Radiation Physics and Chemistry 75 (2006) 1087–1092

CCRI supplementary comparison of standards for

absorbed dose to water in 60Co gamma radiation

at radiation processing dose levels

D.T. Burnsa,, P.J. Allisy-Robertsa, M.F. Desrosiersb, V.Yu. Nagyb,

P.H.G. Sharpec

, R.F. Laitanod, K. Mehtae, M.K.H. Schneiderf, Y.L. Zhangg

a

Bureau International des Poids et Mesures, Pavillon de Breteuil,F- 92312 Se`vres Cedex, France b

National Institute of Standards and Technology, Gaithersburg, MD, USA c

National Physical Laboratory, Teddington, Middlesex, UK d

Istituto Nazionale di Metrologia delle Radiazioni Ionizzanti, Rome, Italy e

International Atomic Energy Agency, Vienna f

Physikalisch-Technische Bundesanstalt, Braunschweig, Germany g

National Institute of Metrology, Beijing, China

Received 7 December 2005; accepted 4 February 2006

Abstract

Six national standards for absorbed dose to water in 60Co gamma radiation at the dose levels used in radiation

processing have been compared over the range from 5 to 30 kGy using the alanine dosimeters of the NIST and the NPL

as the transfer dosimeters. The standards are in agreement at the level of around 0.5%, which is significantly smaller

than the stated standard uncertainties.

r 2006 Elsevier Ltd. All rights reserved.

1. Introduction

At its meeting in July 1997, the Consultative

Committee for Ionizing Radiation (CCRI), Section I

(for X- and gamma-rays and electrons), proposed a

comparison of the high-dose standards for absorbed

dose to water in 60Co gamma radiation among the

primary dosimetry laboratories operating standards and

services in this field. The comparison would be

organized by the Bureau International des Poids et

Mesures (BIPM) and would complement an earlier

CCRI comparison organized by the International

Atomic Energy Agency (IAEA) in collaboration with

the BIPM (Mehta and Girzikowsky, 2000). In particular, the new comparison would involve a more robust

transfer dosimeter with a view to reducing the uncertainties. In the framework of the Mutual Recognition

Arrangement of the International Committee for

Weights and Measures (CIPM MRA) (CIPM, 1999),

the comparison is classed as a supplementary comparison, with reference CCRI(I)-S1, and as such the present

report will be referenced in the BIPM key comparison

database (KCDB).

Six institutes offering a high-dose irradiation service

took part in the present comparison; the Istituto

Nazionale di Metrologia delle Radiazioni Ionizzanti

(ENEA-INMRI, Italy), the National Institute of Metrology (NIM, China), the National Institute of

Standards and Technology (NIST, USA), the National

ARTICLE IN PRESS

www.elsevier.com/locate/radphyschem

0969-806X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.radphyschem.2006.02.003

Corresponding author. Fax: +33 1 4534 2021.

E-mail address: dburns@bipm.org (D.T. Burns).
Physical Laboratory (NPL, UK), the PhysikalischTechnische Bundesanstalt (PTB, Germany) and the

IAEA (Vienna). In addition, the BIPM, although it

does not offer a high-dose service, took part at a lower

dose level (1 kGy) to provide a direct link to the

international reference for absorbed dose to water in

60Co. To render the comparison more robust, two

transfer dosimeters were selected for the comparison; the

alanine/ESR dosimetry system of the NIST (Humphries

et al., 1998) and that of the NPL (Sharpe and Sephton,

2000) (the previous CCRI comparison was conducted

using the IAEA alanine as the transfer dosimeter).

2. High-dose standards and transfer dosimeters

For each of the seven participating institutes, the basis

of the 60Co standard for absorbed dose to water and the

means of transfer of the dosimetry to an industrial

irradiator are summarized in Table 1. Also given in the

table is the combined relative standard uncertainty ui of

the mean absorbed dose to water Dw,i over the

dimension of each alanine transfer dosimeter, as

estimated by each irradiating institute. It should be

noted that the information in Table 1 reflects the highdose standards at the time of the comparison.

The NIST alanine dosimeters for use in 60Co are

supplied in watertight cylindrical holders nominally

12 mm in diameter and 29 mm in length. The stated

relative standard uncertainty of dose estimates derived

using the NIST dosimeters is 1.1%. The NPL alanine

dosimeters are also supplied in cylindrical holders,

nominally 12 mm in diameter and 17 mm in length;

these are not normally watertight but may be made so if

requested. The stated relative standard uncertainty of

dose estimates derived using the NPL dosimeters is also

1.1%. Over the temperature range of the comparison,

both the NIST dosimeter (Nagy et al., 2000) and the

NPL dosimeter have a temperature coefficient of

approximately 0.15%/K.

