A.K. Pearce

α-particle emission probabilities in the decay of 235U

235U decays by α-particle emission to 231Th. The decay scheme of this nuclide is very complex, with more than 20 alpha branches. Recommended values for Pα of this nuclide are based on measurements carried out in 1975. This work presents the results of new measurements made with Si detectors and sources of enriched uranium in the frame of the EUROMET 591 cooperation project. The use of improved measurement techniques and numerical analysis of spectra allowed a new set of Pα values for 13 lines with improved uncertainties to be obtained.

Direct measurement of the half-life of 223Ra

Radioactive decay half-life measurements of (223)Ra, a member of the (235)U naturally occurring radioactive decay series, have been performed of a radiochemically pure solution with an ionisation chamber. The radioactive decay of (223)Ra was followed for 50 days, approximately 4.4 half-lives. The deduced half-life of (223)Ra was found to be 11.4358 (28) days, supporting the other published direct measurements. A detailed uncertainty budget is presented. A new evaluation of the published half-life values was performed, indicating significant variation across the existing published values, suggesting that further measurements of the half-life of (223)Ra are required. A new evaluated half-life has been calculated using a power moderated weighted mean of selected experimental values, with a new value of the recommended half-life for (223)Ra of 11.4354 (17) days.

Standardisation of 11C.

The increasing use of positron emission tomography for medical imaging and the availability of short-lived positron emitters has raised concerns about the accuracy of calibration of secondary standard measurement systems and the viability of using a single long-lived positron emitter as a reference calibration source for all positron emitters. Potential problems arise because the 511 keV quanta arising from positron annihilation are not generally produced at the same point as the original disintegration. In addition, the secondary standard may also be responsive to the associated bremsstrahlung radiation. The magnitude of both effects depends on the positron end-point energy. In order to resolve these problems, it is necessary to produce absolute standards of these positron-emitting radionuclides and the work presented here details the results of such work with 11C.

Precise measurements of the absolute γ-ray emission probabilities of (223)Ra and decay progeny in equilibrium.

Precise measurements of the absolute γ-ray emission probabilities have been made of radiochemically pure solutions of (223)Ra in equilibrium with its decay progeny, which had been previously standardised by 4π(liquid scintillation)-γ digital coincidence counting techniques. Two high-purity germanium γ-ray spectrometers were used which had been accurately calibrated using a suite of primary and secondary radioactive standards. Comparison of the activity concentration determined by the primary technique against γ-ray spectrometry measurements using the nuclear data evaluations of the Decay Data Evaluation Project exhibited a range of ~18% in the most intense γ-ray emissions (>1% probability) of the (223)Ra decay series. Absolute γ-ray emission probabilities and standard uncertainties have been determined for the decay of (223)Ra, (219)Rn, (215)Po, (211)Pb, (211)Bi and (207)Tl in equilibrium. The standard uncertainties of the measured γ-ray emission probabilities quoted in this work show a significant improvement over previously reported γ-ray emission probabilities. Correlation coefficients for pairs of the measured γ-ray emission probabilities from the decays of the radionuclides (223)Ra, (219)Rn and (211)Pb have been determined and are presented. The α-transition probabilities of the (223)Ra have been deduced from P(γ+ce) balance using the γ-ray emission probabilities determined in this work with some agreement observed with the published experimental values of the α-emission probabilities.

Standardisation of 223Ra by liquid scintillation counting techniques and comparison with secondary measurements

An aqueous solution of 223Ra chloride in equilibrium with its decay progeny was standardised by liquid scintillation counting techniques. Since secular equilibrium with the decay progeny of 223Ra had been established by the time of measurement, the apparent detection efficiency of 223Ra was approximately 6 and was determined by both the CIEMAT/NIST efficiency tracing technique and the 4π(LS)-γ digital coincidence counting techniques. The results obtained were compared with γ-spectrometry and ionisation chamber measurements. Whilst the γ-spectrometry measurements were in agreement (albeit exhibiting a large spread (18%) in the individual activity estimations using the main γ-emissions), a significant discrepancy of the order of 9% was identified between the liquid scintillation counting results and those obtained using published calibration factors for a variety of radionuclide calibrators.

