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The effects of high gamma doses on the response of plastic track detectors
N U C L E A R I N S T R U M E N T S AND METHODS
THE
EFFECTS
OF HIGH
I27
([975)
GAMMA
PLASTIC
TRACK
I05-I08;
DOSES
(~) N O R T H - H O L L A N D
ON THE
RESPONSE
P U B L I S H I N G CO.
OF
DETECTORS
HAMEED A. KHAN, M. ASHRAF ATTA, SHANKAT YAMEEN, M. RAFIQ HAROON and ATHER HUSAIN
Reactor School, Nuclear Engineering Division, Pakistan Institute of Nuclear Science and Technology ( P1NSTECIt), Nilore, Rawalpindi, Pakistan Received 8 January 1975 and in revised form 21 April 1975 Systematic studies have been made concerning the effects of high doses of gamma rays on the track-registration properties of various plastic Solid State Track Detectors (SSTDs). The detectors were first exposed to 25zCf fission fragments and then to gamma doses ranging between 30 and 160 megarad. The results show that both the length and the width of latent damage trails are affected when exposed to gamma doses of the order of a few megarads. Furthermore, it was found that the general
etching velocity, Vg, increases with increasing gamma dose. The above mentioned results suggest that: (a) gamma doses below the megarad region do not affect the track-registration characteristics of plastic SSTDs, and (b) the change in track parameters (in the megarad region) can be employed as an index of gamma dose. This new type of gamma dosimeter, due mostly to its extremely small size and flexibility, offers greater advantages over other detectors for in-core gamma dosimetry.
CA 80-15 A
/
Etching Time =15 min
I-
0
A=25l Cf + Gammas B = G a m zszmCf a s ~ . t -
20
J
40
60
80
100
GAMMA DOSE (Mr)
Fig. 1. Variation of WD/WO, the ratio of average values of fission-fragment etched track widths in CA80-15 cellulose-nitrate track detectors at gamma doses o f " D-megarads" and "zero-megarads ", as a function of dose, D. The "group A " of detectors was exposed first to z52cf fission fragments and then to various gamma doses. The detectors of "group B" were exposed in the reverse order. The etching was carried out for 15 min in 30% solution of NaOH kept at (50+0.5)°C. 105
106
H.A. KHAN
I. Introduction
252Cf fission fragments and then to gamma rays, than
Solid State Track Detectors (SSTDs) have found an increasing use during the last fifteen years or sol-S). The detectors have an edge over the conventional type of detectors for the detection of charged particles, like fission fragments, in the presence of a high background of alpha, beta, and gamma radiation. In the past, it was tacitly assumed that even very high doses of these background radiations are ineffective in changing the track-registration properties of SSTDs. The validity of this assumption has been questioned and some work has already been reported that indicates the possible effects of high doses of alpha particles6). This paper describes the experiments carried out (a) to study the effects of high doses of gamma rays in changing the track-registration properties of some plastic SSTDs, and (b) to find the "safe limits" of gamma background below which the basic properties of these detectors remain unchanged so that they can be used without any gamma interference.
the ones exposed in the reverse order (set B). One might conclude from the figure that the gamma doses in the megarad region cause significant enhancement in the damaged region. The enhancement increases with increasing gamma dose. Thus, it appears that gamma doses in the megarad region effectively 5
=
I
5min
CA80-15
4
3i -..
21
~
0
~
20
40
60
80
100
10mir
8
E
CA80- 15
6
2. Experimental procedure
Four plastic track detectors (CA80-15", LR-115*, Makrofol N ÷ and Makrofol E ÷) were used in the present studies. Three sets (A, B, and C) were prepared from these detectors. The "Set A " was first exposed, in 2re geometry, to fission fragments from 252Cf, and then to various doses of 6°Co gamma rays. The "Set B" was exposed in the reverse order, while the "Set C " was not exposed to gamma rays at all, and was kept as a reference. After the irradiations all the detectors were etched simultaneously in a 30% solution of NaOH kept at (50+0.5)°C for various time intervals. The length, l, and the width, IV, of etched fissionfragment tracks were measured at every etching stage. Measurements were also made of the variation of the general etching velocity, Vg, as a function of the gamma dose.
=
15 rain d . 20min
O0 16
2D
40
60
E0
100
80
120
160
200
LR - 115
E
0 0
o
4]
2.6 2.2
Makrofol N
,
,300myf
1.8
"P"
3. Results and discussion
Fig. 1 shows the variation of Wo/Woas a function of dose, D. Here W stands for the average etched width of fission-fragment tracks in CA80-15, cellulose-nitrate track detector and the subscripts indicate the doses in megarads. It is interesting to note that there is a definite increase in the ratio with increasing gamma dose. Also, the increase is always more in the case of the "set A " detectors, which were exposed first to * Manufactured by K o d a k Path6, France. + Manufactured by Bayer Chemicals, Germany.
