- Email: [email protected]
Tunnel afterglow, fading and infrared emission in thermoluminescence of feldspars
Radiarion Mm.wremm~s, Vol. 23, Nos 213, pp. 371-385, 1994 Copyright Q 1994 Else&r !ScicnceLtd Printedin Great Britain. All righta reserved 1350-4487/94 f7.00 + .oo
Pergamon
1350-4487(93)E4lo34-J
TUNNEL AFTERGLOW, FADING AND INFRARED EMISSION IN THERMOLUMINESCENCE OF FELDSPARS R. VISOCEKAS,* N. A. SmoNaa,t A.
ZINK*
and P. BLANCH
*LUAP,
UniversitC Paris 7, case 7087, 2 Place Jussieu, 75251 Paris Cedex 05 (F), France; TRLAHA, University of Oxford, 6 Keble Rd. Oxford OX1 3QJ, U.K.; and $Setvice MEB, UFR928, Universitk Pierre et Marie Curie, boite 104, 4 Place Jussieu, 75252 Paris Cedex 05 (F). France Abstract-Anomalous fading of TL, OSL and IRSL has been observed in many samples of feldspars and attributed to the tunnel effect. Investigations do show expected tunnel afterglow except for samples with no fading. Its intensity, quite noticeable at LNT, is in proportion with reported rate of fading. The emission is entirely in the red and infrared part of the spectrum. An important thermal quenching is observed. Low temperature storage results in fading of TL. Cathodoluminescence emission spectra, monitored from 200 to 900 nm, do confirm TL observations. They show two we&separated ranges: one, “blue”, from W to yellow (the only one observed in usual TL dating), displays various emission bands; the other, “IR”, red and infrared, shows a well-characterized narrow Gaussian emission band, with a maximum around 720nm. Models are proposed, relating fading with disorder in crystals. Tunnel afterglow appears as a good criterion of fading in feldspars.
1. INTRODUCTION THE USE of feldspars in TL and OSL dating, attractive due to their high luminescence and other qualities, is hampered by fading. In addition, it had been in the course of attempts to date feldspars, mostly plagioclases of volcanic origin (e.g. labradorite), that the phenomenon of “anomalous fading” was characterized. It had been attributed to a quantum mechanical phenomenon of tunnelling recombination between opposite trapped charges (Garlick and Robinson, 1972; Wintle, 1977). This model was confirmed by direct observation of tunnelling afterglow in labradorite (Visocekas, 1985). However, other feldspars, typically potassium feldspars of granitic origin, did not display TL fading and could be dated (Hiitt et al., 1993). Due to the wide variety of feldspars, the situation appeared, at best, confused, but dating of feldspars could seem not quite hopeless and at least worth some clarification (Aitken, 1985, pp. 181, 189). The recent advent of IRSL bore a hope of enabling to circumvent fading. Investigations of fading in feldspars were done by Spooner (1992, 1993, 1994), and were systematic. They included not only sediments but some 24 mineralogically well defined samples, the compositions of which are distributed all around the feldspar ternary system. The three methods of optical dating, i.e. TL, OSL (with green light) and IRSL, were used. Results were that fading was quite similar in the three methods, and that out
of these 24 samples, only four or five appeared not to fade. In the present work, the same samples are studied to see whether tunnelling afterglow is always associated with fading.
2. EXPERIMENTAL The equipment used is the same as described previously (Visocekas, 1993). Irradiations are made with a Sr-Y source, usually 240 Gy in 30 min. The temperature! of the sample may be fixed, or vary from 77 to 760 K, raised or lowered linearly. The heating rate used is the low value 10 K/min (0.16 K/s). Hence, glow peak temperatures measured here are lower by some 100-I 50°C than usual reported values. The photomultiplier used is a RCA 31034 with an AsGa photocathode with flat spectral sensitivity extending from 200 to 890 nm. It has to be cooled to -20°C. It is associated with various low pass Schott filters on a carousel. The filter OG 590 monitors only the emission from 590 to 890 nm, i.e. from red to IR (termed here for short: “IR”). Using the filter WG 305, what is transmitted on top of OG 590 is the emission from W to yellow, termed here “blue” emission. It must be noted that with PMTs commonly used in TL dosimetry, only this “blue” domain is detected, while mostly the “IR” emission is not.
377
.^
_^
Dated Dated Dated Dated Dated
sediment (Finland) id id id id
K-feldspar id id id id
HJ HJ HJ HJ HJ
K3 N4 R4 R3 R6
Yes ? (Not ? (Not ? (Not ? (Not ? (Not ? (Not ? (Not
BA #6 (Oxf.) Chaine Puys (F) Be=, Puys (F) id (gangue) Madagascar id Santa Fiora (I) Krupfer, Eifel (D) Leilenkopf, Eifel
Labradorhe Plagioclase? Sanidine Sanidine Microcline &those Sanidine Sanidine Sanidine
(NSl9) GV MCI MC2 MC3 MC4 (NS227) EM2 EM7 EM8
_^
(100 ka) (89 ka) (99 ka) (95 ka) (95 ka)
measured) measured) measured) measured) measured) measured) measured)
Yes (15%)
(8%)
(8%) (12%) too weak)
(12%)
No No Yes No Yes Yes (TL No Yes
Pennsylvania (U.S.A.) Conn. (U.S.A.). Silesia ST Gothard (CH) Crystal Bay (U.S.A.) Patmos (Greece) Alps (F.) Zimbabwe Sweden
Microcline Microcline Microcline Adulaire Sanidine Sanidine Albite Albite Oligoclase
NS13 NSS NS3 NSI NS9 NSl2 NS17 NSIS NS23
Fading (reported)
Origin
Mineral
Reference
-
Yes Yes Yes Yes Yes
_
Yes Yes Yes Yes Yes Yes Yes (weak) Yes Yes
Very weak No Yes No Yes Yes Yes No Yes (weak)
Tunnel afterglow
_
12% 13% 21% 15% 15%
$$; 31% 100% 7% 180% 280%
32% 100%
4% < 1% 230% < I% 150% 110% 7% <2% 6%
-
-.-
I% I% 2% I% I%
20%
5%
15% 8% 30% 30% 2.5% 8% 1%
0.3% 0.1% 18% < 0. I % 12% 9% I% <0.2% 1%
Rate of tunnelling Apparent Corrected
-
_
Important Relatively important Relatively important Relatively important Weak, typical The only band in CL Wide CL spectrum The main one The main one
Very weak, atypical Relatively important Not visible Relatively important Relatively important The only band in CL Minor Weak, atypical
Cathodohtminescence “IR” emission band
Table I. List of the samples studied with a summary of their main characteristics
350 350 350 370 360
II0 85 80 40 90
430 0.01 430 0.16 43&460 7 id 7 370 (No blue emiszn) 470 126 430 5.5 470 2
340 360 3: (No blue emission) 350 I40 470 9 440 (No blue emisi&r) 450 30 370480 0.75
“Blue” TL peak T (K) I (nA)
32 30 30 I2 35
IO I5 8 7
370 430-460 380 370 370 370 370 370 370
2.25 25 25 I8
100 4.5 5 0.002
:;
IOZ
0.45 7
370 380 380 370
370 360-460 350 420
370
370
350 340
“IR” TL peak T (K) I (nA)
%
g 2
2
#
P
FADING
AND IR TUNNELLING
The spectral sensitivity of the system monitored using a calibrated tungsten ribbon lamp shows a flat response up to a sharp cut-off at 890 nm as expected. A scanning electron microscope JEOL type JSM 840 A is used. This equipment enables X-ray microanalysis of the samples. Cathodoluminescence spectra are measured as well using a Jobin Yvon H 10 UV monochromator attached to the SEM, with a low resolution, but good luminosity. A PMT Hamamatsu type R 636 is used, with a spectral sensitivity up to 900 nm. The spectral response of the system was also calibrated with a tungsten filament lamp. Other equipment (at University of Latvia) for cathodoluminescence at 80 K, photoluminescence spectra, X-ray luminescence, and fractional glow have also been used for measurements on a crystal of sanidine.
