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Remote real-time dose measurements of gamma-ray photon beams with radiochromic sensors.
Nuclear Instruments and Methods in Physics Research B79 (1993) 835437 North-Holland
Remote real-time dose measurements with radiochromic sensors. *
Beam Intsractionr with Materials&Atoms
of gamma-ray photon beams
M.L. Walker, C.E. Dick and W.L. McLaughlin IonizingRadiation Diuision, Physics Laboratory * *, National Institute
of Standards and Technology, Gaithersbwg, MD 20899, USA
A method of evaluating intense ionizing radiation fields remotely and in real-time has been developed. The system employs radiochromic sensors, materials that change color in a known fashion upon exposure to radiation, and a helium-neon laser/photodiode detector combination to measure the radiation-induced absorbance change at 632.8 nm. The sensor, placed at the point of interest in the radiation field, is probed by the laser to assess the absorbed dose at a given time as a function of any absorbance changes in the sensor. The resultant attenuation of the probe beam is registered by the photodiode and recorded and evaluated with a data acquisition system. Results from gamma-ray irradiations are presented.
1. Introduction The medical field has utilized the penetrating and tumor-destroying capabilities of ionizing radiation for many years. The advent of the use of ionizing radiation in certain industrial processing applications such as polymer crosslinking, product sterilization, and food preservation has exemplified the need for accurate and rapid dosimetry to verify the amount of radiation needed to accomplish a particular task or to realize a desired effect [l]. Existing methods are suitable for passive dosimetry needs, but new production methods or more efficient process environments mandate the development of real-time dosimetric techniques or systems for on-line process control. One potential system, the laser telemetering dosimetry system, has shown promise in remote, real-time monitoring of ionizing radiation fields such as electron beams and X-rays [2,3]. This study demonstrates applicability of the system to gala-ray fields.
ChromicTM Dosimetry Media #l, was used as the sensing material [4]. This fihn, consisting of a proprietary 6-pm thick sensing layer coated on a NO-pm thick polyester backing, visibly darkens upon exposure to ionizing radiation. A 3 X 1 cm piece of this film was sandwiched between suprasil quartz plates 2 mm thick to provide conditions approximating electron equilibrium 151. The sensor was located 70.3 cm from a vertical beam 6o~ gamma-ray source for which the dose rate in silicon dioxide (quartz) at this distance was 2.57 f 0.02 Gy/min. The 632.8 nm red light from a 7-mW helium-neon laser was directed through the sensor and then reflected back to a photodiode detector. The signal was amplified and input into a personal computer via a commercially available interface board, and was analyzed using coercibly available software. A timed shutter was used so that the laser was incident on the sensor for 15 s every detection interval of 3 min. The total path traveled by the laser beam was 10 m.
2. Experimental
3. Results and discussion
Inherent in the telemetering dosimetry system is a wide number of configurations to fit specific needs f2.31. The setup used for the gamma-ray experiments is shown in fig. 1. A new radiochromic film, Gaf-
A representative plot of the raw data is shown in fig. 2 in terms of the photodiode output versus irradiation time. The GafChromic film has an absorption band shoulder around 633 nm that increases proportionally to radiation dosage in the range l-1000 Gy. The data exhibit a smooth exponential decrease. In fig. 3 are given the transformed data with respect to dose.
* Work supported in part by the U.S. Department of the Army, Redstone Arsenal. ** Technology Administration, U.S. Department of Commerce. Elsevier Science Publishers B.V.
#‘Commercial or trade names used here are for identification purposes only and do not constitute recommendation or endorsement by NIST. XIII. DETECTORS/SPECTROMETERS
ML. Walker et al. /Remote real-time dose measurements
836
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jC/Llirror Fig. 1. Schematic diagram of the experimental setup for real-time, remote dose assessment of a %Zo source beam.
Here, optical density is defined as the log of the ratio of the initial transmitted intensity to the transmitted intensity of the laser light at a specified time. The dose rate at a given distance in the collimated beam from the 6oGo source is known, thus allowing a direct conversion from elapsed irradiation time to dose. A linear regression analysis fits the data reasonably well between 1 and 600 Gy, with a correlation coefficient of 0.999. In table 1 is a representative sampling of mean values spanning the dose range, and the associated maximum deviations from the mean. It can be seen that the maximum deviation is less than 5% of the mean value.
This particular radiochromic film is known to increase in optical density in a 24 h period immediately after irradiation with a significant portion of this increase occurring in the first 2-3 h [41, accounting for the upward curvature of the data curve at higher doses and the increased deviation from linearity. The sensing material that has already reacted to the ionixing radiation continues to develop and increase in color, creating an additive effect on the overall optical density. Despite this limitation, it can be readily used over short time spans as an immediate, real-time dosimeter. Examples of processes that’would benefit from a realtime dose assessment in the range of l-1000 Gy are
Fig. 2. Photodiode output during gamma-ray irradiation of the radiochromic sensor.