120rain ~:
1.0 0.6
0
40
80
120
160
200
GAMMA DOSE { Mr)
Fig. 2. A set o f curves showing the variation o f WD/Wo and Wmax,o/Wmax, 0 for CAB0-15, LR-115, and Makrofol N plastic track detectors, as a function of g a m m a dose. Both the ratios increase with the increasing g a m m a dose. It is interesting to note that cellulose nitrates (CA80-15 and LR-115) show a decrease, while the polycarbonate (Makrofol N) shows an increase in the ratio with the prolongation of the etching process.
107
T H E R E S P O N S E OF P L A S T I C T R A C K D E T E C T O R S 5C
30 > a
2O
JT
~ K
~ / " ~ ~ " " o
40
80
MakrofoI-N 120
160
GAMMA DOSE(Mr)
Fig. 3. The variation of Vg,o/Vg,0, the ratio of the general etching velocities of a number of plastic detectors at gamma doses of "Dmegarads" and "zero-megarads", as a function of dose, D. After the exposure all the detectors were simultaneously etched in 30% solution of NaOH kept at (50±0.5)°C.
increase the "volume" and the "density" of the damaged region. Also, the results of set B (fig. 1) indicate that even the general nature of the plastic is changed by such high gamma doses. Fig. 2 gives a set of curves showing the variations of Wo/Wo and W.... o/Wmax, o as a function of D, the gamma dose. Here Wmax,o stands for the maximum etched track width obtained after exposing the detectors to dose, "D-megarads". All the curves show an increase in the ratios with increasing gamma doses and etching times. In most of the work, Wmax, D w a s chosen as the main parameter instead of Wo, because the former is much easier to compute than the latter. The maximum increase in W. . . . o/W .... 0 is obtained for Makrofol N. The effect of high gamma doses on the general etching velocity, Vg, was also looked intoT'8). For this purpose, detectors exposed to gamma rays only were used. Fig. 3 shows the variation of Vg,o/Vg, o as a function of gamma dose, D. CA80-15 (a cellulose nitrate) and Makrofol N (a polycarbonate) were found to be the most and the least sensitive, respectively, to
gamma rays. The effectiveness of Makrofol E and LR-115 lies in between these two. Fig. 3 suggests the use of CA80-15, cellulose nitrate as a gamma dosimeter. The method seems to be quite simple. First, a calibration curve of Vg vs dose is obtained. The determination of the unknown gamma dose is then achieved by finding out Vg after gamma irradiation and reading off the dose against this Vg value from the calibration curve.
4. Conclusions The following conclusions can be drawn from the above results: 1) High gamma doses in the region of a few megarads (and above) bring substantial changes not only in the general structure of the plastic track detector, but also in the damaged region of the latent damage trail. Thus, effectively, it increases the "volume" of the damaged region. 2) The degree to which both the damaged and the undamaged regions are modified increases with increasing gamma dose.
108
H . A . KHAN
3) G a m m a doses below " m e g a r a d r e g i o n " do n o t alter the structure to any significant extent. This shows that the detectors can be safely used for the n o r m a l detection purposes, p r o v i d e d the level o f the g a m m a b a c k g r o u n d is below the m e g a r a d region. 4) The systematic increase in the etched t r a c k width, W, a n d the general etching velocity, Vg, suggests the use o f these detectors for the m e a s u r e m e n t o f large g a m m a doses. D u e to their small size, the detectors can be a d v a n t a g e o u s l y used in o d d locations like a reactor core. The a u t h o r s are grateful to Drs I n a m - u r - R e h m a n and Ishfaq A h m a d for their m o r a l s u p p o r t a n d the experimental facilities. T h a n k s are also due to other m e m b e r s o f the track-analysis g r o u p for extremely interesting
discussions, a n d to D r K. A. S h o a i b for his constructive c o m m e n t s on the manuscript.
References
1) R. L. Fleischer, P. B. Price and R. M. Walker, Sci. Am. 220 (1969) 30. 2) H. Berger, Ann. Rev. Nucl. Sci. 21 (1971) 335. 3) R.L. Fleischer, P. B. Price and R. M. Walker, Science 178 (1972) 255. 4) M. De Coster and D. Langela, Nucl. Appl. Technol. 9 (1970) 229. 5) H. A. Khan, Ph.D. Thesis, Physics Department, University of Birmingham, England (1972). 6) H.A. Khan and S.A. Durrani, Radiation Effects 17 (1973) 133. 7) A. L. Frank and E.V. Benton, University of San Francisco Report no. DASA 2573 (Nov. 30, 1970). 8) M. Nicolae, Rev. Roum. Phys. 15 (1970) 881.
