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CO2 diffusion from X-ray and γ-ray irradiated CR-39 plastic
Radiation Measurements 35 (2002) 203 – 206
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CO2 di usion from X-ray and -ray irradiated CR-39 plastic M.A. Maleka; ∗ , C.S. Chongb a Health
Physics Division, Atomic Energy Center, 4, Kazi Nazrul Islam Avenue, GPO Box 164, Ramna Dhaka 1000, Bangladesh b School of Physics, Universiti Sains Malaysia, 11800 USM Penang, Malaysia Received 30 April 2001; received in revised form 3 August 2001; accepted 18 October 2001
Abstract The CO2 gas produced inside CR-39 plastic track detectors irradiated by both X-rays and -rays is found to di use out with time. The di usion half-time of the gas was measured for both the irradiation cases. For X-rays, the irradiation doses ranged from 220 to 500 kGy and the half-time was found to decrease with increase in irradiation dose from 7.7 to 6.0 days. For -rays, the irradiation doses ranged from 130 to 950 kGy and the half-time of the gas di usion was also found to decrease with increase in irradiation dose from 9.0 to 4.0 days. The results can be attributed to the degradation action of the energetic photons on the plastic with increase in dose, the CO2 di used out more readily at a higher rate through the more degraded plastic. No signi5cant di erence in the di usion half-time as a function of dose between X-ray and -ray irradiation was found. c 2002 Published by Elsevier Science Ltd. Keywords: CR-39; X-ray; ray; CO2 di usion; Half-time
1. Introduction Irradiation of polymeric substances by high-energy ionizing radiation such as X-rays or -rays causes many changes in the polymer. It may degrade polymeric materials by random fracture of the main chain with the number of fractures being proportional to the radiation dose (Charlseby, 1955). The bond breaking may give rise to free radicals (Hawkins, 1984), ionic species, water molecule, gaseous products, etc. (Reichmanis et al., 1993). It has been reported that CO2 molecules are produced in irradiated CR-39 plastic (polyallyl diglycol carbonate) by X-ray and the production of CO2 was attributed to bond breaking in the CR-39 plastic by the radiation (Chong et al., 1997). If this CO2 is trapped inside, this can lead to subsequent crazing and cracking of the plastic due to accumulated local stresses (Reichmanis et al., 1993). The presence of the CO2 molecule inside the plastic can be detected using FTIR spectroscopy and the area under
∗
Corresponding author. Fax: +880-2-8613051. E-mail addresses: [email protected] (M.A. Malek), [email protected] (C.S. Chong).
the IR absorption band is a direct measure of the amount of gas. The measurement of the gas present inside the plastic at schedule intervals of time therefore may yield the necessary information on the di usion rate. -rays from 60 Co is more energetic than 50 kVp X-rays used in this study and comparison of the results obtained from irradiation by both X-ray and -ray would be of interest. In this paper, the details of the procedures for determining the half-time of the gas diffusion from the irradiated CR-39 plastic by both X-rays and -rays are presented.
2. Experimental CR-39 strips of dimension 1:5 cm ×2:5 cm were prepared from CR-39 sheet (Pershore UK, production year: 1984), 500 m thick. The strips were then exposed to X-rays at di erent doses (Table 1) using an XRF machine (Shimadzu VF 310) with a Rh target operated at 50 kVp and 40 mAs. With a particular dose, only one strip was exposed. The dose rate of the XRF machine was measured using LiF TLD chips (Harshaw–Bicron TLD-700) which were exposed separately.
c 2002 Published by Elsevier Science Ltd. 1350-4487/02/$ - see front matter PII: S 1 3 5 0 - 4 4 8 7 ( 0 1 ) 0 0 2 8 9 - X
204
M.A. Malek, C.S. Chong / Radiation Measurements 35 (2002) 203 – 206
Table 1 Di usion constants and corresponding half-times of CO2 gas from CR-39 plastic irradiated with X-rays Sample no.
Dose (kGy)
(day−1 ) 2350 cm−1
670 cm−1
2350 cm−1
670 cm−1
1 2 3 4
220 300 400 500
0.0890 0.0914 0.0952 0.1100
0.0910 0.0943 0.0994 0.1200
7.79 7.58 7.28 6.30
7.62 7.35 6.97 5.78
t1=2 (day)
Table 2 Di usion constants and the corresponding half-times of CO2 gas from CR-39 plastic irradiated with 60 Co -rays (for 2350 cm−1 band only) Sample no.
