پتا نسیل لومینسانس قرمز برای کار بردهای سن یابی: ترمولومینسانس قرمز(RTL)

دسته ژئوفیزیک
گروه سازمان زمین شناسی و اکتشافات معدنی کشور
مکان برگزاری بیست و ششمین گردهمایی علوم زمین
نویسنده مرتضی فتاحی
تاريخ برگزاری ۰۱ آبان ۱۳۸۴

Abstract                                            
     Following the successful application of thermoluminescence (TL) dating to fired archaeological materials in the ۱۹۶۰s, similar techniques were applied to geologically-fired volcanic deposits that have had their luminescence ‘clocks’ reset by volcanism. Luminescence dating methods have undergone extensive development and refinement during last ۳۰ years.  Today, thermoluminescence techniques are potentially highly useful methods for dating volcanic and related materials and events over timescales ranging from ۱۰۲-۱۰۶a.  The potential of red thermoluminescence (RTL) emission from quartz and feldspar has attracted some attention during the last decay.  RTL from heated quartz has shown important physical characteristics such as signal stability, low sensitivity changes and the slow onset of saturation with dose.
     This paper presents the result of a series of experiment exploring the suitability of RTL from unheated quartz for dating geological sediments.  Outlines of the single-aliquot regenerative dose (SAR) protocol for equivalent dose determination using RTL signal are discussed.  It is shown that, for the samples under study, SAR can successfully and accurately recover a known laboratory dose in RTL from unheated quartz separates.  Natural RTL De was also calculated.  It is found subtracting RTL De estimated using bleached aliquots, from RTL De estimates using non bleached aliquots provides results that match the UV OSL De estimated from the same sample.
 
 
Keywords: luminescence dating; quartz; red emission; Equivalent dose; RTL
 
 
۱.  INTRODUCTION
     Red Thermoluminescence (RTL) in quartz (which possesses a broad emission band with a peak around ۶۲۰ nm) was first observed in samples collected from volcanic ash layers (Hashimoto et al, ۱۹۸۷).  The potential of RTL in natural quartz for dating applications was examined after the failure of blue thermoluminescence (BTL) for dating old volcanic quartz (e.g., Miallier et al ۱۹۹۱). Since then the potential of this signal as a dosimeter for dating heated quartz was reported by limited authors (e.g., Hashimoto et al., ۱۹۹۱, ۱۹۹۶; Miallier et al., ۱۹۹۴; Fattahi and Stokes ۲۰۰۱).
      The presence of red emissions (۶۵۰-۷۵۰ nm) in feldspar TL has been shown in several studies (e.g., Huntley et al., ۱۹۸۸; Krbetschek et al., ۱۹۹۷).  Previous studies have shown that red thermoluminescence (RTL) of feldspar does not suffer from anomalous fading whereas blue-UV emissions from the same sample do demonstrate the effect (Zink and Visocekas ۱۹۹۷; Fattahi, ۲۰۰۱).  Using this advantageous, Zink and Visocekas (۱۹۹۷) successfully dated three volcanically derived feldspars using the far-red TL and additive dose method.
     Despite of these successes the main reason for not using wide spread RTL of quartz and feldspar was related to unwanted incandescence and the technological difficulties involved in blocking these emissions (e.g., Miallier et al., ۱۹۹۱).  For temperatures greater than ۳۵۰°C, the incandescence of the sample and heater plate has an intense emission at wavelengths of > ۷۹۰ nm (Fattahi ۲۰۰۱); this has limited routine measurements to temperatures below ۴۰۰°C (Zink and Visocekas, ۱۹۹۷; Scholefield and Prescott, ۱۹۹۹). 
       Results following technical developments in quartz and feldspar RTL have enabled Fattahi (۲۰۰۱) to observe RTL peaks up to ۶۰۰°C for quartz and feldspar and to produce successful dates for heated quartz with ages between ۳۰۰ to۱۲۰۰ ka (Fattahi and Stokes, ۲۰۰۰a, b).  Successful dating has been due to the reproducibility of quartz glow curves; stability and long lifetime of its trap components and high dose saturation of its growth curves.
 
       As a part of general study exploring the suitability of RTL from naturally occurring unheated quartz for geological dating applications, the result of a series of experiments is presented in this paper which investigate the suitability of single aliquot regeneration method for equivalent dose determination.
 
