BIPM comparison BIPM.RI(II)-K1.Eu-155 of the activity measurements of the radionuclide 155 Eu. G. Ratel and C. Michotte BIPM

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1 BIPM comparson BIPM.RI(II)-K1.Eu-155 of the actvty measurements of the radonuclde 155 Eu G. Ratel and C. Mchotte BIPM Abstract In 1993, a natonal metrology nsttute, the NPL (UK), submtted a sample of known actvty of 155 Eu to the Internatonal Reference System (SIR) for actvty comparson at the BIPM. The value of the actvty submtted was about.5 MBq. The result of ths comparson has been approved by Secton II of the Consultatve Commttee for Ionzng Radaton (CCRI(II)), comparson dentfer BIPM.RI(II)-K1.Eu Introducton The SIR for actvty measurements of γ-ray-emttng radonucldes was establshed n Each natonal metrology nsttute (NMI) may request a standard ampoule from the BIPM that s then flled (3.6 g) wth the radonuclde n lqud (or gaseous) form. The NMI completes a submsson form that detals the standardzaton method used to determne the absolute actvty of the radonuclde and the full uncertanty budget for the evaluaton. The ampoules are sent to the BIPM where they are compared wth standard sources of 6 Ra usng pressurzed onzaton chambers. Detals of the SIR method, expermental set-up and the determnaton of the equvalent actvty are all gven n [1]. Snce ts ncepton, the SIR has measured over 818 ampoules to gve 590 ndependent results for 6 dfferent radonucldes. The SIR makes t possble for natonal laboratores to check the relablty of ther actvty measurements at any tme. Ths s acheved by the determnaton of the equvalent actvty of the radonuclde and by comparson of the result wth the key comparson reference value determned from the results of prmary realzatons. These comparsons are descrbed as BIPM ongong comparsons and the results form the bass of the BIPM key comparson database (KCDB) of the Mutual Recognton Arrangement (MRA) []. The comparson descrbed n ths report s known as the BIPM.RI(II)-K1.Eu-155 key comparson.. Partcpants The NPL s the only partcpant to date for the comparson of 155 Eu actvty measurements and one ampoule was submtted n The laboratory detals are gven n Table 1. 1/6

2 Table 1. Detals of the partcpants n the BIPM.RI(II)-K1.Eu-155 Orgnal acronym NMI Full name Country Regonal metrology organzaton Date of measurement at the BIPM NPL Natonal Physcal Laboratory Unted Kngdom EUROMET NMI standardzaton methods Each NMI that submts ampoules to the SIR has measured the actvty ether by a prmary standardzaton method or by usng a secondary method, for example a calbrated onzaton chamber. In the latter case, the traceablty of the calbraton needs to be clearly dentfed to ensure that any correlatons are taken nto account. A bref descrpton of the standardzaton methods for each laboratory, the actvtes submtted and the relatve standard uncertantes (k = 1) are gven n Table. Full uncertanty budgets have been requested as part of the comparson protocol only snce Consequently, no detaled uncertanty budget was provded n ths case. The orgnal half-lfe used by the BIPM was 181 (4) days [3] and ths s n agreement wth the presently adopted value as recommended by the IAEA, 1770 (50) d [4]. Table. Standardzaton method of the partcpant for 155 Eu NMI Method used Half-lfe / d Actvty / kbq Reference date Relatve standard uncertanty 100 YY-MM-DD Type A Type B NPL 4πβ-γ concdence countng h00 UT Detals regardng the soluton submtted are shown n Table 3, ncludng any mpurtes, when present, as dentfed by the laboratory. When gven, the standard uncertantes on the evaluatons are shown. Recently the BIPM has developed a standard method for evaluatng the actvty of mpurtes usng a calbrated Ge(L) spectrometer [5]. The CCRI(II) agreed n 1999 [6] that ths method should be followed accordng to the protocol descrbed n [7] when an NMI makes such a request or when there appear to be dscrepances. /6

3 Table 3. Detals of the soluton of 155 Eu submtted NMI Chemcal composton Solvent conc. / (mol dm 3 ) Carrer: conc. /(µg g 1 ) Densty /(g cm 3 ) NPL Eu n HCl 0.1 Eu: 50 1 Relatve actvty of mpurty 15 Eu (0.00 ) % 154 Eu (0.035) % the rato of the actvty of the mpurty to the actvty of 155 Eu at the reference date 4. Results All the submssons to the SIR snce ts ncepton n 1976 are mantaned n a database known as the "mother-fle". The actvty measurement for 155 Eu arses from one ampoule and the SIR equvalent actvty for the ampoule, A e, s gven n Table 4 for the NMI,. The dates of measurement n the SIR are gven n Table 1. Although actvtes submtted are compared wth a gven source of 6 Ra, all the SIR results are normalzed to the radum source number 5 [1]. The relatve standard uncertantes arsng from the measurements n the SIR are also shown. Ths uncertanty s addtonal to that declared by the NMI for the actvty measurement shown n Table. In ths case, the SIR uncertanty s relatvely hgh because the SIR onzaton chamber s much less senstve to 155 Eu than to the two cted mpurtes, whch contrbute one thrd of the measured onzaton current. Table 4. Results of SIR measurements of 155 Eu NMI Mass of soluton /g Actvty submtted/ kbq N of Ra source used SIR A e /MBq Relatve uncertanty from the SIR Total standard uncertanty u / MBq NPL Measurements repeated at the BIPM after sx months produced the same comparson result, wthn Ths confrms the valdty of the half-lfe value now used [4] and the evaluaton of the mpurty correctons. As ths submsson has not been dentfed as a plot study, the result of the NPL s elgble for Appendx B of the MRA. However, as only one NMI has submtted an actvty for ths comparson, a key comparson reference value cannot be computed and consequently no degrees of equvalence can be derved. Nevertheless, detals of how ths wll be done once another NMI submts a sample are gven n the next secton of ths report for completeness. 3/6

