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.

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.

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 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.

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.

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.

Activity standardization and decay data of 64Cu.

The activity of a solution of (64)Cu was measured by 4pibeta(PC)-gamma coincidence counting and liquid scintillation counting using the CIEMAT/NIST method. In addition, 4pi ionization chambers were used to establish and preserve calibration factors for secondary standardizations and for a determination of the half-life, for which a value of 12.704(5)h was measured. The photon emission probabilities were determined using gamma-ray spectrometry, finding values of p(511 keV)=0.3512(22) and p(1346 keV)=0.00474(5). The results were checked for consistency, and an ampoule was submitted to the BIPM to have the activity result entered into the database of the International Reference System (SIR), yielding the first entry for this radionuclide.