Energy-Storage Mechanisms and Thermoluminescence Processes in

Chapter 9

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: November 29, 1990 | doi: 10.1021/bk-1990-0415.ch009

Energy-Storage M e c h a n i s m s a n d T h e r m o l u m i n e s c e n c e Processes i n M i n e r a l s Stephen W. S. McKeever Department of Physics, Oklahoma State University, Stillwater, OK 74078-0444 The paper discusses mechanisms by which energy is stored in minerals following the absorption of ionizing radiation, and is subsequently released during heating to produce thermoluminescence (TL). It is discussed how the primary processes of defect formation during irradiation occur via electronic excitation. This can take the form of either the creation of electron-hole pairs, followed by trapping into localized energy states, or of exciton creation leading to the formation of stable vacancy and interstitial defects. Heating the sample after the irradiation causes the release of this stored energy in the form of phonons or photons. Photon emission, ie. luminescence, results from either electron-hole recombination or from vacancy-interstitial recombination. Several examples of both types are discussed for crystalline CaF and SiO . 2

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In this paper we shall be concerned with those luminescence processes which occur following the absorption of ionizing radiation in some common minerals. In particular, we shall be looking at those processes by which energy from the radiation field is absorbed and stored by the material, and at those processes which result in the emission of visible light from the material as it is then heated (thermoluminescence, TL). For illustration of some of the primary processes involved in these mechanisms we shall use CaF , as an example of a fluorite-structure material, and crystalline SiO (quartz). Both of these materials have significant mineralogical importance. Additionally, we shall occasionally refer to alkali-halide-structured materials, eg. halite (NaCl) to illustrate some of the important principles. 2

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0097-6156/90/0415-0166506.00/0 © 1990 American Chemical Society

In Spectroscopic Characterization of Minerals and Their Surfaces; Coyne, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

