- Email: [email protected]
Fiber optics dosimetry
Nuclear Instruments and Methods 175 (1980) 109-111 © North-Holland Publishing Company
FIBER OPTICS DOSIMETRY Stanley KRONENBERG US Army Electronics Technology and Devices Laboratory, Fort Momnouth, N J, USA
Carl R. SIEBENTRITT Federal Emergency Management Agency, Washington, DC, USA
Nuclear radiation induced darkening of optical fibers has been employed to extend the range of glass dosimetry to that of personal dosimetry. Dosimeters constructed using this principle have proved sufficiently rugged and reliable for civil defense applications.
We constructed working models o f such fiberoptics dosimeters to test their operational parameters using Coming 5010 fiber (lead silicate with borosilicare cladding and a numerical aperture of 0.67). This type o f fiber was chosen because its radiationinduced darkening was more permanent than that of o t h e r commercially available fibers which were tried.
Radiological instruments for emergency operations directed at the survival of citizens in the event of a nuclear disaster or war are designed with certain criteria which must be satisfied. The first is reliability. They should be capable of storage for many years without maintance or recalibration and be instantly available for use in case o f need. Their operational range should extend from 20 rad (tissue) to the lethal dose of about 1000 rad (tissue). They should be selfcontained, requiring neither separate readout devices nor power sources, and they should be low-cost instruments capable o f being mass-produced. Radiation4nduced darkening of glasses has been used as the basis of various glass dosimeters for measurements of ttigh gamma doses (in the range of 10a-107 rads) [1]. We have utilized this effect for personal dosimetry, and constructed devices which satisfy all the requirements for emergency operations. This was accomplished by the use o f optical fibers to increase the thickness o f the glass b y an amount sufficient to provide the requisite sensitivity. Fig. 1 illustrates both the basic principle of such a dosimeter and a finished working prototype. Two pieces of optical fiber, one short and the other much longer, are arranged so that the same light intensity is incident on one end of each. Exposure to gamma rays darkens both pieces, the longer one much more severely. After irradiation, the light intensities emerging from the opposite ends are compared visually and equalized b y attenuating the light incident on the short piece by movement o f a transmission gray scale. The dose can then be read out be means o f a calibrated dose scale attached to the gray scale.
3
i 1
7
Fig. 1. Fiber-optics dosimeter. 1 - Short fiber bundle. 2 - Long fiber bundle. 3 - Light input. 4 - Window (acrylic plastic). 5 - Gray scale (photographic film). 6 - Readout lens. 7 - Rotating top cover (acrylic plastic). 109
VII. EXOELECTRON EMISSION
110
S. Kronenberg, C.R. Siebentritt / Fiber optics dosimetry SCALE 80
40
0
ANGLE
OF THE GRAY SCALE 160
(DEGREES) 200
120
240
280
! 000
900
800
700
"7 0 < n,
3 /
600
500
S
bJ D U~
400
b Ill m 0 (:3 a IXl n" m
300
200
>_
._1 m 121
1 O0
o
g
g
g READOUT
g
g
SCALE
g (TISSUE
o
g
g
g
g g g g g g
RADS)
Fig. 2. Calibration of the fiber-optics dosimeter. Dose of 60Co gamma rays (delivered in 50 rad increments) versus scale angle of the gray scale. The readout scale at bottom was constructed by projecting the dose at left from the best fit curve.
It also exhibited a strong response to radiation, permitting use o f a relatively short length of 160 cm. 1000 rad (tissue) of 6°Co gamma rays changed its transparency (in the blue region) b y a factor o f 400, corresponding to a change in density o f 2.6. This was found to be within a convenient range o f operation. The instrument is contained in a flat cylindrical box 10 cm in diameter and 1.5 cm high (see fig. 1). Both the gray scale and the readout scale are mounted on one edge of the cover, which can be rotated relative to a readout marker on the b o d y of the instrument. A b o u t 400 individual 0.04 mm diameter fibers are bundled in plastic tubing o f approximately 2 mm outside diameter. Since the diameter of the fiber is smaller than the average range of electrons produced by the gamma rays, the electron equilibrium in the fiber is predominantly defined b y the plastic body o f the instrument. Its response is very close to tissue, so that the dosimeter's energy response is approximately tissue equivalent.
The instruments were calibrated b y exposure to gamma rays. Incremental dose was plotted against the average o f two independent visual readings o f the gray scale. The readings were surprisingly reproducible (see fig. 2). The scale obtained in this manner was inscribed on the periphery of the rotatable covers o f the remaining instruments. The darkening of the fibers faded b y 10% within the first several hours at room temperature but then slowed; successive readings after delays o f four months changed by an additional 10%. Exposure to high ambient temperatures accelerated the fading significantly and temperatures o f approximately 60°C reduced the reading b y about 50% within a day. Other types o f glass such as borosilicates show a higher persistence even at elevated temperatures, as do certain plastic fibers. These change color permanently when exposed to radiation, and are routinely used in dosimetry involving large doses such as in food sterilization [2]. 6°Co
S. Kronenberg, C.R. Siebentritt / Fiber optics dosimetry References [1] E.H. Attix and W.C. Roesch, Radiation Dosimetry (Academic Press, New York, 1966) p. 248.
111
[2] M. Rosenstein, H. Levine and W. McLaughlin, in: On Microdosimetry, eds. J. Booz, H. Ebert, R. Eichel and A. Waker, (Comm. Eur. Communities, Directorate Gen. Scientific and Tech. Info. and Info. Mgmt, 1974) p. 935.