Dose (kGy)
(day−1 )
t1=2 (day)
1 2 3 4 5
130 217 391 651 955
0.077 0.075 0.095 0.151 0.173
9.0 9.2 7.3 4.6 4.0
The irradiated CR-39 strips were then placed in a FTIR spectrometer (Perkin–Elmer model 2000) to acquire absorption in the IR spectral range from 370 to 6000 cm−1 . The 2 and 3 bands for CO2 (670 and 2350 cm−1 ) were counted at certain intervals of time up to 23 days. The areas under the curves were measured by a computer software PE Grams Analyst, Galactic Industries corporation, USA. To ensure normal di usion of CO2 gas from the strips, the strips were kept in open air at ambient temperature. The results are shown in Table 1. For the case of -ray irradiation, the strips were irradiated by a 60 Co source Gamma Cell, model G.C. 220, Nordion International Inc., Canada. The nominal activity was calculated to be 58:61 TBq on January 14, 1999 and the corresponding dose rate was found to be 1:24 kGy=h on the irradiation platform located at the center of the chamber. The plastic strips were placed on the irradiation platform for pre5xed periods. One strip was irradiated with a particular dose, the irradiation doses are given in Table 2. The IR absorption bands of CO2 at 2350 cm−1 were measured by the same procedure described above. The peak at 670 cm−1 band was found to be not appropriately well resolved and was thus not used in the calculation. The period of observation was 15 days and the results are shown in Table 2. The details of experimental set up, data and calculation were published elsewhere (Malek, 2000). 3. Results and discussion The absorption band areas for the CO2 found in the X-ray irradiated plastic at the di erent elapsed times are shown
Average t1=2 (day) ± 1 7:70 ± 0:12 7:46 ± 0:16 7:13 ± 0:23 6:04 ± 0:37
in Fig. 1 for the 2350 cm−1 band. The data for CO2 diffusion from each plastic sample 5tted well with the equation Y = Y0 e−t where is the e ective di usion constant and t is the elapsed time in days. The time required to diffuse out half of the initial amount of CO2 may be designated as the ‘half-time’ and was calculated from the value of and recalling that t1=2 = 0:693=. The values of and t1=2 calculated from the absorption band acquired form each irradiated plastic strip are shown in the Table 1. For the 2350 cm−1 band, the values of are found to range from 0.089 to 0:11 day−1 . The corresponding values for t1=2 range from 7.79 to 6.30 days. On the other hand, using the data obtained from 670 cm−1 band, the values of are found to range from 0.091 to 0:12 day−1 with the corresponding values of t1=2 ranging from 7.62 to 5.78 day, which is in good agreement with results of the 2350 cm−1 absorption band. Neglecting the slight di erence (±5%) between the values at 2350 and at 670 cm−1 for each irradiation dose, the di usion half-time is found to decrease with increasing dose from an average value of 7.7 – 6.0 days. Immediately after the irradiation, the top side of the large size peak at 2350 cm−1 in some strip was found to be broken or absent, a problem sometimes to be faced in IR spectroscopy. This is due to the instrumental error of the IR spectrometer and therefore, the missing part of the peak was needed to be reconstructed. In the process, some error might be associated with the area under curve and the slight di erence between the slopes of samples 3 and 4 in Fig. 1 may be attributed to the error. However, as the size of the peak reduces with the elapsed time, the broken peak gradually become well de5ned. The di erence between slopes of samples 1 and 3 in Fig. 2 is attributed to the same kind of error. In Fig. 2, the area under the 2350 cm−1 band for CO2 remaining inside the -ray irradiated plastic is plotted as a function of elapsed time for di erent gamma doses. The result is again a straight line with a negative slope. Table 2 presents the di usion constants and the corresponding half-time for the samples irradiated by di erent doses. It is observed that the di usion half-time decreases with increase in dose. The results can be attributed to the further degradation action of the energetic photons on the plastic with increase in dose. The plastic became more porous as microscopic
M.A. Malek, C.S. Chong / Radiation Measurements 35 (2002) 203 – 206
205
Sample 4 Sample 3
100
Absorbtion band area
Sample 2 Sample 1
50
10
5 0
5
10
15
20
25
Elapsed time (day)
Fig. 1. Di usion of CO2 gas from the CR-39 plastic strips after irradiation with X-rays. The data were obtained from the 2350 cm−1 absorption band.
