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A direct reading exposure monitor for radiation processing
Radiat. Phys. Chem. Vol. 18, No. 5-6, pp. 937-946,19~1 Printed in Great Britain.
0146-5"/24/81/110937-10502.00/0 Persamon Press Ltd.
%
A
DIRECT
READING RADIATION
EXPOSURE MONITOR PROCESSING
FOR
A. D. Kantz and K. C. H u m p h e r y s F a r W e s t Technology, Inc. Goleta, California U S A
ABSTRACT V a r i o u s p l a s t i c f i l m s h a v e been u t i l i z e d to m e a s u r e r a d i a t i o n f i e l d s . In g e n e r a l s u c h f i l m s a r e r u g g e d , e a s i l y h a n d l e d , s m a l l e n o u g h to c a u s e n e l i g i b l e p e r t u r b a t i o n on the r a d i a t i o n f i e l d s , a n d r e l a t i v e l y i n e x p e n s i v e . T h e r a d i a c h r o m i c m a t e r i a l s h a v e b e e n s h o w n to h a v e a d v a n t a g e s o v e r o t h e r p l a s t i c f a b r i c a t i o n s i n s t a b i l ity, r e p r o d u c i b i l i t y , e q u i v a l e n t r e s p o n s e to e l e c t r o n a n d g a m m a r a y p r o c e s s i n g f i e l d s , d o s e r a t e i n d e p e n d e n c e , a n d r e a d y a v a i l a b i l i t y of c a l i b r a t i o n s t a n d a r d s . Using a nylon matrix radiachromic detector, a s y s t e m of direct read-out of absorbed dose has been developed to facilitate monitoring in the m e g a r a d region. W h e n an exposed detector is inserted into the reader, the optical transmission signal is processed through an analog to digital converter. The digitized signal addresses a m e m o r y bank w h e r e the standard response curve is stored. The corresponding absorbed dose is displayed on a digital panel meter. T h e v a r i a t i o n of r e l a t i v e s e n s i t i v i t y of d e t e c t o r s , t h e b a c k g r o u n d of u n i r r a d i a t e d detectors, environmental parameters, a n d the c a p a c i t y o f the m e m o r y b a n k a r e c o n t r i b u t i n g f a c t o r s to the t o t a l p r e c i s i o n of t h e r e a d - o u t s y s t e m .
K E YW OR DS Radiachromic;
megarad dose;
dosimetry;
monitoring;
quality assurance.
IN TR ODUC TION B e f o r e a r a d i a t i o n f a c i l i t y is p l a c e d i n o p e r a t i o n , i n f o r m a t i o n of the r a d i a t i o n t y p e , penetration, spatial distributions, and dose r a t e s is e s s e n t i a l . These radiation parameters d e t e r m i n e o p e r a t i o n a l c o n d i t i o n s of c o n v e y o r s p e e d a n d p a t h t h r o u g h a r a d i a t i o n c h a m b e r to a s s u r e u n i f o r m r a d i a t i o n e x p o s u r e f o r a p r o c e s s e d p r o d u c t .
937
938
A.D. KANTZand K. C. Hu~s~mvs
The radiation quantities of greatest interest to industrial radiation processing are absorbed dose and absoTbed dose rate. The principal reason for this emphasis is the very large quantities of radiation needed to effect a given process, and the limit of dose level variations acceptable for quality control. Basic methods used in calibration of a radiation source are usually those involving p r i m a r y standard systems such as calorimeters, liquid chemical dose meters (Chadwick, Ehlerm a n n and McLaughlin, 1977), or precision ionization chambers (Holm and Berry, 1970). These basic calibrations are relatively difficult, time consuming, and expensive. T h e y also m a y require highly skilled scientists and technicians for the proper interpretation of data. After the initial acceptance of a processing facility, a simple, reliable, and accurate dose m e a s u r e m e n t system is needed that can be related to the basic calibration data. The necessary ingredients for a successful radiation monitoring system have been presented in International Meetings on Radiation Processing (McLaughlin, 1977; H u m p h e r y s and Kantz, 1977). A checklist of parameters include:
l) z) 3) 4) 5) 6) 7)
Properly calibrated dosimeters Radiation effect versus dose relationships Evaluation of systematic errors Sampling populations large enough to specify m a x i m u m and m i n i m u m dosages Appropriate control of process parameters to provide repeatability of processing Adequate record keeping and labeling of both monitoring dosimeters and product inventory Selection of a proven dosimeter system which has an acceptable response and stability for the dosage and environmental conditions of interest
Various plastic and dyed plastic films have been used extensively in radiation m e a s u r e m e n t as a dosimeter m o s t likely to m e e t the above criteria.
