Preparation and characteristics of LiF:Mg,Cu,Na,Si thermoluminescent material

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V N U Joumal o f Science, M athem atics - Physics 23 (2007) 225-231

Preparation and characteristics of LiF:Mg,Cu,Na,Si

thermoluminescent material

V u T h i T h a i H a 1, N g u y e n T h i Q u y H a i1, N g u y e n N g o e L o n g 2 *, L e V an V u 2

1 ỉn s tiíu íe o f M a te ria ls Science, VAST, 18 H o a n g Q u o c Viei, C au Giaỵ, H anoi, Vietnam

2 C en ter f o r M a te ria ls Science, C o lle g e o f Science, VNU, 3 3 4 N g u yên Trai, Thanh X u a n , H anoi, Vìetnam

R eceived 28 N ovem ber 2007

A b s t r a c t L ith iu m A uoride (L iF ) therm o lu m in escen t povvder doped w ith M g, C u, N a, and Si

as ac tiv a to rs vvas p repared X -ray d iíĩra c tio n analysis indicated that d o p in g LiF vvith d iffe ren t

a ctiv ato rs m akes crystal lattice som ew hat expand T h e dosim etric pro p erties o f the povvder w ere

studied T h e m ain d o sim etric peak o f L iF :M g,C u,N a,S i appeared in the range o f 2 2 7 -2 4 7 ° C and

the ra tio o f th e h eig h t o f th e m ain peak to that o f th e o ther sm all peaks in the L iF :M g,C u,N a,S i

is m uch g re ater th an that in the L iF :M g,T i and LiF:M g,C u,P T h e prep aratio n co n d itio n s such

as a n n e a lin g tem p cratu re and duratio n , an n ealin g atm osphere, d o p in g vvith d iíĩe re n t activators

etc vvere investigated

1 I n t r o d u c tio n

L iF - b a s e d th e r m o lu m in e s c e n t ( T L ) m a te r ia ls a r e w id e ly u s e d a s a p e r s o n a l d o s im e tr ic m a te - ria l becausc o f th e ir lovv energy depenđence, h ig h s e n s itiv ity , s ta b ility and tissue e q u iva le n cy T h e

th e rm o lu m in e s c e n t d o s im e try ( T L D ) m a te ria l based on L iF th a t has been stud ie d m ost e x te n s iv e ly is

L iF :M g ,T i, w h ic h is vvidely used in personal d o s im e try and a v a ila b le in the m arket u n d e r trade names lik e T L D -1 0 0 and its v a ria tio n s (T L D - 6 0 0 and T L D - 7 0 0 ) [1]

A lth o u g h L iF :M g ,T i has proven to be a useíul m a te ria l fo r T L D , m any researches fo r d e v e lo p in g the m ore advanced T L m a te ria ls have been a ccom plished N a k ạ ịim a et al [2] vvere the fir s t to d escribe the properties o f L iF doped vvith M g , C u and p im p u ritie s T h is T L m a te ria l co m b in e s tw o a ttra c tiv e

p roperties, n a m ely, a h ig h s e n s itiv ity and a good tissue e q u iva le ncy T h is L iF :M g ,C u ,P m a te ria l has been im p ro ve d and c o m m e rc ia liz e d b y C hinese (G R -2 0 0 ), Japanese (N T L -5 0 0 ) , P o lis h ( M C P - N ) and

U S A ( T L D -6 0 0 H and T L D - 7 0 0 H ) [3]

In the 1980s, K im et al developed a pow dered typ e o f L iF doped w ith fo u r dopants: M g , C u,

N a and Si [4| T h is T L povvders have about 2 tim es h ig h e r s e n s itiv ity in co m p arison w ith L iF :M g ,C u ,P

T h e L iF :M g ,C u ,N a ,S i m a te ria l w ith lin e a r T L response o ver a w id e do se range, lo w re sid u a l s ig n a l, and good s ta b ility to heat tre a tm e n t is fo u n d to be a p ro m is in g m a te ria l fo r th e rm o lu m in e s c e n t d o sim e try

* Corresponding aulhor Tel: 0913038005

E-mail: longnn@vnu.edu.vn

225

In the present w o rk , w e prepared L iF povvders a c tiv a te d w ith M g , C u , N a and Si and studied the dependence o f the g lo w c u rve and the T L in te n s ity on th e rm a l tre a tm c n t and th c co nce n tratio n o f the dopants T h e L iF :M g ,T i and L iF :M g ,C u ,P p ow de rs w e re prepared fo r com parison

