The half-life of 227Th by direct and indirect measurements

Activity standard and calibrations for 227Th with ingrowing progeny

Thorium-227 was separated from its progeny and standardized for activity by the triple-to-double coincidence ratio (TDCR) method of liquid scintillation counting. Confirmatory liquid scintillation-based measurements were made using efficiency tracing with 3H and live-timed anticoincidence counting (LTAC). The separation time and the efficiency of the separation were confirmed by gamma-ray spectrometry. Calibrations for reentrant pressurized ionization chambers, including commercial radionuclide calibrators, and a well-type NaI(Tl) detector are discussed.

A new determination of the 68Ga half-life and evaluation of literature data

Inconsistencies within the 68Ga half-life dataset have been repeatedly identified in nuclear decay data evaluations performed over the past decade. In this paper, a half-life has been determined using ionisation chambers measurements and HPGe gamma-ray spectrometry (T1/2 = 67.783(27) min). The current literature has been reviewed and possible causes of discrepancy have been identified, such as neglected contaminant effects, robustness of analysis, and underestimation of systematic effects. This could lead to a significant improvement of the half-life in future evaluations.

Radionuclide metrology: confidence in radioactivity measurements

Radionuclides, whether naturally occurring or artificially produced, are readily detected through their particle and photon emissions following nuclear decay. Radioanalytical techniques use the radiation as a looking glass into the composition of materials, thus providing valuable information to various scientific disciplines. Absolute quantification of the measurand often relies on accurate knowledge of nuclear decay data and detector calibrations traceable to the SI units. Behind the scenes of the radioanalytical world, there is a small community of radionuclide metrologists who provide the vital tools to convert detection rates into activity values. They perform highly accurate primary standardisations of activity to establish the SI-derived unit becquerel for the most relevant radionuclides, and demonstrate international equivalence of their standards through key comparisons. The trustworthiness of their metrological work crucially depends on painstaking scrutiny of their methods and the elaboration of comprehensive uncertainty budgets. Through meticulous methodology, rigorous data analysis, performance of reference measurements, technological innovation, education and training, and organisation of proficiency tests, they help the user community to achieve confidence in measurements for policy support, science, and trade. The author dedicates the George Hevesy Medal Award 2020 to the current and previous generations of radionuclide metrologists who have devoted their professional lives to this noble endeavour.

Status of the decay data for medical radionuclides: existing and potential diagnostic γ emitters, diagnostic β+ emitters and therapeutic radioisotopes

Recommended half-lives and specific well-defined emission energies and absolute emission probabilities are important input parameters that should be well-defined to assist in ensuring the diagnostic and therapeutic efficacy of individual radionuclides when applied in the field of nuclear medicine. Bearing in mind the nature of these requirements, approximately one hundred radionuclides have been considered and re-assessed as to whether their decay data are either adequately quantified, or require further in-depth measurements to improve their existing status and merit full re-evaluations of their decay schemes. The primary aim of such a review is to provide sufficient information on the existing and future requirements for such atomic and nuclear data.

Production and measurement of fission product noble gases

Gaseous fission products have been produced via thermal neutron irradiation of a highly-enriched uranium target and extracted using a custom gas processing system for measurement on a prototype, high-resolution β - γ coincidence detection system. The gas was extracted and measured in two stages in order to measure the prompt and β^--delayed fission products. This paper presents an overview of the system used to produce gaseous fission products, and the results of the advanced coincidence spectrometry techniques used to identify and quantify decays from the radionuclides produced, including the noble gases ⁸⁵Kr, ^85mKr, ⁸⁸Kr, ¹³³Xe, ¹³⁵Xe, ^133mXe and ^135mXe, as well as ¹³³I and ⁸⁸Rb. The measurements were validated by determination of the nuclear decay half-lives, specifically for the ground state decay of ¹³⁵Xe, which was found to be 9.15(49) hours and consistent with the literature value. This work demonstrates the UK capability to produce gaseous radionuclides for quality assurance and calibration purposes in Radionuclide Laboratories supporting the Comprehensive Nuclear-Test-Ban Treaty (CTBT).

