7

Metrologia. 2013, 50, Tech. Suppl. 06017

Key comparison BIPM.RI(I)-K1 of the air-kerma standards of the ININ, Mexico and the BIPM in 60Co gamma radiation C. Kessler1, J.T. Alvarez Romero2, V.M. Tovar-Muñoz2 1 2

Bureau International des Poids et Mesures, F-92312 Sèvres Cedex Instituto Nacional de Investigaciones Nucleares, Ocoyoacac, Estado de México 52750 México.

Abstract A direct comparison of the standards for air kerma of the Instituto Nacional de Investigaciones Nucleares (ININ), Mexico, and of the Bureau International des Poids et Mesures (BIPM) was carried out in the 60Co radiation beam of the BIPM in 2012. The comparison result, evaluated as a ratio of the ININ and the BIPM standards for air kerma, is 1.0035 with a combined standard uncertainty of 2.1  10–3. The results are analysed and presented in terms of degrees of equivalence for entry in the BIPM key comparison database.

1.

Introduction

A direct comparison of the standards for air kerma of the Instituto Nacional de Inverstigaciones Nucelares (ININ) and the Bureau International des Poids et Mesures (BIPM) for 60Co gamma radiation was carried out in June 2012 in the 60Co reference beam at the BIPM.

2.

Details of the standards

The air-kerma standard of the ININ for 60Co is a cylindrical graphite-walled cavity ionization chamber constructed by the Österreichisches Forschungszentrum (ÖFS), Austria, referenced as CC01-131 [1]. The main characteristics of the standard are listed in Table 1. The BIPM standard is a parallel-plate graphite cavity ionization chamber with a volume of about 6.8 cm3 as described in [2] and [3].

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Table 1.

Characteristics of the ININ standard for air kerma

ININ standard

CC01-131 Nominal value

Chamber

Electrode

Outer diameter / mm

19

Outer height / mm

19

Wall thickness / mm

4

Diameter / mm

2

Height / mm Volume

(1)

Wall

Air cavity / cm

Applied voltage / V

3.

1.0174

Material

High Purity Molded ATJ Graphite

Density / g cm (1)

8.98 3

−3

1.80

Both polarities

250

Volume determined by the ÖFS [1]

Determination of the air kerma

For a cavity chamber with measuring volume V, the air-kerma rate is determined by the relation

K 

W 1  en ( ) a,c sc,a  k i  airV e 1  g  I

,

(1)

where

air I W g

(en/)a,c sc,a  ki

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, 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, is the product of the correction factors to be applied to the standard.

Physical data and correction factors The data concerning the various factors entering in the determination of air kerma in the 60Co beam using the primary standards of the BIPM and of the ININ are shown in Table 2. They include the physical constants [4], the correction factors entering in equation (1), the volume of each chamber cavity and the associated uncertainties. For the BIPM standard, these data are taken from Table 8 of [5]. The data for the ININ standard are described in the following section.

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Table 2. Physical constants and correction factors with their relative standard uncertainties of the BIPM and ININ standards for the 60Co radiation beam at the BIPM CH 6.1

BIPM values

100 si

100 ui

1.2930

–

0.01

0.9989

0.01

0.04

1.0010

–

mean energy per charge / J C 33.97 fraction of energy lost in radiative 0.0031 processes

–

Physical Constants dry air density (2) / kg m–3 a (µen/)a,c sc,a W/e ga

uncertainty

ratio of mass energy-absorption coefficients ratio of mass stopping powers –1

Correction factors: kg re-absorption of radiative loss ks recombination losses kh humidity kst stem scattering kwall wall attenuation and scattering kan axial non-uniformity krn radial non-uniformity Measurement of I / V V chamber volume / cm3 I ionization current / pA

CC01-131

ININ (1)

0.11 (3)

values

uncertainty (1) 100 si

100 ui

1.2930

–

0.01

0.9989

0.01

0.04

1.0010

–

33.97

–

0.11(3)

