Indian Jo urnal of Pure & Applied Phys ics Vol. 40. December 2002. pp. 867-872

Luminescence in divalent impurity activated LiF single crystals R S Kher, S J Dhoh1e*, M S K Khokhar** & M Chougaonkar***

Department of Phy~ic~. Government Auto nomous PG Science Co ll ege, Bila~pur 49500 I

*Kamla Nehru Co ll ege. Sakkardma Square, Nagpur 440009

**Guru Ghas idas Uni versity, Bilaspur 495009

"'**Environment al Assessment Divi sion , Bhabha Ato mi c Research Centre. Mumbai 400085

Received 2 1 January 2002; revi sed 9 August 2002: accepted

LiF single crystal s doped wi th va ryi ng co ncentrations o f Ba. Sr and Ca have heen grown by Bridgman tcchniquc and

their th crmo-Iumine~cence (TL) and TL emiss ion spectrum induced hy y-irradiation at room temperature fo r doses betwecn 103 to 10' R have been recorded. In the TL glow curves, peaks around 380, 425 and 500 K are produced irrespective of the

impurity added , hut. the relative intensity of these glow peaks varies for different dopant s. At low y-exposure the TL

effic iency increases with decrease in the ionic rad iu s of the dopant. The dependencc of va ri ous TL peaks on y-exposun: sho ws that, low te mperature peaks have got the ir optimum intensity as compared to hi gh temperature TL pea ks.

1 Introduction

A lkali ha lide c rystal s with co lour centre are well known to be suitable solid-state, optically active med ia , capable of generating radi a ti on tunable in wide spectral range l

. Recent important tec hno logica l

app licati ons in the area of colour centre laser2."

radiati on dosimetry-l and integrated optics;') have considerab ly increased the attention of the sc ientific community towards the ph ys ical properties of LiF, a mate ri a l, trad iti ona ll y e mpl oyed in the product ion of hi gh-quantity optica l e le me nts to be used in the infrared , visible and particularl y. in the ultrav io le t region of the e lectro magneti c spectrum . Coloured LiF is ofte n preferred among a lka li ha lides because, it is not damaged by moi sture, co lour centres are optica ll y stable, and lase rs can be operated at room tempera ture~ .

LiF material is a lso one of the mos t widely used thermo-luminescent (TL) phosphor in dosimetric applicati on because, it provides a good compromi se between the des ired dos ime tri c properti es. The effect ive atomic numbe r of LiF is suffi c ie ntl y near to that of ti ssue [Zdl (L iF) = 8. 14 and Zdf (Ti ssue) = 7.41 so as to provide a response, whi c h va ri es onl y s li ghtl y with photon e nergy . Camaronx conce ived syste matic preparati on of LiF a nd encouraged the comme rc ial production of LiF: Mg, Ti TL dos i mete rs known as TLO-I 00, TLO-600 and TLO-700 depend ing on their preparation from natura l

lithium or lithium e nric hed with (,Li or 7L1 , respecti ve ly. Harshaw's patte rn" described two preparation methods for LiF: Mg, Ti TL ph osphors, one of the m is so lidi fication and the second is s in g le c rys tal method. Portal II) described a method of

pre paring sod ium-stab ili sed LiF TL dosimete rs. In thi s method, 2 % by weight of sod iulll fluoride are added to the LiF powde r. Azorin el (If .," deve loped a method to pre pare LiF: Mg, Ti TL phosph or powder as well LiF: M g, Ti + PIFE and Li F s inte red Pe lle ts in 1989. Nakajima el al. ,' ~ described a preparation of LiF: M g, C u, P of hi gh sensiti ve and low-fading TL phosphor in ser ies of LiF host TLD mate rial.

Azorin el al. 13 , showed that, Li F: M g, C u, P has a good linear response with gamma ra y dose in the range of IO-~ to 101 Gy.

However, in spite of extens ive researc h in the pre parati on and TL mechani sms or pure and doped LiF crys ta ls '~, the exact ro le o f co lour centres on the TL process has not been we ll unde rstood . This may be due to the fact that, in working w ith comme rcial dos imete r syste m, the s itu ati on is often comp li caterl by the presence of large amount of impuriti es. usually of more than one type, which hinde r the ide ntificat ion of spec ific defects in vo lved. In the present in ves ti gation , the role of divalent impur ities in the TL process is in vest igated by study ing the TL

1568 [NOl A N J PURE & APPL PHYS, VOL 40, DECEMB ER 2002

g low curves and emi ss ion spect ra of laboratory­grown LiF s ingle c rys ta ls.