3. SAMPLE3 The samples studied are shown in Table 1. Our investigations were started with the feidspars (referenced NS 13,5,3,1,9,12,17,15,23 in the order from potassium feldspars to plagioclases) the fading of which was studied by Spooner for storage times from 200 s to one year. The great majority of them displayed anomalous fading with the logarithmic law of decay characteristic of the tunnel effect (Spooner, 1992.1993.1994) with a rate of fading in percent per decade as shown in Table 1. To these samples were added some others known to fade such as NS 19, labradorite BA #6, ah-eady studied by Wintle (1977) and Visocekas (1985). Another sample (“GV”) is a plagioclase from lava studied by Valladas ef al. (1979).
379
IN FELDSPARS
A second set of samples (referenced MC 1,2,3,4; EM 2, 7, 8) are mineralogically well-characterized feldspars, mostly sanidines from various volcanic areas, obtained from several mineralogical collections. Their fading was not monitored. A third distinctive set (referenced as HJ K3, N4, R4, R3, R6) are sediments from western Finland, alkali feldspars considered to be microclines, given to us by G. Hiitt. These have been studied and dated with ages over 100 ka by Hiitt et al. (1993). hence presumed not to fade appreciably.
4. TUNNEL AFI’ERGLOW
AND FADING
Investigations for tunnel afterglow are run as in previous studies (Visocekas, 1985, 1988, 1993). The intensity of afterglow monitored during such an experiment is shown in Fig. 1. After room temperature irradiation, the sample is promptly set into the TL apparatus and the intensity I(t) of emission is monitored. First, I(r) is the plain RT phosphorescence. Then, the temperature is lowered progressively, down to LNT. A classical thermally stimulated intensity is expected to subside down exponentially to zero, that is noise level, at around 250 or 230 K. With feldspars, 1(r) is observed to settle down to a lower but nonzero level around 250 K. This is the first classical feature of a tunnel process. Then, as temperature goes down to 80 K, Z(r) is observed to increase all the way. This is a new, paradoxical effect, not reported so far. Prolonged afterglow at 80 K shows 1(r ) keeping to the kinetics characteristic of tunnel afterglow (Visocekas, 1979, 1988): 1(f) = C In(1 + (O/t)) Tcmpemlure
FIQ. 1. Typical afterglow process for a feldspar (Patmos Sanidinc). Lower scale shows time running from end of 30 min beta irradiation. Higher scale is the corrcaponding programmed temperature. “Blue” is the emission from 305 to 590 nm. “Red-IR” is the emission from 590 to 890 nm.
(1)
R. VISOCEKAS
380
(within an accuracy of a few percent). After some time, I(t) may be approximated by: I(r) = A/t
(2)
(where t is the time since end of irradiation; C is a constant; fJ the duration of irradiation, here usually 30min; A = C-0). After some time, the (final) linear heating commences: first, the same paradoxical effect is observed, the decrease of the intensity as temperature is raised between 80 and 250K. This is a type of thermal quenching (Curie, 1963). Around 250 K, the proper TL glow curves show up, all the way to the tem$eratures, (around 400-500 K), where thermal emission dominates the total emission. Considering now the emission spectra, it is observed that the low temperature tunnelling afterglow is exclusively in the “IR” spectral domain, that is entirely transmitted through the OG 590 filter. Filters transmitting also visible “blue” light do not transmit more light. It is only when TL proper starts that some light is emitted in the “blue” range, i.e. likely to be detected with usual TLD equipment. “IR” emission is observed simultaneously. The two emissions have different glow peaks (this was already observed with labradorite; Visocekas, 1985). The above mentioned statements must be qualified to some degree: plagioclases show more erratic behaviour and some samples have practically no “blue” TL emission (while displaying “IR” TL emission). Observations such as those of Fig. 1 are made with all the feldspars under study, with an important exception: no tunnelling afterglow is detected, and it is with those few feldspars which were reported nor to fade (see Table 1). Conversely, we have observed no case of feldspar reported to display fading which does not display noticeable tunnelling.
er al. 5. RATE OF FADING
Further questions are raised when the rate of fading is considered. There is the case of sediments (at the end of Table l), which have been dated to some 100 ka, hence should not fade, and yet display quite noticeable tunnelling at LNT. A similar question is raised by looking at Fig. 1. The intensity of tunnel afterglow is very important as compared with TL proper, to the extent that, for further experiments with extended storage at LNT, the light-sum of tunnel emission exceeds by far the light-sum of TL alone, and after that decay, the TL light-sum appears as only slightly faded. Now, the accepted relation between tunnel afterglow and fading of TL, backed by many observations, is that the same traps are involved in either process. Hence, the light emitted by tunnelling is so much lost to TL proper. In the above cases, TL should have faded completely. We explain this contradiction by the thermal quenching effect as follows. As expected from theory and as we have observed it up to now in various materials, tunnel afterglow is strictly “athermal”, that is temperature independent. Now, it is no more the case with feldspars, as we saw in Fig. 1. This is further shown in Fig. 2, showing intensity of tunnel afterglows of feldspars monitored from 80 to some 300 K to display a thermal quenching by a factor like 12. We hypothesize that this quenching is also at work in TL proper. A given trap, when emptied in TL has a luminescent efficiency much lower than when it gets emptied by tunnelling. This leads us to propose in Table 1 two figures for the rate of tunnelling per decade, as defined in
FIG. 2. Thermal quenching. Intensities of tunnel afterglows vs temperature for six different experiments of heating up or down, normalized at 80 K.
FADING
AND IR TUNNELLING
' 150
' 200
381
is a sensitive criterion for fading in feldspars, as it is already observable with only deep traps filled, such as in natural samples.
