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Thin film alanine-PE dosimeter for electron beam transfer dosimetry
ARTICLE IN PRESS
Radiation Physics and Chemistry 73 (2005) 280–286 www.elsevier.com/locate/radphyschem
Thin film alanine-PE dosimeter for electron beam transfer dosimetry Min Lina,, Huazhi Lia, Yundong Chenb, Ying Cuib, Zhenhong Xiaob, Kesheng Chena, Juncheng Gaob a
Radiometrology Center, China Institute of Atomic Energy (CIAE), Beijing 102413, China b National Institute of Metrology, Beijing 100013, China Received 6 November 2003; accepted 16 October 2004
Abstract Thin film alanine-PE dosimeter developed at CIAE is expected to be used as transfer dosimetry system particularly for electron beam dose measurement. Its basic dosimetry characteristics were studied under 60Co g-ray and electron beam irradiation. The investigated dose range was between 20 and 3 105 Gy. The background dose for the unirradiated dosimeter was about 4 Gy. The inter-specimen scattering among 10 replicate irradiated dosimeters was less than 0.6% ð1sÞ for the doses of 1 kGy after correction of weight differences of dosimeters. The ratio of EPR signals from two batches was about 1.009170.0049 ð1sÞ for 15 dose points from 70 to 40 kGy. No significant dose response dependent on energy or dose rate was found in both g-ray irradiation (average energy 1.25 MeV, dose rate down to 0.4 Gy/s) and electron beams irradiation (energy up to 14 MeV, dose rate to 108 Gy/s). The irradiation temperature coefficient was about 0.26%/1C between 15 and 50 1C in the dose range from 5 to 50 kGy. Another important feature, post-irradiation stability, was also studied in detail for dosimeters stored at 4 1C, room temperature and 40 1C with relative humidity of 98%, 76%, 58%, 33% and 12%. Uncertainty of the dosimetry system for electron beam dose measurement was 2.9% ðk ¼ 2Þ for dose from 102 to 104 Gy, while 5.6% ðk ¼ 2Þ for dose higher than 104 Gy. The preliminary inter-comparison results between CIAE alanine-PE/EPR dosimetry system and the reference dosimetry systems from RISO, JAERI and INCT were comparable within their combined uncertainty, which showed the potential of CIAE alanine-PE/EPR dosimetry system to be used as transfer dosimetry system. r 2004 Elsevier Ltd. All rights reserved. Keywords: Alanine film dosimeter; EPR; ESR; Dosimetry characteristics; Electron beam; Gamma-ray; Uncertainty
1. Introduction Radiation processing with electron been widely developed in China and has a level of industrialization. It has also of the most promising methods for
beams has now reached become one development
Corresponding author.
E-mail address: [email protected] (M. Lin).
of new products. Throughout China, there are 56 electron beam accelerators established for radiation processing, with energies ranging from 300 keV to 14 MeV. Considering the quality of the processed products and the processing procedure itself, it has become more and more necessary to establish a reliable dosimetry method with high accuracy and traceability to national standards by means of reference dosimetry.
0969-806X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2004.10.004
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In the past several years, several studies on electron beam dosimetry have been carried out in China Institute of Atomic Energy (CIAE). After establishing four kinds of liquid chemical dosimetry systems for 60Co g-ray dosimetry, we began to study the dosimetry systems for electron beam dose measurement. Since 1994, potassium dichromate dosimetry systems were set up as standards for electron beams in the Nuclear Industry System (Lin et al., 1998a, 1999). To meet the requirement of routine dosimetry, a mass-produced FJL-01 CTA thin film dosimetry system was also developed in CIAE. However, in face of international standardization, there is still an urgent need to set up a reliable transfer dosimetry system as the bridge between routine check and calibration, and an alanine-PE/EPR dosimetry system can serve this purpose. Alanine, an organic compound that is near tissue equivalent, was first introduced as a dosimeter in the early 1960s (Bradshaw et al., 1962). It has been extensively studied in several countries and considered as a potential dosimetry system that may substitute the Fricke dosimeter as a reference dosimeter (Regulla et al., 1982, 1983, 1985; Onori et al., 1990; Olsen et al., 1990; Kojima et al., 1993; Feist et al., 1993 ). The merits of an alanine-based dosimetry system are a very wide dose range, high stability of the radiationinduced radicals in alanine and non-destructive readout with good repeatability, so that it can be used for archival purposes.
2. Experimental procedures 2.1. Dosimeter preparation Thin film alanine-PE dosimeter developed at CIAE was mass-produced by extrusion of a mixture of ground polycrystalline DL-a-alanine with low-density polyethylene (LDPE) in the ratio of 2:1 in weight. The alanine crystals were not sorted according to size. The ground DL-a-alanine and LDPE were mixed at 110 1C in a mill to get a homogeneous mixture. The mixture was subsequently extruded at 160–165 1C reproducibly by a Brabender plastogragh and at last produced as a strip about 30 mm in width and 200 mm in thickness (Kojima et al., 1993), which was then cut into dosimeters with 30 mm in length and 7.5 mm in width. Variation of thickness of the strip over the width was less than 715 mm, while thickness variation over the length (300 mm length) was less than 75 mm. Density of the dosimeter was about 1.03 g/cm3. The thickness was measured using a thickness gauge (Heidenhain, resolution 70.5 mm) and the weight using an electric digital balance (Mettler, resolution 70.001 mg).