14 1---------------~

...J ....

12

10

1\ II I

"I I J

bl I I I

Temperature (K)

500 600

Fig. I - Thermo-luminescence glow curve of y-irrad iated (a) pure LiF, (b) LiF: Ba (200 ppm). (e) LiF: Sr (200 ppm) and (eI) LiF: Ca (200 ppm)

2 Experimental Details

S ing le c rysta ls o f LiF doped with di ffe rent concentrati ons of Ba, Sr and Ca were grown by the Bridgman tec hnique under 10 6 to rr vacuum from 99.9 % pure Russ ian LiF powder, furthe r purifi ed by d istill ati on technique to reduce the impurities present be low 5 ppm level. The acti vators were introduced into the latti ce by mi xing appropri ate quantities of the ir A R grade sa lt , the ingredients afte r hav ing been thoroughl y mi xed were then placed in seven di ffe rent ho les, drill ed in a graphite c ruc ibl e. Thi s ensures the growth of c rysta ls w ith diffe rent concentrat ions unde r the same identica l condi t ions. T he concentration quoted a re the added amollnt of dopant. The crys ta ls o f small s izes were c leaved from the grown crysta l bl ocks, annea led at 723 K fo r 2 hr and coo led slow ly. A wCo source

carri ed out the y-irradi ati on. The TL g low curves and TL spectra l emi ss ion measurements were carri ed out using a routine TL reade r (TNDOTHERM) and a monoc hromator, respec ti ve ly.

3 Results and Discussion

Fi g. I (curve a) shows the T L g low curve of Ull ­

doped Li F c rysta l afte r y-ex posure to 1.08 x 10· R . T wo g low-peaks a re observed at 396 and 520 K. The hi gh te mperature g low-peak (520 K) is fi ve times less intense than the one at 396 K. Therefore to de ve lop bette r TL dos imetri c c harac te ri sti cs , i.e: intense TL glow-peaks , Li F hos t mate ri al was doped with different di va lent impurities, such as Ba, S r and Ca .

/~ . / ~~ '"

~. ;o,,~J 30 60 90 120 200 300

Concf'n trct ion of Sa ( ppm )

Fig. 2 - TL intens ity of ,(- irrad iated LiF: Ba crystals as a runction or concentra ti on or Ba for various TL pea ks

TL g low curve of Ba (200 ppm) doped L iF

c rysta l (curve b) a fte r y-ex posure at I .08 x 10" R shows four TL glow-peaks at 376, 400. 42 5 and 503 K . The 400 K peak is prominentl y observed. compared to othe r TL g low peaks. LiF: Ba (30 ppm) and LiF: Ba (60 ppm) do not show prominent hi gh temperature 503 K TL glow-peak . It is a lso observed th at, the TL intensity of 400 and 503 K peaks inc reases with inc reas ing concentrati on of Ba w ithout any appreciab le change in temperature. However, the TL intens ity of peaks 7>76 and 425 K initi a lly increases, atta in s an opti mum va lue for 200 ppm and gets saturated at hi gher concentrati on of Ba impurity in Li F (Fig . 2). Due to the hi ghe r TL

KHER et al. :LiF SINGLE CRYSTALS R69

intensity , the 200 ppm Ba-doped LiF crystal is selected for further studies. Fig. 3 shows the response curve of LiF: Ba (200 ppm) with y-ray ex posure. With the exception of the 425 K glow­peak, all the other ones increase up to 105 Rand thereafter decrease.

~ 100 c ~

.0 )j ;. 150 .~ • c ...J >-

100

50

"Y-Exposurr (RJ

Fi g. :I - TL intensity of LiF:B a (200 ppm) crystals as a function ofy-ex posure for various TL peaks

11 ,----------------------------------,

10

,"\ \ \ \

\ \

\ , , , , " , ,

, I ,

.~I __ -L.-. __ -'I----L __ J ____ .~.,_ ''_'"__ _ __'____' JOO 400 500 600

Wov(' il"l'l9th (nml

Fig . 4 - TL emi ssion spectra of y-irradiated crystals , (a) LiF : Sa (250 ppm ). (b) LiF : Sr (200 ppm), (c) Lir : COl (200 ppm)

Fig. 4 (curve a) shows the TL spectral emission of LiF: Ba (200 ppm). A broad emission peak is observed at 470 nm in the blue region of the spectrum and this is the main characteristic of this TL dosimetric phosphor.

