Key comparison BIPM.RI(I)-K3 of the air-kerma standards of the NRC, Canada and the BIPM in medium-energy x-rays

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1 Key comparison BIPM.RI(I)-K3 of the air-kerma standards of the NRC, Canada and the BIPM in medium-energy x-rays D T Burns 1, C Kessler 1, E Mainegra-Hing 2, H Shen 2 and M R McEwen 2 1 Bureau International des Poids et Mesures (BIPM), Pavillon de Breteuil, F Sèvres 2 Ionizing Radiation Standards, Measurement Science and Standards, National Research Council, Ottawa, Ontario, Canada K1A 0R6 Abstract A key comparison has been made between the air-kerma standards of the NRC, Canada and the BIPM in the medium-energy x-ray range. The results show the standards to be in agreement at the level of the standard uncertainty of the comparison of 3.3 parts in The results are analysed and presented in terms of degrees of equivalence, suitable for entry in the BIPM key comparison database. 1. Introduction An indirect comparison has been made between the air-kerma standards of the National Research Council (NRC), Canada, and the Bureau International des Poids et Mesures (BIPM) in the x-ray range from 100 kv to 250 kv. Three cavity ionization chambers of different types were used as transfer instruments. The measurements at the BIPM took place in September 2014 using the reference conditions recommended by the CCRI (CCEMRI 1972). Final results were supplied by the NRC 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 W K air 1 k i (1) V e g air 1 air 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 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 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 = A factor is also included to account for the compressibility of dry air between T ~ 293 K and T 0 = K. 1/14

2 region. The BIPM air-kerma standard is described in Boutillon (1978) and the changes made to certain correction factors in Burns (2004), Burns et al (2009) and the references therein. The NRC standard is described in Henry and Garrett (1960) and was previously compared with the BIPM standard in an indirect comparison carried out in 1998, the results of which are reported in Burns et al (2002). The main dimensions, the measuring volume and the polarizing voltage for each standard are shown in Table 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 adopted at both laboratories. Table 2. Main characteristics of the standards Standard BIPM M-01 NRC MEES Aperture diameter / mm Air path length / mm Collecting length / mm Electrode separation / mm Collector width / mm Measuring volume / mm Polarizing voltage / V The transfer instruments 4.1 Determination of the calibration coefficient for a transfer instrument The air-kerma calibration coefficient N K for a transfer instrument is given by the relation K (2) N K I tr where K is the air-kerma rate determined by the standard using (1) and I tr is the ionization current measured by the transfer instrument and the associated current-measuring system. The current I tr is corrected to the standard conditions of air temperature, pressure and relative humidity chosen for the comparison. While normally for BIPM comparisons these are chosen as T = K, P = kpa and h = 50 %, at the NRC ionization currents are corrected to dry air using a quality-dependent factor (see footnote d to Table 7). Rather than remove this correction, it was decided to adopt for N K the NRC standard conditions of T = K, P = kpa and h = 0 % (see also footnote a to Table 8). To derive a comparison result from the calibration coefficients N K,BIPM and N K,NMI measured, respectively, at the BIPM and at a national metrology institute (NMI), differences in the 2/14