9. McKEEVER

Energy Storage and Thermoluminescence in Minercds


Energy S t o r a g e F o l l o w i n g I r r a d i a t i o n E l e c t r o n i c E x c i t a t i o n . The s t o r a g e o f energy i n an i n s u l a t i n g m a t e r i a l by t h e a b s o r p t i o n o f i o n i z i n g r a d i a t i o n o c c u r s v i a two major p r o c e s s e s , namely, e l e c t r o n i c e x c i t a t i o n and d i s p l a c e m e n t damage. R a d i a t i o n damage by these p r o c e s s e s i n i n s u l a t o r s has been a t o p i c o f i n t e n s e r e s e a r c h f o r decades and s e v e r a l e x c e l l e n t review a r t i c l e s already e x i s t concerning a v a r i e t y o f m a t e r i a l t y p e s ( e g . o x i d e s ( 1 - 5 ) , a l k a l i h a l i d e s ( 6 - 8 ) and f l u o r i t e s ( 9 ) ) . I t i s r e l e v a n t t o ask what i s t h e r e l a t i v e importance o f t h e above two p r o c e s s e s a t p r o d u c i n g r a d i a t i o n damage and t h e r e b y s t o r i n g energy? To answer t h i s we use S i C ^ as an example m a t e r i a l and use the arguments o f Devine (10) who n o t e s t h a t ^ C o g photons when absorbed i n S i 0 produce p r i m a r y Compton e l e c t r o n s o f e n e r g i e s o f about lMeV. The t h r e s h o l d f o r d i s p l a c e m e n t damage i n S1O2 i s a p p r o x i m a t e l y 150eV and t h e r e f o r e one e x p e c t s about 4 x l 0 displaced oxygens, p e r Gy o f absorbed dose, p e r cnr*. Assuming t h a t o n l y 1/40 o f these produce s t a b l e d e f e c t s , t h i s g i v e s an e x p e c t a t i o n o f about 10* d i s p l a c e d oxygens p e r Gy p e r cnr*. However, one a c t u a l l y g e t s a p p r o x i m a t e l y 10** p e r Gy p e r cnr*, o r two o r d e r s o f magnitude more d e f e c t s than e x p e c t e d . The a d d i t i o n a l d e f e c t s have come from e l e c t r o n i c e x c i t a t i o n and most d e f e c t f o r m a t i o n o c c u r s v i a t h i s p r o c e s s f o l l o w i n g a b s o r p t i o n o f e n e r g e t i c photons. F u r t h e r e v i d e n c e f o r t h e importance o f e l e c t r o n i c e x c i t a t i o n as a p r i m a r y means o f d e f e c t c r e a t i o n comes from s t u d i e s o f " s u b - t h r e s h o l d " damage i n which t h e energy o f t h e incoming p a r t i c l e i s l e s s than t h a t r e q u i r e d f o r a "knock-on" c o l l i s i o n . D e f e c t f o r m a t i o n , s p u t t e r i n g and d e s o r p t i o n o f s u r f a c e a d s o r b a t e s a l l o c c u r and these a r e a l l p r o c e s s e s w h i c h r e q u i r e c o u p l i n g o f t h e e l e c t r o n i c energy i n t o t h e l a t t i c e ( 1 1 ) . As a r e s u l t o f these c o n s i d e r a t i o n s , we s h a l l c o n c e n t r a t e i n t h i s paper on d e f e c t f o r m a t i o n by e l e c t r o n i c e x c i t a t i o n o n l y . Energy s t o r a g e v i a e l e c t r o n i c e x c i t a t i o n c a n be f u r t h e r s u b - d i v i d e d i n t o two c a t e g o r i e s - e l e c t r o n - h o l e p a i r p r o d u c t i o n and e x c i t o n c r e a t i o n . W i t h b o t h o f these e l e c t r o n i c e n t i t i e s , i t i s i m p o r t a n t t o r e a l i z e t h a t t h e i r w a v e f u n c t i o n s a r e n o t l o c a l i z e d on any p a r t i c u l a r d e f e c t and they a r e t h e r e f o r e f r e e t o wander throughout t h e l a t t i c e i m m e d i a t e l y a f t e r i r r a d i a t i o n . As a r e s u l t o f t h i s s i g n i f i c a n t energy m i g r a t i o n may take p l a c e b e f o r e t h e s e e n t i t i e s a r e l o c a l i z e d , o r ' t r a p p e d , a t p a r t i c u l a r l a t t i c e s i t e s . F o r example, e x c i t o n l i f e t i m e s o f microseconds t o m i l l i s e c o n d s c a n be found and i n some m a t e r i a l s m i g r a t i o n d i s t a n c e s o f t h e o r d e r o f m i l l i m e t e r s can o c c u r b e f o r e e x c i t o n a n i h i l a t i o n ( v i a r e c o m b i n a t i o n ) . I n o r d e r t o s t o r e energy o f these d e f e c t s , t h e r e f o r e , i t i s n e c e s s a r y t o f i r s t o f a l l l o c a l i z e the w a v e f u n c t i o n s a t l a t t i c e s i t e s . We now d e a l w i t h each o f these e l e c t r o n i c s p e c i e s i n t u r n i n o r d e r t o see how t h i s i s done. a m m a

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E l e c t r o n - H o l e P a i r s . L o c a l i z a t i o n o f e l e c t r o n s and h o l e s o c c u r s v i a the p r o c e s s e s o f n o n - r a d i a t i v e charge t r a p p i n g . These p r o c e s s e s a r e r e p r e s e n t e d p h e n o m e n o l o g i c a l l y i n F i g u r e 1. An example o f a s i m p l e t r a p p i n g p r o c e s s i s the coulombic a t t r a c t i o n o f an e l e c t r o n and an