Visocekas (1985). The “apparent” one based as usual on afterglow measurements reaches some impressive values, like 400%, corresponding in principle to complete TL fading in a matter of hours. The “corrected” one is reduced by the thermal quenching factor. Its values are much nearer to the reported observed TL fading rate, to which it is expected theoretically to be equal. Such is the case for the dated sediments shown at the end of Table 1: the corrected values of rate of tunnelling, from 1 to 2%, do correspond to a negligible fading of TL. Figure 3 shows afterglows of one of these sediments, the sample “HJ K3” starting from LNT. The first is the afterglow of the natural sample. The second is the afterglow of the natural sample after a preheat at 420 K: TLN is hardly depleted, a weak tunnel afterglow is now visible at lower temperatures. The third is the artificial TL after irradiation at room temperature (like Fig. 1). The first part, from 100 to 250 K, is tunnel afterglow showing the thermal quenching effect. Though it is quite intense, it corresponds to the low 1% corrected rate of tunnelling shown in Table 1. The last curve shows afterglow of the same irradiated sample after preheat to 420 K leaving only the deep stable traps filled: they give rise, nevertheless, to an important lower temperature tunnel afterglow. These are various cases of tunnel afterglow in a “non-fading” sample. It shows how, due to retrapping effects, preheat may result in shorter-life traps being refilled (while preheat is currently expected to empty all “shallow” traps). It shows mainly how IR tunnel afterglow
Oswl 100
IN FELDSPARS
6. CATHODOL UMINESCENCE AND EMISSION BANDS Up until now, “blue” and “IR” spectral domains have been defined somewhat arbitrarily by using the RG 590 filter. More details on the spectral distribution of emission may be expected from cathodoluminescence (CL’) measurements. Of course, CL and TL spectra are not identical, but they are comparable. CL shows all possible emissions which includes TL. CL spectra have been measured at RT with all samples. Some of these are shown in Fig. 4. Figure 4(a) shows CL spectra for sanidines, all known to display important tunnel afterglows, and fading when it is measured (see Table 1). It shows narrow “IR” bands of emission nearly identical for all samples with a well-defined maximum around 720 nm (1.72 eV), a Gaussian shape, with a width of some 0.30 eV. Such “IR” bands are observed as well in spectra of most other fading feldspars not reported here, including some feldspars reported as having a weak TL. Figure 4(b) shows CL spectra of the few nonfading, non-tunnelling samples. They are remarkable by the complete absence of “IR” emission. Figure 4(c) shows spectra of some dated “HJ” sediments, where a small tunnel rate is measured as seen above. It is observed that their emission in the “IR” band is rather weak as compared with their emission in the “blue” range, but always centred on
I
1
'. 250
'. 300
I
I
' 350
'. 400
'. 450
' 500
I 550
FIG. 3. Natural TL and afterglows starting from LNT after RT irradiation and preheats for a K-feldspar Finland sediment (“HJ K?“) dated around 100 ka. showing low temperature thermal quenching and TL peaks.
tunnel afterglows with
R. VISOCEKAS
(4
et al.
_ ._ i 160 i
,Y
_-200
-
EM7Kn1pkr
-‘--c--,
NS Q 215 Cryad S.
--+_
EMSLou9nkopt MC1 S99m
-----w---,
_ __
300
400
500
600
NS ,*
paw
700 i
900
900
(nm)
(W -
15,o
0.0 200
300
400
500
NS15Abite
600
700 k
Cc)
900
900
600
900
Mm)
5.0 : =
-
4.0
200
300
400
HJF63(K)
I
I
600
600
Idaa
700 A
(nm)
FIG. 4. Cathodolummescence spectra of some feldspars. (a) Sanidines, known to display tunnel afterglow and fadmg. with normalization of maxima of JR bands. (b) Feldspars known to display neither fading nor tunnel afterglow (sanidine EM7 shown for comparison), with normalisation of total emission. (c) Grains of feldspar sediments from Finland, dated, with low tunnel afterglow; three are K-feldspars, one is mixed K-Na.
FADING
AND IR TUNNELLING
72Onm. The “blue” spectra are in agreement with reported TL spectra of these same samples (Jungner and Huntley, 1991). The main conclusions derived from these CL spectra are that in the “IR” domain, there is a physically well-defined IR band of emission common to all feldspars, and that it is well-separated from other emissions in the so-called “blue” domain. The “blue” emissions appear as much more varied. These conclusions apply to TL emissions: “blue” emissions and “IR” emissions such as those isolated by the filter RG 590 are physically quite distinct. Figure 5 gathers some TL curves in these two bands for two main groups of alkali feldspars: the first is made up of sanidines, and the second, of the dated “HJ” sediments. In the “blue” band, the temperatures of the peaks as well as their amplitudes are different between the two groups (see also Table 1). The stable TL corresponds to more intense “blue” peaks, with lower temperatures. In the “IR” band, the glow peaks are more similar for the two groups in amplitudes as well as temperatures while being quite distinct from the “blue” peaks.
0 250
300
350
400
450
500
T(K)
1(W 20
0
z50
300
350
400
450T($O0
FIG. 5. TL gIow curves for two main groups of alkali feldspars. Sanidines (broken lines) and K-feldspar sediments (solid lines). Top: TL in the “blue” band; bottom: TL in the “IR” band.
383
IN FELDSPARS .
12
w
.
‘0
g
1c -
1.2 -
1.0 -
0.0 -
02
0.4
02
0
A 0.0L zoo
300
400
PabmrIR
-
QY--YIR 000
100 f(K)
FIG. 6. Activation energies of traps vs temperature for several samples of sanidines and a microcline, obtained by fractional glow (“F.Glow”) or by successive initial rises with separation of emission bands.
Moreover, the respective intensities of “blue” and “IR” TL peaks (see also in the last columns of Table 1) correspond nicely with observations on CL spectra. CL is seen to provide good information on TL. The activation energy of traps has been measured for some alkali feldspars, sanidines and microclines by different methods, either by fractional glow, with monitoring of the whole emission spectrum, or by successive initial rises, with separation of bands with filters (see Fig. 6). They give comparable results, qualitatively and numerically. One feature is the absence of any marked trap level. This corresponds, in our view, to TL processes being of the “thermally stimulated tunnel” type, in accordance with other observations, like the presence of tunnelling. Another feature is that blue emissions call for higher activation energy, further evidence of the difference between the two bands. These observations being completed, it could he objected that if there are actually two quite independent systems of traps-plus-centres, “IR” and “blue”, the reported fading of “blue” TL could not be accounted for by “IR” tunnelling. This objection is contradicted by the correlation shown in Table 1 between reported fading and observed tunnelling. This is further contradicted by some experiments. With sanidines stored after irradiation for days or weeks at RT or in liquid nitrogen, not only the blue TL peak is observed to decrease as well as the “IR” one, but it does so more quickly. There is clearly a link between the two bands.
