Final Report of the SIM 60 Co Absorbed- Dose-to-Water Comparison

Transcription

1 Fnal Report of the SIM 60 Co Absorbed- Dose-to-Water Comparson (KCDB Entry SIM.RI(I)-K4) May 2008 C. K. Ross and K. R. Shortt (NRC, Canada; plot laboratory) M. Sarav (CNEA, Argentna) A. Meghzfene (IAEA) V. M. Tovar (ININ, Mexco) R. A. Barbosa and C. N. da Slva (IRD, Brazl) L. Carrzales 1 (LSCD, Venezuela) S. M. Seltzer (NIST, USA) Report prepared by Carl Ross, Ionzng Radaton Standards, Natonal Research Councl, Ottawa, Canada K1A 0R6 Tel: , ext. 233 Fax: E-mal: carl.ross@nrc-cnrc.gc.ca 1 F. Gutt was head of the laboratory when the measurements were made at the Venezuelan SSDL n 2002 Page 1 of 19

2 Abstract Transfer chambers were used to compare the standards for 60 Co absorbed dose to water mantaned by seven laboratores. Sx of the laboratores were members of the Sstema Interamercano de Metrología (SIM) regonal metrology organzaton whle the seventh was the Internatonal Atomc Energy Agency (IAEA) laboratory n Venna. The Natonal Research Councl (NRC) acted as the plot laboratory for the comparson. Because of the partcpaton of laboratores holdng prmary standards, the comparson results could be lnked to the key comparson reference value mantaned by the Bureau Internatonal des Pods et Mesures (BIPM). The results for all laboratores were wthn the expanded uncertanty (two standard devatons) of the reference value. The estmated relatve standard uncertanty on the comparson between any par of laboratores ranged from 0.6 % to 1.4 %. The largest dscrepancy between any two laboratores was 1.3 %. Page 2 of 19

3 1 Introducton A comparson of standards for 60 Co absorbed dose to water was carred out between seven laboratores. Sx of the laboratores were natonal metrology nsttutes (NMIs) and members of the Sstema Interamercano de Metrología (SIM) regonal metrology organzaton. The seventh was the Internatonal Atomc Energy Agency (IAEA) laboratory n Venna. The Natonal Research Councl (NRC, Canada) was the plot laboratory for the comparson, whch took place between 2000 and The comparson was carred out usng three transfer onzaton chambers that were crculated among the laboratores. Two of the laboratores (the Natonal Insttute of Standards and Technology (NIST) and the NRC) mantan prmary standards for absorbed dose to water and ther partcpaton n the comparson allows the results to be lnked to the key comparson data base (KCDB) [1] of the Mutual Recognton Arrangement of the Internatonal Commttee for Weghts and Measures (CIPM MRA) [2]. Ths report descrbes the protocol used for the comparson and presents the results n tabular and graphcal form. It descrbes how prevous comparsons between the NRC and the BIPM and between the NIST and the BIPM can be used to lnk the data of ths comparson to the key comparson reference value. The degree of equvalence s calculated for each laboratory wth respect to the reference value as well as the degree of equvalence between any par of laboratores. 2 Partcpatng Laboratores The partcpatng laboratores are lsted n Table 1. Page 3 of 19

4 Table 1 Lstng of the laboratores that partcpated n the absorbed-dose-to-water comparson. The NIST and the NRC are prmary standards laboratores, whle the others operate as secondary standards laboratores. Laboratory Country CNEA Comsón Naconal de Energía Atómca Argentna IAEA Internatonal Atomc Energy Agency - ININ Insttuto Naconal de Investgacones Nucleares Mexco IRD 1 Insttuto de Radoproteçao e Dosmetra Brazl LSCD 2 Laboratoro Secundaro de Calbracón Dosmetrca Venezuela NIST Natonal Insttute of Standards and Technology Unted States NRC Natonal Research Councl Canada 1 The desgnated nsttute for the CIPM MRA s known as the Laboratóro Naconal de Metrologa das Radações Ionzantes (LNMRI). 2 Note that Venezuela has not yet sgned the CIPM MRA. 3 Transfer Chambers Three Exradn A12 onzaton chambers were used n the comparson wth seral numbers 101, 149 and 150. These are cylndrcal chambers constructed from C552 plastc and are ntrnscally waterproof. The man characterstcs of the A12 onzaton chamber are summarzed n Table 2. No electrometer was provded wth the chambers so each laboratory was responsble for ther own measurement of the electrcal current or charge arsng from the on pars produced n the ar cavty. Page 4 of 19