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X" i o n vacancy i n a m a t e r i a l o f t h e type M X"" and t h e p r o c e s s e s o f n o n - r a d i a t i v e charge t r a p p i n g have been d i s c u s s e d i n depth by Henry and Lang (12) and by P a s s l e r ( 1 3 ) . W i t h r e f e r e n c e t o F i g u r e 1, t h e a b s o r p t i o n o f a photon w i t h energy g r e a t e r than t h e band gap r e s u l t s i n t h e c r e a t i o n o f a f r e e e l e c t r o n and a c o r r e s p o n d i n g f r e e h o l e . These charges move i n d e p e n d e n t l y throughout t h e c r y s t a l u n t i l they become l o c a l i z e d a t s u i t a b l e t r a p p i n g s i t e s . Each charge may undergo s e v e r a l t r a p p i n g and d e - t r a p p i n g c y c l e s b e f o r e b e i n g l o c a l i z e d a t a c e n t e r i n which the b i n d i n g energy o f t h e t r a p p e d charge i s g r e a t e r t h a n about 25kT. Under these c i r c u m s t a n c e s t h e charges become l o c a l i z e d and t h i s g i v e s r i s e t o a n o n - e q u i l i b r i u m p o p u l a t i o n o f t r a p p e d charge i n m e t a s t a b l e energy l e v e l s ( i e . t r a p p e d e l e c t r o n s i n energy s t a t e s above t h e e q u i l i b r i u m c h e m i c a l p o t e n t i a l ( F e r m i l e v e l ) and t r a p p e d h o l e s s t a t e s below the F e r m i l e v e l ) . The energy l e v e l s i n q u e s t i o n a r i s e from the presence o f l a t t i c e d i s o r d e r w i t h i n the m a t e r i a l , such as f o r e i g n i o n s , l a t t i c e v a c a n c i e s , d i s l o c a t i o n s , e t c . Such l a t t i c e d i s o r d e r i s a p r e r e q u i s i t e f o r charge s t o r a g e i n t h i s manner. E x c i t o n s . L o c a l i z a t i o n o f the e x c i t o n s occurs v i a the process o f ' s e l f - t r a p p i n g * t o produce s o - c a l l e d S e l f Trapped E x c i t o n s (STE). F o r a d e s c r i p t i o n o f S T E s we r e f e r t o F i g u r e 2 i n w h i c h a r e s k e t c h e d t h r e e t y p i c a l c o n f i g u r a t i o n s f o r STE's i n an M X" c r y s t a l . Toyozawa (15) d i s c u s s e s t h e f o r m a t i o n o f STE's i n w h i c h t h e e l e c t r o n and h o l e a r e l o c a l i z e d c o n c e n t r i c a l l y (STE 1 and STE 2) o r eccen t r i c a l l y (STE 3 ) . I n types 2 and 3 t h e h o l e i s t r a p p e d on an X " m o l e c u l e and t h e s t r o n g coulombic r e p u l s i o n between i t and t h e t r a p p e d e l e c t r o n make t h i s type o f STE h i g h l y u n s t a b l e . I n h a l i t e - s t r u c t u r e m a t e r i a l s b o t h STE 2 and STE 3 form, w i t h type 2 c o n v e r t i n g i n t o t y p e 3 v i a t h e p r o c e s s d e p i c t e d i n F i g u r e 3. The coulombic r e p u l s i o n i n d u c e s motion o f the X ~ m o l e c u l e a l o n g a c l o s e - p a c k e d d i r e c t i o n . T h i s i n i t i a t e s a d i s p l a c e m e n t sequence w h i c h , as we w i l l see l a t e r , i s o f fundamental importance i n t h e f o r m a t i o n o f s t a b l e i n t e r s t i t i a l s and v a c a n c i e s . I n f l u o r i t e - s t r u c t u r e compounds t h e s e l f - t r a p p e d e x c i t o n c o n f i g u r a t i o n i s t h a t o f type 3, as d e p i c t e d i n F i g u r e 4. Here i t s h o u l d be noted t h a t the X ~ m o l e c u l e i s a l i g n e d a l o n g a d i r e c t i o n which i s n o t a c l o s e - p a c k e d d i r e c t i o n i n t h e CaF2 s t r u c t u r e . I n S i 0 e c c e n t r i c (STE 3) e x c i t o n s a r e a l s o formed i n which t h e e l e c t r o n i s l o c a l i z e d i n an oxygen v a c a n c y , f o r m i n g an E^' c e n t e r , and t h e h o l e i s l o c a l i z e d on a d j a c e n t oxygens f o r m i n g an 0 m o l e c u l e , o r peroxy r a d i c a l ( s e e Freund's paper i n t h i s publication). The n e x t s t a g e i n the d e f e c t f o r m a t i o n p r o c e s s i n v o l v e s t h e t r a n s f e r o f energy from t h e e l e c t r o n i c e x c i t a t i o n t o t h e l a t t i c e . Although the exact d e t a i l s o f the necessary e x c i t e d s t a t e s which induce t h e i n s t a b i l i t y a r e t h e s u b j e c t o f some c o n t r o v e r s y , i t i s known t h a t t h e b a s i c cause o f t h e t r a n s f o r m a t i o n i s t h e coulombic r e p u l s i v e i n t e r a c t i o n between t h e e l e c t r o n and the X ~ m o l e c u l e . I t appears t h a t t h e r e i s s t r o n g c o u p l i n g between t h e e l e c t r o n i n an e x c i t e d s t a t e and the h o l e , a l s o i n an e x c i t e d s t a t e . The i n s t a b i l i t y m a n i f e s t s i t s e l f by m o t i o n o f the X ~ m o l e c u l e i n t h e d i r e c t i o n ( i n M X~ c r y s t a l s ) . The X ~ m o l e c u l e c a n be 1

