THERMOLCIMINESCENCE OF KCI : KBr MIXED...
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CHAPTER - III THERMOLCIMINESCENCE OF KCI : KBr MIXED CRYSTALS
Transcript of THERMOLCIMINESCENCE OF KCI : KBr MIXED...
CHAPTER - III
THERMOLCIMINESCENCEOF
KCI : KBr MIXED CRYSTALS
3-1 INTRODUCTION
A f t e r I ho 11 i i n::li i 111,1 - Ncn.j,ir>.ik i bombincj in 1 rM , a Jo i n t
U S - J a p a n A t o m i c Boinb C a s u a l t y C o m m i s s i o n was i n s t i t u t e d to
s t u d y the i l l - e f f e c t s of the a t o m i c bomb, and its first m a j o r
t a s k w a s to get to k n o w the am o u n t of r a d i a t i o n (dose)
r e c e i v e d b y indi v i d u a l s . But so m a n y were the u n c e r t a i n t i e s
a n d u n k n o w n s i n v o l v e d that the d o s e e s t i m a t e s v a r i e d widely, a
m e a s u r e d v a l u e w a s b a d l y n e e d e d to w o r k out a p r o p e r b a s i s for
t h e d o s e c a l c u l a t i o n s . A J a p a n e s e P r o f e s s o r h a d the i n g e n u i t y
t o m e a s u r e the t h e r m o l u m i n e s c e n t light e m i t t e d b y r o o f - t i l e
s a m p l e s c o l l e c t e d f r o m v a r i o u s pa r t s of the r u i n e d cities.
B a s e d o n these, he c o u l d tell the r a d i a t i o n l evels that ha d
p r e v a i l e d a f t e r the bombing.
M o r e recently, the o n l y a v a i l a b l e r a d i a t i o n d o s e da t a in
the T h r e e M i l e I s l a n d N u c l e a r P o w e r S t a t i o n a c c i d e n t in the
U S A c a m e f r o m just five t h e r m o l u m i n e s c e n c e d o s i m e t e r s w h i c h
h a p p e n e d to be l y i n g e x p o s e d to the e n v i r o n m e n t a r o u n d the
reactor. T h e d a t a c o u l d h e l p e s t i m a t e r a d i a t i o n - i n d u c e d c a n c e r
r isks to the p o p u l a t i o n in the region.
T h e r m o l u m i n e s c e n c e (TL) is the t h e r m a l l y s t i m u l a t e d light
e m i s s i o n f o l l o w i n g the removal of e x c i t a t i o n (i.e. i o n i z i n g
r a d i ation) f r o m an i n s u l a t o r or semi c o n d u c t o r . This
s h o u l d not be c o n f u s e d w i t h the light s p o n t a n e o u s l y e m i t t e d
f r o m a s u b s t a n c e w h e n it is h e a t e d to i n c a ndescence. The three
e s s e n t i a l i n g r e d i e n t s n e c e s s a r y for the p r o d u c t i o n of
t h e r m o l u m i n e s c e n c e are :
(1) T h e m a t e r i a l mu s t be an i n s u l a t o r or semiconductor,
(2) T h e m a t e r i a l must have at some time a b s o r b e d e n e r g y
d u r i n g e x p o s u r e to radiation, and
(3) T h e l u m i n e s c e n c e e m i s s i o n is t r i g g e r e d b y h e a t i n g
the m a t e r i a l .
It is a n i n t e r e s t i n g c h a r a c t e r i s t i c of t h e r m o l u m i n e s c e n c e
that o n c e h e a t e d to e x c i t e the light emission, the m a t e r i a l
c a n n o t be m a d e to emit t h e r m o l u m i n e s c e n c e a g a i n b y sim p l y
c o o l i n g the s p e c i m e n a n d reheating. The m a t e r i a l has to be
r e - e x p o s e d to r a d i a t i o n in o r d e r to r e - e x h i b i t the
l u m i n e s c e n c e , w h e r e u p o n r a i s i n g of the t e m p e r a t u r e will once
a g a i n p r o d u c e li g h t emission.
A p a r t i c u l a r c h a r a c t e r i s t i c of TL is its e m i s s i o n in the
f o r m of g l o w peaks. W h e n the sam p l e t e m p e r a t u r e is r a i s e d
a f t e r h a v i n g b e e n i r r a d i a t e d (excited) the g l o w p e a k s appear.
U s i n g the t e r m i n o l o g y of d e l a y e d p h o s p h o r e s c e n c e , it is found
that r i s e in the t e m p e r a t u r e of the sa m p l e s r e duces the
l i f e t i m e of the e l e c t r o n s in the trap w h i c h leads to their
e j e c t i o n out of the traps' at an e x p o n e n t i a l l y i n c r e a s i n g rate.
This g i v e s r i s e to a q u i c k i n c r e a s e in the l u m i n e s c e n c e light
e m i s s i o n . T h e li g h t i n t e n s i t y d e c r e a s e s and f i n a l l y falls to
zero as the n u m b e r of the t r a p p e d e l e c t r o n s s t arts g e t t i n g
e x h a u s t e d . T h e li g h t e m i s s i o n in TL thus, a p p e a r s in the form
of a g l o w peak.
F i g . 3.1 r e p r e s e n t s the b a n d model d i a g r a m for the TL
C8
D
YB
rradh
t
£(a)
Fi £
(b) (c)
(d) (e) (0
Fig- i B a n d mo d el d i a g r a m s for the TL p r o c e s s in an
i n s u l a t i n g c r y s ta l : (a) - up on i r r a d ia ti on ,
(b) - (f) a l t e r n a t i v e p r o c e s s e s up on h e a t i n g
p r o c e s s in a n i n s u l a t i n g crystal. F i g . 3 . 1(a) shows the p r o c e s s
u p o n i r r a d i a t i o n a n d Fig. 3.1 (b to f) are the a l t e r n a t i v e
p r o c e s s e s u p o n heating.
T h e b a s i c t h e o r y b e h i n d t h e r m o l u m i n e s c e n c e is the
i n t e r a c t i o n of i o n i z i n g r a d i a t i o n w i t h the s o l i d results in
t r a n s f e r of s u f f i c i e n t e n e r g y to the e l e c t r o n s in the v a l e n c e
ba n d (VB) fo r t r a n s f e r r i n g t h e m to the c o n d u c t i o n b a n d (CB) .
