Piergiorgio Fuochi , Marco Lavalle , Ugo Corda , András Kovács

1. Piergiorgio Fuochi , Marco Lavalle , Ugo Corda , András Kovács First Planning and Coordination Meeting - Technical Coordination Project RER/8/010 Quality Control Methods and procedures for Radiation Technology Warsaw 21-25 February 2005 In-plant calibration and use of power transistors for process control of gamma and electron beam facilities ISOF-CNR, Via P Gobetti, 101 – 40129 Bologna Institute of Isotope and Surface Chemistry, Budapest, Ungheria

2. In-plant calibration and use of power transistors for process control of gamma and electron beam facilities Introduction A bipolar transistor, previously investigated as a possible radiation dosimeter, has been tested under industrial irradiation conditions in high activity gamma and high energy, high power electron beam facilities. In-plant calibrations have been performed against transfer standard alanine and reference standard ethanol-monochlorobenzene (ECB) dosimeters over an absorbed dose range of 5 to 45 kGy. Routine irradiations have been done by placing the transistors, side by side with the routine dosimeters used in the plants, on product boxes during production runs. The results of the studies have shown that the agreement between the absorbed dose measured by the transistors and other well established routine dosimeters is within ±3%. This indicates that this type of transistors could be used as routine radiation dosimeters in the dose range investigated.

3. In-plant calibration and use of power transistors for process control of gamma and electron beam facilities Objectives Silicon devices, as radiation monitors, were already considered since mid-1960s both at low doses, as in the field of radiation therapy, and at high doses as for industrial radiation processes. In a previous work the possibility of using a low cost, small size commercial bipolar power transistor as a routine dosimeter was investigated. Attention was focused on the changes of carrier lifetime with irradiation, knowing that these changes were proportional to the absorbed dose. That study, conducted under standard laboratory conditions, revealed that this “dosimeter” was capable of measuring doses in the range 0.1-25 kGy, both in gamma and electron radiation fields. A = 7,4 ÷ 7,8 mm B = 10,5 ÷ 10,8 mm C = 2,4 ÷ 2,7 mm D = 15,7 mm Since the damage to the silicon crystal structure produced by irradiation (which affect the carrier lifetime) does not anneal at temperatures below 300 °C, the information about the absorbed dose is not lost during transistor readout and therefore they can be used for recording dosimetry history. It was also found that the response of the transistor is energy dependent; therefore if the devices are calibrated and used in radiation fields with significantly different radiation energy spectra, due to diffused and scattered photons and electrons, differences in dosimeter response may be expected. The aim of the present work is to characterize the behaviour and the performance of these transistors for routine dose measurements in high activity gamma irradiators and high energy, high power EB facilities by comparing them with other routine dosimeters used in the plants.

4. In-plant calibration and use of power transistors for process control of gamma and electron beam facilities Irradiation sources Gamma irradiations Gamma irradiations were performed at the multipass, product overlap, pallet type 60Co facility of Gammarad Italia, Ca’ de’ Fabbri, Italy, with current activity of 6.9 x 1016 Bq. Electron irradiations Electron irradiations were carried out with the 10 MeV TT100 type Rhodotron at Gambro Dasco plant, Medolla, Italy, under operating production conditions, i.e. on a conveyor system with a beam width of 83 cm and average pulse current of 1.72 mA

5. In-plant calibration and use of power transistors for process control of gamma and electron beam facilities Dosimetry systems To minimize the contribution of influence quantities to the overall uncertainty and to ensure similar irradiation conditions both for calibration and routine dosimetry during the production run, full in-plant calibration of the transistors was performed. Field trial irradiations were carried out under operating conditions to check the applicability and the performance of the transistors for routine application. Calibration of transistors Thecalibration of transistors in the gamma plant was done against ECB dosimeters traceable to Risø High Dose Reference Laboratory. Two vials containing ECB solutions and four transistors were arranged in line inside a polystyrene phantom. Several of such phantoms were attached to the sides of product pallets, parallel to the source plaques and irradiated to nominal doses from 5 up to 45 kGy. At the EB facility transistors were calibrated against alanine transfer standard dosimeter supplied by Risø High Dose Reference Laboratory. Polystyrene phantoms from Risø Laboratory holding one pack of alanine dosimeters and four transistors, were placed in stainless steel trays generally used to carry product boxes. These phantoms were then irradiated to different nominal doses from 5 up to 45 kGy.