Sample 5 Sample 1
100
Sample 4 Sample 3
50
Absorption band area
Sample 2
10
5 0
2
4
6 8 10 Elapsed time (day)
12
14
16
Fig. 2. Di usion of CO2 gas from the CR-39 plastic strips after irradiation with -rays. The data were obtained from the 2350 cm−1 absorption band.
5ssures and voids are created in the plastic by the radiation, the number of which increases with an increase in the radiation dose (for example, see Hawkins, 1984; Reichmanis et al., 1993). Thus, the CO2 gas di used out more readily at a
higher rate through the more degraded plastic with increasing dose. The various values of half-time of CO2 di usion under both X-ray and -ray irradiation are plotted in the same graph as a function of irradiation dose in Fig. 3. An
206
M.A. Malek, C.S. Chong / Radiation Measurements 35 (2002) 203 – 206 12 Data for gamma ray Data for X-Ray Y=11.3*EXP (-0.00125*X)
Half-time (day)
10
8
6
4
0
200
400
600
800
1000
Irradiation dose (kGy)
Fig. 3. The di usion half-time of CO2 gas in samples irradiated with di erent doses of both X-ray and -ray. The data were obtained from the 2350 cm−1 absorption band.
exponential line (Y = 11:3e−0:00125X ) 5tted the experimental data well. Within experimental error, there is no signi5cant di erence between X-ray and -ray irradiation in the e ects of radiation on CR-39 as a function of dose. Thus it may be concluded that the radiation damage to the plastic does not depends on the type of radiation (electromagnetic) but on the absorbed dose. This work has been done for the 5rst time and no data are available to compare the results with the other. Higher dose produce more CO2 in CR-39 strip having same thickness (Malek et al., 2001). Since radiation damage to the plastic depends only on the absorbed dose, therefore, it may be concluded that the same irradiation dose will produce the same amount of CO2 in the CR-39 strips having di erent thicknesses. Obviously, the higher thickness of the strip might increase the di usion half-time. The half-times will be di erent for the same amount of CO2 produced in di erent strips having di erent thicknesses irradiated with the same dose. CR-39 of 500 m thickness has been used in this study, experiments with di erent thicknesses such as 250 or 750 m irradiated with the same dose are recommended. Larger amount of CO2 trapped inside the plastic might change the di usion half-time. However, production of larger amount of CO2 inside the host plastic could only be possible with higher irradiation dose which in turn, degrade the plastic more readily. Therefore, the di usion half-time decreased noticeably (Fig. 3). The age of the polymer may not a ect the di usion constant unless the polymer is being degraded by high-dose radiation prior to the experiment; however, further study on this issue is recommended.
Acknowledgements The authors are thankful to the Department of Physics and the Department of Geology, University of Malaya, Kuala Lumpur for using their FTIR spectrometer and XRF machine respectively. Thanks are due to Y. Amin and R.H. Mahat for their assistance during this experiment in University of Malaya. Valuable comments and suggestion from the anonymous referee are hereby sincerely acknowledged. The 5nancial support of the Malaysian Government through Malaysian Technical Cooperation Program to one of the authors (Malek) is thankfully acknowledged. References Charlseby, A., 1955. Degradation of cellulose by ionizing radiation. J. Polym. Sci. 15, 263–270. Chong, C.S., Ishak, I., Mahat, R.H., Amin, Y.M., 1997. UV-VIS and FTIR spectral studies of CR-39 irradiated with X-ray. Radiat. Meas. 28 (1– 6), 119–122. Hawkins, W.L., 1984. Degradation and Stabilization of Polymers. Springer, New York. Malek, M.A., 2000. Study of the e ects of X-ray and gamma ray irradiation on CR-39 plastic track detectors. Ph.D. Thesis, Malaysia, Universiti Sains Malaysia. Malek, M.A., Chong, S.C., Abdullah, R., 2001. Mechanistic model for bond scission in a polymeric system by radiation. Radiation Phys. Chem. 60, 603–607. Reichmanis, E., Curtis, W.F., O’Donnell, J.H., Hill, David, J.T., 1993. Radiation e ects on polymeric materials. In: Reichmanis E., Curtis, W.F., O’Donnell, J.H. (Eds.), Irradiation of Polymeric Materials: Process, Mechanism and Application. American Chemical Society, Washington, USA.