Fig. I (curve c) shows the TL glow curve of LiF: Sr (200 ppm). TL peaks are observed at 382, 420, 448 and 490 K. The 420 K peak is prominent and more intense compared to other peaks . Variation of TL intensity with concentration of Sr is shown in Fig. 5. It is observed that, the TL intensity is maximum for about 300 ppm of Sr impurities in LiF. The intensity of 420 K TL glow peak in LiF: Sr (300 ppm) increases more than linear, up to 105 R (Fig. 6). TL emission of the 420 K peak is observed at 460 and 520 nm (Fig. 4, curve b) . Similar results have been obtained for LiF: Ca. A TL intense peak is observed at 425 K (Fig. I, curve d) and concentration-quenching of Ca impurity starts at 200 ppm of Ca (Fig. 7). The 425 K TL peak variation with y-rays exposure is shown in Fig. 8, while the TL emission of the 425 K peak is observed at 375 and 450 nm in the blue region of the spectrum (Fig. 4, curve c).

5

i,20K

10 60 90 120 200 300

Conc~ntrallon of Sr (ppm)

Fig. 5 - TL intensity ofy-irradiated LiF: Sr crystals as a function of concentration of Sr for various TL peaks

As far as the glow-peak temperature for Ba. Sr and Ca-doped LiF crystals are concerned, it is observed that, the glow-peak around 376, 400, 425 and 520 K are produced irrespective of the impurity added . However, the relative intensities of these glow-peaks vary from sample to sample. For LiF:Sr

870 INDIAN J PURE & APPL PHYS, VOL 40, DECEMBER 2002

and LiF:Ca, the 420 and 425 K peaks, respectively, are more intense, than the other one, while for LiF:Ba the 400 K TL glow-peak is the most intense.

150

/ ~

490K ·c ~ 100 .d

~ ~

s ~ .;; c • £ ..J 50 ....

'1-Exposurt (R)

Fig. 6 - TL intensity of LiF: Sr (200 ppm) crystals as a

function ofy-exposure for various TL peaks

15

-;; 'c ~

.d 10 ~ 387K S l:-'iii c " C ..J ....

52 OK

30 60 90 120 200 300

Concpntroticn of Co (ppm)

Fig. 7 - TL intensity ofy-irradiated LiF: Ca crys tal s as a fun ct ion of concenlration of Ca for various TL peaks

Rascon & Rivas Alvarez" observed the TL emission peaks of Ba, Sr and Ca in doped KCI crysta ls at 450, 445 and 450 nm, respectively, and concluded that, the same light emission is produced

in these thermo-luminescence processes of KCI samples doped with Ba, Ca, or Sr, which have rather different ionic radii. It is likely that, the entity affecting this emission is the impurity vacancy dipole, associated with the impurity and not the impurity itself. However, in the results obtained by the authors on TL emission spectra are different. TL emission of LiF:Ba is observed at 470 nm, for LiF:Sr at 450 and 520 nm, and for LiF:Ca at 375 and 450 nm. Hence, in the Ba, Sr, Ca-doped LiF crystals the main TL emission peaks observed prominently around 450-470 nm, are due to the impurity-vacancy dipole associated with the impurity and not the impurity itsel f. Moreover, Sr and Ca-doped LiF crystals show additional TL emission peaks at 520 and 375 nm, respectively, due to the impurities . Hence, the Ba, Sr and Ca-doped KCI TL emissions I) are re lated with the Ba-doped LiF crystal, while the Sr and Ca-doped LiF give different results .

