Activity measurement of 55 Fe within the scope of the BIPM comparison 2006

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1 Activity measurement of 55 Fe within the scope of the BIPM comparison 2006 Karsten Kossert PTB, Department 6.1 Radioactivity BIPM Workshop 2 on CCRI(II) Activity Uncertainties and Comparisons, September 2008, BIPM, Paris Physikalisch-Technische Bundesanstalt

2 Overview Methods: 4πβ-γ coincidence counting using Mn-54 as a tracer (measurements carried out by Reiner Klein) LSC CIEMAT/NIST with H-3 as a tracer LSC CIEMAT/NIST with Mn-54 as a tracer Photon spectrometry (Si(Li), coaxial, 16 mm diameter 1 cm 3 ), no impurity could be detected

3 Coincidence counting Physikalisch-Technische Bundesanstalt

4 4πβ-γ coincidence counting pressurized proportional counter, 1 MPa Argon methane (90/10), part of 4πβ-γ coincidence system with an NaI detector (76 mm ø 76 mm) efficiency variation by means of a threshold adjustment with a programmable discriminator

5 4πβ-γ coincidence counting Description of the method (simplified) weighed portions of the Fe-55 solution plus weighed portions of a Mn-54 solution with known activity concentration were used to prepare solid sources on pretreated VYNS foils (15μg/cm 2 ) coated with Au-Pd measurement yields: N β = N β,fe +N β,mn N γ = N γ,mn N c = N c,mn (only Mn-54 contributes) (only Mn-54 contributes) ε β,mn N c / N γ N β,fe = N β - N β,mn = N β A Mn N c / N γ

6 4πβ-γ coincidence counting Description of the method (simplified, continued) N β,fe = A Fe (P K [ω K ε X,K +[1- ω K ] ε Auger,K ] +P L1 [ω L1 ε X,L1 +[1- ω L1 ] ε Auger,L1 ] +P L2 [ω L2 ε X,L2 +[1- ω L2 ] ε Auger,L2 ] +P L3 [ω L3 ε X,L3 +[1- ω L3 ] ε Auger,L3 ] +P M [ω M ε X,M +[1- ω M ] ε Auger,M ]) (efficiencies include rearrangement processes) Plot with N β,fe vs. (1-N c /N γ )/N c /N γ (1-ε β,mn )/ε β,mn

7 4πβ-γ coincidence counting Physikalisch-Technische Bundesanstalt

8 4πβ-γ coincidence counting Description of the method (simplified, continued) Since E XK,Fe > E XK,Mn and E Auger,K,Fe > E Auger,K,Mn we have ε X,K,Fe > ε X,K,Mn and ε Auger,K,Fe > ε Auger,K,Mn Assuming that all ε 1 would yield: N β,fe = A Fe (P K +P L1 +P L2 + )=A Fe But: The efficiency for L1 processes and higher shells can be very low. Assuming that only K events contribute (worst case) yields: A Fe =P K,Mn /P K,Fe N β,fe N β,fe Since we have no (reliable) information on the efficiency contributions of higher shells a relative uncertainty component of 0.5% is taken into account.

9 4πβ-γ coincidence counting Relative standard uncertainty: 1.50% QUANTITY Q Relative uncert. of Q Type (A/B) statistics A 0.3% weighing B 0.18% background B 0.21% dead time B <0.02% resolving time B <0.02% Gandy effect B 0.01% pile-up B --- decay data B --- quenching B --- tracer B <0.01% extrapolation of efficiency curve B 1.35% calibration factor B --- half-life B <0.01% impurities B <0.05% adsorption B - self-absorption B 0.5% Relative uncert. of act. conc. Comment 2 weighing procedures (Mn-54+Fe-55) Mn-54 contribution to counting rate in the beta channel

10 4πβ-γ coincidence counting Other problems and questions: The approach is based on the assumption that the distribution of the elements Fe and Mn within the samples are identical. It would be nice to have a theoretical basis for the non-linear curve which is used for the extrapolation.

11 Liquid scintillation counting Physikalisch-Technische Bundesanstalt

12 Liquid scintillation counting The KL1L2L3M atomic rearrangement model with the photoelectric correction has been applied. Specifics: calculation were done with the EMILIA code we used H-3 as well as Mn-54 as tracer

13 CIEMAT/NIST computer codes β, β + ok β-γ ok β-γ ok EFFY EBEGA CN200X EC VIASKL model KLM EC-γ EMI EC-γ CN200X EC-γ EMI2 model KLM model KLM model KL 1 L 2 L 3 M 1. The photoionization of the scintillator is not taken into account (important if Z < 30). 2. The atomic rearrangement models are not accurate enough for Z 30. Efficiencies are too large

14 Electron-capture nuclides Atomic rearrangement models : KLM: KLMN: KL 1 L 2 L 3 M: KL 1 L 2 L 3 M 1 M 2 M 3 M 4 M 5 N: 22 pathways 229 pathways 264 pathways > 50,000 pathways (MC model)