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CO2 di usion from X-ray and -ray irradiated CR-39 plastic M.A. Maleka; ∗ , C.S. Chongb a Health
Physics Division, Atomic Energy Center, 4, Kazi Nazrul Islam Avenue, GPO Box 164, Ramna Dhaka 1000, Bangladesh b School of Physics, Universiti Sains Malaysia, 11800 USM Penang, Malaysia Received 30 April 2001; received in revised form 3 August 2001; accepted 18 October 2001
Abstract The CO2 gas produced inside CR-39 plastic track detectors irradiated by both X-rays and -rays is found to di use out with time. The di usion half-time of the gas was measured for both the irradiation cases. For X-rays, the irradiation doses ranged from 220 to 500 kGy and the half-time was found to decrease with increase in irradiation dose from 7.7 to 6.0 days. For -rays, the irradiation doses ranged from 130 to 950 kGy and the half-time of the gas di usion was also found to decrease with increase in irradiation dose from 9.0 to 4.0 days. The results can be attributed to the degradation action of the energetic photons on the plastic with increase in dose, the CO2 di used out more readily at a higher rate through the more degraded plastic. No signi5cant di erence in the di usion half-time as a function of dose between X-ray and -ray irradiation was found. c 2002 Published by Elsevier Science Ltd. Keywords: CR-39; X-ray; ray; CO2 di usion; Half-time
1. Introduction Irradiation of polymeric substances by high-energy ionizing radiation such as X-rays or -rays causes many changes in the polymer. It may degrade polymeric materials by random fracture of the main chain with the number of fractures being proportional to the radiation dose (Charlseby, 1955). The bond breaking may give rise to free radicals (Hawkins, 1984), ionic species, water molecule, gaseous products, etc. (Reichmanis et al., 1993). It has been reported that CO2 molecules are produced in irradiated CR-39 plastic (polyallyl diglycol carbonate) by X-ray and the production of CO2 was attributed to bond breaking in the CR-39 plastic by the radiation (Chong et al., 1997). If this CO2 is trapped inside, this can lead to subsequent crazing and cracking of the plastic due to accumulated local stresses (Reichmanis et al., 1993). The presence of the CO2 molecule inside the plastic can be detected using FTIR spectroscopy and the area under
∗
Corresponding author. Fax: +880-2-8613051. E-mail addresses: [email protected] (M.A. Malek), [email protected] (C.S. Chong).
the IR absorption band is a direct measure of the amount of gas. The measurement of the gas present inside the plastic at schedule intervals of time therefore may yield the necessary information on the di usion rate. -rays from 60 Co is more energetic than 50 kVp X-rays used in this study and comparison of the results obtained from irradiation by both X-ray and -ray would be of interest. In this paper, the details of the procedures for determining the half-time of the gas diffusion from the irradiated CR-39 plastic by both X-rays and -rays are presented.
2. Experimental CR-39 strips of dimension 1:5 cm ×2:5 cm were prepared from CR-39 sheet (Pershore UK, production year: 1984), 500 m thick. The strips were then exposed to X-rays at di erent doses (Table 1) using an XRF machine (Shimadzu VF 310) with a Rh target operated at 50 kVp and 40 mAs. With a particular dose, only one strip was exposed. The dose rate of the XRF machine was measured using LiF TLD chips (Harshaw–Bicron TLD-700) which were exposed separately.
c 2002 Published by Elsevier Science Ltd. 1350-4487/02/$ - see front matter PII: S 1 3 5 0 - 4 4 8 7 ( 0 1 ) 0 0 2 8 9 - X
204
M.A. Malek, C.S. Chong / Radiation Measurements 35 (2002) 203 – 206
Table 1 Di usion constants and corresponding half-times of CO2 gas from CR-39 plastic irradiated with X-rays Sample no.
Dose (kGy)
(day−1 ) 2350 cm−1
670 cm−1
2350 cm−1
670 cm−1
1 2 3 4
220 300 400 500
0.0890 0.0914 0.0952 0.1100
0.0910 0.0943 0.0994 0.1200
7.79 7.58 7.28 6.30
7.62 7.35 6.97 5.78
t1=2 (day)
Table 2 Di usion constants and the corresponding half-times of CO2 gas from CR-39 plastic irradiated with 60 Co -rays (for 2350 cm−1 band only) Sample no.