2 Experimental

F o u r k in d s o f L iF p ow ders have been prepared U n d o p e d L iF was prepared b y means o f the reaction:

L iC I + H F — + L iF + HC1 (1 )

T h e p re c ip ita te d L iF was filte re d , w ashed re p ea ted ly and d rie d

L iF :M g ,T i p ow ders w ere synthesized b y c o -p re c ip ita tio n m e tho d [5] A q u e o u s so lu tio n s o f

L iC l, M g C l2 and T i + H F w ith re q u ire d co n ce n tra tio n w e re in c o rpo ra te d T h e m a te ria l p re cip ita te d

w as íilte re d , w ashed, d rie d , and was subjected to th e rm a l tre a tm e n t in an oven at 6 4 0 °c fo r 1 hour, and then w as annealed at 4 0 0 °c fo r 1 hour

L iF :M g ,C u ,P povvders w ere prepared b y the same m ethod lik e in |6Ị A q u e ou s so lu tio n s o f

M g C b , C u C lí, and ( N H4)2H2P0 4 w ith re q u ire d c o n c e n tra tio n w e re added to L iF p re c ip ita te d b y

re a ction (1 ) T h e m a te ria l o b ta in e d was w ashed, d rie d , and w as a n n e a lin g in an oven at 700 ° c fo r 1

h our, and then at 240 ° c fo r 10 m in

T h e syn th e sis process fo r L iF :M g ,C u ,N a ,S i T L p o w d e rs is as fo llo w s [7] T h e host L iF m aterial was m ix e d vvith th e com pounds c o n ta in in g re q uired a c tiv a to rs M g C l2, C u C Ỉ2 and N a2SiC>3 in d is tille d water T h e m ix tu re was m ix e d on a m agnetic s tirre r and w as then d rie d at 150 ° c fo r 15 hours T h e

d rie d m a te ria l was annealed at tem perature range o f 7 5 0 -8 5 0 ° c fo r in te rv a l o f 10-40 m in u tes and then

it was q u ic k ly co o le d to ro o m tem perature F in a l p ro d u c t w as p u lv e riz e d , sieved to select gra ins h avin g sizes in the range o f 6 0 -1 5 0 /im A c c o rd in g to (4, 7], the o p tim u m co n ce n tra tio n o f the a ctivato rs in

p o w d e r-fo rm L iF :M g C u ,N a ,S i m a te ria l w ere fo u n d to be M g : 0 6 m o l% , Cu: 0 6 -0 8 m o l% , Na: 1.8 - 2 0 m o l% a n d S i: 1 8 -2 0 m o l% In th c p r e s e n t w o r k , w e p r e p a r c d L iF p o w d c r a c tiv a tc d w ith M g

0 2 -1 0 m o l% , C u 0 2 -1 0 m o l% , N a S i 1.6-2.4 m oI% (N a S i w as considered as a s in g le d o p a nt and its

co n ce n tra tio n was ca lcu la te d as to ta l o f N a and Si because the co m p ou n d N a2SiC>3.9H2 0 was added)

T h e X -ra y d iffr a c tio n (X R D ) pattem s o f fo u r k in d s o f the above m e n tio n c d specim ens w ere

ob ta in e d b y u s in g C u - K a (A = 0 1 5 4 0 5 6 n m ) irra d ia tio n on an X -ra y d iíĩra c to m e te r D 5 0 0 5 , B ru ker,

G erm any T h e specim ens w ere irra d ia te d b y h ig h energy ra d ia tio n T h e X -ra y source o f 20 k V - 1

m A was used as an irra d ia tio n source Irra d ia tio n d u ra tio n w as 3 m in u tes T h e T L glovv curves o f the sam ples w ere m easured b y u sin g a H arsh aw m o d e l 3 5 0 0 T L D reader with a lin e a r heating rate o f 2 -

5 ° c / s in tem perature range fro m 50 ° c to 360 ° c

3 R e s u lts a n d d is c u s s io n

F ig u re 1 shovvs X R D pattem s o f fo u r k in d s o f the L iF p ow ders A s sh ow n in F ig 1, a ll the

