Key comparison BIPM.RI(I)-K7 of the air-kerma standards of the ENEA-INMRI, Italy and the BIPM in mammography x-rays

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1 Key comprison BIPM.RI(I)-K7 of the ir-kerm stndrds of the ENEA-INMRI, Itly nd the BIPM in mmmogrphy x-rys C Kessler, D T Burns, P Roger, M P Toni*, M Pinto*, M Bovi*, G Cppdozzi*, C Silvestri* Bureu Interntionl des Poids et Mesures, Pvillon de Breteuil, F Sèvres Cedex * Istituto Nzionle di Metrologi delle Rdizioni Ionizznti, C.R. Cscci, c.p Rome, Itly Abstrct A first key comprison hs been mde between the ir-kerm stndrds of the ENEA-INMRI, Itly nd the BIPM in mmmogrphy x-ry bems. The results show the stndrds to be in greement t the level of the stndrd uncertinty for the comprison of 4.8 prts in The results re nlysed nd presented in terms of degrees of equivlence, suitble for entry in the BIPM key comprison dtbse. 1. Introduction An indirect comprison hs been mde between the ir-kerm stndrds of the Istituto Nzionle di Metrologi delle Rdizioni Ionizznti (INMRI) of the Agenzi Nzionle per le Nuove Tecnologie, l Energi e lo Sviluppo Economico Sostenibile (ENEA), Itly nd the Bureu Interntionl des Poids et Mesures (BIPM) in the W/Mo mmmogrphy bems in the x- ry rnge from 23 kv to 35 kv. A thin-window prllel-plte ioniztion chmber ws used s trnsfer instrument. The mesurements t the BIPM took plce in April 2014 using the reference conditions recommended by the CCRI nd described by Allisy et l (2011). Supporting mesurements t the ENEA-INMRI continued until Mrch To verify the stbility of the trnsfer instrument, dditionl mesurements were done t the BIPM when the ENEA supplied the finl results in April Determintion of the ir-kerm rte For free-ir ioniztion chmber stndrd with mesuring volume V, the ir-kerm rte is determined by the reltion I W K ir 1 k i (1) V e g ir 1 ir i where ir is the density of ir under reference conditions, I is the ioniztion current under the sme conditions, W ir is the men energy expended by n electron of chrge e to produce n ion pir in ir, g ir is the frction of the initil electron energy lost through rditive processes in ir, nd k i is the product of the correction fctors to be pplied to the stndrd. The vlues used for the physicl constnts ir nd W ir /e re given in Tble 1. For use with this dry-ir vlue for ir, the ioniztion current I must be corrected for humidity nd for the difference between the density of the ir of the mesuring volume t the time of mesurement nd the vlue given in the tble. 1 1 For n ir temperture T round 293 K, pressure P nd reltive humidity round 50 % in the mesuring volume, 1/12

2 Tble 1. Physicl constnts used in the determintion of the ir-kerm rte Constnt Vlue u i ir b kg m W ir / e J C b u i is the reltive stndrd uncertinty. Density of dry ir t T 0 = K nd P 0 = kp. 3. Detils of the stndrds Both free-ir chmber stndrds re of the conventionl prllel-plte design. The mesuring volume V is defined by the dimeter of the chmber perture nd the length of the collecting region. The BIPM ir-kerm stndrd is described by Boutillon et l (1969); the correction fctors for the mmmogrphy qulities re described by Kessler (2006) nd the chnges mde to certin correction fctors, by Burns et l (2009). A description of the ENEA-INMRI CAL02 stndrd cn be found in the report of the direct comprison crried out t the BIPM in the W/Al low-energy x-ry bems (Burns et l 2011). The min dimensions, the mesuring volume nd the polrizing voltge for ech stndrd re shown in Tble 2. Tble 2. Min chrcteristics of the stndrds Stndrd BIPM L-01 ENEA- INMRI 4. The trnsfer instrument Aperture dimeter / mm Air pth length / mm Collecting length / mm Electrode seprtion / mm Collector width / mm Mesuring volume / mm Polrizing voltge / V Determintion of the clibrtion coefficient for trnsfer instrument The ir-kerm clibrtion coefficient N K for trnsfer instrument is given by the reltion N K K (2) I tr where K is the ir-kerm rte determined by the stndrd using (1) nd I tr is the ioniztion current mesured by the trnsfer instrument nd the ssocited current-mesuring system. The the correction for ir density involves temperture correction T / T 0, pressure correction P 0 / P nd humidity correction k h = At the BIPM, the fctor is included to ccount for the compressibility of dry ir between T round 293 K nd T 0 = K. 2/12

