Atomic exchange correction in forbidden unique beta transitions.

The theoretical modelling of the atomic exchange effect, until now only available for allowed beta decays, has been extended to the forbidden unique transitions. The required computing power for such calculations being important, an extensive tabulation of the correction factors has been conducted. These tables will be included in the future version of the BetaShape code, which atomic screening has also been revised to ensure the same level of accuracy as for exchange. Results for several radionuclides are discussed.

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 [Formula: see text]  = 0.8954(14) was determined. The new results for Pγ and [Formula: see text] 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).

High precision measurement of the 151Sm beta decay by means of a metallic magnetic calorimeter.

The beta decay of 151Sm was measured by means of a metallic magnetic calorimeter. The measurement and subsequent analysis yielded a beta spectrum with an outstanding high-energy resolution of about 70 eV (FWHM) at 22 keV and a very low energy threshold well below 400 eV. The spectrum exhibited unexpectedly elevated beta emission probabilities at very low energy that we have not been able to reproduce in our theoretical study. The data analysis was thus scrutinized and an independent analysis of the same data set carried out. All new approaches have confirmed the previously found shape of the beta spectrum. The measured spectrum was compared to predictions from an advanced theoretical modeling that includes the atomic exchange effect, precise radiative corrections as well as the realistic nuclear structure that usually plays an important role in first forbidden non-unique transitions. The measured spectrum was then carefully analyzed to determine the maximum beta energy, which was found to be Q = 76.430(68) keV. The dominant beta decay of 151Sm populates the ground state of 151Eu, and a weak beta branch populates the first excited state of 151Eu. From our measurements, the probabilities of these two branches were determined to be 99.31(11)% and 0.69(11)%, respectively.

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.

Simulation of the response of an ionization chamber to 214Bi emission. Application to the measurement of 222Rn.

PENELOPE simulations of a Vinten ionization chamber (IC) were performed to investigate the influence of the thickness of glass-ampoules used in 222Rn standardization. The simulation reveals a non-negligible variation of the energy deposited in the chamber gas region (which may induce a proportional variation of the measured current) when considering the ß transition emissions of the daughters of 222Rn. This reinforces the idea of using a specialist container (made of metal to preserve the integrity of the container) that would circulate between the metrology laboratories in the context of international comparison exercises using the BIPM international reference system (SIR).

Activity determination of 60Co and the importance of its beta spectrum.

The activity concentration of a 60Co solution was measured by means of two 4πß-γ coincidence counting systems using a liquid scintillation counter and a proportional counter (PC) in the beta channel, respectively. Additional liquid scintillation measurements were carried out and CIEMAT/NIST efficiency tracing as well as the triple-to-double coincidence ratio (TDCR) methods were applied to analyse the data. The last two methods require computed beta spectra to determine the counting efficiencies. The results of both 4πß-γ coincidence counting techniques are in very good agreement and yield a robust reference value. The initial activity concentration determined with liquid scintillation counting was found to be significantly lower than the results from 4πß-γ coincidence counting. In addition, the results from TDCR and CIEMAT/NIST show some inconsistency. The discrepancies were resolved by applying new beta spectrum calculations for the dominant allowed beta transition of 60Co. The use of calculations which take screening effects as well as the atomic exchange effect into account leads to good agreement between all four methods; the combination of these techniques delivers an important validation of beta spectra.

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

The activity concentration of a (63)Ni 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 (63)Ni 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.