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Interest in the long-lived radioisotope 236U (T 1/2 = 23.4 million years) has significantly increased recently due to the emergence of environmental and earth science applications. Compared to the previous setup at VERA, which was based on oxygen stripping and a time-of-flight detector, we have improved the sensitivity of VERA by more than an order of magnitude by switching to helium stripping and by installing a second 90° magnet in our analyzer beamline. The new setup has been successfully employed for several research projects. Here, we present the characterization of the upgraded spectrometer. We discuss the design of the new beamline and present benchmark measurements, suggesting an instrumental sensitivity limit well below 236U/238U = 10−14. The yield for the 3+ charge state is 19%, of which 90% are recorded in the detector. In environmental samples, one in 4500 sputtered actinide atoms is detected. While this allows tackling natural 236U, also measurements of anthropogenic 236U or other actinides profit from the higher sensitivity. This allows analysis of smaller samples, but also has made the rare anthropogenic isotope 233U accessible. [ABSTRACT FROM AUTHOR]

Interest in the long-lived radioisotope 236 U ( t 1/2 = 23.4 million years) has significantly increased recently, due to the emergence of environmental and earth science applications. Presently, only a few (AMS) accelerator mass spectrometry instruments are suited for this measurement. One major limitation is the relatively low total detection efficiency (on the order of 10 −4 ), which is partly caused by a low stripping yield of the positive ion charge state selected after the tandem accelerator. It has been shown that high yield can be achieved using helium as stripper gas for uranium ion energies below 0.35 MeV. Here we investigate the potential of He stripping of U at the 3-MV tandem accelerator VERA. Phenomenological charge state distributions for U and Th are presented for terminal voltages from 1.0 to 1.7 MV. These terminal voltages provide better background rejection than possible below 1 MeV, and are suited to the widely used 1–3 MV workhorses of many accelerator mass spectrometry laboratories. The methods can be applied to other actinides also. [ABSTRACT FROM AUTHOR]

Very sensitive methods, presently mainly accelerator mass spectrometry (AMS) are necessary for analysis of cosmogenic 10 Be in the environment. AMS is mainly limited by the stable isobar 10 B, while the requirements for mass separation are the least stringent of all standard isotopes analyzed by AMS. We tested a possibility to measure 10 Be using a small switching magnet as an analyzer of accelerated ions, and an ionization chamber with a silicon nitride foil stack used as a passive absorber. A detection limit of 10 −12 for the 10 Be/ 9 Be isotopic ratio was obtained using this technique, which was mainly determined by scattering of 9 Be +2 ions on residual gas inside the switching magnet. [ABSTRACT FROM AUTHOR]

Abstract: For 14C AMS measurements of samples at the microgram level, ion source related effects start to play a role, while generally the lower sample size limit is set by the carbon background introduced during chemical preparation procedures. Measurements of about 800 graphite targets in the mass range of 1–100μg were performed within 25AMS beam-times during the last three years at VERA, revealing a dependency of measured 14C3+/12C3+ ratios on 12C3+ currents. This dependency can be accounted for by assuming a background current, which was determined for each AMS measurement by least square fitting. 12C- ion currents extracted from microgram graphite samples were typically (1.0±0.5)μA / μg C. On average a 12C3+ background current of (0.14±0.14)μA with F14C=0.22±0.46 (skewness γ1 =3.0) was deduced with significant variations between single measurements. The determination of this background current for each AMS measurement of microgram graphite samples allowed to apply a quantitative correction and thereby to improve the AMS measurement precision. Furthermore, the yield of graphitization and ionization in a Cs sputter ion source of graphitized microgram CO2 samples was investigated. No dependency on the cathode target geometry was observed for 9 differently shaped cathode types. [Copyright &y& Elsevier]