R. VISOCEKAS 7. TUNNEL EMISSION AND CRYSTAL DISORDER “Blue” TL displays a great diversity between the samples, and it is considered that impurities play a major part in these emissions. In contrast, the “IR” emission in TL is quite similar among feldspars, especially among alkali feldspars: same spectra, with a nearly Gaussian shape of band; same peak temperatures (370 K in our experiments). The amplitude varies little from one sample to the next (see Table 1) while the percentage of Fe or other impurities (Eu.. .), is shown by X-ray analysis to be quite variable. All of this leads us to preclude the exclusive role of such impurities in the IR emission as proposed in many papers (Kirsh and Townsend, 1988, etc.). We propose that the IR emission we observe in TL and mostly in tunnelling (that is due to donor-acceptor recombination) is dependent on alkali feldspar crystal structure, as follows. Alkali feldspars are made up of a quartz-like SiO, lattice where one Si4+ out of four is substituted by A13+. Positive ions, like Na+ and K+ (or Ca*+ for plagioclases with a different ratio of AI/Si) enter interstitially into the lattice to ensure electrical neutrality (Barth, 1969). If one Al’+ substitutes for Si4+ in a crystalline site, this site will become an acceptor, i.e. a trap for a hole. Conversely, the Si4+ which will substitute in a site of A13+, will become a donor, i.e. a trap for an electron as shown by Marfunin (1979). We propose tentatively that such D-A pairs, or trap pairs, are the main sources for the observed IR tunnel and TL, while impurities could act only as recombination centres. This exchange of positions between Si and Al atoms from an “ordered” feldspar crystal lattice into a “disordered” one is not wishful thinking. It is a basic feature in crystallography of feldspars (Barth, 1969). For example, “high temperature” feldspars are more disordered than “low temperature” feldspars, sanidines are considered as much more disordered than microclines. etc. The relationship between lavas (and other “disordered” feldspars) and fading on one hand, of microcline (and “ordered” feldspars) and stability of TL on the other had been pointed out previously (Aitken, propose relation.
1985, p. 189). The microscopic model we may be a “missing link” explaining this
8. CONCLUSION
The presence of some IR emission in TL of feldspars as well as the existence of two distinct spectral ranges reported here is not exactly new (Andersson et al., 1991; Kirsh and Townsend, 1988; Huntley er al., 1988). However, the clear Gaussian character of this IR band was not reported before the
Krems Conference (Bos er al.. 1994).
et al.
Tunnel emission in the same IR band had been reported before only for labradorite and sanidine (Visocekas, 1985, 1993). Now, it is observed as a general feature for most feldspars. Its close association with anomalous fading is shown and a relation with crystal disorder is proposed. A practical conclusion is that tunnel monitoring provides a means of checking fading in feldspars which compares favorably with the direct measurement of stored TL vs time, at the double point of view of sensitivity and rapidity of evaluation. Cathodoluminescence spectra and their amplitude in the IR domain are also a valuable test for possible fading. There may exist some non-fading component in TL of feldspars, such as proposed in the localized transition model. A disappointing conclusion of this study is a confirmation that most feldspars do fade somehow, as they show tunnel afterglow. Acknowledgements-We
thank M. J. Aitken, G. Hiitt, I. Tale, V. Tale, M. Springis, P. Kulis, Y. Jansson (U. of Latvia); M. Chayt d’Albissin, G. Marinello, G. Valladas, M. Ouchtne, C. Bourquart, M. Nunez, M. Rambourg for direct contributions to this work. Samples initialled ‘MC” were given by Collection de Mineralogie of UniversitC P.M. Curie (M. Bariand) and “EM” given by Mu&e de I’Ecole Nationale Sup&ieure des Mines de Paris (M. Phan). A sabbatical leave from Universid Paris 7 supported one of us (RV).
REFERENCES Aitken M. J. (1985) Thermoluminescence Darina. Studies in Archaeological Science. Academic Press, London. Andersson M.. Rendell H. M. and Townsend P. D. (1991) Low temperature spectra of TL from sedimentary quartz and feldspars. Nucl. Tracks Radial. Meas. 18, 41-43. Barth T. F. M. (1969) Felabpars. J. Wiley and Sons, London. Bos A. J. J., Piters T. M. and Ypma P. J. (1994) Thermoluminescence emission spectra and optical bleaching of oligoclase. Radiat. Meas. 23. 349-353. Curie D. (1963) Luminescence in Crystals. Methuen and Co., London. Garlick G. F. J. and Robinson 1. (1972) The TL of lunar samples. In The Moon (eds Runcom S. K. and Urey H.), pp. 324-329. I.A.U. Huntley D. J., Godfrey-Smith D. I., Thewalt M. L. W. and Berger G. W. (1988) Thermoluminescence spectra of some mineral samples relevant to thermoluminescence dating. J. Lumine&ence 39, 123-136. Hiitt G.. Junaner H.. Kuiansu R. and Saarnisto M. (1993) OSL and TL dating of buried podsols and overlying sands in Ostrobothnia, western Finland. J. Quat. Sci. 8, 125-132. Jungner H. and Huntley D. J. (1991) Emission spectra of some potassium feldspars under 633 nm stimulation. Nucl. Tracks Radial. Meas. 18, 125-126. Kirsh Y. and Townsend P. D. (1988) Speculation on the blue and red bands in the TL emission spectrum of albite and microcline. Nucl. Tracks Radial. Meas. 14, 43-49.
A. S. (1979) Spectroscopy, Luminescence Radiation Centers in Minerals. Springer, Berlin.
Marfunin
and
Spooner N. A. (1992) Optical dating: preliminary results on the anomalous fading of luminescence from feldspars. Qua/. Sci. Rerl. 10, 139-145.
FADING
AND IR TUNNELLING
Spooner N. A. (1993) The validity of optical dating based on feldspars. Ph.D. thesis, University of Oxford. Spooner N. A. (1994) The anomalous fading of infraredstimulated luminescence from feldspars. Radiur. Meus. 23, 625-632.
Valladas G., Gillot P. V. and Guerin G. (1979) Dating plagioclases? PACT 3, 251-257. Visocekas R. (1979) Miscellaneous aspects of artificial TL of calcite: emission spectra, athermal detrapping and anomalous fading. PACT 3, 258-265. Visocekas R. (1985) Tunnelling radiative recombination in
IN FELDSPARS
385
labradorite: its association with anomalous fading of TL. Nucl. Tracks 10, 521-529. Visocekas R. (1988) Comparison between tunnelling afterglows following alpha or beta irradiations. Nucf. Tracks Radial. Meas. 14, 163-168. Visocekas R. (1993) Tunnelling radiative recombination in K-feldspar Sanidine. Nucl. Tracks Radiar. Meas. 21, 175-178. Wintle A. G. (1977) Detailed study of a thermoluminescent mineral exhibiting anomalous fading. J. Luminescence 15, 385-393.
