The CIEMAT/NIST method is the widely used method for activity determination of radionuclides using tritium as a tracer. This method is very well suited for pure-beta nuclides but, in somes cases, problems connected with the ionisation quenching parameter, kB, may cause some bias in activity determination. These problems are discussed hereafter.

   The quenching level of a scintillating source of a given radionuclide is generally determined using an external gamma-rays source. The Compton interaction spectrum recorded with the source is used to calculate a quenching index, QI. The definition of this index depends on the counter manufacturer. This index could be, for example, the Horrocks number (H*), the SQPE index or the TSIE index.
   The counting efficiency of tritium eH for a given quenching index, is determined after the measurement of a set of quenched tritium standards. The theoretical calculation of the counting efficiency of tritium leads to the determination of the figure of merit ho, related to the observed eH. The counting efficiency of the source of a given radionuclide eNuc are then calculated. In this calculation, a model of the ionisation quenching phenomenon is taken into account. This model is generally the Birks function. This calculation procedure can be resumed schematically as follows :

   In the theoretical calculation of the functions ho(eH,kB) and eNuc(ho,kB) a stable and identical value of the ionisation quenching parameter kB is accepted. The value of this parameter is chosen for a given kind of scintillator. The only variable in the calculation is ho.

On the ground of the theoretical calculation of coincidence counting efficiency as a function of ho (an example is presented in Fig.1) it appears that the procedure (1) leads to different values of counting efficiency eNuc, for the one eH, depending on the value of the kB parameter taken into account for a given source.

   For example, the calculation was performed for ⁵⁵Fe with two different kB values, for one selected eH. Two different theoretical values of counting efficiency eFe were obtained, as presented in Fig.2. Other results for ¹⁴C, ⁵⁴Mn and ⁵⁵Fe are listed in Table 1. Relative discrepancies (D) of calculated extreme counting efficiencies (for kB = 0,006 cm/MeV and kB = 0,012 cm/MeV) are given.

   Thus a systematic bias of the CIEMAT/NIST method appears if the kB value is not exactly known for a given source. One can see that, even in the case of a high counting efficiency like ¹⁴C, this bias could be around 1 or 2 %. For low counting efficiency nuclides, this bias could be ten times higher.
Things are going dramatically worse in the case of ⁵⁵Fe or ⁵⁴Mn activity measurement where the systematic errors on the activity could reach several percents.

   Günther [1] has reported an essential modification of the CIEMAT/NIST method. For ⁵⁵Fe activity measurement he used ⁵⁴Mn as a tracer instead of ³H. Good results of this approach are comprehensible on the basis of our calculation, as the similarities of the functions eMn(ho, kB) and eFe(ho, kB) in the measurement range is visible. Moreover Günther has performed additional measurements using a ⁵⁵Fe standard source for the direct determination of the ionisation quenching parameter value, kB. If this value is not exactly known, a systematic error could reach a few tenth percent, as can be seen in Table 2.

   In conclusion it seams that there could be a bias in the activity determination using the CIEMAT/NIST method if the kB parameter value is not known for a given source. This error decrease with the increase of the detection efficiency.

Bibliography

[1] E. Günther, Standardisation of the EC nuclides 55Fe and 65Zn with the CIEMAT/NIST LSC tracer method, (Conference paper No.005, ICRM’97).

Tables and figures

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