Activity measurements and determination of nuclear decay data of 166Ho in the MRTDosimetry project

Holmium-166 is a high-energy β^--emitter radionuclide (~ 1.8 MeV) with a short half-life (~26.8h) that offers great potential as an alternative to ⁹⁰Y for the treatment of liver cancer based on radioembolization. The possibility of quantitative Single Photon Emission Computed Tomography (SPECT) imaging of the main γ-ray emission at 80.6 keV, in addition to strong paramagnetic properties suitable for Magnetic Resonance Imaging (MRI), complement this therapeutic potential. The present paper describes the measurements carried out in three European radionuclide metrology laboratories for primary standardization of ¹⁶⁶Ho and new determinations of X- and γ-ray photon-emission intensities in the framework of the European EMPIR project MRTDosimetry. New half-life measurements were also performed.

Alpha-particle emission probabilities of 231Pa derived from first semiconductor spectrometric measurements

The nuclide ²³¹Pa is a member of the ²³⁵U decay chain. It is a complex alpha emitter with 25 identified alpha emissions. Formerly published alpha-particle emission probabilities were derived from measurements taken with magnetic spectrometers. This work presents the first measurements made with semiconductor detectors. High-resolution alpha-particle spectrometry was carried out at CIEMAT and JRC using ion-implanted planar silicon detectors. Alpha-particle emission probabilities of 23 transitions were derived from deconvolutions of the spectra. For the major lines, uncertainties are lower than 1%, a significant improvement to existing data. The new data set will allow a more accurate evaluation of the decay scheme of ²³¹Pa.

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 ε_T227h++ 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 ε_T227h++ 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 T_1/2 = 18.681(9) d.

Measurement of absolute γ-ray emission probabilities in the decay of 227Ac in equilibrium with its progeny

The emission probabilities of γ rays produced in the ²²⁷Ac decay series were determined by high-resolution γ-ray spectrometry of sources with standardised activity. The sources were prepared quantitatively on glass discs by drop deposition of a solution with ²²⁷Ac in radioactive equilibrium with its daughter nuclides. Their activity was measured by a primary standardisation technique based on alpha-particle counting at a defined low solid angle. Four laboratories performed γ-ray spectrometry and derived absolute γ-ray intensities. Mean values were calculated and compared with literature data and the currently recommended evaluated data. New values on certain γ-ray emission probabilities are proposed.

Linearity check of an ionisation chamber through 99 mTc half-life measurements

The half-life of ^99 mTc was measured at the JRC using the ionisation chamber 'IC1' (type Centronic IG12). The result, T_1/2(^99 mTc) = 6.00660 (18) h, is in good agreement with literature data. One experiment was performed in IC1's default set-up with the ionisation current being integrated over an air capacitor and read out as a voltage increase over time. This ensured excellent linearity and precision throughout the dynamic range, but the maximum current was limited to 2 nA. In a second test, the current was directly read out with a Keithley 6517 A electrometer. Applying correction factors for the automatic range switching of the electrometer, an acceptable linearity was demonstrated over a range of 12 half-life periods starting at 20 nA. Range switching and autocorrelation of the current readout increase the systematic and random error propagation factors. Piecewise fitting of the decay curve over periods of 6 h yields the same ^99 mTc half-life value within 0.04% (0.0025 h) standard deviation over an activity range spanning at least 10 half-life periods (3 orders of magnitude).

Evaluation of the separation and purification of 227Th from its decay progeny by anion exchange and extraction chromatography

Thorium-227 is currently undergoing evaluation as a potential radionuclide for targeted cancer therapy, and as such a high chemical purity of the material is required. To establish a reliable procedure for radiochemical isolation of ²²⁷Th from the parent ²²⁷Ac and decay progeny, which includes the radiotherapeutic ²²³Ra, the performance of three different separation schemes based on ion-exchange and extraction chromatography have been evaluated. The results suggest that both ion exchange and extraction chromatographic techniques can be successfully used for the separation of ²²⁷Th from its decay progeny, however extraction chromatographic resins demonstrate favourable performance in terms of Th recovery and purification from radionuclide impurities.

