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The physical properties of the LiF-Teflon (TL) dosemeters
b~d. Dad~ Jtad~. Mira., Col. 22, Nm I-4, pp. ISg-4N~, 1993
~Imvlw&ruNe Lid l,mtNl Is Oml IMma. $6.00+.e0
empmo.
THE PHYSICAL PROPERTIES OF THE LiF-TEFIJ3N (TL) DOSEMETERS R. B. AIrJM~ PhysiCSDepmmmt, ~
Family, Tekrm U ~ i ~ t y , Trims, h ~
ABSTRACT Some o f t h e p h y s i c a l c h a r a c t e r i s t i c s o f TLD-LiF h a s been p u b l i s h e d previously by t h e a u t h o r . I n this report we have s t u d i e d t h e internal and e x t e r n a l annealing effect,relative d o s e vs a t o m i c number , b u i l d - up f a c t o r i n n a r r o w and b r o a d beam c o n d i t i o n s , d e p t h d o s e and e l e c t r o n and p o s i t r o n d o s i m e t r y . The glow c u r v e o f TLD - LiF f o r c r y s t a l s t h a t have r e c e i v e d t h e s t a n d a r d a n n e a l i n g p r o c e d u r e compared t o t h o s e i n ~ h i c h t h e 80"C a n n e a l i n g h a s been o m i t t e d , which i s t h e c a s e in t h e most c o m m e r c i a l TLD - R e a d e r s ( with internal annealing } . The p o s s i b i l i t y o f m e a s u r i n g t h e a b s o r b e d d o s e i n s i d e a midium was f u r t h e r s t u d i e d by e x p o s i n g T e f l o n d i s k s p l a c e d between s h e e t s of v a r i o u s m e t a l s , T h e r e s u l t s o f t h i s e x p e r i m e n t show:, t h a t t h e TLD LiF T e f l o n d i s k s measure r e a s o n a b l y w e l l t h e a b s o r b e d d o s e f o r >1 NeV gamma - r a d i a t i o n . KEYWORDS A n n e a l i n g , d o s e , a t o m i c number , d e p t h d o s e , b u i l d up , n a r r o w and b r a o d beam , e l e c t r o n and p o s i t r o n d o s i m e t r y .
I. INTRODUCTION Radiation doseseters utilizing t h e phenomenon o f t h e r m o l u m i n e s c e n c e (TL) has been p r o p o s e d and d e v e l o p e d and a r e r e p o r t e d e l s e w h e r e in t h e l i t e r a t u r e { I , 2 ) • A p r e l i m i n a r y r e p o r t on some p h y s i c a l p r o p e r t i e s o f LiF c r y s t a l s a s a r a d i a t i o n dosemeter d o s e m e t e r h a s been p u b l i s h e d by t h e a u t h o r p r e v i o u s l y ( 3 ) .The earlier report described LiF c r y s t a l characteristics on ; reusability , t h e d e c a y p e r y e a r , r a d i a t i o n and e n e r g y d e p e n d e n c e and t h e e f f e c t s of washing , s t e r i l i z a t i o n ,and t h e m e c h a n i c a l v i b r a t i o n . In t h i s report we have s t u d i e d a b o u t t h e I n t e r n a l and external annealing effects , b u i l d - up f a c t o r , in n a r r o w and b r o a d beam c o n d i t i o n s , r e l a t i v e d o s e vs a t o m i c number , d e p t h d o s e and e l e c t r o n and p o s i t r o n d o s i s e t r y . 2 • .MEASI~E~ENTS AND RESULTS 2 • I • Internal
A n n e a l i n g E f f e c t on TLD - T e f l o n S99
900
it. B. A K B A ~ The glow c u r v e o f TLD - 100 LiF f o r c r y s t a l s t h a t h a v e r e c e i v . ~ d t h e s t a n d a r d a n n e a l i n g p r o c e d u r e c o m p a r e d t o t h o s e i n which t h e 80 e C a n n e a l i n g h a s b e e n o m i t t e d , which i s t h e c a s e in t h e most couercial TLD - R e a d e r s . { w i t h i n t e r n a l a n n e a l i n g } . The main e f f e c t o f t h e 80 ° C t r e a t m e n t i s t o r e d u c e t h e l o w e r temperature p e a k s 1 and 2 r e l a t i v e to the higher temperature peaks . Thus r e n d e r i n g t h e TL o f t h e c r y s t a l s o r e s t a b l e a t room t e m p e r a t u r e . The TLD - 100 which h a s b e e n e x p o s e d t o up t o 500 Gy {50,000 tad} and t h e n s u b j e c t e d to the standard annealing p r o c e d u r e a p p e a r s i d e n t i c a l in i t s TL p r o p e r t i e s t o c r y s t a l s which h a v e n e v e r been i r r a d i a t e d . T h i s makes i t p o s s i b l e t o r e u s e t h e crystals i~efinitely • In the following experiments the standard annealing procedure have been applied for better results .