۲. EXPERIMENTAL SET UP AND SAMPLES USED
       Experiments were carried out using a Risø TA-۱۵ automated TL/OSL system (fitted with a ۹۰Sr/۹۰Y beta source delivering ~ ۷ Gy.minute).  The photomultiplier used is an alternative cooled (~-۲۰°C) extended ۹۶۵۸“red” S۲۰ PMT, that is equipped with an S ۶۰۰ PHOTOCOOL thermoelectric refrigeration chamber which allows active cooling of the photocathode down to ۴۰°C below room temperature.  Luminescence was measured through a range of filter combinations designed to transmit red (= ۶۰۰-۶۵۰ nm) emissions.  All TL glow curves were measured using a heating rate of ۵°C/s. The measurements were made on unheated quartz extracted from samples laboratory number (SQA).
 
۳.   DOSE RECOVERY USING AN ORANGE-RED IRSL SAR
     A key requirement for testing any De determination method is to accurately measure a dose given in the laboratory.  This requirement was examined for SAR protocol (Diagram ۱, step ۱-۶).  After depletion of natural luminescence of a set of aliquots using RTL measurement, a laboratory dose of ۱۱۵۰ Gy was administered.  This dose was treated as a ‘surrogate’ natural dose, and the De was determined via a SAR protocol using a range of regenerated doses from ۳۵۰ –۱۵۰۰ Gy (Figure ۱ & ۲).  To monitor the possible sensitivity changes a fixed test dose was applied during the RTL measurement sequence (Figure ۳ & ۴). 
     To test the accuracy of the sensitivity correction, after the first three cycles, the first regeneration cycle was repeated.  The ratio of the ۴th regeneration to the ۱st regeneration data point (recycling ratio) is ۰.۹۸.  This is well within the acceptability criteria proposed by Murray and Wintle (۲۰۰۰).  This experiment is repeated for ۳ aliquots of sample SA۵Q and the result is ۱۱۴۳.۳±۶.۲۸ Gy.
      These experiments suggest that this modified SAR method as applied using RTL meets a necessary requirement for accurate De determination for red emission.
 
۴. ESTIMATION OF NATURAL ABSORBED DOSE USING AN RTL SAR
     We have applied the SAR protocol (Diagram ۱, step ۱-۶), to SQA quartz samples.  This allows us to estimate RTL De values obtained on RTL emissions.  The De was determined by interpolation and the sensitivity was corrected using the procedure explained in the previous section. The result for De values is shown in Table ۱. The author compares the result of RTL and UV OSL.  Three aliquots of sample SA۷Q was bleached by sunlight in Oxford (۲۰ Nov ۲۰۰۲) and the De was estimated to be ۵۵۳ Gy.  As a result the RTL De since the last thermal resetting is ۸۱۳ Gy, while the De since the last optical resetting is ۲۶۰±۲۰ Gy that matches with the result produced by UV OSL (۲۳۷±۱۱ Gy).
 
۵. CONCLUSION
     SAR RTL from unheated quartz can be used for the equivalent dose estimation.  SAR RTL has provided De for SAQ that are much bigger than the estimated using blue OSL.  This difference is mainly due to thermal residual signal that has not been deleted during past exposure of the sample to sun light. However, RTL De estimated from bleached samples was subtracted from the De estimates of unbleached samples.  The result is comparable with the De produced by UV OSL of same quartz.  
 
ACKNOWLEDGEMENT
The author would like to thank Research Department of Tehran University and The Institute of Geophysics for their general support. Particular thanks are due to Mr Karimkhani and Asgharzadeh for technical support.  I gratefully acknowledge Stephen Stokes, Richard Bailey, Aby Stone and Helen Bray from Oxford Luminescence Research Group, University of Oxford for their close co-operations.
 
