Low temperature spectra of TL from sedimentary quartz and feldspars

Nucl Tracks Radmt Meas, Vol 18, No I/2. pp 41-43, 1991 lnt J ltn_,tm~ Appl Instrum, Part D

0735-245X/91 $3 00 + 00

Perpmon Prm plc

Printed m Great Bntam

LOW TEMPERATURE SPECTRA OF TL FROM SEDIMENTARY QUARTZ AND FELDSPARS M AND[atSSON,H M RE~ELL and P D T O ~ Umvermy of Sussex, Brighton BNI 9QH, U K Abstract--Thermolununescence (TL) spectra from separated fractsons of a multi-component sand have been recorded over the temperature range from 60 to 300 K Mineral fracUons were selected by a combmaUon of density separation and acid etching The feldspar fraction produced an intense red enasston together voth weaker blue/green components By contrast the quartz and heavy nnneral fracUons showed pnmanly the blue/green emission In all cases there were blue/green glow peaks near 90 and 130 K, with addlUonal features in thss wavelength range for the "hight" feldspar fracaon On the basis of peak temperature, or spectral analysts of the conventionally recorded blue/green ermsslon, st is not possible to d~stmgmsh the minerals responsible for the low temperature glow peaks from this sand However, the hghter density feldspar fraction had an intense red emssston relaUve to that in the blue or green region

INTRODUCTION

(c) "heavy" fraction (density > 2 72 g cm-3), contaming a concentrauon of heavy minerals.

SPECTRAL measurements of low temperature (60-300 K) thermoluzranescence (TL) from the different mineral fractions of a Nebraskan dune sand were measured to exemplify the way in which sample pre-treatment modifies TL esmsslons and to underline the tmpossibthty of resolwng emissions from different mineral fractions by "judicious" choice of optical filters in the conventional blue/green detecUon systems The TL of quartz at low temperature is frequently much brighter than that above room temperature This is an advantage when recording the TL emission spectra On the Sussex system, the use of a low temperature TL study improved the signalto-noise ratio for quarz emissions by a factor of 100 AddltionaUy there are no problems of discnrmnatmg between TL signals and black body emission at the red end of the spectrum, as in l~gh temperature TL

Further treatment of the density-separated "quartz" produced two other rmneral fracuons for analysis (d) etched quartz A porUon of the "quartz" fracnon was etched wsth 40% H F for 40 mm to remove subsidiary minerals and any surfacebonded lmpunties, (e) alpha irradiated etched quartz Since chemical etching also removes the outer layer of the quartz grams, any potenUal TL signal from that outer, alpha-damaged layer, was also re. moved In an attempt to compensate for this lost outer layer, a portion of the etched quartz was alpha irradiated under vacuum for 40 days at a distance of 7 mm from a 2SiAm source, of strength 0 5 mCi SPECTRAL ANALYSIS The TL spectra were recorded from 60 to 300 K using the Sussex scanning monochromator system (Andersson et a l , 1990) All the spectra were corrected for the wavelength dependent efiictency of the spectrometer system The samples were mounted on rhodmm-plated copper discs and were irradiated m s u u by an X-ray set operating at 30 kVp and 20 mA and producing a dose rate of 50 Gy mm-~ Typical doses were of the order of 1000 Gy

S A M P L E PREPARATION Samples of sand were pre-treated w~th 1 M HCI to remove carbonates and then thoroughly washed with de-tomsed water The 125-250/~m size fraction of the sand was separated into three different mineral fractions using sodium polytungstate solutions These fractions were (a) "light" fraction (density < 2 62 g cm-3), compnsmg predominantly potassium feldspars, (b) "quartz" fraction (denmy 2 62-2 72 g cm-3), comprising predominantly quartz, with subudtary minerals including plagqoclase feldspars,

RESULTS AND DISCUSSION Spectra for the different rmneral fracUons and for the etched and etched alpha irradiated quartz are 41

M

42

A N D E R S S O N et al

"L~ght" fraction

~

~

"Light" fraction (detail)

,

~o

"Heavy mineral" fraction

"Quartz" fraction

. ~t

500 1

Etched Quartz fraction

Etched Quartz fraction after alpha irradiation

500~

-..f-

~

~o

FIG 1 The TL spectra recorded between 60 and 180 K for X-ray Irradmted components of a sand The denslty separated fractions are termed heavy, quartz and hght (HF, Q, LF) Addmonal HF etching removed contammants from the quartz fraction (EQ) and subsequent room temperature alpha ~rradmt]on modified the low temperature glow curve (EQA) All spectra are corrected for the wavelength dependence ofthe system

shown m Fig 1 only from the more interesting range of 6 0 - 1 8 0 K The data for the "hght", feldspardominated, fracuon are shown at two magmficat~ons to both emphasize and accommodate the large dynamic range m s~gnal levels between the red and the blue poruons o f the spectrum The mare points to note are as follows (a) the density-separated fractions show dastxnct differences m the relauve proportions of red and blue em~sslon For the " h g h t " fraction, the red emission ~s dominant The " q u a r t z " and