3. Comparison procedure

A protocol for the comparison was issued by the

BIPM in May 1998. Each irradiating institute (other

than the BIPM) was sent, in late August 1998, fourteen

alanine transfer dosimeters from the NIST and fourteen

from the NPL. Of each fourteen, two remained

unirradiated (as control dosimeters) and four were

irradiated to each of three nominal dose levels; 5 kGy,

15 kGy1 and 30 kGy. Irradiations at all institutes took

place in the two-week period beginning 7 September

19982

. The dosimeters were returned immediately to the

issuing institutes with information on irradiation temperatures but no information on dose estimates. By

January 1999, all irradiating laboratories and both

ARTICLE IN PRESS

Table 1

Basis of estimates of absorbed dose to water at the various institutes at the time of the comparison

Institute Standard of absorbed dose to

water in 60Co reference field

Transfer to high-dose

irradiator

ui/% Reference

BIPM Primary standard ionization

chamber

–a 0.4 Boutillon and Perroche

(1993)

ENEA Graphite calorimeter+thickwalled ionization chamberDichromate dosimeter via

Fricke dosimeter in

calibration irradiator

1.5 Guerra et al. (1996) and

Laitano (1999)

IAEA Secondary standard ionization

chamberb

NPL dichromate dosimeter 1.2 Mehta and

Girzikowsky (1999)

NIM Fricke dosimeter –a 1.5 Zong Yuda et al. (1998)

NIST Water calorimeter Alanine dosimeter 0.9 Humphries et al. (1998)

NPL Graphite calorimeter+scaling

theorem

Fricke dosimeter 1.1 Burns (1994) and

Sharpe and Burns

(1995)

PTB Total absorption in Fricke

dosimeter

Fricke dosimeter 1.5/1.0c Feist (1982) and

Schneider (2002)

a

No irradiator employed; alanine transfer dosimeters irradiated directly in 60Co reference field. b

Although the IAEA secondary standard ionization chamber is traceable to the BIPM, the high-dose field is traceable to the NPL.

c

First figure refers to irradiation of NIST alanine, second to NPL alanine (difference due to dosimeter sizes).

1

The ENEA irradiated to a dose level of 10 kGy instead of

15 kGy. 2

The IAEA was unable to irradiate the NIST dosimeters

because of a small difference between the nominal size of the

dosimeters and their actual size.

1088 D.T. Burns et al. / Radiation Physics and Chemistry 75 (2006) 1087–1092
issuing laboratories had sent their dose estimates to the

BIPM for analysis, along with an information sheet

giving details of the basis of the dose estimates.

However, documentation of the standards and particularly their uncertainty budgets were not available for all

institutes until some time later.

Unlike the other institutes, the BIPM does not

maintain a high-dose irradiation field and was included

in the comparison principally as the co-ordinator.

Nevertheless the BIPM holds a primary standard for

absorbed dose to water in 60Co gamma radiation at

radiotherapy levels and therefore also took part in the

irradiations, following the same protocol except that

only one dose level (1 kGy) was practicable because of

the very low dose rate (0.013 kGy h–1). In order to

sustain the ‘blind’ nature of the present comparison, the

irradiations at the BIPM were performed independently

of the person responsible for the analysis of the data

from all institutes.

The irradiation geometry was not specified in detail in

the protocol, but rather it was preferred that each

irradiating institute use their normal arrangement. This

policy was adopted in order that the dose estimates be

representative of those routinely disseminated by each

institute, rather than modified for the purpose of the

present comparison. All institutes other than the NIM

and the BIPM employed a laboratory-scale self-shielded

irradiator. The NIM and the BIPM irradiated the

alanine dosimeters in a water phantom under their

reference conditions in 60Co.

4. Results and discussion

The results using the NIST alanine transfer dosimeters are given in Table 2. In this table, Ri,NIST is the

mean value, for each institute and at each dose level, of

the four values for Rdosim, where

Rdosim 1⁄4 Dw;i=Dw;alan. (1)

Here, Dw,i is the mean absorbed dose to water over the

dimension of each transfer dosimeter as estimated by the

irradiating institute i, and Dw,alan is the same quantity as

estimated by the NIST using their alanine dosimeter.

