Re-evaluation of the BIPM international standards for air kerma in x-rays

Abstract

A re-evaluation of the BIPM standards for air kerma in low- and medium-energy x-rays has been made. The changes to the air-kerma rate determination arise from three sources: (i) the results of Monte Carlo calculations of diaphragm correction factors for the standards, (ii) a measurement of the effect of scatter from the diaphragm support and (iii) a re-evaluation of the electron-loss corrections for the medium-energy standard. Scatter from the diaphragm support explains a long-standing trend observed in the results of international comparisons for medium-energy x-rays. The new standards were implemented on 1 September 2009.

Key comparison BIPM.RI(I)-K2 of the air-kerma standards of the NRC, Canada and the BIPM in low-energy x-rays

“…These values are given in Table 9 along with the standard uncertainty of the distribution,  dist , and the standard uncertainty of each mean value,  mean . Also given in the table are the results of the previous, direct comparison of the NRC and BIPM standards [5], revised for the published changes made to the BIPM standard in 2003 [3] and in 2009 [4]. The results and combined uncertainties are discussed in Section 10.…”

“…The measuring volume V is defined by the diameter of the chamber aperture and the length of the collecting region. The BIPM air-kerma standard is described in [2] and the changes made to certain correction factors in October 2003 and September 2009 given in [3,4] and the references therein. Details of the NRC standard are given in [5].…”

SIM.RI(I)-K3 comparison of calibration coefficients at radiotherapy level for orthovoltage x-ray beams

“…The results for the NIST in the BIPM key comparison database (KCDB), based on the NIST/BIPM comparison of 2003 [5], are updated for the re-evaluation of the BIPM international standards for air kerma in x-rays made in 2009 [6]. For consistency, therefore, the N K values supplied by the CNEA and the LNMRI, as shown in Tables 9 to 12, must also be updated for this change, since both NMIs are traceable to the BIPM.…”

“…It is these updated results that are used for Table 13, which shows for each quality the ratios of each calibration coefficient taken from Table 9 through Table 12 relative to the NIST mean value found in Table 5. Table 14 provides the results for the most recent NIST/BIPM comparison [2], updated for the BIPM 2009 revision [6]; it is these results that form the basis of the degrees of equivalence for the present comparison. Table 15 lists the product of the ratios of the calibration coefficients for each chamber to the NIST mean value and the NIST/BIPM 2003 comparison result.…”

“…Using multiple transfer instruments (designated by subscript p), Table 14, a relative uncertainty for the ratio of calibration coefficients of 0.37 % and updated by the BIPM 2009 revision to 0.36 % [5,6]. From their table of the standard uncertainties of the standards, reproduced here in Table 17, the Type A relative uncertainty for the NIST standard is 0.23 % and for the BIPM standard is 0.05 %.…”