THE PARAMETERS OF TRAPS IN K-FELDSPARS AND THE TL BLEACHING EFFICIENCY

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1 GEOCHRONOMETRIA Vol. 2, pp 15-2, 21 Journl on Methods nd Applictions of Asolute Chronology THE PARAMETERS OF TRAPS IN K-FELDSPARS AND THE TL BLEACHING EFFICIENCY ALICJA CHRUŒCIÑSKA 1, HUBERT L. OCZKOWSKI nd KRZYSZTOF R. PRZEGIÊTKA Institute of Physics, Nichols Copernicus University, 87-1 Toruñ, Polnd 1 (e-mil: licj@phys.uni.torun.pl) Key words: THERMOLUMINESCENCE, TRAP SPECTROSCOPY, THERMOLUMINESCENCE DATING, K-FELDSPAR Astrct: The frctionl glow technique (FGT) pplied to the investigtion of opticlly leched smples of K-feldsprs extrcted from sediments revels the coexistence of vrious groups of trps which re ctive in the sme temperture region over 3 C. Significnt differences etween the trp prmeters seem to explin the diversity of TL leching efficiency for different trp groups. 1. INTRODUCTION In the luminescence dting of sediments one ssumes tht grins lose their thermoluminescence (TL) or opticlly stimulted luminescence (OSL) signl efore deposition s result of opticl stimultion which in this cse is cused y sunlight. After such reset of the TL/OSL signl of the sediment the nturl nucler rdition constntly delivers energy which is ccumulted in minerl grins in the form of trpped chrge crriers. The nturl TL/OSL signl mesured in dting lortory is then mesure of the time elpsed from the minerl deposition, i.e. from the moment of opticl leching (Aitken, 1985 nd 1998). For mny yers the min prolem in luminescence dting hs een the lck of n ojective method which llows one to prove tht the sediment TL/OSL signl hs een properly reset efore deposition. It is elieved tht in the OSL method only trps which re very efficiently depopulted y light re used ut still it is unknown whether the sunlight leching ws enough long nd effective. These reset prolems hve given rise to investigtions of leching mechnisms nd crrier trnsfer in minerls. The so-clled residul TL (McKeever, 1994) connected with the difficult-to-lech frction of trps, is prticulrly interesting oject of study. Feldsprs nd qurtz re the most commonly used minerls in sediments dting. This work presents the results of prmeter investigtion for those trps involved in the leching process in K-feldsprs. Opticl leching chnges the shpe of the K-feldspr norml glow curve (Fig. 1). The sensitivities to leching of two TL peks round 2 C differ significntly. The 2-4 C temperture region, eing of interest ecuse of its luminescence kinetics, is investigted here y mens of frctionl glow technique (FGT) (Gorecht nd Hofmnn, 1966; Oczkowski, 1978 nd 1981). FGT mesurements hve een crried out on K-feldspr smple fter vrious times of opticl leching. Chruœciñsk (21) found tht deeper trps over 1.6 ev re much more esily depopulted y light thn shllower trps of depth out 1.4 ev. The experiments presented here concentrte on high temperture edge of the glow curve. Opticl leching revels the complex origin of TL in this region. 2. EXPERIMENTAL TECHNIQUE K-feldspr ws seprted from sediment from Finlnd in the usul mnner for TL dting nd heted to 5 C efore excittion. TL ws excited y n X-ry dose of 36 Gy (6 h of irrdition). Bleching ws undertken in sunlight simultor using the spectrl rnge of 3-75 nm (Oczkowski nd Przegiêtk, 1998). Prehet to 31 C t rte of 1 C/s ws pplied efore the FGT mesurement. The wve nd nm, normlly used in TL dting, ws employed s the spectrl window for ll mesurements. A Schott BG-2 filter ws instlled in the light detection unit. Frctionl TL intensity fter 2 h of leching in the simultor is still ove the sensitivity limit of opticl detection when the EMI 697 B photomultiplier nd lock-in technique is used. The FGT introduced y Gorecht nd Hofmnn (1966) consists in registrtion of the TL intensity during oth liner heting nd cooling of the smple which re crried out in cyclic wy. The smple heting from temperture T 1 to T 2 nd susequent cooling to temperture T 1 + T mke one cycle. In the next cycle heting is crried out to temperture T 2 + T nd cooling to temperture T T. The direct result of the experiment is the

