Uncertainties associated with the activity measurements of 55 Fe by LSC-TDCR

Transcription

1 Uncertainties associated with the activity measurements of 55 Fe by LSC-TDCR Cristina Wätjen, *Philippe Cassette, Maria Sahagia IFIN-HH- Romania, *LNHB-France

2 The following budget of uncertainties for 55 Fe was reported by IFIN-HH in the comparison from 2006 Component Value, % Counting statistics 0.4 Weighing 0.1 Background 0.5 Adsorption 0.1 Impurities Input parameters and statistical model 0.25 Decay scheme parameters 0.2 Half life 0.02 Combined uncertainty 0.73 IFIN-HH

3 Combined uncertainty Most important type A uncertainties including counting statistics 0.2%...0.7% (4 vials) Calculated as standard deviation of the radioactive concentration for the measured vials. Second big source significant fluctuation of background counting (not observed during the counting of the samples).

4 The uncertainty of the Mettler M5 balance used for preparing the vials Weighing component. HPGe spectrometer with the minimum detectable energy 50 kev the impurity check no low energy X and gamma-ray emitters being possible to be detected. 60 Co impurity (activities ratio 60 Co/ 55 Fe of determined with an uncertainty of 60%) was found but we suppose that it came from a contamination of the HPGe system. So, we took an uncertainty of %, if the impurity really exists there.

5 Input parameters and statistical method detailed explanation in the ICRM paper IFIN-HH From the simplified diagram of Fe-55 decaying path to the ground level of Mn-55 by electron capture 3 kinds of events: K shell electron capture when the energy is absorbed, (Auger electrons or absorbed K X rays) releasing 6 kev in scintillator. L shell electron capture releasing about 600 ev in the scintillator. M-N- shells electron capture or KX ray escape releasing less than 100 ev in the scintillator.

6 Events releasing 6 kev in scintillator detection efficiency 60% For 600 ev only 2%. For 100 ev it is negligible. K-shell electron capture events + probability of absorbtion of K X rays in the scintillator 0.2% of uncertainty for the decay scheme.

7 Computational codes used for calculation of the activity: Fe55-4 and Fe55-5. Monte Carlo procedure contribution of the decay scheme parameters: each input data = arithmetic mean and standard deviation. Detection efficiency calculation 1000 times by an iterative process, using Gaussian random fluctuations of each of these parameters. At the end, the detection efficiency is the arithmetic mean of these 1000 values and the uncertainty of the detection efficiency due to input parameters is the empirical standard deviation of these 1000 values.

8 For the half life we took a 0.02% uncertainty, using the half life and its uncertainty recommended in the comparison Form. We didn t consider the influence of the kb value, even if the cases of 3 H and 55 Fe can be considered similar taken into account the mean energy transferred in the scintillator. In the 3 H case, the influence of the kb value is much more important due to the continuous electron energy spectrum, by comparison with the discrete, close energies of X-rays and Auger electrons of 55 Fe.

9 Conclusion: IFIN-HH The TDCR method is very well suited to the standardization of pure low-z electron capture radionuclides like 55 Fe and it is also providing a low uncertainty.