REFERENCES
Fattahi M. and Stokes S., ۲۰۰۰a.  Red thermoluminescence (RTL) in volcanic quartz: development of a high sensitivity detection system and some preliminary findings. Ancient TL ۱۸, ۳۵-۴۴.
Fattahi M. and Stokes S., ۲۰۰۰b.  Extending the time range of luminescence dating using red TL (RTL) from volcanic quartz. Radiation Measurements ۳۲, ۴۷۹-۴۸۵.
Fattahi, M., ۲۰۰۱.  Studies on red thermoluminescence and infrared stimulated red luminescence. Unpublished D. Phil. thesis, Oxford University, Oxford.
Fattahi M. and Stokes S., ۲۰۰۱. Luminescence dating of Quaternary Volcanic events.  A review of previous investigations and some observations on the potential of red luminescence emissions.  School of Geography of university of Oxford Research papers, ۵۸, ۱-۷۲
Hashimoto, T., Yokosaka, K. and Habuki, H., ۱۹۸۷. Emission properties of thermoluminescence from natural quartz - blue and red TL response to absorbed dose. Nuclear Tracks and Radiation Measurements, ۱۳: ۵۷-۶۶.
Hashimoto, T., Kojima, M. and Sakai, T., ۱۹۹۱. Age determination of prehistorical sites in Japan using red thermoluminescence measurements from quartz grains. Archeology and Natural Science, ۲۳: ۱۳-۲۵.
Hashimoto, T., Notoya, S., Komura, K. and Shiral, N., ۱۹۹۶. Red thermoluminescence dating of some volcanic-ash and pyroclastic-flow layers related to Takamori pre-historical sites using quartz inclusion method. Archeology and Natural Science, ۳۳: ۱-۱۵.
Huntley, D.J., Godfrey-Smith, D.I., Thewalt, M.L.W. and Berger, G.W., ۱۹۸۸. Thermoluminescence spectra of some mineral samples relevant to thermoluminescence dating. Journal of Luminescence, ۳۹: ۱۲۳-۱۳۶.
Krbetschek M. R., Götze J., Dietrich A., and Trautmann T., ۱۹۹۷.  Spectral information from minerals relevant for luminescence dating,. Radiation Measurements ۲۷(۵-۶),  ۶۹۵-۷۴۸.
Miallier, D. et al., ۱۹۹۱.  Properties of the red TL peak of quartz relevant to thermoluminescence dating. Nuclear Tracks and Radiation Measurements, ۱۸(۱/۲): ۸۹-۹۴.
Miallier, D. et al., ۱۹۹۴. Attempts at dating pumice deposits around ۵۸۰ ka by use of red TL and ESR of xenolithic quartz inclusions. Radiation Measurements, ۲۳: ۳۹۹-۴۰۴.
Scholefield, R.B. and Prescott, J.R., ۱۹۹۹. The red thermoluminescence of quartz:  ۳-D spectral measurements. Radiation Measurements, ۳۰: ۸۳-۹۵.
Visocekas R., ۲۰۰۰.  Monitoring anomalous fading of TL of feldspars by using far-red emission as a gauge. Radiation Measurements ۳۲, ۴۹۹-۵۰۴.
Zink A. J. C. and Visocekas R., ۱۹۹۷.  Datability of sanidine feldspars using the near-infrared TL emission. Radiation Measurements ۲۷(۲), ۲۵۱-۲۶۱.
 
List of Figures:
Figure ۱. Regenerated RTL Glow curves of sample SA۵Q
Figure ۲. Regenerated RTL growth curves of sample SA۵Q
Figure ۳. Test dose (۴۶۰Gy) RTL Glow curves of sample SA۵Q
Figure ۴. The response of test dose signal following regenerated RTL measurement vs the cycles.
 

خلاصه:
بدنبال موفقیت ترمولومینسانس جهت تعیین سن مواد باستانی طی سالهای 1960 تکنیکهای مشابهی جهت مواد آتشفشانی بکار رفت که ساعت انها توسط آتشفشان صفر شده است. سن یابی توسط لومینسانس طی سی سال گذشته رشد جشمگیری داشته است. امروزه  تکنیکهای لومینسانس پتانسیل بالایی جهت سن یابی آتشفشانها با قدمت یکصد تا یک میلیون سا ل ارایه می کنند.  پتانسیل تابش قرمز ترمولومینسانس((RTL کوارتز و فلدسپار بمنظور کاربردهای سن یابی  طی دهه گذشته توجه زیادی را به خود جلب کرده است. RTL ناشی از کوارتز مشخصه های فیزیکی مهمی از جمله پایداری سیگنال  تغییر حسساسیت کم  ودیر اشباع شدن با دز  از خود نشان داده است.  این مقاله نتایج یک مجموعه ازمایشات را که مناسب بودن RTL ناشی از کوارتزگرم نشده برای سن یابی رسوبات زمین شناسی را جستجو می کند ارائه می دهد.  خلاصه پروتکل SAR برای اندازه گیری دز معادل دز طبیعی  با استفاده از   RTL قرمز بحث می شود.  نشان داده شده است که برای نمونه های مطالعه شده SAR می تواند با موفقیت دز معادل دز طبیعی را که در ازمایشگاه به نمونه داده شده است بدست دهد.  همجنین از SAR جهت یافتن دز طبیعی استفاده گردیده است.  نتایج مطلوب بوده و ارایه گردیده است

کلید واژه ها: لومینسانسقرمز سنیابی آتشفشان لومینسانس ژئوفیزیک سایر موارد