Seasonal variation of background counting rates in liquid scintillation counting.

A liquid scintillation background sample was measured daily in a custom-built TDCR counter for more than 17 months. The double and triple coincidence counting rates exhibit an annual sinusoidal fluctuation with a maximum in winter and a minimum in summer. Possible correlations with air temperature, air humidity, radon concentration and secondary cosmic radiation were investigated. The observation of a correlation with the ambient dose equivalent rate [Formula: see text] originating from the charged component of secondary cosmic radiation and an anti-correlation with the effective atmospheric temperature Teff suggest that the seasonal fluctuations in the background counting rate may be primarily driven by temporal variations in the muon flux at ground level. Additionally, a correlation was found with the indoor 222Rn concentration in air.

Activity standardisation of 32Si at IRA-METAS.

This work explores the primary activity standardisation of 32Si as part of the SINCHRON project that aims at filling the geochronological dating gap by making a new precise measurement of the half-life of this nuclide. The stability of some of the radioactive test solutions, providing 32Si as hexafluorosilicic acid (H232SiF6), was monitored over long periods, pointing to the adequate sample composition and vial type to ensure stability. These solutions were standardised using liquid scintillation counting with the triple to double coincidence ratio (TDCR) technique and the CIEMAT-NIST efficiency tracing (CNET) method. Complementary backup measurements, using 4πß-γ coincidence counting with 60Co as a tracer, were performed with both liquid and plastic scintillation for beta detection. While 60Co coincidence tracing with a liquid scintillator predicted activities in agreement with the TDCR and CNET determinations, using plastic scintillation turned out to be unfeasible as the addition of lanthanum nitrate and ammonia to fix the silicon during the drying process generated large crystals that compromised the linearity of the efficiency function.

Activity standardization of 32Si at PTB.

Within the scope of the SINCRON project, several 32Si solutions were measured by means of liquid scintillation (LS) counting techniques at PTB to determine the activity concentration. Initial results revealed limited long-term stability of the samples, and a discrepancy between the TDCR method and the CIEMAT/NIST efficiency tracing method was found. In some cases, the sample instability could not be completely avoided but there is evidence that the results of the first measurements which are carried out within a few days after sample preparation can be used for an activity determination, though with increased uncertainty. Various sample compositions were tested, and a systematic study of long-term measurements and further experiments indicates that the sample instability is due to an adsorption-like effect. The discrepancies between the two LS methods were significantly lower when measuring other 32Si solutions. The initially observed discrepancies are likely due to low-energetic radioactive impurities that can be present in some of the 32Si solutions. A spectral analysis supports the thesis that tritium is present in the first solution and even allows a rough quantification of the activity ratio A(3H)/A(32Si/32P). This value allows impurity corrections to be applied, which leads to a noticeable improvement in the agreement between TDCR and CIEMAT/NIST efficiency tracing. Finally, a new LS sample composition with 15 mL Ultima Gold and 1 mL of HCl (0.5 mol/L) was found to yield stable LS samples. The activity determinations presented in this paper represent a fundamental step towards a new 32Si half-life determination in the framework of the SINCHRON project.

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.

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.

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.

Activity standardization of two enriched 40K solutions for the determination of decay scheme parameters and the half-life.

In this paper we describe experiments on two enriched 40K solutions to accurately determine decay data. The first solution was measured in 2004/2005 by means of a gamma-ray spectrometer with low background and a liquid scintillation (LS) counter to apply the CIEMAT/NIST efficiency tracing method. A combination of results yields an emission probability of the 1461 keV gamma-rays of Pγ = 0.1030(11) which is lower than current results of data evaluations. The activity concentration of the second solution was also determined by means of LS counting, but here, the CIEMAT/NIST efficiency tracing method as well as the TDCR method were applied. Again, the result was combined with that of independent gamma-ray spectrometry and the gamma-ray emission probability was found to be Pγ = 0.1029(9) in good agreement with the result obtained from the first solution. A combination of both experiments yields Pγ = 0.1029(9). The spectra of a TriCarb LS counter were carefully analyzed and a beta minus emission probability [Formula: see text]  = 0.8954(14) was determined. The new results for Pγ and [Formula: see text] indicate that the overall probability of the decay via EC in recent data evaluations is overestimated. The LS counting efficiencies were computed with a stochastic model and up-to-date calculations of the beta spectrum and fractional EC probabilities were used. The final activity result of the second solution is combined with the outcome of a comprehensive isotopic analysis to determine the half-life of 40K which is found to be 1.2536(27) ·109 years. All above-stated uncertainties are standard uncertainties (k = 1).

