Absolute standardizations of 99mTc and 57Co by 4π electron-gamma liquid scintillation coincidence counting for SIRTI and SIR comparisons

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Absolute standardizations of 99mTc and 57Co by 4π electron-gamma liquid scintillation coincidence counting for SIRTI and SIR comparisons ⁎

M.W. van Rooy , M.J. van Staden, B.R.S. Simpson, J. Lubbe Radioactivity Standards Section, NMISA, 15 Lower Hope Road, Rosebank, 7700 Cape Town, South Africa

H I G H L I G H T S standardizations of Tc via 4π(LS)e-γ and Co via 4π(LS)(e,X)-γ coincidence counting. • Absolute at NMISA for SIRTI and SIR comparisons of Tc and Co, respectively. • Preparation technical report on the standardization of Tc via liquid scintillation coincidence counting. • First • Count rate vs. efficiency simulation for Tc. 99m

57

99m

57

99m

99m

A R T I C L E I N F O

A B S T R A C T

Keywords: Liquid scintillation coincidence counting Extrapolation technique Technetium-99m Cobalt-57 SIRTI SIR

The radionuclides 99mTc and 57Co were standardized via absolute liquid scintillation counting techniques. We provide the first technical report on the absolute standardization of 99mTc using 4π(LS)e-γ coincidence counting. The low detection efficiency of low-energy conversion electrons translates into a large efficiency extrapolation range. A simulation indicated that the γ-ray interacting with the liquid scintillator introduces curvature to the count rate vs. efficiency relationship, the approximation of the functional form used for extrapolation providing the main measurement uncertainty for 99mTc. A detection efficiency analysis for both radionuclides is presented. Results from the standardizations, and SIRTI and SIR comparison exercises are reported.

1. Introduction The activity of 99mTc was standardized by the National Metrology Institute of South Africa (NMISA) as preparation for the BIPM.RI(II)K4.Tc-99m comparison exercise which utilizes the International Reference System Transfer Instrument (SIRTI). 99mTc is used in nuclear medicine for diagnostic purposes. The primary method employed was 4πe-γ coincidence counting using liquid scintillation (LS) in the electron channel. Although we are not the first institute to standardize 99mTc via LS counting (Lee et al., 2012), our work provides the first technical report on this technique. The result of the standardization was used to determine a calibration factor for 99mTc for the NMISA secondary standard ionization chamber (Lubbe et al.), which was later used during the SIRTI comparison exercise. Although we applied LS counting to the 99mTc activity determination, the same detection efficiency analysis is applicable as for proportional counters. The efficiency equations pertaining to various window modes have been summarized by Sahagia (2006) and Brito et al. (2012). We have followed method 2 of Brito et al. (2012), where all events in the 4π channel are counted and a γ-window is set over the ⁎

140.51 keV full-energy peak. This method was pointed out as being more accurate, showing less variation between sources due to a much smaller background uncertainty. There is also no need for applying a correction factor related to decay scheme parameters. The low conversion electron efficiency of the low-energetic γ-transition γ2,1 (Fig. 1) results in a large efficiency extrapolation range and corresponding uncertainty. In an attempt to predict the best functional form of the efficiency extrapolation curve, the efficiency formulae were used in a simulation to extract the expected count rates as a function of efficiency. The simulation involved estimation of the detection efficiency of the 140.51 keV γ-ray with the liquid scintillator via two simple methods. Results of the simulation are compared with the experimental results. The SIRTI comparison results (Michotte et al., 2017) are also discussed. A few months before measuring 99mTc, 57Co was standardized via 4π(e,X)-γ coincidence counting using LS in the electron-channel, as part of the BIPM.RI(II)-K1.Co-57 SIR comparison exercise. Because of the short half-life of 99mTc, it is often more convenient to use the longerlived radionuclide 57Co as a surrogate for calibrating γ-cameras and for instrumental quality control purposes in medical imaging (Zanzonico,

Corresponding author. E-mail address: [email protected] (M.W.v. Rooy).

http://dx.doi.org/10.1016/j.apradiso.2017.07.042 Received 9 March 2017; Received in revised form 24 July 2017; Accepted 24 July 2017 0969-8043/ © 2017 Elsevier Ltd. All rights reserved.

