Activity standardization of 99mTc by digitizer-based 4πce(LS)-γ and 4πce(PC)-γ coincidence counting.

An aqueous solution of 99mTc was standardized by means of 4πce(LS)-γ and 4πce(PC)-γ coincidence counting. The activity concentration was obtained by efficiency extrapolation relying on four different variation techniques. In both setups, fast digitizers were used for data acquisition. Detected events were recorded in list mode format, and their coincidence relationship was analyzed in the subsequent offline analysis. The combined result was used to participate in an international comparison using the Transfer Instrument of the International Reference System.

A bilateral comparison between LNHB and PTB to determine the activity concentration of the same 125I solution.

A bilateral comparison to determine the activity concentration of the same 125I solution was organized. As electron-capture radionuclide with a rather high atomic number, 125I must be regarded as difficult to measure. The situation is partly exacerbated by the fact that some established standardization methods, like photon-photon coincidence counting, can no longer be applied due to the unavailability of appropriate equipment and expertise. One aim of this work is to compare modern liquid scintillation counting methods for the standardization of 125I. Both participating metrology institutes have used their custom-built triple-to-double-coincidence ratio (TDCR) counters and the determined activity concentrations are in excellent agreement even though the ways to analyze the data and to compute counting efficiencies were widely independent. The results also agree with the outcome of 4π-γ counting that was carried out at LNHB. In both laboratories, the measurements were complemented by measurements with several secondary standardization methods which even allow to establish a link to the CCRI(II)-K2.I-125(2) comparison started in 2004. A good agreement between the TDCR results and the key comparison reference value of the 2004/2005 comparison was obtained.

The international reference system for beta-particle emitting radionuclides: Validation through the pilot study CCRI(II)-P1.Co-60.

The Bureau International des Poids et Mesures (BIPM) is developing a new transfer instrument to extend its centralized services for assessing the international equivalence of radioactive standards to new radionuclides. A liquid scintillation counter using the triple/double coincidence ratio method is being studied and tested in the CCRI(II)-P1.Co-60 pilot study. The pilot study, involving 13 participating laboratories with primary calibration capabilities, validated the approach against the original international reference system based on ionization chambers, which has been in operation since 1976. The results are in agreement and an accuracy suitable for purpose, below 5×10-4, is achieved. The pilot study also reveals an issue when impurities emitting low-energy electrons are present in the standard solution, which have a different impact on liquid scintillation counting compared to other primary measurement methods.

Activity standardization by means of liquid scintillation counting and determination of the half-life of 89Zr.

A89Zr solution was measured by means of liquid scintillation counting techniques in order to determine the activity concentration. Two methods were used: the CIEMAT/NIST efficiency tracing method with 3H as a tracer, and the triple-to-double coincidence ratio method. The counting efficiencies were computed with a stochastic model. The very detailed investigation showed that a few corrections are particularly important: Asymmetries in the photodetector responses as well as the backscattering of high-energy gamma rays must be taken into account. Corresponding corrections have therefore been applied. In addition, a detailed uncertainty analysis was carried out and the uncertainties compared with those determined by other research groups. The activity concentrations obtained from the two methods agree well and a combined result was used to establish calibration factors for ionization chambers, which are important secondary standardization instruments. The ionization chambers were combined with a new high-precision current measurement device to provide outstanding linearity. Measurement data from one chamber were used to determine the half-life, which was found to be T1/2=(78.373 ± 0.023) h.

Determination of the activity of 225Ac and of the half-lives of 213Po and 225Ac.

The activity concentration of an 225Ac solution was determined by means of liquid scintillation counting using three custom-built TDCR counters. The efficiency calculation was carried out in the same way as it had been done in an earlier article on 229Th. The computation of the counting efficiency is rather complex and requires a correction to allow for the short-lived 213Po. The experimental deadtime was varied to validate the correction. One of the TDCR counters is equipped with a CAEN N6751C digitizer for data acquisition. In addition, the system comprises a CeBr3 solid scintillator as a gamma detector. The offline analysis was used to obtain a time-difference spectrum, using signals from the 213Po γ-rays at about 440 keV in the gamma channel in coincidence with the preceding beta decay as the start signal, and signals from the subsequent (delayed) 213Po alpha decays as the stop signal. After fitting an exponential function with a constant background, the half-life of 213Po was determined to be 3.709(12) µs, which is in good agreement with the evaluated value. The half-life of 225Ac was determined from long-term measurements using an ionization chamber (IC) and a TDCR system. The combined result was found to be 9.9179(30) d, which is in agreement with the outcome from Pommé et al. (2012).