2 . 2 . Relative
Dose v s Atomic Number
The p o s s i b i l i t y o f s e a n u r i n g t h e a b s o r b e d d o s e i n s i d e a medium was studied by e x p o s i n g 0 . 4 a s t h i c k LiF -Teflon disks , placed between sheets of various metals .
The r e s u l t o f t h i s e x p e r i m e n t s h o ~ s t h a t t h e LiF - T e f l o n d i s k s measure reasonably well t h e a b s o r b e d d o s e f o r > 1MeV gamma radiation. Consequently , t h e y c a n be u t i l i z e d t o m e a s u r e t h e a b s o r b e d d o s e in r a d i o l o g i c a l d o s i m e t r y (where r e l a t i v e l y low a t o m i c numbers a r e of interest ) f o r Co - 60 gamma - r a d i a t i o n o f h i g h e r p h o t o n energies •
2 . 3 . Dose B u i l d - up In a C o l l i m a t e d Beam The d a t a show how t h e d o s e i n c r e a s e s with the depth as the electron flux builds up,it is difficult t o s e e how t h i s e x p e r i m e n t c o u l d be made w i t h a n o t h e r t y p e o f d o s e n e t e r s .
The results also show that the LiF - Teflon d~sk behaves essentially as a dose meter , responding to the flux of electrons
generated outside the This is also true for u n c o v e r e d r o d s showed ounded by 3 mm L u c i t e
d o s e m e t e r f o r p h o t o n e n e r g i e s abo~e 1MeV. t h e r o d s and p o ~ d e r . I n a s i m i l a r e x p e r i m e n t 57% o f t h e s i g n a l o b t a i n e d w i t h r o d s s u r r •
LiP-TEFLON DOSEMETERS
901
2 . 3 . Dose Build - up In Uncollimated Beam B u i l d i n g - up f a c t o r f o r e x p o s u r e i n w h i c h a b r a o d a n d u n f i l t e r e d beams of radiation were used , e.g.,in exposures with radionuclides , to obtain electronic equilibrium , a set of measurements in i d e n t i c a l measuring c o n d i t i o n s were c a r r i e d out t o f i n d o u t t h e b u i l d - up r e g i o n f o r t h e r e s p o n s e o f TLD - L i F a t e n e r g i e s o f ; 1 4 0 , 6 6 2 , and 1250 keV . The maximum d o s a g e b u i l d up w i t h Tc - 99m , Cs - 137 and Co - 60 o ~ u r r e d a t 0 . 7 ms , 2 me and 5 me o f tissue equivalent material, respectively •
2 . 4 .
Depth Dose Disribution of ~'Radiation
A series of experiments were made to check the behaviour of the disks for ~ - radiation . The depth - dose distribution under a nutural Uranium source covered with about 7 mg/cm- of plastic sheet was measured with a number of 0.4 ms thick disks . These had been calibrated against Co - 60 gamma - radiation in signal per Gy in tissue , and the measured data have been transformed into mGy per hour . The value of zero depth , 2.3 0.15 mGy lh (230 ~ 15 mrad / h ) , agree well with other published data on this quantity (5).