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2.2. Dosimeter calibration and irradiation The thin film alanine-PE dosimeter was calibrated by Co g-ray irradiation at the Irradiation Center at CIAE (6PBq) or National Institute of Metrology (NIM) (1.5 PBq). The dose rates of both facilities were calibrated by the Fricke dosimeter that is traceable by means of comparison to the national standard of the Fricke dosimeter made in NIM every year. The calibration irradiation was performed in a standard polystyrene water phantom with 30 30 30 cm3 in size. Three to five pieces of thin film alanine-PE dosimeters were enclosed in lens paper as one group, which were inserted into the slits on one end of the polystyrene phantom that was then inserted into a water-proof tube mounted on the cover plate of the phantom. Before irradiating alanine-PE dosimeters, at least seven Fricke dosimeters were irradiated to different doses at a fixed position to calibrate the dose rate and the transit dose. Different groups of thin film alanine-PE dosimeters were irradiated covering the needed dose range. Thin film alanine-PE dosimeters were measured using an X-band EMX/2.7 EPR spectrometer (BRUKER) with GR 4119 high sensitivity (HS) cavity. The dosimeters were set in the gap of a flat quartz holder for fixing the dosimeter in the cavity (Kojima et al., 1993). The calibration curve was then obtained by the leastsquares method. The irradiation temperature was measured before and after each irradiation needed for irradiation temperature correction. Electron beam irradiation was performed at CIAE under scanned beams of RSA-1 2 MeV and self-made 14 MeV linear accelerators. Before irradiation of alanine-PE dosimeters, FJL-01 CTA thin film dosimeters (Lin et al., 1998b) were used to measure field and depth–dose distribution. After irradiation, CTA dosimeters were measured continuously at a distance of 100 mm by Cary-3E UV–Visible spectrophotometer (Varian). According to the results of field distribution, the homogeneous radiation area was determined. Through depth–dose distribution curve, the thickness of polystyrene at the maximum and linear area of the curve was determined. For 2 MeV electron beam irradiation, 3 pieces of thin film alanine-PE dosimeters were irradiated in parallel with potassium (silver) dichromate dosimeters in the linear range of the depth–dose curve (Sharpe and Miller, 1999). The doses of alanine dosimeters were then calculated by correction of the doses of potassium (silver) dichromate dosimeters to those of the center of alanine-PE dosimeters, while for 14 MeV electron beam irradiation, 3–5 pieces of alaninePE dosimeters enclosed in lens paper were irradiated together in parallel with potassium (silver) dichromate dosimeters in the range of the maximum depth–dose curve. The doses of alanine-PE dosimeters were given by dichromate dosimeters. Dose–response curves and 60
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irradiation temperature were obtained as mentioned above.
3. Results and discussion 3.1. EPR measurement The stability of the EPR spectrometer was checked periodically using a reference alanine-PE dosimeter irradiated to about 1 kGy and a weak pitch EPR intensity standard. The alanine-PE reference dosimeter is stored at 4 1C and is normally used for not more than half a year. Its validity is controlled by comparison with the weak pitch standard. As we know, there are five peaks in the alanine spectrum due to the four hydrogen atoms around the radicals. The EPR signal (H) used for absorbed dose measurement was taken as the relative height of the central peak, which was normalized to the weight of the dosimeter, calibrated gain coefficient and EPR intensity of the reference alanine dosimeter as follows: H¼
hx ðH p2p Þx ms gs ¼ hs ðH p2p Þs mx gx
in which: h is the EPR intensity of the individual dosimeter after correction by its mass and gain coefficient; Hp–p is the main peak-to-peak amplitude of EPR spectrum; m is the mass of dosimeter; g is the gain coefficient of dosimeter; and the subscripts x and s mean the measured dosimeter and standard alanine dosimeter, respectively.
3.1.2. Stability of EPR spectrometer The short-term reproducibility of the EPR spectrometer was studied by measuring it 20 times with one alanine-PE dosimeter (irradiated to 1 kGy) fixed in the cavity for half an hour. The relative standard deviation was 0.10% ð1sÞ: Similarly, long-term reproducibility was studied by measuring the same dosimeter fixed in the cavity after warming up for 3 h. The measurement lasted for about 6 h, in which the standard deviation was found to be 0.23% (see Fig. 1). The thin film alanine dosimeter was also measured 20 times by taking it out and then inserting it into the cavity to the same position before each measurement. The standard deviation of these 20 results was found to be 0.77% ð1sÞ: 3.1.3. Orientation effects of thin film alanine dosimeter The thin film alanine-PE dosimeter is not isotropic; it has an orientation effect due to alignment of alanine microcrystals during the film extrusion (Kojima et al., 1995; Janovsky, 1998). Thus, the dosimeter rotated in clockwise direction was measured repeatedly in 301 intervals for two periods and then measured in 901 intervals for other three periods. The relative intensity of the orientation effects for two periods is shown in Fig. 2, which shows a periodical change of EPR intensity with rotation of the dosimeter. Good reproducibility was found for measurement both in parallel and in perpendicular position. However, it was better to measure in parallel rather than in perpendicular to get higher response and better reproducibility.
1.0100 1.0000
Relative intensity
Relative intensity
3.1.1. Determination of important EPR parameters The thin film alanine-PE dosimeter was fixed in the cavity and measured 1 h after switching on the EPR spectrometer. Every spectrum was recorded in about 42 s and delay time was 300 s. Relative intensity of the EPR signal changed with the boot-strap time as shown in Fig. 1, from which it can be concluded that it takes about 3 h to warm up this EPR spectrometer to be stablized. Microwave power, modulation amplitude, time constant, conversion time and modulation frequency were
also studied, but are not shown here in detail. The operating EPR parameters were set as following: center magnetic field 350.371 mT, sweep width 2 mT, microwave power 4 mW, modulation amplitude 1.0 mT, modulation frequency 100 kHz, time constant 327.68 ms and conversion time 20.48 ms. In order to decrease the uncertainty of dose measurement covering a wide dose range, the alanine-PE dosimeters irradiated to different doses were used to calibrate the gain coefficient from 2 10 to 3.99 105. Each coefficient was measured at least four times to get average values and related standard deviation ð1sÞ: The difference between the original and the calibrated coefficient was from 0.975 to 1.015.
0.9900 0.9800 0.9700 0.9600 0.0
100.0
200.0 300.0 400.0 Boot-strap time / min
500.0
600.0
Fig. 1. Relative intensity of alanine-PE dosimeter after switching on the EPR spectrometer.
1.050 1.000 0.950 0.900 0.850
0
90
180
270
360
450
540
630
720
Orentation/degree
Fig. 2. Orientation effect of thin film alanine-PE dosimeter.