X R D pattem s can be w e ll indexed to the face centered c u b ic s tru ctu re d LiF In the X R D spectra are observed d ifF ra c tio n peaks fro m (1 1 1), (2 0 0), (2 2 0), and (3 1 1 ) c rysta l la ttic e planes

226 V.T.T Ha et aỉ / VNV Journal o f Science, M athematics - Physics 23 (2007) 225-231

V.T.T Ha et al / VNU Journal o f Science, Mathematics - Physics 23 (2007) 225-231 227

Fig 1 X R D p attem s o f four k in d s o f L iF p o w d ers a) U n doped LiF, b) L iF :M g,T i,

c) L iF :M g,C u,P , and d) L iF :M g ,C u ,N a,S i

Table 1 C rystal in terp lan a r spacin g s and lattic e co n stan ts for four kin d s o f LiF povvders

Sam ple r f m ( n m ) GỈ2oo(nm ) d2 2o (n n i) o (n m )

ư n d o p e d L iF 0.2 3 26 7 0 2 0 14 8 0.14245 0 4 0 29 6

L iF :M g ,T i 0.2 3 27 3 0 2 0 1 5 4 0.1 4 24 7 0 40305

L iF :M g ,C u ,P 0 2 3 2 7 9 0 2 0 1 6 6 0.14253 0 40322

L iF :M g ,C u ,N a ,S i 0.23291 0.20171 0.14261 0.40340

T a b le 1 show s the values o f c rysta l in te rp la n a r s p acing and la ttic e constants fo r the fo u r k in d s

o f L iF pow dcrs From the tab le , it can be seen th a t d o p in g L iF w ith d iíĩe re n t a ctiva to rs m akes c ry s ta l

la ttic e som ew hat expand T h e la ttic e co nsta nt o f the undoped L iF is 0.4 0 29 6 n m , vvhile the la ttic e constant o f the L iF :M g C u ,N a ,S i is increased to 0 4 0 3 4 0 n m , i.e is increased 0.1 % T h e reason fo r the la ttic e expansion is m aybe th a t the d o p a n t atom s o c c u p y in tc rs ite p o s itio n s o r the d op a nt atom s

re p la c in g those o f the host la ttic e have b ig g e r a to m ic ra d ii than that o f the host la ttic e atom s

F ig u re 2 shows the T L g lo w curves o f the L iF p ow de rs w ith the o p tim u m c o n c e n tra tio n o f dopants: L iF (0.013 m o l% M g , 0 0003 m o l% T i) , L iF (0 05 m o!% M g , 0.0 0 4 m o l% C u , 2 m o l% P) and L iF (0 6 m o l% M g , 0.6 m o l% C u , 1.8 m o)% N a S i) A s can be seen fro m the fìg u re th a t the g lo w

c u rve o f L iF :M g ,C u ,N a ,S i p o w d e r show s som e peaks in the range fro m 0 to 350 °C : the m a in peak

u s u a lly used in d o s im e try appears at 2 47 ° c and the o th e r s m a ll peaks are at 150, 2 05 , and 2 80 °c

C o m p a rin g (he g lo w curves sh ow n in F ig 2, one can note that the ra tio o f the h e ig h t o f the m a in peak

to th a t o f the o th e r sm all peaks in the L iF :M g ,C u ,N a ,S i is m uch greater than that in the L iF :M g ,T i and

L iF :M g ,C u ,P T h is p ro p e rty m akes L iF :M g ,C u ,N a ,S i becom e an e x c e lle n t m a te r ia l useđ in d o sim etry

In o rd e r to fin d o p tim u m p re p a ra tio n c o n d itio n s , the sam ple L iF : 0.6 m o l% M g , 0.6 m oI% C u, and 1.8 m o l% N a S i was annealed in N2 gas f lo w w ith a rate o f 6 1/m in at va rio u s tem peratures: 750,

800, 830, and 850 ° c T h e results sh o w n in F ig 3 in d ic a te th a t a n n e a lin g tem pe ra ture has s tro n g ly afFected the T L in te n s ity and the g lo w cu rve stru ctu re A t a n n e a lin g tem perature o f 800 ° c , the

in te n s ity o f the m a in peak reaches a m a x im u m value

228 V.T.T Ha et al / VNU Journaỉ o f Science, Mathematics - Physics 23 (2007) 225-231

T em perature (°C)

Fig 2 G lo w curves o f a) L iF:M g,T i, b) L iF :M g,C u,P and c) L iP tM g C u N a.S i pow ders

