Key comparison BIPM.RI(I)-K7 of the air-kerma standards of the CMI, Czech Republic and the BIPM in mammography x-rays

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1 Key comparison BIPM.RI(I)-K7 of the air-kerma standards of the CMI, Czech Republic and the BIPM in mammography x-rays C Kessler 1, D T Burns 1, P Roger 1, V Sochor 2 1 Bureau International des Poids et Mesures, Pavillon de Breteuil, F Sèvres Cedex 2 Czech Metrology Institute, Okružní 31, CZ Brno Abstract A first key comparison has been made between the air-kerma standards of the CMI, Czech Republic and the BIPM in mammography x-ray beams. The results show the standards to be in agreement at the level of the standard uncertainty for the comparison of 3.5 parts in The results for an indirect comparison made at the same time are consistent with the direct results at the level of 1 part in 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 Czech Metrology Institute (CMI), Czech Republic and the Bureau International des Poids et Mesures (BIPM) in the Mo/Mo mammography beams in the x-ray range from 25 kv to 35 kv. The comparison took place at the BIPM in December 2015 using the reference conditions recommended by the CCRI and described by Allisy et al (2011). An indirect comparison was also made using a thinwindow parallel-plate ionization chamber as a transfer instrument. The final results were supplied by the CMI in March 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 Wair 1 K airv e 1 g air i k i where 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 k i is the product of the correction factors to be applied to the standard. The values used for the physical constants air and W air /e are given in Table 1. For use with this dry-air value for 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 (1) 1 For an air temperature T around 293 K, pressure P and relative humidity around 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 = At the BIPM, the factor is included to account for the compressibility of dry air between T around 293 K and T 0 = K. 1/11

2 Table 1. Physical constants used in the determination of the air-kerma rate Constant Value u i a air b kg m W air / e J C a b u i is the relative standard uncertainty. Density of dry air at T 0 = K and P 0 = kpa. 3. Details of the primary 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 L-02 is described in Kessler et al (2010). The CMI standard is newly constructed and its characteristics are detailed in Solc and Sochor (2014) and in the present report. The main dimensions, the measuring volume and the polarizing voltage for each standard are shown in Table 2. Table 2. Main characteristics of the standards Standard BIPM L-02 CMI L1 Aperture diameter / mm Air path length / mm Collecting length / mm Electrode separation / mm Collector width / mm Measuring volume / mm Polarizing voltage / V Comparison procedure 4.1 The BIPM irradiation facility and reference radiation qualities The BIPM low-energy x-ray laboratory houses a constant-potential generator and a molybdenum-anode x-ray tube with an inherent filtration of 0.8 mm beryllium. A molybdenum filter of thickness mm is added for all radiation qualities. A voltage divider is used to measure the generating potential, which is stabilized using an additional feedback system of the BIPM. Rather than use a transmission monitor, the anode current is measured and the ionization chamber current is normalized for any deviation from the reference anode current. The resulting variation in the BIPM FAC-L-02 free-air chamber current over the duration of a comparison is normally not more than in relative terms. The radiation qualities used in the range from 25 kv to 35 kv are given in Table 3 in ascending order, from left to right, of the half-value-layer (HVL) measured using aluminium filters. 2/11

3 The irradiation area is temperature controlled at around 20 C and is stable over the duration of a calibration to better than 0.2 C. Two thermistors, calibrated to a few mk, measure the temperature of the ambient air and the air inside the BIPM standard. Air pressure is measured by means of a calibrated barometer positioned at the height of the beam axis. The relative humidity is controlled within the range 47 % to 53 % and consequently no humidity correction is applied to the current measured using transfer instruments. Table 3. Characteristics of the BIPM mammography radiation qualities Radiation quality Mo-25 Mo-28 Mo-30 Mo-35 Generating potential / kv Additional filtration mm Mo Al HVL / mm (µ/ ) air / cm 2 g Reference distance / mm 600 Beam diameter / mm 100 K BIPM / mgy s Correction factors The correction factors applied to the ionization current measured at each radiation quality, together with their associated uncertainties, are given in Table 4 for the BIPM standard and in Table 5 for the CMI standard. Table 4. Correction factors for the BIPM FAC-L-02 standard Radiation quality Mo-25 Mo-28 Mo-30 Mo-35 u ia u ib Air attenuation k a a Scattered radiation k sc Fluorescence k fl Electron loss k e Saturation k s Polarity k pol Wall transmission k p Field distortion k d Diaphragm correction k dia Humidity k h g air a Values for K and kpa; each measurement is corrected using the air density measured at the time. u ia represents the relative standard uncertainty estimated by statistical methods, type A u ib represents the relative standard uncertainty estimated by other means, type B 3/11