3 current I tr is corrected to the reference conditions of mbient ir temperture, pressure nd reltive humidity chosen for the comprison (T = K, P = kp nd h = 50 %). To derive comprison result from the clibrtion coefficients N K,BIPM nd N K,NMI mesured, respectively, t the BIPM nd t ntionl mesurement institute (NMI), differences in the rdition qulities must be tken into ccount. Normlly, ech qulity used for the comprison hs the sme nominl generting potentil t ech institute, but the hlf-vlue lyers (HVLs) might differ. A rdition qulity correction fctor k Q is derived for ech comprison qulity Q. This corrects the clibrtion coefficient N K,NMI determined t the NMI into one tht pplies t the equivlent BIPM qulity nd is derived by interpoltion of the N K,NMI vlues in terms of HVL. The comprison result t ech qulity is then tken s k N Q K,NMI RK,NMI (3) N K,BIPM For the present comprison, this is discussed in section Detils of the trnsfer instrument A thin-window prllel-plte ioniztion chmber belonging to the ENEA-INMRI ws used s the trnsfer instrument for the comprison. Its min chrcteristics re given in Tble 3. The reference point for the chmber ws tken to be on the xis defined by the entrnce window (pssing through the centre of the entrnce window). The reference plne for the chmber is defined by the red line round the body of the chmber. Tble 3. Min chrcteristics of the trnsfer chmber Chmber type Rdcl RC6M Seril number Window mteril metllized polyester Window thickness / mg cm Collector dimeter / mm 30 Cvity height / mm 8.5 Nominl volume / cm Polrizing potentil / V +300 Potentil pplied to the chmber window, the collector remining t virtul ground potentil. 5. Clibrtion t the BIPM 5.1 The BIPM irrdition fcility nd reference rdition qulities The BIPM low-energy x-ry lbortory houses constnt-potentil genertor nd tungstennode x-ry tube with n inherent filtrtion of 1 mm beryllium. A molybdenum filter of thickness 60 µm is dded for ll rdition qulities. A voltge divider is used to mesure the generting potentil, which is stbilized using n dditionl feedbck system designed t the BIPM. Rther thn use trnsmission monitor, the node current is mesured nd the ioniztion chmber current is normlized for ny devition from the reference node current. The resulting vrition in the BIPM FAC-L-01 free-ir chmber current over the durtion of 3/12

