DEVELOPMENT OF DOSIMETRY TECHNIQUES — I
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IAEA-SM-356/40 A REAL-TIME LOW ENERGY ELECTRON CALORIMETER
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N. MOD ALI*, F.A. SMITH XA9949700 Physics Department,
Queen Mary and Westfield College, University of London,
London, United Kingdom
Abstract
A real-time low energy electron calorimeter with a thin film window has been designed and fabricated to facilitate a reliable method of dose assessment for electron beam energies down to 200 keV. The work was initiated by the Radiation Physics Group of Queen Mary and Westfield College in collaboration with the National Physical Laboratory (NPL), Teddington. Irradiations were performed on the low and medium electron energy electron accelerators at the Malaysian Institute for Nuclear Technology Research (MINT). Calorimeter response was initially tested using the on-line temperature measurements for a 500-keV electron beam. The system was later redesigned by incorporating a data-logger to use on the self-shielded 200-keV beam. In use, the final version of the calorimeter could start logging temperature a short time before the calorimeter passed under the beam and continue measurements throughout the irradiation. Data could be easily retrieved at the end of the exposure.
1. INTRODUCTION
The calorimeter consists of a graphite core of diameter 50 mm and thickness 2 mm, embedded in a graphite scatter ring. Two 0.5-mm diameter glass bead thermistors are inserted in the absorber disc and connected to opposite arms of a DC bridge. Thermal insulation above the graphite core is achieved by a 1-mm thick of polystyrene foam, and a 8-um thick aluminised mylar film is stretched across the aluminium ring. These minimize the upward heat losses from the core, mostly caused by the compressed air cooling of the accelerator window. For the purpose of calibration, the design also provided an opportunity to insert thin film dosimeters between the polystyrene foam and the aluminized mylar film.
The entire graphite core plus the scatter ring rests on a 13-mm thick polystyrene foam block held securely within a PVC casing of dimensions 140 mm x 140 mm x 17 mm (Fig. 1).
Calorimeter response was initially tested using the on-line temperature measurements on the MINT 500-keV electron beam. For these measurements, the thermistor was connected via the high quality co-axial cables directly to a Wheatstone DC bridge circuit and the real-time temperature measurements were made by recording the out-of-balance voltage on digital voltmeters. Voltages were then down-loaded via a Tastronic IEEE 488-R232 converter into a computer memory for analysis.
The system was then redesigned for used with the MINT 200-keV self-shielded accelerator which does not permit on-line measurements. A commercially available voltage data-logger was used and the device set to start logging a short time before the calorimeter passed under the beam. Data were recovered after the calorimeter had emerged from the irradiation. The bridge, lithium batteries and voltage data-logger were all contained within the same box as the calorimeter itself (Fig. 2).
* Present address: Mint-Tech Park (MINT), Block 43, Jalan Dengkil, 43000 Kajang, Selangor, Malaysia.
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la-Airing Graphite absorber
Scatter riig
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FIG.L Schematic cross-section of a low energy calorimeter
Polystyrene
Temperature datalogger
Lead to datalogger
FIG. 2. Diagram of the calorimeter with the data-logger system
2. APPLICATIONS
2.1. Calibration of accelerator output
Typical temperature-time plots for the data-logger system at 200 keV and the on-line system at 500 keV are shown in Fig. 3. A slow temperature drift before the irradiation begins (period I) is followed by a rapid rise during irradiation (period II) and then a slow cooling after the end of the irradiation (period III). The shape of the post-irradiation curve depends on the difference between the calorimeter core and the ambient temperature, the cooling rate being smaller at 200 keV than it was at 500 keV.
The temperature rise, AT in the graphite core was determined from the extrapolation to the mid-irradiation time, of both the pre- and post- mid-irradiation parts of the curve. Values of AT at typical conveyor speeds and beam currents for the 200-and 500-keV beams are shown in Fig. 4. These can be used firstly to relate the accelerator output to the beam settings of energy, beam current and conveyor speed, and secondly, to determine the total absorbed dose in the calorimeter core using Eq. (1):
Dgraphite = — = cg.AT
m s (1)
where, E is the deposited energy, AT is the temperature rise induced by the radiation, m and cg are the mass and specific heat of the graphite core, respectively.
