**Establishment of the SI Activity Unit (Becquerel) by key comparisons.**

*The Activity Unit is established individually for each radionuclide, after the evaluation of the key comparison result, which is expressed as the Key Comparison Reference Value – KCRV*.

Four methods are used to calculate the KCRV (Ratel.2007): arithmetic and weighted mean, median and weighted median. The most adequate variant is selected; in most cases, the arithmetic mean is preferred. The outliers are established after applying the exclusion criteria. The KCRV and its uncertainty are approved by the CCRI(II) and may be modified in time, after accumulating new results for the respective radionuclide. The "outliers" are not eliminated from KCDB, but their difference from KCRV must be taken into account when reporting the calibration uncertainties.

When both the *SIR-BIPM.RI(II)-K1.Radionuclide* and the *CCRI(II)-K2. Radionuclide*  comparison results are registered for the gamma-ray emitters, a link is established between the two types of comparisons, through the measurement in the BIPM ionization chamber of the ampoules sent for the *K2* comparison.

*Equivalence.* An equivalence matrix is calculated. Two main quantities are reported and compared: Difference between the individual result and KCRV, (*D*i) and its uncertainty (*U*i), for a coverage interval *k* =2. The ratio of these quantities is the measure of degree of equivalence. The CCRI(II) approved "Draft B" is published in the KCDB and in *Metrologia, Technical Supplement issues*. IFIN-HH, RML, is registered in KCDB with 28 radionuclides.

*Equivalence validity.* A validity interval was adopted by consensus. Consequently, in order to maintain the equivalence for some radionuclides, IFIN-HH repeated the comparison. It participated in 1983 and in 2007 at the *SIR-BIPM.RI(II)-K1*.Co-60 comparisons. As for 134Cs, the participations were: *CCRI(II)-K2* in 1978 and *SIR-BIPM.RI(II)-K1* in 2006 and for 137Cs, the participations were: *CCRI(II)-K2* in 1982 and *SIR-BIPM.RI(II)-K1* in 2009.

Example: *Analysis of the IFIN-HH, RML result at the medical 131I key comparison SIR-BIPM.RI(II)-K.1.I-131, 2007* (Ratel et al.2008). *KCRV* = (40400 ±40) *MBq*; *IFIN-HH* value = (40371 ± 139) *MBq*; *D*i =0.07%; *U*i=0.65% . Consequently, the IFIN-HH, RML's result was taken into account in the calculation of the *KCRV*.

#### **SI Activity Unit (Becquerel) for other radionuclides.**

A matrix of radionuclides, codified by colors, according to the standardization difficulty by method, was established by the CIPM-CCRI(II) Key Comparison Working Group (KCWG), within the document Grouping Criteria Radionuclides for Supporting CMCs (Karam.2007). It can be used as follows. A red coded radionuclide measured by LSC, such as 3H, supports 89Sr, 90Y, LSC measured, and green color coded.