Abstract: A growing number of AMS laboratories are pursuing applications of actinides. We discuss the basic requirements of the AMS technique of heavy (i.e., above ∼150amu) isotopes, present the setup at the Vienna Environmental Research Accelerator (VERA) which is especially well suited for the isotope 236U, and give a comparison with other AMS facilities. Special emphasis will be put on elaborating the effective detection limits for environmental samples with respect to other mass spectrometric methods. At VERA, we have carried out measurements for radiation protection and environmental monitoring (236U, 239,240,241,242,244Pu), astrophysics (182Hf, 236U, 244Pu, 247Cm), nuclear physics, and a search for long-lived super-heavy elements (Z >100). We are pursuing the environmental distribution of 236U, as a basis for geological applications of natural 236U. [Copyright &y& Elsevier]

Abstract: 36Cl AMS measurements at natural isotopic concentrations have yet been performed only at tandem accelerators with 5MV terminal voltage or beyond. We have developed a method to detect 36Cl at natural terrestrial isotopic concentrations with a 3-MV system, operated above specifications at 3.5MV. An effective separation was obtained with an optimized split-anode ionization chamber design (adopted from the ETH/PSI Zurich AMS group), providing a suppression factor of up to 30,000 for the interfering isobar 36S. Despite the good separation, a relatively high sulfur output from the ion source (36S−/35Cl− ≈4×10−10 for samples prepared from chemically pure reagents), and a possibly cross contamination resulted in a background corresponding to 36Cl/Cl≈3×10−14. The method was applied to samples containing between 105 and 106 atoms 36Cl/g rock from sites in Italy and Iran, which were already investigated by other laboratories for surface exposure dating. The 36Cl/Cl ratios in the range from 2×10−13 to 5×10−12 show a generally good agreement with the previous results. These first measurements demonstrate that also 3-MV tandems, constituting the majority of dedicated AMS facilities, are capable of 36Cl exposure dating, which is presently the domain of larger facilities. [Copyright &y& Elsevier]

Abstract: The interaction of thermal neutrons with 235U results in fission with a probability of ∼85% and in the formation of 236U (t 1/2 =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 0.5mg 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). [Copyright &y& Elsevier]

Abstract: The concentration of the long-lived radioisotope 129I was determined in two Austrian lignite samples through 129I/127I isotopic ratio measurement by AMS, and through a measurement of the total iodine content with an ion-selective electrode. In addition, the uranium concentration of the samples was measured by α-spectrometry. From the mean values of the uranium concentrations the steady-state concentration of 129I in lignite was calculated; the 129I concentrations derived from the measured 129I/127I ratio and the total iodine content, however, were much higher than expected, clearly indicating that lignite does not represent a closed system for 129I. The observed high ratio in the sample from the surface mine suggests the presence of anthropogenic 129I, while the source for the high 129I concentrations in the subsurface lignite is the addition of fissiogenic 129I either from the lignite itself (from layers with much higher U content than in the samples investigated) or from nearby geological formations rich in uranium. [Copyright &y& Elsevier]

Abstract: With the Vienna Environmental Research Accelerator (VERA, based on a 3-MV tandem accelerator) we have systematically explored the limitations due to terminal voltage, both by modelling and by experiments. If no stable atomic isobars exist, e.g. for 236U or 244Pu, the same detection limits as for large machines have been achieved. In cases where isobar separation is required, the achievable energy is the limitation. To study isobar separation experimentally, we use a new method called ΔTOF: the different energy loss in a foil is measured with a time-of-flight (TOF) detector. Separation of 36Cl from 36S is possible. With diamond like carbon (DLC) stripper foils (0.6μg/cm2) and by pushing our Pelletron tandem to 3.6MV, we achieved a significant increase in energy compared to gas stripping at 3MV: e.g. 36Cl7+ at 28.8MeV instead of 36Cl5+ at 18MeV can be used. [Copyright &y& Elsevier]

A detector with sufficient energy resolution at high count rates could alleviate a difficult problem in AMS measurements – the detection of rare particles in the vicinity of a large background. The exceptional properties of diamond lead one to expect that these detectors can outperform silicon detectors and ionization chambers for many applications. Only natural diamond detectors (NDDs) that are single crystals, allow energy spectrometry.We have explored a detector produced from a natural type-II diamond of highest purity mined from deposits in Yakutsk, Russian Federation. To late, we have characterized the energy resolution, linearity, stability and radiation degradation using beams of 13C, 180Hf and 238U in the energy range from 12 to 38 MeV. The influence of “priming” and “polarization” is discussed.Deterioration of the detector resolution was observed after irradiation with several 108 ions/mm2 of 238U at 18 MeV. The small detector size, unexpected high energy losses in the entrance window, and the low electrical strength of the assembly were identified as the main limitations in this first test. However, the results demonstrate its intriguing potential for AMS of heavy isotopes. [Copyright &y& Elsevier]

