ABSTRACT
A method for absolute measurements of the 225Ac activity in equilibrium with its progeny was developed. Measurements were performed using the triple-to-double coincidence ratio (TDCR) method in two different TDCR counters. The activity concentration of an 225Ac solution was determined and the solution was sent to the SIR system for a comparison. The half-life of 225Ac was determined by one of the TDCR counters and found to be 9.9150(63) days.
ABSTRACT
This work presents the first direct measurement of the 93Mo half-life. The measurement is a combination of high-resolution mass spectrometry for the determination of the 93Mo concentration and liquid scintillation counting for determining the specific activity. A 93Mo sample of high purity was obtained from proton irradiated niobium by chemical separation of molybdenum with a decontamination factor larger than 1.6 × 1014 with respect to Nb. The half-life of 93Mo was deduced to be 4839(63) years, which is more than 20% longer than the currently adopted value, whereas the relative uncertainty could be reduced by a factor of 15. The probability that the 93Mo decays to the metastable state 93mNb was determined to be 95.7(16)%. This value is a factor of 8 more precise than previous estimations. Due to the man-made production of 93Mo in nuclear facilities, the result leads to significantly increased precision for modelling the low-level nuclear waste composition. The presented work demonstrates the importance of chemical separations in combination with state-of-the-art analysis techniques, which are inevitable for precise and accurate determinations of nuclear decay data.
ABSTRACT
The 32Si decay rate measurement data of Alburger et al. obtained in 1982-1986 at Brookhaven National Laboratory have been presented repeatedly as evidence for solar neutrino-induced beta decay. The count rates show an annual sinusoidal oscillation of about 0.1% amplitude and maximum at February-March. Several authors have claimed that the annual oscillations could not be explained by environmental influences on the set-up, and they questioned the invariability of the decay constant. They hypothesised a correlation with changes in the solar neutrino flux due to annual variations in the Earth-Sun distance, in spite of an obvious mismatch in amplitude and phase. In this work, environmental conditions at the time of the experiment are presented. The 32Si decay rate measurements appear to be inversely correlated with the dew point in a nearby weather station. Susceptibility of the detection set-up to local temperature and humidity conditions is a likely cause of the observed instabilities in the measured decay rates. Similar conclusions apply to 36Cl decay rates measured at Ohio State University in 2005-2012.
ABSTRACT
Activity measurements of 3H, 241Am and 60Co solutions were performed to compare digital coincidence modules used at PTB and POLATOM for TDCR and 4πß(LS)-γ coincidence counting. The activities determined with various coincidence modules connected in parallel to the same counter at PTB were found to be consistent. Observed discrepancies caused by differences in the coincidence resolving time did not exceed 0.14%. Accidental coincidences simulated by a frequency generator were registered, and the coincidence resolving time was determined.
ABSTRACT
The aim of the European Metrology Research Project MetroBeta is to improve the knowledge of the shapes of beta spectra, both in terms of theoretical calculation and measurement. The precise knowledge of beta spectra is required for the activity standardisation of pure beta emitters. Metallic magnetic calorimeters (MMCs), a type of cryogenic detectors, with the beta emitter embedded in the absorber have proven to be among the best beta spectrometers, in particular for low-energy beta transitions. Within this project, new designs of MMCs optimized for five different beta energy ranges were developed and a new detector module was constructed. The beta spectra of 151Sm, 14C and 99Tc have been measured so far; additional measurements with 36Cl are under preparation. Improved theoretical calculation methods and complementary measurement techniques complete the project.
ABSTRACT
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)ß-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.
ABSTRACT
Some authors have raised doubt about the invariability of decay constants, which would invalidate the exponential-decay law and the foundation on which the common measurement system for radioactivity is based. Claims were made about a new interaction - the fifth force - by which neutrinos could affect decay constants, thus predicting changes in decay rates in correlation with the variations of the solar neutrino flux. Their argument is based on the observation of permille-sized annual modulations in particular decay rate measurements, as well as transient oscillations at frequencies near 11 year-1 and 12.7 year-1 which they speculatively associate with dynamics of the solar interior. In this work, 12 data sets of precise long-term decay rate measurements have been investigated for the presence of systematic modulations at frequencies between 0.08 and 20 year-1. Besides small annual effects, no common oscillations could be observed among α, ß-, ß+ or EC decaying nuclides. The amplitudes of fitted oscillations to residuals from exponential decay do not exceed 3 times their standard uncertainty, which varies from 0.00023 % to 0.023 %. This contradicts the assertion that 'neutrino-induced' beta decay provides information about the deep solar interior.
