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

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1 ey comparison BIPM.RI(I)-1 of the air-kerma standards of the NIM, China and the BIPM in 60 Co gamma radiation C. essler 1, D. Burns 1,. Wang 2,Y. Fan 2,S. Jin 2, X. Yang 2 1 Bureau International des Poids et Mesures, F Sèvres Cedex 2 National Institute of Metrology, Beijing, China Abstract An indirect comparison of the standards for air kerma of the National Institute of Metrology (NIM), China and of the Bureau International des Poids et Mesures (BIPM) was carried out in the 60 Co radiation beam of the BIPM in November The comparison result, evaluated as a ratio of the NIM 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 National Institute of Metrology (NIM), China and of the Bureau International des Poids et Mesures (BIPM) was carried out in November 2015 in the 60 Co radiation beam at the BIPM to update the previous comparison result of 2001 (Allisy-Roberts et al 2009) published in the BIPM key comparison database (CDB 2015) under the reference BIPM.RI(I)-1. Three transfer ionization chambers belonging to the NIM were calibrated by both laboratories but the comparison result was evaluated using the calibration coefficients of two of them. 2. Details of the standards The NIM standard PS-102 for air kerma is a cylindrical graphite-walled spherically-ended cavity ionization chamber of 9.5 cm 3 of similar design as the standard of the Laboratoire National de Métrologie et d'essais Laboratoire National Henri Becquerel (LNE-LNHB), France (Delaunay et al 1993); it was constructed by the NIM in 1983 during a collaboration with the LNE-LNHB. The details of the NIM standard are given in Table 1. The BIPM primary standard is a parallel-plate graphite cavity ionization chamber with a volume of about 6.8 cm 3 (Boutillon et al 1973, Burns et al 2007). 1/11

2 Table 1. Chamber Characteristics of the NIM standard for air kerma Standard PS 102 Outer height / mm Outer diameter / mm Cavity height / mm Cavity diameter / mm Dimensions Wall thickness / mm 3 Electrode Diameter / mm Height / mm 23 Volume Air cavity / cm Wall Material graphite Density / g cm Applied voltage Both polarities 800 V 3. Determination of the air kerma For a cavity chamber with measuring volume V, the air-kerma rate is determined by the relation where air I W g I V air W e 1 en ( ) 1 g a,c s c,a k is the density of air under reference conditions, is the ionization current under the same conditions, is the average energy spent by an electron of charge e to produce an ion pair in dry air, 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, is the ratio of the mean stopping powers of graphite and air, s c,a k i is the product of the correction factors to be applied to the standard. Physical data and correction factors The values used for the physical constants, recommended by the Consultative Committee for Ionizing Radiation (CCEMRI 1985) are given in Table 2. The correction factors entering in equation (1), the volume of the primary standards and the associated uncertainties for the BIPM (Allisy-Roberts et al 2011) and the NIM standards are also included in Table 2. i, (1) 2/11

3 Table 2. Physical Constants Physical constants and correction factors with their relative standard uncertainties of the BIPM and NIM standards for the 60 Co radiation beam at the BIPM BIPM CH 6.1 NIM PS-102 values 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 k g re-absorption of radiative loss k s recombination losses k h humidity k st stem scattering k wall wall attenuation and scattering (4) k an axial non-uniformity (4) k rn radial non-uniformity Measurement of I / V V chamber volume / cm (4) I ionization current / pa Relative standard uncertainty quadratic summation combined uncertainty (1) 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 (2) At Pa and s, and W / e (3) Combined uncertainty for the product of c a (4) The uncertainties for k wall and k an are included in the determination of the effective volume (Burns et al 2007) 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 by Burns et al (2007). The correction factors for the NIM standards were re-evaluated in 2013 (Wang et al 2016). The main change arises from the new value adopted for the wall correction factor, calculated using the Monte Carlo code EGSnrc. The correction factor for saturation was 3/11