VERA, the Vienna Environmental Research Accelerator, was equipped from the start in 1996 with magnets strong enough to bend all known isotopic species. However, considerable improvements were needed to allow AMS analysis of nuclides above mass 150, leading to an almost complete rebuild of the high-energy analyzer. After the analyzer magnet, a new high-resolution electrostatic analyzer (ESA) was installed. First measurements of 182Hf, 236U and 244Pu were successfully performed. The higher background suppression has brought along a significant improvement for 26Al measurements, where a background of 26Al/27Al of ∼6 × 10−16 is reached. [Copyright &y& Elsevier]

The occurrence of 10Be in natural archives is commonly used to date their formation and growth on time scales of million years. Accelerator Mass Spectrometry (AMS) can perform a direct measurement of the 10Be/9Be ratio. The carrier-free method, in which no 9Be carrier is added to the original sample, is especially suitable for 10Be/9Be ratio determination in the marine environment. By normalizing the 10Be content to 9Be, temporal variations of Be uptake processes into the archive are eliminated. Here, we present a simple method for the chemical extraction of beryllium from ferromanganese (FeMn) crusts or nodules, the measurement procedure, and the first carrier-free 10Be/9Be measurements at the 3 MV AMS facility VERA. Several tests of chemical methods are discussed including different options to short-cut and accelerate the procedure for special cases. Results from FeMn crust 237KD from cruise VA13/2 in the Pacific ocean show the known 10Be/9Be distribution with depth that is commonly related to a changing growth rate of the archive. In this context we discuss the potential influence of diffusion and adsorption processes on the age models of FeMn crusts that are based on radioactive nuclides such as 10Be and 230Th. Including an open-system behavior for these isotopes in the description of their profiles allows interpreting the accumulation of crusts with a constant growth rate over millions of years and does not require the assumption of abrupt growth changes. [ABSTRACT FROM AUTHOR]

The environmental abundance of 129I has been significantly increased in the Nuclear Age starting from the 1950s. Tons of anthropogenic 129I have been discharged into the environment through anthropogenic nuclear activities. This fact allows the relative dating of spring water samples, where low concentrations of 129I indicate waters with no surface contact since the 50s. In this regard, the present study aims to identify the relative age of spring waters in the Kaunertal Valley in Western Austria. More than fifty water samples were derived from precipitation collectors, springs, and directly from the Gepatschferner glacier. Measurement results cover 129I concentrations ranging from 1 × 106 to 5 × 108 atoms per litre. The variability of six springs, which were tested in July and September, was found to be negligible given the associated measurement uncertainties. No altitudinal dependence was found along the topographic gradient. Significant differences between high 129I concentrations of precipitation and low 129I contents of glacier samples were found and are considered beneficial to ascribe spring waters to pre-nuclear (older than 1950) or modern times of formation. Additional tritium analyses of selected water samples partly support the usage of 129I for relative dating. [ABSTRACT FROM AUTHOR]

The detection of low abundances of 135Cs in environmental samples is of significant interest in different fields of environmental sciences, especially in combination with its shorter-lived sister isotope 137Cs. The method of Ion–Laser InterAction Mass Spectrometry (ILIAMS) for barium separation at the Vienna Environmental Research Accelerator (VERA) was investigated and further improved for low abundance cesium detection. The difluorides Ba F 2 − and Cs F 2 − differ in their electron detachment energies and make isobar suppression with ILIAMS by more than 7 orders of magnitude possible. By this method, samples with ratios down to the order of 135 , 137 Cs/133Cs ≈ 1 0 − 11 are measurable and the 135Cs/137Cs ratios of first environmental samples were determined by AMS. [ABSTRACT FROM AUTHOR]