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McKEEVER

Energy Storage and Thermoluminescence in Minerals

b— F i g u r e 1. P h e n o m e n o l o g i c a l model o f e l e c t r o n - h o l e p a i r p r o d u c t i o n and charge t r a p p i n g f o l l o w i n g a b s o r p t i o n o f an e n e r g e t i c photon o f energy g r e a t e r than the band gap.

STE 1

STE 2

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F i g u r e 2. Three t y p i c a l c o n f i g u r a t i o n s o f S e l f - T r a p p e d E x c i t o n s (STE), named STE 1, STE 2 and STE 3. (Reproduced w i t h k i n d p e r m i s s i o n from Ref. 14. C o p y r i g h t 1986 Gordon and Breach.)

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F i g u r e 3. T r a n s f o r m a t i o n o f STE 2 i n t o STE 3 i n M X" crystals (eg. h a l i t e ) . (Reproduced w i t h k i n d p e r m i s s i o n from Ref. 14. C o p y r i g h t 1986 Gordon and Breach.)


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s e p a r a t e d from the e l e c t r o n by s e v e r a l l a t t i c e s p a c i n g s i n a m a t t e r o f s e v e r a l p i c o s e c o n d s (11,13) and i n t h i s way t h e e l e c t r o n i c e x c i t a t i o n energy i s t r a n s f e r r e d d i r e c t l y t o t h e l a t t i c e . I t i s i m p o r t a n t t o n o t e t h a t i n the h a l i t e - s t r u c t u r e m a t e r i a l s the d i s p l a c e m e n t sequence i s a l o n g a c l o s e - p a c k e d d i r e c t i o n e n a b l i n g momentum t r a n s f e r t o o c c u r such t h a t t h e i n t e r s t i t i a l becomes sepa r a t e d from the vacancy by s e v e r a l atomic p o s i t i o n s . I n the f l u o r i t e - s t r u c t u r e compounds and i n c r y s t a l l i n e S i 0 , however, such a d i s p l a c e m e n t sequence i s n o t p o s s i b l e s i n c e t h e STE i s n o t a l i g n e d a l o n g a c l o s e - p a c k e d d i r e c t i o n . As a r e s u l t s t a b l e , w e l l - s e p a r a t e d i n t e r s t i t i a l s and v a c a n c i e s a r e v e r y u n l i k e l y i n t h e s e m a t e r i a l s a t a l l b u t the v e r y l o w e s t t e m p e r a t u r e s . Transient defect formation only i s observed (9,16). I n t h e d i s c u s s i o n so f a r we have n e g l e c t e d t o mention any e f f e c t s due t o l a t t i c e d i s o r d e r . T h i s w i l l s u r e l y be o f major importance i n m i n e r a l o g i c a l m a t e r i a l s . We have noted how, i n t h e a l k a l i h a l i d e s , s t a b l e i n t e r s t i t i a l atoms (Η-centers) and e l e c t r o n s i n v a c a n c i e s ( F - c e n t e r s ) may be formed, even i n a p e r f e c t c r y s t a l , whereas i n C a F and q u a r t z o n l y t r a n s i e n t d e f e c t s a r e formed, w i t h i n t e r s t i t i a l - v a c a n c y recombination o c c u r r i n g r a p i d l y . I f lattice d i s o r d e r ( s t r u c t u r a l i m p e r f e c t i o n s , p o l y c r y s t a l l i n i t y , amorphous s t r u c t u r e , i m p u r i t i e s ) i s i n t r o d u c e d i n t o t h e system the i n t e r s t i t i a l s and v a c a n c i e s may be s t a b i l i z e d by these e n t i t i e s f o r m i n g ' p e r t u r b e d H- and F - c e n t e r s . Such p e r t u r b e d c e n t e r s may be formed, f o r example, i f the m o b i l e X m o l e c u l e becomes t r a p p e d by ( s a y ) an i m p u r i t y , thus p r e v e n t i n g i n t e r s t i t i a l / v a c a n c y r e combination. R e c a l l i n g that the e x c i t o n i s a h i g h l y mobile species b e f o r e i t becomes s e l f - t r a p p e d , one might even e x p e c t t h a t t h e e x c i t o n i t s e l f moves through t h e l a t t i c e u n t i l i t f i n d s a l a t t i c e i m p e r f e c t i o n a t w h i c h p o i n t e x c i t o n l o c a l i z a t i o n o c c u r s and t h e d e f e c t c r e a t i o n sequence i s i n i t i a t e d (17). The n e t r e s u l t i s t h a t l a t t i c e d i s o r d e r enhances t h e f o r m a t i o n o f s t a b l e d e f e c t s f o l l o w i n g irradiation. The e x i s t e n c e o f f r e e i n t e r s t i t i a l p o i n t d e f e c t s f o r m i n g t h e complements t o the vacancy c e n t e r s i s g e n e r a l l y n o t observed f o l l o w i n g i r r a d i a t i o n a t room t e m p e r a t u r e . A t t h e s e temperatures the i n t e r s t i t i a l s c l u s t e r t o g e t h e r t o form i n t e r s t i t i a l aggregates and d i s l o c a t i o n l o o p s . However, l a t t i c e d i s o r d e r c a n slow down o r p r e v e n t t h e a g g r e g a t i o n p r o c e s s due t o i n t e r s t i t i a l t r a p p i n g .