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3.2. Dosimetry characteristics 3.2.1. Dose–response The dose–response of thin film alanine-PE dosimeter was studied for the dose range between 20 and 3 105 Gy under both 60Co g-rays and electron beams irradiation. The linear dose range can be up to 1.6 104 Gy with the linear regression coefficient better than 0.9999. Fig. 3 shows the dose–response from 50 to 7 104 Gy. Below 100 Gy, the dose–response deviates from linearity due to background signal before irradiation. This background signal was found to correspond to 4.070.8 Gy. The calibration curve shows gradual saturation effect for doses higher than 104 Gy, which may be due to radical–radical reactions as the radical concentration increases (Krushev et al., 1994; Koizumi et al., 1997). Fig. 5 shows good agreement between the dose responses for the dosimeters irradiated by g-rays (1.25 MeV, dose rate from 0.4 to 10.9 Gy/s) and those irradiated by electron beams (2 MeV, 300 Gy/s and 14 MeV, 108 Gy/s) demonstrating negligible dependence on radiation energy or dose rate. Therefore, it is suitable to use the calibration curve obtained under gamma irradiation for dose evaluation of the dosimeters irradiated by electron beams. 3.2.2. Batch uniformity Batch uniformity is very important for a dosimetry system. In order to check the inter-specimen scattering of one batch, 60 pieces of thin film alanine-PE dosimeters from one batch were selected and divided into six groups. Each group was irradiated to the same dose at the 14 MeV electron accelerator. The standard deviation of the inter-specimen scattering of one batch was found to be between 0.32% and 0.51%. To study the inter-batch homogeneity, two batches of thin film alanine-PE dosimeters were selected (batch number 980207 and 980709). 15 groups (3 pieces for each group) of alanine-PE dosimeters from each batch were irradiated covering the dose range from 70 Gy to 50 kGy. For each dose point, two groups from different
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batches of alanine-PE dosimeters were irradiated together. The ratios of the averaged EPR signals from the two batches were calculated and the averaged ratio was found to be 1.009170.0049 ð1sÞ: 3.2.3. Irradiation temperature increase As we know there is serious temperature effect during electron beams irradiation, which is due to the high dose rate of electron beams. Hence, it is very important to study irradiation temperature effect on the dosimeter. Since it was difficult to get reproducible radiation conditions for each electron beam irradiation, temperature effect was studied under 60Co g-rays irradiation. Five alanine-PE dosimeters were enclosed in lens paper as one group. A special water phantom was designed, in which the irradiation temperature of dosimeters could be controlled by the circulated water of certain temperature. Sixteen groups of thin film alanine-PE dosimeters were irradiated at the Cobalt-60 facility to 5, 10, 30, 50 kGy in the temperature range of (1550) 1C. The EPR signals were all normalized to those of room temperature. The results are shown in Fig. 4. As seen from above, the irradiation temperature coefficient is found to be 0.26%/1C, which seems independent of dose in the selected dose range. This result was same as that reported by Xie et al. (2002) and close to 0.24%/1C reported by Kojima et al. (1993). However, it is much more higher than 0.13%/1C reported by Janovsky (1998). The big difference may be due to different materials between DL- and L-alanine. 3.2.4. Post-irradiation stability As a transfer dosimetry system, it is very important to have good stability after irradiation. There are many factors that affect post-irradiation stability such as temperature, humidity, dose, light, crystallization degree, atmosphere, etc. The first four factors were studied in this paper. Post-irradiation stability studies were performed using thin film alanine-PE dosimeters irradiated to different
EPR intensity/a.u.
100.000 10.000 1.000 0.100 0.010 10
100
1000 Dose/Gy
1.25MeV, gamma
2MeV, e
10000
100000
14MeV, e
Fig. 3. Dose–response of thin film alanine-PE dosimeters irradiated by 60Co g-rays and electron beams. (~) 1.25 Mev, gamma (’) 2 Mev, e (m) 14 Mev, e.
Fig. 4. Irradiation temperature effect on thin film alanine-PE dosimeter. (~) 5 kGy, (’) 10 kGy, (m) 30 kGy, () 50 kGy; (—) 5 kGy; (???) 10 kGy, (– – – –) 30 kGy, (– – ) 50 kGy.
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doses and stored under different temperatures and relative humidities. The dosimeters, three pieces for each group, were irradiated to approximately 1, 10 and 66 kGy. The storage temperature was selected as 4 1C (in refrigerator), room temperature and 40 1C (in oven). Relative humidity (RH) of 98%, 76%, 58%, 33% and 12% were selected by different inorganic salt-saturated
Relative intensity
1.020 1.000 0.980 0.960 0.940 0.920 0.900 0
(a)
50
100
150 200 250 Storage time /day 76%
58%
33%
300
350
400
300
350
400
12%
Relative intensity
1.020 1.000 0.980 0.960 0.940 0.920 0
(b)
50
100
150 200 250 Storage time/day 98%
76%
58%
33%
12%
Fig. 5. (a) Stability results of the thin film alanine dosimeters stored under normal light illumination and at room temperature. (a) (’) 76%, (m) 58%, ( ) 33%, (~) 12%. (b) Stability results of the thin film alanine dosimeters stored in dark at room temperature. (b) (~) 98%, ( ) 76%, (m) 58%, () 33%, (’) 12%.
solutions (98%–CuSO4 5H2O; 76%–NaCl; 58%–NaBr; 33%–MgCl2; 12%–LiCl) in enclosed vessels. Before EPR measurement, the dosimeters were kept at room temperature and humidity for equivalence for at least 1 h. All of the results were normalized to their individual initial EPR signals and corrected by the signals of standard alanine sample irradiated to 1 kGy. We found that when the dosimeters were exposed to 98% RH, decay rate of the EPR signal was fast independent of dose and storage temperature. Decay was about 2% during the first 10 days and 70% for nearly one year, which may be due to the reactions of free radicals with free electrons, hydroxyl or other radicals. When stored in other RH conditions, decay rate was influenced by the combined effect of RH, dose, temperature and even light. When stored in refrigerator in RH below 76%, dose became the decisive factor. Decay was more than 1% in about 120 days for dosimeters irradiated to 66 kGy, while for dosimeters irradiated to 10 kGy it was about 1% for 1 year. For the dosimeters stored at room temperature and under normal light illumination, it was found that decay for one year was about 7% for RH less than 60%, while it was 9% for 76% RH, see Fig. 5(a). For the dosimeters stored in the dark magazine at different RH of less than 58% and room temperature, decay for 1 year was found to be within the measurement uncertainty. At 76% RH, the decay of the dosimeter response was found to be 2% for 1 year, while for 98% RH, it was 2% during the first 20 days and 7% for 1 year, see Fig. 5(b).
