Thermoluminescence spectra of minerals

Transcription

1 Thermoluminescence spectr of minerls H. M. RENDELL 1, M-R. KHANLARY 2, P. D. TOWNSEND 2, T. CALDERON 3 AND B. J. LUrE 2 1 Geogrphy Lbortory, University of Sussex, Flmer, Brighton BN1 9QN, U.K. 2 School of Mthemticl & Physicl Sciences, University of Sussex, Flmer, Brighton BN1 9QH, U.K. 3 Deprtmento de Quimic Agricol, Geologi y Geoquimic, Autonom Universidd de Mdrid, Cntoblnco, Mdrid, Spin Abstrct Thermoluminescence (TL) studies of insultors, including crystls of minerls such s clcite, qurtz or zircon, hve resulted in the development of wider rnge of pplictions in the fields of rdition dosimetry nd rcheologicl nd geologicl dting. Most conventionl TL mesurements re mde by recording light emission during heting by mens of brod-bnd opticl filters nd blue-sensitive photomultiplier tubes. Much more informtion my be gined by monitoring the detils of the emission spectrum during thermoluminescence. TL spectr of minerls exhibit chnges s result of crystl purity, rdition dose, dose rte nd therml history. This pper exemplifies the dditionl informtion vilble s result of spectrl studies, nd focuses on investigtions undertken by the University of Sussex TL group. Exmples include work on clcite, fluorite, zircon nd feldsprs. KEYWORDS: thermoluminescence, insultors, dting, clcite, zircon, fluorite, feldspr. Introduction THERMOLUMINESCENCE (TL) is sensitive technique for studying defects nd defect concentrtions in insultors. Nturlly-occurring crystls of rock-forming minerls contin both intrinsic defects nd extrinsic impurities. The presence of impurities in concentrtions of the order of p.p.b. to p.p.m, is, for exmple, responsible for the colours exhibited by nturl forms of trnsprent minerls such s qurtz nd fluorite. TL spectr provide importnt informtion on the nture of both intrinsic nd extrinsic defects, nd on their sptil reltionships within crystls, by llowing both temperture-dispersive seprtion of different trps (defects) nd wvelength-dispersive seprtion of rditive recombintion centres. A review of the potentil informtion tht cn be deduced from TL spectr is given by Townsend nd Kirsh (1989). There is vriety of techniques for studying defects nd not ll methods revel the sme defects. Luminescence methods of TL, CL (cthodoluminescence) nd PL (photoluminescence) re complementry in tht TL records metstble defects, dispersed throughout the bulk of the mteril, with chrge sttes within the bnd gp. By contrst, CL provides dt from trnsient defects, in thin surfce lyer, fter ionistion within the lttice. Minerlogicl Mgzine, June 1993, 17ol. 57, pp (~ Copyright the Minerlogicl Society Experimentl methods Conventionl TL mesurements re reltively simple to mke. An irrdited smple is plced on heter strip, in n inert tmosphere or under vcuum, the temperture is then rised t constnt rte (e.g. 150 ~ min -1) nd the emitted light is detected by photomultiplier tube with either current or photon counting electronics. In principle, different depth trps relese chrge t different tempertures so tht the TL signl gives temperture-dispersive mesure of the trp popultion. This in turn reltes to the irrdition history of the smple, nd TL is therefore sensitive rdition dosimetric method. In relity, the processes re not so simple, nd one requires more complete understnding of the possible processes involved, s discussed by McKeever (1985), Horowitz (1984) nd Aitken (1985). To obtin mximum signl, most simple TL systems use brod bnd filter, which rejects both short wvelength UV nd the longer wvelength red signls, The ltter re rejected in order to reduce sturtion from blck body emission. Blue-sensitive photomultiplier tubes lso limit the spectrl rnge, but re used becuse they hve low drk current. With these experimentl constrints, one typiclly detects between 1% nd 20% of the vilble signl. The simple filter plus blue PM