2 . 5 . JBeta - Radiation Effect On Surface Exposure The results of measurements on the syringe contazning radioactive samples , using TLD - LIF showed higher values than the calculated figures for the surface exposure of a cylinderical source{Table1}, It was felt that this can be probably due to the beta emission of the source , because for Tc - 99m , which does not emit betas , calculation and measured values were in reasonable agreeement . The largest discrepancy app#ared between the measured and calcu fated values for (Fe - 52 + Mn - 52m) . So the range of the positrons emitted from Fe - 52 and Mn -52m was calculated by using an emperical relationship between R {practical range} and Ec (incident energy of electron} for 1 - 3 MeV (Harder 1964}. R m p --__0.528 E o - 0.098 knowing
E o
~
2.26
MeV
R m {positron} ~ The
density
of
g / cm
( I00 % )
1.092
polystyrene
--
g / cm 1.063
g
/ cm , so the range of
positrons emitted from the Fe - 52 samples (in cm} was found to be R 1 (positron)
~
1.02
Cm
The t h i c k n e s s o f a p l a s t i c s y r i n g e (5 e l } u s e d i n t h e m e a s u r e m e n t s was 0 . 1 c e . Thus t h e r a n g e o f p o s i t r o n s i s s u c h g r e a t e r t h a n t h e t h i c k n e s s o r t h e w a l l o f t h e c o n t a i n e r and t h e r e f o r e t h e p o s i t r o n s a r e t r a n s m i t e d t h r o u g h t h e t h i c k n e s s o f t h e s y r i n g e and a f f e c t e d
902
R . B . AKBARI t h e measured d o s e on t h e s u r f a c e o f t h e s y r i n g e . The p e r c e n t a g e o f p o s i t r o n s t r a n s , i t t e d was f o u n d by t h e e x p e . r i l m n t . The r e s u l t s a r e shown i n T a b l e 1.
T a b l e I . D a t a o b t a i n e d f r o , TLD - LiF R e a s u r e , e n t s bution of electrons to the surface cylinderical source . TLD - LiF Doseseter no.
Condition
TLD R e a d i n g in counts
of the contri exposure of
Z Transmission relative to the max.value at 7 and 0 c a .
1 2 3
no p l a s t i c on f r o n t a t 7 cm
178 179 184
4 5 6
1.5 u t h i c k p l a s t i c a t 7 cm . f r o n t and back
124 111 108
7 8 9
1.5 mm t h i c k plastic attached to the bottom
1110 840 700
60 45 37
180
114
100 {7 cm)
63 (7 cm)
10 11 12
t h r e e on t o g e t h e r 1.5 mm p l a s t i c on t h e back o n l y
+
1135 1487 1884
60 79 I00
13
on t h e s u r f a c e a l o n e
1863
99
Background Beading Calibration factor
50 c o u n t s . 2 2 . 5 ~Gy / c o u n t
a
( 2 . 2 5 mrad / c o u n t ) .
REFERENCES
1 - MOREHEAD F.T and DANIELS F. (1957) S c . C h e n . P h y s . , 27 , ! 3 1 8 . 2 - CAMERON J.R.,DANIELS F.,JOHNSON N.,KENNEY G . ( 1 9 6 1 ) , S c i e n c e , 134,333. 3 - AKB~I R.B. (1988)Nucl.Tracks.Radiat.Meas.,Vol.15,~oa.l-4pp. 783-786 . l n t . J . R a d i a t Appl. I n s t r u n . P a r t A. 4 - RITZ V . H . , a n d ATTIX F . H . ( 1 9 6 1 ) A s o l i d - s t a t e Bragg-Gray Cavity C h a l b e r . in S e l l e c t e d T o p i c s in R a d i a t i o n D o s i s e t r y . p p 4 8 1 - 4 9 0 , IAEA , Vienna. 5 - L.~4ER J . F . and BARKER R . F . ( 1 9 5 9 ) u p p e r i n t e n d e n t o f Doccuments. U.S. Government p r i n t i n g O f f i c e , Washington ,D.C.