Standardisation of 85Sr with digital anticoincidence counting and half-life determination of the 514 keV level of 85Rb.

The activity of a 85Sr solution was determined by means of fully digital 4πß(LS)-γ anticoincidence counting. The measurements were carried out in a custom-built, combined TDCR / 4πß(LS)-γ coincidence system, utilising a commercially available CAEN N6751C digitizer. The analysis of the experimental data, collected in list-mode format, was performed off-line by using the SoftKAM computer code developed at PTB. The data were also used to determine the half-life of the 514 keV level of 85Rb which was found to be (1020.2 ± 6.0) ns.

Determination of the activity and half-life of 227Th.

Liquid scintillation samples with 227Th were prepared a few hours after the separation of the progeny. During the measurements, 227Th and its daughters are not in radioactive equilibrium. The counting efficiencies of the individual radionuclides of the decay chain differ from each other and the activity of an individual progeny relative to the activity of 227Th varies with time. Hence, the overall counting efficiency varies with time as well. The counting efficiency εT227h++ of 227Th and its progeny was determined by means of the CIEMAT/NIST efficiency tracing method. The free parameter is derived from the quench-indicating parameter, SQP(E), and from 3H tracer measurements. This makes it possible to compute the efficiency εT227h++ as a function of time. The individual efficiencies of all progeny are to be combined, taking correction factors and activity ratios into account. Thereby, a new, time-dependent correction, namely for the decay during the measurements, is applied. With this method, activity results are obtained that are stable over a long period of time. A least-squares method yields the time of the chemical separation as well as the 227Th half-life, which was also obtained by means of measurements in an ionization chamber. The weighted mean of the two methods (CIEMAT/NIST efficiency tracing and measurements in ionization chambers) was found to be T1/2 = 18.681(9) d.

The half-life of 129I.

The radionuclide 129I is a long-lived fission product that decays to 129Xe by beta-particle emission. It is an important tracer in geological and biological processes and is considered one of the most important radionuclides to be assessed in studies of global circulation. It is also one of the major contributors to radiation dose from nuclear waste in a deep geological repository. Its half-life has been obtained by a combination of activity and mass concentration measurements in the frame of a cooperation of 6 European metrology institutes. The value obtained for the half-life of 129I is 16.14 (12) × 106 a, in good agreement with recommended data but with a significant improvement in the uncertainty.

Activity determination of 60Co and the importance of its beta spectrum.

The activity concentration of a 60Co solution was measured by means of two 4πß-γ coincidence counting systems using a liquid scintillation counter and a proportional counter (PC) in the beta channel, respectively. Additional liquid scintillation measurements were carried out and CIEMAT/NIST efficiency tracing as well as the triple-to-double coincidence ratio (TDCR) methods were applied to analyse the data. The last two methods require computed beta spectra to determine the counting efficiencies. The results of both 4πß-γ coincidence counting techniques are in very good agreement and yield a robust reference value. The initial activity concentration determined with liquid scintillation counting was found to be significantly lower than the results from 4πß-γ coincidence counting. In addition, the results from TDCR and CIEMAT/NIST show some inconsistency. The discrepancies were resolved by applying new beta spectrum calculations for the dominant allowed beta transition of 60Co. The use of calculations which take screening effects as well as the atomic exchange effect into account leads to good agreement between all four methods; the combination of these techniques delivers an important validation of beta spectra.

Activity determination of 67Ga using 4πß‒γ coincidence counting.

The activity of a 67Ga solution was measured by means of the 4πß-γ coincidence counting technique. A setup with a proportional counter and non-extending dead-time modules in the two detector channels was used. The influence of the delayed state affecting the measured count rates was taken into account by using appropriate correction formulae. The analysis requires a variation of the dead time and a subsequent extrapolation to infinite dead time. An uncertainty of about 0.5% was obtained and the result was used to make an intercomparison with the SIR of the BIPM.

Activity determination of 68Ge/68Ga by means of 4πß(C)-γ coincidence counting.

Germanium-68 is an important radionuclide since it is used to generate its daughter 68Ga which is frequently used for positron emission tomography (PET). In addition, 68Ge/68Ga sources are often used as surrogates for short-lived PET isotopes when calibrating instruments. In this work, 4πß(C)-γ coincidence counting was used to determine the activity concentration of a 68Ge/68Ga solution. The presented measurements were made by means of a new PTB-custom-built 4π(LS)ß-γ coincidence counting system, where a liquid scintillation detector in the ß channel can also be used as a Cerenkov counter, and a NaI(Tl) crystal detects annihilation radiation. The arithmetic mean of two results (from double ND and triple NT coincidences in the ß channel) was adopted as the final result for the activity concentration of the solution under study. The overall relative uncertainty was estimated to be 0.82% and the uncertainty consideration as well as details about the measurement and the analysis are discussed. The measurements were carried out within the scope of the international CCRI(II)-K2.Ge-68 comparison.