THE
EFFECTS
OF HIGH
I27
([975)
GAMMA
PLASTIC
TRACK
I05-I08;
DOSES
(~) N O R T H - H O L L A N D
ON THE
RESPONSE
P U B L I S H I N G CO.
OF
DETECTORS
HAMEED A. KHAN, M. ASHRAF ATTA, SHANKAT YAMEEN, M. RAFIQ HAROON and ATHER HUSAIN
Reactor School, Nuclear Engineering Division, Pakistan Institute of Nuclear Science and Technology ( P1NSTECIt), Nilore, Rawalpindi, Pakistan Received 8 January 1975 and in revised form 21 April 1975 Systematic studies have been made concerning the effects of high doses of gamma rays on the track-registration properties of various plastic Solid State Track Detectors (SSTDs). The detectors were first exposed to 25zCf fission fragments and then to gamma doses ranging between 30 and 160 megarad. The results show that both the length and the width of latent damage trails are affected when exposed to gamma doses of the order of a few megarads. Furthermore, it was found that the general
etching velocity, Vg, increases with increasing gamma dose. The above mentioned results suggest that: (a) gamma doses below the megarad region do not affect the track-registration characteristics of plastic SSTDs, and (b) the change in track parameters (in the megarad region) can be employed as an index of gamma dose. This new type of gamma dosimeter, due mostly to its extremely small size and flexibility, offers greater advantages over other detectors for in-core gamma dosimetry.
CA 80-15 A
/
Etching Time =15 min
I-
0
A=25l Cf + Gammas B = G a m zszmCf a s ~ . t -
20
J
40
60
80
100
GAMMA DOSE (Mr)
Fig. 1. Variation of WD/WO, the ratio of average values of fission-fragment etched track widths in CA80-15 cellulose-nitrate track detectors at gamma doses o f " D-megarads" and "zero-megarads ", as a function of dose, D. The "group A " of detectors was exposed first to z52cf fission fragments and then to various gamma doses. The detectors of "group B" were exposed in the reverse order. The etching was carried out for 15 min in 30% solution of NaOH kept at (50+0.5)°C. 105
106
H.A. KHAN
I. Introduction
252Cf fission fragments and then to gamma rays, than
Solid State Track Detectors (SSTDs) have found an increasing use during the last fifteen years or sol-S). The detectors have an edge over the conventional type of detectors for the detection of charged particles, like fission fragments, in the presence of a high background of alpha, beta, and gamma radiation. In the past, it was tacitly assumed that even very high doses of these background radiations are ineffective in changing the track-registration properties of SSTDs. The validity of this assumption has been questioned and some work has already been reported that indicates the possible effects of high doses of alpha particles6). This paper describes the experiments carried out (a) to study the effects of high doses of gamma rays in changing the track-registration properties of some plastic SSTDs, and (b) to find the "safe limits" of gamma background below which the basic properties of these detectors remain unchanged so that they can be used without any gamma interference.
the ones exposed in the reverse order (set B). One might conclude from the figure that the gamma doses in the megarad region cause significant enhancement in the damaged region. The enhancement increases with increasing gamma dose. Thus, it appears that gamma doses in the megarad region effectively 5
=
I
5min
CA80-15
4
3i -..
21
~
0
~
20
40
60
80
100
10mir
8
E
CA80- 15
6
2. Experimental procedure
Four plastic track detectors (CA80-15", LR-115*, Makrofol N ÷ and Makrofol E ÷) were used in the present studies. Three sets (A, B, and C) were prepared from these detectors. The "Set A " was first exposed, in 2re geometry, to fission fragments from 252Cf, and then to various doses of 6°Co gamma rays. The "Set B" was exposed in the reverse order, while the "Set C " was not exposed to gamma rays at all, and was kept as a reference. After the irradiations all the detectors were etched simultaneously in a 30% solution of NaOH kept at (50+0.5)°C for various time intervals. The length, l, and the width, IV, of etched fissionfragment tracks were measured at every etching stage. Measurements were also made of the variation of the general etching velocity, Vg, as a function of the gamma dose.
=
15 rain d . 20min
O0 16
2D
40
60
E0
100
80
120
160
200
LR - 115
E
0 0
o
4]
2.6 2.2
Makrofol N
,
,300myf
1.8
"P"
3. Results and discussion
Fig. 1 shows the variation of Wo/Woas a function of dose, D. Here W stands for the average etched width of fission-fragment tracks in CA80-15, cellulose-nitrate track detector and the subscripts indicate the doses in megarads. It is interesting to note that there is a definite increase in the ratio with increasing gamma dose. Also, the increase is always more in the case of the "set A " detectors, which were exposed first to * Manufactured by K o d a k Path6, France. + Manufactured by Bayer Chemicals, Germany.