3 radiation qualities must be taken into account. Normally, each quality used for the comparison has the same nominal generating potential at each institute, but the half-value layers (HVLs) may differ. A radiation quality correction factor k Q is derived for each comparison quality Q. This corrects the calibration coefficient N K,NMI determined at the NMI into one that applies at the equivalent BIPM quality and is derived by interpolation of the N K,NMI values in terms of log(hvl). The comparison result at each quality is then taken as k N Q K,NMI RK,NMI (3) NK,BIPM In practice, the half-value layers normally differ by only a small amount and k Q is close to unity. 4.2 Details of the transfer instruments Three cavity ionization chambers belonging to the NRC were used as transfer instruments for the comparison, none of which were used in the 1998 comparison. Their main characteristics are given in Table 3. Chamber type Table 3. Main characteristics of the transfer chambers NE 2571 NE 2611A Exradin A12 Serial number XA Geometry thimble thimble thimble External diameter / mm Wall thickness / mm Wall material graphite graphite C552 Nominal volume / cm Reference point (on chamber axis) 13 mm from tip 5 mm from tip 13 mm from tip Polarizing potential / V +300 a +300 a +300 a a Potential of the chamber wall with respect to the central electrode. At the BIPM a potential of +300 V is applied to the chamber wall, the central electrode remaining at virtual ground, while at the NRC the chamber wall remains at virtual ground and the central electrode is at 300 V. 5. Calibration at the BIPM 5.1 The BIPM irradiation facility and reference radiation qualities The BIPM medium-energy x-ray laboratory houses a high-stability generator and a tungstenanode x-ray tube with a 3 mm beryllium window. An aluminium filter of thickness mm is added (for all radiation qualities) to compensate for the decrease in attenuation that occurred when the original BIPM x-ray tube (with an aluminium window of approximately 3 mm) was replaced in June Two voltage dividers monitor the tube voltage and a voltage-tofrequency converter combined with data transfer by optical fibre measures the anode current. No transmission monitor is used. For a given radiation quality, the standard uncertainty of the distribution of repeat air-kerma rate determinations over many months is better than 3 parts in The radiation qualities used in the range from 100 kv to 250 kv are those recommended by the CCRI (CCEMRI 1972) and are given in Table 4. 3/14

4 The irradiation area is temperature controlled at around 20 C and is stable over the duration of a calibration to better than 0.1 C. Two calibrated thermistors measure the temperature of the ambient air and the air inside the BIPM standard (which is controlled at 25 C). 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 %. Table 4. Characteristics of the BIPM reference radiation qualities Radiation quality 100 kv 135 kv 180 kv 250 kv Generating potential / kv Inherent Be filtration / mm Additional Al filtration / mm Additional Cu filtration / mm Al HVL / mm Cu HVL / mm (µ/ ) air a / cm 2 g K BIPM / mgy s a Measured at the BIPM for an air path length of 280 mm. 5.2 The BIPM standard and correction factors The reference plane for the BIPM standard was positioned at 1200 mm from the radiation source, with a reproducibility of 0.03 mm. The standard was aligned on the beam axis to an estimated uncertainty of 0.1 mm. The beam diameter in the reference plane is 98 mm for all radiation qualities. During the calibration of the transfer chambers, measurements using the BIPM standard were made using positive polarity only. A correction factor of is applied to correct for the known polarity effect in the standard. The leakage current for the BIPM standard, relative to the ionization current, was measured to be around 1 part in The correction factors applied to the ionization current measured at each radiation quality using the BIPM standard, together with their associated uncertainties, are given in Table 5. The factor k a corrects for the attenuation of the x-ray fluence along the air path between the reference plane and the centre of the collecting volume. It is evaluated using the measured air-attenuation coefficients given in Table 4. In practice, the values used for k a take account of the temperature and pressure of the air in the standard. Ionization current measurements (both for the standard and for transfer chambers) 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. 5.3 Transfer chamber positioning and calibration at the BIPM The reference point for each chamber was positioned in the reference plane (1200 mm from the radiation source), with a reproducibility of 0.03 mm. Each transfer chamber was aligned on the beam axis to an estimated uncertainty of 0.1 mm. The leakage current was measured before and after each series of ionization current measurements and a correction made using the mean value. The relative leakage current for each transfer chamber was around 1 part in /14