Please cite this article as: van Rooy, M.W., Applied Radiation and Isotopes (2017), http://dx.doi.org/10.1016/j.apradiso.2017.07.042

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142.68 keV peaks. Thus, the γ-ray count rate given by the NaI detector is:

NG = N0 [a (1−f ) ε γ2 + (1−a)(1−g ) ε γ3]

(2)

where ε γ2 and ε γ3 are the efficiencies for detecting events in the 140.51 keV and 142.68 keV peaks, respectively (the efficiencies include the appropriate escape probability, (1−pi ) , from the LS vial). Setting ε γ2 = ε γ3 = ε γ , the 4πe-γ coincidence count rate is given by

NC = N0 [a (1−f ) ε1 ε γ ]

(3)

and consequently

a (1−f ) ε1 NC ≅ ε1 = NG a (1−f ) + (1−a)(1−g )

(4)

since the term (1 − a)(1 − g ) contributes negligibly. Rearranging and setting ε3 = ε2 , εβγ2 = εβγ3 = εβγ and p2 = p3 = p, the following equality similar to that given by Brito et al. (2012) is obtained: Fig. 1. Decay scheme of Morillon et al. (2004).

99m

Tc showing the γ-transitions to the ground state of

(1−ε1) N4π NG {[af + (1−a) g ][ε2 =a+ NC N0 ε1 + (1−ε2) εAX ] + [a (1−f ) + (1−a)(1−g )] pεβγ }

99

Tc,

+ (1−a){g [ε2 + (1−ε2) εAX ] + (1−g ) pεβγ }

ε1→1 so does When = a+(1 − a){g+(1 − g ) pεβγ } .

2008). 57Co is suitable as a “mock” 99mTc source because of its similar γray intensity and energy, ensuring comparable instrument responses. Standardizing 57Co is complicated by a structured electron-capture spectrum comprising X-rays and Auger electrons superimposed on top of internal conversion electrons, which can lead to higher order efficiency functions (Baerg, 1973). We report on the results of our 57Co standardization and SIR comparison. Details of the primary technique employed by NMISA have been published previously by Simpson (1986). A summary is provided here. 2. Standardization of

99m

N4π NG NC N0

2.1.1. Summary of formulae 99m Tc decays 99.0 (4) % through a 2.17 keV γ-transition, γ2,1, to the first excited state of 99Tc and subsequently through a 140.51 keV γtransition, γ1,0 , to the ground state (Fig. 1, Morillon et al., 2004). The remaining 1.0 (1) % decays directly to the ground state through a 142.68 keV γ-transition, γ2,0 . All three γ-transitions show significant conversion electron components. The 4π channel count rate N4π for 99mTc is given by (Sahagia, 2006):

⎣

(1−a) {g [ε3 + (1−ε3) εAX ] + (1−g ) p3 εβγ3}

⎤ ⎦

(1)

where N0 is the source activity; a = 0.99 is the probability for γ2,1 and γ1,0 transitions; ε1, ε2 and ε3 are the efficiencies for detecting conversion electrons (ec2,1, ec1,0 and ec2,0 respectively); εAX is the efficiency for detecting Auger-electrons and X-rays; ε βγi are the detection efficiencies for γ-rays from γ1,0 and γ2,0 interacting with the liquid scintillator source and pi are the appropriate γ-ray interaction probabilities. The parameters f and g indicate the total internal conversion portion of the γ1,0 and γ2,0 α g= transitions, respectively ( f = 1 +2αT = 0.1063 (27) and α3T 1 + α3T