ABSTRACT
The hypothesis that proximity to the Sun causes variation of decay constants at permille level has been tested and disproved. Repeated activity measurements of mono-radionuclide sources were performed over periods from 200 days up to four decades at 14 laboratories across the globe. Residuals from the exponential nuclear decay curves were inspected for annual oscillations. Systematic deviations from a purely exponential decay curve differ from one data set to another and are attributable to instabilities in the instrumentation and measurement conditions. The most stable activity measurements of alpha, beta-minus, electron capture, and beta-plus decaying sources set an upper limit of 0.0006% to 0.008% to the amplitude of annual oscillations in the decay rate. Oscillations in phase with Earth's orbital distance to the Sun could not be observed within a 10-6 to 10-5 range of precision. There are also no apparent modulations over periods of weeks or months. Consequently, there is no indication of a natural impediment against sub-permille accuracy in half-life determinations, renormalisation of activity to a distant reference date, application of nuclear dating for archaeology, geo- and cosmochronology, nor in establishing the SI unit becquerel and seeking international equivalence of activity standards.
ABSTRACT
In the past few years, two Cerenkov methods were developed to make activity measurements of high-energy beta emitters in liquid scintillation counters with two or three photomultiplier tubes (PMTs) possible. Both methods are based on a free parameter model and make use of the Frank and Tamm theory for the emission of Cerenkov light. In this article, additional effects are discussed and further improvements are presented. The dependence of the refractive index of water on the wavelength can now be taken into account, which has also an influence on the upper limit of the wavelength region for the production of Cerenkov light. In addition, the dependence of the PMT response on the wavelength is taken into account. Finally, it is possible to take a potential asymmetry of efficiencies in a system with three PMTs into account. To this end, three free parameters are assigned to each individual PMT and then determined by means of a downhill simplex optimization algorithm. The computed counting efficiencies for a triple-to-double coincidence ratio (TDCR) system were compared with experimental data for (32)P, (89)Sr, and (90)Y.
ABSTRACT
Liquid scintillation measurements of (229)Th in radioactive equilibrium with its progenies were carried out. The counting efficiency was determined by means of a free parameter model. The measurements were made in a custom-built triple-to-double coincidence ratio (TDCR) system. In addition, a new portable mini-TDCR system with three channel photomultipliers and two commercial counters were tested. The decay of the short-lived (213)Po requires great care, since it often occurs during the dead time of the counter systems. Also the rather short-lived (217)At may decay during the dead time caused by (221)Fr decay events. The overall counting efficiency of the TDCR system of PTB was found to be about 700% (depending on the degree of chemical quenching) and the relative standard uncertainty of the activity concentration was found to be about 0.23%. The determined activity concentration was compared with the outcome of alpha spectrometry under defined solid angle and excellent agreement was found. The TDCR efficiency calculations can be easily adapted to activity determinations of (225)Ra or (225)Ac in equilibrium with their progenies.
ABSTRACT
The triple-to-double coincidence ratio (TDCR) method is frequently used to measure the activity of radionuclides decaying by pure ß emission or electron capture (EC). Some radionuclides with more complex decays have also been studied, but accurate calculations of decay branches which are accompanied by many coincident γ transitions have not yet been investigated. This paper describes recent extensions of the model to make efficiency computations for more complex decay schemes possible. In particular, the MICELLE2 program that applies a stochastic approach of the free parameter model was extended. With an improved code, efficiencies for ß(-), ß(+) and EC branches with up to seven coincident γ transitions can be calculated. Moreover, a new parametrization for the computation of electron stopping powers has been implemented to compute the ionization quenching function of 10 commercial scintillation cocktails. In order to demonstrate the capabilities of the TDCR method, the following radionuclides are discussed: (166m)Ho (complex ß(-)/γ), (59)Fe (complex ß(-)/γ), (64)Cu (ß(-), ß(+), EC and EC/γ) and (229)Th in equilibrium with its progenies (decay chain with many α, ß and complex ß(-)/γ transitions).
ABSTRACT
A procedure has been developed to calibrate photon-emitting wide area reference sources in terms of activity, surface emission rate and uniformity considering the requirements introduced by the recent revision of ISO standard 8769. The method makes use of a large volume NaI detector with a cross table scanning system, a radiography system and it applies Monte Carlo techniques to assess the surface emission rate. The method was successfully applied to sources of (241)Am, (57)Co, (137)Cs and (60)Co. In addition, problems with definitions and the practical use of standard ISO 8769:2010 are highlighted.
Subject(s)
Models, Chemical , Photons , Radioisotopes/analysis , Radioisotopes/standards , Radiometry/instrumentation , Radiometry/standards , Computer Simulation , Half-Life , Internationality , Monte Carlo Method , Radiation Dosage , Radioisotopes/chemistry , Reference Standards , Reference ValuesABSTRACT
An international Key Comparison of (177)Lu has recently been carried out. Twelve laboratories performed assays for radioactivity content on aliquots of a common master solution of (177)Lu, leading to eleven results submitted for entry into the Key Comparison Database of the Mutual Recognition Arrangement. A proposed Comparison Reference Value (CRV) was calculated to be 3.288(4)MBq/g using all eleven results. Degrees of equivalence and their uncertainties were calculated for each laboratory based on the CRV. Most of the values reported by the participating laboratories were within 0.6% of the CRV.