Table 1 Uncertainty evaluation of electron beam dose measurement using thin film alanine-PE/EPR dosimetry system Source
Uncertainty of reference Fricke dosimeter Irradiation temperature correction for reference Fricke dosimeter Irradiation time of reference Fricke dosimeter Decay of cobalt sources Irradiation time of alanine dosimeter Calibration curve Irradiation temperature correction of alanine dosimeter Weight measurement of reference alanine dosimeter ðn ¼ 20Þ Weight measurement of irradiated alanine dosimeter ðn ¼ 3Þ ESR measurement repeatability of standard alanine dosimeter ESR measurement repeatability of irradiated dosimeter ðn ¼ 3Þ Stability of irradiated alanine dosimeter (within 50 days) Ununiformity of radiation field (including decay and geometry) (ASTM Standard E1707, 1995) Standard uncertainty of the above sources Combined standard uncertainty Expanded uncertainty ðk ¼ 2Þ
102–104 Gy
4104 Gy
A
B
A
B
0.83 — — — — 0.25 — 0.0015 0.0035 0.77 — — 0.5
— 0.18 0.058 0.072 0.052 — 0.30 — — — 0.33 0.50 —
0.83 — — — — 1.25 — 0.0015 0.0035 0.77 — — 0.5
— 0.18 0.0032 0.11 0.0032 — 0.75 — — — 0.27 2.00 —
0.70
1.76
1.26 1.45 2.9
2.16 2.79 5.6
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For the dosimeters stored in dark and at RH below 60% and for temperatures up to 40 1C, decay of the EPR signal can be neglected for the storage time up to 140 days for 1 kGy dosimeters, 50 days for 10 kGy dosimeters and 30 days for 66 kGy dosimeters. 3.3. Uncertainty evaluation Many factors affect the uncertainty of the dosimetry system such as the uncertainty of the reference dosimetry system, calibration procedure, fitting of the calibration curve, irradiation procedure, instability of instrumentation and instability of dosimetry system. Table 1 shows the results of uncertainty evaluation of the dose measurement of thin film alanine-PE/EPR dosimetry system irradiated by electron beams. In order to check the reliability of our thin film alanine-PE/EPR dosimetry system, it was compared with the alanine-PE/EPR dosimetry systems from Risø National Laboratory (Denmark), Environmental Conservation Process Laboratory in JAERI and Laboratory for Measurement of Technological Doses in INCT (Kojima et al., 1999). The preliminary results were shown in detail by Peimel-Stuglik et al. (2002) and Lin et al. (2004). It demonstrated that the CIAE film alanine-PE/EPR dosimetry system has the potential to be used as a transfer dosimetry system.
4. Conclusion From the detailed studies we conclude that the thin film alanine-PE dosimeter developed at CIAE has the potential to be used as a transfer dosimetry system particularly for electron beam dose measurement. Its basic dosimetry characteristics were studied under 60Co g-ray and electron beam irradiation. The inter-specimen scattering among 10 replicate irradiated dosimeters was less than 0.6% ð1sÞ for the doses of 1 kGy after correction due to weight differences of dosimeters. The ratio of EPR signal from two batches was about 1.009170.0049 ð1sÞ for 15 dose points from 70 Gy to 40 kGy. No significant difference in dose–response as a function of energy or dose rate was found from g-ray irradiation (average energy 1.25 MeV, dose rate down to 0.4 Gy/s) to electron beam irradiation (energy up to 14 MeV, dose rate up to 108 Gy/s). The irradiation temperature coefficient was about 0.26%/1C between 15 and 50 1C in the dose range from 5 to 50 kGy. Postirradiation stability was studied in detail for dosimeters stored at temperature of 4 1C , room temperature and 40 1C and relative humidities of 98%, 76%, 58%, 33% and 12% and decay of the EPR signal was found to be significant at high relative humidities and controllable at others. Typical uncertainty of the dosimeter was 2.9% ðk ¼ 2Þ for electron beam dose measurement.
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Acknowledgements The authors hope to thank Prof. Huilin Yuan (Beijing University of Chemical Engineering) for his devotion to thin film alanine-PE dosimeter preparation. They are also grateful to Mr. Yanli Zhang (National Institute of Metrology), Mr. Guicheng Chen, Mr. Lian Cao and Ms. Lanzhen Liu (China Institute of Atomic Energy) for their cooperation on gamma ray irradiation and electron beam irradiation.
References ASTM Standard E1707, 1995. Standard Guide for Estimating Uncertainties in Dosimetry for Radiation Processing. American Society for Testing and Materials, 100 Barr Harbor Drive, West Conshohocken, PA 19428, USA. Bradshaw, W.W., Cadena, D.G., Crawford, E.W., Spetzler, H.A.W., 1962. The use of alanine as a solid dosimeter. Radiat. Res. 17 (1), 11–21. Feist, H., Regulla, D., Wieser-, A., 1993. Is alanine/ESR dosimetry now an alternative to ferrous sulphate dosimetry. Appl. Radiat. Isot. 44 (1–2), 47–51. Janovsky, I., 1998. Progress in alanine film/ESR dosimetry. IAEA-SM-314/47, 173–187. Koizumi, H., Ichikawa, T., Yoshida, H., Namba, H., Taguchi, M., Kojima, T., 1997. Effect of radiation quality on radical formation in ion-irradiated solid alanine. JAERI-Conference 97-003, pp. 286–288. Kojima, T., Ranjith, H.L.A., Haruyama, Y., Kashiwazaki, S., Tanaka, R., 1993. Thin film alanine-polyethylene dosimeter. Appl. Radiat. Isot. 44, 41–45. Kojima, T., Kashiwazaki, S., Tachibana, H., Tanaka, R., Desrosiers, M.F., McLaughlin, W.L., 1995. Orientation effects on ESR analysis of alanine-polymer dosimeters. Appl. Radiat. Isot. 46, 1407–1411. Kojima, T., Tachibana, H., Haneda, N., Kawashima, I., Sharpe, P.H.G., 1999. Uncertainty estimation in 60Co gamma-ray dosimetry at JAERI involving a two-way intercomparison study with NPL in the dose range 1–50 kGy. Radiat. Phys. Chem. 54, 619–626. Krushev, V.V., Koizumi, H., Ichikawa, T., Yoshida, H., Shibata, H., Tagawa, S., Yoshida, Y., 1994. Relation between track structure and LET effect on free radical formation for ion beam-irradiated alanine dosimeter. Radiat. Phys. Chem. 44 (5), 521–526. Lin, M., Ye, H. Sh., Lin, J., Chen, Y., Zhang, G., Xiao, Z.N., Li, H., 1998a. Dose measurements with liquid chemical dosimeters for electron beams. At. Energy Sci. Technol. 32 (6), 510–516. Lin, J., Long, S., Ye, H.Sh., Xiao, Z.N., Lin, M., Zhang, G., Chen, Y., 1998b. Research on the dosimetry of FJL-01 CTA film dosimeter. At. Energy Sci. Technol. 32 (6), 524–529. Lin, M., Lin, J., Chen, Y., Li, H., Xiao, Z., 1999. A set of dosimetry systems for electron beam irradiation. Proceedings of Techniques for High Dose Dosimetry in Industry, Agriculture and Medicine, Vienna, 2–5 November 1998, IAEA-TEDOC-1070, pp. 77–82.