5 For each transfer chamber and at each radiation quality, two sets of seven measurements were made, each measurement with integration time 60 s. The relative standard uncertainty of the mean ionization current for each pair was typically below 2 parts in Repeat calibrations for all three chambers (after repositioning) showed a reproducibility consistent with the uncertainty of 3 parts in 10 4 included in Table 11 for the short-term reproducibility of calibration coefficients in medium-energy x-rays at the BIPM. Table 5. Correction factors for the BIPM M-01 standard Radiation quality 100 kv 135 kv 180 kv 250 kv u ia u ib Air attenuation k a a Photon scatter k sc Fluorescence k fl Electron loss k e b Ion recombination k s Polarity k pol Field distortion k d Diaphragm correction k dia b a b Wall transmission k p Humidity k h Radiative loss 1 g air Values for the BIPM reference conditions of K and kpa; each measurement is corrected using the air density measured at the time. Values for k e and k dia adopted in September 2009 (Burns et al 2009). The diaphragm correction, described in Burns and Kessler (2009), is evaluated by Monte Carlo calculation and includes the effect of photon transmission and scatter in the diaphragm as well as fluorescence and secondary electron production in the diaphragm. 6. Calibration at the NRC 6.1 The NRC irradiation facility and reference radiation qualities The medium-energy x-ray facility at the NRC comprises two constant-potential generators (Glassman, ±160 kv) and a tungsten-anode x-ray tube (Comet, bipolar, 320 kv) with a 3 mm beryllium window and a focal spot size of approximately 3 mm by 3 mm. The generating potential is measured at 3 s intervals using two voltage dividers (Park 1962), calibrated to 3 parts in 10 5, and is constant for a given radiation quality to better than 5 V. The x-ray tube current is stabilized over a wide dynamic range (µa to ma) using a feedback system developed at the NRC that controls the filament current. Stability over the short term is around 5 parts in 10 5 and long-term stability (> 5 year) is better than around 5 parts in The characteristics of the NRC realization of the CCRI comparison qualities (CCEMRI 1972) are given in Table 6. The x-ray output is switched on and off using a mechanical shutter with a timing uncertainty of approximately 15 ms. The combination of tube current and shutter time serves as a beam monitor. A parallel-plate transmission ionization chamber provides a second, independent beam 5/14

6 monitor. This is located 34 cm from the focal spot and consists of five layers of aluminized Mylar, totalling 5.6 mg cm 2 of Mylar and 0.21 mg cm 2 of aluminium. The air temperature for this monitor is measured by a sensor mounted inside the chamber. An analysis of the calibration coefficients obtained using the two monitors over the last ten years yielded an average difference of 1 part in 10 4 with a standard deviation of 3 parts in 10 4, with no significant trend with x-ray energy or time. Ambient air pressure was recorded using a barometer positioned at the height of the beam axis. For the transfer chambers, an air temperature sensor is placed at the height of the beam axis, at the same perpendicular distance from the x-ray source, outside the radiation field. The relative humidity was measured throughout the ion chamber measurements to be (41 ± 4) %. Table 6. Characteristics of the NRC reference radiation qualities Radiation quality 100 kv 135 kv 180 kv 250 kv Generating potential / kv Additional Al filtration / mm Additional Cu filtration / mm Al HVL / mm Cu HVL / mm (µ/ ) air a / cm 2 g K NRC / mgy s a Measured at the NRC for an air path length of 419 mm. 6.2 The NRC standard and correction factors The defining plane for the NRC standard was positioned at 1000 mm from the radiation source, with a reproducibility of 0.1 mm. The standard was aligned on the beam axis to an estimated uncertainty of 0.2 mm. The beam diameter in the reference plane is 90 mm for all radiation qualities. During the calibration of the transfer chambers, measurements using the NRC standard were made at a single polarity. The polarity correction for the standard was measured previously and is within 3 parts in 10 4 of unity. The relative leakage current was measured to be less than 1 part in The correction factors applied to the ionization current measured at each radiation quality using the NRC standard, together with their associated uncertainties, are given in Table 7. The correction factor k a is derived from experimental data for tube voltages between 50 kv and 135 kv. For higher kv values, the attenuation is calculated using XCOM attenuation coefficients (Berger and Hubbell 1987). The mean mass attenuation coefficients (µ/ ) air for air given in Table 6 were derived from the corresponding k a values. In practice the values used for k a account for the temperature and pressure of the air in the standard at the time of the measurements. A difference in the NRC and BIPM correction factors is noted. The NRC correction factor for ionization gain k sc includes not only the effect of scattered photons, but also that of fluorescence photons (treated separately as k fl for the BIPM standard). 6/14

7 6.3 Transfer chamber positioning and calibration at the NRC The reference point for each transfer chamber was positioned at the reference distance (at the NRC 1000 mm from the radiation source), with a reproducibility of 0.1 mm. Alignment on the beam axis was to an estimated uncertainty of 0.2 mm. The leakage current was measured after the chamber reading stabilized. The relative leakage current for each transfer chamber was less than 5 parts in The relative standard uncertainty of the sample of five repeat current measurements at each radiation quality was well below 3 parts in 10 4 except for the NE 2611 at the 100 kv beam quality where a relative standard uncertainty of 7 parts in 10 4 was observed. The observed repeatability, estimated from two series of measurements, before and after the measurements at the BIPM, was better than 2 parts in Table 7. Correction factors for the NRC MEES standard a Radiation quality 100 kv 135 kv 180 kv 250 kv u ia u ib Air attenuation k a b Ionization gain k sc c Electron loss k e Ion recombination k s Polarity k pol Field distortion k d Humidity k h d a b c d Radiative loss 1 g air Component uncertainties below have been neglected. It is also noted that the NRC do not apply corrections corresponding to the BIPM corrections k dia and k p for diaphragm effects and for wall transmission, respectively. Values for the NRC reference conditions of K and kpa; each measurement is corrected using the air density measured at the time. This corrects for the re-absorption of scattered radiation and fluorescence photons. Nominal values; each measurement is corrected using the relative humidity measured at the time. The small beamquality dependence arises from the change with energy of the mass energy-absorption coefficient for air at 50 % relative humidity. 7. Additional corrections to transfer chamber measurements 7.1 Ion recombination, polarity, radial non-uniformity, distance and field size As can be seen from Tables 4 and 6, the air-kerma rates at the NRC are about two times higher than those at the BIPM. Thus volume recombination effects will be greater for the transfer chamber calibrations at the NRC, although no recombination corrections have been applied at either laboratory. Based on previous measurements of recombination corrections for thimble chambers, this effect is small and is accounted for by introducing a component of uncertainty of 2 parts in Each transfer chamber was used with the same polarity at each laboratory and so no corrections are applied for polarity effects in the transfer chambers. No correction is applied at either laboratory for the radial non-uniformity of the radiation fields. For small cylindrical transfer chambers, the effect will be very small and similar at the two laboratories. 7/14

8 The reference distance is 1000 mm at the NRC and 1200 mm at the BIPM, while the field size of 90 mm at the NRC is smaller than that of 98 mm at the BIPM. It is known that transfer chambers respond to scattered radiation in a way that free-air chambers do not, so that calibration coefficients can show some sensitivity to field size. However, the magnitude of this effect for thimble chambers is relatively small, as is the field-size difference. An uncertainty component of 1 part in 10 3 is introduced in Table 12 for this effect. 7.2 Radiation quality correction factors k Q As noted in Section 4.1, slight differences in radiation qualities may require a correction factor k Q. However, from Tables 4 and 6 it is evident that the BIPM and NRC radiation qualities are closely matched in terms of HVL, such that in the worst case the correction is less than 2 parts in 10 4 and the mean value is less than 5 parts in Consequently, k Q is taken to be unity for all chambers and qualities, with a negligible uncertainty. 8. Comparison results The calibration coefficients N K,NRC and N K,BIPM for the transfer chambers are presented in Table 8. The values N K,NRC measured before and after the measurements at the BIPM give rise to the relative standard uncertainties s tr,1, s tr,2 and s tr,3 for the three chambers, which represent the uncertainty in N K arising from transfer chamber stability. For each chamber at each radiation quality, the mean of the NRC results before and after the BIPM measurements is used to evaluate the comparison results N K,NRC / N K,BIPM given in Table 9. The final results R K,NRC in Table 9 are evaluated as the mean for the three transfer chambers. For each quality, the corresponding uncertainty s tr is the standard uncertainty of the mean (using (n 1.4) as described in the footnote to Table 8), or taken as s tr s s s / 3 (4) tr,1 tr,2 tr,3 if this is larger (on the basis that the agreement between transfer chambers should, on average, not be better than their combined stability estimated using s tr,1, s tr,2 and s tr,3 from Table 8). The r.m.s. value of s tr for the four qualities, s tr,comp = , is a global representation of the comparison uncertainty arising from the transfer chambers and is included in Table 12. Also given in Table 9 are the results of the previous comparison of the NRC and BIPM standards (Burns et al 2002), revised for the published changes made to the BIPM standard in 2003 (Burns 2004) and in 2009 (Burns et al 2009). 9. Uncertainties The uncertainties associated with the primary standards are listed in Table 10 and those for the transfer chamber calibrations in Table 11. The combined standard uncertainty u c for the comparison results R K,NRC is presented in Table 12. This combined uncertainty takes into account correlation in the type B uncertainties associated with the physical constants and the humidity correction. Correlation in the values for k e and k sc (including k fl at the BIPM), derived from Monte Carlo calculations in each laboratory, are taken into account in an approximate way by assuming half of the uncertainty value for each factor at each laboratory. This is consistent with the analysis of the results of BIPM comparisons in medium-energy x-rays in terms of degrees of equivalence described in Burns (2003). 8/14