therefore

N

2.1.2. Rate simulation To estimate the expected change in source count rate with efficiency variation, Eq. (5) was populated with the 99mTc decay scheme parameter values from Morillon et al. (2004) and efficiency estimates. The simulated source activity N0 was 30 000 Bq. A Monte-Carlo simulation was used to determine p for a 140.51 keV γ-ray (Simpson, 1994). Since the energy of conversion electron 2 (ec1,0 ) is in the range 119.47 – 140.51 keV, the efficiency ε2 was taken as 1 for all thresholds. It is clear that if p is approximately zero (the case for proportional counters), then Eq. (5) is linear with respect to (1−ε1)/ ε1. This would also be the case for a high-energy γ-ray interacting with the scintillator, since pεβγ would be essentially constant for all thresholds. However, since the γ-rays have energies in the range 140.51 – 142.68 keV and the resulting Compton electrons have energies in the range 0 – 50 keV, εβγ varies with the electronic threshold setting. This variation in the Compton electron efficiency introduces curvature to the observed rate vs. efficiency relationship. These threshold settings were calibrated in terms of energy using a measured liquid scintillation pulse height spectrum and energy estimations of the first and second mono-electron peaks, as illustrated in Fig. 2 (a). The εβγ efficiency was then estimated for each threshold, using two methods.

Tc

aε1 + a (1−ε1) {f [ε2 + (1−ε2) εAX ] + (1−f ) p2 εβγ2}+

and

Thus 4Nπ G ≅0.99976N0 at ε1 = 1 since the term containing εβγ is C negligible. The factor 0.99976 has an uncertainty of 0.00391.

2.1. Detection efficiency analysis

N4π = N0 ⎡

N

ε2

(5)

2.1.2.1. Method 1: Double coincidence Compton electron efficiency. The first method used to estimate the efficiency of Compton electrons interacting with the liquid scintillator was based on an approximation of the double coincidence electron detection efficiency used by Simpson et al. (2012). The approximate efficiency is given by:

εD ≅

2T

= 0.9761 (191) , where α2T and α3T are the appropriate total internal conversion coefficients). Applying the recommended method 2 (Brito et al., 2012), a γ -window was set over the combined unresolved 140.51 keV and

∫E

Emax

t

N (E )[1−e−PEQ (E ) ]2 dE = εβγ ,

(6)

where E is the electron energy, N(E) is the theoretical Compton spectrum (illustrated in Fig. 2 (b) in terms of normalized cross section) for a 140.5 keV γ-ray calculated from the Klein-Nishina

2

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Fig. 2. (a) Fifteen energy calibrated threshold settings applied to a 4π LS spectrum of 99mTc. (b) An illustration of the theoretical LS Compton spectrum and energy calibrated threshold settings.

activity. For method 1 a third order polynomial with the second order coefficient set to zero (Miyahara et al., 1986), implemented in a locally written Fortran program called ALTFIT, specifically for this kind of fit, gave the closest extrapolated value to that expected. For method 2 a second order polynomial gave the most accurate result. Fitting was done using a least squares minimization, weighted by the uncertainties. These results are displayed in Fig. 3. The simulation results indicate that the best fit to the measurement data is expected to be a second order polynomial or third order polynomial with the second order coefficient set to zero.

formula, Q(E) is the ionization quenching function for Quicksafe A and P is the figure-of-merit (Simpson and Meyer, 1994). The efficiency above a given threshold was found by integrating from the threshold energy Et up to the maximum Compton electron energy Emax, where t = 1 – 15. A typical value for P of 0.29 e/keV (obtained from our previous 4πβ-γ measurements) was used in Eq. (6) to obtain estimates of εβγ above each threshold. 2.1.2.2. Method 2: Compton spectrum fractional area. The second efficiency estimate was based on a fractional area scheme of the theoretical Compton spectrum. The fractional area A of the Compton spectrum above each threshold relative to the lowest threshold E1 was determined. Each fractional area was then multiplied by εβγ (E1) = 0.8070 obtained from Eq. (6) for the lowest threshold:

εβγ = 0.8070 × A.