2 The prmeters of trps in K-feldsprs nd the TL leching efficiency sequence of trp energies E i (i numers of the cycles; symol plots in Figs 2 nd 5), nd integrls of the TL intensity, L i, mesured in the cycle. The trp energy is clculted seprtely for heting (E ih ) nd cooling (E ic ) y mens of liner regression pplied to dependencies é ê ë é ê ë d ln I d(1/ T) d ln I d(1/ T) ù = - ú û ih ù = - ú û ic E k E k ih ic, (2.1), (2.2) where indexes h nd c denote dependence nd vlues otined respectively for heting nd cooling, k is the Boltzmnn constnt, T is the temperture, I is the TL intensity. E i is the men of vlues E ih nd E ic tken with the fctors dependent on the heting (w ih ) nd cooling (E ic ) rtes in the cycle w ic Ei E + ih ( E - ic Eih ) w + ih wic =. (2.3) The detils of resoning s well s the clcultion leding to the ove formul re presented in erlier work (Chruœciñsk, 1994). The histogrms of the trp occuption spectr (histogrms in Figs 2 nd 5) re clculted using the distriution function (Chruœciñsk, 1994): H E L i i ( ) =, (2.4) E - i -1 Ei where H i (E) denotes the reltive numer of trps of energy E emptied during i-th cycle. The finl trp occuption spectrum is clculted tking into considertion the possiility of overlpping of H i (E) otined for different cycles. The TL fcilities used for FGT with heting-cooling correction (Chruœciñsk, 1994 nd Chruœciñsk et l., 1996) enles liner heting nd cooling with the verge rte of out.1 C/s. The trp depth ws determined with use of dt collected only in tht prt of the cycle, during which the heting nd cooling were liner. The temperture ws repetedly incresed y 12 C nd lowered y 1 C strting t 2 C, up to 33 C. It is the high temperture edge of the frctionl glow curve. The whole procedure consists of out 4 heting-cooling cycles. 3. MEASUREMENT RESULTS One complex pek cn e oserved in the glow curve of n 1 mg liquot of unleched K-feldspr fter fst prehet (crried out t the heting rte of 1 C/s) to 31 C (Fig. 1). The TL curve shpe chnges fter leching ut still only one mximum is detected. Fig. 2 (upper prt) presents the results of FGT mesurements crried out fter dose of 36 Gy nd prehet to 31 C. Energy levels of out 165 nd 18 mev cn e distinguished. The lower prt of Fig. 2 shows the results otined for smples fter dose of 36 Gy, 2 hours of leching nd prehet to 31 C. Two trp groups re gin oserved ut their depths re out 138 nd 146 mev. The glow curve shown in Fig. 3 ws mesured with the heting rte of.5 C/s fter dose of 36 Gy, 2 hours of leching nd prehet to 31 C. Fig. 4 shows the glow curve mesured with the lowest heting rte of.1 C/s fter dose of 36 Gy nd prehet to 31 C. The clculted curves were fitted to the experimentl glow curves in oth cses. The fitted curves were clculted using the trp energy spectrum otined from FGT. Initilly monomoleculr kinetics nd qusi-continuous trp depth distriution were ssumed. However, the fitting results were highly disppointing. Fig.1 8 c d 8 6 < x TL intensity [.u.] Fig.1. Glow curves of K-feldspr mesured fter dose of 36 Gy: fter prehet to 2 C (), fter prehet to 31 C (), fter 2 hours of sunlight leching nd prehet to 2 C (c) or to 31 C (d). Heting rte.8 C/s. Inset presents the chnges of the curve shpe fter leching more clerly. 16