RADIACHROMIC
MATERIALS
Of the m a n y plastic films used in radiation m e a s u r e m e n t , radiachromic materials developed at the National B u r e a u of Standards (McLaughlin and Chalkley, 1965), with additional i m p r o v e m e n t s m a d e at the Riso National Laboratory in D e n m a r k (McLaughlin, Miller, Fijian, Pejtersen and Pedersen, 1977), and developed into an applied monitoring system {Kantz and Humpherys, 1979) are well suited for radiation monitoring. A m o n g the advantages of the radiachromic system over other m e g a r a d d o s e u r e m e n t systems are:
i) Z) 3)
large dynamic range 14 dose rate independent to over i0 rads/sec. equivalent response to various spectra
meas-
A direct reading exposure monitor for radiation processing
4)
939
ruggedness and stability both before and a/ter irradiation a c c e p t a b l e r e p r o d u c i b i l i t y o v e r l o n g p e r i o d s of t i m e high spatial resolution characteristics ( M c L a u g h l i n , 1974)
s)
6)
T h e r e is a n e n t i r e f a m i l y of a m i n o t r i p h e n y l d y e d e r i v a t i v e s w h i c h e x h i b i t t h e p r o p e r t y of b e i n g r a d i a c h r o m i c ; t h a t is u p o n a n e x p o s u r e to r a d i a t i o n , t h e y u n d e r g o a n induced color by photoionization. T h e c h a n g e f r o m c o l o r l e s s to a d e e p l y c o l o r e d s t a t e is a d i r e c t f u n c t i o n of the a m o u n t of r a d i a t i o n e x p o s u r e . The 'dye' which h a s e x h i b i t e d the g r e a t e s t s t a b i l i t y f o r r a d i a t i o n p r o c e s s i n g is h e x a h y d r o x y e t h y l aminotriphenylnitrile. F i g u r e I i l l u s t r a t e s the r e a c t i o n of the c o m p o u n d to r a d i a tion.
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Radiation induced reaction.
T h e o p t i c a l a b s o r p t i o n s p e c t r a of t h i s c o m p o u n d a f t e r i r r a d i a t i o n is s h o w n in F i g . 2. T h e m a j o r a b s o r p t i o n b a n d i s c e n t e r e d n e a r 600 n a n o m e t e r s . A s e r i e s of a b s o r p t i o n c u r v e s s h o w s the i n c r e a s e i n a b s o r p t i o n ( d e c r e a s e i n t r a n s m i s s i o n ) as the e x p o s u r e l e v e l i n c r e a s e s . These curves are those measured when a filmdet e c t o r f a b r i c a t e d w i t h a n y l o n m a t r i x is e x p o s e d to a r a d i a t i o n s o u r c e . W h e n the o p t i c a l d e n s i t y c h a n g e s ( A OD) due to the r a d i a t i o n e x p o s u r e a r e p l o t t e d v e r s u s t h e d o s a g e , the t y p i c a l r e s p o n s e c u r v e is s h o w n in F i g . 3. This figu r e i l l u s t r a t e s the r e s p o n s e f o r the m a i n a b s o r p t i o n p e a k a t 600 n a n o m e t e r s and a s e c o n d a r y r e s p o n s e a t 510 n a n o m e t e r s . W i t h the u s e of t h i s s e c o n d a r y res p o n s e , the s a m e d e t e c t o r e x t e n d s the r a n g e of u s e f u l m e a s u r e m e n t to a b s o r b e d doses above twenty megarads.
A.D. KAwrz and K. C. HUMPHERYS
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Optical absorption spectra for different radiation exposures.
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ABSORBED DOSE-RADS
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Optical response of radiachromic detectors.
A direct reading exposure monitor for radiation p r o c e s s i n g
RADIATION
941
PROCESSLNG
A n absorbed dose of approximately 2.5 rnegarads is a typical requirement for radiation processing. F r o m the response curves s h o w n above, the nylon detectors exhibit a large A O D at the 600 n m peak and a small A O D f o r the 510 n m line. Neither region exhibit A O D values favorable for m a x i m u m spectrographic accuracy. For large A O D ' s (e. g., ~ 1.0) less than ten percent of the light is transmitted, and stray light starts to decrease the accuracy of m e a s u r e m e n t s . For small A O D ' s the accuracy is limited by the difficulties in m e a s u r i n g small quantities and background levels. For greatest spectrographic accuracy the ~ O D should be in the region 0.5 < & O D < I. 0. A series of experiments w e r e undertaken to find a suitable fabrication which would result in a favorable detector response near the 2.5 m e g a r a d s u s e d in radiation processing. Various types of plastic matrix materials have been reported (Levine, M c L a u g h l i n and Miller, 1979). Matrices of gels, nylon, polyvinylchloride, polyvinylbuteral, and polyhalostyrenes have been used for special m e a s u r e m e n t requirements. N y lon w a s selected for radiation processing f r o m its properties of strength, clarity, flexibility, and m i n i m a l environmental dependence. With a nylon matrix, the 'sensitivity' of the detector is dependent on the n u m b e r of potential color centers that m a y interact with the radiation flux. A m o r e sensitive detector could be produced by either increasing thethicknessofthefilmdetector, or increasing the a m o u n t of radiachromic 'dye'. Figure 4 shows the variation in response as the thickness of the film detectors is varied, keeping the 'dye' content constant. A s would be expected, the response curves are parallel and directly proportional to thickness. A suitable processing dosimeter could be m a d e by selecting a thickness of approximately I. 3 mils. Unfortunately, this presents two additional problems. The nylon matrix is so flexible that detectors are hard to handle, and films of uniform thickness are m o r e difficult to fabricate and m e a s u r e accurately. 2 A
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942
A.D. KAN'rzand K. C. Hu.,~m~ERys
T h e o t h e r a p p r o a c h of c h a n g i n g s e n s i t i v i t y b y c h a n g i n g t h e a m o u n t of r a d i a c h r o m i c ' d y e ~ is s h o w n in F i g . 5. T y p i c a l r e s p o n s e c u r v e s a r e s h o w n f o r f [ l m s u s i n g ' d y e ' c o n t e n t s of 5%, 8%, a n d 10%. O v e r a l i m i t e d e x p o s u r e t h e c u r v e s a r e d i r e c t l y p r o p o r t i o n a l to t h e a m o u n t of d y e . At h i g h e r d o s e s , the c u r v e s a r e s i m i l a r but s t a r t to b e n d f r o m a l i n e a r r e s p o n s e . F r o m t h e s e c u r v e s it a p p e a r s that a d e t e c t o r w i t h a 5% d y e c o n t e n t w o u l d b e f a v o r a b l e f o r r a d i a t i o n p r o c e s s i n g d o s e s . I
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Radiation response as a function of radiachromic
DIGITAL
DIRECT
READ-OUT
OF
ABSORBED
'dye' content.