–

0.02

0.0031

–

0.02

0.9996 1.0022 0.9970 1.0000 1.0011 1.0020 1.0015

– 0.01 – 0.01 – – –

0.01 0.02 0.03 – – (4) – (4) 0.02

– 1.0021 0.9970 0.9995 1.0211 1.0000 1.0001

– 0.03 – 0.02 0.04 – –

– 0.02 0.03 0.02 –

6.8855

–

0.08 (4)

1.0174

0.01

0.02

– 0.01

0.15 0.02

0.15

0.06

Relative standard uncertainty quadratic summation combined uncertainty

0.02 0.15

0.10 0.02

0.22 0.23

(1)

Expressed as one standard deviation si represents the type A relative standard uncertainty estimated by statistical methods, ui represents the type B relative standard uncertainty estimated by other means

(2)

At 101 325 Pa and 273.15 K

(3)

Combined uncertainty for the product of

(4)

The uncertainties for kwall and kan are included in the determination of the effective volume [3]

s c ,a and W / e

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 standards, a re-evaluation of the correction factor for saturation and a new evaluation of the air volume of the standards 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 1.0054 and a reduction of the relative standard uncertainty of this determination to 1.5 parts in 103. A full description of the changes to the standard is given in [3]. The corrections for the ININ standard are briefly described in the following paragraphs.  Attenuation and scattering in the chamber wall (kwall)

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The effects of attenuation and scatter in the graphite walls of the CC01-131 standard were determined by the ININ using Monte Carlo calculations [6].  Polarity effect (kpol) The polarity effect determined at the BIPM for the ININ primary standard was 5 parts in 104. However, as all measurements were made with both polarities at the BIPM, no explicit correction for polarity was applied.  Scatter from the stem (kst) The correction for stem scatter was assumed to be the same as for the BEV primary standard, both chambers being of the same type and manufacturer; the correction measured at the BEV was 0.9995 (0.0003) and is taken from [7].  Axial non-uniformity (kan) The axial non-uniformity correction factor is taken to be unity, as for the BEV standard [7].  Radial non-uniformity of the beam (krn) The correction factor krn for the radial non-uniformity of the BIPM beam over the crosssection of the ININ standard is estimated to be 1.0001(2).  Recombination loss (ks) The correction factor for losses due to ion recombination was determined at the BIPM during the present comparison using the method of Niatel as described in [8]. The recombination correction ks can be expressed as ks  1  kinit  k vol I V

(2)

and Table 3 gives the values for kinit and kvol and the uncertainty for ks. Consequently, a correction factor of 1.0021(3) for ion recombination at 250 V was applied to the ININ standard CC01-131 in the BIPM 60Co beam. Table 3.

Ion recombination for the ININ standard ININ Standard

CC01-131

Initial recombination and diffusion, kinit

19.3  10–4

Volume recombination coefficient, kvol / pA–1

9.8  10–7

ks in the BIPM beam

1.0021

Standard uncertainty

3  10–4

Reference conditions The reference conditions for the air-kerma determination at the BIPM are given in Table 7 of [5]:  the distance from source to reference plane is 1 m,

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 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. Reference values The BIPM reference air-kerma rate K BIPM is taken from 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 2012-01-01, 0 h UTC. The half-life of 60Co was taken as 1925.21 days (u = 0.29 days) [9]. Beam characteristics The characteristics of the BIPM and the ININ beams are given in Table 4. The ININ parameters are for information only as all measurements for the present direct comparison were made in the BIPM beam. Table 4.

Characteristics of the 60Co beams at the ININ and the BIPM

60

Co beam

ININ source BIPM source

4.