Comparison of (90)Y activity measurements in nuclear medicine in Germany.

In 2014, PTB and the company Eckert & Ziegler organized a national comparison exercise to determine the activity of a (90)Y solution. One aim of the comparison was to assess the measurement capability of hospitals and medical practices in Germany. P6-type vials were filled with aliquots of a radioactive (90)Y solution and then sent to 19 participants who were asked to measure the activity in the ampoules as well as in their own standard geometry using syringes. Most of the submitted results have a deviation of less than ±10% from the PTB reference activity when measured in the P6-type vials. The spread is somewhat larger when measured in a syringe geometry. The comparison revealed that some participants have difficulties in applying decay corrections and only a few participants were capable of estimating realistic measurement uncertainties.

Calibration and efficiency curve of SANAEM ionization chamber for activity measurements.

A commercially available Fidelis ionization chamber was calibrated and assessed in PTB with activity standard solutions. The long-term stability and linearity of the system was checked. Energy-dependent efficiency curves for photons and beta particles were determined, using an iterative method in Excel™, to enable calibration factors to be calculated for radionuclides which were not used in the calibration. Relative deviations between experimental and calculated radionuclide efficiencies are of the order of 1% for most photon emitters and below 5% for pure beta emitters. The system will enable TAEK-SANAEM to provide traceable activity measurements.

Activity determination of 227Ac and 223Ra by means of liquid scintillation counting and determination of nuclear decay data.

The activity concentrations of solutions containing 227Ac and 223Ra in equilibrium with their progenies, respectively, were measured by means of liquid scintillation counting. The counting efficiencies were determined with the aid of a free parameter model. The corresponding calculations comprise the computation of several alpha, beta and beta/gamma branches. For short-lived progenies like 215Po the counting efficiency depends on the counter dead time. Measurements were made in custom-built triple-to-double coincidence ratio (TDCR) systems and various dead-time adjustments were used. In addition, two commercial counters were used to apply the CIEMAT/NIST efficiency tracing technique using 3H as a tracer. For the 227Ac solution, the overall relative standard uncertainty of the activity concentration was found to be 0.93%. The dominant uncertainty components are assigned to the efficiency computation of the low-energy beta transitions of 227Ac. We have identified a need for improved 227Ac decay data to achieve a significant reduction in the overall uncertainty. In the case of 223Ra, the activity concentrations were determined with relative standard uncertainties below 0.3%. Hence, PTB is prepared to provide calibration services for 223Ra, which is an isotope of increasing interest in nuclear medicine. The TDCR measurements were also used to determine the half-life of 223Ra. The decay was followed for about 58 days and a half-life T1/2=11.4362(50)d was obtained.

Bilateral comparison between PTB and ENEA to check the performance of a commercial TDCR system for activity measurements.

The only commercial TDCR counter from Hidex Oy (Finland), comprising three photomultiplier tubes, was tested at the two National Metrology Institutes (NMIs) PTB and ENEA. To this end, the two NMIs purchased a Hidex 300 SL TDCR counter (METRO version) each and carried out various tests at their laboratories. In addition, the two institutions agreed to organize a bilateral comparison in order to acquire information on the reproducibility of the results obtained with the counters. To achieve this, PTB prepared some (89)Sr liquid scintillation samples, which were first measured in various counters at PTB and then shipped to ENEA for comparative measurements. The aim of this paper is to summarize the findings on the counter characteristics and adjustments. In addition, the results of the bilateral comparison between PTB and ENEA are presented and the results from various commercial counters using the CIEMAT/NIST efficiency tracing and the TDCR method are discussed.

Activity determination of (59)Fe.