Dose (kGy)
(day−1 )
t1=2 (day)
1 2 3 4 5
130 217 391 651 955
0.077 0.075 0.095 0.151 0.173
9.0 9.2 7.3 4.6 4.0
The irradiated CR-39 strips were then placed in a FTIR spectrometer (Perkin–Elmer model 2000) to acquire absorption in the IR spectral range from 370 to 6000 cm−1 . The 2 and 3 bands for CO2 (670 and 2350 cm−1 ) were counted at certain intervals of time up to 23 days. The areas under the curves were measured by a computer software PE Grams Analyst, Galactic Industries corporation, USA. To ensure normal di usion of CO2 gas from the strips, the strips were kept in open air at ambient temperature. The results are shown in Table 1. For the case of -ray irradiation, the strips were irradiated by a 60 Co source Gamma Cell, model G.C. 220, Nordion International Inc., Canada. The nominal activity was calculated to be 58:61 TBq on January 14, 1999 and the corresponding dose rate was found to be 1:24 kGy=h on the irradiation platform located at the center of the chamber. The plastic strips were placed on the irradiation platform for pre5xed periods. One strip was irradiated with a particular dose, the irradiation doses are given in Table 2. The IR absorption bands of CO2 at 2350 cm−1 were measured by the same procedure described above. The peak at 670 cm−1 band was found to be not appropriately well resolved and was thus not used in the calculation. The period of observation was 15 days and the results are shown in Table 2. The details of experimental set up, data and calculation were published elsewhere (Malek, 2000). 3. Results and discussion The absorption band areas for the CO2 found in the X-ray irradiated plastic at the di erent elapsed times are shown
Average t1=2 (day) ± 1 7:70 ± 0:12 7:46 ± 0:16 7:13 ± 0:23 6:04 ± 0:37
in Fig. 1 for the 2350 cm−1 band. The data for CO2 diffusion from each plastic sample 5tted well with the equation Y = Y0 e−t where is the e ective di usion constant and t is the elapsed time in days. The time required to diffuse out half of the initial amount of CO2 may be designated as the ‘half-time’ and was calculated from the value of and recalling that t1=2 = 0:693=. The values of and t1=2 calculated from the absorption band acquired form each irradiated plastic strip are shown in the Table 1. For the 2350 cm−1 band, the values of are found to range from 0.089 to 0:11 day−1 . The corresponding values for t1=2 range from 7.79 to 6.30 days. On the other hand, using the data obtained from 670 cm−1 band, the values of are found to range from 0.091 to 0:12 day−1 with the corresponding values of t1=2 ranging from 7.62 to 5.78 day, which is in good agreement with results of the 2350 cm−1 absorption band. Neglecting the slight di erence (±5%) between the values at 2350 and at 670 cm−1 for each irradiation dose, the di usion half-time is found to decrease with increasing dose from an average value of 7.7 – 6.0 days. Immediately after the irradiation, the top side of the large size peak at 2350 cm−1 in some strip was found to be broken or absent, a problem sometimes to be faced in IR spectroscopy. This is due to the instrumental error of the IR spectrometer and therefore, the missing part of the peak was needed to be reconstructed. In the process, some error might be associated with the area under curve and the slight di erence between the slopes of samples 3 and 4 in Fig. 1 may be attributed to the error. However, as the size of the peak reduces with the elapsed time, the broken peak gradually become well de5ned. The di erence between slopes of samples 1 and 3 in Fig. 2 is attributed to the same kind of error. In Fig. 2, the area under the 2350 cm−1 band for CO2 remaining inside the -ray irradiated plastic is plotted as a function of elapsed time for di erent gamma doses. The result is again a straight line with a negative slope. Table 2 presents the di usion constants and the corresponding half-time for the samples irradiated by di erent doses. It is observed that the di usion half-time decreases with increase in dose. The results can be attributed to the further degradation action of the energetic photons on the plastic with increase in dose. The plastic became more porous as microscopic
M.A. Malek, C.S. Chong / Radiation Measurements 35 (2002) 203 – 206
205
Sample 4 Sample 3
100
Absorbtion band area
Sample 2 Sample 1
50
10
5 0
5
10
15
20
25
Elapsed time (day)
Fig. 1. Di usion of CO2 gas from the CR-39 plastic strips after irradiation with X-rays. The data were obtained from the 2350 cm−1 absorption band.