Nominal K (2012-01-01) 5 mGy s-1 5 mGy s

–1

Source dimensions / mm diameter

length

Scatter contribution / energy fluence

20

19.5

30 %

10 cm  10 cm

20

14

21 %

10 cm  10 cm

Field size at 1 m

Experimental method

The experimental method for measurements at the BIPM is described in [5] and the essential details are reproduced here. Positioning The centre of the chamber was positioned in the reference plane of the beam at 1 m from the source. Applied voltage and polarity A collecting voltage of 250 V (both polarities) was applied to the outer electrode of the chamber at least 30 min before any measurements were made. Charge and leakage measurements The charge Q collected by the ININ standard was measured using a Keithley electrometer, model 642. The source is operational during the entire exposure series and the charge is collected for the appropriate, electronically controlled, time interval. The standard was preirradiated for at least 40 min before any measurements were made. The ionization current measured for the standard was corrected for the leakage current. This correction was less than 1  10–4 in relative value. Ambient conditions

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During a series of measurements, the air temperature is measured for each current measurement and it was stable to better than 0.1 °C. Relative humidity is controlled at (50  5) %. No correction for humidity is applied to the ionization current measured. 5.

Result of the comparison

The ININ standard was set-up and measured in the BIPM 60Co beam on two separate occasions. The results were reproducible to better than 1 × 10–4. The values of the ionization currents measured at the BIPM for the ININ standard are given in Table 5. They have been normalized to standard temperature and pressure and corrected to the reference date for the decay of the 60Co source and for an evacuated path between source and standard.

Table 5.

The experimental results for the ININ standard in the BIPM beam ININ standard

I /pA

CC01-131

Imean /pA

-187.273

187.471

187.372

-187.294

187.484

187.389

Mean current

187.380

The result of the comparison, RK , is expressed in the form RK  K ININ / K BIPM

(3)

The comparison result and the combined standard uncertainty uc associated with the ratio RK are presented in Table 6. Table 6.

Final result of the ININ/BIPM comparison of standards for 60Co air kerma –1 K ININ / mGy s

–1 K BIPM / mGy s

RK

uc

4.9493

4.9319

1.0035

0.0021

The mean ratio of the values of the air-kerma rate determined by the ININ and the BIPM standards taken from Table 6 is 1.0035 with a combined standard uncertainty, uc, of 0.0021. Some of the uncertainties in K that appear in both the BIPM and the ININ determinations (such as air density, W/e, en/, g , sc,a and kh) cancel when evaluating the uncertainty of RK (see Table 2).

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 KBIPM, is taken as the key comparison reference value (KCRV) [10]. It follows that for each

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NMI i having a BIPM comparison result RK,i (denoted xi in the KCDB) with combined standard uncertainty, ui, the degree of equivalence with respect to the reference value is given by a pair of terms: Di = (Ki – KBIPM,i )/ KBIPM,i = RK,i – 1,

the relative difference

(4)

where Ki is the value measured by the NMI during the comparison, and the expanded uncertainty (k = 2) of this difference, Ui = 2 ui.

(5)

The results for Di and Ui are expressed in mGy/Gy. Table 7 gives the values for Di and Ui for each NMI, i, taken from the KCDB of the CIPM MRA [11] and this report, using (4) and (5). These data are presented graphically in Figure 1. Table 7.

Degrees of equivalence For each laboratory i, the degree of equivalence with respect to the key comparison reference value is the difference Di and its expanded uncertainty Ui. 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

Di

Ui

Lab i

/ (mGy/Gy)

Di

Ui

/ (mGy/Gy)