4 Table 5. Correction factors for the CMI FAC L1 standard a used at the BIPM Radiation quality Mo-25 Mo-28 Mo-30 Mo-35 u ia u ib Air attenuation k a b Scattered radiation k sc Fluorescence k fl Electron loss k e Saturation k s Polarity k pol Wall transmission k p Field distortion k d Diaphragm correction k dia Humidity k h g air a For this comparison, the CMI considered (but finally rejected) adoption of an initial charge correction factor k ii (see Burns et al 2016 for a fuller explanation) b Values for K and kpa; each measurement is corrected using the air density measured at the time. u ia represents the relative standard uncertainty estimated by statistical methods, type A u ib represents the relative standard uncertainty estimated by other means, type B The correction factor k a for the BIPM standard is evaluated using the measured mass attenuation coefficients ( air given in Table 3. In practice, the values used for k a take account of the temperature and pressure of the air in the standard at the time of the measurements. Ionization measurements are also corrected for changes in air attenuation arising from variations in the temperature and pressure of the ambient air between the radiation source and the reference plane. The correction factor k a is usually evaluated for both standards using the BIPM ( air values given in Table 3. However, for this comparison, the correction factors k a shown in Table 5 (evaluated using the CMI µ values) were used for the CMI standard. The reason for this choice is in relation to the different values determined at the CMI, as explained by Burns et al (2016), and for consistency in evaluating the direct and the indirect comparison results. Measurements using the BIPM standard were made using positive polarity only as the polarity effect in the standard is less than 1 part in The leakage current for the BIPM standard, relative to the ionization current, was measured to be less than Similarly, measurements using the CMI standard were made using positive polarity only and a polarity correction of was applied (see Table 5). All measured ionization currents are corrected for ion recombination. The measured values for the ion recombination correction k s for the BIPM standard are given in Table 4. For the CMI standard, the values for k s given in Table 5 for the BIPM air-kerma rates are derived from measurements at the CMI. 4.3 Chamber positioning and measurement procedure The reference plane for the BIPM and the CMI standards were positioned at 600 mm from the radiation source; this distance was measured to 0.03 mm and was reproducible to 0.01 mm. 4/11

5 Alignment on the beam axis was measured to around 0.1 mm and this position was reproducible to better than 0.01 mm The beam diameter in the reference plane is 100 mm for all radiation qualities. No correction is applied for the radial non-uniformity of the beam as both standards were used with the same aperture diameter. The CMI standard does not incorporate a temperature sensor. The internal air temperature for measurements using the CMI chamber was taken to be the temperature of the surrounding air over the period of each set of measurements. The difference between this temperature and the temperature of the BIPM standard around the same time was around 0.1 K; a temperature uncertainty of 3 parts in 10 4 is included in the type A uncertainty for the CMI ionization current in Table 8. The leakage current was measured before and after each series of ionization current measurements and a correction made based on the mean of these leakage measurements. For both standards the leakage current, relative to the ionization current of around 90 pa, was below 1 part in For the CMI chamber, the standard uncertainty of the mean of a series of seven measurements, each with integration time 40 s, was less than 1 part in Two series were made for each comparison. For the BIPM standard, a similar series was made for each comparison with a standard uncertainty below 1 part in The 28 kv quality was repeated on a subsequent day. The observed reproducibility of around 3 parts in 10 4 is consistent with the type A uncertainty for the CMI ionization current given in Table Supporting measurements using a transfer chamber A thin-window parallel-plate transfer ionization chamber belonging to the CMI, type Radcal RC6M, serial number 10242, was calibrated in both laboratories to obtain an indirect comparison result. The characteristics of the CMI realization of the mammography comparison qualities are given in Table 6. Table 6. Characteristics of the CMI reference radiation qualities Radiation quality RQR-M1 (Mo-25) RQR-M2 (Mo-28) RQR-M3 (Mo-30) RQR-M4 (Mo-35) Generating potential / kv Additional filtration mm Mo Al HVL / mm Reference distance / mm 1111 Beam diameter / mm 385 K CMI / mgy s The essential details of the measurements at each laboratory are explained here Distance: The chamber was positioned in each laboratory with the red line around the body of the chamber in the reference plane. At the CMI, it was calibrated at the reference distance of 1111 mm; at the BIPM, the chamber was calibrated at the reference distance of 600 mm (it is not possible to measure at another distance in these beams). To evaluate the effect of distance on the calibration coefficients, measurements were made during previous comparisons using 5/11

