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Determination of the activity of Ce-139 and Y-88 within the scope of international comparison measurements

29.09.2008

The Mutual Recognition Arrangement (MRA) under the auspices of the Comité International des Poids et Mesures (CIPM) is a worldwide agreement on the cross-border mutual recognition of calibration certificates issued by national metrology institutes for all commercially or socially relevant measuring quantities. The agreement requires that the participants, besides proving the efficiency of their quality management system, shall regularly take part in comparison measurements in order to attest their ability to perform the measurements with the required uncertainties.

The international reference system SIR for activity measurements was established in 1976 at the BIPM. The participants thereby fill ampoules, which are provided by the Bureau International des Poids et Mesures (BIPM), with their national activity standards in the form of solutions or gases, and send these to the Bureau in Paris. There, the ampoules are measured in an ionization chamber and compared with a long-lived 226Ra standard. By tracing back the results to a radium reference source and thanks to the corresponding long-term stability of the measuring system, the results of the participants can be compared with each other over periods of several years to several decades. Up to now, 905 samples of 63 different radionuclides have been submitted and measured in the ionization chambers of the SIR (as of June 2008). At present, the database contains 662 independent entries.

In the period under report, PTB submitted ampoules with solutions of the radionuclides 139Ce and 88Y to SIR. 88Y is an important calibration standard, e.g. for semiconductors in the field of gamma spectrometry, due to its comparatively high γ energies of 898 keV and 1836 keV. With γ radiation with an energy of 166 keV and the corresponding conversion electrons with energies of 127 keV or 160 keV, 139Ce is also an important calibration standard.

At the same time, these two nuclides as electron capture emitters offer a good possibility of testing new models in the field of liquid scintillation counting (LSC) and its applicability. The activity determination according to the CIEMAT/NIST method has been playing an ever greater role for years since progress in modeling increasingly enables activity determination for electron capture emitters [1,2]. These improved theoretical fundaments will also benefit the TDCR method which is presently being established within the Working Group as a new absolute measurement method.

The specific activity of the solution was determined both by means of the 4πβ-γ coincidence method in the proportional counter operated at ambient pressure or the pressurized proportional counter and with the LSC method according to the CIEMAT/NIST method, whereby the activity was established on the basis of the coincidence measurements. The result of the comparison measurements allows us to draw conclusions with regard to the applicability of the new models and the application spectrum of the LSC method can thus be enlarged by a further class of nuclides.

At PTB, the results of the activity measurements are kept as calibration factors for a 4π ionization chamber. For this purpose, similar to the SIR system, the measurements of the ionization current are related to the ionization current of a long-lived 226Ra reference source and are - as relative measurements - thus less sensitive to modifications in the detector system or in the subsequent electronic system. Even if several components are replaced, the reproducibility of the measurement results remains ensured - SIR’s whole read-out electronic unit, which was newly set up a few years ago, gives proof of this.

Literature

  1. Grau Carles, A.:
    MICELLE, the micelle size effect on the LS counting efficiency,
    Comput. Phys. Commun. 176 (2007) 305.
  2. Kossert, K., Grau Carles, A.:
    Study of a Monte-Carlo rearrangement model for the activity determination of electron-capture nuclides by means of liquid scintillation counting,
    Appl. Radiat. Isotopes 66 (2008) 998-1005.