Temperature (”C)

F ig 3 G lo w curves o f L iF (0.6 m ol% M g, 0.6 m ol% C u , 1.8 m ol% N aS i)

T L povvder annealed in N 2 gas flow w ith a rate o f 6 1/min at v arious tem peratures

A c c o rd in g to [8], a n n e a lin g atm osphere m ay m o d iĩy T L glovv cu rve shape, and so ann e aling

in in e rt atm ospheres (h e liu m (H e ) o r n itro ge n ( N ỉ ) ) has been recom m ended to a v o id the changes that

w ere observed w h e n the ( L iF :M g ,T i) T L D - 1 0 0 ch ip s vvere annealed in air O n th c contrary, authors [9, 1 0] have re p orte d that no s ig n ific a n t d iíĩe re n ce s w ere fo u n d betw een g lo w cu rve shapcs o f T L D -1 0 0

ch ip s annealed e ith e r in H e, N2 o r in air It m ay be o f interest to in ve stig a te the in ílu e n c e o f annealing atm osphere on T L g lo w c u rve o f o u r L iF :M g ,C u ,N a ,S i pow der T h e pow ders w ere annealed at 800

° c fo r 15 m in in a ir (no N2 gas flo w e d ) and in N2 gas f lo w w ith d iíĩe re n t rates A fte r irra d ia tio n w ith

X -ra y , g lo w curves o f the pow ders w ere recorded in air F ig 4 illu s tra te s g lo w curves o f L iF (0.6

m o l% M g , 0.6 m o l% C u , 1.8 m o l% N a S i) T L p o w d e r annealed at 8 00 ° c fo r 15 m in as a íu n c tio n o f

N2 f lo w rate T h e results sh o w n in F ig 4 in d ic a te that a n n e a lin g atm osphere has s tro n g ly a ffected the

T L in te n s ity and glovv c u rv e structure T h e pow ders annealed in N2 flo w have stro ng e r T L in te n s ity than the p ow de rs annealed in a ir do F rom the íìgu re , it can be seen th a t the o p tim u m N2 f lo w rate is

4 1/min T h e h e ig h t o f the m a in peak in the sam ple annealed in N2 flo w rate o f 4 l/m in is 10 tim es greater than th a t in the sam ple annealed in air

A n n e a lin g tim e is an im p o rta n t fa c to r as vvell F ig 5 show s the glovv curves fo r 1.5 g o f L iF (0.6 m o l% M g , 0.6 m o l% C u, 1.8 m o l% N a S i) T L p o w d e r annealed at 800 °c in N2 gas flo w w ith the rate o f 4 l/m in fo r va rio u s tim e in te rva ls: 10, 20, 30, and 40 m in F rom Fig 5 and the inset on it,

it can be seen th a t fo r a sam ple am ount o f 1.5 g the o p tim u m a n n e a lin g tim e in te rv a ls at 800 ° c are

2 0 - 3 0 m in

V.T.T Ha et al. / VNU Journal o f Science, Mathematics - Physics 23 (2007) 225-231 229

.2x10’ „ 0x10*

.0*105 >

.0x10* I

s

.0x10 I .0x1 o5 pỊ

.0

100 150 200 250 300 350 Temperature (°C) Fig 4 G lo w cu rv es o f L iF (0.6 m ol% M g, 0.6 m ol% C u, 1.8 m ol% N aS i)

T L p o w d e r an n e aled at 800 °c for 15 m in as a íu n ctio n o f N2 gas flow rate

Fig 5 G low cu rv es o f L iF (0 6 m ol% M g, 0.6 m ol% C u, 1.8 m ol% N aS i) T L povvder an n ealed at 800 °c

in N2 gas flow rate o f 4 l/m in for various tim e intervals: a) 10, b) 20, c) 30, and d ) 40 m in

A fte r a n n e aling process, the p o w d e r was s tu c k together and had the same blue-green c o lo u r as that o f C u C lo T h e h ig h e r C u co n ce n tra tio n was, the c o lo u r was darker It seems th a t the C u dopants

d iffu s e d not c o m p le te ly in to L iF , the excess C u dopant co m pound a g g lu tin a te d to the surface o f the