2.2. Experimental 2.2.1. Source preparation A 99mTc solution (containing 99mTc eluted from a generator into saline solution) was used to prepare five liquid scintillation counting sources. Accurately weighed aliquots of the solution were dispensed into 12 mL of scintillation cocktail inside custom made flat-faced cylindrical glass vials. Source masses (buoyancy corrected) ranged between 42 mg and 155 mg. Quicksafe A from Zinsser Analytic was used as the liquid scintillation cocktail, to which 3 mL L−1 of 1 M HCl had been added as a precaution against possible adsorption. The 99mTc solution was also used to prepare a sample with mass

(7)

This method results in εβγ values that are at most 19 % lower compared to method 1. The differences are reasonable considering that the two methods only provide rough estimations to allow an attempt at investigating the behaviour of the count rate vs. efficiency curves. Polynomials were fitted to the simulated values of N4π NG vs. (1 − ε1) NC

and extrapolated to ε1 = 1, or equivalently

(1 − ε1) ε1

ε1

= 0 , to obtain the

Fig. 3. Illustration of the fits to the 99mTc simulated count rates that gave the most accurate extrapolation results, a third order polynomial with the second order coefficient set to zero, applicable to method 1 and a second order polynomial applicable to method 2. The simulated source activity N0 was 30 000 Bq.

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evaluation showed that a second order polynomial provided the best fit for all the measurement data, although visually the fits were only marginally different. Fig. 4 provides an example of the fit and extrapolation of a second order polynomial and a third order polynomial with the second order coefficient set to zero applied to measurement data from source 1. The extrapolation results from a third order polynomial with the second order coefficient set to zero were 1 – 3 % higher compared to the second order extrapolation results. The final activity concentration of our stock solution was determined as 8 039.31 Bq mg−1 on the reference date of 15 September 2015 12h00 SAST. The corresponding uncertainty budget is provided in Table 1. The main contribution to the total uncertainty is due to the uncertainty in the fitting and efficiency extrapolation, contributing by 1.3 % to the total of 1.3 % at k = 1. The extrapolation uncertainty was determined by the difference in average results between second order polynomials and third order polynomials with the second order coefficient set to zero, divided by 3 . The result from the primary standardization was used to determine a 99mTc calibration factor of 341 for the NMISA secondary standard ionization chamber with a standard relative uncertainty of 1.4 %. This calibration factor agrees with the normalized manufacturer’s factor of 341 for the NMISA chamber (Lubbe et al.). One month after the primary standardization the pre-determined calibration factor was used to measure an accurately weighed solution in an ampoule for the SIRTI comparison. The NMISA equivalent activity Ae = 156 100 (2 200) kBq is 1.9 % higher than the KCRV of 153 170 (310) kBq for 99mTc and in agreement within two standard deviations (Michotte et al., 2017).

3.35371 (34) g in a 5 mL ampoule used for calibration of the NMISA secondary standard ionization chamber via current measurements. 2.2.2. Measurements Repeat measurements of 900 s were undertaken on the five sources with a locally developed double-phototube and NaI detector system viewing each LS source in turn, details of which are given in Simpson and Meyer (1988). Gamma counts were recorded using the NaI detector with a window set over both the unresolved 140.51 keV and 142.68 keV peaks. The 4πe counting efficiency variation was achieved through fifteen simultaneous electronic threshold settings, counted integrally, starting below the first mono-electron peak. The main contribution to the electron count rate came from γ-transition γ2,1 which is almost 100 % converted to electron emission with energy between 1.73 keV and 2.13 keV. Therefore, these low energy conversion electrons were concentrated within the first mono-electron peak if detected. The electron counting efficiency was optimized by setting the first six thresholds within the first mono-electron peak where the highest efficiency is expected. The highest efficiency obtained was approximately 18 %, decreasing to a minimum of approximately 5 %. The typical electron count rate at the highest efficiency was approximately 9 000 s−1 with a background count rate of 4.8 s−1. The coincidence resolving time decreased from 492 ns for the lowest threshold to 479 ns for the highest threshold. The system deadtime varied between 1.0 μs and 1.3 μs depending on the threshold setting. Corrections were applied for deadtime, coincidence resolving time and background. A correction was applied for the spurious pulse (Simpson, 2002; Godfrey et al., 1951) contribution from accidental coincidences, which was the greatest within the first mono-electron peak, ranging from 0.6 % to 0.2 % and decreasing to a minimum of 0.07 % above the highest threshold. It is well known that for a counting interval not significantly shorter than the half-life of a radionuclide, a correction should be applied to account for decay during the measurement. Most commonly, either the start time or the midpoint time of the counting interval is used as the reference time (Fitzgerald, 2016; Mann et al., 1991; NCRP Report No. 58, 1986). Using the midpoint time as our reference, the average count rates during the 900 s counting intervals agree within 0.003 % of the instantaneous (decay corrected) count rates at the midpoint time as obtained from either of the corrections proposed by Fitzgerald (2016), Mann et al. (1991) and NCRP Report No. 58 (1986). The discrepancy of 0.003 % is negligible compared to the uncertainty in the counting statistics, therefore we used the average count rates which did not need further corrections for decay during the counting interval. An adsorption test was performed on one of the sources by decanting the source material, filling the vial with LS cocktail and measuring the electron-electron coincidence count rate and γ-ray count rate in the 4π(LS)e–γ counting system. This process was repeated four times. The remaining count rates were comparable to background count rates, indicating negligible adsorption to the glass walls. Measurements made with a HPGe detector indicated that the 99mTc source material was free from photon-emitting radioactive impurities.