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Prompt Luminescence. B e f o r e g o i n g on t o d i s c u s s the mechanisms by w h i c h t h e s t o r e d energy c a n be r e l e a s e d from t h e l a t t i c e we s h a l l pause here t o d i s c u s s what happens t o t h e e l e c t r o n i c e x c i t a t i o n energy i f i t i s n o t s t o r e d i n t h e l a t t i c e by any o f the mechanisms d i s c u s s e d above. I n wide-band-gap i n s u l a t o r s such as those d i s c u s s e d here d i r e c t band-to-band r e c o m b i n a t i o n o f e l e c t r o n s w i t h h o l e s i s n o t g e n e r a l l y observed and i n d i r e c t , Shockley-Read r e c o m b i n a t i o n v i a l o c a l i z e d s t a t e s i n the band gap i s the main cause o f e l e c t r o n - h o l e r e c o m b i n a t i o n . The r e c o m b i n a t i o n p r o c e s s r e s u l t s i n t h e e m i s s i o n o f phonons o r photons. S i n c e we a r e i n t e r e s t e d i n luminescence e m i s s i o n i n t h i s paper i t i s t h e e m i s s i o n o f photons t h a t we s h a l l c o n s i d e r . The e m i s s i o n wavelength c a n be c h a r a c t e r i s t i c o f t h e p o s i t i o n ( i n terms o f energy) o f t h e l o c a l i z e d s t a t e w i t h i n t h e band gap, o r v i a t h e p r o c e s s o f energy t r a n s f e r , i t may



