The new value for air kerma in 60Co gamma fields for radiotherapy was adopted on 1 November 2007 following the publication of the re-evaluation of the BIPM standard in Metrologia. This was in accordance with the recommendation of the CCRI. In parallel, the higher activity (CIS-Bio) 60Co source was adopted as the reference field. A new series of BIPM graphite cavity primary standards is under construction and the first chambers agree with the variable-volume chamber at the level of 2 parts in 104.
The results of a study of low pressure (60 kPa) effects on graphite cavity standards, which was carried out in conjunction with some calibrations for the ININ (Mexico), have implications for all graphite-walled standards at the level of several parts in 103, and further work is planned.
The results of a significant study of aperture effects (transmission, scatter and fluorescence) were presented to the KCWG(I) and have been prepared for publication prior to proposing a change of up to 7 parts in 103 to the medium-energy air-kerma standard at the CCRI in 2009.
The prototype graphite calorimeter for absorbed dose to water is now constructed and the first trials show that the novel design works well in practice in terms of absorbed dose to graphite. The CCRI Accelerator Dosimetry Working Group has recommended recently that the BIPM calorimeter be used in a series of bilateral comparisons at the eight NMIs that have accelerators. The calorimeter will be tested in a trial comparison at the LNE-LNHB (France) later this year before this series of on-going comparisons is launched.
The primary standard free air chamber for mammography dosimetry does not appear to be as stable as the existing low-energy free air standard and this is under investigation. The mammography comparison facility will then be completed.
Three new dosimetry comparisons have been made and three other comparison reports have been published. We have just now succeeded in obtaining a guest scientist, from the ININ (Mexico), for the project on the CCRI brachytherapy comparisons, so progress should now be made. Twenty-eight national secondary standards have been calibrated and the Quality System for calibrations successfully underwent its internal audit. Support for the IAEA continues with regular irradiations of their dosimeters for the IAEA/WHO measurement service.
Last year, only five laboratories submitted eleven ampoules to eight of the BIPM ongoing activity comparisons using the International Reference System (SIR). The implication is that the NMIs are finding it more difficult to comply with the transport regulations and may now be relying more on the “Mesurement methods grouping criteria for radionuclides”, to support their CMCs. However, as 24 old comparison results will disappear from the KCDB this year there may be a resurgence of submissions next year. Impurity activity levels were measured using the BIPM Ge(Li) gamma spectrometer for four radionuclides submitted for comparisons and a guest worker from Albania is presently calibrating the replacement HpGe spectrometer. The BIPM is grateful to the NPL (UK) for their generous donation of over 5000 glass ampoules which will assure the SIR's continuation. An internal audit of the SIR was successful and the new SIR measurement system has been adopted with the final verification due at the end of 2008.
Studies of 85Kr measurements in the SIR are continuing and the SIR Transfer Instrument has been tested for the short-lived 99Tcm radionuclide at the NPL prior to making comparisons at more distant NMIs. We have devoted much effort to the extension of the SIR for pure beta emitters and the system should be in place by the end of the present programme. The BIPM is presently piloting two CCRI key comparisons and completing the reports for some earlier radionuclide activity comparisons.
Summary