3. Standardization of

A detailed mathematical analysis for 4π(LS)(e,X)−γ coincidence counting of 57Co is provided in a laboratory research report (Simpson, 1986), which is briefly summarized here with reference to a schematic showing the essential features of the decay scheme (Fig. 5), where a = 0.9982 and b = 0.0018 are the branching ratios. The detection efficiency analysis assumes that radiation due to L-, M- or higher-shell electron capture events is not registered due to the very low energy and -detection efficiency of particles associated with these events. This situation could arise if the counting source is heavily quenched or when the lowest electronic threshold is set above the first mono-electron peak. The analysis results in a term to correct for the afore mentioned assumption. With a window set to select the 122 keV and 136 keV peaks, it can be shown that

1 − P1 1 − P2 XG G ′ N∙ (1 − φ − φ2) ⎤ N + φ2 PKb εXAK φ+ =⎡ ⎥ ⎢ 1 + α2 C C ⎦ ⎣ 1 + α1 α P G 1 1 ⎤ φN ∙ + +⎡ ⎥ ⎢ 1 + α1 1 + α1 ⎦ C ⎣ α2 P2 ⎤ G (1 − φ − φ2) N ∙ , + +⎡ ⎥ ⎢ 1 + α2 1 + α2 ⎦ C ⎣

NC

(8)

where X is the LS count rate, G is the γ count rate, C is the coincidence count rate and N is the source disintegration rate. The intensity of the 122 keV and 692 keV transition states are given by φ and φ2 respectively, and (1 − φ−φ2) is the intensity of the 136 keV transition; α1 and α2 are the total internal conversion coefficients for the 122 keV and 136 keV transitions, respectively; P1 and P2 are the liquid scintillation interaction probabilities for the 122 keV and 136 keV γ-rays, respectively; and PKb is the capture probability of a K-electron in branch b, with ′ being the efficiency for sum pulses due to X-rays or Auger electrons εXAK

The activity concentration was obtained by extrapolating polynomials fitted to the measured rates in the form of N4π NG vs. NG to 100 %

(

Co

3.1. Detection efficiency analysis

2.3. Activity determination and SIRTI result

N

57

NC

)

beta efficiency ε1 = NC = 1 . The measured N4π NG vs. NG curves showed NC NC G apparent linear behaviour in the higher efficiency range of 10–17 %, with slight curvature below 10 % efficiency. Based on the simulation results, only fitting and extrapolation results from a second order polynomial and a third order polynomial with the second order coefficient set to zero were considered and evaluated based on χ2-tests. This

4

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Fig. 4. A comparison of a second order polynomial and a third order polynomial with the second order coefficient set to zero applied to 99mTc measurement N data from source 1 extrapolated to G = 1. NC

coincident with the 14 keV transition events. Since the second term in the expression is very small, a linear relationship is expected. When G/C ′ = 1 (Simpson, = 1 (non-physical region), it can be inferred that PKa εXAK 1986), where PKa is the capture probability of a K-electron in branch a. ′ here compensates for the non-detection of LIn effect, the value of εXAK and higher orbital de-excitation particles. Then N is given by the extrapolated value at G/C = 1:

φ (PKb − PKa ) ⎤ XG 1+ 2 = N⎡ ⎥, ⎢ C PKa ⎦ ⎣

(9)

where the correction term is negligible for 57Co but can be significant for some electron-capture-decaying radionuclides (Funck and Nylandstedt Larsen, 1983). By assuming that some L- and higher-shell de-excitation particles are also registered, the correction term falls away. 3.2. Effect of the metastable level in

57

Fig. 5. Simplified decay scheme of

57

Co, Chechev and Kuzmenko (2004).