DOSE
Using the 5 % dye film as the basis for d e v e l o p m e n t of a detector system., several batches of film w e r e m a d e to establish the repeatabilityofthe response curve. Figure 6 shows the standard response m e a s u r e d for these detectors. The dashed line indicates the change of O D with exposure. The solid line is the response of the film including a constant background. T h e b a c k g r o u n d is presentfrornreflections, sca±tering, and absorption in the unirradiated film detector. To m a k e a direct reading instrument, the response curve with the b a c k g r o u n d included has b e e n stored in two m e m o r y cores. The schematic circuit for addressing the m e m o r y is s h o w n in Fig. 7. The m o n o c h r o m a t i c light source transmitted through the sample, is incident upon a photodetector. The signal is p r o c e s s e d through an operational amplifier circuit to an analog to digital converter (ADC). The digitized binary n u m b e r serves as address to the m e m o r y bank. The m e m o r y b a n k is two m e m o r y cores with the output byte divided into 4 s e g m e n t nibbles. E a c h of these four nibbles are used to drive a binary coder driver (BCD). E a c h B C D activates a seven s e g m e n t digital display module. The coded display reads out absorbed doses f r o m . 010 to 9.99 rnegarads. A n over range s y m b o l (o) is activated for film receiving m o r e than 9. 99 m e g a r a d s . The s a m e overrange s y m b o l is displayed w h e n the light path is completely blocked by the detector holder. A n u n d e r r a n g e (u) s y m b o l is displayed w h e n there is no exposed film in the light path, or the s a m p l e holder is r e m o v e d f r o m the instrument.
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A.D. KAN'rz and K. C. Humpm~RYS
944
ACCURACY
OF
DISPLAYED
DOSE
The a c c u r a c y o f the direct display of absorbed dose is d e t e r m i n e d by the following factors:
Uniformity of Thickness of the F i l m Detectors W h e n film detectors are cast in nylon sheets, there isavariation in thickness. As indicated in Fig. 4, the thickness and sensitivity will be directly proportional. F o r this system, the film detectors are sorted into u n i f o r m thickness bins such that each bin will have a uniformity within approximately :~ 2.5%. O n e individual m e a s u r e m e n t would have the uncertainty of the bin variation. A s with other types of nylon detectors, the use of several detectors will average out the variation in thickness. Alternately the user m a y also m e a s u r e the thickness of each detector. Table I indicates the accuracy of a group of thirty detectors all exposed toa 1.50 m e g a r a d dose.
Table I Displayed Dose
Standard Amount
i. 50
Deviation (Percent)
Measurement System
O. 015
(1.0%)
Thirty Detectors--each thickness m e a s u r e d
1.49
±
O. 025
(1. 7°7'0)
Thirty Detectors f r o m one bin--in groups of three
1.49
±
O. 04
(2. 7%)
Thirty Single Detectors f r o m one bin
B a c k g r o u n d of Unirradiated F i l m Detectors B a c k g r o u n d of nylon film over a period of several years have been fabricated with a b a c k g r o u n d of 0.06 ± 0 . 0 1 O D units. This is the uncertainty inthe f i n a l A O D m e a s u r e m e n t s w h i c h is p r o c e s s e d through the display circuit.
Resolution of M e m o r y
Bank
T h e m e m o r y b a n k is a digitized step function to change the continuous optical transm i s s i o n signal to a digital address. T h e steps are correlated to equal changes in optical transmission. In the region of radiation processing m e a s u r e m e n t s (I to 3 m e g a r a d s ) these steps are ± 1.0%0. N e a r absorbed dose of 9.9 the width of the steps increased to ± 5%. The lowest displayed dose is . 0 1 0 m e g a r a d s . A s t h e n e x t display would be .011, the accuracy is ± 10%0.