5 Table 2. Characterstcs of the cylndrcal A12 onzaton chamber. Characterstc Nomnal value Dmensons Inner dameter 6.1 mm Wall thckness 0.5 mm Cavty length 24.7 mm Tp to center of collectng volume 12.9 mm Electrode Dameter 1.0 mm Heght 21.6 mm Volume Ar cavty 0.65 cm 3 Wall Materal C552 plastc Densty 1.76 g cm -3 Buldup cap Materal C552 plastc Thckness 2.7 mm Appled voltage 300 V Sgn of collected charge Postve 4 Calbraton Coeffcents The comparson of the absorbed-dose-to-water standards was made ndrectly by comparng the calbraton coeffcents, N D,w, of the three transfer chambers as determned by the ndvdual laboratores. The calbraton coeffcent s gven by N = D Q, (1) D,w w / where Q s the charge per unt tme or current, I, due to postve ons produced n the cavty gas when the delvered absorbed dose to water rate s D w. All laboratores were asked to report values of N D,w that would apply f postve charge were collected. Each chamber was postoned so that the centre of ts senstve volume was at the reference pont and D w s the absorbed dose to water that would be delvered to the reference pont n the absence of the chamber. In order to get the current, I, from the measured current, I m, a number of correctons must be consdered. These nclude: Page 5 of 19

6 Leakage current: Ths s the current measured when the prmary radaton feld s blocked. For all the chambers at all the laboratores, the leakage current was less than 0.01 % of the current measured when the chamber was exposed to the 60 Co beam. In practce, the measured leakage current ncludes contrbutons from background radaton. Recombnaton: No correcton for recombnaton was appled. The volume recombnaton s neglgble for absorbed dose to water rates less than 15 mgy s -1 for ths chamber type and polarzng voltage and the ntal recombnaton wll be the same for all the laboratores. Temperature and pressure normalzaton: For all the laboratores, the measured onzaton current of the transfer chambers was normalzed to a temperature of K and a pressure of kpa. (Ths s consstent wth normal practce at the NIST and the NRC but several of the other laboratores would normally use a reference temperature of K.) Humdty: None of the laboratores appled a correcton to ther measured current (or charge) for humdty. As long as the relatve humdty s wthn the range from 10 to 80 % for all the laboratores, the effect on the chamber calbraton coeffcent of varatons n the humdty s less than 0.1 %. Radal non-unformty: It was assumed than any correcton for radal nonunformty would be smlar for all the 60 Co beams and thus need not be appled when comparng calbraton coeffcents. Page 6 of 19

7 5 Absorbed-Dose-to-Water Standards The absorbed-dose-to-water standards of the secondary laboratores are all traceable to the absorbed-dose-to-water standard mantaned by the BIPM. Three of the laboratores (the CNEA, the IAEA and the IRD) have ther secondary standards calbrated drectly by the BIPM. The other two (the ININ and the LSCD) have ther chambers calbrated at the IAEA, whch n turn s traceable to the BIPM. The NIST standard for 60 Co absorbed dose to water s based on a water calormeter operatng at room temperature [3-5]. The NRC standard s also based on a water calormeter but t s operated at 4 C [6,7]. The BIPM standard s based on a graphte cavty chamber of pancake geometry [8,9] and ths s the standard to whch the secondary laboratores are traceable. Results of recent comparsons between the BIPM and several NMIs, ncludng the NIST and the NRC, are gven n [9]. 6 Chamber Calbratons The chambers were crculated among the varous laboratores as follows: the NRC to the IAEA; the IAEA to the NRC; the NRC to the CNEA; the CNEA to the IRD; the IRD to the NRC; the NRC to the LSCD; the LSCD to the ININ; the ININ to the NRC; the NRC to the NIST; the NIST to the IAEA; the IAEA to the NRC. By havng the chambers return several tmes to the NRC, ther stablty could be verfed. The condtons under whch the chambers were calbrated were smlar at all of the laboratores. The calbratons were carred out n a sutable water phantom Page 7 of 19