Pergamon
1350-4487(93)E4lo34-J
TUNNEL AFTERGLOW, FADING AND INFRARED EMISSION IN THERMOLUMINESCENCE OF FELDSPARS R. VISOCEKAS,* N. A. SmoNaa,t A.
ZINK*
and P. BLANCH
*LUAP,
UniversitC Paris 7, case 7087, 2 Place Jussieu, 75251 Paris Cedex 05 (F), France; TRLAHA, University of Oxford, 6 Keble Rd. Oxford OX1 3QJ, U.K.; and $Setvice MEB, UFR928, Universitk Pierre et Marie Curie, boite 104, 4 Place Jussieu, 75252 Paris Cedex 05 (F). France Abstract-Anomalous fading of TL, OSL and IRSL has been observed in many samples of feldspars and attributed to the tunnel effect. Investigations do show expected tunnel afterglow except for samples with no fading. Its intensity, quite noticeable at LNT, is in proportion with reported rate of fading. The emission is entirely in the red and infrared part of the spectrum. An important thermal quenching is observed. Low temperature storage results in fading of TL. Cathodoluminescence emission spectra, monitored from 200 to 900 nm, do confirm TL observations. They show two we&separated ranges: one, “blue”, from W to yellow (the only one observed in usual TL dating), displays various emission bands; the other, “IR”, red and infrared, shows a well-characterized narrow Gaussian emission band, with a maximum around 720nm. Models are proposed, relating fading with disorder in crystals. Tunnel afterglow appears as a good criterion of fading in feldspars.
1. INTRODUCTION THE USE of feldspars in TL and OSL dating, attractive due to their high luminescence and other qualities, is hampered by fading. In addition, it had been in the course of attempts to date feldspars, mostly plagioclases of volcanic origin (e.g. labradorite), that the phenomenon of “anomalous fading” was characterized. It had been attributed to a quantum mechanical phenomenon of tunnelling recombination between opposite trapped charges (Garlick and Robinson, 1972; Wintle, 1977). This model was confirmed by direct observation of tunnelling afterglow in labradorite (Visocekas, 1985). However, other feldspars, typically potassium feldspars of granitic origin, did not display TL fading and could be dated (Hiitt et al., 1993). Due to the wide variety of feldspars, the situation appeared, at best, confused, but dating of feldspars could seem not quite hopeless and at least worth some clarification (Aitken, 1985, pp. 181, 189). The recent advent of IRSL bore a hope of enabling to circumvent fading. Investigations of fading in feldspars were done by Spooner (1992, 1993, 1994), and were systematic. They included not only sediments but some 24 mineralogically well defined samples, the compositions of which are distributed all around the feldspar ternary system. The three methods of optical dating, i.e. TL, OSL (with green light) and IRSL, were used. Results were that fading was quite similar in the three methods, and that out
of these 24 samples, only four or five appeared not to fade. In the present work, the same samples are studied to see whether tunnelling afterglow is always associated with fading.
2. EXPERIMENTAL The equipment used is the same as described previously (Visocekas, 1993). Irradiations are made with a Sr-Y source, usually 240 Gy in 30 min. The temperature! of the sample may be fixed, or vary from 77 to 760 K, raised or lowered linearly. The heating rate used is the low value 10 K/min (0.16 K/s). Hence, glow peak temperatures measured here are lower by some 100-I 50°C than usual reported values. The photomultiplier used is a RCA 31034 with an AsGa photocathode with flat spectral sensitivity extending from 200 to 890 nm. It has to be cooled to -20°C. It is associated with various low pass Schott filters on a carousel. The filter OG 590 monitors only the emission from 590 to 890 nm, i.e. from red to IR (termed here for short: “IR”). Using the filter WG 305, what is transmitted on top of OG 590 is the emission from W to yellow, termed here “blue” emission. It must be noted that with PMTs commonly used in TL dosimetry, only this “blue” domain is detected, while mostly the “IR” emission is not.
377
.^
_^
Dated Dated Dated Dated Dated
sediment (Finland) id id id id
K-feldspar id id id id
HJ HJ HJ HJ HJ
K3 N4 R4 R3 R6
Yes ? (Not ? (Not ? (Not ? (Not ? (Not ? (Not ? (Not
BA #6 (Oxf.) Chaine Puys (F) Be=, Puys (F) id (gangue) Madagascar id Santa Fiora (I) Krupfer, Eifel (D) Leilenkopf, Eifel
Labradorhe Plagioclase? Sanidine Sanidine Microcline &those Sanidine Sanidine Sanidine
(NSl9) GV MCI MC2 MC3 MC4 (NS227) EM2 EM7 EM8
_^
(100 ka) (89 ka) (99 ka) (95 ka) (95 ka)
measured) measured) measured) measured) measured) measured) measured)
Yes (15%)
(8%)
(8%) (12%) too weak)
(12%)
No No Yes No Yes Yes (TL No Yes
Pennsylvania (U.S.A.) Conn. (U.S.A.). Silesia ST Gothard (CH) Crystal Bay (U.S.A.) Patmos (Greece) Alps (F.) Zimbabwe Sweden
Microcline Microcline Microcline Adulaire Sanidine Sanidine Albite Albite Oligoclase
NS13 NSS NS3 NSI NS9 NSl2 NS17 NSIS NS23
Fading (reported)
Origin
Mineral
Reference
-
Yes Yes Yes Yes Yes
_
Yes Yes Yes Yes Yes Yes Yes (weak) Yes Yes
Very weak No Yes No Yes Yes Yes No Yes (weak)
Tunnel afterglow
_
12% 13% 21% 15% 15%
$$; 31% 100% 7% 180% 280%
32% 100%
4% < 1% 230% < I% 150% 110% 7% <2% 6%
-
-.-
I% I% 2% I% I%
20%
5%
15% 8% 30% 30% 2.5% 8% 1%
0.3% 0.1% 18% < 0. I % 12% 9% I% <0.2% 1%
Rate of tunnelling Apparent Corrected
-
_
Important Relatively important Relatively important Relatively important Weak, typical The only band in CL Wide CL spectrum The main one The main one
Very weak, atypical Relatively important Not visible Relatively important Relatively important The only band in CL Minor Weak, atypical
Cathodohtminescence “IR” emission band
Table I. List of the samples studied with a summary of their main characteristics
350 350 350 370 360
II0 85 80 40 90
430 0.01 430 0.16 43&460 7 id 7 370 (No blue emiszn) 470 126 430 5.5 470 2
340 360 3: (No blue emission) 350 I40 470 9 440 (No blue emisi&r) 450 30 370480 0.75
“Blue” TL peak T (K) I (nA)
32 30 30 I2 35
IO I5 8 7
370 430-460 380 370 370 370 370 370 370
2.25 25 25 I8
100 4.5 5 0.002
:;
IOZ
0.45 7
370 380 380 370
370 360-460 350 420
370
370
350 340
“IR” TL peak T (K) I (nA)
%
g 2
2
#
P
FADING
AND IR TUNNELLING
The spectral sensitivity of the system monitored using a calibrated tungsten ribbon lamp shows a flat response up to a sharp cut-off at 890 nm as expected. A scanning electron microscope JEOL type JSM 840 A is used. This equipment enables X-ray microanalysis of the samples. Cathodoluminescence spectra are measured as well using a Jobin Yvon H 10 UV monochromator attached to the SEM, with a low resolution, but good luminosity. A PMT Hamamatsu type R 636 is used, with a spectral sensitivity up to 900 nm. The spectral response of the system was also calibrated with a tungsten filament lamp. Other equipment (at University of Latvia) for cathodoluminescence at 80 K, photoluminescence spectra, X-ray luminescence, and fractional glow have also been used for measurements on a crystal of sanidine.