9. McKEEVER

171 Energy Storage and Thermoluminescence in Minerals

be a p r o p e r t y o f t h e type o f d e f e c t which g i v e s r i s e t o t h e l o c a l i z e d s t a t e . F o r example, r e c o m b i n a t i o n s t a t e s a r i s i n g from the presence o f r a r e - e a r t h i m p u r i t i e s o f t e n r e s u l t i n l i n e e m i s s i o n s which a r e c h a r a c t e r i s t i c o f those i o n s . The d e t a i l s o f the e m i s s i o n can o f t e n g i v e i n f o r m a t i o n about t h e e x a c t s t r u c t u r e o f t h e s i t e i n which t h e i o n i s l o c a t e d . (See W r i g h t ' s paper e l s e w h e r e i n t h i s p u b l i c a t i o n . ) Once a g a i n we see t h a t l a t t i c e d i s o r d e r i s r e q u i r e d f o r prompt e m i s s i o n from e l e c t r o n - h o l e r e c o m b i n a t i o n . F o r e x c i t o n s , t h e e l e c t r o n and h o l e c a n a n n i h i l a t e each o t h e r w i t h e m i s s i o n wavelengths which a r e a p r o p e r t y o f t h e p a r t i c u l a r e x c i t e d s t a t e s o f t h e e x c i t o n and t h e l o c a l l a t t i c e environment. F o r example, " i n t r i n s i c " luminescence due t o e x c i t o n a n n i h i l a t i o n i n pure N a C l i s a t 250nm (low-energy t r i p l e t s t a t e ) and 370nm ( h i g h energy s i n g l e t s t a t e ) ( L 8 ) . I n C a F t h e i n t r i n s i c e m i s s i o n appears a t 280nm ( 9 ) , w h i l e i n S i 0 i t i s a t 440nm ( 1 6 ) . W i t h t h e i n t r o d u c t i o n o f i m p u r i t i e s t h e i n t r i n s i c luminescence becomes p e r t u r b e d . F o r example, i n NaCl p e r t u r b e d t r i p l e t - s t a t e e m i s s i o n s appear a t 440nm and 424nm, depending on t h e i n d e n t i t y o f t h e p e r turbing impurity (18). S i m i l a r l y , impurity-perturbed exciton a n n i h i l a t i o n e m i s s i o n s o c c u r i n C a F a t a p p r o x i m a t e l y 290nm ( 1 9 ) . 2

2

2

T h e r m a l l y S t i m u l a t e d Energy R e l e a s e ;

Thermoluminescence ( T L )

E l e c t r o n - H o l e Recombination. A phenomenological r e p r e s e n t a t i o n o f an e l e c t r o n - h o l e r e c o m b i n a t i o n p r o c e s s i s shown i n F i g u r e 5. F o r i l l u s t r a t i o n we assume t h a t i t i s t h e e l e c t r o n t h a t i s t h e t h e r m a l l y - f r e e d c a r r i e r a l t h o u g h t h e d i s c u s s i o n c o u l d j u s t as w e l l proceed on t h e b a s i s t h a t the h o l e i s t h e r m a l l y l i b e r a t e d . The p r o c e s s b e g i n s when phonon c o u p l i n g w i t h t h e l a t t i c e e n a b l e s the t r a p p e d e l e c t r o n s t o absorb an amount o f t h e r m a l energy kT. The p r o b a b i l i t y t h a t t h i s energy w i l l surmount t h e p o t e n t i a l energy b a r r i e r Ε i s then ρ = s e x p [ - E / k T ] , where s i s a c o n s t a n t r e l a t e d t o t h e l a t t i c e v i b r a t i o n f r e q u e n c y and t h e e n t r o p y change a s s o c i a t e d w i t h t h e r e a c t i o n . A t a s u i t a b l y h i g h temperature t h e t r a p p e d e l e c t r o n w i l l be l i b e r a t e d i n t o t h e c o n d u c t i o n band and be d e l o c a l i z e d . Recombination w i t h t r a p p e d h o l e s c a n then t a k e p l a c e and l i g h t may be e m i t t e d i n a s i m i l a r f a s h i o n t o t h a t d e s c r i b e d i n the p r e v i o u s s e c t i o n . As a r e s u l t we have t h e t h e r m a l l y s t i m u l a t e d r e t u r n o f t h e system from i t s m e t a s t a b l e s t a t e t o i t s e q u i l i b r i u m s t a t e , w i t h t h e excess energy l i b e r a t e d as l i g h t . Not a l l such r e c o m b i n a t i o n p r o c e s s e s a r e r a d i a t i v e , o f c o u r s e , b u t , as w i t h prompt l u m i n e s c e n c e , we a r e i n t e r e s t e d here o n l y i n those p r o c e s s e s t h a t l e a d t o luminescence. There a r e many examples o f t h i s s i m p l e p r o c e s s i n a wide v a r i e t y o f m a t e r i a l s . F o r i l l u s t r a t i o n we d i s c u s s here the TL e m i s s i o n t h a t o c c u r s a t a p p r o x i m a t e l y 100°C i n S i 0 f o l l o w i n g t h e a b s o r p t i o n o f i o n i z i n g r a d i a t i o n a t room t e m p e r a t u r e . The e m i s s i o n i s c h a r a c t e r i z e d by two, o v e r l a p p i n g , broad bands, p e a k i n g a t 470nm and 380nm ( F i g u r e 6 ) . The p i c t u r e t h a t has emerged from a d e t a i l e d s t u d y o f t h i s TL peak i s t h a t d u r i n g i r r a d i a t i o n e l e c t r o n s a r e t r a p p e d a t Ge i m p u r i t i e s and h o l e s a r e t r a p p e d a t A l and H c e n t e r s , according t o : 2