~Imvlw&ruNe Lid l,mtNl Is Oml IMma. $6.00+.e0
empmo.
THE PHYSICAL PROPERTIES OF THE LiF-TEFIJ3N (TL) DOSEMETERS R. B. AIrJM~ PhysiCSDepmmmt, ~
Family, Tekrm U ~ i ~ t y , Trims, h ~
ABSTRACT Some o f t h e p h y s i c a l c h a r a c t e r i s t i c s o f TLD-LiF h a s been p u b l i s h e d previously by t h e a u t h o r . I n this report we have s t u d i e d t h e internal and e x t e r n a l annealing effect,relative d o s e vs a t o m i c number , b u i l d - up f a c t o r i n n a r r o w and b r o a d beam c o n d i t i o n s , d e p t h d o s e and e l e c t r o n and p o s i t r o n d o s i m e t r y . The glow c u r v e o f TLD - LiF f o r c r y s t a l s t h a t have r e c e i v e d t h e s t a n d a r d a n n e a l i n g p r o c e d u r e compared t o t h o s e i n ~ h i c h t h e 80"C a n n e a l i n g h a s been o m i t t e d , which i s t h e c a s e in t h e most c o m m e r c i a l TLD - R e a d e r s ( with internal annealing } . The p o s s i b i l i t y o f m e a s u r i n g t h e a b s o r b e d d o s e i n s i d e a midium was f u r t h e r s t u d i e d by e x p o s i n g T e f l o n d i s k s p l a c e d between s h e e t s of v a r i o u s m e t a l s , T h e r e s u l t s o f t h i s e x p e r i m e n t show:, t h a t t h e TLD LiF T e f l o n d i s k s measure r e a s o n a b l y w e l l t h e a b s o r b e d d o s e f o r >1 NeV gamma - r a d i a t i o n . KEYWORDS A n n e a l i n g , d o s e , a t o m i c number , d e p t h d o s e , b u i l d up , n a r r o w and b r a o d beam , e l e c t r o n and p o s i t r o n d o s i m e t r y .
I. INTRODUCTION Radiation doseseters utilizing t h e phenomenon o f t h e r m o l u m i n e s c e n c e (TL) has been p r o p o s e d and d e v e l o p e d and a r e r e p o r t e d e l s e w h e r e in t h e l i t e r a t u r e { I , 2 ) • A p r e l i m i n a r y r e p o r t on some p h y s i c a l p r o p e r t i e s o f LiF c r y s t a l s a s a r a d i a t i o n dosemeter d o s e m e t e r h a s been p u b l i s h e d by t h e a u t h o r p r e v i o u s l y ( 3 ) .The earlier report described LiF c r y s t a l characteristics on ; reusability , t h e d e c a y p e r y e a r , r a d i a t i o n and e n e r g y d e p e n d e n c e and t h e e f f e c t s of washing , s t e r i l i z a t i o n ,and t h e m e c h a n i c a l v i b r a t i o n . In t h i s report we have s t u d i e d a b o u t t h e I n t e r n a l and external annealing effects , b u i l d - up f a c t o r , in n a r r o w and b r o a d beam c o n d i t i o n s , r e l a t i v e d o s e vs a t o m i c number , d e p t h d o s e and e l e c t r o n and p o s i t r o n d o s i s e t r y . 2 • .MEASI~E~ENTS AND RESULTS 2 • I • Internal
A n n e a l i n g E f f e c t on TLD - T e f l o n S99
900
it. B. A K B A ~ The glow c u r v e o f TLD - 100 LiF f o r c r y s t a l s t h a t h a v e r e c e i v . ~ d t h e s t a n d a r d a n n e a l i n g p r o c e d u r e c o m p a r e d t o t h o s e i n which t h e 80 e C a n n e a l i n g h a s b e e n o m i t t e d , which i s t h e c a s e in t h e most couercial TLD - R e a d e r s . { w i t h i n t e r n a l a n n e a l i n g } . The main e f f e c t o f t h e 80 ° C t r e a t m e n t i s t o r e d u c e t h e l o w e r temperature p e a k s 1 and 2 r e l a t i v e to the higher temperature peaks . Thus r e n d e r i n g t h e TL o f t h e c r y s t a l s o r e s t a b l e a t room t e m p e r a t u r e . The TLD - 100 which h a s b e e n e x p o s e d t o up t o 500 Gy {50,000 tad} and t h e n s u b j e c t e d to the standard annealing p r o c e d u r e a p p e a r s i d e n t i c a l in i t s TL p r o p e r t i e s t o c r y s t a l s which h a v e n e v e r been i r r a d i a t e d . T h i s makes i t p o s s i b l e t o r e u s e t h e crystals i~efinitely • In the following experiments the standard annealing procedure have been applied for better results .