A direct reading exposure monitor for radiation processing
945
G a i n A d j u s t m e n t s of A m p l i f i e r C i r c u i t N e u t r a l density filters are used to set the electronic gain adjustments. Theseadjustments are m a d e w h e r e the resolution of the m e m o r y bank is approximately ± 170.
S t a b i l i t y of Read-out S y s t e m Electronic components and neutral density filters are selected for maximum stab i l i t y . N e v e r t h e l e s s , n e u t r a l d e n s i t y f i l t e r s ( e s p e c i a l l y t h o s e w i t h low t r a n s m i s sion) will slowly change with t i m e . This m a y be m i n i m i z e d by s t o r i n g such f i l t e r s a t low t e m p e r a t u r e s b e t w e e n u s e s . E l e c t r o n i c c o m p o n e n t s u s u a l l y show a s m a l l t e m p e r a t u r e d e p e n d e n c e which v a r i e s a s a f u n c t i o n of a g e . B o t h e l e c t r o n i c a n d f i l t e r s t a b i l i t y c a n b e c h e c k e d w i t h S t a n d a r d s M e a s u r e m e n t a n d C a l i b r a t i o n s S e r v i c e s p r o v i d e d b y t h e N a t i o n a l B u r e a u of Standards.
Ultraviolet Sensitivity T h e d e t e c t o r s w i l l a l s o be s e n s i t i v e to u l t r a v i o l e t l i g h t w h i c h m a y b e p r e s e n t t h r o u g h l a b o r a t o r y w i n d o w s o r f l o r e s c e n t l i g h t i n g . T h e u s e r m u s t e s t a b l i s h a r o u t i n e to h a n d l e d e t e c t o r s i n a r e a s w h e r e UV l i g h t c o m p o n e n t s h a v e b e e n e l i m i n a t e d , o r e v a l u a t e the a m o u n t of c o l o r a t i o n t h a t m a y o c c u r w i t h r o u t i n e h a n d l i n g .
F i l m Factor A c o r r e c t i o n f a c t o r is s u p p l i e d w i t h the f i l m d e t e c t o r d e s i g n e d f o r the D i r e c t R e a d out system. The film factor should be adjusted by a Quality Assurance Engineer to assure proper response of the display circuit.
Environmental Effects The final absorbed dose reading m a y be modified by environmental parametersof temperature and humidity (Levine, McLaughlin and Miller, 1979).
C ON C LUSION W i t h a 5% dye c o n t e n t i n a n y l o n m a t r i x r a d i a t i o n d e t e c t o r , a d i r e c t r e a d i n g of a b s o r b e d d o s e c a n b e a c h i e v e d . U s i n g r o u t i n e q u a l i t y c o n t r o l p r o c e d u r e s , the s y s t e m s h o u l d p r o d u c e o v e r a l l a c c u r a c i e s of ± 5%. T h e g r e a t e s t a c c u r a c i e s of a b s o r b e d d o s e r e a d i n g s w i l l b e i n the r a d i a t i o n p r o c e s s i n g r e g i o n f r o m 1 to 3 r n e g a r a d s . W i t h t h e r e s p o n s e c u r v e s t o r e d i n the i n s t r u m e n t m e m o r y , c a l i b r a t i o n c a n b e a c h i e v e d b y c h e c k i n g a s i n g l e d o s e d i s p l a y i n the r e g i o n of i n t e r e s t .
946
A.D. ~
and K. C. HUMPREaYS
Although the detectors have heen designed for use in the direct read-out instrument, they are equally useful in standard photometric instruments by constructing the response curve with radiation dose.
REFERENCES Chadwick, K. H., D.A.E. E h l e r m a n n and W. L. McLaughlin (1977). Manual for Food Irradiation Processing. Internation Atomic E n e r g y Agency. Holm, N. W. and R. S. B e r r y (Eds.)(1970). Manual on Radiation Dosimetry Marcel Dekker, N e w York. H u m p h e r y s , K. C. and A. D. Kantz (1977). Radiachromic, A Radiation Monitoring System. Radiat. Phys..Chem., 9, 737-747. Kantz, A. D. and K. C.~ H u m p h e r y s {1979). Quality Assurance for Radiation Processing. Radiat. Phys. C h e m . , i__~4,575-584. Levine, H., W . L. McLaughlin and A. Miller {1979). T e m p e r a t u r e and Humidity Effectsonthe G a m m a R a y Response and Stability of Plastic and Dyed Plastic Dosimeters. Radiat. Phys. C h e m . , 14, 551-574. McLaughlin, W. L. (1974). Radiation Dosimetry with Thin Films 'Industrial Applications of Small Accelerators. Proc. 3rd Conf., Denton, TX. McLaughlin, W . L. (1977). Radiation M e a s u r e m e n t s andQualityControL Radiat. Phys. C h e m . , 9, 147-181. McLaughlin, W. L. and L. Chalkley (1965). L o w Atomic N u m b e r Dye Systems for lonizing Radiation Measurement. Phot. Sci. Engng., 9, 159-166. IVIcLaughlin, W . L., A. Miller, S. Fidan, K. Pejtersen and W . Pedersen (1977). Radiochromic Plastic F i l m for Accurate M e a s u r e m e n t of Radiation Absorbed Dose and Dose Distributions. Radiat. Phys. C h e m . , iO, 119-127.