150

VI

'c ~

.ci :; 100

?: .;;; ~ C ...J

.... 50

y- Exposure (R)

Fig. 8 - TL intensity o f LiF: Ca (200 ppm) crystal s as a function ofy-exposure for various TL peaks

Baldacchini et a/1f>., performed the detailed study on the radiative and non-radiative processes in the optical cycle of the F, + centre. It has been found that, a metastable triplet state (TS), plays an important role in the optical cycle of this centre, as shown in Fig. 9. This new state decreases the optical efficiency of the radiative emissi on by trapping a sizeable fraction of the active centres. The following relations describe the increase and the decrease of

KH ER el al .:LiF SINGLE CRYSTALS 87 1

the triplet popul ation after switching on and off the pumping beam:

where, Tp and T, are the triplet built-up and decay time respecti ve ly, and 7;, is the radi ati ve life-time of the re laxed, excited state . Un is the pump-rate out of the ground-state f ixed by the ex perimental conditions, and W, and W2 are the transition probabilities to form the tri plet state. T he know ledge of W, and W2 is very important to establi sh the radi at ive yie ld of Ft emi ssi on, and there fore, the ir va lues should be known precisely. Ba ldacchini et a L., I7·IX reported rel iable data on co loured LiF crystal, the radi ati ve li fe- time of the F/ and a lso of the F2 centres in the temperature range between 15 and 300 K. The F.\+ and F2 absorpti on bands are strongly overlapping in the region around 450 nm, but their emi ss ions at 532 and 674 nm are mostl y resolved . The co louration of pure Li F crystals was perfo rmed at low temperature (-60°C) and fo ll owed by a room temperature bleaching with an exc imer laser fo r about one hour . Indeed, the UV bleaching des troys the F2 centres through the fo ll owing reacti on:

and the centre diffusion at RT produces an increase of the F.\+ defects:

F/ + F-) F/

The energy levels of the F.\' centre showing radi ative, non-radi ati ve and re laxati on trans iti on are reported in Fig. 9. In the present paper, the resul ts on TL emi ss ion also ex hibit the F2 and F,+ centres in divalent impurity doped Li F crysta ls . TL e mi ss ion of high intense TL peaks in Ba, Sr and Ca-doped L iF crysta ls are seen at 470, 460 and 450 nm due to F/ and f~ centres, respec ti ve ly. Tn the Li F:Sr crystal , the second T L emi ss ion peak at 520 nm was observed due to only F/ centre l X

•

The princ ipa l e ffect of optical and thermal treatments in an ionic so lid is to alter the lattice defec t equilibrium, inc luding the concentration and arrangement of cati on and ani on vacancies, impuriti es , impurity vacancy assoc iates and assorted electrons and ho les whic h may be assoc iated w ith such de Fects. In the present in vestigation, un-doped and doped crystals were annealed at 723 K fo r 2 hr

and cooled slow ly. It is specul ated that, due to pre­irradiation annealing, simple complexes such as dimers and trimers are broken up to d ipo les, produc ing the relati ve ly large low te mpe rature peak and due to the slow coo ling, the formation of hi gher order complexes and precipitati on of impurity reduces the intens ity of hi gh temperature peaks. T he increase of the TL intens ity by impurities may be due to the creation of e mi ss ion centres or due to trapping centres in the lattice. The comparative study of TL in Ba, Sr and Ca-doped LiF crysta ls indicates that, the dopant hav ing smaller ionic radiu s is more efFecti ve in caus ing intense TL , whic h is in agreement with the idea that the impurities of larger ionic radiu s do not enter the lattice eas ily whil e growing the crysta l I ') .

-r-..

Uo

\ :4.. RES

l/ro

~\~ I I

JW2 ~ ........ -

GS ... l -,--.'

Fig. 9 - Energy level d iagram o f the F/ centre showing the radiati ve ( _ ), non-rad iati ve (-----) and re laxation ( .... .. ) transitions

In the emi ss ion spectra of di va lent impurity doped Li F crystals (Fig. 4) a ll emiss ion peaks are observed near blue to green region of the spectmm and this is the main characteri sti cs of TL dos imetric materi als which avoid the mixing of red emi ss ion of heating cantho l pl ate during TL mea~urements. Earlier Nahum & Weigard ' 9 have shown that, the excitati on at 450 nm of coloured Li F crystals produces a green emi ss ion peaked at 530 nm and a red one at 670 nm attributed to the F/ and F2 centres, respect ive ly, which have the absorpti on bands almost coinc ident with F,' centre in doped Li F.

872 INDIAN J PURE & APPL PHYS, VOL 40, DECEMBER 2002

Acknowledgements

The authors are thankful to Dr S V Moharil, Nagpur University, Nagpur and Prof B P Chandra, Vice Chancellor of Pt Ravishankar Shukla University, Raipur, for helpful discussions. One of the authors (S1D), is grateful to Dr S G Wanjari, Director, Kamala Nehru College, Nagpur, for her encouragement during the work.

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