2 218 tube systems do not offer ny informtion on the spectr of the emitted light, nd so utomticlly reject welth of informtion from the recombintion sites. Although these simplified TL mesurements re often mde for resons of cost, the mesurement of trnsient emission spectr t the reltively low intensities provided by TL is difficult experiment. Severl different strtegies hve been employed to mesure TL spectr including rotting set of brod bnd filters, scnning monochromtors, Fourier trnsform spectrometer, commercil opticl multi-chnnel nlysers (OMA) nd, most recently new Sussex system employing pir of high perture spectrometers coupled to position sensitive photomultiplier tubes. Some of the reltive dvntges of these different experimentl pproches re discussed by Luff nd Townsend (1992). The TL spectr discussed in this pper were lmost ll obtined using scnning monochromtor system over wvelength rnge of nm with bndpss of 10 nm nd heting rtes of the order of ~ rin ]. The new spectrl system t the University of Sussex, mentioned bove, used two position-sensitive photomultipliers to record the spectrum simultneously throughout the wvelength rnge nm, chieving sensitivity t lest 200 times greter thn the erlier Sussex monochromtor system. All the spectr presented here re corrected for the wvelength dependence of the sensitivity of the spectrometer. This is n essentil requirement becuse neither the diffrction grting nor the detector perform eqully efficiently t ll wvelengths. The scle of the corrections cn be very lrge. For exmple, in the Sussex scnning monochromtor system, the efficiency vries by fctor of lmost 200 within H. M. RENDELL ETAL. the region scnned. Further signl mnipultion is lso required s the system hs fixed entrnce slit defining fixed wvelength spred (AX), but spectr need conversion to the physiclly more meningful photon energy xis nd hence require djustment to equivlent AE pertures. Filure to observe these essentil corrections results in very distorted view of the dt but, nevertheless, the need for corrections is not universlly pprecited in descriptions of TL, photoluminescence or cthodoluminescence dt. Experimentl Results nd Discussion TL spectr of minerls exhibit chnges s result of crystl purity (Chee et l., 1988; Down et l., 1985), rdition dose (Rendell et l., 1985), dose rte nd therml history (Strin et l., 1985). In the following section we present results for number of different rock-forming minerls. These results exemplify the potentil vlue of TL spectr for provennce studies, in the cse of minerls like fluorite nd zircon, nd for studies of impurities within minerls. Clcite. Clcite is bright TL phosphor which emits strongly in the ornge portion of the visible spectrum (c nm) when doped with mngnese. TL spectr of nturl Mn-rich clcite (Joplin, USA) nd pure synthetic Icelnd spr show pronounced differences. After X-ry irrdition the nturl clcite smple showed brod peks t 150 ~ nd ner 400 ~ (Fig. l). The spectrum for synthetic Icelnd spr under identicl experimentl conditions exhibited line spectr (Fig. lb). The wvelength region of the emission confirms tht light is emitted t Mn sites (Medlin, 1968), but to understnd why the spectrum hs line fetures or is brod bnd requires n..-:-t /N b 4 9 ~ 44) "~ 0.(1 Fro. l(). X-ry irrdited clcite (Joplin, U.S.A.) (b) X-ry irrdited synthetic Icelnd spr.

3 explntion of the lterntive Mn-relted recombintion sites. These differences re interpreted in the detiled study by Down et l. (1985) in terms of the concentrtion nd solubility of Mn impurity ions. At low concentrtions Mn ions exist s isolted defects within the crystl lttice. At higher concentrtions, of the order of 1000 ppm. the solubility limit is exceeded nd Mn precipittes in clusters. But solubility is temperture dependent, nd s the mteril is heted Mn clusters dissolve nd the number of isolted Mn defects increses. Consequently the line fetures from isolted Mn sites re pprent t ll tempertures in the purer Icelnd spr crystls, wheres in the hevily doped clcite, line fetures only emerge t high temperture, nd t low temperture the Mn exists s clusters which give brod emission bnds. Therml processing of smples hs been used to modify the dynmics of the clustering process (Down et l., 1985). Crushing the clcite lso produces chnges in intensity of emission which pper to be relted to Mnmigrtion (Khnlry nd Townsend, 1991). Fluorite. Studies of thermoluminescence, photoluminescence nd opticl bsorption of nturl fluorites from Spin (MI, M2) nd USA (M3) (Clderon et l., 1992) hve demonstrted tht different colour fluorites re chrcterised by different combintions of rre erth impurities. A key observtion is tht ll three techniques re required, s seprtely they ech only detect selection of the impurities, despite the fct tht the impurities re ll rte erth ions. One obvious reson for this is tht nturlly cquired rdition doses or lbortory experiments using X-ry irrdition cn lter the chrge stte of the RE ion from RE 2+ to RE 3+. The smple of green fluorite (M1) exhibited opticl bsorption chrc- THERMOLUMINESCENCE SPECTRA 219 teristic of smrium (Sm 2+) nd photoluminescence peks chrcteristic of smrium nd europium (Eu2+). The spectrum of the nturl TL showed mixture of brod peks nd line fetures from nm (Fig. 2). When the sme smple ws cooled, irrdited with X-rys nd then reheted, series of line fetures, chrcteristic of Dy 3+ trnsitions, ws present t 50~ together with series of brod peks in the blue region t higher tempertures (Fig. 2b). By contrst, the smple of yellow fluorite (M2) showed opticl bsorption chrcteristic of 0 3- substitution for two djcent F- ions, nd photoluminescence chrcteristic of cerium (Ce 3+) nd Eu 2+. The spectrum of the nturl TL shows well defined brod pek t 380 nm t high temperture but no clerly defined line fetures (Fig. 3) After X-ry irrdition, line fetures were reveled t low tempertures in the wvelength rnge nm (Fig. 3b). The mjority of these fetures re t wvelengths chrcteristic of dysprosium (Dy3+). The drk-blue fluorite (M3) showed opticl bsorption chrcteristic of ggregted colloids of toms of clcium nd no photoluminescence. The spectrum of nturl TL is dominted by line spectr t high temperture (Fig. 4). After X-ry irrdition, the line fetures repper t low tempertures nd re gin chrcteristic of Dy 3+ (Fig. 4b). In ll three cses the spectr from the nturl nd the nneled nd X-ry irrdited smples llow the smples to be distinguished from ech other. In ddition, the roles plyed by the rre erth impurities pper complex nd they my be vried by therml nd/or rdition tretments. All three techniques give informtion on the REE impurities but, in the cse of TL, Dy 3+ ppers to dominte emission of the line spectr. b 4.oJ. ~ 1.o ~ o z~..~. ~'~'~ ~_. ~ ~ o..~.- ~''~ F16.2(). Nturl TL of green fluorite (Spin) (b) X-ry irrdited green fluorite.