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Lin, M., Chen, Y., Cui, Y., Chen, K., Li, H., Xiao, Z., Kojima, T., Peimel-Stuglik, Z., Fabisiak, S., 2004. Dose Intercomparison Studies for 60Co Gamma-ray and Electron Beam Irradiation in 2002. Nucl. Sci. Tech. 15 (3), 166–173. Olsen, K.J., Hansen, J.W., Wille, M., 1990. Response of the alanine radiation dosimeter to high-energy photon and electron beams. Phys. Med. Biol. 35 (1), 43–52. Onori, S., Bartolotta, A., Caccia, B., Indovina, P.L., Milano, F., Renzis, R., Scarpa, G., Caporali, C., Moscati-, M., 1990. Dosimetric characteristics of alanine-based ESR detectors in electron beams used in radiotherapy. Radiat. Prot. Dosi. 34 (1/4), 287–289. Peimel-Stuglik, Z., Lin, M., Fabisiak, S., Cui, Y., Li, H., Xiao, Z., Chen, Y., 2002. Polish–Chinese intercomparison of high-
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Radiation Physics and Chemistry 73 (2005) 280–286 www.elsevier.com/locate/radphyschem
Thin film alanine-PE dosimeter for electron beam transfer dosimetry Min Lina,, Huazhi Lia, Yundong Chenb, Ying Cuib, Zhenhong Xiaob, Kesheng Chena, Juncheng Gaob a
Radiometrology Center, China Institute of Atomic Energy (CIAE), Beijing 102413, China b National Institute of Metrology, Beijing 100013, China Received 6 November 2003; accepted 16 October 2004
Abstract Thin film alanine-PE dosimeter developed at CIAE is expected to be used as transfer dosimetry system particularly for electron beam dose measurement. Its basic dosimetry characteristics were studied under 60Co g-ray and electron beam irradiation. The investigated dose range was between 20 and 3 105 Gy. The background dose for the unirradiated dosimeter was about 4 Gy. The inter-specimen scattering among 10 replicate irradiated dosimeters was less than 0.6% ð1sÞ for the doses of 1 kGy after correction of weight differences of dosimeters. The ratio of EPR signals from two batches was about 1.009170.0049 ð1sÞ for 15 dose points from 70 to 40 kGy. No significant dose response dependent on energy or dose rate was found in both g-ray irradiation (average energy 1.25 MeV, dose rate down to 0.4 Gy/s) and electron beams irradiation (energy up to 14 MeV, dose rate to 108 Gy/s). The irradiation temperature coefficient was about 0.26%/1C between 15 and 50 1C in the dose range from 5 to 50 kGy. Another important feature, post-irradiation stability, was also studied in detail for dosimeters stored at 4 1C, room temperature and 40 1C with relative humidity of 98%, 76%, 58%, 33% and 12%. Uncertainty of the dosimetry system for electron beam dose measurement was 2.9% ðk ¼ 2Þ for dose from 102 to 104 Gy, while 5.6% ðk ¼ 2Þ for dose higher than 104 Gy. The preliminary inter-comparison results between CIAE alanine-PE/EPR dosimetry system and the reference dosimetry systems from RISO, JAERI and INCT were comparable within their combined uncertainty, which showed the potential of CIAE alanine-PE/EPR dosimetry system to be used as transfer dosimetry system. r 2004 Elsevier Ltd. All rights reserved. Keywords: Alanine film dosimeter; EPR; ESR; Dosimetry characteristics; Electron beam; Gamma-ray; Uncertainty
1. Introduction Radiation processing with electron been widely developed in China and has a level of industrialization. It has also of the most promising methods for
beams has now reached become one development
Corresponding author.
E-mail address: [email protected] (M. Lin).
of new products. Throughout China, there are 56 electron beam accelerators established for radiation processing, with energies ranging from 300 keV to 14 MeV. Considering the quality of the processed products and the processing procedure itself, it has become more and more necessary to establish a reliable dosimetry method with high accuracy and traceability to national standards by means of reference dosimetry.
0969-806X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2004.10.004
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In the past several years, several studies on electron beam dosimetry have been carried out in China Institute of Atomic Energy (CIAE). After establishing four kinds of liquid chemical dosimetry systems for 60Co g-ray dosimetry, we began to study the dosimetry systems for electron beam dose measurement. Since 1994, potassium dichromate dosimetry systems were set up as standards for electron beams in the Nuclear Industry System (Lin et al., 1998a, 1999). To meet the requirement of routine dosimetry, a mass-produced FJL-01 CTA thin film dosimetry system was also developed in CIAE. However, in face of international standardization, there is still an urgent need to set up a reliable transfer dosimetry system as the bridge between routine check and calibration, and an alanine-PE/EPR dosimetry system can serve this purpose. Alanine, an organic compound that is near tissue equivalent, was first introduced as a dosimeter in the early 1960s (Bradshaw et al., 1962). It has been extensively studied in several countries and considered as a potential dosimetry system that may substitute the Fricke dosimeter as a reference dosimeter (Regulla et al., 1982, 1983, 1985; Onori et al., 1990; Olsen et al., 1990; Kojima et al., 1993; Feist et al., 1993 ). The merits of an alanine-based dosimetry system are a very wide dose range, high stability of the radiationinduced radicals in alanine and non-destructive readout with good repeatability, so that it can be used for archival purposes.
