements for astronomy, this is a minimal problem for a CCD system as the signal can be accumulated for 20 min and readout noise is then a small percentage of the total. So, for astronomy a CCD may be preferable to a PM tube even if the sensitivity is lower. For TL with high sample heating rates, spectra must be read at least once a second which exacerbates the noise problem, and one must consider the choice of detector in terms of the state of the art. There is not an absolute answer, and even within the last few years, detector performance, area and costs have improved considerably. Having recorded the signal, a good data processing package is essential to correct the observed spectra for the wavelength dependent sensitivity of the entire system. Commercial packages are often excellent but one must be cautious that high quality software is matched by high quality optics. This is not the case on all instruments. A further factor in the spectrometer design will be cost. In fact, all the types of spectrometer discussed so far come to rather similar cost for equivalent performance. Finally, if one wishes to build or purchase a TL spectrometer, then a reappraisal of the instruments produced so far in terms of modem detectors and spectrometers could result in spectacular advances in TL spectral sensitivity.
6. SUMMARY In the future, it is certain that every major TL laboratory will routinely run studies of emission spectra, even for material that is being dated by existing methods. The examples of information revealed by the spectral studies suggest that the effort in introducing such sophistication is worthwhile and essential. There is no unique solution but the present article has indicated some of the problems that must be addressed. Acknowledgements-1 would particularly like to acknowledge my debt to Dr Paul Levy who introduced me to the study of TL and whose initial examples of TL spectral analysis offered a challenging standard and inspiration for all subsequent work.
REFERENCES Bailiff 1. K., Morris D. A. and Aitken M. J. (1977) A rapid scanning interference spectrometer: application to low level TL emission. J. Whys. E. IO, 1156-l 160. Bakas G. V. (1984) A new optical muhichannel analyser using a charge coupled device as detector for thermoluminescence emission measurements. Rad. Prorecr. Dosim. 9, 301-305.
Bctts D. S., Couturier L., Khyarat A. H., Luff B. J. and Townsend P. D. (1993) Temperature distribution in thermoluminescence experiments I: experimental results. J. Phys. D X,843-848; Temperature distribution in thermoluminescence experiments II: some cakulation models. J. Phys. D 26, 849-857. Harris A. M. and Jackson J. H. (1970) A rapid scanning spectrometer for the region ZOO-850 nm: application to thermoluminescent emission spectra. J. Phys. E 3, 374-376. Homyak W. F. and Franklin A. D. (1988) A second generation 3d-spectrophotometer for TL analysis. J. Lumin. 42, 89-96.
Jensen H. E. and Prescott J. R. (1982) A Fourier transform spectrometer for the measurement of thermoluminescence emission spectra. PACT 6, 542-549. Kirsh Y. (1992) Kinetic analysis of thermoluminescence. Phys. Slut. Sol. (a) 129, 15-48. Luff B. J. and Townsend P. D. (1993) High sensitivity thermoluminescence spectrometer. Meas. Sci. Technof. 4, 65-71.
Mattem P. L., Lengweiler K. and Levy P. W. (1971) Apparatus for the simultaneous determination of thermoluminescent intensity of spectral distribution. Mod. Geol. 2, 293-294; Three dimensional thermoluminescence analysis of minerals. Mod. Geol. 2, 295-297. Oczkowski H. L. (1992) Hadamard spectrometry for thermoluminescence investigations. Acru phys. Pol. A 82, 367-375. Piters T. M., Met&mans W. H. and Bos A. J. (1993) An automated research facility for measuring thermoluminescence emission spectra using an optical multichannel analyzer. Rev. Sci. Instrum. 64, 109-117. Prescott J. R., Fox P. J., Akber R. A. and Jensen H. E. (1988) Thermoluminescence emission spectrometer. Appl. Optics 27, 3496-3501.
Rasheedy M. S., Nishimura F. and Ichimori T. (1991) On the thermoluminescence emission spectra of CaF, : Tm. Nucl. Instrum. Meth. B61, 67-71. Rendell H. M., Townsend P. D., Luff B. J., Wintle A. G. and Balescu S. (1993) Spectral analysis of thermoluminescence in the dating of potassium feldspars. Phys. Stut. Sol. (II) 138, 335-341.
Townsend P. D. and Kirsh Y. (1989) Spectral measurement during thermoluminescenccan essential requirement. Contemp. Phys. 30, 337-354.