0146-5"/24/81/110937-10502.00/0 Persamon Press Ltd.
%
A
DIRECT
READING RADIATION
EXPOSURE MONITOR PROCESSING
FOR
A. D. Kantz and K. C. H u m p h e r y s F a r W e s t Technology, Inc. Goleta, California U S A
ABSTRACT V a r i o u s p l a s t i c f i l m s h a v e been u t i l i z e d to m e a s u r e r a d i a t i o n f i e l d s . In g e n e r a l s u c h f i l m s a r e r u g g e d , e a s i l y h a n d l e d , s m a l l e n o u g h to c a u s e n e l i g i b l e p e r t u r b a t i o n on the r a d i a t i o n f i e l d s , a n d r e l a t i v e l y i n e x p e n s i v e . T h e r a d i a c h r o m i c m a t e r i a l s h a v e b e e n s h o w n to h a v e a d v a n t a g e s o v e r o t h e r p l a s t i c f a b r i c a t i o n s i n s t a b i l ity, r e p r o d u c i b i l i t y , e q u i v a l e n t r e s p o n s e to e l e c t r o n a n d g a m m a r a y p r o c e s s i n g f i e l d s , d o s e r a t e i n d e p e n d e n c e , a n d r e a d y a v a i l a b i l i t y of c a l i b r a t i o n s t a n d a r d s . Using a nylon matrix radiachromic detector, a s y s t e m of direct read-out of absorbed dose has been developed to facilitate monitoring in the m e g a r a d region. W h e n an exposed detector is inserted into the reader, the optical transmission signal is processed through an analog to digital converter. The digitized signal addresses a m e m o r y bank w h e r e the standard response curve is stored. The corresponding absorbed dose is displayed on a digital panel meter. T h e v a r i a t i o n of r e l a t i v e s e n s i t i v i t y of d e t e c t o r s , t h e b a c k g r o u n d of u n i r r a d i a t e d detectors, environmental parameters, a n d the c a p a c i t y o f the m e m o r y b a n k a r e c o n t r i b u t i n g f a c t o r s to the t o t a l p r e c i s i o n of t h e r e a d - o u t s y s t e m .
K E YW OR DS Radiachromic;
megarad dose;
dosimetry;
monitoring;
quality assurance.
IN TR ODUC TION B e f o r e a r a d i a t i o n f a c i l i t y is p l a c e d i n o p e r a t i o n , i n f o r m a t i o n of the r a d i a t i o n t y p e , penetration, spatial distributions, and dose r a t e s is e s s e n t i a l . These radiation parameters d e t e r m i n e o p e r a t i o n a l c o n d i t i o n s of c o n v e y o r s p e e d a n d p a t h t h r o u g h a r a d i a t i o n c h a m b e r to a s s u r e u n i f o r m r a d i a t i o n e x p o s u r e f o r a p r o c e s s e d p r o d u c t .
937
938
A.D. KANTZand K. C. Hu~s~mvs
The radiation quantities of greatest interest to industrial radiation processing are absorbed dose and absoTbed dose rate. The principal reason for this emphasis is the very large quantities of radiation needed to effect a given process, and the limit of dose level variations acceptable for quality control. Basic methods used in calibration of a radiation source are usually those involving p r i m a r y standard systems such as calorimeters, liquid chemical dose meters (Chadwick, Ehlerm a n n and McLaughlin, 1977), or precision ionization chambers (Holm and Berry, 1970). These basic calibrations are relatively difficult, time consuming, and expensive. T h e y also m a y require highly skilled scientists and technicians for the proper interpretation of data. After the initial acceptance of a processing facility, a simple, reliable, and accurate dose m e a s u r e m e n t system is needed that can be related to the basic calibration data. The necessary ingredients for a successful radiation monitoring system have been presented in International Meetings on Radiation Processing (McLaughlin, 1977; H u m p h e r y s and Kantz, 1977). A checklist of parameters include:
l) z) 3) 4) 5) 6) 7)
Properly calibrated dosimeters Radiation effect versus dose relationships Evaluation of systematic errors Sampling populations large enough to specify m a x i m u m and m i n i m u m dosages Appropriate control of process parameters to provide repeatability of processing Adequate record keeping and labeling of both monitoring dosimeters and product inventory Selection of a proven dosimeter system which has an acceptable response and stability for the dosage and environmental conditions of interest
Various plastic and dyed plastic films have been used extensively in radiation m e a s u r e m e n t as a dosimeter m o s t likely to m e e t the above criteria.