3. SAMPLE3 The samples studied are shown in Table 1. Our investigations were started with the feidspars (referenced NS 13,5,3,1,9,12,17,15,23 in the order from potassium feldspars to plagioclases) the fading of which was studied by Spooner for storage times from 200 s to one year. The great majority of them displayed anomalous fading with the logarithmic law of decay characteristic of the tunnel effect (Spooner, 1992.1993.1994) with a rate of fading in percent per decade as shown in Table 1. To these samples were added some others known to fade such as NS 19, labradorite BA #6, ah-eady studied by Wintle (1977) and Visocekas (1985). Another sample (“GV”) is a plagioclase from lava studied by Valladas ef al. (1979).
379
IN FELDSPARS
A second set of samples (referenced MC 1,2,3,4; EM 2, 7, 8) are mineralogically well-characterized feldspars, mostly sanidines from various volcanic areas, obtained from several mineralogical collections. Their fading was not monitored. A third distinctive set (referenced as HJ K3, N4, R4, R3, R6) are sediments from western Finland, alkali feldspars considered to be microclines, given to us by G. Hiitt. These have been studied and dated with ages over 100 ka by Hiitt et al. (1993). hence presumed not to fade appreciably.
4. TUNNEL AFI’ERGLOW
AND FADING
Investigations for tunnel afterglow are run as in previous studies (Visocekas, 1985, 1988, 1993). The intensity of afterglow monitored during such an experiment is shown in Fig. 1. After room temperature irradiation, the sample is promptly set into the TL apparatus and the intensity I(t) of emission is monitored. First, I(r) is the plain RT phosphorescence. Then, the temperature is lowered progressively, down to LNT. A classical thermally stimulated intensity is expected to subside down exponentially to zero, that is noise level, at around 250 or 230 K. With feldspars, 1(r) is observed to settle down to a lower but nonzero level around 250 K. This is the first classical feature of a tunnel process. Then, as temperature goes down to 80 K, Z(r) is observed to increase all the way. This is a new, paradoxical effect, not reported so far. Prolonged afterglow at 80 K shows 1(r ) keeping to the kinetics characteristic of tunnel afterglow (Visocekas, 1979, 1988): 1(f) = C In(1 + (O/t)) Tcmpemlure
FIQ. 1. Typical afterglow process for a feldspar (Patmos Sanidinc). Lower scale shows time running from end of 30 min beta irradiation. Higher scale is the corrcaponding programmed temperature. “Blue” is the emission from 305 to 590 nm. “Red-IR” is the emission from 590 to 890 nm.
(1)
R. VISOCEKAS
380
(within an accuracy of a few percent). After some time, I(t) may be approximated by: I(r) = A/t
(2)
(where t is the time since end of irradiation; C is a constant; fJ the duration of irradiation, here usually 30min; A = C-0). After some time, the (final) linear heating commences: first, the same paradoxical effect is observed, the decrease of the intensity as temperature is raised between 80 and 250K. This is a type of thermal quenching (Curie, 1963). Around 250 K, the proper TL glow curves show up, all the way to the tem$eratures, (around 400-500 K), where thermal emission dominates the total emission. Considering now the emission spectra, it is observed that the low temperature tunnelling afterglow is exclusively in the “IR” spectral domain, that is entirely transmitted through the OG 590 filter. Filters transmitting also visible “blue” light do not transmit more light. It is only when TL proper starts that some light is emitted in the “blue” range, i.e. likely to be detected with usual TLD equipment. “IR” emission is observed simultaneously. The two emissions have different glow peaks (this was already observed with labradorite; Visocekas, 1985). The above mentioned statements must be qualified to some degree: plagioclases show more erratic behaviour and some samples have practically no “blue” TL emission (while displaying “IR” TL emission). Observations such as those of Fig. 1 are made with all the feldspars under study, with an important exception: no tunnelling afterglow is detected, and it is with those few feldspars which were reported nor to fade (see Table 1). Conversely, we have observed no case of feldspar reported to display fading which does not display noticeable tunnelling.
er al. 5. RATE OF FADING
Further questions are raised when the rate of fading is considered. There is the case of sediments (at the end of Table l), which have been dated to some 100 ka, hence should not fade, and yet display quite noticeable tunnelling at LNT. A similar question is raised by looking at Fig. 1. The intensity of tunnel afterglow is very important as compared with TL proper, to the extent that, for further experiments with extended storage at LNT, the light-sum of tunnel emission exceeds by far the light-sum of TL alone, and after that decay, the TL light-sum appears as only slightly faded. Now, the accepted relation between tunnel afterglow and fading of TL, backed by many observations, is that the same traps are involved in either process. Hence, the light emitted by tunnelling is so much lost to TL proper. In the above cases, TL should have faded completely. We explain this contradiction by the thermal quenching effect as follows. As expected from theory and as we have observed it up to now in various materials, tunnel afterglow is strictly “athermal”, that is temperature independent. Now, it is no more the case with feldspars, as we saw in Fig. 1. This is further shown in Fig. 2, showing intensity of tunnel afterglows of feldspars monitored from 80 to some 300 K to display a thermal quenching by a factor like 12. We hypothesize that this quenching is also at work in TL proper. A given trap, when emptied in TL has a luminescent efficiency much lower than when it gets emptied by tunnelling. This leads us to propose in Table 1 two figures for the rate of tunnelling per decade, as defined in
FIG. 2. Thermal quenching. Intensities of tunnel afterglows vs temperature for six different experiments of heating up or down, normalized at 80 K.
FADING
AND IR TUNNELLING
' 150
' 200
381
is a sensitive criterion for fading in feldspars, as it is already observable with only deep traps filled, such as in natural samples.