This p r o c e s s u s u a l l y r e q u i r e s e n e r g i e s a r o u n d 10 eV in a
typa.cal i o n i c c r y s t a l an d is a f f e c t e d b y s e c o n d a r y e l e c t r o n s
w h i c h a r e p r o d u c e d in the e n v i r o n m e n t of the p r i m a r y
p h o t o e l e c t r o n of the c h a r g e d p a r t i c l e tracks. A suffic i e n t
n u m b e r of t h e s e l i b e r a t e d e l e c t r o n s r e t u r n i m m e d i a t e l y to the
g r o u n d s t a t e a c c o m p a n y i n g e n e r g y e m i s s i o n ca u s i n g
p h o s p h o r e s c e n c e / i n t e r n a l heating, however, a f r a c t i o n of these
c a n b e c a p t u r e d at d o n o r level D w i t h the c o r r e s p o n d i n g holes
at a c c e p t o r level A.
If the traps are not v e r y deep, d e t r a p p i n g and
r e c o m b i n a t i o n m a y a l r e a d y o c c u r at a s u b s t a n t i a l rate a r o u n d
r o o m t e m p e r a t u r e r e s u l t i n g in the r e lease of the s t ored
energy. G e n e r a l l y this is c a l l e d p h o s p h o r e s c e n c e , but str i c t l y
it is r o o m t e m p e r a t u r e t h e r m o l u m i n o o c e n c e . On the o ther hand,
if the t r a p s are d e e p e n o u g h to cause s u f f i c i e n t storage
s t a b i l i t y at r o o m t e m p e r a t u r e the effect b e c o m e s of d o s i m e t r i c
i n t erest. T h e d o n o r / a c c e p t o r levels are s i m p l y m e t a s t a b l e
s t a t e s a s s o c i a t e d w i t h crystal d e f e c t s i n c l u d i n g impurities.
As the c r y s t a l is b e i n g h e a t e d e i t h e r the e l e c t r o n or the hole
w h i c h e v e r is t r a p p e d w i t h less e n e r g y will be t h e r m a l l y
e j e c t e d out of the trap and the c o r r e s p o n d i n g TL g l o w pe a k
t e m p e r a t u r e w i l l be a m e a s u r e of the thermal a c t i v a t i o n e n e r g y
E_̂ or E^, r e s p e c t i v e l y . The r e l e a s e d c harge car r i e r s can
r e c o m b i n e at the still t r a p p e d counter-part, e m i t t i n g TL as
s h o w n in (b) a n d (c) of F i g . 3.1. If e l e c t r o n s are d e t r a p p e d
and r e c o m b i n e at the hole t r a p p e d at the a c c e p t o r level, the
T L g l o w c u r v e will r e p r e s e n t the thermal a c t i v a t i o n e n e r g y of
tK- e l e c t r o n traps and the TL s p e ctral e m i s s i o n curve will
c h a r a c t e r i z e the h o l e c entre and vic e - v e r s a . In m a n y p h o s p h o r s
it i.s p o s s i b l e that the e l e c t r o n s and h oles are d e t r a p p e d mo r e
cr ,i.ess s i m u l t a n e o u s l y a n d they r e c o m b i n e at an e n t i r e l y new
r e c o m b i n a t i o n centre. In this case the TL g l o w curve will
r e p r e s e n t a n e f f e c t i v e thermal a c t i v a t i o n e n e r g y n e e d e d b y the
t r a p p e d e l e c t r o n / h o l e to s u r m o u n t a p o t e n t i a l b a r r i e r b e t w e e n
the t r a p a n d the r e c o m b i n a t i o n centre.
A t r a p p e d c h a r g e c a r r i e r w h e n d e t r a p p e d has a finite
p r o b a b i l i t y of g e t t i n g r e t r a p p e d ( F i g . 3 . 1 e ) . W h e n this
r e t r a p p i n g p r o b a b i l i t y is significant, the shape of the gl o w
curve is d i f f e r e n t f r o m the case w h e n it is absent. Also, the
r e c o m b i n a t i o n p r o b a b i l i t y of the d e t r a p p e d ch a r g e c a r r i e r may
in m o s t c a s e s c h a n g e w i t h time (i.e. as the h e a t i n g proceeds)
d e p e n d i n g o n the n u m b e r of a v a i l a b l e u n u s e d r e c o m b i n a t i o n
centres. S u c h a p r o c e s s is c a l l e d a s e c o n d or d e r or
b i m o l e c u l a r p r o c e s s usually, w h i l e the simplest case wh e r e the
r e c o m b i n a t i o n has a c o n s t a n t p r o b a b i l i t y wi t h time is called a
first o r d e r or m o n o - m o l e c u l a r process. T h e r e are also
p r a c t i c a l s i t u a t i o n s w h e r e the d e t r a p p e d c h a r g e carriers
r e c o m b i n e d i r e c t l y wi t h o u t h a v i n g to be e x c i t e d into the
co n d u c t .ion/valence bands. This is a case i n v o l v i n g isolated
l u m i n e s c e n c e c e n t r e a n d the p r o c e s s follows first order
k i n e t i c s (F i g . 3 . If).
T h e TL of a l k a l i h a l i d e s u n d e r v a r i o u s p a r a m e t e r s have
b e e n s t u d i e d in d e t a i l (Halperin and K r i s t i a n P o l l e r 1958,
A u s i n a n d A l v a r e z - R i v a s 1972) . In the past y e a r s it has b e e n
fo u n d th a t d i v a l e n t i m p u r i t y ions can s t r o n g l y a ffect the
m e c h a n i c a l p r o p e r t i e s of a l kali h a l i d e crystals. F l e i s c h e r
(1962) h a s i n v e s t i g a t e d the ca u s e for this i n c r e a s e in
h a r d n e s s a n d c o n c l u d e d that i m p u r i t y v a c a n c y d i p o l e w h i c h give
rise to t e t r a g o n a l d i s t o r t i o n c o n t r i b u t e m o r e to hardness.
