Browsing by Author "Bichler, M"

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    High-sensitivity isobar-free AMS measurements and reference materials for 55Fe, 68Ge and 202gPb
    (Elsevier B.V., 2013-01-01) Wallner, A; Bichler, M; Buczak, K; Fink, D; Forstner, O; Golser, R; Hotchkis, MAC; Klix, A; Krása, A; Kutschera, W; Lederer, C; Plompen, AJM; Priller, A; Schumann, D; Semkova, VM; Steier, P
    Isobaric interference represents one of the major limitations in mass spectrometry. For a few cases in AMS with tandem accelerators, isobaric interference is completely excluded like the well-known major isotopes 14C, 26Al, 129I. Additional isotopes are 55Fe (t1/2=2.74years), 68Ge (t1/2=270.9days) and 202Pb (t1/2=52.5kyr), with 68Ge and 202Pb never been used in AMS so far. Their respective stable isobars, 55Mn, 68Zn and 202Hg do not form stable negative ions. The exceptional sensitivity of AMS for 55Fe, 68Ge and 202gPb offers important insights into such different fields like nuclear astrophysics, fundamental nuclear physics and technological applications. VERA, a dedicated AMS facility is well suited for developing procedures for new and non-standard isotopes. AMS measurements at the VERA facility established low backgrounds for these radionuclides in natural samples. Limits for isotope ratios of <10−15, <10−16 and ⩽2×10−14 were measured for 55Fe/56Fe, 68Ge/70Ge and 202Pb/Pb, respectively. In order to generate accurate isotope ratios of sample materials, AMS relies on the parallel measurement of reference materials with well-known ratios. A new and highly accurate reference material for 55Fe measurements with an uncertainty of ±1.6% was produced from a certified reference solution. In case of 68Ge dedicated neutron activations produced a sufficiently large number of 68Ge atoms that allowed quantifying them through the activity of its decay product 68Ga. Finally, for 202Pb, the short-lived isobar 202Tl was produced via neutron activation and served as a proxy for 202Pb AMS measurements. © 2012 Elsevier B.V.
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    Natural and anthropogenic 236U in environmental samples
    (Elsevier, 2008-03-06) Steier, P; Bichler, M; Fifield, LK; Golser, R; Kutschera, W; Priller, A; Quinto, F; Richter, S; Srncik, M; Terrasi, P; Wacker, L; Wallner, A; Wilcken, KM; Wild, EM
    The interaction of thermal neutrons with 235U results in fission with a probability of ∼85% and in the formation of 236U (t1/2 = 2.3 × 107 yr) with a probability of ∼15%. While anthropogenic 236U is, therefore, present in spent nuclear fuel at levels of 236U/U up to 10−2, the expected natural ratios in the pre-anthropogenic environment range from 10−14 to 10−10. At VERA, systematic investigations suggest a detection limit below 236U/U = 5 × 10−12 for samples of 0.5 mg U, while chemistry blanks of ∼2 × 107 atoms 236U per sample limit the sensitivity for smaller samples. We have found natural isotopic ratios in uranium reagents separated before the onset of human nuclear activities, in uranium ores from various origins and in water from a subsurface well in Bad Gastein, Austria. Anthropogenic contamination was clearly visible in soil and rivulet samples from Salzburg, Austria, whereas river sediments from Garigliano river (Southern Italy) were close to the detection limit. Finally, our natural in-house standard Vienna-KkU was calibrated against a certified reference material (IRMM REIMEP-18 A). © 2008 Elsevier B.V.
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    A novel approach for neutron-capture studies of 235U and 238U
    (GNS Science, 2011-03-25) Wallner, A; Belgya, T; Bichler, M; Buczak, K; Dillmann, I; Käppeler, F; Mengoni, A; Quinto, F; Steier, P; Szentmiklósi, L
    Improved and highly accurate nuclear data are urgently required for the design of advanced reactor concepts. This demand holds for minor actinides but also for the main fuel materials. Existing data were measured by detection of the prompt capture γ-rays. A major difficulty in these experiments is the safe discrimination against the strong γ-background from the competing fission channel. Recent studies exhibit critical discrepancies at keV energies for both, 235U(n,γ) and 238U(n,γ) with great impact on the keff-value of fission reactors. Neutron activation with subsequent accelerator mass spectrometry (AMS) measurement of the reaction product represents an independent technique, where interference from fission is completely excluded. Within the European EFNUDAT project, new measurements were performed with the goal to determine the neutron capture cross sections of 235U and 238U via neutron irradiations at thermal (cold) and keV energies: Activations were performed with cold neutrons (Budapest Research Reactor), thermal (Atominstitut, Vienna) and with neutrons of 25 and 450 keV (Karlsruhe Institute of Technology). The produced long-lived 236U and the decay product of 239U, 239Pu were subsequently counted by AMS at the Vienna Environmental Research Accelerator (VERA). This method for measuring the neutron capture cross section has the advantages that the involved systematic uncertainties are in no way correlated with those inherent to previous techniques. Therefore, this experiment provides important and independent information for these key reactions of reactor physics with uncertainties expected below 5%. The high sensitivity of AMS requires only very small samples. New results for 235U(n,γ)236U and 238U(n,γ)239U in the energy range from thermal to 500 keV will be presented. The challenges of measuring 236U/238U isotope ratios at the 10-12 level and to quantify 239Pu with high precision will be highlighted. Finally, the potential for extending that method to other isotopes in that mass range will be discussed. Copyright (c) 2011 AMS12