High precision measurement of the 151Sm beta decay by means of a metallic magnetic calorimeter.

The beta decay of 151Sm was measured by means of a metallic magnetic calorimeter. The measurement and subsequent analysis yielded a beta spectrum with an outstanding high-energy resolution of about 70 eV (FWHM) at 22 keV and a very low energy threshold well below 400 eV. The spectrum exhibited unexpectedly elevated beta emission probabilities at very low energy that we have not been able to reproduce in our theoretical study. The data analysis was thus scrutinized and an independent analysis of the same data set carried out. All new approaches have confirmed the previously found shape of the beta spectrum. The measured spectrum was compared to predictions from an advanced theoretical modeling that includes the atomic exchange effect, precise radiative corrections as well as the realistic nuclear structure that usually plays an important role in first forbidden non-unique transitions. The measured spectrum was then carefully analyzed to determine the maximum beta energy, which was found to be Q = 76.430(68) keV. The dominant beta decay of 151Sm populates the ground state of 151Eu, and a weak beta branch populates the first excited state of 151Eu. From our measurements, the probabilities of these two branches were determined to be 99.31(11)% and 0.69(11)%, respectively.

Realization and dissemination of activity standards for medically important alpha-emitting radionuclides.

Interest in targeted cancer therapy with alpha-emitting radionuclides is growing. To evaluate emerging radiotherapeutic agents requires precise activity measurements for consistent dose-response relationships and patient-specific dosimetry. National metrology institutes around the world have reported on the development and comparison of activity standards for medically important alpha emitters. This review describes the relevant methods and models underpinning these standards, the generation of new nuclear decay data, and the impacts on preclinical and clinical activity assays using radionuclide calibrators and γ-ray spectrometry.

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.

Half-life determination of short-lived nuclear levels in 237Np (59.54 keV), 233Pa (86.47 keV) and 227Ac (27.37 keV).

New half-life values for isomeric states in 237Np, 233Pa and 227Ac were measured by means of 4πα(LS)-γ coincidence counting with digital data acquisition. A careful assessment of uncertainties was carried out, and the new results are found to be much more precise than any previous measurement result. The half-lives were found to be T1/2Np237,E=59.54keV=67.86±0.09ns, T1/2Pa233,E=86.47keV=36.44±0.10ns and T1/2Ac227,E=27.37keV=38.56±0.15ns, respectively. A comprehensive study of literature data was undertaken for a new data evaluation that includes the results from this work.

On the photomultiplier-tube asymmetry in liquid scintillation counters for the CIEMAT/NIST efficiency tracing method.

The triple-to-double coincidence ratio (TDCR) and the CIEMAT/NIST efficiency tracing (CNET) methods are important techniques for accurate activity determination in radionuclide metrology and other fields. The methods require liquid scintillation (LS) counters with three (TDCR) or two (CNET) photomultiplier tubes (PMTs), respectively, and the original techniques assume that the responses of all PMTs in a counter system are identical. This assumption is, however, often not fulfilled. This may have significant consequences for the activity determination, and dedicated corrections are required. Corresponding effects and possible corrections for the TDCR method were discussed in a recent article (Kossert et al., 2020) while the CNET method was not included in that work. The intention of this paper is to present a brief theoretical consideration of this problem. This paper makes clear that for most radionuclides potential PMT asymmetries in CNET counters are of minor importance. In addition, measures to identify potential asymmetries in these counters are discussed.

Improved activity standardization of 90Sr/90Y by means of liquid scintillation counting.