8 provded by each laboratory. Because the A12 onzaton chamber s waterproof, t can be mounted drectly n water. The center of the chamber was postoned at a depth of 5 cm. The surface of the water was postoned approxmately 1 m from the 60 Co source and the feld sze was approxmately 10 cm by 10 cm at 1 m. The polarzng voltage was set to 300 V and laboratores were asked to report values of ND,w that would apply f postve charge were collected. None of the laboratores used a shutter or source transfer system to defne the rradaton tme. Instead, each chamber was rradated contnuously durng the measurement sesson and the charge accumulated by the electrometer was measured at welldefned tmes. 7 Results The calbraton coeffcents, N D,w, obtaned by the dfferent laboratores for each of the three transfer chambers are gven n Table 3. The chambers were calbrated on two separate occasons at the IAEA and on fve separate occasons at the NRC. The values reported n Table 3 for these laboratores are the averages of the repeated calbratons. The ratos of the calbraton coeffcents for each laboratory to that of the NRC are gven n columns 3, 5 and 7. The mean values of the ratos for the three chambers and for each of the laboratores are gven n the fnal column. The spread of these mean values s about 1.3 %. Page 8 of 19

9 Table 3. The absorbed-dose-to-water calbraton coeffcents, expressed n mgy/nc, obtaned by each laboratory for each onzaton chamber are gven n columns 2, 4 and 6. Columns 3, 5 and 7 report the rato of the calbraton coeffcent for a gven NMI to that of the NRC whle column 8 gves the mean value of the rato for all three chambers. Laboratory #101 #149 #150 Mean CNEA IAEA ININ IRD LSCD NIST NRC The ratos of the calbraton coeffcents obtaned by each laboratory for each chamber to that obtaned by the NRC are shown graphcally n Fgure 1. Ideally, all three calbraton-coeffcent ratos for each laboratory should be approxmately the same. Instead, dfferences of up to 0.8 % are apparent. Calbratons were repeated at the NRC fve tmes durng the course of the comparson, and these results are shown n Fgure 2. The calbraton coeffcents for the three chambers were always consstent to better than 0.15 %. Thus, the dfferences apparent n Fgure 1 are probably not due to changes n chamber response due to transport. 8 Uncertantes Each laboratory reported the key components that contrbuted to ther uncertanty budget, and the results are summarzed n Table 4. The uncertanty on the prmary standards as reported by the NIST and the NRC are lsted n the row labeled N D,w of reference chamber. Components 2, 3, 5 and 6 n ths secton of the table have already been ncorporated nto ther respectve overall uncertanty. However, there wll be a component due to source decay because both NIST and NRC use the 60 Co half-lfe to track the absorbed dose rate as a functon of tme. Page 9 of 19

10 Fgure 1. Graphcal summary of the absorbed-dose-to-water calbraton coeffcents reported by each laboratory to that of the NRC for each of the onzaton chambers. N D,w (NMI)/N D,w (NRC) #101 #149 # CNEA IAEA ININ IRD LSCD NIST NRC Fgure 2. Summary of the results obtaned for repeated chamber calbratons at the NRC. The calbraton coeffcent for each chamber s repeat measurement s shown wth respect to the average of all fve calbraton coeffcents N D,w (NRC)/Average N D,w (NRC) Repeat dentfer #101 #149 #150 Page 10 of 19