Possible ion output enhancement for C−, C 4 −, Al−, AlO−, CaF 3 − and SrF 3 − by pulsed operation of an NEC-MC-SNICS II ion source under AMS-conditions has been investigated. The cathode voltage was switched periodically between −5.0 kV for regular sputtering and an idle value of −0.8 kV. With normal Cs-supply, all investigated species showed a peak ion current in pulsed mode similar to their DC ion current. A study of ionization efficiency of Al− from Al 2 O 3 likewise revealed no enhancement. However, current enhancements of up to 400 % for C− and C 4 − were observed in pulsed mode when strongly reducing the cesium supply. This is not reflected in ionization efficiency. Under Cs-scarcity, the ionization efficiency of C− in pulsed mode was 12–13 % for samples graphitized with AGE-3. Irrespective of Cs-level, AGE-3 samples and powder mixed samples in DC operation yielded 15–16 % and 16–21 %, respectively. Summarizing, no immediate benefits for AMS could be identified. [ABSTRACT FROM AUTHOR]

Atmospheric radiocarbon has been monitored around the Nuclear Power Plant (NPP) Jaslovské Bohunice (Slovakia) using CO 2 absorption in NaOH solution since 1969. In 2012, tree ring samples were collected from Tilia cordata using an increment borer at Žlkovce monitoring station situated close to the Bohunice NPP. Each tree ring was identified and graphite targets were produced for 14 C analysis by accelerator mass spectrometry. The 14 C concentrations obtained from the tree-ring samples have been in a reasonable agreement with the averaged annual 14 C concentrations in atmospheric CO 2 . [ABSTRACT FROM AUTHOR]

The experimental setup at the Vienna Environmental Research Accelerator (VERA) was extended with an ion beam cooler and a 18 W laser to perform Ion-Laser InterAction Mass Spectrometry (ILIAMS). Gas collisions of the decelerated negative ion beam with He buffer gas inside a radio-frequency quadrupole slow down the ions, which facilitates element selective photodetachment to remove isobaric interferences. Here we present the first successful implementation of a suppression of the isobar of 36Cl via laser-photodetachment into an AMS measurement. Whereas conventional AMS measurements require separation of the isobar 36S at high beam energy, ILIAMS takes advantage of the higher electron affinities of Cl− (3.6 eV) compared to S− (2.1 eV). Using 532 nm photons (E ph = 2.3 eV) the isobar 36S is suppressed by 10 orders of magnitude. A high transmission of the Cl− beam through the ion cooler (up to 80%) and a quantitative suppression of the isobaric interference allows for competitive measurements at low accelerator voltages and with lower charge states. This results in high yields for 36Cl transport to the detector. The ILIAMS system is stable over several days of beamtime and blank levels below 36Cl/Cl = 10−15 are reached. The accomplishment of a 36Cl AMS measurement at a final beam energy as low as 5.4 MeV with full separation of isobars and m/q interferences shows the potential of the new isobar suppression method to widen the capabilities of smaller and middle-sized AMS facilities. [ABSTRACT FROM AUTHOR]

Selective laser photodetachment of anions is a novel technique for isobar suppression in Accelerator Mass Spectrometry (AMS). Ion-laser interaction times on the order of ms required for near-complete isobar suppression are achieved by retarding the ions in a gas-filled radio frequency quadrupole cooler. Inside this RFQ, the cooled anion beam is overlapped collinearly with an intense cw-laser beam. Within the Ion Laser InterAction Mass Spectrometry (ILIAMS) project at the University of Vienna, a dedicated injector beamline has been coupled to the VERA-AMS facility to explore and develop this method. In this work, experimental investigations on ion beam transmission, stability and elemental selectivity of the new setup are presented. A 532 nm laser at 10 W transmitted power provides suppression factors larger than ten orders of magnitude for S− and MgO− under AMS conditions with simultaneous beam transmission for the ions of interest of up to 80%. The excellent ion identification capabilities of the subsequent AMS system also facilitate the study of destruction and formation of molecular anions inside the ion cooler. These kinetic and chemical reactions with the buffer gas provide additional elemental selectivity in certain cases, whereas others constitute a source of background. [ABSTRACT FROM AUTHOR]