Also given in the table is the statistical standard

uncertainty si,NIST of Ri,NIST.

The results for Ri,NIST are shown in Fig. 1. The

uncertainty bars represent the statistical standard

uncertainty si,NIST only. For each of the three dose

levels, the relative standard deviation of the four results

for Ri,NIST is 0.5%. Recalling the values for the standard

uncertainty ui given in Table 1, the results for Ri,NIST for

each dose level show agreement between the institutes at

a level that is well within one standard uncertainty ui for

each institute.

The results using the NPL alanine are given in Table 3

and Fig. 2. The relative standard deviation of the five

results for Ri,NPL at each dose level is 1.1%. However, it

is evident from both figures that the NIST dose

estimates are relatively high. Removing the NIST results

from Fig. 2 reduces the relative standard deviation to

0.5%, 0.6% and 1.0% for the three dose levels 5, 15 and

30 kGy, respectively. Again, the values for 5 and 15 kGy

are lower than one would expect from the standard

uncertainties ui given in Table 1 and may indicate that

some uncertainty components have been overestimated.

The result for the BIPM at 1 kGy relative to the NPL

alanine is consistent at the 0.2% level with the results of

international comparisons of reference absorbed dose to

water in 60Co. For the BIPM relative to the NIST the

difference from the international comparison result is

ARTICLE IN PRESS

Table 2

Absorbed-dose estimates for irradiations involving the NIST

transfer dosimeters

Institute Dose level/kGy Ri,NIST si,NIST

ENEA 5 0.9798 0.0020

10 0.9784 0.0014

30 0.9923 0.0024

NIM 5 0.9802 0.0029

15 0.9830 0.0038

30 0.9981 0.0031

NPL 5 0.9794 0.0022

15 0.9736 0.0026

30 0.9896 0.0011

PTB 5 0.9693 0.0017

15 0.9715 0.0016

30 0.9858 0.0020

BIPM 1 0.9949 0.0017

Nominal dose / kGy

1 10

Ri,NIST

0.96

0.97

0.98

0.99

1.00

1.01

1.02

1.03

1.04

ENEA

NIM

NPL

PTB

BIPM

Fig. 1. Results for the comparison ratios Ri,NIST using the

alanine transfer dosimeter of the NIST. The uncertainty bars

represent the statistical standard uncertainty si,NIST.

D.T. Burns et al. / Radiation Physics and Chemistry 75 (2006) 1087–1092 1089
around 0.7%, which is within the stated uncertainty ui of

Table 1.

It is also of note that the standard deviation of the

results at each dose level is significantly smaller than the

values of around 2% observed in the previous high-dose

comparison (Mehta and Girzikowsky, 2000).

The data demonstrate that the NIST alanine dose

estimates are higher than the NPL alanine estimates. This

has been seen previously in validation comparisons

conducted between the two institutes (Desrosiers and

Sharpe, private communication). The best estimate of the

ratio Dw,NIST/Dw,NPL for each dose level is evaluated from

the present data by taking the weighted mean of the five

estimates derived from the Ri,NIST and Ri,NPL values for

the ENEA, NIM, PTB, NIST and the NPL (the IAEA did

not irradiate the NIST dosimeters and the BIPM did not

use a comparable dose level). For this purpose, only the

statistical uncertainties si,NIST and si,NPL have been used

for the weighting. The values for Dw,NIST/Dw,NPL so

derived are 1.019 (statistical uncertainty 0.002) for the 5

and 15 kGy dose levels and 1.010 (0.003) for the 30 kGy

dose level. Although measurable within the statistical

uncertainties, these differences in dose estimates are within

the combined standard uncertainty (0.016).

Following the present comparison, a NIST internal

re-evaluation of its calibration service dose rates was

facilitated by improvements to the detector technology

and the experimental design (Desrosiers and Puhl, 2001).

This resulted in a decrease of 1.8% in the NIST dose

estimates for the particular arrangement used for the

present comparison, in good agreement with the ratio

Dw,NIST/Dw,NPL observed in the present work.

5. Registration in the KCDB

In the framework of the CIPM MRA, this comparison is registered as the supplementary comparison

CCRI(I)-S1. Although no degrees of equivalence are

registered for supplementary comparisons, the present

report will be referenced in Appendix B of the KCDB.