Fe

If the 14 keV transition (T½≅0.1 μs) is delayed for a sufficient period, a separate pulse could be generated so that two counts per disintegration occur. The percentage of decays occurring after 1 μs (the inherent dead time of the counting system) is however minimal at 0.1 %. An experiment was conducted to check this by introducing a fixed dead time of 1.5 μs (Simpson, 1986). No significant change in the measured Table 1 Uncertainty budget for the primary standardization of

99m

source activity was observed. 3.3. Experimental 3.3.1. Source preparation An aliquot with mass 1.41217 (14) g of a master solution containing

Tc at k = 1.

Component

Evaluation method

Description

Relative uncertainty (%)

Counting statistics Weighing (source mass) Counting time Dead time Coincidence resolving time Afterpulses Background Adsorption Decay correction using half-life Extrapolation of efficiency curve

A B B B B B B B B B

Standard deviation of repeats Calibration of balance Calibration of timer Formula with ΔτD ± 0.05 µs Formula with ΔτR ± 0.01 µs Based on formula for Δθ Square root statistics Count rate measurements after multiple rinsings comparable to background count rate 6.0067 (10) h from Morillon et al. (2004) Difference between 2nd O polynomial and third order polynomial with the second order

0.23 0.05 0.001 0.05 0.05 0.02 0.06 0.0 0.07 1.3

Impurities Combined uncertainty

B

coefficient set to zero divided by HPGe measurements Summed in quadrature

0.0 1.3

5

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~ 5 MBq 57Co dissolved in 5 mL 0.1 M HCl was used to prepare a dilution with a mass of 4.08909 (82) g. The carrier solution used to prepare the dilution contained approximately 450 mg CoCl2 per litre 1 M HCl. The 57Co dilution was used to prepare five liquid scintillation counting sources. Accurately weighed aliquots of the dilution were dispensed into 12 mL of scintillation cocktail inside custom made flatfaced cylindrical glass vials. Source masses (buoyancy corrected) ranged between 45 mg and 69 mg. Small glass stirring rods were used to disperse the 57Co source material in the scintillation cocktail. The liquid scintillation cocktail used was Quicksafe A from Zinsser Analytic, to which 3 mL L−1 of 3 M HCl had been added to prevent adsorption of the active material to the vial surface. An aliquot with mass 3.52319 (70) g of the master solution was dispensed into a special 5 mL ionization chamber ampoule (supplied by the BIPM) and the ampoule was flame-sealed. The sample was used to obtain a 57Co calibration factor for the NMISA secondary standard ionization chamber. The same sample, together with the NMISA absolute standardization result, was subsequently sent to the SIR at the BIPM for comparison purposes and establishing international equivalence.