11 The onzaton-chamber current s obtaned by all of the laboratores by measurng the charge collected n a known tme nterval. Most of the laboratores assume the uncertanty of the measured tme nterval s neglgble and that the uncertanty of the current s domnated by the uncertanty of the charge. If the laboratory reported separate uncertantes for the charge and tme, they have been combned n quadrature to obtan the uncertanty of the current. 9 Degrees of Equvalence Two of the laboratores partcpatng n the present comparson (the NRC and the NIST) mantan prmary standards for absorbed dose to water and have partcpated n prevous comparsons wth the BIPM. Thus, we can lnk the results of ths comparson to the absorbed-dose-to-water standard mantaned by the BIPM. For the NIST, usng the comparson results reported n [9] gves (51) for Dw,NIST / D w,bipm, whle the correspondng result for the NRC s (51). Note that D w,nist, D w,bipm and, more generally, D w,nmi, are the values of the absorbed dose to water that would be reported by each natonal standard for dentcal rradaton condtons. The numbers n parentheses represent the standard uncertantes of the last two dgts of the ratos. Although both the NIST and the NRC could be used as the lnk, n what follows, we have used the NRC result whch s more robust as they measured the chambers fve tmes. Ths also enables the NIST to update ther 1997 comparson value n the KCDB. Page 11 of 19

12 Table 4. Summary of the standard uncertanty estmates reported by the varous laboratores partcpatng n the comparson. Relatve standard uncertanty (%) Source of uncertanty CNEA IAEA ININ IRD LSCD NIST NRC A B A B A B A B A B A B A B Related to dose to water rate 1 N D,w of ref. chamber Long-term stablty of ref. chamber Postonng of ref. chamber Source decay Temperature and pressure Charge Related to the transfer nstrument 7 Chamber postonng Temperature, pressure, humdty Charge Quadratc summaton Combned standard uncertanty a 0.42 a The overall uncertanty on absorbed dose-to-water calbratons by the NIST was ncreased to 0.6 % n 2001, due to uncertanty related to the feld sze [5]. Page 12 of 19

13 In ths comparson, each NMI reported calbraton coeffcents. However, the calbraton coeffcents are proportonal to the absorbed dose to water as determned by the natonal standard of the NMI, so ratos of calbraton coeffcents wll be equal to absorbed-dose-to-water ratos. In the followng, any absorbed-dose-towater rato can be replaced by the numercal value of the equvalent rato of calbraton coeffcents. The CCRI meetng n 1999 agreed that, n an absorbed-dose-to-water key comparson, the BIPM value of the comparson quantty would be taken as the key comparson reference value (KCRV). Furthermore, the Key Comparson Workng Group of the CCRI(I) confrmed at ts meetng n Aprl 2008 that, for these dosmetry comparsons, the degree of equvalence, D, s defned as the dfference between the absorbed dose to water measured by a partcpatng NMI and the KCRV, dvded by the KCRV and the expanded uncertanty of ths dfference. That s, D = ( D D )/ D = D / D 1= R 1, (2) w, w,bipm w,bipm w, w,bipm where the ndex,, s used to dentfy the NMI and R D / D. (3) = w, w,bipm R for each NMI can be found by multplyng NDw,NMI / N Dw,NRC as reported n Table 3 by Dw,NRC / D w,bipm, as gven above. The uncertanty, u R,, of R s obtaned by combnng the uncertanty of the calbraton coeffcents reported by the NMI, the absorbed-dose-to-water uncertanty of the BIPM standard and the uncertanty of the lnk through the rato D / D, ncludng the effects of correlatons between the laboratores. We w,nrc w,bipm denote by u the overall relatve uncertanty reported by a partcular NMI of ts calbraton coeffcent and by u ( k ) a partcular component, k, of the uncertanty. Page 13 of 19