L iF c ry s ta llite s T h en the p o w d e r was washed w ith HC1 s o lu tio n A fte r w a s h in g w ith HC1 s o lu tio n the c o lo u r o f the L iF :M g ,C u ,N a ,S i T L p o w d e r was b lu is h , b u t tu m e d w h ite and the c ry s ta llite s w ere separated fro m each o th e r (F ig 6) It indicates that the excess C u co m p o u n d was d is s o lv e d o u t o f the

p o w d e r d u rin g the HC1 tre atm e n t

In the povvder w ith o u t H C l treatm ent, the excess C u com pounds are not T L m a te ria l b u t are

an obstacle to the e m issio n o f T L , so T L in te n s ity o f th is p o w d e r was w eak F o r the p o w d e r a fte r tre atm e n t vvith HC1 s o lu tio n , in w h ic h the excess C u co m p ou n d was d isso lve d o u t o f the p ow de r, T L

in te n s ity becomes e v id e n tly s tro n g e r (F ig 7)

T h e p re lim in a ry s tu d y on the e ffe ct o f the dopants in L iF :M g ,C u ,N a ,S i T L p o w d e r upon T L

g lo w c u rve in d ica te d th a t d o p in g w ith a ctivato rs has s tro n g ly a íĩe c te d the T L in te n s ity and the g Io w curve s tru ctu re o f the p ow der F ig 8 illu s tra te s g lo w curves o f L iF T L pow ders undoped and doped

w ith d iíĩe re n t a ctiva to rs In the g io w cu rve o f the undoped L iF sam ple, the o n ly s tro n g peak was observed at 140 °c F o r the L iF :C u ,N a S i sam ple, tw o stro ng peaks w ere observed at 140, 2 1 2 °c ,

w h ile in the g lo w cu rve o f the L iF :M g ,N a S i sam ple, 3 stro ng peaks w ere observed at 185, 230, 292

230 V.T.T Ha et al / VNU Journal o f Science, Mathematics - Physics 23 (2007) 225-231

Fig 6 T h e optical m icroscope im ages o f L iF :M g, C u ,N a,S i T L povvder:

a) B eíore vvashing w ith HC1, b) A fte r vvashing w ith HC1

Temperature (*C)

Fig 7 Glovv curves o f L iF :M g,C u,N a,S i T L povvders:

a) B eíore w a sh in g w ith HCI solution, b) A fte r w a sh in g w ith HC1 solution

° c F o r the L iF :M g ,C u sam ple, 2 o v e rla id stro n g peaks vvere observeđ at 240, 253 ° c E specially, the

g lo w c u rv e fo r L iF :M g ,C u ,N a S i sam ple e x h ib its v e ry stro ng d o s im e tric peak at 2 2 8 °c. T h e e íĩe ct o f the dopants in L iF :M g ,C u ,N a ,S i T L povvder upon T L g lo w c u rve w il l be reported in d e ta il in oth e r paper

4 C o n c l u s i o n

L ith iu m ílu o rid e ( L iF ) th e rm o lu m in e s c e n t povvder doped w ith M g , C u , N a , and Si as activators vvas prepared X - r a y d iữ ra c tio n a nalysis in d icate d th a t the L iF has the face centered c u b ic structure and d o p in g L iF w it h d iíĩe re n t a ctivato rs makes crysta l la ttic e so m e w ha t expand

T h e d o s im e tric p roperties o f the povvder w ere stud ie d In th e T L g !o w c u rve o f L iF :M g ,C u ,N a ,S i

p o w d e r the m a in d o s im e tric peak appeared in the range o f 2 2 7 -2 4 7 ° c and the ra tio o f the h eigh t o f the m a in peak to th a t o f the o th e r s m a ll peaks is ra the r great

In o rd e r to o b la in h ig h se n sitive L iF :M g ,C u ,N a ,S i T L p o w d e r, the o p tim u m pre pa ra tio n c o n d i- tio n s have been fou n d T h e o p tim u m ann e aling tre a tm e n t was to be at 800 ° c , fo r 2 0 - 3 0 m in in N2

gas flo w w ith rate o f 4 l/m in A fte r ann e aling tre atm e n t, th e sam ples w ere vvashed w ith HC1 so lu tio n

V.T.T Ha et al / VNU Journal o f Science, Maihematics - Physics 23 (2007) 225-231

Temperature <aC)

Fig 8 G low curves o f LiF T L povvders undoped and doped w ith d iíĩe re n t activators

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