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

Utilising a chemically purified solution the radioactive half-life of (227)Th has been determined indirectly by observation of the ingrowth of (223)Ra using an ionisation chamber (IC) and for the first time by direct observation of the change in activity with time using a high-purity germanium (HPGe) γ-ray spectrometer. The radioactive decay was observed for ~104 days (~5.6 half-lives) by γ-ray spectrometry and approximately 63 days and 72 days (~3.4 and ~3.9 half-lives) using an ionisation chamber (IC). The resulting half-life values - 18.695 (4) days (IC) and 18.683 (20) days (HPGe) - are consistent and detailed uncertainty budgets are presented for the two measurement techniques. A weighted mean of our results of 18.695 (4) days is inconsistent with the most precise published half-life value of 18.7176 (52) days (Jordan and Blanke, 1967). A critical evaluation of literature data has been performed, indicating a paucity of reliable and independent measurements. Selected independent published values have been used to determine a recommended half-life of 18.697 (7) days. A method has been introduced in the course of this work so that the recommended half-life of (227)Th as determined by ingrowth can be modified if a different (223)Ra half-life has been determined, evaluated and adopted.

Standardisation of 210Pb by Čerenkov counting

A standard of (210)Pb in solution was produced at the National Physical Laboratory by a novel technique combining Čerenkov counting with the established liquid scintillation efficiency tracing technique known as the CIEMAT/NIST method. Coincidence counting was applied in order to validate the measurements and the activity concentrations of the solution determined with each technique are shown to be in agreement. Radiochemical separation of the (210)Pb from its daughters was also necessary and the scheme for the separation is described. After performing this two-stage standardisation, the uncertainty was successfully lowered to 0.66% (k=1). This uncertainty is approximately a factor of four lower than previously achieved at NPL by the classical method of standardisation of radionuclides, i.e. coincidence counting.

Determination of the gamma emission intensities of 111Ag

A radioactive solution of (111)Ag, standardised by the absolute measurement methods 4π(PC)-γ and 4π(LS)-γ coincidence counting at the National Physical Laboratory (NPL), was measured by two independently calibrated HPGe γ spectrometers in order to estimate the γ emission intensities and to determine the absolute intensity, with the aim of improving the currently published values. An absolute intensity value of 6.68 (7)% was obtained for the 342.1 keV γ emission, which is in agreement with previously reported values, but greatly reduces the uncertainty. Additionally, this work proposes a new emission intensity for the 450.0 keV γ emission that has not been previously reported, with an absolute intensity of 0.000482 (12)%. An investigation of the published γ emission intensities shows significant discrepancies that require resolution.

Development of NANA: A Fast-Scintillator, Coincidence Gamma-ray Array for Radioactive Source Characterisation and Absolute Activity Measurements at the UK National Physical Laboratory

A multi-detector modular coincidence gamma-ray spectrometer is being designed and constructed for use at the UKs National Physical Laboratory (NPL) for use in direct measurement and metrological standardisation of nuclear decay activities. In its first generation, the NPL National Nuclear Array (NANA) will consist of twelve individual halide scintillation detectors placed in a high-efficiency geometry around a well-defined central point source position. This brief conference paper provides details of the measured detector module and coincidence energy and timing responses for the LaBr3(Ce) detectors which will be used in the NANA array. Preliminary GEANT4 simulations of the arrays full energy peak efficiency and expected gamma-ray coincidence response are also presented.

Results of an international comparison of activity measurements of 68 Ge

An international key comparison, identifier CCRI(II)-K2.Ge-68, has been performed. The National Institute of Standards and Technology (NIST) served as the pilot laboratory, distributing aliquots of a 68Ge/68Ga solution. Results for the activity concentration, CA, of 68Ge at a reference date of 12h00 UTC 14 November 2014 were submitted by 17 laboratories, encompassing many variants of coincidence methods and liquid-scintillation counting methods. The first use of 4π(Cherenkov)β-γ coincidence and anticoincidence methods in an international comparison is reported. One participant reported results by secondary methods only. Two results, both utilizing pure liquid-scintillation methods, were identified as outliers. Evaluation using the Power-Moderated Mean method results in a proposed Comparison Reference Value (CRV) of 621.7(11)kBqg-1, based on 14 results. The degrees of equivalence and their associated uncertainties are evaluated for each participant. Several participants submitted 3.6mL ampoules to the BIPM to link the comparison to the International Reference System (SIR) which may lead to the evaluation of a Key Comparison Reference Value and associated degrees of equivalence.