4 comprison is normlly not more thn in reltive terms. The rdition qulities used in the rnge from 23 kv to 35 kv re given in Tble 4 in scending order, from left to right, of the hlf-vlue-lyer (HVL) mesured using luminium filters. The irrdition re is temperture controlled t round 20 C nd is stble over the durtion of clibrtion to better thn 0.2 C. Two thermistors, clibrted to few mk, mesure the temperture of the mbient ir nd the ir inside the BIPM stndrd. Air pressure is mesured by mens of clibrted brometer positioned t the height of the bem xis. The reltive humidity is controlled within the rnge 47 % to 53 % nd consequently no humidity correction is pplied to the current mesured using trnsfer instruments. Tble 4. Chrcteristics of the BIPM mmmogrphy rdition qulities Rdition qulity W/Mo-23 W/Mo-28 W/Mo-30 W/Mo-35 Generting potentil / kv Additionl filtrtion 60 µm Mo Al HVL / mm (µ/ ) ir / cm 2 g Reference distnce / mm 500 K BIPM / mgy s The BIPM stndrd nd correction fctors The reference plne for the BIPM stndrd ws positioned t 500 mm from the rdition source, with reproducibility of 0.03 mm. The stndrd ws ligned on the bem xis to n estimted uncertinty of 0.1 mm. The stndrd bem dimeter in the reference plne is 100 mm for ll rdition qulities. For this comprison, tungsten collimtor tht hlves the bem dimeter ws used to mtch the ENEA-INMRI field size. During the clibrtion of the trnsfer chmber, mesurements using the BIPM stndrd were mde using positive polrity only. A correction fctor of ws pplied to correct for the known polrity effect in the stndrd. The lekge current for the BIPM stndrd, reltive to the ioniztion current, ws mesured to be less thn The correction fctors pplied to the ioniztion current mesured t ech rdition qulity using the BIPM stndrd, together with their ssocited uncertinties, re given in Tble 5. The correction fctor k is evluted for the reference distnce of 500 mm using the mesured mss ttenution coefficients ( ir given in Tble 4. In prctice, the vlues used for k tke ccount of the temperture nd pressure of the ir in the stndrd t the time of the mesurements. Ioniztion mesurements (both for the stndrd nd for trnsfer chmbers) re lso corrected for chnges in ir ttenution rising from vritions in the temperture nd pressure of the mbient ir between the rdition source nd the reference plne. 5.3 Trnsfer chmber positioning nd clibrtion t the BIPM The reference point of the chmber ws positioned in the reference plne t 500 mm with reproducibility of 0.03 mm. The trnsfer chmber ws ligned on the bem xis to n estimted uncertinty of 0.1 mm. The lekge current ws mesured before nd fter ech series of ioniztion current 4/12

5 mesurements nd correction mde using the men vlue. The lekge current, reltive to the ioniztion current of round 200 pa, ws less thn 1 prt in The stndrd uncertinty of the men of series of seven mesurements, ech with integrtion time 30 s, ws less thn 2 prts in Tble 5. Correction fctors for the BIPM FAC-L-01 stndrd Rdition qulity W/Mo-23 W/Mo-28 W/Mo-30 W/Mo-35 u ia u ib Air ttenution k Scttered rdition k sc Fluorescence k fl Electron loss k e Sturtion k s Polrity k pol Wll trnsmission k p Field distortion k d Diphrgm correction k di Humidity k h g ir Vlues for K nd kp; ech mesurement is corrected using the ir density mesured t the time. u ia represents the reltive stndrd uncertinty estimted by sttisticl methods, type A u ib represents the reltive stndrd uncertinty estimted by other mens, type B 6. Clibrtion t the ENEA-INMRI 6.1 The ENEA-INMRI irrdition fcility nd reference rdition qulities The mmmogrphy x-ry fcility t the ENEA-INMRI is comprised of constnt-potentil genertor nd W-node tube with n inherent filtrtion of 1.0 mm beryllium. The x-ry output is monitored continuously by mens of free-ir monitor chmber. The chrcteristics of the ENEA-INMRI reliztion of the mmmogrphy comprison qulities re given in Tble 6. Two clibrted NTC type thermometers were used to mesure the ir temperture, one positioned inside the stndrd chmber (or close to the trnsfer chmber during clibrtion) nd the second close to the free-ir monitor chmber. Air pressure ws mesured using clibrted brometer positioned t the height of the bem xis. The reltive humidity ws in the rnge from 45 % to 55 %. No humidity correction hs been pplied to the trnsfer chmber current mesurements. 5/12

