Efficiency Tracing Method Research Articles

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 4πβ-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 and half-life measurement of 177Lu

The activity of the 177Lu solution has been measured by means of the CIEMAT/NIST efficiency tracing method. This result has been compared to the previous obtained results received from 4πβ(LS)–γ coincidence and anticoincidence counting. The activities determined with various methods have been found to be consistent.The decay curve of the 177Lu solution has been followed in the TDCR counter to determine the half-life of this isotope. The half-life has been separately determined for double and triple coincidence events. The arithmetic mean value of these two results has been found to be T1/2 = 6.6489(52) d.

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 Pβ− = 0.8954(14) was determined. The new results for Pγ and Pβ− 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).

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.

Standardization of 129I using the movable 4πβ(LS)-X(NaI(Tl)) system

The 129I standardization, using the movable 4πβ(LS)−X(NaI(Tl)) coincidence system, was performed for two 129I radioactive sources – one was dissolved in 0.1M NaOH solution and the other in 0.1M HNO3 solution. The system incorporates three movable PM tubes for a β–counter placed on a plane and a X-ray detector that can be moved up to the bottom of the vial.The β-efficiency depending on the amount of radioactive solution was investigated with 14 liquid scintillation samples prepared by gravimetrically dispensing 4.4–145 mg of 129I radioactive solution. The β-efficiencies above 90% were observed at less than 56 mg, but it was at most 70% at 145 mg. This occurred regardless of the activity of the sample or the type of chemical solution used to dissolve 129I source. The activity concentration of each 129I source was determined by efficiency-extrapolation method for samples with an activity range of 0.28–4.5 kBq. The β-efficiency points were derived over 10 intervals by moving 3-PM tubes in fine steps of about 1 mm from the sample. The highest value for β-efficiency was 95%. The combined uncertainty were 0.25% and 0.26%, respectively. The stated precision obtained using the system is better than that previously reported in the literature obtained by the triple to double coincidence ratio (TDCR) or the CIEMAT/NIST efficiency tracing method.

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.

Determination of 90Sr activity concentration in black tea powder used as proficiency test sample by the modified liquid scintillation counting method

Environmental monitoring is very important in nuclear facilities and its surroundings during the construction and operation. It requires many and different radioactivity measurements in order to radiation protection, radioactive waste management and environmental protection. An accurate and reliable determination of Strontium-90 in various environment matrices has been studied with many techniques such as co-precipitation, ion exchange, solvent extraction and extraction chromatography for environmental contamination monitoring purposes. In this study, the method used for dissolution and radiochemical separation to determine strontium-90 activity concentration in tea matrix used as proficiency test sample with liquid scintillation counter was investigated. As a result of these studies, the average mean activity concentration of strontium-90 in proficiency test material was determined as 155 ± 16 Bq kg−1 dry matter (k = 2) by liquid scintillation efficiency tracing (CIEMAT/NIST) method for seven selected samples. This value was also confirmed by Joint Research Center-Geel with the CIEMAT/NIST method. The proficiency test results for consistency were checked by Dioxon's Q-test. Uncertainties arising from counting statistics, background, weighing, half-life, chemical recovery, efficiency and homogeneity were calculated.

Activity determination of 231Pa by means of liquid scintillation counting

Protactinium-231 decays through a chain of daughter nuclides to stable 207Pb, resulting in a number of alpha and beta transitions. A method based on liquid scintillation (LS) counting was developed to determine the activity concentration of 231Pa. For α emissions, the corresponding LS counting efficiency was assumed to be unity. A potential loss of the noble gas 219Rn was evaluated by an additional experiment. In the case of the short-lived 215Po, a dead-time-dependent correction factor was taken into account in the calculation. For beta emitters 227Ac, 223Fr, 211Pb and 207Tl, the counting efficiencies were determined by the CIEMAT/NIST efficiency tracing (CNET) method with 3H standard as a tracer. The impurities were checked and found that the possible impurity was 227Ac, with an initial activity ratio A(227Ac)/A (231Pa) between 0.5% and 0.8%. Finally, the activity concentration of the 231Pa solution was determined with a combined relative standard uncertainty of 0.39%.

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.

Measurement of 160Tb and 161Tb in nuclear forensics samples

160Tb and 161Tb are important radionuclides to measure when analysing a Nuclear Forensics sample. An analytical method for the measurement of both 160Tb and 161Tb was developed in this study. Terbium was separated and purified using exchange resin and TrisKem LN Resin. The purified fraction containing 160Tb and 161Tb was measured by gamma spectrometry and liquid scintillation counting. The counting efficiencies of 160Tb and 161Tb were determined using the CIEMAT/NIST efficiency tracing method. The LSC count rate ratio, \({{R_{{{}^{160}{\text{Tb}}}} } \mathord{\left/ {\vphantom {{R_{{{}^{160}{\text{Tb}}}} } {R_{{{}^{161}{\text{Tb}}}} }}} \right. \kern-0pt} {R_{{{}^{161}{\text{Tb}}}} }}\), on the reference date was determined by sequential counting and calculated using a custom script based on their half-lives.