6 the same type of transfer chamber at 500 mm and at 1000 mm in the BIPM W/Mo beam (with the same field size at both distances); the calibration coefficients measured at the 23 kv W/Mo quality ( mm Al HVL) differed by 2.4 parts in 10 3 (N K, 1000 mm / N K, 500 mm = (2)). Assuming that the same effect is present in the Mo/Mo beams, the same correction factor k dist,tr = was applied to the N K measured at the BIPM at the distance of 600 mm; a relative standard uncertainty of is introduced for this effect. Ion recombination: No correction for ion recombination k s,tr is applied. Based on previous measurements for this chamber type at different air kerma rates, a relative standard uncertainty of 5 parts in 10 4 is introduced to account for this effect as the kerma rates are different at the two laboratories. Field size: The beam diameter at the reference distance differs considerably at the two laboratories. The effect for this field size difference has not been studied; a relative standard uncertainty of is included. Polarity: The transfer chamber was used with the same polarity at each laboratory and so no corrections are applied for polarity effects in the transfer chamber. Radial non-uniformity: No correction k rn,tr is applied at either laboratory for the radial nonuniformity of the radiation field. For a chamber with collector diameter 60 mm, the correction factor for the BIPM reference field (relative to the aperture diameter of 10 mm) is around and this effect is likely to cancel to some extent at the two laboratories. A relative standard uncertainty of is introduced for this effect. HVL considerations: From Tables 3 and 6 it is evident that the radiation qualities at the BIPM and the CMI are not well matched in terms of HVL, despite the use of the same calibrated generating potentials and similar molybdenum filters. To derive a comparison result for the BIPM HVL values, a fit was made to the CMI results and a set of k Q values with a relative standard uncertainty of was derived to correct each CMI calibration coefficient into one that applies at the equivalent BIPM quality. The calibration results are shown in Table 7. Table 7. Calibration coefficients for the transfer chamber Radiation quality Mo-25 Mo-28 Mo-30 Mo-35 N K,CMI (pre-bipm) / Gy C N K,CMI (post-bipm) / Gy C dist (relative) N K,CMI k Q / Gy C N K,BIPM / Gy C N K,BIPM k dist,tr / Gy C Indirect comparison result /11

7 6. Uncertainties The uncertainties associated with the primary standards and with the results of the direct comparison are listed in Table 8. The uncertainties associated with the measurement of the ionization current and with chamber positioning are those that apply to measurements at the BIPM. The uncertainty associated with air attenuation is that which applies at each laboratory, as discussed in 4.2. The combined standard uncertainty u c of the ratio K CMI K BIPM takes into account correlation in the type B uncertainties associated with the humidity correction and the physical constants. Correlation in the values for k sc, k fl and the diaphragm corrections is taken into account in an approximate way by assuming half of the uncertainty value for each factor at each laboratory. The uncertainties associated with the results of the indirect comparison are listed in Table 9. The combined standard uncertainty u c of the ratio N N takes into account correlation K, CMI K, BIPM in the type B uncertainties associated with the primary standards. Table 8. Uncertainties associated with the direct comparison Standard BIPM CMI Relative standard uncertainty u ia u ib u ia u ib Ionization current a a Positioning b b Volume Correction factors (excl. k h ) c c Humidity k h Physical constants K Standard d K CMI K BIPM u c = e a For measurements at the CMI, the uncertainty components for ionization current are u ia = and u ib = b At the CMI, the uncertainty for positioning is u ib = (no type A uncertainty). c Following the discussion of Section 4.2 the uncertainty of the attenuation correction is that of the CMI determination. d The uncertainty of the air-kerma determination at the CMI is , rather than the value tabulated here. It is the former value that appears as u Lab i in the KCDB. e Takes account of correlation in the type B uncertainties. 7/11

8 Table 9. Uncertainties associated with the indirect comparison Transfer chamber BIPM CMI Relative standard uncertainty u ia u ib u ia u ib K Standard Positioning I tr Reproducibility N K Indirect comparison result u ia u ib N K ratio a Ion recombination k s,tr Radial non-uniformity k rn,tr Distance k dist,tr Field size k field,tr Fitting procedure k Q N N u c = K, CMI K,BIPM a Takes account of correlation in the type B uncertainties. 7. Results and discussion The comparison results are those derived from the direct comparison and are given in Table 10. Agreement at the level of around 2.6 parts in 10 3 is observed, which is within the standard uncertainty of the comparison of 3.5 parts in 10 3 given in Table 8. These results can be compared with those of the indirect comparison given in Table 7, which are higher by around 1 part in 10 3 ; this level of agreement with the direct comparison results is within the standard uncertainty of the calibration process. Table 10. Comparison results Radiation quality Mo-25 Mo-28 Mo-30 Mo-35 K CMI K BIPM 8/11