120rain ~:
1.0 0.6
0
40
80
120
160
200
GAMMA DOSE { Mr)
Fig. 2. A set o f curves showing the variation o f WD/Wo and Wmax,o/Wmax, 0 for CAB0-15, LR-115, and Makrofol N plastic track detectors, as a function of g a m m a dose. Both the ratios increase with the increasing g a m m a dose. It is interesting to note that cellulose nitrates (CA80-15 and LR-115) show a decrease, while the polycarbonate (Makrofol N) shows an increase in the ratio with the prolongation of the etching process.
107
T H E R E S P O N S E OF P L A S T I C T R A C K D E T E C T O R S 5C
30 > a
2O
JT
~ K
~ / " ~ ~ " " o
40
80
MakrofoI-N 120
160
GAMMA DOSE(Mr)
Fig. 3. The variation of Vg,o/Vg,0, the ratio of the general etching velocities of a number of plastic detectors at gamma doses of "Dmegarads" and "zero-megarads", as a function of dose, D. After the exposure all the detectors were simultaneously etched in 30% solution of NaOH kept at (50±0.5)°C.
increase the "volume" and the "density" of the damaged region. Also, the results of set B (fig. 1) indicate that even the general nature of the plastic is changed by such high gamma doses. Fig. 2 gives a set of curves showing the variations of Wo/Wo and W.... o/Wmax, o as a function of D, the gamma dose. Here Wmax,o stands for the maximum etched track width obtained after exposing the detectors to dose, "D-megarads". All the curves show an increase in the ratios with increasing gamma doses and etching times. In most of the work, Wmax, D w a s chosen as the main parameter instead of Wo, because the former is much easier to compute than the latter. The maximum increase in W. . . . o/W .... 0 is obtained for Makrofol N. The effect of high gamma doses on the general etching velocity, Vg, was also looked intoT'8). For this purpose, detectors exposed to gamma rays only were used. Fig. 3 shows the variation of Vg,o/Vg, o as a function of gamma dose, D. CA80-15 (a cellulose nitrate) and Makrofol N (a polycarbonate) were found to be the most and the least sensitive, respectively, to
gamma rays. The effectiveness of Makrofol E and LR-115 lies in between these two. Fig. 3 suggests the use of CA80-15, cellulose nitrate as a gamma dosimeter. The method seems to be quite simple. First, a calibration curve of Vg vs dose is obtained. The determination of the unknown gamma dose is then achieved by finding out Vg after gamma irradiation and reading off the dose against this Vg value from the calibration curve.
4. Conclusions The following conclusions can be drawn from the above results: 1) High gamma doses in the region of a few megarads (and above) bring substantial changes not only in the general structure of the plastic track detector, but also in the damaged region of the latent damage trail. Thus, effectively, it increases the "volume" of the damaged region. 2) The degree to which both the damaged and the undamaged regions are modified increases with increasing gamma dose.
108
H . A . KHAN
3) G a m m a doses below " m e g a r a d r e g i o n " do n o t alter the structure to any significant extent. This shows that the detectors can be safely used for the n o r m a l detection purposes, p r o v i d e d the level o f the g a m m a b a c k g r o u n d is below the m e g a r a d region. 4) The systematic increase in the etched t r a c k width, W, a n d the general etching velocity, Vg, suggests the use o f these detectors for the m e a s u r e m e n t o f large g a m m a doses. D u e to their small size, the detectors can be a d v a n t a g e o u s l y used in o d d locations like a reactor core. The a u t h o r s are grateful to Drs I n a m - u r - R e h m a n and Ishfaq A h m a d for their m o r a l s u p p o r t a n d the experimental facilities. T h a n k s are also due to other m e m b e r s o f the track-analysis g r o u p for extremely interesting
discussions, a n d to D r K. A. S h o a i b for his constructive c o m m e n t s on the manuscript.
References
1) R. L. Fleischer, P. B. Price and R. M. Walker, Sci. Am. 220 (1969) 30. 2) H. Berger, Ann. Rev. Nucl. Sci. 21 (1971) 335. 3) R.L. Fleischer, P. B. Price and R. M. Walker, Science 178 (1972) 255. 4) M. De Coster and D. Langela, Nucl. Appl. Technol. 9 (1970) 229. 5) H. A. Khan, Ph.D. Thesis, Physics Department, University of Birmingham, England (1972). 6) H.A. Khan and S.A. Durrani, Radiation Effects 17 (1973) 133. 7) A. L. Frank and E.V. Benton, University of San Francisco Report no. DASA 2573 (Nov. 30, 1970). 8) M. Nicolae, Rev. Roum. Phys. 15 (1970) 881.