VII. EXOELECTRON EMISSION
FIBER OPTICS DOSIMETRY Stanley KRONENBERG US Army Electronics Technology and Devices Laboratory, Fort Momnouth, N J, USA
Carl R. SIEBENTRITT Federal Emergency Management Agency, Washington, DC, USA
Nuclear radiation induced darkening of optical fibers has been employed to extend the range of glass dosimetry to that of personal dosimetry. Dosimeters constructed using this principle have proved sufficiently rugged and reliable for civil defense applications.
We constructed working models o f such fiberoptics dosimeters to test their operational parameters using Coming 5010 fiber (lead silicate with borosilicare cladding and a numerical aperture of 0.67). This type o f fiber was chosen because its radiationinduced darkening was more permanent than that of o t h e r commercially available fibers which were tried.
Radiological instruments for emergency operations directed at the survival of citizens in the event of a nuclear disaster or war are designed with certain criteria which must be satisfied. The first is reliability. They should be capable of storage for many years without maintance or recalibration and be instantly available for use in case o f need. Their operational range should extend from 20 rad (tissue) to the lethal dose of about 1000 rad (tissue). They should be selfcontained, requiring neither separate readout devices nor power sources, and they should be low-cost instruments capable o f being mass-produced. Radiation4nduced darkening of glasses has been used as the basis of various glass dosimeters for measurements of ttigh gamma doses (in the range of 10a-107 rads) [1]. We have utilized this effect for personal dosimetry, and constructed devices which satisfy all the requirements for emergency operations. This was accomplished by the use o f optical fibers to increase the thickness o f the glass b y an amount sufficient to provide the requisite sensitivity. Fig. 1 illustrates both the basic principle of such a dosimeter and a finished working prototype. Two pieces of optical fiber, one short and the other much longer, are arranged so that the same light intensity is incident on one end of each. Exposure to gamma rays darkens both pieces, the longer one much more severely. After irradiation, the light intensities emerging from the opposite ends are compared visually and equalized b y attenuating the light incident on the short piece by movement o f a transmission gray scale. The dose can then be read out be means o f a calibrated dose scale attached to the gray scale.
3
i 1
7
Fig. 1. Fiber-optics dosimeter. 1 - Short fiber bundle. 2 - Long fiber bundle. 3 - Light input. 4 - Window (acrylic plastic). 5 - Gray scale (photographic film). 6 - Readout lens. 7 - Rotating top cover (acrylic plastic). 109
VII. EXOELECTRON EMISSION
110
S. Kronenberg, C.R. Siebentritt / Fiber optics dosimetry SCALE 80
40
0
ANGLE
OF THE GRAY SCALE 160
(DEGREES) 200
120
240
280
! 000
900
800
700
"7 0 < n,
3 /
600
500
S
bJ D U~
400
b Ill m 0 (:3 a IXl n" m
300
200
>_
._1 m 121
1 O0
o
g
g
g READOUT
g
g
SCALE
g (TISSUE
o
g
g
g
g g g g g g
RADS)
Fig. 2. Calibration of the fiber-optics dosimeter. Dose of 60Co gamma rays (delivered in 50 rad increments) versus scale angle of the gray scale. The readout scale at bottom was constructed by projecting the dose at left from the best fit curve.
It also exhibited a strong response to radiation, permitting use o f a relatively short length of 160 cm. 1000 rad (tissue) of 6°Co gamma rays changed its transparency (in the blue region) b y a factor o f 400, corresponding to a change in density o f 2.6. This was found to be within a convenient range o f operation. The instrument is contained in a flat cylindrical box 10 cm in diameter and 1.5 cm high (see fig. 1). Both the gray scale and the readout scale are mounted on one edge of the cover, which can be rotated relative to a readout marker on the b o d y of the instrument. A b o u t 400 individual 0.04 mm diameter fibers are bundled in plastic tubing o f approximately 2 mm outside diameter. Since the diameter of the fiber is smaller than the average range of electrons produced by the gamma rays, the electron equilibrium in the fiber is predominantly defined b y the plastic body o f the instrument. Its response is very close to tissue, so that the dosimeter's energy response is approximately tissue equivalent.
The instruments were calibrated b y exposure to gamma rays. Incremental dose was plotted against the average o f two independent visual readings o f the gray scale. The readings were surprisingly reproducible (see fig. 2). The scale obtained in this manner was inscribed on the periphery of the rotatable covers o f the remaining instruments. The darkening of the fibers faded b y 10% within the first several hours at room temperature but then slowed; successive readings after delays o f four months changed by an additional 10%. Exposure to high ambient temperatures accelerated the fading significantly and temperatures o f approximately 60°C reduced the reading b y about 50% within a day. Other types o f glass such as borosilicates show a higher persistence even at elevated temperatures, as do certain plastic fibers. These change color permanently when exposed to radiation, and are routinely used in dosimetry involving large doses such as in food sterilization [2]. 6°Co
S. Kronenberg, C.R. Siebentritt / Fiber optics dosimetry References [1] E.H. Attix and W.C. Roesch, Radiation Dosimetry (Academic Press, New York, 1966) p. 248.
111
[2] M. Rosenstein, H. Levine and W. McLaughlin, in: On Microdosimetry, eds. J. Booz, H. Ebert, R. Eichel and A. Waker, (Comm. Eur. Communities, Directorate Gen. Scientific and Tech. Info. and Info. Mgmt, 1974) p. 935.
VII. EXOELECTRON EMISSION