COOMET Project 445: Comparison of national measurement standards of air kerma for Cs-137 gamma radiation at protection level

A COOMET supplementary comparison of the national measurement standards of air kerma for Cs-137 gamma radiation at protection level (~ 10 mGy/h) was carried out between May 2011 and February 2013. Participants were VNIIM (Russia), BelGIM (Belarus), CPHR (Cuba), GEOSTM (Georgia), INSM (Moldova), NSC-"IM" (Ukraine), SMU (Slovakia), PTB (Germany), BIM (Bulgaria), VMT/FTMC (Lithuania) and the International Atomic Energy Agency (IAEA). The PTB acted as the pilot laboratory. The comparison reference value (CRV) was obtained as the mean of the results obtained by PTB and VNIIM, both of which had previously taken part in the key comparison BIPM-RI(I)-K5. Results will be published in Appendix B of the BIPM key comparison database (KCDB) using the identifier COOMET.RI(I)-S1 as soon as the Draft B report is accepted.

The comparison was organized within an extended COOMET project (identified as Project 445/DE/08), the aim of which was not only to compare national standards but also for educational purposes. Some of the participants had taken part in such a comparison for the first time and, therefore, an introductory seminar was held at PTB in May 2011. The seminar was open for participation also to those countries of the COOMET region which did not participate in the comparison and to non-MRA signatories and countries without an approved quality management system in order to acquire measurement routine and theoretical as well as organizational knowledge for future comparisons. The main goal of this seminar was to prepare the participants for the procedure of having entries accepted for the CMC list in the metrological area of Ionising Radiation, Section I, x and gamma rays. The seminar was very successful and the 21 participants from 15 different countries (among them were delegates from Armenia, Azerbaijan, Kyrgyzstan, Tajikistan and Uzbekistan) found it to be extremely helpful.

The goal of the supplementary comparison was to confirm the calibration and measurement capabilities of the participating NMIs for air kerma calibrations at protection level Cs-137 gamma radiation. For this purpose the calibration coefficients and the corresponding uncertainties of three circulating ionization chambers with sensitive volumes of about 1000 cm3 were compared at air kerma rates of about 10 mGy/h. This type of comparison differs significantly from the established key comparison BIPM.RI(I)-K5 due to the lower dose rates and larger ionization chambers and the expected larger uncertainties. Therefore, it was decided by COOMET-TC1.9 and confirmed by the key comparison working group (KCWG) to regard this comparison as a supplementary one with the identifier COOMET.RI(I)-S1. PTB and VNIIM had previously participated in the key comparison BIPM.RI(I)-K5. Therefore, it was decided that their mean result shall be regarded as the comparison reference value (CRV). The comparison was conducted by at least three different transfer ionization chambers. Each participant determined the calibration coefficients of the three transfer ionization chambers under reference conditions. The degrees of equivalence with the CRV were evaluated based on these results. The three transfer chambers were circulated star-shaped between PTB and the participants. After each participantxs calibration, PTB performed chamber constancy checks. The comparison started in May 2011 with PTB's measurements and was completed in February 2013 with the last stability measurements at PTB.

The Draft A report of results was discussed and agreed upon by the participants during a concluding seminar within Project 445, held at PTB in September 2013. The concluding seminar was again open to non-comparison participants of those countries which had already participated in the introductory seminar described above. In addition, Igor Gomola from IAEA was invited and reported on the status of cooperation between the IAEA and SSDLs in COOMET member countries. The seminar offered the opportunity to discuss and coordinate the work of the PTB Technical Cooperation and the IAEA, together with concerned delegates from countries within the COOMET region. This second seminar within Project 445 was a great success because it resulted not only in the preparation of the Draft B report of the current comparison but also in several future comparison projects piloted by other COOMET NMIs who will assume this responsibility for the first time. These new pilots are taking advantage of the knowledge they have gained during the two seminars at PTB.

Acknowledgement:
COOMET Project 445 was strongly supported by the PTB Technical Cooperation within a project entitled "Support of regional cooperation of member states in regional metrology organizations (COOMET)" funded by the Federal Ministry for Economic Cooperation and Development, Germany. The organizational work of Annette Kögler, Katrin Hoffmann and Olga Krasota is highly acknowleged.