Fig. 3. Plot of optical density versus absorbed dose in the radiochromic sensor.
M.L. Walker et al. / Remote real-time dose measurements Table 1 Mean value of the optical density at a specific dose Dose
KY] 0.00 8.01
41.43 83.19 116.60 158.63 200.13 250.24 300.36 400.59 500.82 600.05
Optical density
Maximum percentage deviation from mean optical density
0.00
0.0
0.012 0.064 0.125 0.171 0.229 0.287 0.359 0.433 0.577 0.741 0.918
3.3 4.5 2.4 0.9 1.6 1.6 1.8 1.5 2.0 2.1 2.4
fruit disinfestation, sprout inhibition, and blood leukocyte deactivation. Other radiochromic films are available for use in real-time remote dose assessment that would be applicable in the range of l-50 kGy for such processes as medical device sterilization [l].
4. Summary
831
dose. The system can be used for electrons, X-rays, and gamma-rays. Further experiments are planned to explore the telemetering range as well as the dose range of the system, and the sensitivity of the method to neutrons.
References t11 W.L. McLaughlin, A.W. Boyd, K.H. Chadwick, J.C. McDonald and A. Miller, Dosimetry For Radiation Processing (Taylor and Francis, London, 1989). 121M.L. Walker and W.L. McLaughlin, RadTech ‘90 North America Conf. Proc., vol. II, Chicago (1990) 189. [31 M.L. Walker, C.E. Dick and W.L. McLaughlin, Development and Applications of the Laser Telemetering Dosimetry System, Proc. 14th Annual Meeting of The National Society of Black Physicists, 1991, in press. [41 W.L. McLaughlin, Chen Yun-Dong, C.G. Soares, A. Miller, G. van Dyk and D.F. Lewis, Nucl. Instr. and Meth. A302 (1991) 165. [51 International Commission on Radiation Units and Measurements, Report 37, Stopping Powers for Electrons and Positrons (1984).
A remote, real-time dosimetry system has been developed for this measurement of ionizing radiation
XIII. DETECTORS/SPECTROMETERS
Remote real-time dose measurements with radiochromic sensors. *
Beam Intsractionr with Materials&Atoms
of gamma-ray photon beams
M.L. Walker, C.E. Dick and W.L. McLaughlin IonizingRadiation Diuision, Physics Laboratory * *, National Institute
of Standards and Technology, Gaithersbwg, MD 20899, USA
A method of evaluating intense ionizing radiation fields remotely and in real-time has been developed. The system employs radiochromic sensors, materials that change color in a known fashion upon exposure to radiation, and a helium-neon laser/photodiode detector combination to measure the radiation-induced absorbance change at 632.8 nm. The sensor, placed at the point of interest in the radiation field, is probed by the laser to assess the absorbed dose at a given time as a function of any absorbance changes in the sensor. The resultant attenuation of the probe beam is registered by the photodiode and recorded and evaluated with a data acquisition system. Results from gamma-ray irradiations are presented.
1. Introduction The medical field has utilized the penetrating and tumor-destroying capabilities of ionizing radiation for many years. The advent of the use of ionizing radiation in certain industrial processing applications such as polymer crosslinking, product sterilization, and food preservation has exemplified the need for accurate and rapid dosimetry to verify the amount of radiation needed to accomplish a particular task or to realize a desired effect [l]. Existing methods are suitable for passive dosimetry needs, but new production methods or more efficient process environments mandate the development of real-time dosimetric techniques or systems for on-line process control. One potential system, the laser telemetering dosimetry system, has shown promise in remote, real-time monitoring of ionizing radiation fields such as electron beams and X-rays [2,3]. This study demonstrates applicability of the system to gala-ray fields.
ChromicTM Dosimetry Media #l, was used as the sensing material [4]. This fihn, consisting of a proprietary 6-pm thick sensing layer coated on a NO-pm thick polyester backing, visibly darkens upon exposure to ionizing radiation. A 3 X 1 cm piece of this film was sandwiched between suprasil quartz plates 2 mm thick to provide conditions approximating electron equilibrium 151. The sensor was located 70.3 cm from a vertical beam 6o~ gamma-ray source for which the dose rate in silicon dioxide (quartz) at this distance was 2.57 f 0.02 Gy/min. The 632.8 nm red light from a 7-mW helium-neon laser was directed through the sensor and then reflected back to a photodiode detector. The signal was amplified and input into a personal computer via a commercially available interface board, and was analyzed using coercibly available software. A timed shutter was used so that the laser was incident on the sensor for 15 s every detection interval of 3 min. The total path traveled by the laser beam was 10 m.