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0 100 200 300 Time (seconds)
(b) 500 keV, 1.45 m.min'1 and 2 mA
FIG. 3. Temperature-time pattern using (a) the data-logger system at 200 keV and (b) the on-line system at 500 keVat the stated values of conveyor speed and beam current.
2.2. Calibration of thin film dosimeter
In order to establish a procedure for the calibration of the thin film dosimeters for low energy irradiations, films of cellulose triacetate (CTA) and blue cellophane (BC) were arranged in a stack between the thin aluminised mylar window and the calorimeter core. Both film materials were irradiated at 200 and 500 keV and the difference of temperature rise recorded with and without the insertion of the film stacks. This provided an accurate determination of absorbed dose by the films (Figs 3 and 5). A calibration factor could then be determined to relate the radiation-induced absorbance of the film to the total dose deposited in the film.
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FIG. 4. Relation between temperature rise and beam current for 200-keV and 500-keV electrons at the stated conveyor speeds
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On-line: 500 keV
Datalogger:200 keV
0 50 100 150 200 250 300 350 400 Time (seconds)
FIG. 5. Typical temperature-time pattern with film stack with (a) 500 keV, m.min1 and 3 mA (20 pieces ofBC) and (b) 200 keV, 22.5 m.min'1
and 12 mA (5 pieces ofBC).
Film response was expressed as the net change in the absorbance per unit film thickness and when plotted against absorbed dose gave the calibration curves for CTA and BC films at 200 and 500 keV (Fig. 6). There is a linear response of the CTA film at both energies although the smaller electron penetration at 200 keV gives a smaller range of absorbed dose even for beam currents up to 18 mA (~1 kGy) (Fig. 6a). The 500-keV beam requires beam currents only up to 5 mA to achieves doses up to 4 kGy. A measured calibration factor of 0.0062 + 0.0002 kGy"1 was found to be in good agreement with measurements at other institutions [1,2,3] made under different conditions. A slight non-linearity was observed in the calibration curve for the BC film (Fig. 6b). Although a second order polynomial could be fitted without difficulty, the absorbance was found to be much higher than that obtained using higher energy electrons and photons [4].
E Total dose (kGy) Total dose (kGy)
(a) CTA (b) BC
FIG. 6. Calibration curve for (a) CTA and (b) EC film dosimeter.
3. CONCLUSIONS AND FUTURE WORK
The simple and accurate method of dose determination using the real-time low energy calorimeter provides a reliable method of dosimetric calibration and standardization for low energy electron beam facilities. The advantages of the calorimeter are that:
• it permits use on either on-line and off-line facilities
• it is suitable for daily use since the data can be retrieved at any time after it conies from the beam
• it provides for a precise calibration of the beam and film dosimeters under actual irradiation conditions
ACKNOWLEDGEMENTS
The authors wish to thank the Malaysian Government for the award of a scholarship to NMA, and Dr. David Burns, Dr. Peter Sharpe, Mr. Alan Dusatouy and Mr Malcolm McEwen for assistance both with the calorimeter development and the data interpretation. The work forms part of the thesis (Low Energy Electron Calorimetry for Industrial Processing) presented by NMA to the University of London for the award of the Ph.D degree (1998).
REFERENCES
[1] SUNAGA.H., TANAKA, R., MOD ALI, N AND YOTSUMOTO, K.(1995). Radial. Phys. Chem.
46, pp 1283-1286.
[2] JANOVSKY, I AND MILLER, A. (1987). Appl. Radiat. Isot. 38, pp 931-937
[3] GEHRINGER, P., PROKSCH, E. AND ESCHWEILER, H. (1985). Proc.High Dose Dosimetry Symp. Vienna 333-344.
[4] MCLAUGHLIN, W.L., HUMPHREYS J.C., RADAK, B.B., MILLER, A. AND OLEJNIK, T.A.
(1979) Radiat. Phys. Chem. 14, pp 535-550.