3 A. Chruœciñsk et l. Figs 3 nd 4 show the results of fitting, ssuming second-order ehviour. A discrete trp spectrum is lso ssumed. The TL intensity ws clculted ccordingly to the following formul: I = n -2 ù -E s h - i i i Ei ås h exp( ) ê1 + i i ( ) exp( ) kt w ' ' ú ò kt dt, (3.5) i 1 2 é = ê o ú ë where i is the trp kind index, σ i nd η i re numericl prmeters relted, respectively, to the pre-exponentil fctor s nd to the initil trp occuption, E i is the trp depth, k is the Boltzmnn constnt, T is the temperture, T is the initil temperture nd w is the heting rte. T T û In the cse of leched curve (Fig. 3) the clculted curve is the sum of two imoleculr curves (n=2 in Eq. (3.5)) clculted for the energy vlues of 138 mev nd 146 mev in ccordnce with two energy levels mrked in the FGT histogrm (Fig. 2B). The prmeters σ nd η used for fitting re presented in Tle1. The question rises whether the shllower (<15 mev) trps were lredy occupied fter irrdition or, lterntively, they retined very low popultion, ut the leching process cused the crrier trnsfer from other trps. The second possiility seems to e rther unrelistic. The glow curve chrcteristic for long opticl leching is very similr to the glow curve fter smll dose Fig.2 2 Trp occuption [.u.] Energy [mev] A 11 B Fig.2. The FGT results. Symol plots (smll letters) present the energy dependence on the mximum temperture in the cycle. Histogrms (cpitl letters) re the trp occuption spectr. The smples of K-feldspr were first given 36 Gy dose. Then smple:, A ws preheted to 31 C; smple, B ws leched for 2 hours nd preheted to 31 C. Fig.3 16 experimentl curve sum of second order curves individul second order curves TL intensity [.u.] Fig.3. The experimentl nd fitted glow curves for K-feldspr. The experimentl curve ws mesured fter dose of 36 Gy, 2 hours of leching nd prehet to 31 C. Heting rte ws.5 C/s. The energy vlues tken from the FGT mesurements were 138 () nd 146 mev () The respective fitting prmeters re presented in Tle1. 17

4 The prmeters of trps in K-feldsprs nd the TL leching efficiency the results of FGT fter ~15 Gy dose (Fig. 5) do not show the existence of the deeper trps. This indictes tht the shllower trps (<15 mev) re populted efficiently during the first stge of the trp filling process nd it is unlikely tht they re empty fter long irrdition, lthough the FGT mesurements (Fig. 2A) do not detect them. For this reson four individul second-order curves (n=4) were fitted to the glow curve otined fter long irrdition nd without leching (Fig. 4). Two secondorder curves were clculted for two energy levels out 165 nd 18 mev distinguished in the FGT results presented in the upper prt of Fig. 2. The other two were clculted for the E nd s prmeters used previously to fit the TL curve otined fter long leching (Fig. 3). The vlue of h i (i=3, 4) representing the initil trp occuption of the two shllower trps, were the only fitting prmeters in these cses. 4. DISCUSSION The results of our previous work (Chruœciñsk, 21) showed tht the shllower trps (~138 nd 146 mev) re the most resistnt to opticl emptying. The leching experiments presented here nd the results of FGT mesurements simultion (Chruœciñsk, 21) show clerly tht in this K-feldspr there re trps which re ctive in the sme temperture region C (heting rte ~1 C/s) ut they hve significntly different prmeters. These trps re lso filled t different rtes during irrdition. As is cler from the FGT mesurements for low dosed smples (Fig. 5), the shllower trps with energy of 138 nd 146 mev re filled much more efficiently thn the deeper ones with energy of out 165 nd 18 mev. For the ltter trps (165 nd 18 mev) the TL signl is lso detected ove 3 C, ut only fter lrge dose (36 Gy). Ahmed nd Grti ( 1985) lso determined the kinetics of the TL in microcline s imoleculr nd they otined trp depth