A direct comparison of the standards for air kerma of the Instituto Nacional de Investigaciones Nucleares (ININ), Mexico, and of the Bureau International des Poids et Mesures (BIPM) was carried out in the 137Cs radiation beam of the BIPM in February 2015. The comparison result, evaluated as a ratio of the ININ and the BIPM standards for air kerma, is 1.0048 with a combined standard uncertainty of 2.0 × 10-3. The results are analysed and presented in terms of degrees of equivalence for entry in the BIPM key comparison database. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCRI, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

The results of the comparison between SSDL-ININ and SSDL-CPHR (pilot laboratory) demonstrates the competence of the SSDL-ININ for the performance of the KR in 192Ir. The RININ/CHPR ratio for the calibration coefficients is 0.989 ± 0.005. The comparison uses three SI-HDR 1000-Plus as transfer chambers, series: A02423, A941755 and A973052. CPHR used a secondary standard PTW 3304 chamber, s/n 154, calibrated at PTB and ININ employed a secondary standard SI-90008 s/n A963391, calibrated at NPL. To determine KR, the SSDL-CPHR used the IAEA TEC-DOC-1274 and the SSDL-ININ used the IPEM (UK) code of practice. The latter uses a correction factor by source's geometry, ksg. The results show that both codes are equivalent; however, for the use of well chambers in the highlands or in locations with reduced atmospheric pressure, it is needed to apply an additional factor k'P, or, to design a well chamber with air-equivalent walls for the application of the conventional kPT.

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By using the statistical techniques of ANOVA: mean test and regression, it was found that the NKR calibration factor of Standard Imaging (SI) model HDR 1000 plus chambers presents a quadratic dependence with the reference air kerma rate KR (from 6.9 mGy h-1 to 43.9 mGy h-1). In order to understand and correct this dependency one model is presented for total recombination: ks=I300/I150=1+kini+ kd +kvol I300+kscreenI300 2 , where kini is the initial recombination, kd the thermal diffusion recombination, kvol the volumetric recombination and kscreen the screening for the currents/charges collected at the potential differences of 300 and 150 V. In conclusion, the total recombination ks is composed by one kini with a constant contribution of 0.019%, one kd contribution of 0.017%, one kvol I300 contribution from 0.022% to 0.138%, and the kscreenI300 2 effects from 0.002% to 0.09% in the range of KR rate above. However, when this model for ks is applied to try to correct the quadratic dependence of the NKR vs KR , explicitly there is no improvement in the variation range of 0.5% of the NKR vs KR .Nonetheless, it allows to obtain NKR values consistent with a uc ≤ 0.7 %, which is less than 1.25% reported in the literature by ADCL or SSDL.