2 . 2 . Relative
Dose v s Atomic Number
The p o s s i b i l i t y o f s e a n u r i n g t h e a b s o r b e d d o s e i n s i d e a medium was studied by e x p o s i n g 0 . 4 a s t h i c k LiF -Teflon disks , placed between sheets of various metals .
The r e s u l t o f t h i s e x p e r i m e n t s h o ~ s t h a t t h e LiF - T e f l o n d i s k s measure reasonably well t h e a b s o r b e d d o s e f o r > 1MeV gamma radiation. Consequently , t h e y c a n be u t i l i z e d t o m e a s u r e t h e a b s o r b e d d o s e in r a d i o l o g i c a l d o s i m e t r y (where r e l a t i v e l y low a t o m i c numbers a r e of interest ) f o r Co - 60 gamma - r a d i a t i o n o f h i g h e r p h o t o n energies •
2 . 3 . Dose B u i l d - up In a C o l l i m a t e d Beam The d a t a show how t h e d o s e i n c r e a s e s with the depth as the electron flux builds up,it is difficult t o s e e how t h i s e x p e r i m e n t c o u l d be made w i t h a n o t h e r t y p e o f d o s e n e t e r s .
The results also show that the LiF - Teflon d~sk behaves essentially as a dose meter , responding to the flux of electrons
generated outside the This is also true for u n c o v e r e d r o d s showed ounded by 3 mm L u c i t e
d o s e m e t e r f o r p h o t o n e n e r g i e s abo~e 1MeV. t h e r o d s and p o ~ d e r . I n a s i m i l a r e x p e r i m e n t 57% o f t h e s i g n a l o b t a i n e d w i t h r o d s s u r r •
LiP-TEFLON DOSEMETERS
901
2 . 3 . Dose Build - up In Uncollimated Beam B u i l d i n g - up f a c t o r f o r e x p o s u r e i n w h i c h a b r a o d a n d u n f i l t e r e d beams of radiation were used , e.g.,in exposures with radionuclides , to obtain electronic equilibrium , a set of measurements in i d e n t i c a l measuring c o n d i t i o n s were c a r r i e d out t o f i n d o u t t h e b u i l d - up r e g i o n f o r t h e r e s p o n s e o f TLD - L i F a t e n e r g i e s o f ; 1 4 0 , 6 6 2 , and 1250 keV . The maximum d o s a g e b u i l d up w i t h Tc - 99m , Cs - 137 and Co - 60 o ~ u r r e d a t 0 . 7 ms , 2 me and 5 me o f tissue equivalent material, respectively •
2 . 4 .
Depth Dose Disribution of ~'Radiation
A series of experiments were made to check the behaviour of the disks for ~ - radiation . The depth - dose distribution under a nutural Uranium source covered with about 7 mg/cm- of plastic sheet was measured with a number of 0.4 ms thick disks . These had been calibrated against Co - 60 gamma - radiation in signal per Gy in tissue , and the measured data have been transformed into mGy per hour . The value of zero depth , 2.3 0.15 mGy lh (230 ~ 15 mrad / h ) , agree well with other published data on this quantity (5).