2. Experimental procedures 2.1. Dosimeter preparation Thin film alanine-PE dosimeter developed at CIAE was mass-produced by extrusion of a mixture of ground polycrystalline DL-a-alanine with low-density polyethylene (LDPE) in the ratio of 2:1 in weight. The alanine crystals were not sorted according to size. The ground DL-a-alanine and LDPE were mixed at 110 1C in a mill to get a homogeneous mixture. The mixture was subsequently extruded at 160–165 1C reproducibly by a Brabender plastogragh and at last produced as a strip about 30 mm in width and 200 mm in thickness (Kojima et al., 1993), which was then cut into dosimeters with 30 mm in length and 7.5 mm in width. Variation of thickness of the strip over the width was less than 715 mm, while thickness variation over the length (300 mm length) was less than 75 mm. Density of the dosimeter was about 1.03 g/cm3. The thickness was measured using a thickness gauge (Heidenhain, resolution 70.5 mm) and the weight using an electric digital balance (Mettler, resolution 70.001 mg).
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2.2. Dosimeter calibration and irradiation The thin film alanine-PE dosimeter was calibrated by Co g-ray irradiation at the Irradiation Center at CIAE (6PBq) or National Institute of Metrology (NIM) (1.5 PBq). The dose rates of both facilities were calibrated by the Fricke dosimeter that is traceable by means of comparison to the national standard of the Fricke dosimeter made in NIM every year. The calibration irradiation was performed in a standard polystyrene water phantom with 30 30 30 cm3 in size. Three to five pieces of thin film alanine-PE dosimeters were enclosed in lens paper as one group, which were inserted into the slits on one end of the polystyrene phantom that was then inserted into a water-proof tube mounted on the cover plate of the phantom. Before irradiating alanine-PE dosimeters, at least seven Fricke dosimeters were irradiated to different doses at a fixed position to calibrate the dose rate and the transit dose. Different groups of thin film alanine-PE dosimeters were irradiated covering the needed dose range. Thin film alanine-PE dosimeters were measured using an X-band EMX/2.7 EPR spectrometer (BRUKER) with GR 4119 high sensitivity (HS) cavity. The dosimeters were set in the gap of a flat quartz holder for fixing the dosimeter in the cavity (Kojima et al., 1993). The calibration curve was then obtained by the leastsquares method. The irradiation temperature was measured before and after each irradiation needed for irradiation temperature correction. Electron beam irradiation was performed at CIAE under scanned beams of RSA-1 2 MeV and self-made 14 MeV linear accelerators. Before irradiation of alanine-PE dosimeters, FJL-01 CTA thin film dosimeters (Lin et al., 1998b) were used to measure field and depth–dose distribution. After irradiation, CTA dosimeters were measured continuously at a distance of 100 mm by Cary-3E UV–Visible spectrophotometer (Varian). According to the results of field distribution, the homogeneous radiation area was determined. Through depth–dose distribution curve, the thickness of polystyrene at the maximum and linear area of the curve was determined. For 2 MeV electron beam irradiation, 3 pieces of thin film alanine-PE dosimeters were irradiated in parallel with potassium (silver) dichromate dosimeters in the linear range of the depth–dose curve (Sharpe and Miller, 1999). The doses of alanine dosimeters were then calculated by correction of the doses of potassium (silver) dichromate dosimeters to those of the center of alanine-PE dosimeters, while for 14 MeV electron beam irradiation, 3–5 pieces of alaninePE dosimeters enclosed in lens paper were irradiated together in parallel with potassium (silver) dichromate dosimeters in the range of the maximum depth–dose curve. The doses of alanine-PE dosimeters were given by dichromate dosimeters. Dose–response curves and 60
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irradiation temperature were obtained as mentioned above.
3. Results and discussion 3.1. EPR measurement The stability of the EPR spectrometer was checked periodically using a reference alanine-PE dosimeter irradiated to about 1 kGy and a weak pitch EPR intensity standard. The alanine-PE reference dosimeter is stored at 4 1C and is normally used for not more than half a year. Its validity is controlled by comparison with the weak pitch standard. As we know, there are five peaks in the alanine spectrum due to the four hydrogen atoms around the radicals. The EPR signal (H) used for absorbed dose measurement was taken as the relative height of the central peak, which was normalized to the weight of the dosimeter, calibrated gain coefficient and EPR intensity of the reference alanine dosimeter as follows: H¼
hx ðH p2p Þx ms gs ¼ hs ðH p2p Þs mx gx
in which: h is the EPR intensity of the individual dosimeter after correction by its mass and gain coefficient; Hp–p is the main peak-to-peak amplitude of EPR spectrum; m is the mass of dosimeter; g is the gain coefficient of dosimeter; and the subscripts x and s mean the measured dosimeter and standard alanine dosimeter, respectively.
3.1.2. Stability of EPR spectrometer The short-term reproducibility of the EPR spectrometer was studied by measuring it 20 times with one alanine-PE dosimeter (irradiated to 1 kGy) fixed in the cavity for half an hour. The relative standard deviation was 0.10% ð1sÞ: Similarly, long-term reproducibility was studied by measuring the same dosimeter fixed in the cavity after warming up for 3 h. The measurement lasted for about 6 h, in which the standard deviation was found to be 0.23% (see Fig. 1). The thin film alanine dosimeter was also measured 20 times by taking it out and then inserting it into the cavity to the same position before each measurement. The standard deviation of these 20 results was found to be 0.77% ð1sÞ: 3.1.3. Orientation effects of thin film alanine dosimeter The thin film alanine-PE dosimeter is not isotropic; it has an orientation effect due to alignment of alanine microcrystals during the film extrusion (Kojima et al., 1995; Janovsky, 1998). Thus, the dosimeter rotated in clockwise direction was measured repeatedly in 301 intervals for two periods and then measured in 901 intervals for other three periods. The relative intensity of the orientation effects for two periods is shown in Fig. 2, which shows a periodical change of EPR intensity with rotation of the dosimeter. Good reproducibility was found for measurement both in parallel and in perpendicular position. However, it was better to measure in parallel rather than in perpendicular to get higher response and better reproducibility.