4 also re-evaluated. The new values adopted by the NIM result in an increase of the air kerma determination by a factor or with a relative standard uncertainty of 2.1 parts in Reference conditions The reference conditions for the air-kerma determination at the BIPM are described by Allisy-Roberts et al (2011): 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. The reference conditions at the NIM are the same to those at the BIPM. the distance from source to reference plane is 1 m, the field size in air at the reference plane is 10 cm in diameter, defined by the radius where the photon fluence rate is 50 % of the photon fluence rate measured at the beam axis. Reference values The BIPM reference air-kerma rate BIPM is taken as the mean of the four measurements made around the period of the comparison. The BIPM values refer to an evacuated path length between source and standard corrected to the reference date of , 0 h UTC. The half-life of 60 Co was taken as days (u = 0.29 days) (Bé et al 2006). The NIM value is the mean of the measurements made over a period of 2 months before and 1 month after the comparison. It is given at the same reference date as the BIPM and corrected by the decay of the 60 Co source using the same half-life. Beam characteristics The characteristics of the BIPM and NIM beams are given in Table 3. Table Co beam Characteristics of the 60 Co beams at the NIM and the BIPM Nominal / mgy s 1 ( ) Source dimensions / mm diameter length Scatter contribution in terms of energy fluence Field size at 1 m NIM source % 10 cm diameter BIPM source % 10 cm 10 cm 4. The transfer chambers and their calibration The comparison of the NIM and BIPM standards was made indirectly using transfer chambers belonging to the NIM. Three ionization chambers were measured at both laboratories; the calibration coefficients N are given by, (2) N, lab lab Ilab where lab is the air kerma rate at each lab and I lab is the ionization current of a transfer chamber measured at the NIM or the BIPM. The current is corrected for the effects and influences described in this section. 4/11

5 The ionization chambers NE 2571, serial number 708, NE 2611A, serial number 226 and PTW 30010, serial number 2369, belonging to the NIM, were the transfer chambers calibrated at each laboratory. Their main characteristics are listed in Table 4. Table 4. Characteristics of the NIM transfer chambers (1) Characteristic/Nominal values NE 2571 NE 2611A PTW Thimble dimensions Electrode dimensions Outer diameter / mm Outer height / mm Wall thickness / mm Diameter / mm Height / mm Volume Air cavity / cm Wall Material graphite graphite Applied voltage PMMA with thin graphite inner layer Negative polarity (1) negative polarity to the outer electrode For the reason explained in Section 5, only the calibration coefficients determined for the NE chambers were used to evaluate the comparison result. The experimental method for measurements at the BIPM is described by Allisy-Roberts et al (2011). The essential details of the measurements done with the NE chambers are reproduced here. Positioning At each laboratory the chamber was positioned with the stem perpendicular to the beam direction and with the appropriate marking on the stem (engraved line or text) facing the source. Applied voltage and polarity At the BIPM and at the NIM, a collecting voltage of 250 V and 200 V (negative polarity) was applied to the outer electrode of the chambers NE 2571 and NE 2611, respectively, at least 30 min before any measurements were made. No corrections were applied at either laboratory for polarity. Volume recombination Volume recombination is negligible at a kerma rate of less than 15 mgy s 1 for these chamber types at this polarizing voltage, and the initial recombination loss will be the same in the two laboratories. No correction for ion recombination was applied and a relative uncertainty component of is included in Table 7. Radial non-uniformity correction The correction for the radial non-uniformity of the beam for the transfer chambers is less than at the BIPM; at the NIM, this correction would be less than ; no radial non-uniformity correction was applied and a relative uncertainty component of is included in Table 7. Charge and leakage measurements The charge Q collected for each transfer chamber was measured at the NIM using a eithley electrometer, model 6517A; at the BIPM, the charge is measured using a eithley 5/11