T h e t h e r m o l u m i n e s c e n c e s t u d i e s e x p l a i n s two important
a s p e c t s :
(1) the u t i l i z a t i o n of t h e r m o l u m i n e s c e n t p h o s p h o r for
v a r i o u s a p p l i c a t i o n s e.g. in dosimetry, geology,
g e o c h r o n o l o g y , f o r e n s i c s c i e n c e s etc, w h e r e the TL
o u t p u t a n d its c o n s i s t e n c y w h i c h changes in
p a r a m e t e r s , is the d e c i d i n g f a c t o r in the choice of
the pho s p h o r s ,
(2) the i m p r o v e m e n t of our b a s i c u n d e r s t a n d i n g of the TL
p h e n o m e n o n b y an a n a l y s i s of TL g l o w curves, TL
e m i s s i o n spectra, s u p p l e m e n t a r y - c o m p l e m e n t a r y
e x p e r i m e n t a l results, l e a d i n g to i d e n t i f i c a t i o n of
T L p r o c e s s and the p r o b a b l e m o d e l s d e p i c t i n g the TL
m e c h a n i s m .
T h e r e are v a r i o u s a p p l i c a t i o n s of t h e r m o l u m i n e s c e n c e . One
of: the i m p o r t a n t a p p l i c a t i o n of TL is in the field of
a r c h a e o l o g y - d a t i n g e s p e c i a l l y for d a t i n g p o t t e r y and ot h e r
field c e r a m i c s w h e r e the m e t h o d of r a d i o - c a r b o n d a t i n g cannot
be a p p l i e d . In n u c l e a r industry, t h e r m o l u m i n e s c e n t d o s i m e t e r s
have f o u n d t h e i r us e for m e a s u r i n g dose in the range
0 . 0 0 0 1 - 1 0 0 0 rads. Th e d e t e c t o r c o n s i s t s of a c r y s t a l l i n e
p o w d e r of C a F 2 a n d L i F in a p l a s t i c m a t e r i a l like teflon. The
i r r a d i a t i o n l eads to i o n i z a t i o n a n d t r a p p i n g of the el e c t r o n s
in the c r y s t a l lattice. U p o n heating, r e c o m b i n a t i o n occurs
w i t h l i g h t emission, w h i c h is m e a s u r e p h o t o m e t r i c a l l y . Th e s e
d o s i m e t e r s h a v e the a d v a n t a g e that t h e y can be d e s i g n e d like a
pen, w h e r e s c r e e n s can d i f f e r e n t i a t e b e t w e e n d i f f e r e n t kinds
of r a d i a t i o n . B y u s i n g a L e a d (Pb) filter, the d o s i m e t e r can
be m a d e e n e r g y d e p e n d e n t in the 0 . 0 2 - 2 0 M e V for y - i r r a d i a t i o n .
T h e r m o l u m i n e s c e n c e is one of the mo s t u s e f u l m e t h o d s of
l o c a t i n g a n d s t u d y i n g m o l e c u l a r m o t i o n a n d structural
t r a n s i t i o n in polymers. Th e d e t a i l e d s tudy of
t h e r m o l u m i n e s c e n c e can give i n f o r m a t i o n on c h a r g e d i f f u s i o n
and t r a p p i n g in p o l y m e r s and i n f l u e n c e of a d d i t i v e s and
i m p u r i t i e s , tem p e r a t u r e , p h y s i c a l and c h e mical s t r u c t u r e and
o t h e r f a c t o r s on th e s e pro p e r t i e s . Th e s e d i f f e r e n t types of
traps h a v e b e e n o b s e r v e d in polymers, namely, d i e l e c t r i c
cavities, n e u t r a l m o l e c u l e s w i t h p o s i t i v e e l e c t r o n a f f i n i t y
and m f e w r a d i c a l c a v i t y traps. T h e s e can be p i c t u r e d as an
i r r e g u l a r s p a c e b o u n d e d by a p a r t i c u l a r local a r r a n g e m e n t of
m o l e c u l a r chains. In the case of i r r a d i a t e d p o l yethylene, a
b r o a d a b s o r p t i o n b a n d is o b s e r v e d w h i c h has b e e n a t t r i b u t e d in
c a v i t y traps. M o l e c u l a r m o t i o n a s s o c i a t e d w i t h structural
t r a n s i t i o n s a l s o g i v e d i f f e r e n t rates of ion rec o m b i n a t i o n . A
TL, s p e c t r u m p r o v i d e the m o s t c o n v i n c i n g e v i d e n c e of
g l a s s - r u b b e r t r a n s i t i o n b e c a u s e of i n c r e a s e d m o l e c u l a r motion.
This is d u e to the fact that p o l y m e r at t a i n s cri t i c a l volume
d u e to t h e r m a l e xpansion.
I n the p r e s e n t investigation, we have s t u d i e d the effect
of p l a s t i c d e f o r m a t i o n on the TL of K C l : K B r m i x e d crystals
p r i m a r i l y to get h e l p to u n d e r s t a n d the m e c h a n i s m and
c h a r a c t e r i s t i c s of TL. The p r e s e n t c h a p t e r r e p o r t s the TL of
KC.l: K B r m i x e d c r y s t a l s and the m o s t p r o b a b l e m e c h a n i s m of TL
e x c i t a t i o n ha s a l s o b e e n d i s c u s s e d on the basis of
e x p e r i m e n t a l results.
3 -2 EXPERIMENTAL
F i g . 3 . 2 (a) r e p r e s e n t s the b l o c k d i a g r a m of
t h e r m o l u m i n e s c e n c e g l o w curve reader. The e x p e r i m e n t a l set-up
c o n s i s t s of an a r r a n g e m e n t for h e a t i n g the samples, light
d e t e ctor, s i g n a l p r o c e s s o r and signal recorder. W i t h the help
of t h i s s e t - u p the t h e r m o l u m i n e s c e n c e glow curves were
recorded. T h e li g h t e m i t t e d b y the p h o s p h o r was detected by
Fig. 3.2(a): The blocK diagram of TL reader
RCA 9 3 1A p h o t o m u l t i p l i e r tube (PMT) a n d the i n d o t h e r m
t e m p e r a t u r e p r o g r a m m e r was u s e d for h e a t i n g the K a n t h a l plate.