Sample 5 Sample 1
100
Sample 4 Sample 3
50
Absorption band area
Sample 2
10
5 0
2
4
6 8 10 Elapsed time (day)
12
14
16
Fig. 2. Di usion of CO2 gas from the CR-39 plastic strips after irradiation with -rays. The data were obtained from the 2350 cm−1 absorption band.
5ssures and voids are created in the plastic by the radiation, the number of which increases with an increase in the radiation dose (for example, see Hawkins, 1984; Reichmanis et al., 1993). Thus, the CO2 gas di used out more readily at a
higher rate through the more degraded plastic with increasing dose. The various values of half-time of CO2 di usion under both X-ray and -ray irradiation are plotted in the same graph as a function of irradiation dose in Fig. 3. An
206
M.A. Malek, C.S. Chong / Radiation Measurements 35 (2002) 203 – 206 12 Data for gamma ray Data for X-Ray Y=11.3*EXP (-0.00125*X)
Half-time (day)
10
8
6
4
0
200
400
600
800
1000
Irradiation dose (kGy)
Fig. 3. The di usion half-time of CO2 gas in samples irradiated with di erent doses of both X-ray and -ray. The data were obtained from the 2350 cm−1 absorption band.
exponential line (Y = 11:3e−0:00125X ) 5tted the experimental data well. Within experimental error, there is no signi5cant di erence between X-ray and -ray irradiation in the e ects of radiation on CR-39 as a function of dose. Thus it may be concluded that the radiation damage to the plastic does not depends on the type of radiation (electromagnetic) but on the absorbed dose. This work has been done for the 5rst time and no data are available to compare the results with the other. Higher dose produce more CO2 in CR-39 strip having same thickness (Malek et al., 2001). Since radiation damage to the plastic depends only on the absorbed dose, therefore, it may be concluded that the same irradiation dose will produce the same amount of CO2 in the CR-39 strips having di erent thicknesses. Obviously, the higher thickness of the strip might increase the di usion half-time. The half-times will be di erent for the same amount of CO2 produced in di erent strips having di erent thicknesses irradiated with the same dose. CR-39 of 500 m thickness has been used in this study, experiments with di erent thicknesses such as 250 or 750 m irradiated with the same dose are recommended. Larger amount of CO2 trapped inside the plastic might change the di usion half-time. However, production of larger amount of CO2 inside the host plastic could only be possible with higher irradiation dose which in turn, degrade the plastic more readily. Therefore, the di usion half-time decreased noticeably (Fig. 3). The age of the polymer may not a ect the di usion constant unless the polymer is being degraded by high-dose radiation prior to the experiment; however, further study on this issue is recommended.
Acknowledgements The authors are thankful to the Department of Physics and the Department of Geology, University of Malaya, Kuala Lumpur for using their FTIR spectrometer and XRF machine respectively. Thanks are due to Y. Amin and R.H. Mahat for their assistance during this experiment in University of Malaya. Valuable comments and suggestion from the anonymous referee are hereby sincerely acknowledged. The 5nancial support of the Malaysian Government through Malaysian Technical Cooperation Program to one of the authors (Malek) is thankfully acknowledged. References Charlseby, A., 1955. Degradation of cellulose by ionizing radiation. J. Polym. Sci. 15, 263–270. Chong, C.S., Ishak, I., Mahat, R.H., Amin, Y.M., 1997. UV-VIS and FTIR spectral studies of CR-39 irradiated with X-ray. Radiat. Meas. 28 (1– 6), 119–122. Hawkins, W.L., 1984. Degradation and Stabilization of Polymers. Springer, New York. Malek, M.A., 2000. Study of the e ects of X-ray and gamma ray irradiation on CR-39 plastic track detectors. Ph.D. Thesis, Malaysia, Universiti Sains Malaysia. Malek, M.A., Chong, S.C., Abdullah, R., 2001. Mechanistic model for bond scission in a polymeric system by radiation. Radiation Phys. Chem. 60, 603–607. Reichmanis, E., Curtis, W.F., O’Donnell, J.H., Hill, David, J.T., 1993. Radiation e ects on polymeric materials. In: Reichmanis E., Curtis, W.F., O’Donnell, J.H. (Eds.), Irradiation of Polymeric Materials: Process, Mechanism and Application. American Chemical Society, Washington, USA.