9 8. Degrees of Equivalence The analysis of the results of BIPM comparisons in low-energy x-rays in terms of degrees of equivalence is described by Burns (2003) and a similar analysis is adopted for comparisons in mammography x-ray beams. Following a decision of the CCRI, the BIPM determination of the air-kerma rate is taken as the key comparison reference value, for each of the CCRI radiation qualities. It follows that for each laboratory i having a BIPM comparison result x i with combined standard uncertainty u i, the degree of equivalence with respect to the reference value is the relative difference D i = (K i K BIPM,i ) / K BIPM,i = x i 1 and its expanded uncertainty U i = 2 u i. The results for D i and U i expressed in mgy/gy, are shown in Table 11. These data are presented graphically in Figure 1. Table 11. Degrees of equivalence Mo/Mo-25 Mo/Mo-28 Mo/Mo-30 Mo/Mo-35 D i U i D i U i D i U i D i U i /(mgy/gy) /(mgy/gy) /(mgy/gy) /(mgy/gy) NMIJ PTB NIST VNIIM VSL BEV CMI W/Mo-23 W/Mo-28 W/Mo-30 W/Mo-50 D i U i D i U i D i U i D i U i /(mgy/gy) /(mgy/gy) /(mgy/gy) /(mgy/gy) NRC W/Mo-23 W/Mo-28 W/Mo-30 W/Mo-35 ENEA- INMRI Note that the data presented in the table, while correct at the time of publication of the present report, will become out of date when a laboratory makes a new comparison with the BIPM. The formal results under the CIPM MRA are those available in the BIPM key comparison database. When required, the degree of equivalence between two laboratories i and j can be evaluated as the difference D ij = D i D j = x i x j and its expanded uncertainty U ij = 2 u ij, both expressed in mgy/gy. In evaluating u ij, account should be taken of correlation between u i and u j (Burns 2003). 9/11

10 D i /(mgy/gy) Metrologia 53 (2016) Tech. Suppl Figure Graph of degrees of equivalence with the KCRV BIPM.RI(I)-K7 Degrees of equivalence, D i and U i (k = 2) NRC NMIJ PTB NIST VNIIM VSL BEV ENEA-INMRI CMI W/Mo qualities Open black square: radiation quality W/Mo 23 kv Open blue square: radiation quality W/Mo 28 kv Open green circle: radiation quality W/Mo 30 kv Open purple circle: radiation quality W/Mo 35 kv Open green square: radiation quality W/Mo 50 kv Mo/Mo qualities Red triangle: radiation quality Mo/Mo 25 kv Blue square: radiation quality Mo/Mo 28 kv Green circle: radiation quality Mo/Mo 30 kv Purple circle: radiation quality Mo/Mo 35 kv 9. Conclusion The key comparison BIPM.RI(I)-K7 for the determination of air kerma in mammography x-ray beams shows the standards of the CMI and the BIPM to be in agreement at the level of the standard uncertainty for the comparison of 3.5 parts in The results of the indirect comparison made at the same time are consistent with the direct results at the level of 1 part in Degrees of equivalence, including those for the CMI, are presented for entry in the BIPM key comparison database. The formal results under the CIPM MRA are those available in the BIPM key comparison database. 10/11

11 References Allisy P J, Burns D T and Kessler C 2011 Measuring conditions and uncertainties for the comparison and calibration of national dosimetric standards at the BIPM Rapport BIPM- 2011/04 Burns D T 2003 Degrees of equivalence for the key comparison BIPM.RI(I)-K2 between national primary standards for low-energy x-rays Metrologia 40 Technical Supplement Burns D T, Kessler C and Sochor V 2016 Key comparison BIPM.RI(I)-K2 of the air-kerma standards of the CMI, Czech Republic and the BIPM in low-energy x-rays Metrologia 53 Technical Supplement KCDB 2016 The BIPM key comparison database is available online at Kessler C, Roger P and Burns D T 2010 Establishment of reference radiation qualities for mammography Rapport BIPM-2010/01 Solc J and Sochor V 2014 Characterization of the new free-air primary standard for low-energy X-rays at CMI Rad. Phys. Chem /11