vlues of 1.36 nd 1.44 ev for the high temperture edge of TL curve. They mesured the TL following reltively smll dose. Much lrger doses were used y Visocecs et l. (1996) who investigted K- feldspr using simple initil rise method. The energy vlues otined y them re comprle with those presented here for high-dosed smples. In comintion, those previous results indicte tht fter low dose the shllower trps re occupied, nd the deeper trps re populted y higher dose. The efficiency of trpping nd leching for prticulr kind of trp is proportionl to the rtio of the recomintion coefficient to the retrpping coefficient A. If one includes the rtio /A in the second-order eqution, the vlue of the imoleculr pre-exponentil fctors s = s β/ N A (4.6) is otined, where N is the trp concentrtion (Chen nd McKeever, 1997). Low vlues of β/a indicte the predominnce of retrpping over recomintion (for prticulr type of trp). If trps of severl kinds tke prt in the filling process during excittion, the fstest populted one is tht with the smllest β/a rtio. Similrly, crriers opticlly relesed from the trps during leching process repopulte the trps with the smllest β/a rtio most efficiently. Hence, these trps re the most resistnt to leching. This model is confirmed y the results shown in tle1, for the K-feldspr studied here. In Tle 1 the product σ η hs the dimension of [s -1 ] nd, ccording to Eq. (4.6), is equl to sa/β for the fully occupied trps. The vlues of 1 12 nd 1 1 otined here for this product, for the deeper nd shllower trps respectively, seem to e relistic. The vlues of η for fully occupied trps re those determined fter 36 Gy dose. Fig.4 experimentl curve sum of second order curves 6 individul second order curves 5 TL intensity [.u.] c 1 d Fig.4. The experimentl nd fitted glow curves for K-feldspr. The experimentl curve ws mesured fter dose of 36 Gy nd prehet to 31 C. Heting rte ws.1 C/s. The energy vlues tken from the FGT mesurements were 18 (), 165 (), 146 (c) nd 138 mev (d). The respective fitting prmeters re presented in Tle1. 18

5 A. Chruœciñsk et l. The TL intensity dependence on dose implies tht fter dose of 36 Gy the sturtion level is reched in the K-feldspr smples investigted here. Hence, the h vlues determined fter 36 Gy dose cn y treted s N, expressed in the ritrry units of our experiments. Tle 1 includes the initil component of lifetime which is interesting for dting specilist. As one cn see from the second-order decy eqution (see Chen nd McKeever, 1997): I = 2 æ æ E ö ö ç1 + s' n expç - t è è kt ø ø I, (4.7) where t is time, I nd n re the initil luminescence intensity nd trp occuption respectively; the presented vlues re the estimtion of minimum vlues of lifetimes otined for fully occupied trps. The rtio β/a for the shllower trps is lower thn in the cse of other trps for two resons. The prmeter σ (proportionl to pre-exponentil fctor s ) is comprtively low (much lower thn σ determined for the deeper trps) nd η (proportionl to N) is not higher thn for the deeper trps. The shllower trps re oth the most resistnt to leching nd the most esily populted trps. This suggests tht the differences etween the β/a rtios of individul trps is responsile for the vriety of TL leching efficiencies oserved for this smple. The TL itself is not simple reflection of the chrge crrier distriution in trps, so investigtion of TL leching phenomen using simple comprison of TL curves does not give ny direct informtion out the efficiency of the opticl relese of crriers from individul types of trps. Two processes could e responsile for the decrese of the TL signl mesured fter leching: the lowered trp popultion or the reduced recomintion proility ecuse of lck of recomintion centres (or excess crriers; McKeever, 1994). The FGT experiments presented here show clerly tht different types of trps dominte the TL process efore nd fter leching. So emptying of trps is responsile for the opticl leching of the TL signl from this K-feldspr y visile light, rther thn chnges in recomintion proility. Chnges in the initil concentrtion of excess crriers cnnot influence the trp energy vlue determined y FGT. It only cuses decrese of the totl TL intensity. 