6 Tble 6. Chrcteristics of the ENEA-INMRI reference rdition qulities Rdition qulity W/Mo-23 W/Mo-28 W/Mo-30 W/Mo-35 Generting potentil / kv Additionl filtrtion 60 µm Mo Al HVL / mm (µ/ ) ir / cm 2 g Reference distnce / mm 500 K ENEA / mgy s Air ttenution coefficient t K nd kp, mesured for n ir pth length of mm. 6.2 The ENEA-INMRI stndrd nd correction fctors The reference plne for the ENEA-INMRI stndrd ws positioned t 500 mm from the node of the x-ry tube, with reproducibility of 0.2 mm. The stndrd ws ligned on the bem xis to n estimted uncertinty of 0.2 mm. The bem dimeter in the reference plne is 50 mm for ll rdition qulities. The correction fctors pplied to the ioniztion current mesured t ech rdition qulity using the ENEA-INMRI stndrd, together with their ssocited uncertinties, re given in Tble 7. Tble 7. Correction fctors for the ENEA-INMRI CAL02 stndrd Rdition qulity W/Mo-23 W/Mo-28 W/Mo-30 W/Mo-35 u ia u ib Air ttenution k Scttered rdition k sc Fluorescence k fl Electron loss k e Sturtion k s Polrity k pol Wll trnsmission k p Field distortion k d Diphrgm correction k di Humidity k h g ir Vlues for K nd kp; ech mesurement is corrected using the ir density mesured t the time. 6/12

7 The correction fctor k is evluted using the mesured mss ttenution coefficients ( ir given in Tble 6. The vlues used for k tke ccount of the temperture nd pressure of the ir in the stndrd t the time of the mesurements. Ioniztion mesurements (stndrd nd trnsfer chmbers) re lso corrected for vritions in the temperture nd pressure of the mbient ir between the rdition source nd the reference plne. 6.3 Trnsfer chmber positioning nd clibrtion t the ENEA-INMRI The reference point of the trnsfer chmber ws positioned t the reference distnce with reproducibility of 0.3 mm. Alignment on the bem xis ws to n estimted uncertinty of 0.5 mm. For ech rdition qulity, two sets of mesurements were done using the primry stndrd nd in between, one set of mesurements using the trnsfer chmber. Ech set consists of five mesurements with n integrtion time of 100 s. The reltive stndrd uncertinty of ech set ws less thn 1 prt in The lekge current ws mesured before nd fter ech series of ioniztion current mesurements nd correction mde using the men vlue. The reltive lekge current ws less thn 5 prts in The chmber ws clibrted two times for ech rdition qulity before the mesurements t the BIPM nd two times following the BIPM mesurements. The stted reltive stndrd uncertinty for the reproducibility of the clibrtions t the ENEA-INMRI is 2.3 prts in Additionl considertions for trnsfer chmber clibrtions 7.1 Ion recombintion, polrity, field size nd rdil non-uniformity The ENEA pplies correction for ion recombintion clculted for ech rdition qulity s k s,tr = 1 + k init + k vol I (4) where k init is the initil recombintion (k init = ) nd k vol is the volume recombintion (k vol = A 1 ) A reltive stndrd uncertinty of 5 prts in 10 4 is included in Tble 12 to ccount for this effect. As the ENEA-INMRI kerm rtes re different for ech qulity nd lso different t the two lbortories (see Tbles 4 nd 6), the BIPM results hve been corrected by k s using the ENEA-INMRI determintion. The correction pplied to the current mesured t the BIPM is The trnsfer chmber ws used with the sme polrity t ech lbortory nd so no corrections re pplied for polrity effects in the trnsfer chmber. No correction is pplied for field size s the BIPM field dimeter ws reduced to 50 mm to mtch the ENEA-INMRI field size. No correction k rn,tr is pplied t either lbortory for the rdil non-uniformity of the rdition field s this effect is likely to cncel to some extent t the two lbortories. A reltive stndrd uncertinty of is introduced for this effect. 7.2 Rdition qulity correction fctors k Q As noted in Section 4.1, differences in rdition qulities must be tken into ccount to evlute comprison result. Tbles 4 nd 6 show slight differences in the HVL t the BIPM nd the ENEA-INMRI. To derive comprison result for the BIPM HVL vlues, qudrtic fit ws mde to the ENEA-INMRI dt set, s shown in Figure 1. From the fit, set of k Q vlues ws derived to correct ech ENEA-INMRI clibrtion coefficients into one tht pplies t the equivlent BIPM qulity. 7/12