RADIACHROMIC
MATERIALS
Of the m a n y plastic films used in radiation m e a s u r e m e n t , radiachromic materials developed at the National B u r e a u of Standards (McLaughlin and Chalkley, 1965), with additional i m p r o v e m e n t s m a d e at the Riso National Laboratory in D e n m a r k (McLaughlin, Miller, Fijian, Pejtersen and Pedersen, 1977), and developed into an applied monitoring system {Kantz and Humpherys, 1979) are well suited for radiation monitoring. A m o n g the advantages of the radiachromic system over other m e g a r a d d o s e u r e m e n t systems are:
i) Z) 3)
large dynamic range 14 dose rate independent to over i0 rads/sec. equivalent response to various spectra
meas-
A direct reading exposure monitor for radiation processing
4)
939
ruggedness and stability both before and a/ter irradiation a c c e p t a b l e r e p r o d u c i b i l i t y o v e r l o n g p e r i o d s of t i m e high spatial resolution characteristics ( M c L a u g h l i n , 1974)
s)
6)
T h e r e is a n e n t i r e f a m i l y of a m i n o t r i p h e n y l d y e d e r i v a t i v e s w h i c h e x h i b i t t h e p r o p e r t y of b e i n g r a d i a c h r o m i c ; t h a t is u p o n a n e x p o s u r e to r a d i a t i o n , t h e y u n d e r g o a n induced color by photoionization. T h e c h a n g e f r o m c o l o r l e s s to a d e e p l y c o l o r e d s t a t e is a d i r e c t f u n c t i o n of the a m o u n t of r a d i a t i o n e x p o s u r e . The 'dye' which h a s e x h i b i t e d the g r e a t e s t s t a b i l i t y f o r r a d i a t i o n p r o c e s s i n g is h e x a h y d r o x y e t h y l aminotriphenylnitrile. F i g u r e I i l l u s t r a t e s the r e a c t i o n of the c o m p o u n d to r a d i a tion.
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Radiation induced reaction.
T h e o p t i c a l a b s o r p t i o n s p e c t r a of t h i s c o m p o u n d a f t e r i r r a d i a t i o n is s h o w n in F i g . 2. T h e m a j o r a b s o r p t i o n b a n d i s c e n t e r e d n e a r 600 n a n o m e t e r s . A s e r i e s of a b s o r p t i o n c u r v e s s h o w s the i n c r e a s e i n a b s o r p t i o n ( d e c r e a s e i n t r a n s m i s s i o n ) as the e x p o s u r e l e v e l i n c r e a s e s . These curves are those measured when a filmdet e c t o r f a b r i c a t e d w i t h a n y l o n m a t r i x is e x p o s e d to a r a d i a t i o n s o u r c e . W h e n the o p t i c a l d e n s i t y c h a n g e s ( A OD) due to the r a d i a t i o n e x p o s u r e a r e p l o t t e d v e r s u s t h e d o s a g e , the t y p i c a l r e s p o n s e c u r v e is s h o w n in F i g . 3. This figu r e i l l u s t r a t e s the r e s p o n s e f o r the m a i n a b s o r p t i o n p e a k a t 600 n a n o m e t e r s and a s e c o n d a r y r e s p o n s e a t 510 n a n o m e t e r s . W i t h the u s e of t h i s s e c o n d a r y res p o n s e , the s a m e d e t e c t o r e x t e n d s the r a n g e of u s e f u l m e a s u r e m e n t to a b s o r b e d doses above twenty megarads.
A.D. KAwrz and K. C. HUMPHERYS
940
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Optical absorption spectra for different radiation exposures.
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Fig. 3.
Optical response of radiachromic detectors.
A direct reading exposure monitor for radiation p r o c e s s i n g
RADIATION
941
PROCESSLNG
A n absorbed dose of approximately 2.5 rnegarads is a typical requirement for radiation processing. F r o m the response curves s h o w n above, the nylon detectors exhibit a large A O D at the 600 n m peak and a small A O D f o r the 510 n m line. Neither region exhibit A O D values favorable for m a x i m u m spectrographic accuracy. For large A O D ' s (e. g., ~ 1.0) less than ten percent of the light is transmitted, and stray light starts to decrease the accuracy of m e a s u r e m e n t s . For small A O D ' s the accuracy is limited by the difficulties in m e a s u r i n g small quantities and background levels. For greatest spectrographic accuracy the ~ O D should be in the region 0.5 < & O D < I. 0. A series of experiments w e r e undertaken to find a suitable fabrication which would result in a favorable detector response near the 2.5 m e g a r a d s u s e d in radiation processing. Various types of plastic matrix materials have been reported (Levine, M c L a u g h l i n and Miller, 1979). Matrices of gels, nylon, polyvinylchloride, polyvinylbuteral, and polyhalostyrenes have been used for special m e a s u r e m e n t requirements. N y lon w a s selected for radiation processing f r o m its properties of strength, clarity, flexibility, and m i n i m a l environmental dependence. With a nylon matrix, the 'sensitivity' of the detector is dependent on the n u m b e r of potential color centers that m a y interact with the radiation flux. A m o r e sensitive detector could be produced by either increasing thethicknessofthefilmdetector, or increasing the a m o u n t of radiachromic 'dye'. Figure 4 shows the variation in response as the thickness of the film detectors is varied, keeping the 'dye' content constant. A s would be expected, the response curves are parallel and directly proportional to thickness. A suitable processing dosimeter could be m a d e by selecting a thickness of approximately I. 3 mils. Unfortunately, this presents two additional problems. The nylon matrix is so flexible that detectors are hard to handle, and films of uniform thickness are m o r e difficult to fabricate and m e a s u r e accurately. 2 A
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942
A.D. KAN'rzand K. C. Hu.,~m~ERys
T h e o t h e r a p p r o a c h of c h a n g i n g s e n s i t i v i t y b y c h a n g i n g t h e a m o u n t of r a d i a c h r o m i c ' d y e ~ is s h o w n in F i g . 5. T y p i c a l r e s p o n s e c u r v e s a r e s h o w n f o r f [ l m s u s i n g ' d y e ' c o n t e n t s of 5%, 8%, a n d 10%. O v e r a l i m i t e d e x p o s u r e t h e c u r v e s a r e d i r e c t l y p r o p o r t i o n a l to t h e a m o u n t of d y e . At h i g h e r d o s e s , the c u r v e s a r e s i m i l a r but s t a r t to b e n d f r o m a l i n e a r r e s p o n s e . F r o m t h e s e c u r v e s it a p p e a r s that a d e t e c t o r w i t h a 5% d y e c o n t e n t w o u l d b e f a v o r a b l e f o r r a d i a t i o n p r o c e s s i n g d o s e s . I
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DIGITAL
DIRECT
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'dye' content.
