Metrologia 48 (2011) Tech. Suppl. 06010 Key comparison BIPM.RI(I)-K2 of the air-kerma standards of the ENEA-INMRI, Italy and the BIPM in low-energy x-rays D T Burns, C Kessler, P Roger, M P Toni*, M Pinto*, M Bovi*, G Cappadozzi*, C Silvestri* Bureau International des Poids et Mesures, Pavillon de Breteuil, F-92312 Sèvres Cedex * Istituto Nazionale di Metrologia delle Radiazioni Ionizzanti, C.R. Casaccia, c.p. 2400 Rome, Italy Abstract A key comparison has been made between the air-kerma standards of the ENEA-INMRI, Italy and the BIPM in the low-energy x-ray range. The results show the standards to be in agreement at the level of the standard uncertainty for the comparison of 1.9 parts in 10 3 . No significant trend with radiation quality is observed. The results are analysed and presented in terms of degrees of equivalence, suitable for entry in the BIPM key comparison database. 1. Introduction A direct comparison has been made between the air-kerma standards of the Istituto Nazionale di Metrologia delle Radiazioni Ionizzanti (INMRI) of the Agenzia Nazionale per le Nuove Tecnologie, l’Energia e lo Sviluppo Economico Sostenibile (ENEA), Italy and the Bureau International des Poids et Mesures (BIPM) in the x-ray range from 10 kV to 50 kV. The comparison took place at the BIPM in January 2011 using the reference conditions recommended by the CCRI [1]. 2. Determination of the air-kerma rate For a free-air ionization chamber standard with measuring volume V, the air-kerma rate is determined by the relation ∏ − = i i k g e W V I K air air air 1 1 ρ & (1) where ρ air is the density of air under reference conditions, I is the ionization current under the same conditions, W air is the mean energy expended by an electron of charge e to produce an ion pair in air, g air is the fraction of the initial electron energy lost through radiative processes in air, and Π k i is the product of the correction factors to be applied to the standard. The values used for the physical constants ρ air and W air /e are given in Table 1. For use with this dry-air value for ρ air , the ionization current I must be corrected for humidity and for the difference between the density of the air of the measuring volume at the time of measurement and the value given in the table 1 . 3. Details of the standards Both free-air chamber standards are of the conventional parallel-plate design. 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 1 For an air temperature T ~ 293 K, pressure P and relative humidity ~50 % in the measuring volume, the correction for air density involves a temperature correction T / T 0 , a pressure correction P 0 / P and a humidity correction k h = 0.9980. In addition, a factor 1.0002 is included to account for the compressibility of dry air between T ~ 293 K and T 0 = 273.15 K. 1/10