8 The comprison result is then evluted s N K,ENEA kq RK,ENEA (5) N K,BIPM An dditionl uncertinty of 2 prts in 10 4 is included in Tble 12 for this fitting procedure. Figure 1. Clibrtion coefficients determined t the ENEA-INMRI nd the BIPM Rdcl RC6M N K / Gy µc BIPM ENEA ENEA for BIPM HVL Al HVL / mm 8. Comprison results The trnsfer chmber ws clibrted t the ENEA-INMRI before nd fter the BIPM mesurements. The ENEA-INMRI nd BIPM results re given in Tble 8. Tble 8. Clibrtion coefficients for the trnsfer chmber Rdition qulity W/Mo-23 W/Mo-28 W/Mo-30 W/Mo-35 N K,ENEA-INMRI (pre-bipm) / Gy C N K,ENEA-INMRI (post-bipm) / Gy C s tr (reltive) s men N K,ENEA-INMRI k Q / Gy C N K,BIPM / Gy C To be consistent with the ENEA results, the BIPM clibrtion coefficients N K hve been corrected for ion recombintion using k s,tr = The vlues N K,ENEA-INMRI mesured before nd fter the mesurements t the BIPM give rise to the men vlue for s men in Tble 8 of Nevertheless, bsed on the typicl 8/12

9 reproducibility t the ENEA-INMR, the stted reproducibility of their clibrtion coefficients is This ltter vlue is included in Tble 11 with no dditionl component in Tble 12. The rtios N K,ENEA-INMRI / N K,BIPM for the trnsfer chmber re given in Tble 9. The finl results R K,ENEA-INMRI include the correction fctors k Q ccording to eqution (5). 9. Uncertinties Tble 9. Comprison results Rdition qulity W/Mo-23 W/Mo-28 W/Mo-30 W/Mo-35 R K,ENEA-INMRI The uncertinties ssocited with the primry stndrds re listed in Tble 10 nd those for the trnsfer chmber clibrtions in Tble 11. The combined stndrd uncertinty u c for the comprison results R K,ENEA-INMRI is presented in Tble 12. Tble 10. Uncertinties ssocited with the stndrds t ech lbortory Stndrd BIPM L-01 ENEA-INMRI Reltive stndrd uncertinty u ia u ib u ia u ib Ioniztion current Volume Positioning Correction fctors (excl. k h ) Humidity k h Physicl constnts K Tble 11. Uncertinties ssocited with the clibrtion of the trnsfer chmbers Institute BIPM ENEA-INMRI Reltive stndrd uncertinty u ia u ib u ia u ib K Positioning of trnsfer chmber I tr Reproducibility N K /12

10 Tble 12. Uncertinties ssocited with the comprison results Reltive stndrd uncertinty u ia u ib N K,ENEA-INMRI / N K,BIPM Ion recombintion k s,tr Rdil non-uniformity k rn,tr Fitting procedure k Q R K,ENEA-INMRI u c = Tkes ccount of correltion in type B uncertinties. The combined stndrd uncertinty u c of the comprison result tkes into ccount correltion in the type B uncertinties ssocited with the physicl constnts nd the humidity correction. In the nlysis of the results of BIPM comprisons in low-energy x-rys in terms of degrees of equivlence described by Burns (2006), correltion in the vlues for the correction fctors k e, k sc nd k fl is tken into ccount if the NMI hs used vlues derived from Monte Crlo clcultions, s is the cse for the ENEA-INMRI stndrd. 10. Discussion The comprison results presented in Tble 9 show greement between the ENEA-INMRI nd BIPM stndrds t the level of round 3.5 prts in 10 3 with combined stndrd uncertinty of 4.8 prts in This comprison ws conducted using one trnsfer chmber rther thn by direct comprison of the primry stndrds. While the use of trnsfer chmbers introduces more uncertinty in the comprison results, the results obtined re more directly relted to the disseminted quntity. The ENEA-INMRI prticipted in 2011 in the BIPM.RI(I)-K2 comprison using the primry stndrds, being the sme stndrds used for the mmmogrphy bems for both the ENEA- INMRI nd the BIPM (Burns et l 2011). The direct comprison result for the qulity 25 kv, W/Al bem, the closest in HVL to the mmmogrphy W/Mo bems, ws (28); this result is in greement, within the uncertinties, with the results presented in Tble Degrees of Equivlence The nlysis of the results of BIPM comprisons in low-energy x-rys in terms of degrees of equivlence is described by Burns (2003) nd similr nlysis is dopted for comprisons in mmmogrphy x-ry bems. Following decision of the CCRI, the BIPM determintion of the ir-kerm rte is tken s the key comprison reference vlue, for ech of the CCRI rdition qulities. It follows tht for ech lbortory i hving BIPM comprison result x i with combined stndrd uncertinty u i, the degree of equivlence with respect to the reference vlue is the reltive difference D i = (K i K BIPM,i ) / K BIPM,i = x i 1 nd its expnded uncertinty U i = 2 u i. The results for D i nd U i expressed in mgy/gy, re shown in Tble /12