R e g u l a t e d p o w e r s u p p l y (1050 volts, 5 mA) was u s e d to b i a s the
v a r i o u s d i o d e s of PMT. U s i n g E C I L d i g i t a l p i c o - a m m e t e r the
PMT o u t p u t w a s a m p l i f i e d . Th i s a m p l i f i e d o u t p u t wa s r e c o r d e d
on d u a l t r a c e r e c o r d e r (Omini S c r i b e ,D i g i t a l E l e c t r o n i c s ) . The
s e c o n d c h a n n e l of the r e c o r d e r was u s e d to r e c o r d the o u t p u t
f r o m t h e i r o n - c o n s t a n t a t h e r m o c o u p l e w h i c h w a s spot w e l d e d to
t h e h e a t e r p l a t e . Th e h e a t e r p l a t e a n d the h o u s i n g of PMT we r e
f a b r i cated. H e a t e r w a s in the f o r m of a small K a n t h a l p l a t e of
3size .Hi X 5.5 X 0.25 m m w i t h a c i r c u l a r d e p r e s s i o n of about 5
m m d i a m e t e r a n d d e p t h 0.5 m m at its c e n t r e w h i c h was fed
t h r o u g h a s t e p d o w n t r a n s f o r m e r c o n n e c t e d to the t e m p e r a t u r e
p r o g r a m m e r . T h e h e a t e r p l a t e was s c r e w e d on b r a s s rods of 5 m m
d i a m e t e r p l a c e d one b e l o w the o t h e r w i t h a sp a c e in between.
The o t h e r e n d of the r o d was f i x e d o n a c i r c u l a r b a k e l i t e disc
wh i c h in t u r n w a s f i x e d in a n a l u m i n i u m r i n g support.
F i g . 3 . 2(b) s h o w s the b l o c k d i a g r a m of the P M T h o u s i n g a n d the
l i g h t - t i g h t b o x of the TL reader.
The K a n t h a l p l a t e a n d s p o t - w e l d e d t h e r m o c o u p l e was u s e d to
connect the b a k e l i t e d i n e t h r o u g h tho n u i t n b l e c o n n n ctorn. A
h o l l o w c y l i n d e r (20 m m o u t e r di a m e t e r ) w a s f i x e d on the
b a k e l i t e d i s c as a s l i d i n g guide. All t h e s e p a r t s were
e n c l o s e d in a c y l i n d r i c a l l i g h t - t i g h t b o x m a d e up of m i l d
steel body, w h i c h w a s c o n c e n t r i c w i t h the a l u m i n i u m ring
support. T h e o t h e r e n d of the c y l i n d r i c a l light ti g h t b o x was
c l o s e d w i t h p o l y v i n y l c h l o r i d e (PVC) cap. M e t a l (copper)
tubing f o r ga s i n l e t a n d o u t l e t a n d PVC s u p p o r t tube for two
sliding g u i d e of the b a k e l i t e d i s c w e r e a l s o f i x e d on PVC cap.
A n a l u m i n i u m g u i d e was m a d e at the o t h e r e n d of the
c y l i n d r i c a l l i g h t - t i g h t b o x in s u c h a w a y that it c o u l d act as
a g u i d e f o r t h e a l u m i n i u m r i n g s u p p o r t of the b a k e l i t e disc.
Thus the b r a s s rods h o l d i n g the K a n t h a l p l a t e a n d the spot
w e l d e d t h e r m o c o u p l e c o u l d b e m o v e d b a c k a n d f o r t h w i t h the
help of a l u m i n i u m g u i d e support, a n d the two c o n c e n t r i c
PVC-tubes, w e r e f i x e d on the PV C cap a n d b a k e l i t e plate,
r e s p e c t i v e l y . A slit w a s cut on the top of the l i g h t - t i g h t b o x
and the s u r f a c e n e a r b y w a s f l a t t e n e d a n d a p o l i s h e d flat b r a s s
plate w i t h a w i n d o w f o r P M T m o u n t w a s fixed. T h e l i g h t - t i g h t
box was m o u n t e d o n a b a s e p l a t e s u p p o r t .
Th e p h o t o m u l t i p l i e r tu b e a l o n g w i t h its b a s e wa s f i x e d in
a n o t h e r s m a l l c y l i n d r i c a l l i g h t - t i g h t b o x of a l u m i n i u m . The
sockets f o r E H T a n d o u t p u t s i g n a l s f r o m P M T w e r e f i x e d on this
box it s e l f . A slit wa s cut at the p o s i t i o n of the
p h o t o s e n s i t i v e c a t h o d e a n d its n e a r b y s u r f a c e wa s m a d e plane.
A flat p o l i s h e d b r a s s p l a t e wa s f i x e d s u r r o u n d i n g the slit. It
e x a c t l y m a t c h e s w i t h the p l a t e on the box, c o n t a i n i n g heater.
The b o x h a v i n g the P M T c o u l d b e m o u n t e d on the b o x c o n t a i n i n g
screws. T h e r e w a s a l s o a p r o v i s i o n for a c c o m m o d a t i n g a fil t e r
of size 12 X 22 m m 2 t r a n s m i t t i n g d e s i r e d w a v e l e n g t h on its
brass p l a t e . W h e n the K a n t h a l p l a t e was s l i d e d to e x t r e m e
position, th e d e p r e s s i o n f a c e d the c e n t r e of the c a t h o d e of
the PMT. F o r p l a c i n g the s a m p l e on the heater, the ri n g
a l u m i n i u m l i d c o u l d b e p u l l e d out so that the K a n t h a l p l a t e
could be s l i d e d out. T h e s a m p l e c o u l d b e p u t in the d e p r e s s i o n
in the K a n t h a l plate.
In the p r e s e n t s t u d y the t e m p e r a t u r e p r o g r a m m e r u s e d was
capable of g i v i n g l i n e a r h e a t i n g r a t e s f r o m 5 to 400°K/min.
The t e m p e r a t u r e of the K a n t h a l s t r i p wa s d i s p l a y e d on a
digital p a n e l m a t t e r of the p r o g r a m m e r . T h e t e m p e r a t u r e
p r o g r a m m e r h a s the f a c i l i t y to o p e r a t e in the i s o t h e r m a l or
p r o g r a m m e m o d e .