Subject(s)
Lutetium/analysis , Lutetium/chemistry , Radioisotopes/analysis , Radioisotopes/chemistry , Radiometry/instrumentation , Radiometry/standards , Lutetium/standards , Radiation Dosage , Radioisotopes/standards , Reference Standards , Reference ValuesABSTRACT
The purposes of this study were to create national activity standards of (64)Cu, to make possible the definition of an international key comparison reference value and to determine the decay data in order to improve the decay scheme. Four laboratories measured the activity of a (64)Cu solution; these results were compared through the International Reference System. Moreover, the laboratories carried out new measurements of the photon emission intensities and of the half-life. A new decay scheme was derived from these new values and the previously published ones.
Subject(s)
Copper Radioisotopes/analysis , Copper Radioisotopes/chemistry , Radiometry/standards , Half-Life , Internationality , Radiation Dosage , Reference Standards , Reference ValuesABSTRACT
The radionuclide (113m)Cd was extracted from a Cd component, which has been irradiated in a research reactor during its routine operation. A validated separation procedure was then used to obtain a (113m)Cd solution of high radiochemical purity. The solution was measured by means of liquid scintillation counting for activity determination. To this end, the CIEMAT/NIST efficiency tracing method as well as the triple-to-double coincidence (TDCR) counting technique were used. Measurements in a low-background gamma-ray spectrometer were applied to identify any potential photon-emitting impurities and to measure the 263.7 keV photons from the isomer transition. A gamma emission probability of 0.01839(29)% was determined for this line. The liquid scintillation measurements were also used to evaluate the half-life of (113m)Cd. The decay was followed for about 11 months and a half-life T(1/2)=13.97(13) years was obtained.
ABSTRACT
An international exercise, registered as EUROMET project no. 907, was launched to measure both the activity of a solution of (124)Sb and the photon emission intensities of its decay. The same solution was sent by LNE-LNHB to eight participating laboratories, six of which sent results for photon emission intensities both in absolute and in relative terms. From these results and including previous published values, a consistent decay scheme was worked out, proving that problems in activity measurements have not been due to decay scheme data.
ABSTRACT
An international exercise, registered as EUROMET project no. 907, was launched to measure both the activity of a solution of (124)Sb and the photon emission intensities of its decay. The same solution was sent by LNE-LNHB to eight participating laboratories. In order to identify possible biases, the participants were asked to use all possible activity measurement methods available in their laboratory and then to determine their reference value for comparison. Thus, measurement results from 4pibeta-gamma coincidence/anti-coincidence counting, CIEMAT/NIST liquid-scintillation counting, 4pigamma counting with well-type ionization chambers and well-type crystal detectors were given. The results are compared and show a maximum discrepancy of about 1.6%: possible explanations are proposed.
Subject(s)
Antimony/analysis , Antimony/standards , International Cooperation , Photons , Reference Values , Reproducibility of Results , Scintillation Counting , Solutions , Weights and MeasuresABSTRACT
Samples containing high-energy beta emitters in hydrochloric acid were measured in an in-house built triple-to-double coincidence counter. For the analysis of the results, a new method was developed that is based on a free parameter model. Formulas for the determination of the counting efficiencies were derived assuming a Poisson distribution for the generation of photoelectrons at the photocathodes and applying the Frank and Tamm theory to the production of Cerenkov light. The anisotropy effect for the emission of Cerenkov light was taken into account. The activities determined with the new method were compared with the results from other primary standardization techniques. The following radionuclides were successfully studied: (32)P, (89)Sr, (90)Y, (204)Tl and (106)Rh.
ABSTRACT
The methods to compute the counting efficiency of electron-capture nuclides in liquid scintillation counting have been improved in several previous studies. The main improvements comprise a more realistic treatment of the ejection of photoelectrons and subsequent rearrangement processes in the atomic shell as well as a more detailed atomic rearrangement model. The latter was realized in the MICELLE code by means of a new stochastic approach. This new model was also developed to account for energy deposits within micelles. The recent improvements have now been combined in an updated version of the MICELLE code, which also makes the computation of the counting efficiency of complex decay schemes possible. In this paper, we describe and discuss recent extensions and improvements of the models and further corrections. The calculated counting efficiencies of selected radionuclides are compared with the experimental data obtained by liquid scintillation counting. For the measurements, we use standard solutions, which were calibrated by other methods.
ABSTRACT
A triple-to-double coincidence ratio (TDCR) liquid scintillation counting system (Pochwalski et al., 1988; Grau Malonda and Coursey, 1988) was set up at the Physikalisch-Technische Bundesanstalt (PTB) to be used as an additional technique for primary activity standardization. The optical chamber of the system was designed in our laboratory, expending great effort to achieve maximum light transmission to the photomultiplier tubes (PMT) and, consequently, a maximum counting efficiency and low uncertainties. A programme to compute the counting efficiencies and to analyse the measurement data was developed and tested. The software can also be used to allow for PMT asymmetries. The system and recently developed analysis procedures were used for the activity standardization of a tritiated water solution within the scope of an international comparison under the auspices of the Bureau International des Poids et Mesures (BIPM).