In order that the results be representative of each

institute’s present ability to disseminate absorbed dose

at high-dose levels, the following analysis uses dose

estimates Dw,alan for the NIST alanine that are reduced

by 1.8% from those used to evaluate Ri,NIST in Table 2

and a similar reduction of the estimates Dw,i used to

evaluate Ri,NPL for the NIST in Table 3.

Although the use of two transfer dosimeter systems is

more robust, only one result is permitted for each

institute at each dose level and so the two sets of data

Ri,NIST and Ri,NPL have been combined. To this end, the

best estimate of the ratio Dw,NIST/Dw,NPL evaluated

above (for each dose level) has been used to relate the

results Ri,NIST and Ri,NPL obtained for the ENEA, the

NIM and the PTB. A similar procedure has been used

for the NPL irradiation of the NIST alanine and for the

NIST irradiation of the NPL alanine. Furthermore, the

value Ri,NIST 1⁄4 1 has been included (at each dose level)

for the NIST in the data of Table 2 and Fig. 1, since the

horizontal line in the figure represents valid NIST dose

estimates that should not be lost from the analysis.

Similarly for the NPL (Table 3 and Fig. 2).

In this way, two values are obtained (at each dose

level) for each of the ENEA, NIM, PTB, NIST and the

NPL and the arithmetic mean of each pair is taken. Note

that only one estimate exists for the IAEA and that the

BIPM data are not included in this analysis since no

high-dose standard is maintained at the BIPM.

ARTICLE IN PRESS

Table 3

Absorbed-dose estimates for irradiations involving the NPL

transfer dosimeters

Institute Dose level/kGy Ri,NPL si,NPL

ENEA 5 0.9915 0.0029

10 0.9899 0.0028

30 0.9875 0.0018

NIM 5 0.9971 0.0024

15 1.0027 0.0014

30 1.0113 0.0051

NIST 5 1.0181 0.0025

15 1.0169 0.0021

30 1.0139 0.0021

PTB 5 0.9896 0.0023

15 0.9921 0.0023

30 0.9960 0.0017

IAEA 5 1.0004 0.0018

15 0.9991 0.0020

30 1.0040 0.0029

BIPM 1 1.0029 0.0014

Nominal dose / kGy

10

Ri,NPL

0.96

0.97

0.98

0.99

1.00

1.01

1.02

1.03

1.04

ENEA

NIM

NIST

PTB

IAEA

BIPM

1

Fig. 2. Results for the comparison ratios Ri,NPL using the

alanine transfer dosimeter of the NPL. The uncertainty bars

represent the statistical standard uncertainty si,NPL.

1090 D.T. Burns et al. / Radiation Physics and Chemistry 75 (2006) 1087–1092
Thus a single set of values is obtained (at each dose

level) that represents the combined data. In the final

stage of the analysis, these values are re-normalized to

give a set of values Ri (at each dose level) such that the

arithmetic mean value Rmean is unity, and the set of

differences xi 1⁄4 (Ri – Rmean) is evaluated. The standard

uncertainty uR,i of each Ri is taken to be the

corresponding ui of Table 1 combined with the small

uncertainty arising in the above analysis from the

statistical uncertainties si,NIST and si,NPL. The standard

uncertainty uR,mean of Rmean is taken as the statistical

standard uncertainty of the mean of the Ri. This is

considered to be appropriate because the standards are

almost completely uncorrelated.

The results for the differences xi and the standard

uncertainties ux,i are given in Table 4.

6. Conclusions

The evaluation of the differences xi in Section 5 reduces

the results for each institute to a single value for each dose

level. The statistical standard uncertainty of the distribution of the xi therefore represents the general level of

agreement between the institutes. The values obtained are

0.4% for the 5 and 15 kGy dose levels and 0.6% for the

30 kGy level. These results demonstrate that the high-dose

standards are in agreement at a level that is well within the

standard uncertainty ui for each institute, as given in Table

1. It should be noted, however, that this level of agreement

was obtained following the change to the NIST dose

estimates noted in Section 4.

Acknowledgements

The authors would like to acknowledge the help of

their colleagues at each institute, in particular; G. Ratel

at the BIPM, J. M. Puhl at the NIST, K. Rajendran at

the NPL, A. Guerra and M. Pimpinella at the ENEA, R.

Girzikowsky at the IAEA and D. M. Boche at the PTB.

References3

Boutillon, M., Perroche, A.-M., 1993. Ionometric determination of absorbed dose to water for cobalt-60 gamma rays.

Phys. Med. Biol. 38, 439–454.