Iron-59 was measured in three commercial and two custom-built liquid scintillation counters. The counting efficiencies were determined using CIEMAT/NIST efficiency tracing and the triple-to-double coincidence ratio (TDCR) method, respectively. The efficiency computation for the TDCR method was realized by means of the MICELLE2 program, applying a stochastic model for the computation of electron emission spectra. The program was extended to make calculations of spectra originating from complex decay schemes possible. In addition, a new parameterization of electron stopping powers for 10 commercial liquid scintillation cocktails was included in the software. The activities determined with the two methods were in very good agreement; the relative standard uncertainty of the combined result was found to be 0.16%. It was used to calibrate a 4π ionization chamber at PTB for future calibrations of this isotope which is used for investigations of iron metabolism. A standardized solution was submitted to the Bureau International des Poids et Mesures (BIPM) to be measured in the ionization chambers of the International Reference System (SIR) for comparison purposes. The liquid scintillation samples were also measured in a new portable TDCR system with three channel photomultipliers. Although this system has a much lower counting efficiency, the activity was in satisfactory agreement with the conventional TDCR system. The usage of the portable TDCR system, thus, provides an important test of the free parameter model.

A portable TDCR system.

The triple-to-double-coincidence ratio method (TDCR) is an important method for activity standardization in metrology institutes worldwide. There is an increasing interest in portable systems that allow activity determination outside of specialized laboratories with high accuracy. Within the framework of the EMRP "MetroFission" project, several portable systems using different designs were developed. The PTB system described here is based on channel photomultipliers incorporated in a portable detection module, a separate electronics bin and a computer for data acquisition and storage. This miniature TDCR system was extensively tested and compared to the PTB reference TDCR system that is very well characterized and has been used in several intercomparisons.

Investigations on TDCR measurements with the HIDEX 300 SL using a free parameter model.

The Triple to Double Coincidence Ratio (TDCR) method requires a special counting system with three photodetectors. The systems currently used by National Metrology Institutes for activity standardizations are custom built, and up to now the HIDEX 300 SL counter is the only TDCR counter commercially available. At PTB, measurements with a special metrology version of this counter were carried out to investigate its applicability for activity standardizations. The activity results of measurements with the HIDEX counter are compared to those obtained with a PTB-TDCR counter, as such a comparison reduces the model dependence. In addition, a spectrometry method was applied to measure (109)Cd samples and a new TDCR-Cerenkov method was tested with (32)P samples.

Activity determination and nuclear decay data of 177Lu.

The activity concentration of a (177)Lu solution was measured within the scope of the international comparison CCRI(II)-K2.Lu-177, starting in 2009. At PTB, the solution was measured by means of 4πß-γ coincidence counting using a proportional counter and a NaI detector. In addition, liquid scintillation counting using the CIEMAT/NIST efficiency tracing method as well as the triple-to-double coincidence ratio (TDCR) method was applied. The efficiency computation for the TDCR method was realized by means of the MICELLE2 program, applying a stochastic model for the computation of electron emission spectra. The activity concentrations derived from the three methods were found to be in good agreement and the relative standard uncertainty of the combined result was found to be 0.19%. At PTB, the combined result was used to calibrate a 4π ionization chamber for future calibrations of this isotope which is frequently used in nuclear medicine. In addition, activity standardizations were combined with gamma-ray spectrometry to determine photon emission probabilities. To this end, the comparison solution as well as another (177)Lu solution was used. The results are in good agreement with previous measurements at PTB but show a considerable discrepancy to recently published values from Deepa et al. (2011). The decay curve of a third solution was followed by liquid scintillation counting for about 66 days to determine the half-life of (177)Lu, which was found to be T(1/2)=6.639(9) d.

Beta shape-factor function and activity determination of 241Pu.

The Physikalisch-Technische Bundesanstalt (PTB) investigated the low-energy beta emitter (241)Pu within the scope of an international key comparison on the activity concentration of the same solution. The activity concentration was measured by means of liquid scintillation counters with two and three photomultiplier tubes (PMT). The counting efficiencies were determined with two established techniques, which are based on a free parameter model. The free parameter is determined via (3)H-efficiency tracing in systems with two PMTs, or it is derived from the triple-to-double coincidence ratio (TDCR) in a system with three PMTs. Both methods require an accurate computation of the beta emission spectrum of the first-forbidden (non-unique) transition. In this work, the experimental outcome of a recent measurement from Loidl et al. (2010) with cryogenic magnetic calorimeters was used to determine a shape-factor function. The computed beta spectrum is in good agreement with the measured data when the shape-factor function C(W)=1-1.9582W+0.96078 W(2) and an end-point energy E(ß,max)=21.6 keV are used. The activity concentrations determined with the two methods agree well when using the new shape-factor function, whereas a considerable discrepancy is found when assuming C(W)=1, as for an allowed beta transition. Consequently, the difference between the efficiency tracing method and the TDCR method, as observed by other researchers, could be resolved.