DOSE
Using the 5 % dye film as the basis for d e v e l o p m e n t of a detector system., several batches of film w e r e m a d e to establish the repeatabilityofthe response curve. Figure 6 shows the standard response m e a s u r e d for these detectors. The dashed line indicates the change of O D with exposure. The solid line is the response of the film including a constant background. T h e b a c k g r o u n d is presentfrornreflections, sca±tering, and absorption in the unirradiated film detector. To m a k e a direct reading instrument, the response curve with the b a c k g r o u n d included has b e e n stored in two m e m o r y cores. The schematic circuit for addressing the m e m o r y is s h o w n in Fig. 7. The m o n o c h r o m a t i c light source transmitted through the sample, is incident upon a photodetector. The signal is p r o c e s s e d through an operational amplifier circuit to an analog to digital converter (ADC). The digitized binary n u m b e r serves as address to the m e m o r y bank. The m e m o r y b a n k is two m e m o r y cores with the output byte divided into 4 s e g m e n t nibbles. E a c h of these four nibbles are used to drive a binary coder driver (BCD). E a c h B C D activates a seven s e g m e n t digital display module. The coded display reads out absorbed doses f r o m . 010 to 9.99 rnegarads. A n over range s y m b o l (o) is activated for film receiving m o r e than 9. 99 m e g a r a d s . The s a m e overrange s y m b o l is displayed w h e n the light path is completely blocked by the detector holder. A n u n d e r r a n g e (u) s y m b o l is displayed w h e n there is no exposed film in the light path, or the s a m p l e holder is r e m o v e d f r o m the instrument.
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944
ACCURACY
OF
DISPLAYED
DOSE
The a c c u r a c y o f the direct display of absorbed dose is d e t e r m i n e d by the following factors:
Uniformity of Thickness of the F i l m Detectors W h e n film detectors are cast in nylon sheets, there isavariation in thickness. As indicated in Fig. 4, the thickness and sensitivity will be directly proportional. F o r this system, the film detectors are sorted into u n i f o r m thickness bins such that each bin will have a uniformity within approximately :~ 2.5%. O n e individual m e a s u r e m e n t would have the uncertainty of the bin variation. A s with other types of nylon detectors, the use of several detectors will average out the variation in thickness. Alternately the user m a y also m e a s u r e the thickness of each detector. Table I indicates the accuracy of a group of thirty detectors all exposed toa 1.50 m e g a r a d dose.
Table I Displayed Dose
Standard Amount
i. 50
Deviation (Percent)
Measurement System
O. 015
(1.0%)
Thirty Detectors--each thickness m e a s u r e d
1.49
±
O. 025
(1. 7°7'0)
Thirty Detectors f r o m one bin--in groups of three
1.49
±
O. 04
(2. 7%)
Thirty Single Detectors f r o m one bin
B a c k g r o u n d of Unirradiated F i l m Detectors B a c k g r o u n d of nylon film over a period of several years have been fabricated with a b a c k g r o u n d of 0.06 ± 0 . 0 1 O D units. This is the uncertainty inthe f i n a l A O D m e a s u r e m e n t s w h i c h is p r o c e s s e d through the display circuit.
Resolution of M e m o r y
Bank
T h e m e m o r y b a n k is a digitized step function to change the continuous optical transm i s s i o n signal to a digital address. T h e steps are correlated to equal changes in optical transmission. In the region of radiation processing m e a s u r e m e n t s (I to 3 m e g a r a d s ) these steps are ± 1.0%0. N e a r absorbed dose of 9.9 the width of the steps increased to ± 5%. The lowest displayed dose is . 0 1 0 m e g a r a d s . A s t h e n e x t display would be .011, the accuracy is ± 10%0.
A direct reading exposure monitor for radiation processing
945
G a i n A d j u s t m e n t s of A m p l i f i e r C i r c u i t N e u t r a l density filters are used to set the electronic gain adjustments. Theseadjustments are m a d e w h e r e the resolution of the m e m o r y bank is approximately ± 170.