15 Electron-capture nuclides Recent improvements: low-z EC-γ EMILIA ok KL 1 L 2 L 3 M model in combination with photoelectric correction Results are satisfactory for low-z nuclides (e.g. 54 Mn, 55 Fe-55) Grau Carles, CPC 174 (2006) 35; Kossert & Grau Carles, ARI 64 (2006) 1446 EC-γ ok MICELLE Stochastic KL 1 L 2 L 3 M 1 M 2 M 3 M 4 M 5 N atomic rearrangement model. Results are satisfactory for nuclides like 109 Cd, 125 I Grau Carles, CPC 176 (2007) 305; Kossert & Grau Carles, ARI 66 (2008) 998. Physikalisch-Technische Bundesanstalt

16 Photoelectric correction Example: Photoelectric interaction of X-ray with energy E XY with atom of the scintillator e.g. EMI considers E XY Q(E XY ) but we must consider the binding energy of the photo electron and we have rearrangement processes after the emission of a photo electron => If the photo-electron stems from the K-shell: (E XY -E K s )Q(E XY -E K s ) + rearrangement terms Physikalisch-Technische Bundesanstalt

17 Application of EMILIA Nuclide: 55 Fe Tracer: 3 H Kossert & Grau Carles, ARI 64 (2006) 1446

18 Application of EMILIA Nuclide: 55 Fe Tracer: 54 Mn Kossert & Grau Carles, ARI 64 (2006) 1446

19 LSC measurements Spectrometer: Wallac ml Ultima Gold + 1 ml water Quenching agent: Nitromethane Tracer: H-3 and Mn-54 from primary activity standardization (traceable to international comparisons) Physikalisch-Technische Bundesanstalt

20 LSC measurements General problems: model dependence: ionization quenching (stopping power and kb) dependence on the tracer activity Both problems should be reduced when Mn-54 is used as tracer, but we observed a slope when efficiency variation was applied Fe, CN, H-3 Tracer Fe, CN, Mn-54 Tracer (a i -a mean )/ a mean in % (a i -a mean )/ a mean in % ε tracer ε tracer Physikalisch-Technische Bundesanstalt

21 Liquid scintillation counting, H-3 Relative standard uncertainty: 1.54% QUANTITY Q Relative uncert. of Q Type (A/B) statistics A 0.095% weighing B 0.106% background B <0.05% dead time B 0.1% resolving time B - Gandy effect B n.a. pile-up B n.a. decay data B 0.503% quenching B 0.4% tracer B 1.38% extrapolation of efficiency curve B - calibration factor B n.a. half-life B <0.01% impurities B <0.05% adsorption B - Relative uncert. of act. conc. Comment including H-3 decay data SQP(E) measurement and ionization quenching (model dependence) no extrapolation no impurity detected time and duration of measurements B <0.01%

22 Liquid scintillation counting, Mn-54 Relative standard uncertainty: 1.11% QUANTITY Q Relative uncert. of Q Type (A/B) statistics A 0.1% weighing B 0.106% background B <0.05% dead time B 0.1% resolving time B - Gandy effect B n.a. pile-up B n.a. decay data B 1.0% quenching B 0.18% tracer B 0.39% extrapolation of efficiency curve B - calibration factor B n.a. half-life B <0.01% impurities B <0.05% adsorption B - Relative uncert. of act. conc. Comment combined with uncertainty component for model dependence SQP(E) measurement and ionization quenching (model dependence) no extrapolation no impurity detected time and duration of measurements B <0.01%

23 Comparison of results Reference date 1. Dec. 2005, 0h00 UTC Coincidence counting (509.1 ± 7.7) kbq/g LSC, tracer H-3 (509.7 ± 7.9) kbq/g LSC, tracer Mn-54 (511.7 ± 5.7) kbq/g Weighted mean taking correlations into account: (510.5 ± 4.3) kbq/g relative standard uncertainty of the final result 0.84%

24 Summary and Outlook The three results determined at PTB are in good agreement. In the near future we will apply the new models with the TDCR technique. A new stochastic model is available which is currently extended. It is desirable to implement the photoelectric correction to this new approach.

25 Conclusions for Fe-55 For LSC the photoelectric correction must be taken into account. The KLM atomic rearrangement model is not adequate. The L sub-shells must be taken into account. Although the influence of the new models on the TDCR method is expected to be low, the effects should be investigated.

26 Thank you for your attention K. Kossert Physikalisch-Technische Bundesanstalt

BUREAU INTERNATIONAL DES POIDS ET MESURES

1 BUREAU INTERNATIONAL DES POIDS ET MESURES International comparison of activity measurements of a solution of 3 H (January 2009) Participating laboratory: T ½ = (4 496.862 d; u = 9.131 d)* Ampoule number