4 220 H. M. RENDELL ETAL. '~'~..o_,-,..'~'~,~ ~o ~'~"~'~<~ ~'~'..o_~. "~"~'~, ~.~.-~, 9 ~'~'~ ~ ~'o.~..r ~_ -~,~" ~ <'~" ~S'~r Fro. 3(). Nturl TL of yellow fluorite (Spin) (b) X-ry irrdited yellow fluorite. Zircon. Zircon is common component of hevy minerl suites, nd fission trck dting of zircons is now widely-ccepted dting technique. TL spectr of nturl zircons re dominted by the line spectr chrcteristic of the rre erth Yb 3+ nd/or Dy 3+ (Chee et l., 1988). The emission spectr of two smples of nturl zircon show brod fetures in the blue region ( nm) nd series of line spectr in the region nm (Fig. 5 nd b). Both spectr were recorded fter 750 Gy X-r_y irrdition nd t heting rtes of 20~ min-. The Sri Lnkn zircon smple shown in Fig. 5 shows strong emission t the red end of the spectrum which is chrcteristic of Eu 3+ wheres the Mexicn zircon smple shown in Fig. 5b exhibits strong line fetures chrcteristic of Dy 3+ t 100 ~ nd of Tb 3+ t higher tempertures. Both spectr contin brod feture t bout 100 ~ with pek t 385 nm. This brod spectrl feture is chrcteristic of minerls contining SiO4 groups, including qurtz nd feldsprs, nd my be relted to defective Si-O bonds (Chee et l., 1988). As in the cse of the fluorite smples discussed bove, it is pprent tht while severl rre erth impurity ions my be present in zircon crystl, the ions contribute to the TL signl in different wys. The low-temperture signls tend to be dominted by Dy 3+, but storge fter irrdition nd before heting cn result in the growth of Tb 3+ relted emission fetures t the expense of the Dy 3+ ones. Such conversions cn tke plce t unexpectedly low tempertures, nd we hve monitored the growth of 90~ 630 nm Tb 3+ line, t the expense of the Dy 3+ lines. This work hs served to underline the importnce of rre erth impurities s rditive recombintion centres s well s the complexity of such centres. The feldspr group. In contrst to the shrp, clerly difined spectrl fetures exhibited by fluorites, TL emissions from the feldspr group re dominted by brod bnd emissions. All the 0,1 Fie. 4(). Nturl TL of drk-blue fluorite (U.S.A.) (b) X-ry irrdited drk-blue fluorite.