P i c o - a m m e t e r ( m a n u f a c t u r e d b y E C I L H y d e rabad) was u s e d to
a m p l i f y t h e s i g n a l f r o m PMT. Th i s si g n a l was d i r e c t l y fed to
the p i c o - a m m e t e r a n d the p i c o - a m m e t e r w a s c o n n e c t e d to the
_ 3dual p e n r e c o r d e r . T h e c u r r e n t r e c o r d e d v a r i e d b e t w e e n 10 to
10 a m p e r e s . T h e r e c o r d e r p u r c h a s e d f r o m M/ s D i g i t a l
E l e c t r o n i c s Ltd., B o m b a y has r e s p o n s e ti m e of 0.5 second, full
scale c h a r t w i d t h 25 cm a n d v a r i a b l e c h a r t s p e e d f r o m 2.5
cm/min to 25 c m /min. R a t e of f l o w c h a r g e w a s k e p t at 5 cm/min.
The t h e r m o l u m i n e s c e n c e g l o w c u r v e s of y - i r r a d i a t e d K C l : K B r
m i x e d c r y s t a l s w e r e r e c o r d e d at a h e a t i n g ra t e of 90°C/min.
For m e a s u r i n g the e f f e c t of y - d o s e on the T L i n t e n s i t y of
K C 1 :K B r m i x e d c r y s tals, the c r y s t a l s w e r e e x p o s e d to d i f f e r e n t
levels of y - d o s e u s i n g G a m m a Cell [Fig.3.2(c)] an d the TL
output w a s r e c o r d e d .
For all the T L o b s e r v a t i o n at le a s t th r e e or f o u r cry s t a l s
°f small s i z e 4 X 2 X 2 m m w e r e used. To a c h i e v e a m o r e
F i g . 3 . 2 (C ) S c h e m a t i c v i e w of g a m m a c h a m b e r 900
X6mm
3 -3 Bl ock d i a g r a m of TL o m i s s i o n s p ec tr a r e c o r d e r
c o n c l u s i v e p i c t u r e of the m e c h a n i s m of T L e x c i t a t i o n .
The TL s p e c t r a of K C l : K B r m i x e d c r y s t a l s w e r e r e c o r d e d u s i n g a
s c a n n i n g m o n o - c h r o m a t o r a n d a p h o t o m u l t i p l i e r tube. The
a p p a r a t u s ( F i g . 3.3) c o n s i s t of a c o n t i n u o u s s c a n n i n g tap 0.25
m e t e r f o c a l l e n g t h J a r r e l - A s h m o n o c h r o m a t o r w i t h a p e r t u r e
(spped) of F / 3 . 6 a n d l i n e a r d i s p e r s i o n of 3.3 m m p e r nm. It
has two i n t e r c h a n g e a b l e gratings, o n e b l a z z e d at 3 00 n m an d
the o t h e r at 600 nm. T h e i n s t r u m e n t ca n b e u s e d in the
w a v e l e n g t h r a n g e of 200 to 1000 nm. T h e m o n o c h r o m a t o r was
p o s i t i o n e d v e r t i c a l l y o n a s t a n d v i e w i n g the s a m p l e p l a c e d in
the h o r i z o n t a l p o s i t i o n c l o s e to the slit. A S-20 r e s p o n s e
p h o t o m u l t i p l i e r t u b e (EMI-9558 2B) w i t h q u a r t z w i n d o w was u s e d
to r e c o r d s p e c t r a in the 2 0 0 - 7 5 0 n m range. T h e m o n o c h r o m a t o r
had a set of s c a n n i n g m o tors. A s c a n n i n g s p e e d of 100 n m p e r
m inute w a s u s e d to r e c o r d the T L s p e c t r a of t h e crystals.
R e c o r d i n g of T L s p e c t r u m u s i n g a u t o m a t i c s c a n n i n g as d e s c r i b e d
above w a s n o t a l w a y s p o s s i b l e if the T L li g h t wa s too w e a k to
s u s t a i n f o r t h e r e c o r d i n g p e r i o d (Sunta et al 1973) .
3 -3 RESULTS
Fig. 3.4 (a) r e p r e s e n t the T L g l o w c u r v e of K C l r K B r m i x e d
crystal f o r d i f f e r e n t p e r c e n t of K B r in KC1 cr y s t a l s . It is
seen th a t a l l th e T L g l o w cu r v e s h a v e two d o m i n a n t peaks.
F i g . 3 . 4(b) r e p r e s e n t s T L i n t e n s i t y v a r i a t i o n w i t h
d i f f e r e n t p e r c e n t a g e of K B r in KC1 crystals. It is o b s e r v e d
from t h e f i g u r e that the T L i n t e n s i t y i n c r e a s e s w i t h
i n c r e a s i n g the c o n c e n t r a t i o n of KBr and at t a i n a m a x i m u m value
for 50% of K B r in KC1 crystals and then d e c r eases w i t h further
i n c r e a s e i n c o n c e n t r a t i o n of KBr.
F i g . 3 . 4(c) r e p r e s e n t s the v a r i a t i o n in p e a k temp e r a t u r e in
TL glow c urve of K C 1 :KBr mi x e d crystals w i t h different
percentage of KBr in K C 1 . It is seen that the TL peak
temperature d e c r e a s e s w i t h inc r e a s i n g p e r c e n t a g e of KBr in
K C 1 , act. a .in a m i n i m u m v a l u e at p a r t i c u l a r p e r c e n t a g e of KBr in
KC1 crystal an d then increases w i t h further increasing
p ercentage of KB r in K C 1 . Two peaks in the TL gl o w curve for
KC1 crystal are o b s e r v e d one ar o u n d 130°C (peak I) and the
other a r o u n d 179°C (peak II) . Wi t h the inc r e a s i n g p ercentage
of KBr in KC1 crystal, the glow p e a k tem p e r a t u r e decreases
from 130°C to 89°C at KC1 : KBr (40:60) for peak I, and from
179 C to 149°C at K C 1 :KBr [50:50] for pe a k II, then the glow
peak t e m p e r a t u r e i n c r eases up to 139°C and 187°C for peak I
and II, resp e c t i v e l y .