the reference date of 24 March 2015 12h00 SAST. The corresponding uncertainty budget is provided in Table 2. The main contribution to the combined uncertainty is due to the uncertainty in the fitting and efficiency extrapolation, contributing by 0.2 % to the total of 0.21 % at k = 1. The extrapolation uncertainty was determined from the relative difference in average results between first and second order polynomials. The result from the primary standardization was used to determine a 57Co calibration factor of 331 for the NMISA secondary standard ionization chamber with a standard uncertainty of 1.0 %. This calibration factor is 1.6 % higher than the normalized manufacturer’s factor (Lubbe et al.). For the SIR comparison, the NMISA equivalent activity Ae = 170 640 (450) kBq is 0.9 % higher than the latest (2017) KCRV of 169 080 (260) kBq (Michotte, 2017) and passes the BIPM k = 2.5 test for identifying outliers. 4. Conclusions We provide the first technical paper on the absolute standardization of 99mTc via 4π(LS)e-γ coincidence counting. Following method 2 of Brito et al. (2012), measurement data confirmed the curved nature of the count rate vs. efficiency function that was predicted by our simulation. The low detection efficiency of low-energy conversion electrons translates into a large efficiency extrapolation range. The slight curvature causes difficulty in deciding the functional form, resulting in the main contribution (1.3 %) to the total standard uncertainty of 1.3 %. This makes the standardization of 99mTc more problematic by LSC than proportional counting where a linear fit is normally obtained. Our SIRTI comparison result is 1.9 % higher than the KCRV for 99mTc and in agreement within two standard deviations (Michotte et al., 2017). Our absolute standardization of 57Co resulted in a small standard uncertainty of 0.21 % due to the high electron efficiency achieved, resulting in a small extrapolation range. Our SIR comparison result is 0.9 % higher than the KCRV for 57Co (Michotte, 2017). Since 1978, 57 Co SIR results vary within 1.7 % of the corresponding KCRV, using various methods of proportional- and liquid scintillation counting techniques, indicating the difficulty in establishing a credible KCRV and preference cannot be given to a particular technique. This statement is supported by the addition of an ad-hoc uncertainty in the powermoderated mean process for establishing the KCRV. Our result indicates the difficulty in finding an appropriate fit to the measurement data that will also realistically represent the extrapolation region. Based on the comparison result, our extrapolation uncertainty is probably too small

3.3.2. Measurements Measurements were performed on the five 57Co sources using the same detector system and procedure described in Section 2.2.2. The γ-window comprised the 122 keV and 136 keV peaks as well as the ~100 keV iodine escape peak. The electron efficiency range and distribution was varied through electronic threshold settings, starting below the first mono-electron peak, from ~30 % to ~ 86 %, with similar count rates as observed in Section 2.2.2. The spurious pulse contribution remained below 0.2 %. An adsorption test performed on one of the sources indicated no significant loss of activity to the vial walls. The stock solution supplier certificate stated impurities of 0.0128 % 56Co and 0.00216 % 58Co on the provided reference date. The impurity contribution to the master solution was subsequently determined as 0.002 %. 3.4. Activity determination and SIR result The activity concentration was obtained by extrapolating second order polynomials fitted to the measured rates in the form of XG/C vs. G/C to 100 % efficiency. Fig. 6 presents an example of a second order polynomial applied to measurement data from source 2. The final activity concentration of our 57Co dilution (stock solution) was determined as 217.62 Bq mg−1 on

Fig. 6. Example of a second order polynomial applied to 57Co measurement data from source 2.

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Table 2 Uncertainty budget for the primary standardization of

57

Co at k = 1.

Component

Evaluation method

Description

Relative uncertainty (%)

Counting statistics Weighing (source mass) Counting time Dead time Coincidence resolving time Afterpulses Background Adsorption Decay correction using half-life Extrapolation of efficiency curve Impurities Combined uncertainty

A B B B B B B B B B B

Standard deviation of repeats Calibration of balance Calibration of timer Formula with ΔτD ± 0.05 µs Formula with ΔτR ± 0.01 µs Based on formula for Δθ Square root statistics Count rate measurements after multiple rinsings comparable to background count rate 271.80 (5) h from Chechev and Kuzmenko (2004) Difference between 1st and 2nd O polynomial Determined from master solution supplier certificate Summed in quadrature