1.0100 1.0000
Relative intensity
Relative intensity
3.1.1. Determination of important EPR parameters The thin film alanine-PE dosimeter was fixed in the cavity and measured 1 h after switching on the EPR spectrometer. Every spectrum was recorded in about 42 s and delay time was 300 s. Relative intensity of the EPR signal changed with the boot-strap time as shown in Fig. 1, from which it can be concluded that it takes about 3 h to warm up this EPR spectrometer to be stablized. Microwave power, modulation amplitude, time constant, conversion time and modulation frequency were
also studied, but are not shown here in detail. The operating EPR parameters were set as following: center magnetic field 350.371 mT, sweep width 2 mT, microwave power 4 mW, modulation amplitude 1.0 mT, modulation frequency 100 kHz, time constant 327.68 ms and conversion time 20.48 ms. In order to decrease the uncertainty of dose measurement covering a wide dose range, the alanine-PE dosimeters irradiated to different doses were used to calibrate the gain coefficient from 2 10 to 3.99 105. Each coefficient was measured at least four times to get average values and related standard deviation ð1sÞ: The difference between the original and the calibrated coefficient was from 0.975 to 1.015.
0.9900 0.9800 0.9700 0.9600 0.0
100.0
200.0 300.0 400.0 Boot-strap time / min
500.0
600.0
Fig. 1. Relative intensity of alanine-PE dosimeter after switching on the EPR spectrometer.
1.050 1.000 0.950 0.900 0.850
0
90
180
270
360
450
540
630
720
Orentation/degree
Fig. 2. Orientation effect of thin film alanine-PE dosimeter.
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3.2. Dosimetry characteristics 3.2.1. Dose–response The dose–response of thin film alanine-PE dosimeter was studied for the dose range between 20 and 3 105 Gy under both 60Co g-rays and electron beams irradiation. The linear dose range can be up to 1.6 104 Gy with the linear regression coefficient better than 0.9999. Fig. 3 shows the dose–response from 50 to 7 104 Gy. Below 100 Gy, the dose–response deviates from linearity due to background signal before irradiation. This background signal was found to correspond to 4.070.8 Gy. The calibration curve shows gradual saturation effect for doses higher than 104 Gy, which may be due to radical–radical reactions as the radical concentration increases (Krushev et al., 1994; Koizumi et al., 1997). Fig. 5 shows good agreement between the dose responses for the dosimeters irradiated by g-rays (1.25 MeV, dose rate from 0.4 to 10.9 Gy/s) and those irradiated by electron beams (2 MeV, 300 Gy/s and 14 MeV, 108 Gy/s) demonstrating negligible dependence on radiation energy or dose rate. Therefore, it is suitable to use the calibration curve obtained under gamma irradiation for dose evaluation of the dosimeters irradiated by electron beams. 3.2.2. Batch uniformity Batch uniformity is very important for a dosimetry system. In order to check the inter-specimen scattering of one batch, 60 pieces of thin film alanine-PE dosimeters from one batch were selected and divided into six groups. Each group was irradiated to the same dose at the 14 MeV electron accelerator. The standard deviation of the inter-specimen scattering of one batch was found to be between 0.32% and 0.51%. To study the inter-batch homogeneity, two batches of thin film alanine-PE dosimeters were selected (batch number 980207 and 980709). 15 groups (3 pieces for each group) of alanine-PE dosimeters from each batch were irradiated covering the dose range from 70 Gy to 50 kGy. For each dose point, two groups from different
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batches of alanine-PE dosimeters were irradiated together. The ratios of the averaged EPR signals from the two batches were calculated and the averaged ratio was found to be 1.009170.0049 ð1sÞ: 3.2.3. Irradiation temperature increase As we know there is serious temperature effect during electron beams irradiation, which is due to the high dose rate of electron beams. Hence, it is very important to study irradiation temperature effect on the dosimeter. Since it was difficult to get reproducible radiation conditions for each electron beam irradiation, temperature effect was studied under 60Co g-rays irradiation. Five alanine-PE dosimeters were enclosed in lens paper as one group. A special water phantom was designed, in which the irradiation temperature of dosimeters could be controlled by the circulated water of certain temperature. Sixteen groups of thin film alanine-PE dosimeters were irradiated at the Cobalt-60 facility to 5, 10, 30, 50 kGy in the temperature range of (1550) 1C. The EPR signals were all normalized to those of room temperature. The results are shown in Fig. 4. As seen from above, the irradiation temperature coefficient is found to be 0.26%/1C, which seems independent of dose in the selected dose range. This result was same as that reported by Xie et al. (2002) and close to 0.24%/1C reported by Kojima et al. (1993). However, it is much more higher than 0.13%/1C reported by Janovsky (1998). The big difference may be due to different materials between DL- and L-alanine. 3.2.4. Post-irradiation stability As a transfer dosimetry system, it is very important to have good stability after irradiation. There are many factors that affect post-irradiation stability such as temperature, humidity, dose, light, crystallization degree, atmosphere, etc. The first four factors were studied in this paper. Post-irradiation stability studies were performed using thin film alanine-PE dosimeters irradiated to different
EPR intensity/a.u.
100.000 10.000 1.000 0.100 0.010 10
100
1000 Dose/Gy
1.25MeV, gamma
2MeV, e
10000
100000
14MeV, e
Fig. 3. Dose–response of thin film alanine-PE dosimeters irradiated by 60Co g-rays and electron beams. (~) 1.25 Mev, gamma (’) 2 Mev, e (m) 14 Mev, e.
Fig. 4. Irradiation temperature effect on thin film alanine-PE dosimeter. (~) 5 kGy, (’) 10 kGy, (m) 30 kGy, () 50 kGy; (—) 5 kGy; (???) 10 kGy, (– – – –) 30 kGy, (– – ) 50 kGy.