F i g . 3.5 (a,b,c) r e p r esent the TL g l o w curves of pure K C 1 ,
pure KBr an d K C 1 :KBr [50:50] m i x e d crystals r e c o r d e d after
giving d i f f e r e n t levels of “jr-dose. No significant var i a t i o n
observed in the p o s i t i o n of bo t h peaks wi t h changing the
? ~radia t i o n dose.
F i g . 3.6 r e p r e s e n t s the y-dose d e p e n d e n c e of the TL peak
intensities for the pu r e KC1, pure KBr and K C 1 : KBr [50:50]
mixed crystals. It is evident from the figure that the TL peak
TL
INTE
NSI
TY
(AR
B.
UN
ITS
)
PEAK INTENSITY
III - X k
12-
10-
KCl : KBrioo : o - i
100 150TEMPERATURE
- TL GLOW C U R V E S OF K C l ! KBr M I X E D CRYSTALS FOR D I F F E R E N T PERCENTAGE OF KBr IN KCl .
TL
INTE
NSIT
Y ( A
RB
- UNI
TS
)
% OF KBr IN KCl CRYSTALS
f '9 3 k ( b ) _ T L I NTENSI TY VARI ATI ON W I T H D I FF ER E NT PERCENTAGE OF K B r IN KCl C R Y S T A L S .
TL
PEAK
TE
MPE
RATU
RE
( *C
)
f ' 9 3 - M e ) . V A R I A T I O N IN P E A K T E M P E R A T U R E T mi & T m 2 INTHE TL GLOW CURVES OF KCl : KBr MI X E D CRYSTALS W I T H D I F F E R E N T PERCENTAGE OF K B r IN KCL
TL
INTE
NS
fTY
( ARB
. UN
ITS
)
T E M P E R A T U R E ( °C ) ------- ►
F'g 3 - 5 ( a ) _ T L GLOW CURES OF PURE KCl CRYSTALS FOR DIFFERENT V- DOSES.
TL
INTE
NSIT
Y (A
RB.
UN
ITS)
100 150T E M P E R A T U R E ( °C )
200 250
Fig 3 5 ( b )_ TL GLOW CURVES OF PURE K B r C R Y S T A L S FOR DI FF ERE NT Y ' - DOSES.
TL
INTE
NSIT
Y (
AR
B.
UN
ITS)
TEMPERATURE ( *C ) — ►
p i g - 3 . 5 ( c ) . T L GLOW CURVES OF KCl KBr ( 5 0 : 5 0 ) MI XED CRYSTALS FOR D I F F E R E N T DOSES.
TL
INTE
NSIT
Y fA
RB
.UN
ITS
) KC
I !
KBr
( 5 0
: 50)
V - DOSE ( G y ) — ►
Fig. 3 . 6 _ / - DOSE DEPENDENCE OF THE TL PEAK INTENSI TY I m & I m,
OF K C l , KBr AND KCl : KBr ( 50 50) MI XED CRYSTALS.
TL IN
TEN
SITY
( AR
B. U
NITS
) K
Cl:
KB
TOTA
L TL
fN
TENS
fTY
(ARB
. UN
ITS
) KC
I : K
Br
(50
:50
)
F i g . 3 . 7 _ f - DOSE DEPENDENCE OF THE TOTAL TL I N T E N S I T Y I T OF
KCl : K B r MI XED CRYSTALS.
TOTA
L TL
IN
TENS
ITY!
ARB
-UNI
TS)
KCl,
KBr
350
Fig. 3 .
k00 £+50 500 550WAVELENGTH ( n m ) — ►
8 _ T L SPECTRA OF PURE K C U K B r A N D KCl KBr (50 ! M I X E D CRYSTALS.
i n t e n s i t i e s I ^ a n d I 2 / i n c r e a s e s w i t h i n c r e a s i n g y - d o s e an d
then a t t a i n a s a t u r a t i o n value.
Fig. 3.7 r e p r e s e n t the “ar-dose d e p e n d e n c e of the total TL
intensity in t h e g l o w c u r v e for th e p u r e KCl, p u r e K B r an d
KCl: KB r [50:50] m i x e d crystals. It is s e e n that the t otal TL
i n t e n s i t y i n c r e a s e s w i t h i n c r e a s i n g y - d o s e a n d t h e n a t t a i n a
s a t u r a t i o n v alue.
Fig, 3.8 s h o w s the T L s p e c t r a of p u r e KCl, p u r e K B r an d
K C l : K B r m i x e d c r y s t a l s . Th e p e a k of T L s p e c t r a a r e o b s e r v e d at
4 65 nm, 4 85 n m a n d 475 n m for p u r e KCl, p u r e K B r a n d K C l : KBr
[50:50] m i x e d cry s t a l s , r e s p e c t i v e l y . T h e p o s i t i o n of Am for
K C l : K B r m i x e d c r y s t a l s lies b e t w e e n that of p u r e KCl a n d p u r e
KBr c r y s t a l s .
3 .4 DISCUSSION
3-4-1 THEROLUMINESCENCE OF ALKALI HALIDES
In a l k a l i halides, t h e r m o l u m i n e s c e n c e e m i s s i o n is
c o r r e l a t e d w i t h c o l o u r centres. O u r k n o w l e d g e of c o l o u r
centres, t h e i r s t r u c t u r e s a n d p r o d u c t i o n m e c h a n i s m s etc. has
a d v a n c e d a g r e a t d e a l . S a m e t h i n g c a n n o t b e s a i d a b o u t our
u n d e r s t a n d i n g of the m e c h a n i s m of TL, p a r t i c u l a r l y a b o u t the
p r o c e s s a b o v e 300°K.
In t h e e a r l y days, it wa s t h o u g h t that the p r i m a r y
d e f e c t s in a l k a l i h a l i d e s are a n i o n a n d c a t i o n vacanc i e s .
I r r a d i a t i o n c r e a t e s fr e e e l e c t r o n s a n d h o l e s w h i c h ar e t r a p p e d
in these vacanc i e s . I r r a d i a t i o n itself creates vacancies. The
colour c e n t r e s in the i r r a diated alkali halides, thus, were
thought to be F-centres, e l e c t r o n t r apped at the anion
vacancies an d th e i r ant i m o r p h s (Seitz 1954). Accordingly,
th e r m o l u m i n e s c e n c e was i n t e r p r e t e d as t h e r m a l l y stimul a t e d
release of e l e c t r o n (or hole) from traps into v a l e n c e b a n d and
its eve n t u a l r a d i a t i v e r e c o m b i n a t i o n wi t h hole (or electron)
at the r e c o m b i n a t i o n centre. This model of TL pro c e s s is
referred to as the m o b i l e (3 model.
