The importance of the beta spectrum calculation for accurate activity determination of 63 Ni by means of liquid scintillation counting

International audienceThe activity concentration of a Ni-63 solution was determined by means of two liquid scintillation counting techniques: the TDCR method and the CIEMAT/NIST efficiency tracing technique. The results of both methods are in excellent agreement, provided that the Ni-63 beta spectrum calculation accounts for the atomic exchange effect. Thus, new beta spectrum calculations resolve a discrepancy that has been found in previous analyses. The influence of the computed beta spectrum on the final uncertainty of the activity concentration is discussed

Do radioactive decay rates depend on the distance between the Earth and the Sun?

The potential influence of solar neutrinos on beta decay rates was investigated at PTB. To this end, new experiments have been carried out for the beta emitters \nuc{36}{Cl} and \nuc{90}{Sr}/\nuc{90}{Y}, respectively. The measurements were performed using custom-built liquid scintillation counters with three photomultiplier tubes (PMTs). The data were analyzed applying the TDCR method which yields information on the counting efficiency and the activity. The activities corrected for decay were found to be stable and no oscillation could be observed. Also frequency analyses do not show any significant periodicity. Thus, we disprove the findings from several previous works of a research group working with Fischbach, Jenkins, Sturrock et al. who used data from relative measurement methods only. The data they use are not suitable to claim evidence for variations of decay rates, since the measurement techniques do not provide information on the instrument efficiency. That group also used data from our laboratory which were obtained by means of ionization chambers. We can show that the observed effects cannot be explained with an influence of solar neutrinos, whereas an influence of climate data in the corresponding measurement room appears to be a more plausible reason

Improved activity standardization of 90^{90}Sr/90^{90}Y by means of liquid scintillation counting

International audienceRadioactive 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 $^{90}$Sr/$^{90}$Y 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 $^{90}$Sr/$^{90}$Y and the external standard source

Improved method for the calculation of the counting efficiency of electron-capture nuclides in liquid scintillation samples

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. © 2009 Elsevier Ltd.Peer Reviewe

Liquid scintillation counting: A valuable tool to determine half-lives

In the past two decades, the radionuclide metrology group at PTB has carried out a number of half-life determinations using liquid scintillation counting. The half-lives, which were often determined in collaboration with other institutions, range from a few ten nanoseconds (e.g., 86 keV level of 233Pa) to several billion years (e.g., 87Rb). This review aims to give an overview of the various half-life determination techniques that have been used and to demonstrate the great potential of liquid scintillation counting as an experimental tool for such measurements

Beta shape-factor function and activity determination of 241Pu

The Physikalisch-Technische Bundesanstalt (PTB) investigated the low-energy beta emitter 241Pu within the scope of an international key comparison on the activity concentration of the same solution. The activity concentration was measured by means of liquid scintillation counters with two and three photomultiplier tubes (PMT). The counting efficiencies were determined with two established techniques, which are based on a free parameter model. The free parameter is determined via 3H-efficiency tracing in systems with two PMTs, or it is derived from the triple-to-double coincidence ratio (TDCR) in a system with three PMTs. Both methods require an accurate computation of the beta emission spectrum of the first-forbidden (non-unique) transition. In this work, the experimental outcome of a recent measurement from Loidl et al. (2010) with cryogenic magnetic calorimeters was used to determine a shape-factor function. The computed beta spectrum is in good agreement with the measured data when the shape-factor function C(W)=1-1.9582W+0.96078W2 and an end-point energy EΒ,max=21.6keV are used.The activity concentrations determined with the two methods agree well when using the new shape-factor function, whereas a considerable discrepancy is found when assuming C(W)=1, as for an allowed beta transition. Consequently, the difference between the efficiency tracing method and the TDCR method, as observed by other researchers, could be resolved. © 2011 Elsevier Ltd.The authors wish to thank Lena Johansson and her colleagues from the National Physical Laboratory for providing the 241Pu solution, which was used in this work. The authors are also indebted to Oliver Ott for his impurity determination by means of gamma-ray spectrometry.Peer Reviewe

Precise and direct determination of the half-life of 41Ca

Calcium-41 plays an important role in the long-term evaluation of the safety of final repositories for nuclear waste and is used to study the fine-scale chronology of the formation of the Solar System. Both applications are hindered by insufficient precision and poor consistency of previous determinations of the half-life. This work reports a half-life for41Ca of (9.94±0.15)×104years, which was determined with a combination of methods, chosen to provide the best possible precision. The activity was measured by liquid scintillation counting (LSC) exploiting the triple-to-double coincidence ratio method (TDCR); the absolute isotopic composition was determined by thermal ionization mass spectrometry (TIMS) and isotope dilution. Enhanced precision and accuracy of the41Ca half-life will allow the improvement of safety analyses for final deposit sites of nuclear waste and of dating first solids, and better constrain the stellar environment of the formation of the Solar System