4 4.1 The key comparson reference value The key comparson reference value s derved from the unweghted mean of all the results submtted to the SIR wth the followng provsons: a) only prmary standardzed solutons are accepted, or onzaton chamber measurements that are drectly traceable to a prmary measurement n the laboratory; b) each NMI has only one result (normally the most recent result or the mean f more than one ampoule s submtted); c) any outlers are dentfed usng a reduced ch-squared test and, f necessary, excluded from the KCRV usng the normalzed error test wth a test value of four; d) exclusons must be approved by the CCRI(II). The reduced data set used for the evaluaton of the KCRVs s known as the KCRV fle and s the reduced data set from the SIR mother-fle. Although the KCRV may be modfed when other NMIs partcpate, on the advce of the Key Comparson Workng Group of the CCRI(II), such modfcatons are only made by the CCRI(II), normally durng one of ts bennal meetngs. However, a KCRV for 155 Eu cannot be computed untl another NMI fulfls the crtera descrbed. 4. Degrees of equvalence Every NMI that has submtted ampoules to the SIR s enttled to have one result ncluded n Appendx B of the KCDB as long as the NMI s a sgnatory or desgnated nsttute lsted n the MRA. Normally, the most recent result s the one ncluded. Any NMI may wthdraw ts result only f all the partcpants agree. The degree of equvalence of a gven measurement standard s the degree to whch ths standard s consstent wth the key comparson reference value []. The degree of equvalence s expressed quanttatvely n terms of the devaton from the key comparson reference value and the expanded uncertanty of ths devaton (k = ). The degree of equvalence between any par of natonal measurement standards s expressed n terms of ther dfference and the expanded uncertanty of ths dfference and s ndependent of the choce of key comparson reference value Comparson of a gven NMI wth the KCRV The degree of equvalence of a partcular NMI,, wth the key comparson reference value s expressed as the dfference between the results D = A e KCRV (1) and the expanded uncertanty (k = ) of ths dfference, U, known as the equvalence uncertanty, hence U = u D, () takng correlatons nto account as approprate. 4/6

5 4.. Comparson of any two NMIs wth each other The degree of equvalence, D j, between any par of NMIs, and j, s expressed as the dfference n ther results D = D D = A A (3) and the expanded uncertanty of ths dfference U j where u D j = u + u j j j e e j ( f k ucorr, ) ( f k u ) k corr, k k and any obvous correlatons n the standard uncertantes for a gven component, u corr,k,, between the NMIs (such as a traceable calbraton) are subtracted usng an approprate correlaton coeffcent, f k, as are normally those correlatons comng from the SIR. The uncertantes of the dfferences between the values assgned by ndvdual NMIs and the key comparson reference value (KCRV) are not necessarly the same uncertantes that enter nto the calculaton of the uncertantes n the degrees of equvalence between a par of partcpants. k j (4) 5. Concluson The BIPM ongong key comparson for 155 Eu, BIPM.RI(II)-K1.Eu-155 currently comprses one result. Consequently, no KCRV s defned for ths radonuclde, nor can degrees of equvalence be calculated for publcaton n the BIPM key comparson database. Once another NMI contrbutes 155 Eu actvty measurements to ths comparson, the KCRV and degrees of equvalence wll be determned and the approval of the CCRI(II) sought pror to publcaton. Acknowledgements The authors would lke to thank the NMIs for ther partcpaton n ths comparson, Mr Chrstan Colas of the BIPM for hs dedcated work n mantanng the SIR snce ts ncepton and for the thousands of measurements he has made over the years, and Dr P.J. Allsy-Roberts of the BIPM for edtoral assstance. References [1] Ratel G. The nternatonal reference system for actvty measurements of γ- emttng radonucldes (SIR), BIPM Monograph XX, 00, (n preparaton). [] MRA: Mutual recognton of natonal measurement standards and of calbraton and measurement certfcates ssued by natonal metrology nsttutes, Internatonal Commttee for Weghts and Measures, 14 October 1999, 45 pp. Avalable from the BIPM webste: [3] Emery J.F. et al., Nucl. Sc. Eng. 48, 197, /6

6 [4] IAEA-TECDOC-619, X-ray and gamma-ray standards for detector calbraton, (1991), Venna, IAEA. [5] Mchotte C., Effcency calbraton of the Ge(L) detector of the BIPM for SIRtype ampoules, Rapport BIPM-1999/03, 15 pp. [6] CCRI Rapport de la 16 e sesson (June 1999), CCRI(II) (Pars: Off. Lb.) [7] Mchotte C., Protocol on the use of the calbrated spectrometer of the BIPM for the measurement of mpurtes n ampoules submtted to the SIR, CCRI(II)/01-01, 001, p. 6/6