2 . 5 . JBeta - Radiation Effect On Surface Exposure The results of measurements on the syringe contazning radioactive samples , using TLD - LIF showed higher values than the calculated figures for the surface exposure of a cylinderical source{Table1}, It was felt that this can be probably due to the beta emission of the source , because for Tc - 99m , which does not emit betas , calculation and measured values were in reasonable agreeement . The largest discrepancy app#ared between the measured and calcu fated values for (Fe - 52 + Mn - 52m) . So the range of the positrons emitted from Fe - 52 and Mn -52m was calculated by using an emperical relationship between R {practical range} and Ec (incident energy of electron} for 1 - 3 MeV (Harder 1964}. R m p --__0.528 E o - 0.098 knowing
E o
~
2.26
MeV
R m {positron} ~ The
density
of
g / cm
( I00 % )
1.092
polystyrene
--
g / cm 1.063
g
/ cm , so the range of
positrons emitted from the Fe - 52 samples (in cm} was found to be R 1 (positron)
~
1.02
Cm
The t h i c k n e s s o f a p l a s t i c s y r i n g e (5 e l } u s e d i n t h e m e a s u r e m e n t s was 0 . 1 c e . Thus t h e r a n g e o f p o s i t r o n s i s s u c h g r e a t e r t h a n t h e t h i c k n e s s o r t h e w a l l o f t h e c o n t a i n e r and t h e r e f o r e t h e p o s i t r o n s a r e t r a n s m i t e d t h r o u g h t h e t h i c k n e s s o f t h e s y r i n g e and a f f e c t e d
902
R . B . AKBARI t h e measured d o s e on t h e s u r f a c e o f t h e s y r i n g e . The p e r c e n t a g e o f p o s i t r o n s t r a n s , i t t e d was f o u n d by t h e e x p e . r i l m n t . The r e s u l t s a r e shown i n T a b l e 1.
T a b l e I . D a t a o b t a i n e d f r o , TLD - LiF R e a s u r e , e n t s bution of electrons to the surface cylinderical source . TLD - LiF Doseseter no.
Condition
TLD R e a d i n g in counts
of the contri exposure of
Z Transmission relative to the max.value at 7 and 0 c a .
1 2 3
no p l a s t i c on f r o n t a t 7 cm
178 179 184
4 5 6
1.5 u t h i c k p l a s t i c a t 7 cm . f r o n t and back
124 111 108
7 8 9
1.5 mm t h i c k plastic attached to the bottom
1110 840 700
60 45 37
180
114
100 {7 cm)
63 (7 cm)
10 11 12
t h r e e on t o g e t h e r 1.5 mm p l a s t i c on t h e back o n l y
+
1135 1487 1884
60 79 I00
13
on t h e s u r f a c e a l o n e
1863
99
Background Beading Calibration factor
50 c o u n t s . 2 2 . 5 ~Gy / c o u n t
a
( 2 . 2 5 mrad / c o u n t ) .
REFERENCES
1 - MOREHEAD F.T and DANIELS F. (1957) S c . C h e n . P h y s . , 27 , ! 3 1 8 . 2 - CAMERON J.R.,DANIELS F.,JOHNSON N.,KENNEY G . ( 1 9 6 1 ) , S c i e n c e , 134,333. 3 - AKB~I R.B. (1988)Nucl.Tracks.Radiat.Meas.,Vol.15,~oa.l-4pp. 783-786 . l n t . J . R a d i a t Appl. I n s t r u n . P a r t A. 4 - RITZ V . H . , a n d ATTIX F . H . ( 1 9 6 1 ) A s o l i d - s t a t e Bragg-Gray Cavity C h a l b e r . in S e l l e c t e d T o p i c s in R a d i a t i o n D o s i s e t r y . p p 4 8 1 - 4 9 0 , IAEA , Vienna. 5 - L.~4ER J . F . and BARKER R . F . ( 1 9 5 9 ) u p p e r i n t e n d e n t o f Doccuments. U.S. Government p r i n t i n g O f f i c e , Washington ,D.C.