Digital liquid-scintillation counting and effective pulse-shape discrimination with artificial neural networks

A typical problem in low-level liquid scintillation (LS) counting is the identification of α particles in the presence of a high background of β and γ particles. Especially the occurrence of β-β and β-γ pile-ups may prevent the unambiguous identification of an α signal by commonly used analog electronics. In this case, pulse-shape discrimination (PSD) and pile-up rejection (PUR) units show an insufficient performance. This problem was also observed in own earlier experiments on the chemical behaviour of transactinide elements using the liquid-liquid extraction system SISAK in combination with LS counting. α-particle signals from the decay of the transactinides could not be unambiguously assigned. However, the availability of instruments for the digital recording of LS pulses changes the situation and provides possibilities for new approaches in the treatment of LS pulse shapes. In a SISAK experiment performed at PSI, Villigen, a fast transient recorder, a PC card with oscilloscope characteristics and a sampling rate of 1 giga samples s−1 (1 ns per point), was used for the first time to record LS signals. It turned out, that the recorded signals were predominantly α, β-β and β-γ pile up, and fission events. This paper describes the subsequent development and use of artificial neural networks (ANN) based on the method of “back-propagation of errors” to automatically distinguish between different pulse shapes. Such networks can “learn” pulse shapes and classify hitherto unknown pulses correctly after a learning period. The results show that ANN in combination with fast digital recording of pulse shapes can be a powerful tool in LS spectrometry even at high background count rates.

On-line anion exchange of rutherfordium in HF/HNO3 and HF solutions

The fluoride complexation of the group-4 elements Zr, Hf, and Rf, and of the pseudo-homolog Th, was previously investigated in mixed HNO3/HF solutions by studying K d values on both cation-exchange resins (CIX) and anion-exchange resins (AIX) using the automated rapid chemistry apparatus ARCA. On the CIX, the fluoride complexation of Rf was found to be weaker than that of Zr and Hf but stronger than that of Th. On the AIX, the competition for the binding sites by the counter ion NO3 - was found to be stronger for the fluoride complexes of Rf than for those of Zr and Hf. The aim of the present work is to add independent evidence to the latter result by measuring K d values for Rf on the AIX in 0.1 M HNO3/0.5 M HF and in 0.01 M HF without any HNO3. The results are obtained via an activity ratio of the long-lived α-decay descendant of 261mRf, 20-d 253Es, using the multi-column technique (MCT). These experiments corroborate the seemingly much more pronounced competition of NO3 - for the exchanging sites of the AIX with respect to [RfF x ]( x -4)- than with [ZrF x ]( x -4)- and [HfF x ]( x -4)-.

An EC-branch in the decay of 27-s 263Db: Evidence for the isotope 263Rf

27-s 263Db was produced in the 249Bk ( 18O, 4n) reaction at 93 MeV. The activity was transported by a He/KCl-jet to the laboratory where it was collected for 15 min and then subjected to a chemical separation specific for group-4 elements. The activity was dissolved in 0.5 M unbuffered α-HiB and eluted from a cation-exchange column. The effluent was made 9 M in HCl and group-4 tetrachlorides were extracted into TBP/Cyclohexane which was evaporated to dryness on a Ta disc. The Ta discs were assayed for α and SF activity. A SF activity with a half life on the order of 20 min was observed and assigned to the nuclide 263Rf. It is formed by electron-capture decay of 263Db with a decay branch of 3+4 -1%.

Continuous on-line chromatography of short lived isotopes of tungsten as homolog of seaborgium (element 106)

We have studied the sorption of W on anion-exchange resins from HNO3/HF solutions under on-line conditions using continuous chromatography with the multi-column technique. Kd values and the charge of the W species were determined. In order to achieve an effective separation of mother (W) and daughter (Ta), also the sorption of Ta from HNO3/HF solutions on various anion exchange resins with different functional groups was studied. This investigation serves the purpose to select a suitable anion exchange resin for planned experiments with Sg.

Fluoride complexation of rutherfordium (Rf, element 104)

The fluoride complexation of the group-4 elements Zr, Hf and Rf, and of the pseudo-homolog Th, has been investigated in mixed HNO3/HF solutions by studying Kd values on both cation exchange resins (CIX) and anion exchange resins (AIX) using the automated rapid chemistry apparatus ARCA. On the CIX, the four elements are strongly retained as cations below 10-3M HF. For Zr and Hf, the decrease of the Kd values due to the formation of fluoride complexes occurs between 10-3M HF and 10-2M HF. For Rf and Th, this decrease is observed at one order of magnitude higher HF concentrations. On the AIX, for Zr and Hf, a rise of the Kd values due to the formation of anionic fluoride complexes is observed between 10-3M HF and 10-2M HF, i.e. in the same range of HF concentrations where the decrease of the Kd values on the CIX is observed, yielding a consistent picture. For Rf and Th, on the AIX, no rise of the Kd values is observed even if the HF concentration is increased up to 1 M. By varying the concentration of the counter ion NO3-which is competing for the binding sites on the AIX resin, it could be shown, nevertheless, that Rf does form anionic fluoride complexes. Apparently, there is a more specific competition of NO3-with respect to [RfFx](x-4)-than with [ZrFy](y-4)-and [HfFz](z-4)-.

Trace detection of 41Ca in nuclear reactor concrete by diode-laser-based resonance ionization mass spectrometry

The content of the long-lived isotope 41Ca in concrete samples obtained from the biological shield of a shut-down nuclear research reactor has been determined by diode-laser-based resonance ionization mass spectrometry (RIMS). Standard procedures were applied for the chemical separation of calcium. A radioactive tracer of 47Ca was used to determine the chemical yield. The total calcium concentration in the final nitric acid solution was measured by x-ray fluorescence (XRF) spectroscopy. The RIMS measurement yielded the abundance of 41Ca relative to the total calcium content with a detection limit of 5×10-10, limited by background effects at mass 41. The detection limit corresponds to a minimum detectable specific 41Ca activity of ~100 mBq/g in the concrete. Reproducibility and accuracy were determined with 41Ca spikes and found to be in the range of 15%, limited predominantly by ion counting statistics.

41Ca ultratrace determination with isotopic selectivity > 10(12) by diode-laser-based RIMS

41Ca ultratrace determination by diode-laser-based resonance ionization mass spectrometry with extremely high isotopic selectivity is presented. Application to environmental dosimetry of nuclear reactor components, to cosmochemical investigations of production cross sections, and biomedical isotope-tracer studies of human calcium kinetics are discussed. Future investigations are possible use in 41Ca-radiodating. Depending on the application, 41Ca isotopic abundances in the range of 10(-9) to 10(-15) relative to the dominant stable isotope 40Ca must be determined. Either double- or triple-resonance optical excitation with narrow-band extended cavity diode lasers and subsequent non-resonant photoionization of calcium in a collimated atomic beam were used. The resulting photoions are detected with a quadrupole mass spectrometer optimized for background reduction and neighboring mass suppression. Applying the full triple-resonance scheme provides a selectivity of approximately 5 x 10(12) in the suppression of neighboring isotopes and > 10(8) for isobars, together with an overall detection efficiency of approximately 5 x 10(-5). Measurements on a variety of sample types are discussed; the accuracy and reproducibility of the resulting 41Ca/40Ca isotope ratios was better than 5%.