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Blind test of the alanine dosimetry system

  • Fundamentals of Metrology

To check the reliability of the dosimetry system based on alanine/ESR, a blind test was carried out with the support of the BIPM.

Nine alanine probes were irradiated with different doses between 10 Gy and 20 Gy at the BIPM’s 60Co reference field. The doses applied were not indicated at first.

Each probe consists of four alanine pellets which are assembled to form a cylinder; for the purpose of the irradiation, these are impermeably encapsulated in 0.2 mm thick polyethylene foil. The probes were analyzed at PTB. The dose values were determined on the basis of PTB’s primary standard for absorbed dose to water. The measured dose values and their uncertainties were then transmitted to the BIPM.

On average, the nine dose values determined by PTB are 0.27 % lower than those indicated by the BIPM. According to the results of the key comparison BIPM.RI(I)-K4, a difference of -0.39 % would have had to be expected; the doses determined were, thus, 0.12 % higher than expected. Hereby, the scattering of the nine values of the ratio of the determined dose to the indicated dose amounts to 0.09 % (relative standard deviation of the single measurement), the relative uncertainty of the measurement results lies (depending on the dose) between 0.4 % and 0.45 %, including the uncertainty of the primary standard. Hence, within the measurement uncertainties, the agreement is very good.

The results are shown in Fig. 1 as the ratio of the determined dose to the indicated dose, as a function of the indicated dose. The error bars represent the (standard) uncertainty component which is solely contributed by the ESR measuring system. The uncertainty of the primary standard is not included.

Figure 1 : Ratio of determined dose DPTB to indicated dose DBIPM as a function of the indicated dose for the blind test of the alanine secondary standard measuring system. The error bars represent exclusively the standard uncertainty of the alanine/ESR measuring system; the uncertainty of the primary standard is not included.


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