5 THERMOLUMINESCENCE SPECTRA 221 i,o] ' I NNN ~.~ ~.~ d Fro. 5(). X-ry irrdited zircon (Sri Lnk) (b) X-ry irrdited zircon (Mexico). feldsprs show strong emission t the red end of the spectrum ( nm), nd this emission hs generlly been ttributed to conversion between Fe 2+ nd Fe 3+ impurities tht ct s recombintion sites for either holes or electrons, depending on vlency. In contrst to lbite, microcline nd orthoclse, the smples of plgioclse studied by the Sussex group hve shown no strong emissions in the blue spectrl region (Dli et l., 1988; Kirsh nd Townsend, 1988) (Fig. 6, b). Brod emissions in the blue spectrl region pper chrcteristic of intrinsic defects modified by ssocition with impurities such s n AI-O- AI group (Kirsh nd Townsend, 1988). If the blue emission is cused by electron-hole recombintion then defect sites cn influence the frction of the recombintions which re rditive, for exmple emission intensity my be enhnced where the defects become more complex (e.g. by the presence of Cu 2+ impurities ner to the hole trp). Dt from cthodoluminescence studies of feldsprs indicte tht emission bnds in the blue region my be enhnced by the presence of Cu 2+ (420 nm _+ 5 nm) nd Ti 4+ (460 nm _+ 10 nm) (Mrshll, 1988). The reltive strength of the blue nd red emissions vries with the prticulr feldspr smples mesured, but no line spectr, chrcteristic of rre erth impurity ions, hve been detected in ny of the spectr tht we hve exmined. There is some evidence from CL studies of nturl nd synthetic feldsprs tht RE ions rrely if ever occur in sufficient concentrtions in nturl feldsprs to be effective ctivtors (Mrshll, 1988). Conclusions The exmples given bove hve served to emphsise the vlue of TL spectrl studies of minerls. TL is very sensitive tool for probing defects within crystls nd the spectr llow both the nture of the electron nd hole trps nd of b ~.~ ~i 4) 0.0.~ Fro. 6(). X-ry irrdited orthoclse feldspr (b) X-ry irrdited plgioclse feldspr.

6 222 the rditive recombintion centres to be explored. The bility to detect low concentrtions of impurity ions is of vlue in its own right, nd of potentil vlue for provennce studies. Acknowledgements We re grteful to the SERC nd the Irnin government for finncil support. H. M. RENDELL ETAL. References Aitken, M. J. (1985) Thermoluminescence dting, Acdemic Press, London. Clderon, T., Khnlry, M-R., Rendell, H. M., nd Townsend, P. D. (1992) Luminescence from nturl fluorite crystls. Nucler Trcks nd Rdition Mesurements, 20, Chee, J., Oczkowski, H. L., Kirsh, Y., Scott, A., Siynbol, W. O., nd Townsend, P. D. (1988) TL spectr of nturl zircons. Ibid., 14, Dll, M. L., Kirsh, Y., Rendell, H. M., nd Townsend, P. D. (1988) TL emission spectr of nturl feldspr. Ibid., 14, Down, J. S. Flower, R., Strin, J. A., nd Townsend, P. D. (1985) Thermoluminescence emission spectr of clcite. Nucler Trcks, 10, Horowitz, Y. S. (1984) Thermoluminescence ndphotoluminescent dosimetry, CRC Press, Boc Rton, Florid. 3 vols. Khnlry, M.-R. nd Townsend, P. D. (1991) TL spectr of single crystl nd crushed clcite. Nucler Trcks nd Rdition Mesurements, 18, Kirsh, Y. nd Townsend, P. D. (1988) Specultions on the blue nd red bnds in the TL emission spectrum of lbite nd microcline. Ibid., 14, Luff, B. J. nd Townsend, P. D. (1992) A novel high sensitivity TL spectrometer. J. Mes. Sci. Techn., 3, Mrshll, D. J. (1988) Cthodoluminescence of geologicl mterils, Unwin Hymn, Boston. 146pp. McKeever, S. W. S. (1985) Thermoluminescence of solids, Cmbridge University Press, Cmbridge. Medlin, W. L. (1968) The nture of trps nd emission centers in thermoluminescent rock mterils. In Thermoluminescence of Geologicl Mterils (McDougll, D. J., ed.). Acdemic Press, London, Rendell, H. M., Steer, D. C., Townsend, P. D., nd Wintle, A. G. (1985) TL emission spectr of obsidin. Nucler Trcks, 10, (4-6), Strin, J. A., Townsend, P. D., Jssemnejd, B. nd McKeever, S. W. S. (1985) Emission spectr of meteorites during TL. Erth Plnet. Sci. Lett., 77, Townsend, P. D. nd Kirsh, Y. (1989) Spectrl mesurement during thermoluminescence-- An essentil requirement. Contemporry Physics, 30, [Mnuscript received 9 Mrch 1992: revised 20 July 1992]