S t a b i l i t y of Read-out S y s t e m Electronic components and neutral density filters are selected for maximum stab i l i t y . N e v e r t h e l e s s , n e u t r a l d e n s i t y f i l t e r s ( e s p e c i a l l y t h o s e w i t h low t r a n s m i s sion) will slowly change with t i m e . This m a y be m i n i m i z e d by s t o r i n g such f i l t e r s a t low t e m p e r a t u r e s b e t w e e n u s e s . E l e c t r o n i c c o m p o n e n t s u s u a l l y show a s m a l l t e m p e r a t u r e d e p e n d e n c e which v a r i e s a s a f u n c t i o n of a g e . B o t h e l e c t r o n i c a n d f i l t e r s t a b i l i t y c a n b e c h e c k e d w i t h S t a n d a r d s M e a s u r e m e n t a n d C a l i b r a t i o n s S e r v i c e s p r o v i d e d b y t h e N a t i o n a l B u r e a u of Standards.
Ultraviolet Sensitivity T h e d e t e c t o r s w i l l a l s o be s e n s i t i v e to u l t r a v i o l e t l i g h t w h i c h m a y b e p r e s e n t t h r o u g h l a b o r a t o r y w i n d o w s o r f l o r e s c e n t l i g h t i n g . T h e u s e r m u s t e s t a b l i s h a r o u t i n e to h a n d l e d e t e c t o r s i n a r e a s w h e r e UV l i g h t c o m p o n e n t s h a v e b e e n e l i m i n a t e d , o r e v a l u a t e the a m o u n t of c o l o r a t i o n t h a t m a y o c c u r w i t h r o u t i n e h a n d l i n g .
F i l m Factor A c o r r e c t i o n f a c t o r is s u p p l i e d w i t h the f i l m d e t e c t o r d e s i g n e d f o r the D i r e c t R e a d out system. The film factor should be adjusted by a Quality Assurance Engineer to assure proper response of the display circuit.
Environmental Effects The final absorbed dose reading m a y be modified by environmental parametersof temperature and humidity (Levine, McLaughlin and Miller, 1979).
C ON C LUSION W i t h a 5% dye c o n t e n t i n a n y l o n m a t r i x r a d i a t i o n d e t e c t o r , a d i r e c t r e a d i n g of a b s o r b e d d o s e c a n b e a c h i e v e d . U s i n g r o u t i n e q u a l i t y c o n t r o l p r o c e d u r e s , the s y s t e m s h o u l d p r o d u c e o v e r a l l a c c u r a c i e s of ± 5%. T h e g r e a t e s t a c c u r a c i e s of a b s o r b e d d o s e r e a d i n g s w i l l b e i n the r a d i a t i o n p r o c e s s i n g r e g i o n f r o m 1 to 3 r n e g a r a d s . W i t h t h e r e s p o n s e c u r v e s t o r e d i n the i n s t r u m e n t m e m o r y , c a l i b r a t i o n c a n b e a c h i e v e d b y c h e c k i n g a s i n g l e d o s e d i s p l a y i n the r e g i o n of i n t e r e s t .
946
A.D. ~
and K. C. HUMPREaYS
Although the detectors have heen designed for use in the direct read-out instrument, they are equally useful in standard photometric instruments by constructing the response curve with radiation dose.
REFERENCES Chadwick, K. H., D.A.E. E h l e r m a n n and W. L. McLaughlin (1977). Manual for Food Irradiation Processing. Internation Atomic E n e r g y Agency. Holm, N. W. and R. S. B e r r y (Eds.)(1970). Manual on Radiation Dosimetry Marcel Dekker, N e w York. H u m p h e r y s , K. C. and A. D. Kantz (1977). Radiachromic, A Radiation Monitoring System. Radiat. Phys..Chem., 9, 737-747. Kantz, A. D. and K. C.~ H u m p h e r y s {1979). Quality Assurance for Radiation Processing. Radiat. Phys. C h e m . , i__~4,575-584. Levine, H., W . L. McLaughlin and A. Miller {1979). T e m p e r a t u r e and Humidity Effectsonthe G a m m a R a y Response and Stability of Plastic and Dyed Plastic Dosimeters. Radiat. Phys. C h e m . , 14, 551-574. McLaughlin, W. L. (1974). Radiation Dosimetry with Thin Films 'Industrial Applications of Small Accelerators. Proc. 3rd Conf., Denton, TX. McLaughlin, W . L. (1977). Radiation M e a s u r e m e n t s andQualityControL Radiat. Phys. C h e m . , 9, 147-181. McLaughlin, W. L. and L. Chalkley (1965). L o w Atomic N u m b e r Dye Systems for lonizing Radiation Measurement. Phot. Sci. Engng., 9, 159-166. IVIcLaughlin, W . L., A. Miller, S. Fidan, K. Pejtersen and W . Pedersen (1977). Radiochromic Plastic F i l m for Accurate M e a s u r e m e n t of Radiation Absorbed Dose and Dose Distributions. Radiat. Phys. C h e m . , iO, 119-127.