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Determination of the response of the alanine dosimeter for high-energy electrons in collaboration with the Swiss Federal Office of Metrology (METAS)

23.09.2011

The response of the alanine dosimeter with regard to the absorbed dose to water Dw for high-energy electron radiation has been determined in collaboration with the Swiss Federal Office of Metrology (METAS). It lies within the relative standard measurement uncertainty of 1 %, regardless of the energy. The resulting correction factor for MeV electron radiation amounts to kE= 1.012 ± 0.010.

The response of the alanine dosimeter with regard to the absorbed dose to water Dw for high-energy electron radiation has been determined in collaboration with the Swiss Federal Office of Metrology (METAS).

To date, the most precise measurements have been published by the Canadian Institute for National Measurement Standards (NRC) [1]. The absorbed dose to water for electron radiation was thereby determined via the local primary standard (water calorimeter) with 60Co and ionisation chambers using protocol TG-51 [2]. In the above-mentioned publication, the influence of the support used by NRC for the detectors was stated as being 0.3 %. Alanine probes of PTB were used to investigate this effect. These probes were shrink-wrapped impermeably in 0.2 mm thick polyethylene (PE) foil, which was a good approximation of a fully "naked" irradiation. The irradiation took place at METAS’ electron accelerator. The absorbed dose to water for electron radiation was determined using the Swiss primary standard which is based on Fricke dosimetry. The probes were analysed at PTB; the calibration of the ESR spectrometer was traced back to PTB’s primary standard (water calorimeter [3]) with 60Co.

The total uncertainty for the response with regard to Dw, relative to 60Co, of 1 % which was thereby achieved is, thus, only slightly lower than that obtained by NRC. For electron energies of 6 MeV, 9 MeV, 12 MeV, 15 MeV and 20 MeV (nominally), it turned out that, within the uncertainty, the response does not significantly depend on the energy. The mean value of the relative response for the five energies considered (after a correction of approx. 0.1 % for the influence of the PE foil) is r = 0.988 ± 0.010. For the corresponding radiation qualities, a Monte Carlo simulation yielded a mean value of rMC = 0.989. Both values are in excellent agreement with NRC’s results. Hereby, rNRC = 0.987 ± 0.011 is the mean value of the data determined by NRC, minus the correction due to the influence of the support. The agreement is all the more remarkable as the results have been obtained in two different manners: METAS/PTB: Fricke primary standard for electrons + water calorimeter for 60Co; NRC: TG-51 for electrons + on the basis of water calorimetry for 60Co.

The current data are shown in Figure 1 for comparison with those obtained by NRC. The response r with regard to Dw, relative to 60Co, is hereby stated in MeV as a function of the nominal energy (acceleration voltage). The error bars represent the total uncertainty, i.e. the standard uncertainty including the uncertainties of the primary standards.

Figure 1 : Response of PTB’s alanine dosimeter with regard to Dw for high-energy electrons, relative to 60Co, as a function of the acceleration voltage in MeV. (For further explanations, please cf. text).

Literature

  1. Zeng, G. G. et al., Phys. Med. Biol. 50 (2005), pp. 1119-1129
  2. Almond, P. R. et al., Med. Phys. 26 (9), (1999), pp. 1847-1870
  3. Krauss, A., Metrologia 43 (2006), pp. 259-272