F i g u r e 5. Phenomenological r e p r e s e n t a t i o n o f the t h e r m a l r e l e a s e o f t r a p p e d e l e c t r o n s and t h e i r r e c o m b i n a t i o n w i t h t r a p p e d h o l e s t o y i e l d TL.


Energy Storage and Thermoluminescence in Minerals


McKEEVER

F i g u r e 6. I s o m e t r i c p l o t o f TL e m i s s i o n v e r s u s w a v e l e n g t h v e r s u s temperature f o r a sample o f n a t u r a l q u a r t z . The TL s i g n a l i s produced v i a the mechanism d e s c r i b e d i n E q u a t i o n s (1-6). (Reproduced w i t h p e r m i s s i o n from Ref. 20. Copyright 1988 Pergammon P r e s s . )


174

SPECTROSCOPIC CHARACTERIZATION OF MINERALS AND THEIR SURFACES (Ge0 )° + e"

(Ge0 )"

4

+

(A10 M)° + h 4

(H 0 )° + h 4

(1)

4

—(A10 )° + M

+

(2)

(H 0 )° + H

+

(3)

4

+

4

3

4

where M i s an a l k a l i m e t a l , u s u a l l y L i o r Na (21,22) . heating the e l e c t r o n i s r e l e a s e d thus: (Ge0 )" Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: November 29, 1990 | doi: 10.1021/bk-1990-0415.ch009

4

and

—

During

(Ge0 )° + e"

(4)

4

recombines w i t h t h e t r a p p e d h o l e s t o produce TL: e" + (A10 )° + M

+

— ( A 1 0 M ) ° + photon

4

e" + ( H 0 ) ° 3

4

+

H

+

4

-

(H 0 )° + P h o t o n 4

4

(5)

4 7 Q n m

3 8 0 n m

( 6 )

The e x a c t energy s t a t e s w h i c h g i v e r i s e t o t h e 470nm and 380nm e m i s s i o n s a r e unknown. 3+ A second example i s p r o v i d e d by CaF2 doped w i t h Ce ions. Ce^ e n t e r s the C a F l a t t i c e s u b s t i t u t i o n a l l y f o r t h e h o s t 2+ 2 Ca i o n s . S e v e r a l s i t e symmetries a r e p o s s i b l e depending upon t h e charge compensation. N o n - l o c a l l y compensated C e ^ produces a c e n t e r o f 0^ symmetry, whereas C symmetry r e s u l t s when F~ i o n s i n i n t e r s t i t i a l p o s i t i o n s t a k e up nn p o s i t i o n s t o t h e i m p u r i t i e s a l o n g d i r e c t i o n s . I n a d d i t i o n many h i g h e r - o r d e r c l u s t e r c o n f i g u r a t i o n s o f these i m p u r i t y - i n t e r s t i t i a l p a i r s e x i s t ( 2 3 ) . When i r r a d i a t e d a t room temperature t h e n o n - l o c a l l y compensated centers trap electrons to y i e l d C e i n 0^ symmetry, w h i l e t h e h o l e s become s e l f - t r a p p e d p r o d u c i n g p e r t u r b e d centers (24), thus: 3

+

0

+

4 v

2 +

e

3

2

" + Ce + Oh —

Ce + Oh

(7)

and h

+

—•

self-trapped

—•

(8)

z

Energy-Storage Mechanisms and Thermoluminescence Processes in

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