Burns, J.E., 1994. Absorbed-dose calibrations in high-energy

photon beams at the National Physical Laboratory:

conversion procedure. Phys. Med. Biol. 39, 1555–1575.

CIPM, 1999. Mutual recognition of national measurements

standards and of measurement certificates issued by

national metrology institutes. Bureau International des

Poids et Mesures, Se`vres.

Desrosiers, M.F., Puhl, J.C., 2001. NIST 60Co radiationprocessing calibration fields. CCRI(I)/01-16, Bureau International des Poids et Mesures, Se`vres.

Feist, H., 1982. Determination of the absorbed dose to water

for high-energy photons and electrons by total absorption

of electrons in ferrous sulphate solution. Phys. Med. Biol.

27, 1435–1447.

Guerra, A.S., Laitano, R.F., Pimpinella, M., 1996. Characteristics of the absorbed-dose-to-water standard at ENEA.

Phys. Med. Biol. 41, 657–674.

Humphries, J.C., Puhl, J.M., Seltzer, S.M., McLaughlin, W.L.,

Desrosiers, M.F., Bensen, D.L., Walker, M.L., 1998. Radiation

processing dosimetry calibration services and measurement

assurance program. NIST Special Publication 250-44. National

Institute of Standards and Technology, Gathersburg, MD.

Laitano, R.F., 1999. Dosimetry standards at the ENEAINMRI. CCRI(I)/99-21, Bureau International des Poids et

Mesures, Se`vres.

Mehta, K., Girzikowsky, R., 1999. IAEA reference dosimeter:

alanine-ESR. Proceedings of a Symposium on Techniques

ARTICLE IN PRESS

Table 4

Results at each dose level for the difference xi 1⁄4 (Ri – Rmean), where Ri is the combined comparison result for institute i and Rmean 1⁄4 1,

and the combined standard uncertainty ux,i of this difference. These results are obtained following adjustment of the NIST dose

estimates as noted in Section 4

Institute Dose level

5 kGy 15 kGy 30 kGy

xi ux,i xi ux,i xi ux,i

ENEA 0.002 0.015 0.004 0.015 0.005 0.015

IAEA 0.003 0.012 0.002 0.012 0.004 0.013

NIM 0.001 0.015 0.005 0.015 0.010 0.016

NIST 0.004 0.010 0.003 0.010 0.006 0.011

NPL 0.002 0.011 0.001 0.011 0.000 0.011

PTB 0.008 0.013 0.006 0.013 0.004 0.013

3

CCRI documents from 2001 onwards are available on the

BIPM website (www.bipm.org). Older BIPM references

(including CCRI and CCEMRI reports) are available on

request from the BIPM or from the corresponding author

(dburns@bipm.org).

D.T. Burns et al. / Radiation Physics and Chemistry 75 (2006) 1087–1092 1091
for High-Dose Dosimetry in Industry, Agriculture and

Medicine, Vienna 2–5 Nov. 1998, IAEA-SM-356/63. International Atomic Energy Agency, Vienna, pp. 299–304.

Mehta, K., Girzikowsky, R., 2000. IAEA high-dose intercomparison in 60Co field. Appl. Radiat. Isot. 52, 1179–1184.

Nagy, V.Yu., Puhl, J.M., Desrosiers, M.F., 2000. Advancements in

accuracy of the alanine dosimetry system. Part 2. The influence

of the irradiation temperature. Radiat. Phys. Chem. 57, 1–9.

Schneider, M.K.H., 2002. Die Gammacell-Eine Bestrahlungsanlage fu ̈r hohe Dosen und Dosisleistungen. PTB-Berichte

der Serie Dos (Dosimetrie) Dos-42, ISBN 3-89701-912-4.

Sharpe, P., Sephton, J., 2000. An automated system for the

measurement of alanine/EPR dosimeters. Appl. Radiat.

Isot. 52, 1185–1188.

Sharpe, P.H.G., Burns, D.T., 1995. The relative response of

Fricke, dichromate and alanine dosimeters to Co-60 and

high energy electron beam radiation. Radiat. Phys. Chem.

46, 1273–1277.

Zong Yuda, Zhang Yanli, Gao Juncheng, Liu Zhimian, Xia

Xuan, Li Chenghua, Xie Liqing, 1998. Calorimeters for

absorbed dose standard of electron beam radiation processing. Radiat. Phys. Chem. 53, 549–553.

ARTICLE IN PRESS

1092 D.T. Burns et al. / Radiation Physics and Chemistry 75 (2006) 1087–1092