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doses and stored under different temperatures and relative humidities. The dosimeters, three pieces for each group, were irradiated to approximately 1, 10 and 66 kGy. The storage temperature was selected as 4 1C (in refrigerator), room temperature and 40 1C (in oven). Relative humidity (RH) of 98%, 76%, 58%, 33% and 12% were selected by different inorganic salt-saturated
Relative intensity
1.020 1.000 0.980 0.960 0.940 0.920 0.900 0
(a)
50
100
150 200 250 Storage time /day 76%
58%
33%
300
350
400
300
350
400
12%
Relative intensity
1.020 1.000 0.980 0.960 0.940 0.920 0
(b)
50
100
150 200 250 Storage time/day 98%
76%
58%
33%
12%
Fig. 5. (a) Stability results of the thin film alanine dosimeters stored under normal light illumination and at room temperature. (a) (’) 76%, (m) 58%, ( ) 33%, (~) 12%. (b) Stability results of the thin film alanine dosimeters stored in dark at room temperature. (b) (~) 98%, ( ) 76%, (m) 58%, () 33%, (’) 12%.
solutions (98%–CuSO4 5H2O; 76%–NaCl; 58%–NaBr; 33%–MgCl2; 12%–LiCl) in enclosed vessels. Before EPR measurement, the dosimeters were kept at room temperature and humidity for equivalence for at least 1 h. All of the results were normalized to their individual initial EPR signals and corrected by the signals of standard alanine sample irradiated to 1 kGy. We found that when the dosimeters were exposed to 98% RH, decay rate of the EPR signal was fast independent of dose and storage temperature. Decay was about 2% during the first 10 days and 70% for nearly one year, which may be due to the reactions of free radicals with free electrons, hydroxyl or other radicals. When stored in other RH conditions, decay rate was influenced by the combined effect of RH, dose, temperature and even light. When stored in refrigerator in RH below 76%, dose became the decisive factor. Decay was more than 1% in about 120 days for dosimeters irradiated to 66 kGy, while for dosimeters irradiated to 10 kGy it was about 1% for 1 year. For the dosimeters stored at room temperature and under normal light illumination, it was found that decay for one year was about 7% for RH less than 60%, while it was 9% for 76% RH, see Fig. 5(a). For the dosimeters stored in the dark magazine at different RH of less than 58% and room temperature, decay for 1 year was found to be within the measurement uncertainty. At 76% RH, the decay of the dosimeter response was found to be 2% for 1 year, while for 98% RH, it was 2% during the first 20 days and 7% for 1 year, see Fig. 5(b).
Table 1 Uncertainty evaluation of electron beam dose measurement using thin film alanine-PE/EPR dosimetry system Source
Uncertainty of reference Fricke dosimeter Irradiation temperature correction for reference Fricke dosimeter Irradiation time of reference Fricke dosimeter Decay of cobalt sources Irradiation time of alanine dosimeter Calibration curve Irradiation temperature correction of alanine dosimeter Weight measurement of reference alanine dosimeter ðn ¼ 20Þ Weight measurement of irradiated alanine dosimeter ðn ¼ 3Þ ESR measurement repeatability of standard alanine dosimeter ESR measurement repeatability of irradiated dosimeter ðn ¼ 3Þ Stability of irradiated alanine dosimeter (within 50 days) Ununiformity of radiation field (including decay and geometry) (ASTM Standard E1707, 1995) Standard uncertainty of the above sources Combined standard uncertainty Expanded uncertainty ðk ¼ 2Þ
102–104 Gy
4104 Gy
A
B
A
B
0.83 — — — — 0.25 — 0.0015 0.0035 0.77 — — 0.5
— 0.18 0.058 0.072 0.052 — 0.30 — — — 0.33 0.50 —
0.83 — — — — 1.25 — 0.0015 0.0035 0.77 — — 0.5
— 0.18 0.0032 0.11 0.0032 — 0.75 — — — 0.27 2.00 —
0.70
1.76
1.26 1.45 2.9
2.16 2.79 5.6
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For the dosimeters stored in dark and at RH below 60% and for temperatures up to 40 1C, decay of the EPR signal can be neglected for the storage time up to 140 days for 1 kGy dosimeters, 50 days for 10 kGy dosimeters and 30 days for 66 kGy dosimeters. 3.3. Uncertainty evaluation Many factors affect the uncertainty of the dosimetry system such as the uncertainty of the reference dosimetry system, calibration procedure, fitting of the calibration curve, irradiation procedure, instability of instrumentation and instability of dosimetry system. Table 1 shows the results of uncertainty evaluation of the dose measurement of thin film alanine-PE/EPR dosimetry system irradiated by electron beams. In order to check the reliability of our thin film alanine-PE/EPR dosimetry system, it was compared with the alanine-PE/EPR dosimetry systems from Risø National Laboratory (Denmark), Environmental Conservation Process Laboratory in JAERI and Laboratory for Measurement of Technological Doses in INCT (Kojima et al., 1999). The preliminary results were shown in detail by Peimel-Stuglik et al. (2002) and Lin et al. (2004). It demonstrated that the CIAE film alanine-PE/EPR dosimetry system has the potential to be used as a transfer dosimetry system.
4. Conclusion From the detailed studies we conclude that the thin film alanine-PE dosimeter developed at CIAE has the potential to be used as a transfer dosimetry system particularly for electron beam dose measurement. Its basic dosimetry characteristics were studied under 60Co g-ray and electron beam irradiation. The inter-specimen scattering among 10 replicate irradiated dosimeters was less than 0.6% ð1sÞ for the doses of 1 kGy after correction due to weight differences of dosimeters. The ratio of EPR signal from two batches was about 1.009170.0049 ð1sÞ for 15 dose points from 70 Gy to 40 kGy. No significant difference in dose–response as a function of energy or dose rate was found from g-ray irradiation (average energy 1.25 MeV, dose rate down to 0.4 Gy/s) to electron beam irradiation (energy up to 14 MeV, dose rate up to 108 Gy/s). The irradiation temperature coefficient was about 0.26%/1C between 15 and 50 1C in the dose range from 5 to 50 kGy. Postirradiation stability was studied in detail for dosimeters stored at temperature of 4 1C , room temperature and 40 1C and relative humidities of 98%, 76%, 58%, 33% and 12% and decay of the EPR signal was found to be significant at high relative humidities and controllable at others. Typical uncertainty of the dosimeter was 2.9% ðk ¼ 2Þ for electron beam dose measurement.
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Acknowledgements The authors hope to thank Prof. Huilin Yuan (Beijing University of Chemical Engineering) for his devotion to thin film alanine-PE dosimeter preparation. They are also grateful to Mr. Yanli Zhang (National Institute of Metrology), Mr. Guicheng Chen, Mr. Lian Cao and Ms. Lanzhen Liu (China Institute of Atomic Energy) for their cooperation on gamma ray irradiation and electron beam irradiation.
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