2. Experimental
3. Results and discussion
Inherent in the telemetering dosimetry system is a wide number of configurations to fit specific needs f2.31. The setup used for the gamma-ray experiments is shown in fig. 1. A new radiochromic film, Gaf-
A representative plot of the raw data is shown in fig. 2 in terms of the photodiode output versus irradiation time. The GafChromic film has an absorption band shoulder around 633 nm that increases proportionally to radiation dosage in the range l-1000 Gy. The data exhibit a smooth exponential decrease. In fig. 3 are given the transformed data with respect to dose.
* Work supported in part by the U.S. Department of the Army, Redstone Arsenal. ** Technology Administration, U.S. Department of Commerce. Elsevier Science Publishers B.V.
#‘Commercial or trade names used here are for identification purposes only and do not constitute recommendation or endorsement by NIST. XIII. DETECTORS/SPECTROMETERS
ML. Walker et al. /Remote real-time dose measurements
836
Vertical Beam Co-60 Source
He-Ne Laser
‘. ‘.
\ \ \ \ \
‘.
‘.
‘.
‘.
‘\
Mirror ‘.
Mirror ‘\
‘.
Photodiode Detector
‘\
Radiochromic
‘.
-.
%T+ To Data Collection Computer
I
.,j*.,
‘.
@*a.$ ‘.
‘.
..
y;
‘.
4.
‘.
i i ‘.
‘.
‘.
‘.
I f f
‘x
“’
jC/Llirror Fig. 1. Schematic diagram of the experimental setup for real-time, remote dose assessment of a %Zo source beam.
Here, optical density is defined as the log of the ratio of the initial transmitted intensity to the transmitted intensity of the laser light at a specified time. The dose rate at a given distance in the collimated beam from the 6oGo source is known, thus allowing a direct conversion from elapsed irradiation time to dose. A linear regression analysis fits the data reasonably well between 1 and 600 Gy, with a correlation coefficient of 0.999. In table 1 is a representative sampling of mean values spanning the dose range, and the associated maximum deviations from the mean. It can be seen that the maximum deviation is less than 5% of the mean value.
This particular radiochromic film is known to increase in optical density in a 24 h period immediately after irradiation with a significant portion of this increase occurring in the first 2-3 h [41, accounting for the upward curvature of the data curve at higher doses and the increased deviation from linearity. The sensing material that has already reacted to the ionixing radiation continues to develop and increase in color, creating an additive effect on the overall optical density. Despite this limitation, it can be readily used over short time spans as an immediate, real-time dosimeter. Examples of processes that’would benefit from a realtime dose assessment in the range of l-1000 Gy are
Fig. 2. Photodiode output during gamma-ray irradiation of the radiochromic sensor.
Fig. 3. Plot of optical density versus absorbed dose in the radiochromic sensor.
M.L. Walker et al. / Remote real-time dose measurements Table 1 Mean value of the optical density at a specific dose Dose
KY] 0.00 8.01
41.43 83.19 116.60 158.63 200.13 250.24 300.36 400.59 500.82 600.05
Optical density
Maximum percentage deviation from mean optical density
0.00
0.0
0.012 0.064 0.125 0.171 0.229 0.287 0.359 0.433 0.577 0.741 0.918
3.3 4.5 2.4 0.9 1.6 1.6 1.8 1.5 2.0 2.1 2.4
fruit disinfestation, sprout inhibition, and blood leukocyte deactivation. Other radiochromic films are available for use in real-time remote dose assessment that would be applicable in the range of l-50 kGy for such processes as medical device sterilization [l].
4. Summary
831
dose. The system can be used for electrons, X-rays, and gamma-rays. Further experiments are planned to explore the telemetering range as well as the dose range of the system, and the sensitivity of the method to neutrons.
References t11 W.L. McLaughlin, A.W. Boyd, K.H. Chadwick, J.C. McDonald and A. Miller, Dosimetry For Radiation Processing (Taylor and Francis, London, 1989). 121M.L. Walker and W.L. McLaughlin, RadTech ‘90 North America Conf. Proc., vol. II, Chicago (1990) 189. [31 M.L. Walker, C.E. Dick and W.L. McLaughlin, Development and Applications of the Laser Telemetering Dosimetry System, Proc. 14th Annual Meeting of The National Society of Black Physicists, 1991, in press. [41 W.L. McLaughlin, Chen Yun-Dong, C.G. Soares, A. Miller, G. van Dyk and D.F. Lewis, Nucl. Instr. and Meth. A302 (1991) 165. [51 International Commission on Radiation Units and Measurements, Report 37, Stopping Powers for Electrons and Positrons (1984).
A remote, real-time dosimetry system has been developed for this measurement of ionizing radiation
XIII. DETECTORS/SPECTROMETERS