11 Tble 13. Degrees of equivlence Mo/Mo-25 Mo/Mo-28 Mo/Mo-30 Mo/Mo-35 D i U i D i U i D i U i D i U i /(mgy/gy) /(mgy/gy) /(mgy/gy) /(mgy/gy) NMIJ PTB NIST VNIIM VSL W/Mo-23 W/Mo-28 W/Mo-30 W/Mo-50 D i U i D i U i D i U i D i U i /(mgy/gy) /(mgy/gy) /(mgy/gy) /(mgy/gy) NRC W/Mo-23 W/Mo-28 W/Mo-30 W/Mo-35 ENEA- INMRI Note tht the dt presented in the tble, while correct t the time of publiction of the present report, will become out of dte when lbortory mkes new comprison with the BIPM. The forml results under the CIPM MRA re those vilble in the BIPM key comprison dtbse. When required, the degree of equivlence between two lbortories i nd j cn be evluted s the difference D ij = D i D j = x i x j nd its expnded uncertinty U ij = 2 u ij, both expressed in mgy/gy. In evluting u ij, ccount should be tken of correltion between u i nd u j (Burns 2003). 12. Conclusion The key comprison BIPM.RI(I)-K7 for the determintion of ir kerm in mmmogrphy x-ry bems shows the stndrds of the ENEA-INMRI nd the BIPM to be in greement within the uncertinty of 4.9 prts in Degrees of equivlence, including those for the ENEA-INMRI, re presented for entry in the BIPM key comprison dtbse. The forml results under the CIPM MRA re those vilble in the BIPM key comprison dtbse. 11/12

12 References Allisy P J, Burns D T, nd Kessler C 2011 Mesuring conditions nd uncertinties for the comprison nd clibrtion of ntionl dosimetric stndrds t the BIPM Rpport BIPM- 2011/04 Boutillon M, Henry W H nd Lmperti P J 1969 Comprison of exposure stndrds in the kv x-ry region Metrologi Burns D T nd Kessler C 2009 Diphrgm correction fctors for free-ir chmber stndrds for ir-kerm in x-rys, 2009, Phys. Med. Biol. 54, Burns D T 2003 Degrees of equivlence for the key comprison BIPM.RI(I)-K2 between ntionl primry stndrds for low-energy x-rys Metrologi 40 Technicl Supplement Burns D T, Kessler C, Roger P, Toni M P, Pinto M, Bovi M, Cppdozzi G nd Silvestri C 2011 Key comprison BIPM.RI(I)-K2 of the ir-kerm stndrds of the ENEA-INMRI, Itly nd the BIPM in low-energy x-rys Metrologi 48 Technicl Supplement Kessler C 2006 Estblishment of simulted mmmogrphy rdition qulities t the BIPM Rpport BIPM-2006/08 12/12

Key comparison BIPM.RI(I)-K2 of the air-kerma standards of the PTB, Germany and the BIPM in low-energy x-rays

Key comparison BIPM.RI(I)-K2 of the air-kerma standards of the PTB, Germany and the BIPM in low-energy x-rays Key comprison BIPM.RI(I)-K2 of the ir-kerm stndrds of the PTB, Germny nd the BIPM in low-energy x-rys D.T. Burns 1, C Kessler 1 nd L Büermnn 2 1 Bureu Interntionl des Poids et Mesures, Pvillon de Breteuil,