I n 1958, K a n z i n g and Woodruf r e p o r t e d that the e l e ctron
deficient ce n t r e in alkali halides are not the ant i m o r p h s of F
and F - a g g r e g a t e centres. The v a r i o u s colour centres (as
V - c e n t r e ) , w e r e fo u n d to be m o l e c u l a r ions (Spaeth and
K o s c hnick 1991), for example, V v centre is a s e l f - t r a p p e d
hole (h a l o g e n ) 2 (Castner et al 1958) wh i l e the H-centres
consist of four h a l o g e n atoms sha r i n g three lattice sites and
three e l e c t r o n s b e t w e e n th e m (Kanzing and Woo d r u f f 1958).
Moreover, these centres were not stable above room
temperature. The so c alled V - c e n t r e s o b s e r v e d above room
temp e r a t u r e w e r e found to be H - c e n t r e s (or some m o l e c u l a r
ions) e s t a b l i s h e d n e a r the i m p u r i t i e s (Crawford 1968).
H-centres can s t a b i l i s e by c l u s t e r i n g also (Itoh and Tanimura
1986 an d Itoh 1982) . The clusters however, ha v e not been
ide n t i f i e d as co l o u r centres in that they do not lead to
c h a r a c t e r i s t i c s optical abs o r p t i o n band. As early as 1966, the
excitonic m e c h a n i s m of colour centre p r o d u c t i o n which
e n v i s a g e d F - H p a i r as p r i m a r y d e f e c t w a s p r o p o s e d b y H e r s h
(1 9 6 6 ) a n d P o o l e y (196 6) .
T h e m e c h a n i s m of t h e r m o l u m i n e s c e n c e w a s no t a c c o r d i n g l y
m o d i f i e d d u r i n g this period. Infact, m o s t w o r k s p r i o r to 1972
(and m a n y of l a t e r dates) s t u c k to ' the m o b i l e (3 m o d e l s
(Muriani a n d A l v a r e z - R i v a s 1978, R a s c o n a n d A l v a r e z - R i v a s
1978, A l v a r e z - R i v a s 1980, H o d g s o n et al 1978, C a s t r o a n d
A l v a r e z - R i v a s 1979, A u s i n a n d A l v a r e z - R i v a s 1972, A u s i n an d
A l v a r e z - R i v a s 1974) . In p a r t i c u l a r , t h e y p o i n t e d out that
there w a s n o t h e r m a l l y s t i m u l a t e d c o n d u c t i v i t y (TSC)
a s s o c i a t e d w i t h the g l o w p e a k a b o v e 300°K (Mariani an d
A l v a r e z - R i v a s 1978) as w o u l d b e e x p e c t e d f r o m a n e l e c t r o n (or
hole) r e l e a s e d f r o m the t r a p in the c o n d u c t i o n band. A g a i n
there a r e s e v e r a l g l o w p e a k s p r e s e n t in the g l o w curve, all of
w h i c h u s u a l l y c o r r e l a t e w i t h F - c e n t r e s . It h a d not b e e n
s a t i s f a c t o r i l y e x p l a i n e d as to w h y the F - c e n t r e s s h o u l d
r e l e a s e e l e c t r o n s d u r i n g v a r i o u s t e m p e r a t u r e int e r v a l s . J a i n
and M a h e n d r u (1965) p r o p o s e d e x i s t e n c e of d i f f e r e n t types of
F - c e n t r e s r e s p o n s i b l e fo r v a r i o u s peaks, b u t no e v i d e n c e c o u l d
be g a t h e r e d w h i c h w o u l d s u p p o r t t y p e s of F - c e n t r e s an d n u m b e r
of g l o w p e a k s f o r a l k a l i h a l i d e s in g e n e r a l . A n o t h e r i m p o r t a n t
f e a t u r e w a s e v o l u t i o n of the g l o w c u r v e (Mariani a n d
A l v a r e z - R i v a s 1978, A u s i n a n d A l v a r e z - R i v a s 1972). It was
o b s e r v e d th a t w i t h the i n c r e a s i n g exposure, the g l o w p e a k s
grow, s a t u r a t e a n d t h e n decline. Th e d e c l i n e is u s u a l l y
a c c o m p a n i e d b y e m e r g e n c e of a d d i t i o n a l g l o w p e a k s at h i g h e r
temperature. U l t i m a t e l y , at v e r y h i g h e x p o s u r e s ( > 10 C/ k g
) , o n l y s i n g l e g l o w p e a k at h i g h e s t t e m p e r a t u r e c o u l d be
o b s e r v e d .
A l v a r e z - R i v a s a n d c o - w o r k e r s a r g u e d that the m o b i l e /3
model c a n n o t e x p l a i n the evo l u t i o n . T h e m o d e l e x p e c t s the g l o w
peaks to g r o w w i t h the dose. A t the most, the p e a k m a y
saturate, b u t n o t dec l i n e . C o n s i d e r i n g s e v e r a l s u c h facts,
they p r o p o s e d th a t the e n t i t y m o b i l e d u r i n g the T L p r o c e s s is
the n e u t r a l h a l o g e n a t o m r e l e a s e d f r o m the i n t e r s t i t i a l
pos i t i o n s . T h i s is c o n s i s t e n t w i t h the fact that a f t e r r o o m
t e m p e r a t u r e i r r a d i a t i o n , the d e f e c t s i n c o r p o r a t e d in a l k a l i
ha l i d e s a r e F - c e n t r e s (and t h e i r a g g r e g a t e s ) a n d the c l u s t e r s
of i n t e r s t i t i a l h a l o g e n atoms. T h e v a r i o u s g l o w p e a k s ca n be
i n t e r p r e t e d as r e l e a s e of i n t e r s t i t i a l h a l o g e n a t o m s f r o m
c l u s t e r s of v a r i o u s si z e s a n d t h e i r s u b s e q u e n t r a d i a t i v e
r e c o m b i n a t i o n w i t h F - c e n t r e s . In fact, the e n t i r e e v o l u t i o n of
the g l o w c u r v e c a n b e e x p l a i n e d . Th e m o d e l p r o p o s e d b y
A l v a r e z - R i v a s a n d c o - w o r k e r s is r e f e r r e d to as the m o b i l e
i n t e r s t i t i a l mo d e l .