5. CONCLUSIONS The light from rod-nd of spectrum ws used to lech the TL signl in K-feldspr. This non-selective ction gives rise to selective effects relted to trp fetures. There coexist two different types of trps giving the TL signl in the sme temperture region (over 3 C) in this minerl. The correltion of low pre-exponentil fctor with the high filling efficiency nd low lechility Tle 1. Trp prmeters determined y the FGT (E-energy) nd fitting procedures (σ nd η) using the Eq. (3,5) E [mev] σ η x 1 4 σ η [s -1 ] τ = (σ η) -1 exp(e/kt) [yers] for T=2 C Smples without x x x 1 1 leching x x x x x x x x x 1 6 Smples fter 2h x of leching x Fig.5 18 Trp occuption [.u.] Energy [mev] Fig.5. The smple ws given dose of ~15 Gy nd preheted to 31 C. The symol plot shows the energy dependence on the mximum temperture in the cycle. The histogrm presents the trp occuption spectrum. 19

6 The prmeters of trps in K-feldsprs nd the TL leching efficiency of TL, oserved for the trps with energy of out 138 nd 146 mev, shows tht the rtio of recomintion to retrpping β/a cn e responsile for the selective effects of opticl leching. In dosimetric mesurements, potentil prolems rising from the TL intensity dependence on dose should e tken into ccount. Trps which contriute to the sme glow pek ut which hve significntly different kinetic prmeters cn e source of prolems, ecuse in this cse non-liner dose response curve cn develop. When the shllower trps re filled fster thn the deeper ones, the sturtion prt of the growth curve for the shllower trps cn pper t similr doses s the liner prt of the growth curve for the deeper trps. REFERENCES Ahmed A.B. nd Grti R.K., 1985: Trpping levels in KlSi 3 O 8. Physic Sttus Solidi (A) 94: Aitken M.J., 1985: Thermoluminescence Dting. Acdemic Press, London. Aitken M.J., 1998: An Introduction to Opticl Dting. Oxford University Press, Oxford. Chen R. nd McKeever S.W.S., 1997: Theory of Thermoluminescence nd Relted Phenomen. Word Scientific, Singpore. Chruœciñsk A., 1994: The frctionl thermoluminescence: some spects concerning the experimentl dt nlysis. Journl of Luminescence 62: Chruœciñsk A., 21: The frctionl thermoluminescence of leched K-feldsprs. Journl of Luminescence 92: Chruœciñsk A., 21: The diversity of TL leching efficiency in K-feldsprs investigted y frctionl glow technique, in press. Chruœciñsk A., Oczkowski H.L. nd Przegiêtk K.R.,1996: Trp spectr of nneled qurtz. Act Physic Polonic A 89: Gorecht H. nd Hofmnn D., 1966: Spectroscopy of Trps y the Frctionl Glow Technique. Journl of Physics nd Chemistry of Solids 27: Kiersted J.A. nd Levy P.W. 1991: Vlidity of repeted initil rise thermoluminescence kinetic prmeter determintions. Nucler Trcks nd Rdition Mesurements 18: McKeever S.W.S., 1994: Models for opticl leching of thermoluminescence in sediments. Rdition Mesurements 23: Oczkowski H.L., 1978: Thermoluminescence in Ag(Cl, I) mixed crystls. Journl of Luminescence 17: Oczkowski H.L., 1981:Thermoluminescence in conductive n-type ZnSe crystls. Physic Sttus Solidi (A) 68 (1981) Oczkowski H.L. nd Przegiêtk K.R., 1998: TL dting of young eolin deposits from Kêp Kujwsk. Rdition Mesurements 28 (1998) Visoceks R., Tle V., Zink A., Spooner N.A. nd Tle I., 1996: Trp spectroscopy nd TSL in Feldsprs. Rdition Protection Dosimetry 66: