Key comparison BIPM.RI(I)-K1 of the air-kerma standards of the LNE-LNHB, France and the BIPM in 60 Co gamma radiation

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1 Key comparison BIPM.RI(I)-K1 of the air-kerma standards of the LNE-LNHB, France and the BIPM in 60 Co gamma radiation C. Kessler 1, D.T. Burns 1, F. Delaunay 2, M. Donois 2 1 Bureau International des Poids et Mesures, F Sèvres Cedex 2 Laboratoire National de métrologie et d'essais Laboratoire National Henri Becquerel CEA-Saclay PtC 104, Gif-sur-Yvette Cedex Abstract An indirect comparison of the standards for air kerma of the Laboratoire National de Métrologie et d'essais Laboratoire National Henri Becquerel (LNE-LNHB), France and of the Bureau International des Poids et Mesures (BIPM) was carried out in the 60 Co radiation beam of the BIPM in April The comparison result, evaluated as a ratio of the LNE-LNHB and the BIPM standards for air kerma, is with a combined standard uncertainty of The results are analysed and presented in terms of degrees of equivalence for entry in the BIPM key comparison database. 1. Introduction An indirect comparison of the standards for air kerma of the Laboratoire National de Métrologie et d'essais Laboratoire National Henri Becquerel (LNE-LNHB), France and of the Bureau International des Poids et Mesures (BIPM) was carried out in April 2013 in the 60 Co radiation beam at the BIPM to update the previous comparison result of 2003 [1, 2] published in the BIPM.RI(I)-K1 comparison series [3]. The comparison was undertaken using two transfer ionization chambers of the LNE-LNHB. 2. Details of the standards The LNE-LNHB standard for air kerma is comprised of six graphite-walled cavity ionization chambers of different size and shape, each with a graphite inner electrode, constructed at the LNE-LNHB [4]. The main characteristics of the standards are listed in Table 1. The standards CSp03 and Sp03 were used for the present comparison, but they were considered as transfer 1/13

2 chambers rather than primary standards. The BIPM primary standard is a parallel-plate graphite cavity ionization chamber with a volume of about 6.8 cm 3 as described in [5] and [6]. Table 1. Characteristics of the LNHB standards for air kerma Shape Dimensions / mm CSp01 CSp02 CSp03 Sp01 Sp02 Sp03 Cylindrospherical Cylindrospherical Cylindrospherical Spherical Spherical Spherical Outer height / mm Outer diameter / mm Inner height / mm Inner diameter / mm Wall thickness / mm Graphite wall density / g cm Electrode diameter / mm Electrode height / mm Air cavity volume / cm Applied voltage (both polarities) / V Determination of the air kerma For a cavity chamber with measuring volume V, the air-kerma rate is determined by the relation I K V air W e 1 en ( ) 1 g a,c s c,a k i, (1) where air is the density of air under reference conditions, I is the ionization current under the same conditions, W is the average energy spent by an electron of charge e to produce an ion pair in dry air, g is the fraction of electron energy lost by bremsstrahlung production in air, ( en / ) a,c is the ratio of the mean mass energy-absorption coefficients of air and graphite, s c,a is the ratio of the mean stopping powers of graphite and air, is the product of the correction factors to be applied to the standard. k i 2/13

3 Physical data and correction factors The values used for the physical constants [7] are given in Table 2. The correction factors entering in equation (1), the volume of the primary standard and the associated uncertainties for the BIPM standard are also included in Table 2. These data are taken from Table 8 of [8]. The LNE-LNHB air-kerma determination is based on six primary standards; the correction factors for each standard can be found in [4] and only the uncertainties of the corrected current per unit volume (I k i / V) are presented in Table 2. Table 2. Physical constants and correction factors with their relative standard uncertainties of the BIPM standard for the 60 Co radiation beam at the BIPM Physical Constants BIPM values LNHB uncertainty (1) uncertainty (1) values 100 s i 100 u i 100 s i 100 u i a dry air density (2) / kg m (µ en / ) a,c ratio of mass energy-absorption coefficients s c,a ratio of mass stopping powers (3) W/e mean energy per charge / J C g a Correction factors: fraction of energy lost in radiative processes 0.27 (3) k g re-absorption of radiative loss k s recombination losses (5) 0.02 k h humidity k st stem scattering k wall wall attenuation and scattering (4) (5) 0.10 k an axial non-uniformity (4) (5) 0.05 (1) (2) (3) k rn radial non-uniformity k pol polarity --- Measurement of I / V (5) (5) (5) V chamber volume / cm (4) (5) I ionization current / pa Relative standard uncertainty (5) quadratic summation (6) 0.31 combined uncertainty Expressed as one standard deviation s i represents the type A relative standard uncertainty estimated by statistical methods, u i represents the type B relative standard uncertainty estimated by other means At Pa and K for the BIPM; at Pa and K for the LNHB Combined uncertainty for the product of s, and W / e c a (4) The uncertainties for k wall and k an are included in the determination of the effective volume [6] (5) The reference air-kerma rate is derived from the six values for (I k s k st k wall k an k rn k pol ) /V determined for the six ionization chambers. Values for each chamber are given in [4] (6) A global type A uncertainty of 8 parts in 10 4 is determined from the spread of the six values for (I k s k st k wall k an k rn k pol ) /V 3/13

4 The correction factors for the BIPM standards were re-evaluated in 2007 and the changes to the air-kerma rate determination arise from the results of Monte Carlo calculations of correction factors for the standard, a re-evaluation of the correction factor for saturation and a new evaluation of the air volume of the standard using an experimental chamber of variable volume. The combined effect of these changes is an increase in the BIPM determination of air kerma by the factor and a reduction of the relative standard uncertainty of this determination to 1.5 parts in A full description of the changes to the standard is given in [6]. The corrections for the LNE-LNHB standards are described in [4] and are briefly summarized in the following paragraphs. Attenuation and scattering in the chamber wall (k wall ) The effects of attenuation and scatter in the graphite walls of the CSp and Sp chambers were determined by the LNE-LNHB using Monte Carlo calculations (EGSnrc v3 and PENELOPE 2001 codes). The maximum difference between the results obtained with the two codes is with a corresponding type A uncertainty of 1.2 parts in The type A uncertainty of k wall varies from to Axial non-uniformity (k an ) The axial non-uniformity correction factor is taken to be unity [9, 10]. Scatter from the stem (k st ) This correction was determined by measurement using a dummy stem. It varies from to for the spherical chambers and from to for the cylindrospherical chambers, with a relative type A standard uncertainty of Radial non-uniformity of the LNE-LNHB beam (k rn ) The correction for the radial non-uniformity of the beam over the cross-section of the CSp standards varies from to , and for the Sp chambers is in the range from to The relative type A standard uncertainty is Recombination loss (k s ) The correction factors k s for losses due to ion recombination were determined using the method of Niatel as described in [11]. Reference conditions The reference conditions for the air-kerma determination at the BIPM are given in Table 7 of [8]: the distance from source to reference plane is 1 m, the field size in air at the reference plane is 10 cm 10 cm, defined by the photon fluence rate at the centre of each side of the square being 50 % of the photon fluence rate at the centre of the square. At the LNE-LNHB: the distance from source to reference plane is 1 m, the field size in air at the reference plane is 16 cm diameter, defined by the photon fluence rate being 50 % of the photon fluence rate at the centre. 4/13

5 Reference values The BIPM reference air-kerma rate K BIPM is taken as the mean of the four measurements made around the period of the comparison. The K BIPM values refer to an evacuated path length between source and standard and are given at the reference date of , 0 h UTC. The half-life of 60 Co was taken as days (u = 0.29 days) [12]. The LNE-LNHB reference air-kerma rate is taken as the mean of the air-kerma rates determined using the six new primary standards. Measurements were made with the six ionization chambers in 2006 and 2008, and with two of the chambers in These measurements have been used to determine the reference air-kerma rate, which is given for the reference date of The half-life is taken as (8) years or (29) days ( Beam characteristics The characteristics of the BIPM and LNE-LNHB beams are given in Table 3. Table Co beam Characteristics of the 60 Co beams at the LNE-LNHB and the BIPM Nominal K / mgy s 1 ( ) Source dimensions / mm diameter length Scatter contribution in terms of energy fluence Field size at 1 m LNE-LNHB source % Ø 16 cm BIPM source % 10 cm 10 cm 4. Experimental method The comparison of the LNE-LNHB and BIPM standards was made indirectly using the calibration coefficients N for the two transfer standards CSp03 and Sp03 given by K, (2) N K, lab Klab Ilab where K lab is the air-kerma rate at each laboratory and I lab is the ionization current for each transfer chamber measured at the LNE-LNHB or the BIPM. The experimental method for measurements at the BIPM is described in [8]; the essential details for the determination of the calibration coefficients N K for the transfer chambers CSp03 and Sp03 are reproduced here. Positioning The centre of the chambers was positioned in the reference plane of the beam at 1 m from the source at both laboratories. Applied voltage and polarity At the BIPM, a collecting voltage of 850 V (both polarities) was applied to the outer electrode of the chambers at least 30 min before any measurements were made. The polarity effect determined at the BIPM for the LNE-LNHB transfer chambers are shown in Table 4. However, as all the measurements were made with both polarities at the BIPM, no explicit correction for polarity was applied. The LNE-LNHB applies positive polarity and a correction 5/13

6 for the polarity effect k pol is introduced in equation (1) and consequently in the determination of the calibration coefficients N K,LNHB. The values for k pol applied at the LNE-LNHB are also included in Table 4. Table 4. Polarity correction for the LNE-LNHB standards LNE-LNHB Standards Sp03 CSp03 LNE-LNHB polarity correction, k pol (9) (9) BIPM polarity correction, k pol (2) (2) Recombination loss The correction factor for losses due to ion recombination was determined at the LNE-LNHB using the method of Niatel as described in [11]. The recombination correction k s can be expressed as ks 1 kinit kvoli V (3) and Table 5 gives the values for k init and k vol and the uncertainty of k s for the two standards. This factor was also determined at the BIPM during the present comparison for the standard CSp03; the results obtained at the BIPM are also shown in Table 5. Table 5. Ion recombination for the LNE-LNHB standards LNE-LNHB Standard Sp03 CSp03 CSp03 Values determined at the LNE-LNHB BIPM Initial recombination and diffusion, k init Volume recombination coefficient, k vol, / pa k s in the BIPM beam Standard uncertainty Consequently, a correction factor of (2) and (2) for ion recombination at 850 V was applied to the standards Sp03 and CSp03, respectively, in the BIPM 60 Co beam. Radial non-uniformity correction The correction factors for radial non-uniformity of the beam over the cross-section of the transfer chambers applied at the LNE-LNHB and the BIPM are presented in Table 6. A relative uncertainty component of is included in Table 10. 6/13

7 Table 6. Radial non-uniformity correction for the LNE-LNHB standards LNE-LNHB Standards Sp03 CSp03 LNE-LNHB k rn correction (22) (22) BIPM k rn correction (10) (10) Stem scatter correction No correction for stem scatter has been applied to the current measured with the transfer chambers at either laboratory. Charge and leakage measurements The charge Q collected by the LNE-LNHB chambers was measured at the BIPM using a Keithley electrometer, model 642. The source is exposed during the entire measurement series and the charge is collected for the appropriate, electronically controlled, time interval. A preirradiation was made for at least 40 min before any measurements. The ionization current measured for the standard was corrected for the leakage current. This correction was less than in relative value. At the LNE-LNHB, the charge was measured using a Keithley model 2001 electrometer. The leakage current was less than 1 x 10 4 in relative value. There was no fixed pre-irradiation time; the measurements were considered valid when the current stabilized; for the Sp03 and CSp03 ionization chambers, a pre-irradiation of only few minutes was needed in the LNE- LNHB beam. Ambient conditions During a series of measurements at the BIPM, the air temperature is recorded for each current measurement and was stable to better than 0.1 K. Relative humidity is controlled at (50 5) %. No correction for humidity is applied to the ionization current measured. At the LNE-LNHB, the atmospheric conditions are also recorded for each current measurement. The irradiation room is controlled in temperature (20 ± 2) K and, in principle, in humidity (50 ± 10) %. If the relative humidity is lower than 35 % or higher than 65 % the measurements are discarded. 5. Results of the comparison The CSp03 and Sp03 standards were set-up and measured in the BIPM 60 Co beam on three and two separate occasions, respectively. The results were reproducible to better than The values of the ionization currents measured at the BIPM for the LNE-LNHB transfer chambers are given in Table 7. They have been normalized to standard temperature and pressure and corrected to the reference date for the decay of the 60 Co source. 7/13

8 Table 7. The experimental results from the LNE-LNHB transfer standards in the BIPM beam LNE-LNHB standard CSp03 Sp03 I + and I - /pa I mean / pa Mean current Mean current The result of the comparison, R, is expressed in the form K K R N N (4) K, LNHB K, BIPM in which the average value of measurements made at the LNE-LNHB before and after those made at the BIPM for each chamber is compared with the mean of the measurements made at the BIPM. The results are presented in Table 8. The small difference in the comparison result for the two transfer chambers results in an additional relative uncertainty component tr of included in Table 10. Contributions to the relative standard uncertainty of N K, lab are listed in Table 9 and the combined standard uncertainty u c for the comparison result R K is presented in Table 10. Table 8. Final result of the LNE-LNHB/BIPM comparison of 60 Co air-kerma standards Transfer standard N K,LNHB / Gy µc 1 N K, BIPM / Gy µc 1 R K CSp Sp u c Mean values /13

9 Table 9. Uncertainties associated with the calibration of the transfer standards LNE-LNHB BIPM Relative standard uncertainty 100 s i 100 u i 100 s i 100 u i Air kerma Ionization current of the transfer standards 0.05 (1) Positioning (1) 0.01 Short-term stability (1) 0.01 Polarity correction Relative standard uncertainty of N K, lab Quadratic summation Combined uncertainty (1) The ionisation current is the mean of measurements made at different times. Consequently, the type A uncertainty of the mean current includes components related to chamber positioning and short-term stability. Table 10. Uncertainties associated with the comparison result Relative standard uncertainty 100 s i 100 u i N K,LNHB / N K,BIPM a Radial non-uniformity k rn,tr Different chambers tr Relative standard uncertainty of R K u c = 0.18 a Takes account of correlation in type B uncertainties. The comparison result is taken as the unweighted mean value for the two transfer chambers, R K = with a combined standard uncertainty for the comparison of Some of the uncertainties in K that appear in both the BIPM and the LNE-LNHB determinations (such as air density, W/e, en /, g, s c,a and k h ) cancel when evaluating the uncertainty of R K. 6. Degrees of equivalence Comparison of a given NMI with the key comparison reference value Following a decision of the CCRI, the BIPM determination of the dosimetric quantity, here K BIPM, is taken as the key comparison reference value (KCRV) [13]. It follows that for each NMI i having a BIPM comparison result R K,i (denoted x i in the KCDB) with combined standard uncertainty, u i, the degree of equivalence with respect to the reference value is given by a pair of terms: the relative difference D i = (K i K BIPM,i )/ K BIPM,i = R K,i 1, (5) 9/13

10 where K i is the value measured by the NMI during the comparison, and the expanded uncertainty (k = 2) of this difference, U i = 2 u i. (6) The results for D i and U i are expressed in mgy/gy. Table 11 gives the values for D i and U i for each NMI, i, taken from the KCDB of the CIPM MRA [3] and this report, using (5) and (6). These data are presented graphically in Figure 1. Table 11. Degrees of equivalence For each laboratory i, the degree of equivalence with respect to the key comparison reference value is the difference D i and its expanded uncertainty U i. Tables formatted as they appear in the BIPM key comparison database BIPM.RI(I)-K1 - COOMET.RI(I)-K1 (2006) - EURAMET.RI(I)-K1 (2005 to 2008) - APMP.RI(I)-K1 (2004 to 2005) Lab i D i U i Lab i / (mgy/gy) / (mgy/gy) DMDM CIEMAT ENEA-INMRI CMI VSL SSM MKEH STUK GUM NRPA NPL SMU NRC IAEA BEV HIRCL VNIIM BIM KRISS IST/ITN ARPANSA PTB NIST METAS NMIJ LNMRI ININ CNEA LNE-LNHB BARC BelGIM INER CPHR Nuclear Malasya RMTC NIM NMISA PNRI D i U i 10/13

11 DMDM ENEA-INMRI VSL MKEH GUM NPL NRC BELGIM CPHR RMTC CIEMAT CMI SSM STUK NRPA SMU IAEA HIRCL BIM IST/ITN PTB METAS LNMRI CNEA BARC INER Nuclear Malasya NIM NMISA PNRI BEV VNIIM KRISS ARPANSA NIST NMIJ ININ LNE-LNHB Metrologia. 2013, 50, Tech. Suppl Figure 1. Graph of degrees of equivalence with the KCRV 20 BIPM.RI(I)-K1 Degrees of equivalence with the KCRV for air kerma in 60 Co Di / (mgy / Gy) Di / (mgy / Gy) N.B. Black squares indicate results that are more than 10 years old. COOMET.RI(I)-K1 (2006), EUROMET.RI(I)-K1 (2005 to 2008) and APMP.RI(I)-K1 ( ) Degrees of equivalence with the KCRV for air kerma in 60 Co -35 Comparison of any two NMIs with each other The degree of equivalence between any pair of national measurement standards, when required, is expressed in terms of the difference between the two comparison results and the 11/13

12 expanded uncertainty of this difference; consequently, it is independent of the choice of key comparison reference value. The degree of equivalence, D ij, between any pair of NMIs, i and j, is thus expressed as the difference D ij D D R R (7) and the expanded uncertainty (k = 2) of this difference, U ij = 2 u ij, where i j i j u 2 ij u 2 c, i u 2 c, j 2 f u f u k k k,corr i k k k,corr 2 j (8) and the final two terms are used to take into account correlation between the primary standards, notably that arising from the physical constants and correction factors for similar types of standard. Following the advice of the CCRI(I) in 2011, results for D ij and U ij are no longer published in the KCDB. Note that the data presented in the table, while correct at the time of publication of the present report, become out-of-date as NMIs make new comparisons. The formal results under the CIPM MRA [14] are those available in the key comparison database [3]. 7. Conclusion The previous comparison of the air-kerma standards for 60 Co gamma radiation of the LNE- LNHB and of the BIPM was made directly in The comparison result, based on the LNE- LNHB primary standard GCS-10-1, is (27) when updated in the key comparison database for the changes made to the BIPM standard. The LNE-LNHB has also changed its air-kerma standards since 2003, resulting in an increase of 0.09 % in the air-kerma determination. Taking into account this change, the comparison result becomes (27). The new LNE-LNHB standard for air kerma in 60 Co gamma radiation compared with the present BIPM air-kerma standard gives a comparison result of (18) and so is in agreement within the uncertainties with the KCRV and with the previous comparison result. 12/13

13 References [1] Allisy-Roberts P.J., Kessler, C., Burns D.T., Delaunay, F., Leroy, E., Comparison of the standards for air kerma of the LNE-LNHB and the BIPM for 60 Co gamma radiation, Rapport BIPM-2006/02. [2] Allisy-Roberts P.J., Burns D.T., Kessler C., Summary of the BIPM.RI(I)-K1 comparison for air kerma in 60 Co gamma radiation, Metrologia, 2007, 44, Tech. Suppl., [3] BIPM Key Comparison Database KCDB, 60 Co air kerma comparisons, BIPM.RI(I)-K1 [4] Delaunay, F., Donois, M., Gouriou, J., Leroy, E., Ostrowsky, A., New LNHB primary standard for 60 Co air kerma, Metrologia, 2010, 47, No 6, 652 [5] Boutillon M. and Niatel M.-TA., Study of a graphite cavity chamber for absolute measurements of 60 Co gamma rays, 1973, Metrologia, 9, [6] Burns D.T, Allisy P.J., Kessler C., 2007, Re-evaluation of the BIPM international standard for air kerma in 60 Co gamma radiation, Metrologia, 2007, 44, L53-L56 [7] Comité Consultatif pour les Étalons de Mesures des Rayonnements Ionisants, Constantes physiques pour les étalons de mesure de rayonnement, 1985, CCEMRI Section (I), 11, R45. [8] Allisy-Roberts P.J., Burns D.T., Kessler C., 2011, Measuring conditions and uncertainties for the comparison and calibration of national dosimetric standards at the BIPM, Rapport BIPM-11/04, 20 pp. [9] Bielajew A.F., 1990, An analytic theory of the point-source non-uniformity correction factor for thick-walled ionisation chambers in photon beams Phys. Med. Biol [10] Bielajew A.F. and Rogers D.W.O., 1992, Implications of new correction factors on primary air kerma standards in 60Co-beams Phys. Med. Biol [11] Boutillon M., Volume recombination parameter in ionization chambers, 1998, Phys. Med. Biol., 43, [12] Bé M.-M., Chisté V, Dulieu C., Browne E., Baglin C., Chechev V., Kuzmenco N., Helmer R., Kondev F., MacMahon D., Lee K.B., Table of Radionuclides (Vol. 3 A = 3 to 244) Monographie BIPM-5. [13] Allisy P.J., Burns D.R., Andreo P., International framework of traceability for radiation dosimetry quantities, Metrologia, 2009, 46(2), S1-S8. [14] CIPM MRA: Mutual recognition of national measurement standards and of calibration and measurement certificates issued by national metrology institutes, International Committee for Weights and Measures, 1999, 45 pp. 13/13