Fundamental uncertainty equations for nuclear dating applied to the 140Ba-140La and 227Th-223Ra chronometers

Basic equations for age dating through activity ratio measurements are presented and applied to nuclear chronometers based on parent-daughter decay. Uncertainty propagation formulae are derived which relate the relative uncertainty on the half-lives and measured activity ratios with the relative uncertainty on the calculated time of a nuclear event. Particular attention is paid to the case of relatively short-lived radionuclides for which the change in decay rate during the measurement is non-negligible. Mathematical solutions are presented to correct the perceived activity ratio and adapt the uncertainty propagation formulae to complete the uncertainty budget. The formulae have been applied to ¹⁴⁰Ba-¹⁴⁰La chronometry, which is particularly useful for dating a nuclear explosion through measurement of the produced activity ratio of ¹⁴⁰La and ¹⁴⁰Ba in a finite time interval. They were also applied to the ²²⁷Th-²²³Ra parent-daughter pair produced for therapeutic use. The impact of inaccuracies in the nuclear decay data on the performance of these nuclear chronometers is shown and discussed.

Development of separation technology for the removal of radium-223 from targeted thorium conjugate formulations. Part I: purification of decayed thorium-227 on cation exchange columns

Targeted thorium conjugates (TTCs) are being explored as a potential future platform for specific tumor targeting pharmaceuticals. In TTCs, the alpha emitting radionuclide thorium-227 (²²⁷Th) with a half-life of 18.697 d is labeled to targeting moieties, such as monoclonal antibodies (mAbs). The amount of daughter nuclide radium-223 (²²³Ra, t_1/2=11.435 d) will increase during manufacture and distribution, and so a technology for purification is required to assure an acceptable level of ²²³Ra is administrated to the patient. Since ²²³Ra is the only progeny of ²²⁷Th with a long half-life (days), the progenies of ²²³Ra will have a very limited stay in the formulation once ²²³Ra is removed. The focus in this study has, therefore, been on the removal of ²²³Ra. In this study, the sorption and separation of ²²³Ra (radium(II)) and ²²⁷Th (thorium(IV)) on cation exchange columns has been evaluated as a purification method of decayed ²²⁷Th (i.e. prior to radiolabelling of a mAb and formation of TTC). The goal is to minimize the sorption of ²²⁷Th and maximize the sorption of ²²³Ra. Statistical experimental design with formulation and process parameters, including buffered formulations comprising citrate and acetate, at various concentrations and pH, presence of free radical scavenger and chelator, and resin amount have been evaluated for impact on the purification process. The studies have been interpreted by the aid of multivariate data analysis. The correlations between design of experimental variables and sorption are summarized by regression models. The predictive accuracy of radionuclide sorption was given by standard deviation and 95% confidence intervals originating from statistical cross validation. Experimental results and statistical models for citrate-buffered formulations verified reproducible and acceptable sorption levels of ²²³Ra and ²²⁷Th under selected conditions. For acetate-buffered formulations, prediction of ²²⁷Th sorption was influenced by complex variable relationships and hence a risk of obtaining irreproducibility. Fine-tuned variable levels showed, however, variable combinations predicting high sorption of ²²³Ra (>90%) and low sorption of ²²⁷Th (<3%) also for the acetate-buffered formulations. The optimal separation conditions should be decided based on tuning the variables levels for ²²³Ra in the citrate-buffered formulations, while for acetate, the optimal separation should be based on tuning variable levels for ²²⁷Th sorption. The ionic strength of the formulation also seemed to affect the radionuclide sorption. Labeling of an antibody-chelator conjugate with purified ²²⁷Th (i.e. preparation of TTC) was successful in the selected citrate-buffered formulations tested.