DMDM

2.5

3.6

CIEMAT

-1.5

3.9

ENEA-INMRI

-0.3

5.2

CMI

-5.8

14.1

VSL

-1.5

4.4

SSM

1.0

7.5

-2.3

7.3

MKEH

5.5

4.4

STUK

GUM

2.3

4.8

NRPA

5.1

7.1

NPL

1.1

7.6

SMU

5.2

6.5

0.0

7.5

NRC

3.2

5.6

IAEA

BEV

3.4

4.2

HIRCL

4.2

11.9

VNIIM

0.8

3.6

BIM

-4.5

13.0 6.0

KRISS

-0.5

3.2

IST/ITN

-0.4

ARPANSA

0.9

6.2

PTB

8.4

3.4

NIST

3.9

6.8

METAS

-1.3

4.6

NMIJ

1.2

4.4

LNMRI

2.4

13.7

CNEA

1.8

10.0

BARC

0.7

7.6

ININ

3.5

4.2

LNE-LNHB

-0.6

3.6

BelGIM

12.5

21.8

INER

-3.2

5.4

CPHR

1.1

9.7

Nuclear Malasya

-0.1

7.4

RMTC

-3.6

9.7

NIM

-4.9

6.6

NMISA PNRI

7/10

0.9

6.9

14.6

11.4


Figure 1. Graph of degrees of equivalence with the KCRV BIPM.RI(I)-K1 Degrees of equivalence with the KCRV for air kerma in

60

Co

20 15

Di / (mGy / Gy)

10 5 0 -5 -10 -15

LNE-LNHB

ININ

NMIJ

NIST

ARPANSA

KRISS

VNIIM

BEV

NRC

NPL

GUM

MKEH

VSL

DMDM

ENEA-INMRI

-20

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 (2004-2005) Degrees of equivalence with the KCRV for air kerma in

60

Co

35

15 5 -5 -15

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PNRI

NMISA

NIM

Nuclear Malasya

INER

BARC

CNEA

LNMRI

METAS

PTB

IST/ITN

BIM

HIRCL

IAEA

SMU

NRPA

STUK

SSM

CMI

CIEMAT

CPHR

-35

RMTC

-25 BELGIM

Di / (mGy / Gy)

25


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 expanded uncertainty of this difference; consequently, it is independent of the choice of key comparison reference value. The degree of equivalence, Dij, between any pair of NMIs, i and j, is thus expressed as the difference Dij  Di  D j  Ri  R j

(6)

and the expanded uncertainty (k = 2) of this difference, Uij = 2 uij, where uij 2  u c,i 2  u c, j 2    f k u k ,corr i2    f k u k ,corr 2j k

(7)

k

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 Dij and Uij 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 [12] are those available in the key comparison database [11].

7.

Conclusion

The ININ standard for air kerma in 60Co gamma radiation compared with the BIPM air-kerma standard gives a comparison result of 1.0035 (0.0021) and so is in agreement with the KCRV within the expanded uncertainty of the comparison.

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References [1]

BEV. Primary Standard Graphite Cavity chamber Forschunzentrum Seibersdorf, 1992 YW022.DOC

[2]

Boutillon M. and Niatel M.-TA., Study of a graphite cavity chamber for absolute measurements of 60Co gamma rays, 1973, Metrologia, 9, 139-146.

[3]

Burns D.T, Allisy P.J., Kessler C., 2007, Re-evaluation of the BIPM international standard for air kerma in 60Co gamma radiation, Metrologia, 2007, 44, L53-L56

[4]

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.

[5]

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.

[6]

Álvarez J.T., De La Cruz H.D., and Tovar V.M., Simulación de Montecarlo del Factor de Corrección por Atenuación y Dispersión en la pared kwall para Patrones Primarios de Kerma en aire, ID 1403, Memorias Simposio de Metrología 2012, CENAM, Octubre 2012, México.

[7]

Kessler C., Allisy-Roberts P.J., Steurer A., Tiefenboeck W., Gabris F., Comparison of the standards for air kerma of the BEV and the BIPM for 60Co gamma radiation, Metrologia, 2010, 47, Tech. Suppl., 06006

[8]

Boutillon M., Volume recombination parameter in ionization chambers, 1998, Phys. Med. Biol., 43, 2061-2072

[9]

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.

[10]

Allisy P.J., Burns D.R., Andreo P., International framework of traceability for radiation dosimetry quantities, Metrologia, 2009, 46(2), S1-S8.

[11]

BIPM Key Comparison Database KCDB, K1.

[12]

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. http://www.bipm.org/pdf/mra.pdf .

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60

type

CC01

ser

131.

Co air kerma comparisons, BIPM.RI(I)-