Standardization of 142Pr activity concentration

142Pr (praseodymium-142), a potential radionuclide for brachytherapy, was produced after irradiating high purity natural Pr2O3 powder in a research reactor. The irradiated powder was then dissolved in acids and diluted for measurement. Several radioactivity measurement methods were used in the standardization of 142Pr radionuclide. This work reports the results of standardization of 142Pr with CIEMAT/NIST efficiency tracing method, gamma spectrometry and a calibrated ionization chamber. The activity measured from different standardization methods are compatible with each other within the uncertainty limits. The CIEMAT/NIST efficiency tracing method gives the lowest uncertainty and is a reliable method for the standardization of this radionuclide.

Standardisation of the 129I, 151Sm and 166mHo activity concentration using the CIEMAT/NIST efficiency tracing method

The 129I, 151Sm and 166mHo standardisations using the CIEMAT/NIST efficiency tracing method, that have been carried out in the frame of the European Metrology Research Program project “Metrology for Radioactive Waste Management” are described. The radionuclide beta counting efficiencies were calculated using two computer codes CN2005 and MICELLE2. The sensitivity analysis of the code input parameters (ionization quenching factor, beta shape factor) on the calculated efficiencies was performed, and the results are discussed. The combined relative standard uncertainty of the standardisations of the 129I, 151Sm and 166mHo solutions were 0.4%, 0.5% and 0.4%, respectively. The stated precision obtained using the CIEMAT/NIST method is better than that previously reported in the literature obtained by the TDCR (129I), the 4πγ-NaI (166mHo) counting or the CIEMAT/NIST method (151Sm).

Analysis of 161Tb by radiochemical separation and liquid scintillation counting

The determination of 161Tb activity is problematic due to its very low fission yield, short half-life, and the complication of its gamma spectrum. At AWE, radiochemically purified 161Tb solution was measured on a PerkinElmer 1220 QuantulusTM Liquid Scintillation Spectrometer. Since there was no 161Tb certified standard solution available commercially, the counting efficiency was determined by the CIEMAT/NIST Efficiency Tracing method. The method was validated during a recent inter-laboratory comparison exercise involving the analysis of a uranium sample irradiated with thermal neutrons. The measured 161Tb result was in excellent agreement with the result using gamma spectrometry and the result obtained by Pacific Northwest National Laboratory.

Revision of the NIST Standard for (223)Ra: New Measurements and Review of 2008 Data.

After discovering a discrepancy in the transfer standard currently being disseminated by the National Institute of Standards and Technology (NIST), we have performed a new primary standardization of the alpha-emitter 223Ra using Live-timed Anticoincidence Counting (LTAC) and the Triple-to-Double Coincidence Ratio Method (TDCR). Additional confirmatory measurements were made with the CIEMAT-NIST efficiency tracing method (CNET) of liquid scintillation counting, integral γ-ray counting using a NaI(Tl) well counter, and several High Purity Germanium (HPGe) detectors in an attempt to understand the origin of the discrepancy and to provide a correction. The results indicate that a −9.5 % difference exists between activity values obtained using the former transfer standard relative to the new primary standardization. During one of the experiments, a 2 % difference in activity was observed between dilutions of the 223Ra master solution prepared using the composition used in the original standardization and those prepared using 1 mol·L−1 HCl. This effect appeared to be dependent on the number of dilutions or the total dilution factor to the master solution, but the magnitude was not reproducible. A new calibration factor (“K-value”) has been determined for the NIST Secondary Standard Ionization Chamber (IC “A”), thereby correcting the discrepancy between the primary and secondary standards.

Standardization of 237Np

The standardization of 237Np was investigated. The certified massic activity for 237Np was obtained by 4παβ liquid scintillation (LS) counting with correction for the 233Pa daughter using the CIEMAT/NIST efficiency tracing method using a 3H standard. Confirmatory measurements were also performed by high-resolution HPGe gamma-ray spectrometry, and by 4παβ(LS)-γ(NaI) anticoincidence counting. All results agree within the respective method's uncertainties. It was confirmed that the 237Np/233Pa radioactive equilibrium is disturbed when making dilutions and/or removing aliquots.

An advanced method of activity determination of large area beta emitting sources

The presented advanced method of activity determination of large area beta emitting sources is based on a version of efficiency tracing method using a test foil placed between the source and a conventional large area detector. It is shown that the total efficiency of the measuring system may depend on a dimensionless parameter derived from the difference in count rates caused by inserting the test foil while other disturbing effects are mostly reduced or compensated.

Standardization of Tc-99 by three liquid scintillation counting methods

The NMISA participated in the international key comparison of the pure beta-emitter Technetium-99, CCRI(II)-K2.Tc-99. The comparison solution was standardized using three methods, namely the TDCR efficiency calculation method, the CIEMAT/NIST efficiency tracing method and the 4π(LS)β–γ coincidence tracing method with Co-60 as tracer. Excellent agreement between results obtained with the three methods confirmed the applicability of the beta spectral shape given by the latest (2011) DDEP evaluation of Tc-99 decay data, rather than the earlier (2004) evaluation.

Activity standardisation of 45Ca and 204Tl using the new TDCR system at CMI

A new triple-to-double coincidence ratio (TDCR) system has been established at CMI as an additional technique for primary activity standardisation. Details of the detectors and the electronics are given. Construction of the optical chamber was validated using measurement of a 3H standard—the efficiency achieved with Ultima Gold was 58%. Several methods of efficiency variation (coloured bands painted on vial, positioning and photomultipliers defocussing) are compared. Activity concentrations of 204Tl and 45Ca were determined with the TDCR method and with the efficiency tracer technique. Advantages of the TDCR method compared to the efficiency tracing method and agreement between results are shown.