Radioactive strontium isotopes play an important role in environmental radioactivity. Reliable activity standards are required in order to validate radioanalytical techniques and related measurements. In this paper, improved methods for the primary activity standardization of 90Sr/90Y based on liquid scintillation counting are presented. To this end, two methods were used: the CIEMAT/NIST efficiency tracing technique with 3H as a tracer and the triple-to-double coincidence ratio method. Non-negligible discrepancies between the two methods were found when applying existing analysis techniques. A detailed study was carried out to identify and eliminate the causes of these discrepancies. Eventually, excellent agreement between the two methods was obtained. This required advanced beta spectrum calculations which were carried out with a specific version of the BetaShape program taking the atomic exchange effect into account. In addition, it was found that the quench-indicating parameters determined in commercial liquid scintillation counters are biased, which can cause significant problems for the CIEMAT/NIST efficiency tracing method. The effect depends on the counting rate and can be explained by a superposition of the LS spectra generated by 90Sr/90Y and the external standard source.

Half-life determination of 213Bi and 209Pb by means of Cherenkov counting and detection with a NaI detector.

An 225Ac source was prepared and combined with a dedicated setup to collect recoil atoms which are ejected as a consequence of alpha decay. Some targets were made of acrylic glass and were then placed in custom-built triple-to-double coincidence ratio (TDCR) counters to measure the Cherenkov counting rates as a function of time. Half-lives of 213Bi and 209Pb were derived from the data. In addition, polyethylene films were used as targets to collect recoil atoms. These films were then measured in a well-type NaI detector to detect gamma rays. The analysis of the data yielded an independent result for the half-life of 213Bi. Combining the results of both measurement techniques, T1/2(213Bi) = (45.60 ± 0.09) min was obtained as a final value for the half-life of 213Bi. The half-life of 209Pb was found to be T1/2(209Pb) = (195.1 ± 2.6) min.

On the photomultiplier-tube asymmetry in TDCR systems.

The responses of the three photomultiplier tubes (PMTs) in a triple-to-double coincidence ratio (TDCR) liquid scintillation (LS) system are often not identical. Such asymmetries can have a significant influence on activity determinations. The problem is often solved by means of a minimization algorithm which can easily be applied when analytical methods are used for the efficiency calculation, as is usually done for pure beta emitters. However, for radionuclides with more complex decay schemes, the counting efficiencies are often calculated with stochastic methods and the computation of the required corrections becomes very challenging. This paper presents a new numerical method to overcome the asymmetry problem for such complex decays and a comprehensive study on 55Fe is described in detail. For the measurements, the asymmetry was varied by means of grey filter films which were placed in front of one of the photomultiplier tubes. In the case of the pure electron-capture (EC) radionuclide 55Fe, the asymmetry can also be taken into account with a very simple correction which is derived assuming monoenergetic emissions. This work is also of great importance for the planned extension of the International Reference System (SIR) at the BIPM which will be used for international comparisons in radionuclide metrology.

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).

TDCR measurements to determine the half-life of 55Fe.

Several measurements with aliquots from a 55Fe solution were carried out using two custom-built TDCR counters. The time between the initial and the last measurements is 2304 d, i.e. a length of more than two half-lives. The data were analysed to determine the half-life of 55Fe, which was found to be (1006.2 ±â€¯1.0) d. The determined half-life is longer than the current values from nuclear data evaluations, but it is in good agreement with recent results obtained by Pommé et al. (2019).

Half-life determination and comparison of activity standards of 231Pa.

The results of an international comparison of activity measurements of a solution of 231Pa are reported and analysed. Prior to this, no known standardisation of 231Pa by activity measurement had been carried out. The comparison was run in 2017-2018 involving eight laboratories, and returned results with no identifiable inconsistencies between methods or laboratories. The results, including one mass determination, gave a231Pa activity concentration of 41.461(48) kBq g-1 and a231Pa atom concentration of 61.48(23) × 1015 atoms g-1, from which a half-life value of 32 570(130) years was derived.

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.