6 Normalized current Metrologia 53 (2016) Tech. Suppl electrometer, model 642. The chambers were pre-irradiated for at least 30 min ( 1 Gy) at the NIM, and for at least 30 min ( 5 Gy) at the BIPM before any measurements were made. The ionization current measured for each transfer chamber was corrected for the leakage current; at both laboratories, this correction was less than in relative value. Ambient conditions During a series of measurements, the air temperature is measured for each current measurement and was stable to better than 0.01 C at the BIPM. At the NIM, the air temperature was stable to better than 0.1 o C. The ionization current is corrected to the reference conditions of and kpa at both laboratories. Relative humidity is controlled at (50 5) % at the BIPM. At the NIM, relative humidity is normally in the range (50 10) %. Consequently, no correction for humidity is applied to the ionization current measured at either laboratory. 5. Results of the comparison The NE 2571 and the NE 2611 transfer chambers were set-up and measured in the BIPM 60 Co beam on two and three separate occasions, respectively. The results were reproducible to better than The PTW chamber was positioned in the reference plane of the 60 Co gamma-ray beam on four occasions over a period of 5 days, registering on each occasion 90 measurements. For each series of 90 measurements, the chamber was stable, with a relative standard uncertainty of the mean ionization current of around However, the measured response of the chamber was observed to drift between each independent calibration, a relative drift of being measured between the first and the fourth calibration. Two weeks later, the chamber was again repositioned, registering the same number of measurements; the chamber was less stable, with a relative standard uncertainty of the mean ionization current of around The data, normalized to the first measurement, are shown in Figure 1; different colours identify the five times the chamber was placed in the beam. Figure 1. Normalized current PTW Number of measurements 6/11

7 Similar behaviour has been observed at the BIPM for chambers of the same type when measured in air but they have showed a stable response when calibrated in terms of absorbed dose to water. The implication is that the drift in the measurements might be due to an accumulation of charge in the build-up cap (Takata et al 2011), an effect that seems to decay slowly when the chamber is not irradiated for some time. Because of this behavior, the calibration coefficients for the PTW chamber were not included in the evaluation of the comparison result. The result of the comparison, R, is expressed in the form R N N, (3), NIM, BIPM in which the average value of measurements made at the NIM prior to those made at the BIPM (pre-bipm) and those made afterwards (post-bipm) for each chamber is compared with the mean of the measurements made at the BIPM. The NIM calibration coefficients of the transfer chambers are the average of two calibrations made before and after the measurements at the BIPM. Table 5 lists the relevant values of N at the stated reference conditions ( and kpa). The uncertainties associated with the calibration of the transfer chambers are presented in Table 6 and for the comparison result R in Table 7. Table 5. Results of the comparison Transfer chamber N,NIM / Gy µc 1 pre-bipm N,NIM / Gy µc 1 post-bipm N,NIM / Gy µc 1 overall mean N, BIPM / Gy µc 1 R NE NE 2611A u c Mean value Table 6. Uncertainties associated with the calibration coefficient, N, lab, of the transfer chambers BIPM NIM Relative standard uncertainty 100 s i 100 u i 100 s i 100 u i Air kerma rate Ionization current for the transfer chambers Distance Short-term stability Pressure, temperature 0.07 Decay 0.02 Relative standard uncertainties of N, lab The final result R is evaluated as the arithmetic mean for the two transfer chambers. 7/11

8 The individual result for each chamber differs by This difference results in an additional relative uncertainty component tr of , included in Table 7. Some of the uncertainties in that appear in both the BIPM and the NIM determinations (such as air density, W/e, en /, g, s c,a and k h ) cancel when evaluating the uncertainty of R. Table 7. Uncertainties associated with the comparison result R Relative standard uncertainty 100 s i 100 u i (1) N,NIM / N,BIPM Ion recombination 0.02 Radial non-uniformity 0.02 Different transfer chambers tr 0.14 Relative standard uncertainty of R combined uncertainty 0.27 (1) The combined standard uncertainty of the comparison result takes into account correlation in the type B uncertainties associated with the physical constants and the humidity correction 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 BIPM, is taken as the key comparison reference value (CRV) (Allisy-Roberts et al 2009). It follows that for each NMI i having a BIPM comparison result R,i (denoted x i in the CDB) with combined standard uncertainty u i, the degree of equivalence with respect to the reference value is the relative difference D i = ( i BIPM,i ) / BIPM,i = R,i 1 and its expanded uncertainty U i = 2 u i. The results for D i and U i are usually expressed in mgy/gy. Table 8 gives the values for D i and U i for each NMI, i, taken from the CDB of the CIPM MRA (1999) and this report. These data are presented graphically in Figure 2. 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. Following the advice of the CCRI(I) in 2011, results for D ij and U ij are no longer published in the CDB. 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 are those available in the key comparison database. 8/11