9 Table 8. Calibration coefficients for the transfer chambers a Radiation quality 100 kv 135 kv 180 kv 250 kv NE N K,NRC (pre-comp) / Gy C N K,NRC (post-comp) / Gy C s tr,1 (relative) b N K,BIPM / Gy C NE 2611A-136 N K,NRC (pre-comp) / Gy C N K,NRC (post-comp) / Gy C s tr,2 (relative) b N K,BIPM / Gy C Exradin A12-XA N K,NRC (pre-comp) / Gy C N K,NRC (post-comp) / Gy C s tr,3 (relative) b N K,BIPM / Gy C a For the reasons stated in Section 4.1, the calibration coefficients are given for the reference conditions T = K, P = kpa and h = 0 %. b For each pre-post pair of N K,NRC values with half-difference d, the standard uncertainty of the mean is taken to be s tr,i = d / (n 1.4), where the term (n 1.4) is found empirically to be a better choice than (n 1) to estimate the standard uncertainty for low values of n. For n = 2, s tr,i = 1.3d. 10. Discussion The comparison results presented in Table 9 show agreement between the NRC and BIPM standards at the level of the standard uncertainty of the comparison of 3.3 parts in A trend is evident in the results for the different radiation qualities, amounting to almost 3 parts in 10 3 over the energy range. The results for the two 0.6 cm 3 transfer chambers (NE 2571 and Exradin A12) agree at the level of 1 part in 10 3 (2 parts in 10 3 at 100 kv). However, the results for the NE 2611A are higher by around 4 parts in 10 3 (7 parts in 10 3 at 100 kv) and it is this difference that dominates the uncertainty component s tr,comp of Table 12. No explanation has been found for this difference. The present results are higher than those obtained in the 1998 comparison by around 3 parts in 10 3 (5 parts in 10 3 at 100 kv). This result is again largely due to the high comparison results for the NE 2611A; if this chamber is removed from the analysis, the results agree with those for the 1998 comparison at the level of around 1 part in 10 3 (3 parts in 10 3 at 100 kv). It is perhaps relevant to note that the previous comparison was carried out using three transfer chambers of the type NE 2571 (none of which was the NE 2571 chamber used for the present comparison). 9/14

10 Table 9. Comparison results Radiation quality 100 kv 135 kv 180 kv 250 kv N K,NRC /N K,BIPM using NE N K,NRC /N K,BIPM using NE 2611A N K,NRC /N K,BIPM using Exradin A s tr R K,NRC Previous results for R K,NRC Table 10. Uncertainties associated with the standards Standard BIPM NRC Relative standard uncertainty u ia u ib u ia u ib Ionization current Volume Positioning Correction factors (excl. k h ) Humidity k h Physical constants K std Table 11. Uncertainties associated with the calibration of the transfer chambers Institute BIPM NRC Relative standard uncertainty u ia u ib u ia u ib K std Positioning of transfer chamber I tr Short-term reproducibility a - N K,std a The reproducibility of the NRC transfer chamber calibrations over the duration of the comparison is implicitly included in s tr in Table 9 and consequently in s tr,comp in Table /14