Visocekas (1985). The “apparent” one based as usual on afterglow measurements reaches some impressive values, like 400%, corresponding in principle to complete TL fading in a matter of hours. The “corrected” one is reduced by the thermal quenching factor. Its values are much nearer to the reported observed TL fading rate, to which it is expected theoretically to be equal. Such is the case for the dated sediments shown at the end of Table 1: the corrected values of rate of tunnelling, from 1 to 2%, do correspond to a negligible fading of TL. Figure 3 shows afterglows of one of these sediments, the sample “HJ K3” starting from LNT. The first is the afterglow of the natural sample. The second is the afterglow of the natural sample after a preheat at 420 K: TLN is hardly depleted, a weak tunnel afterglow is now visible at lower temperatures. The third is the artificial TL after irradiation at room temperature (like Fig. 1). The first part, from 100 to 250 K, is tunnel afterglow showing the thermal quenching effect. Though it is quite intense, it corresponds to the low 1% corrected rate of tunnelling shown in Table 1. The last curve shows afterglow of the same irradiated sample after preheat to 420 K leaving only the deep stable traps filled: they give rise, nevertheless, to an important lower temperature tunnel afterglow. These are various cases of tunnel afterglow in a “non-fading” sample. It shows how, due to retrapping effects, preheat may result in shorter-life traps being refilled (while preheat is currently expected to empty all “shallow” traps). It shows mainly how IR tunnel afterglow
Oswl 100
IN FELDSPARS
6. CATHODOL UMINESCENCE AND EMISSION BANDS Up until now, “blue” and “IR” spectral domains have been defined somewhat arbitrarily by using the RG 590 filter. More details on the spectral distribution of emission may be expected from cathodoluminescence (CL’) measurements. Of course, CL and TL spectra are not identical, but they are comparable. CL shows all possible emissions which includes TL. CL spectra have been measured at RT with all samples. Some of these are shown in Fig. 4. Figure 4(a) shows CL spectra for sanidines, all known to display important tunnel afterglows, and fading when it is measured (see Table 1). It shows narrow “IR” bands of emission nearly identical for all samples with a well-defined maximum around 720 nm (1.72 eV), a Gaussian shape, with a width of some 0.30 eV. Such “IR” bands are observed as well in spectra of most other fading feldspars not reported here, including some feldspars reported as having a weak TL. Figure 4(b) shows CL spectra of the few nonfading, non-tunnelling samples. They are remarkable by the complete absence of “IR” emission. Figure 4(c) shows spectra of some dated “HJ” sediments, where a small tunnel rate is measured as seen above. It is observed that their emission in the “IR” band is rather weak as compared with their emission in the “blue” range, but always centred on
I
1
'. 250
'. 300
I
I
' 350
'. 400
'. 450
' 500
I 550
FIG. 3. Natural TL and afterglows starting from LNT after RT irradiation and preheats for a K-feldspar Finland sediment (“HJ K?“) dated around 100 ka. showing low temperature thermal quenching and TL peaks.
tunnel afterglows with
R. VISOCEKAS
(4
et al.
_ ._ i 160 i
,Y
_-200
-
EM7Kn1pkr
-‘--c--,
NS Q 215 Cryad S.
--+_
EMSLou9nkopt MC1 S99m
-----w---,
_ __
300
400
500
600
NS ,*
paw
700 i
900
900
(nm)
(W -
15,o
0.0 200
300
400
500
NS15Abite
600
700 k
Cc)
900
900
600
900
Mm)
5.0 : =
-
4.0
200
300
400
HJF63(K)
I
I
600
600
Idaa
700 A
(nm)
FIG. 4. Cathodolummescence spectra of some feldspars. (a) Sanidines, known to display tunnel afterglow and fadmg. with normalization of maxima of JR bands. (b) Feldspars known to display neither fading nor tunnel afterglow (sanidine EM7 shown for comparison), with normalisation of total emission. (c) Grains of feldspar sediments from Finland, dated, with low tunnel afterglow; three are K-feldspars, one is mixed K-Na.
FADING
AND IR TUNNELLING
72Onm. The “blue” spectra are in agreement with reported TL spectra of these same samples (Jungner and Huntley, 1991). The main conclusions derived from these CL spectra are that in the “IR” domain, there is a physically well-defined IR band of emission common to all feldspars, and that it is well-separated from other emissions in the so-called “blue” domain. The “blue” emissions appear as much more varied. These conclusions apply to TL emissions: “blue” emissions and “IR” emissions such as those isolated by the filter RG 590 are physically quite distinct. Figure 5 gathers some TL curves in these two bands for two main groups of alkali feldspars: the first is made up of sanidines, and the second, of the dated “HJ” sediments. In the “blue” band, the temperatures of the peaks as well as their amplitudes are different between the two groups (see also Table 1). The stable TL corresponds to more intense “blue” peaks, with lower temperatures. In the “IR” band, the glow peaks are more similar for the two groups in amplitudes as well as temperatures while being quite distinct from the “blue” peaks.
0 250
300
350
400
450
500
T(K)
1(W 20
0
z50
300
350
400
450T($O0
FIG. 5. TL gIow curves for two main groups of alkali feldspars. Sanidines (broken lines) and K-feldspar sediments (solid lines). Top: TL in the “blue” band; bottom: TL in the “IR” band.
383
IN FELDSPARS .
12
w
.
‘0
g
1c -
1.2 -
1.0 -
0.0 -
02
0.4
02
0
A 0.0L zoo
300
400
PabmrIR
-
QY--YIR 000
100 f(K)
FIG. 6. Activation energies of traps vs temperature for several samples of sanidines and a microcline, obtained by fractional glow (“F.Glow”) or by successive initial rises with separation of emission bands.
Moreover, the respective intensities of “blue” and “IR” TL peaks (see also in the last columns of Table 1) correspond nicely with observations on CL spectra. CL is seen to provide good information on TL. The activation energy of traps has been measured for some alkali feldspars, sanidines and microclines by different methods, either by fractional glow, with monitoring of the whole emission spectrum, or by successive initial rises, with separation of bands with filters (see Fig. 6). They give comparable results, qualitatively and numerically. One feature is the absence of any marked trap level. This corresponds, in our view, to TL processes being of the “thermally stimulated tunnel” type, in accordance with other observations, like the presence of tunnelling. Another feature is that blue emissions call for higher activation energy, further evidence of the difference between the two bands. These observations being completed, it could he objected that if there are actually two quite independent systems of traps-plus-centres, “IR” and “blue”, the reported fading of “blue” TL could not be accounted for by “IR” tunnelling. This objection is contradicted by the correlation shown in Table 1 between reported fading and observed tunnelling. This is further contradicted by some experiments. With sanidines stored after irradiation for days or weeks at RT or in liquid nitrogen, not only the blue TL peak is observed to decrease as well as the “IR” one, but it does so more quickly. There is clearly a link between the two bands.