" h e a v y " fractions both show broad emission m the blue, peaked at 130K, with some red emission which may reflect the presence of feldspar as a contaminant, (b) on etching the " q u a r t z " fraction to obtain only SIO 2, the red emission is greatly reduced and the dominant emission peak at 120--130 K is in the blue/green poruon of the emission spectrum The intensity of the blue/green signal was not slgmficantly altered, (c) alpha ~rradmuon of the etched quartz produces further small modifications to the intensity and

LOW TEMPERATURE Q U A R T Z TL SPECTRA shape of the enussmn spectrum, by redumng the red s~gnal and reducing s~gnals below 100 K m the blue/green portmn of the ermssmn spectrum Two obwous features are ~mmedmtely apparent The first is that all density fracuons ermt hght m the blue/green region of the spectrum The second is that the red enusslon mmally recorded from the quartz fraction Is most probably assocmted w~th ~mpuntaes bonded to the density separated quartz grams, or It ongmates m an outer layer of the quartz The results from the alpha lrradmted etched quartz appear to show that the alpha damage further quenches the long wavelength luminescence efficiency of the etched quartz These results are perhaps unexpected, as one might have hypothesized that red emlsstons came from impunues which should have been totally removed by the H F etching An alternative consideration is that some Impurities remain bonded rote the quartz grams and the consequent red emission ts produced wRh reduced et~ctency m alpha damaged material It should be noted that, m conventtonal TL systems based on the low noise blue sensmve photomultlpher tubes combined w~th broad band blue filters, emission at the red end of the spectrum was dehberately rejected m an attempt to achteve high s~gnal-to-no~se glow curves As emphasized m a recent rewew of TL spectral measurements (Townsend and K~rsh, 1989), the loss of spectral mformat~on ~s a most unfortunate consequence of thts experimental approach The present data underhne the value of a w~der spectral range system Indeed, the mtense red emlss~on from the hght feldspar fractton would have been rmssed by standard blue PM/filter systems Constdenng only the blue/green em~sston indicates that all samples have a complex, but spectrally nond~stmgutshable set of glow peaks and em~sston bands Consequently, one cannot hope to d~scnmmate between emtsstons from the dtfferent mineral components of mineral m~xtures on the bas~s of emission wavelength, etther m the present case w~th a monochromator, or w~th conventtonal broad band filters These data are reported for the less commonly recorded low temperature TL but, at least for the brighter stgnals, a stmdar conclusion ts drawn from

43

a detmled d~scusslon of the data, coupled wRh results for TL signals from above room temperature for these minerals (Andersson et a l , 1990) CONCLUSIONS All of the rmneral fractions were used to show enusslon m the blue/green region of the spectrum The differences between the spectra are differences m magratude of component features rather than m the presence or absence of parUcular emlssmn bands Th~s observation suggests that using only that region of the spectrum, separaUon of TL rote components from different rmnerals ~s not feasible w~th opucal filters Other work at th~s laboratory indicates that the problem ~s not hnuted just to th~s specific sand sample It ~s true that vanous sands wall vary to some extent m gram s~ze chstnbut~on, mineral composmon and ~mpunty content, hence there may fortmtously be favourable cases m which blue/green regmn filters might parhally dlscnmmate between components Such a posslblhty was descnbed for a wind blown sediment (Debenham and Walton, 1983) m which an attempt at spectral recordmg and filtered TL suggested that for that mixture the quartz fracuon was separable Their data emphasized that there was considerable complexRy m the glow curves which was potentmlly resolvable w~th specml analys~s The inescapable conclusion ~s that for dating work revolving polymmerahc m~xtures it ts ~mposs~ble to d~stlngmsh between mineral fracttons s~mply by using filters, unless the filter choice has been made on the basts of detmled measurements of the spectra over the whole temperature range of TL emJsslon Unfortunately, at the present t~me, almost no laboratones, including our own, have the necessary detecuon sensmvny

REFERENCES Andersson M, Jeanm M, Rendell H M, Tardot A and Townsend P D (1990) TL spectra of mineral mixtures dlscnmmatlon between dtfferent components Nucl Tracks Bad, at Meas 17, 569-577 Debenham N C and Walton A J (1983) TL properties of some wind-blown sediments PACT J 9, 531-538 Townsend P D and Kirsh Y (1989) Spectral measurement dunng thennolummescence--an essentml reqmrement Cent Phys 30, 337-354