6. In-plant calibration and use of power transistors for process control of gamma and electron beam facilities Field trial irradiations For the field trial irradiations at gamma plant, eight packs of dosimeters were attached to product pallets at a reference position and irradiated during production runs. Each dosimeter pack contained four transistors and a Red 4034 PMMA routine dosimeter in close contact, without shielding each other. Evaluation of the absorbed dose from the PMMA dosimeters was done by Gammarad using their own dosimetry system traceable to NPL. Field trial irradiations at EB facility were performed by placing dosimeter packs, each consisting four transistors in close contact, on top of nineteen product boxes evenly distributed during the production run. The routine dose measurements were done with polystyrene calorimeter, traceable to Risø High Dose Reference Laboratory, according to the usual irradiation procedure used by the facility.

7. In-plant calibration and use of power transistors for process control of gamma and electron beam facilities Field trial irradiations Since the routine calorimeters were located at different distance from the accelerator window, as compared to the transistors, a separate experiment was carried out to check the influence of this difference on the dose measurements. In a tray, along with a routine polystyrene calorimeter another polystyrene calorimeter was placed at the same height as the transistors. The tray was then sent through the irradiation zone together with other product boxes during the field trial. Both the calorimeters registered almost the same dose: 28.44 kGy and 28.48 kGy. This allowed us to compare the dose measured by the calorimeters and by the transistors during routine irradiations. Measurements of the parameter T for the irradiated transistors were done at constant temperature (25 °C) and the changes of T were plotted as (1/T)=1/T-1/T0, where T0 and T are the pre-irradiation and post-irradiation values respectively, against the dose registered by the reference standard ECB and transfer standard alanine dosimeters. Portable instrument used to measure a physical parameter T directly related to the carrier lifetime t. Resistive load switching test circuit for measuring the T parameter.

8. In-plant calibration and use of power transistors for process control of gamma and electron beam facilities Results 1Response curves The results of the calibration at the gamma plant and at the EB facility are shown in figures respectively. These data were used to establish the calibration curves following the least squares fitting procedure. The polynomial functions were selected according to the correlation coefficients and distribution of the percentage residuals. The calibration curves thus obtained were then used for response evaluation of the transistors irradiated together with other types of routine dosimeters used in the plants during actual production runs. GAMMARAD response curve (gamma irradiation plant) GAMBRO DASCO response curve (electron beam irradiation plant)

9. In-plant calibration and use of power transistors for process control of gamma and electron beam facilities Results 2Comparison of dose mesured by transistors and routine dosimeters Linear regression was applied to compare the absorbed dose results obtained in the gamma plant, measured by the transistors and by the Red 4034 PMMA dosimeters (Fig. 10). The overall combined uncertainty associated to the Red PMMA is 6% at 95% confidence level, as reported by Gammarad. As for the electron irradiation trial, the dose values determined by the irradiated transistors placed on the top of nineteen boxes in non consecutive trays are plotted against the average dose measured by the polystyrene calorimeters during the trial (Fig. 11). The overall combined uncertainty associated with the polystyrene calorimeter is 3% at 95% confidence level, as reported by Gambro Dasco. GAMMARAD results (gamma irradiation plant) GAMBRO DASCO results (electron beam irradiation plant)

10. In-plant calibration and use of power transistors for process control of gamma and electron beam facilities Conclusions The tests conducted on this type of transistors show the good performances of this device in the production irradiators and if they are calibrated in the plant they are reliable dosimeters thus confirming that they can be used as routine dosimeters in the dose range of 5 to 45 kGy under production irradiation conditions at high activity gamma and high energy, high power electron beam facilities. As shown in figure, the slope of the line is close to unity (0.972) with a correlation coefficient of 0.992, indicating that the doses measured in the gamma plant were in agreement within ±2.8%. Similarly the results of the transistors irradiated at EB facility during actual production runs demonstrate the good agreement among them (0.6 kGy standard deviation) and with the dose measured by the calorimeters (only 2.8% higher than the average value measured by the calorimeter). However, it has to be pointed out that these transistors are not suitable for dose measurements in EB irradiation facilities with energy <2 MeV. Based on the response values of the transistors irradiated in the gamma facility for routine measurements, the coefficient of variation is found to be 4.5% (1) even including a few outliers. For electron irradiation, this value is much better, namely 2.1% (1) confirming the results obtained previously under standard laboratory conditions.