9 Table 8. 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)-1 - COOMET.RI(I)-1 (2006) - EURAMET.RI(I)-1 (2005 to 2008) APMP.RI(I)-1 (2004 to 2006) Lab i D i U i Lab i / (mgy/gy) / (mgy/gy) DMDM CIEMAT VSL CMI MEH SSM GUM STU NPL NRPA NRC SMU BEV IAEA VNIIM HIRCL RISS BIM ARPANSA IST/ITN NIST METAS NMIJ LNMRI ININ CNEA LNE-LNHB PTB ENEA BARC NIM INER Nuclear Malasya BelGIM NMISA CPHR RMTC D i U i 9/11

10 BELGIM CPHR RMTC CIEMAT CMI SSM STU NRPA SMU IAEA HIRCL BIM IST/ITN METAS LNMRI CNEA BARC INER Nuclear Malasya NMISA D i / (mgy / Gy) DMDM VSL MEH GUM NPL NRC BEV VNIIM RISS ARPANSA NIST NMIJ ININ LNE-LNHB PTB ENEA-INMRI NIM D i / (mgy / Gy) Metrologia 53 (2016) Tech. Suppl Figure 2. Graph of degrees of equivalence with the CRV 20 BIPM.RI(I)-1 Degrees of equivalence with the CRV for air kerma in 60 Co N.B. Black squares indicate results that are more than 10 years old COOMET.RI(I)-1 (2006), EUROMET.RI(I)-1 (2005 to 2008) and APMP.RI(I)-1 ( ) Degrees of equivalence with the CRV for air kerma in 60 Co Conclusion The previous comparison of the air-kerma standards for 60 Co gamma radiation of the NIM and of the BIPM was made directly in The comparison result, based on the same primary standard and updated for the changes made to the BIPM standard (Burns et al 2007), is (18); this updated value is given in the key comparison database. For the present comparison, the NIM standard for air kerma in 60 Co gamma radiation compared with the BIPM air-kerma standard gives a comparison result of (27) and so is in agreement within the uncertainties with the previous comparison result. 10/11

11 References Allisy-Roberts P J, essler C, Burns D, Zhongqing T, Jiacheng H, Xiaoyuan Y and un W 2009 Comparison of the standards for air kerma of the NIM and the BIPM for 60 Co gamma rays Metrologia 46 Technical Supplement Allisy-Roberts P J, Burns D and essler C 2011 Measuring conditions and uncertainties for the comparison and calibration of national dosimetric standards at the BIPM Rapport BIPM-11/04 20 pp Allisy P J, Burns D and Andreo P 2009 International framework of traceability for radiation dosimetry quantities Metrologia 46(2) S1-S8 Bé M-M, Chisté V, Dulieu C, Browne E, Baglin C, Chechev V, uzmenco N, Helmer R, ondev F, MacMahon D and Lee B 2006 Table of Radionuclides (Vol. 3 A = 3 to 244) Monographie BIPM-5. Boutillon M and Niatel M T 1973 Study of a graphite cavity chamber for absolute measurements of 60 Co gamma rays Metrologia Burns D, Allisy P J and essler C 2007 Re-evaluation of the BIPM international standard for air kerma in 60 Co gamma radiation Metrologia 44, L53-L56 CCEMRI 1985 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 CIPM MRA 1999 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. Delaunay F and Ostrowsky A 1993 Caractérisation primaire en débit de kerma dans l air du faisceau de 60 Co n IIB Note technique LPRI/93/012 CDB 2015 BIPM ey Comparison Database CDB 60 Co air kerma comparisons BIPM.RI(I)-1 Takata N and Morishita Y 2011 Effect of radiation-induced charge accumulation on buildup cap on the signal current from an ionization chamber Radiation Protection Dosimetry (1) Wang, Li T and Jin S 2016 Monte Carlo simulation of physical parameters for air kerma measurement by graphite pancake ionization chamber Atomic Energy Science and Technology (accepeted for publication) 11/11