14 We use u r to denote the relatve uncertanty of the lnk through Dw,NRC / D w,bipm and u stab to denote the uncertanty due to the long term stablty of the transfer chambers. Then u 2 = u 2 + u 2 + u 2 + u 2 ( f u ( k)) 2 ( f u ( k )) 2, (4) R, BIPM r stab k k BIPM k k where the last two terms account for any correlated quanttes between the NMI and the BIPM. The factor, f k, whch can range from zero to unty, accounts for the possblty that the quanttes are not fully correlated. The only correlated quantty between the varous NMIs and the BIPM s the BIPM calbraton coeffcent of the natonal secondary standards. All of the secondary standards laboratores partcpatng n ths comparson are traceable to the BIPM and thus the uncertanty assocated wth the BIPM absorbed-dose-to-water standard must be subtracted. Reference [10] gves a value of 0.29 % for ths quantty and f k s taken to be unty. At frst sght, one mght set ur to 0.51 %, whch s the uncertanty gven earler fordw,nrc / D w,bipm. However, the uncertanty of the lnk through the NRC to the BIPM standard depends only on the stablty of the results obtaned usng transfer chambers and not on any estmate of how well ether laboratory can determne the absorbed dose to water. Accordng to [10] a reasonable estmate for ur s 0.11 %. The value for u stab was obtaned as recommended by Burns and Allsy-Roberts [11] by calculatng the standard devaton of the repeated measurements at the NRC for each of the chambers. The data are shown graphcally n Fgure 2 and gve values of the relatve standard devaton for each of the chambers of 0.06 %, 0.03 % and 0.05 %. The mean value of 0.05 % was used for u stab. It may be that Page 14 of 19

15 some of the uncertanty attrbuted to u stab s already ncluded n the uncertantes quoted by the NMIs. We have not tred to quantty these effects. The standard uncertanty of of wth R because D s approxmately equal to the relatve uncertanty R s close to unty. By conventon, the uncertanty, D s gven as twce the standard uncertanty. Values for D and U, assocated U are gven n the shaded columns of Table 5 and are shown n graphcal form n Fgure 3. The degree of equvalence, D j, between any par of NMIs, and j, s gven by D = D D = R R, (5) j j j wth expanded uncertanty good approxmaton by U j.the standard uncertanty of u = u + u + 2 u ( f u ( k)) ( f u ( k)), j j stab k k j k k D j s gven to a very (6) where the notaton s smlar to that used wth equaton (4). The form of equaton (6) may seem surprsng because t nvolves only the relatve uncertantes, u and u j, of the NMI absorbed-dose-to-water dssemnatons and there s no term related to the uncertanty of D w,bipm. Ths follows by makng the reasonable assumpton that, for purposes of estmatng the uncertantes, all of the absorbeddose-to-water values can be consdered equal. Note that u stab enters twce n equaton (6) (once for each laboratory) but only once n equaton (4). One correlaton to be consdered when evaluatng equaton (6) s due to the fact that all the secondary standard calbraton coeffcents are traceable to the BIPM. Thus, the uncertanty contrbuted by the BIPM absorbed-dose-to-water standard must be subtracted. Its value s taken to be 0.29 % [10] and the correlaton s assumed to be complete ( f k = 1). A second correlaton s due to the heat defect for water, whch s common to the water calormeters operated by the NIST and the Page 15 of 19

16 NRC, and ts contrbuton to the uncertanty must be subtracted (0.30 %). In ths case, f k s taken to be 0.7 [9]. Values for D j and ts assocated expanded uncertanty are gven n Table 5 for each par of laboratores partcpatng n the comparson. The largest dscrepancy between any par of laboratores s almost 1.3 %. However, n no case s the degree of equvalence between any par of laboratores larger than the expanded uncertanty. 10 Conclusons A comparson of absorbed-dose-to-water standards has been carred out between seven laboratores. The partcpatng laboratores ncluded the IAEA and sx NMIs that are members of the SIM regonal metrology organzaton. Three onzaton chambers were crculated among the seven laboratores and each laboratory was asked to provde calbraton coeffcents and assocated uncertantes. The onzaton chambers were returned several tmes to NRC durng the comparson and they showed satsfactory stablty. Because two laboratores mantanng prmary standards for absorbed dose to water partcpated n the comparson, and because these laboratores have partcpated n earler comparsons wth the BIPM, the results of all the laboratores partcpatng n ths comparson could be compared to the key comparson reference value mantaned by the BIPM. For results to be ncluded n the KCDB, the partcpant must be a sgnatory to the CIPM MRA and, unfortunately, ths s not yet the case of Venezuela. The dfference n each result wth respect to the KCRV, expressed as a fracton, was calculated for each laboratory and n each case ths dfference was smaller Page 16 of 19