11 11. Degrees of Equivalence The analysis of the results of BIPM comparisons in medium-energy x-rays in terms of degrees of equivalence is described in Burns (2003). 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 and including those of the present comparison, are shown in Table 13 and in Figure 1, which include the linked results of the corresponding regional key comparison APMP.RI(I)-K3 (Lee et al 2008). When required, the degree of equivalence between two laboratories i and j can be evaluated as the difference D ij = D i D j and its expanded uncertainty U ij = 2u ij, both expressed in mgy/gy. In evaluating u ij, account should be taken of correlation between u i and u j (Burns 2003). Table 12. Uncertainties associated with the comparison results Relative standard uncertainty u ia u ib N N a K, NRC K,BIPM Ion recombination Field size / distance Transfer chambers s tr,comp R K,NRC u c = a Takes account of correlation in type B uncertainties as noted in Section Conclusions The key comparison BIPM.RI(I)-K3 for the determination of air kerma in medium-energy x-rays shows agreement between the NRC and BIPM standards at the level of the standard uncertainty of the comparison of 3.3 parts in A trend is evident in the results for the different radiation qualities, amounting to almost 3 parts in 10 3 over the energy range. The present results are higher than those obtained in the 1998 comparison by around 3 parts in 10 3 (5 parts in 10 3 at 100 kv). This difference is largely removed if one of the three transfer chambers is excluded from the comparison, although no reason has been found to support this exclusion. Tables and graphs of degrees of equivalence, including those for the NRC, are presented for entry in the BIPM key comparison database. Note that the data presented in the tables, while correct at the time of publication of the present report, become out of date as laboratories make new comparisons with the BIPM. The formal results under the CIPM MRA are those available in the BIPM key comparison database. 11/14

12 12/14 Metrologia 53 (2016) Tech. Suppl

13 D i / (mgy/gy) NPL NIM VSL NIST LNE-LNHB GUM ARPANSA MKEH VNIIM PTB ENEA BEV NRC NMIJ INER Nuc. Malaysia DMSc BARC NMISA KRISS IAEA kv 135 kv 180 kv 250 kv Figure 1. Degrees of equivalence for each laboratory i with respect to the key comparison reference value. Results to the left of the dashed line are for the ongoing comparison BIPM.RI(I)-K3 and those to the right are for the regional comparison APMP.RI(I)-K3. 13/14

14 References Berger M J and Hubbell J H 1987 XCOM: Photon cross sections on a personal computer NBSIR Boutillon M 1978 Mesure de l exposition au BIPM dans le domaine des rayons X de 100 à 250 kv Rapport BIPM-78/3 Burns D T 2003 Degrees of equivalence for the key comparison BIPM.RI(I)-K3 between national primary standards for medium-energy x-rays Metrologia 40 Tech. Suppl Burns D T 2004 Changes to the BIPM primary air-kerma standards for x-rays Metrologia 41 L3 Burns D T, Shortt K R and VanderZwan L 2002 Comparison of the air-kerma standards of the NRC and the BIPM in the medium-energy x-ray range Rapport BIPM-02/15 Burns D T and Kessler C 2009 Diaphragm correction factors for free-air chamber standards for air kerma in x-rays Phys. Med. Biol Burns D T, Kessler C and Allisy P J 2009 Re-evaluation of the BIPM international standards for air kerma in x-rays Metrologia 46 L21 23 CCEMRI 1972 Qualités de rayonnement Comité Consultatif pour les Étalons de Mesures des Rayonnements Ionisants (Section I) 2 nd meeting R15 16 Henry W H and Garrett C 1960 The Canadian standard free-air chamber for medium quality x-rays Canadian Journal of Physics 38 (12) KCDB 2014 The BIPM key comparison database is available online at Lee J H, Hwang W S, Kotler L H, Webb D V, Büermann L, Burns D T, Takeyeddin M, Shaha V V, Srimanoroth S, Meghzifene A, Hah S H, Chun K J, Kadni T B, Takata N and Msimang Z 2008 APMP/TCRI key comparison report of measurement of air kerma for medium-energy x-rays (APMP.RI(I)-K3) Metrologia 45 Tech. Suppl Park J H 1962 Special Shielded Resistor for High-Voltage D-C Measurements J. Res. Natl. Bur. Stand. 66C 19 14/14