0.04 0.05 0.001 0.03 0.008 0.03 0.004 0.0 0.0003 0.2 0.003 0.21

and could be increased for future standardizations of

57

Michotte, C., 2017. Proposals for KCRV updates for the SIR (BIPM.RI(II)-K1) comparisons, Working document of the Consultative Committee for Ionizing Radiation Section II – Measurement of radionuclides, CCRI(II)/17-20. Miyahara, H., Momose, T., Watanabe, T., 1986. Optimisation of efficiency extrapolation functions in radioactivity standardization. Appl. Radiat. Isot. 37, 1–5. Morillon, C., Bé, M.M., Chechev, V., Egorov, A., 2004. 99mTc - Monographie BIPM-5. In: Bé, M.-M., Chisté, V., Dulieu, C., Browne, E., Chechev, V., Kuzmenko, N., Helmer, R., Nichols, A., Schönfeld, E., Dersch, R. (Eds.), Table of Radionuclides (vol. 1 - A = 1 to 150). Bureau International des Poids et Mesures. NCRP Report No. 58, 1986. A Handbook of Radioactivity Measurements Procedures, Second Edition. NCRP Publicationspp. 72. Sahagia, M., 2006. Standardization of 99mTc. Appl. Radiat. Isot. 64, 1234–1237. Simpson, B.R.S., 1986. The Standardization of 57Co by Liquid Scintillation Coincidence Counting. CSIR Research Report 597. Council for Scientific and Industrial Research, Faure, South Africa. Simpson, B.R.S., 1994. Monte Carlo calculation of the γ-ray interaction probability pertaining to liquid scintillator samples of cylindrically-based shape NAC Report NAC/ 94-04. National Accelerator Centre, Faure, South Africa. Simpson, B.R.S., 2002. Radioactivity standardization in South Africa. Appl. Radiat. Isot. 56, 301–305. Simpson, B.R.S., Meyer, B.R., 1988. A multiple-channel 2- and 3-fold coincidence counting system for radioactivity standardization. Nucl. Instrum. Methods Phys. Res. A 263, 436–440. Simpson, B.R.S., Meyer, B.R., 1994. Direct activity measurement of pure beta-emitting radionuclides by the TDCR efficiency calculation technique. Nucl. Instrum. Methods Phys. Res. A339, 14–20. Simpson, B.R.S., van Staden, M.J., Lubbe, J., van Wyngaardt, W.M., 2012. Accurate activity measurement of Lu-177 by the liquid scintillation 4πβ-γ coincidence counting method. Appl. Radiat. Isot. 70, 2209. Zanzonico, P., 2008. Routine quality control of clinical nuclear medicine instrumentation: a brief review. J. Nucl. Med. 49, 1114–1131.

Co.

References Baerg, A.P., 1973. The efficiency extrapolation method in coincidence counting. Nucl. Instrum. Methods 112, 143–150. Brito, A.B., Koskinas, M.F., Litvak, F., Toledo, F., Dias, M.S., 2012. Standardization of 99m Tc by means of a software coincidence system. Appl. Radiat. Isot. 70, 2097–2102. Chechev, V.P., Kuzmenko, N.K., 2004. 57Co - Monographie BIPM-5. In: Bé, M.-M., Chisté, V., Dulieu, C., Browne, E., Chechev, V., Kuzmenko, N., Helmer, R., Nichols, A., Schönfeld, E., Dersch, R. (Eds.), Table of Radionuclides (vol. 1 - A = 1 to 150). Bureau International des Poids et Mesures. Fitzgerald, R., 2016. Corrections for the combined effects of decay and dead time in livetimed counting of short-lived radionuclides. Appl. Radiat. Isot. 109, 335–340. Funck, E., Nylandstedt Larsen, A., 1983. The influence from low energy X-rays and Auger electrons on 4πβ-γ coincidence measurements of electron-capture-decaying nuclides. Appl. Radiat. Isot. 34 (3), 565–569. Godfrey, T.N.K., Harrison, F.B., Keuffel, J.W., 1951. Satellite pulses from photomultipliers. Phys. Rev. 84, 1248. Lee, J.M., Lee, K.B., Lee, S.H., Park, T.S., 2012. Calibration of KRISS reference ionization chamber for key comparison of 99mTc measurement. Appl. Radiat. Isot. 70, 1853–1855. Lubbe, J., Simpson, B.R.S., van Staden, M.J., van Rooy, M.W. Utilising normalized manufacturer’s calibration factors for an ionization chamber with depleted gas. (in preparation). Mann, W.B., Rytz, A., Spernol, A., 1991. Radioactivity Measurements. Principles and Practice. Pergamon Press, Oxford, pp. 69. Michotte, C., Nonis, M., Van Rooy, M.W., van Staden, M.J., Lubbe, J., 2017. Activity measurements of the radionuclides 18F and 99mTc for the NMISA, South Africa in the ongoing comparisons BIPM.RI(II)-K4.F-18 and BIPM.RI(II)-K4.Tc-99m 54 Metrologia Tech. Suppl. 06001.

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