17 than the expanded uncertanty, ndcatng that each laboratory has a satsfactory realzaton of the gray for 60 Co absorbed dose to water. The largest dscrepancy between any par of laboratores was almost 1.3 %. Ths suggests that there s stll room for mprovement n dssemnatng the 60 Co absorbed dose to water, as the uncertantes assocated wth the transfer system are almost neglgble. Table 5. The degree of equvalence of each laboratory wth respect to the reference value s gven n the shaded columns. The degree of equvalence s the dfference between the value obtaned by a partcular NMI and that obtaned by the BIPM, dvded by the BIPM value, along wth the expanded uncertanty on ths fractonal dfference. The degrees of equvalence between any par of laboratores are gven n the rest of the table. Lab Lab j CNEA IAEA ININ IRD LSCD D U D j U j D j U j D j U j D j U j D j U j /(mgy/gy) /(mgy/gy) /(mgy/gy) /(mgy/gy) /(mgy/gy) /(mgy/gy) CNEA IAEA ININ IRD LSCD NIST NRC Lab Lab j NIST NRC D U D j U j D j U j /(mgy/gy) /(mgy/gy) /(mgy/gy) CNEA IAEA ININ IRD LSCD NIST NRC Page 17 of 19

18 Fgure 3. Graphcal representaton of the degrees of equvalence for the varous laboratores partcpatng n the comparson. The degree of equvalence s the dfference between the value obtaned by a partcular NMI and that obtaned by the BIPM, dvded by the BIPM value, along wth the expanded uncertanty on ths fractonal dfference. The expanded uncertanty corresponds to twce the standard uncertanty. SIM.RI(I)-K4 30 Degrees of equvalence for 60 Co absorbed dose to water D NMI / (mgy/gy) CNEA IAEA ININ IRD LSCD NIST NRC 11 Acknowledgements We are grateful to Drs. Penny Allsy-Roberts and Davd Burns of the BIPM for revewng the manuscrpt and provdng many helpful comments. Page 18 of 19

19 References [1] BIPM, (2002), The BIPM key comparson database, [2] BIPM, (1999), Mutual recognton of natonal measurement standards and of calbraton and measurement certfcates ssued by natonal metrology nsttutes, [3] DOMEN S.R., (1994), A sealed water calormeter for measurng absorbed dose, J. Res. Natl. Inst. Stand. Technol., 99, [4] SELTZER S.M. and SHOBE J., (1999), Present status of the NIST standard for absorbed dose to water n 60 Co gamma-ray beams, CCRI(I) 99-8, (BIPM, Pars). [5] SHOBE J., (2001), Addtonal uncertanty n NIST 60 Co absorbed-dose-towater calbratons, CCRI(I) 01-14, (BIPM, Pars). [6] SEUNTJENS J.P., ROSS C.K., KLASSEN N.V. and SHORTT K.R., (1999), A status report on the NRC sealed water calormeter, NRC Report PIRS- 0584, (Natonal Research Councl, Ottawa). [7] MEDIN J., ROSS C.K., STUCKI G., KLASSEN N.V. and SEUNTJENS J.P., (2004), Commssonng of an NRC-type sealed water calormeter at METAS usng 60 Co γ-rays, Phys. Med. Bol., 49, [8] BOUTILLON M. and PERROCHE A.M., (1993), Ionometrc determnaton of absorbed dose to water for cobalt-60 gamma rays, Phys. Med. Bol., 38, [9] ALLISY-ROBERTS P.J. and BURNS D.T., (2005), Summary of the BIPM.RI(I)-K4 comparson for absorbed dose to water n 60 Co gamma radaton, Metrologa, 42, [10] ALLISY-ROBERTS P.J., BURNS D.T., SHORTT K.R. and ROSS C.K., (2000), Comparson of the standards of absorbed dose to water of the NRC, Canada and the BIPM for 60 Co γ rays, BIPM-99/13, (BIPM, Pars). [11] BURNS D.T. and ALLISY-ROBERTS P.J., (2007), The evaluaton of degrees of equvalence n regonal dosmetry comparsons, CCRI(I) 07-04, (BIPM, Pars). Page 19 of 19