Intercomparison of NMI air kerma standards for ISO 4037 narrow spectrum series radiation qualities

Project Description

It is the objective of the proposed project to extend the mutual traceability between the air kerma standards of NMIs to ISO 4037 narrow spectrum series radiation qualities. The project is intended for NMIs having participated in recent years in a BIPM comparison of their primary standards for BIPM radiation qualities.

Two transfer chambers, a large one ISO narrow spectra series and a small one for both series of radiation qualities, will be calibrated by each of the participating laboratories for a number of previously selected radiation qualities. In this exercice the dosimetric procedure lower than those at therapy level will be tested together with the influence of larger radiation field sizes. Important parameters are extrafocal radiation, filter inhomogeneities and scattered radiation generated in the filters. The properties of the radiation fields with respect to the three parametrers do not matter substantially as long as the cross-sectional area of the aperture of the free air chamber is comparable to the cross-sectional area of the dosimeter to be calibrated. This is the case for the calibration of the mammography chamber and usually also for calibrations with BIPM radiation qualities. However, with a large protection level chamber of a volume of typically 1 l these effects may play an important role. The program of measurement will be designed in a way to allow conclusions concerning the origin of possible differences

Remark: The mammography radiation qualities will no longer be covered in this project. (see EURAMET Project 526)

Final Report 2009-02-18

The comparison has been finished and the results are availbale in the KCDB.

For the first time results are presented of an indirect comparison of ten national standards for air kerma for ten radiation qualities of the ISO 4037 narrow spectrum series in the range from 30 kV to 300 kV. Nine of the ten participants maintain primary air kerma standards, one is traceable to PTB. The comparison was conducted in the time period from February 2004 until October 2005. For each radiation quality, the results are analysed in terms of the degrees of equivalence of each national standard with respect to the key comparison reference value. These data are suggested to form the basis of the results entered into the BIPM key comparison database for comparison EUROMET.RI(I)-S3. In addition, results are presented of differences in the results due to different realizations of beam qualities at the participants sites and of the influence of different transfer chamber sizes on the variance of the comparison results.

Progress report 2005-11-24

All participants have completed their measurements and the results have been received by the pilot institute. The comparison report is in preparation.

The comparison was aimed at two main points:

• Indirect comparison of the primary air kerma standards for the ISO 4037 narrow-spectrum series (30 kV - 300 kV)
• Comparison of calibration coefficients for this qualities of three different sized chambers with diameters 4.4 cm (TK30), 14 cm (LS01) and 27.5 cm (LS 10).

Different evaluation procedures will be used for (i) and (ii). The indirect comparison of the primary standards (i) will be based on the results obtained with the TK30, because the diameter of this chamber is closer to the aperture diameters of the primary standards and hence field size dependent effects are minimized. To examine the special influence of different field sizes, the calibration coefficients of the LS01 and LS10 chambers will be normalised to those of the TK30. The normalised results will then be analysed separately. More details will be given in the report.