R. VISOCEKAS 7. TUNNEL EMISSION AND CRYSTAL DISORDER “Blue” TL displays a great diversity between the samples, and it is considered that impurities play a major part in these emissions. In contrast, the “IR” emission in TL is quite similar among feldspars, especially among alkali feldspars: same spectra, with a nearly Gaussian shape of band; same peak temperatures (370 K in our experiments). The amplitude varies little from one sample to the next (see Table 1) while the percentage of Fe or other impurities (Eu.. .), is shown by X-ray analysis to be quite variable. All of this leads us to preclude the exclusive role of such impurities in the IR emission as proposed in many papers (Kirsh and Townsend, 1988, etc.). We propose that the IR emission we observe in TL and mostly in tunnelling (that is due to donor-acceptor recombination) is dependent on alkali feldspar crystal structure, as follows. Alkali feldspars are made up of a quartz-like SiO, lattice where one Si4+ out of four is substituted by A13+. Positive ions, like Na+ and K+ (or Ca*+ for plagioclases with a different ratio of AI/Si) enter interstitially into the lattice to ensure electrical neutrality (Barth, 1969). If one Al’+ substitutes for Si4+ in a crystalline site, this site will become an acceptor, i.e. a trap for a hole. Conversely, the Si4+ which will substitute in a site of A13+, will become a donor, i.e. a trap for an electron as shown by Marfunin (1979). We propose tentatively that such D-A pairs, or trap pairs, are the main sources for the observed IR tunnel and TL, while impurities could act only as recombination centres. This exchange of positions between Si and Al atoms from an “ordered” feldspar crystal lattice into a “disordered” one is not wishful thinking. It is a basic feature in crystallography of feldspars (Barth, 1969). For example, “high temperature” feldspars are more disordered than “low temperature” feldspars, sanidines are considered as much more disordered than microclines. etc. The relationship between lavas (and other “disordered” feldspars) and fading on one hand, of microcline (and “ordered” feldspars) and stability of TL on the other had been pointed out previously (Aitken, propose relation.
1985, p. 189). The microscopic model we may be a “missing link” explaining this
8. CONCLUSION
The presence of some IR emission in TL of feldspars as well as the existence of two distinct spectral ranges reported here is not exactly new (Andersson et al., 1991; Kirsh and Townsend, 1988; Huntley er al., 1988). However, the clear Gaussian character of this IR band was not reported before the
Krems Conference (Bos er al.. 1994).
et al.
Tunnel emission in the same IR band had been reported before only for labradorite and sanidine (Visocekas, 1985, 1993). Now, it is observed as a general feature for most feldspars. Its close association with anomalous fading is shown and a relation with crystal disorder is proposed. A practical conclusion is that tunnel monitoring provides a means of checking fading in feldspars which compares favorably with the direct measurement of stored TL vs time, at the double point of view of sensitivity and rapidity of evaluation. Cathodoluminescence spectra and their amplitude in the IR domain are also a valuable test for possible fading. There may exist some non-fading component in TL of feldspars, such as proposed in the localized transition model. A disappointing conclusion of this study is a confirmation that most feldspars do fade somehow, as they show tunnel afterglow. Acknowledgements-We
thank M. J. Aitken, G. Hiitt, I. Tale, V. Tale, M. Springis, P. Kulis, Y. Jansson (U. of Latvia); M. Chayt d’Albissin, G. Marinello, G. Valladas, M. Ouchtne, C. Bourquart, M. Nunez, M. Rambourg for direct contributions to this work. Samples initialled ‘MC” were given by Collection de Mineralogie of UniversitC P.M. Curie (M. Bariand) and “EM” given by Mu&e de I’Ecole Nationale Sup&ieure des Mines de Paris (M. Phan). A sabbatical leave from Universid Paris 7 supported one of us (RV).
REFERENCES Aitken M. J. (1985) Thermoluminescence Darina. Studies in Archaeological Science. Academic Press, London. Andersson M.. Rendell H. M. and Townsend P. D. (1991) Low temperature spectra of TL from sedimentary quartz and feldspars. Nucl. Tracks Radial. Meas. 18, 41-43. Barth T. F. M. (1969) Felabpars. J. Wiley and Sons, London. Bos A. J. J., Piters T. M. and Ypma P. J. (1994) Thermoluminescence emission spectra and optical bleaching of oligoclase. Radiat. Meas. 23. 349-353. Curie D. (1963) Luminescence in Crystals. Methuen and Co., London. Garlick G. F. J. and Robinson 1. (1972) The TL of lunar samples. In The Moon (eds Runcom S. K. and Urey H.), pp. 324-329. I.A.U. Huntley D. J., Godfrey-Smith D. I., Thewalt M. L. W. and Berger G. W. (1988) Thermoluminescence spectra of some mineral samples relevant to thermoluminescence dating. J. Lumine&ence 39, 123-136. Hiitt G.. Junaner H.. Kuiansu R. and Saarnisto M. (1993) OSL and TL dating of buried podsols and overlying sands in Ostrobothnia, western Finland. J. Quat. Sci. 8, 125-132. Jungner H. and Huntley D. J. (1991) Emission spectra of some potassium feldspars under 633 nm stimulation. Nucl. Tracks Radial. Meas. 18, 125-126. Kirsh Y. and Townsend P. D. (1988) Speculation on the blue and red bands in the TL emission spectrum of albite and microcline. Nucl. Tracks Radial. Meas. 14, 43-49.
A. S. (1979) Spectroscopy, Luminescence Radiation Centers in Minerals. Springer, Berlin.
Marfunin
and
Spooner N. A. (1992) Optical dating: preliminary results on the anomalous fading of luminescence from feldspars. Qua/. Sci. Rerl. 10, 139-145.
FADING
AND IR TUNNELLING
Spooner N. A. (1993) The validity of optical dating based on feldspars. Ph.D. thesis, University of Oxford. Spooner N. A. (1994) The anomalous fading of infraredstimulated luminescence from feldspars. Radiur. Meus. 23, 625-632.
Valladas G., Gillot P. V. and Guerin G. (1979) Dating plagioclases? PACT 3, 251-257. Visocekas R. (1979) Miscellaneous aspects of artificial TL of calcite: emission spectra, athermal detrapping and anomalous fading. PACT 3, 258-265. Visocekas R. (1985) Tunnelling radiative recombination in
IN FELDSPARS
385
labradorite: its association with anomalous fading of TL. Nucl. Tracks 10, 521-529. Visocekas R. (1988) Comparison between tunnelling afterglows following alpha or beta irradiations. Nucf. Tracks Radial. Meas. 14, 163-168. Visocekas R. (1993) Tunnelling radiative recombination in K-feldspar Sanidine. Nucl. Tracks Radiar. Meas. 21, 175-178. Wintle A. G. (1977) Detailed study of a thermoluminescent mineral exhibiting anomalous fading. J. Luminescence 15, 385-393.