In both, the m o b i l e £ m o d e l a n d the m o b i l e i n t e r s t i t i a l
model, it h a s b e e n a s s u m e d that f o l l o w i n g the d e t r a p p i n g , the
e l e c t r o n (or hole, o r the h a l o g e n atom) is free to m o v e and it
can r e c o m b i n e a n y w h e r e in the c r y s t a l u n l e s s it is lost in
some o t h e r n o n - r a d i a t i v e p r o cess. Th e m o b i l e i n t e r s t i t i a l
model m e n t i o n so m e " c o r r e l a t e d p r o c e s s but it has not m o d i f i e d
the e q u a t i o n f o r T L k i n e t i c s w h i c h m a k e the a b o v e ass u m p t i o n .
It has b e e n s h o w n th a t the T L in a l k a l i h a l i d e s is a qu a s i
l o c a l i z e d p r o c e s s at le a s t a b o v e 300°K. A n e e d for r e v i s i n g
the TL m e c h a n i s m a n d the c o r r e s p o n d i n g k i n e t i c e q u a t i o n is
n e c e s s a r y .
3*4-2 THERMOLUMINESCENCE OF k c i : K B r MIXED CRYSTALS
It is g e n e r a l l y a c c e p t e d that the h a r d e n i n g e f f e c t m u s t
be u n d e r s t o o d no t in te r m s of s t a t i c p i n n i n g d u e to
s e g r e g a t i o n of i m p u r i t i e s a n d v a c a n c i e s to d i s l o c a t i o n , but in
terms of t h e i n t e r a c t i o n of the d e f e c t s w i t h f r e s h m o v i n g
d i s l o c a t i o n s . P r a t t et al (1964) h a v e s u g g e s t e d that d i p o l e
i n t e r a c t i o n w i t h d i s l o c a t i o n m a i n l y a c c o u n t for the h a r d e n i n g
of the a l k a l i h a l i d e crystals, s i n c e i m p u r i t y - v a c a n c y d i p o l e s
in a l k a l i h a l i d e s p r o d u c e a s y m m e t r i c s t r a i n in the matrix.
T h e T L of K C l : K B r m i x e d c r y s t a l s c o n s i s t of two g l o w
peaks, o n e at h i g h e r t e m p e r a t u r e a n d the o t h e r at l o w e r
t e m p e r a t u r e . It is s u g g e s t e d that the c e n t r e s r e s p o n s i b l e for
h i g h e r t e m p e r a t u r e g l o w p e a k c o n s i s t s of a c o m b i n a t i o n of
di p o l e s w i t h a n e g a t i v e ion vac a n c y , the c o m p l e x b e i n g
s i t u a t e d in t h e c l o s e n e i g h b o u r h o o d of a d i s l o c a t i o n w h e r e
both e l a s t i c a n d e l e c t r o s t a t i c i n t e r a c t i o n s b e t w e e n the p o i n t
defect c o m p l e x a n d t h e d i s l o c a t i o n a r e i m p o r t a n t (Deshmukh and
M oh a r i l 1985) . T h e r e is n o w g e n e r a l a g r e e m e n t that b e l o w
50 7°c , d i s l o c a t i o n in a l k a l i h a l i d e s are n e g a t i v e l y c h a r g e d
with a s u r r o u n d i n g c l o u d of p o s i t i v e charges, n a m e l y n e g a t i v e
ion v a c a n c i e s . T h e first g l o w p e a k at l o w e r t e m p e r a t u r e is
m ainly o b s e r v e d in s l o w l y c o o l e d spe c i m e n . It is, therefore,
s u g g e s t e d t h a t d i p o l e in the d i s l o c a t i o n free r e g i o n of the
lattice a r e r e s p o n s i b l e for the s e c o n d l o w t e m p e r a t u r e g l o w
peak. T h e s h i f t in t h e p e a k p o s i t i o n w i t h m i x i n g is du e to the
l o c a t i o n of d i p o l e s at d i f f e r e n t l a t t i c e d i s t a n c e s f r o m the
d i s l o c a t i o n .
T h e s t r a i n p r o d u c e d du e to the d i f f e r e n c e in l a t t i c e
constant is m a x i m u m for K C l :K B r [50:50] m i x e d crystals.
Therefore, v a c a n c i e s w i l l be m o r e an d t h e r e b y m o r e c o l o u r
centres w i l l b e p r o d u c e d for this c o m p o s i t i o n . Thus, the T L
i n t e n s i t y is m a x i m u m for K C l : K B r [50:50] m i x e d c r y s tals. For
lower a n d h i g h e r c o n c e n t r a t i o n of K B r in KCl crystals, le a s t
strain is p r o d u c e d d u e to the l a t t i c e m i s m a t c h and, therefore,
the T L i n t e n s i t y is c o m p a r a t i v e l y lower.
F r o m t h e s t u d y of r a d i a t i o n d o s e d e p e n d e n c e of the T L
intensity, it is f o u n d that the TL i n t e n s i t y i n i t i a l l y
i n c r e a s e s w i t h t h e r a d i a t i o n d o e s b e c a u s e the n u m b e r of
F - c e n t r e s i n c r e a s e s . T h e i n t e n s i t y t h e n t ends to a t t a i n a
s a t u r a t i o n v a l u e f o r h i g h e r r a d i a t i o n d o s e b e c a u s e the n u m b e r
of F - c e n t r e s t e n d s to s a t u r a t e du e to the r e c o m b i n a t i o n
b e t w e e n h o l e s a n d e l e c t r o n s .
T h e a p p e a r a n c e of one p e a k in T L s p e c t r a i n d i c a t e s that
only o n e t y p e of c a r r i e r r e l a x a t i o n p r o c e s s e s are i n v o l v e d for
the T L e m i s s i o n in K C l :K B r m i x e d crystals.