Abstract Accelerator Mass Spectrometry (AMS) has been a frequently used technique for analysis of low-level concentrations of 26Al in environmental, biological and space samples with detection limits down to 0.01 fg. Al 2 O 3 targets have been mostly used in ion sources for the production of aluminium negative ions because of high-temperature stability, non-toxicity, in-air stability, and relatively easy production procedures. It is well known, however, that aluminium does not yield as intense negative ion beams as other elements used in tandem AMS accelerators. An alternative solution could be to use as the target material AlN, which can yield higher Al− currents. On the other hand, the AlN targets are more difficult to synthesize, and they decompose with water in the air to form Al(OH) 3 and ammonia. Commercially available compounds of Al 2 O 3 and AlN were mixed with copper, silver and iron high-purity powders and sputtered in a MC-SNICS ion source for studying ionization yields. Since the production of magnesium and nitrogen negative ions is negligible, a production of MgN− molecules has been questionable. Obtained results indicate that the aluminium nitride matrix could be a suitable material for AMS measurements as the production of 27Al− is higher by a factor of 1.8 from the aluminium oxide matrix, while aluminium sulphate and aluminium fluoride showed a very low sputtering efficiency. The formation probability of the MgN− was tested using AlN and Mg 3 N 2 matrices. The AMS measurements of 26Al using AlN− as injected ions from the AlN matrix, similar to AlO− ions from Al 2 O 3 matrix, have shown that further isobar suppression due to a high 26Mg3+ production rate will be needed. [ABSTRACT FROM AUTHOR]

93 Zr with a half-life of 1.6 Ma is produced with high yield in nuclear fission, and thus should be present as a natural or anthropogenic trace isotope in all compartments of the general environment. Sensitive measurements of this isotope would immediately find numerous applications, however, its detection at sufficiently low levels has not yet been achieved. AMS measurements of 93 Zr suffer from the interference of the stable isobar 93 Nb. At the Vienna Environmental Research Accelerator VERA a new multi-anode ionization chamber was built. It is optimized for isobar separation in the medium mass range and is based on the experience from AMS experiments of 36 Cl at our 3-MV tandem accelerator facility. The design provides high flexibility in anode configuration and detector geometry. After validating the excellent energy resolution of the detector with 36 S, it was recently used to study iron–nickel and zirconium–niobium–molybdenum isobar separation. To our surprise, the separation of 94 Zr ( Z = 40) from 94 Mo ( Z = 42) was found to be much better than that of 58 Fe ( Z = 26) from 58 Ni ( Z = 28), despite the significantly larger Δ Z / Z of the latter pair. This clearly contradicts results from SRIM-simulations and suggests that differences in the stopping behavior may unexpectedly favor identification of 93 Zr. At 24 MeV particle energy, a 93 Nb ( Z = 41) suppression factor of 1000 is expected based on a synthetic 93 Zr spectrum obtained by interpolation between experimental spectra from the two neighboring stable isotopes 92 Zr and 94 Zr. Assuming realistic numbers for chemical niobium reduction, a detection level of 93 Zr/Zr below 10 −9 seems feasible. [ABSTRACT FROM AUTHOR]

Neutron activation analysis using decay counting of the activated element is a well-established method in elemental analysis. However, for chlorine there is a better alternative to measuring decay of the short-lived activation product chlorine-38 ( t 1/2 = 37.24 min) – accelerator mass spectrometry (AMS) of 36 Cl: the relatively high neutron capture cross section of chlorine-35 for thermal neutrons (43.7 b) and combined the AMS technique for chlorine-36 ( t 1/2 = 301 ka) allow for determination of chlorine down to ppb-levels using practical sample sizes and common exposure durations. The combination of neutron activation and AMS can be employed for a few other elements (nitrogen, thorium, and uranium) as well. For bulk solid samples an advantage of the method is that lab contamination can be rendered irrelevant. The chlorine-35 in the sample is activated to chlorine-36, and surface chlorine can be removed after the irradiation. Subsequent laboratory contamination, however, will not carry a prominent chlorine-36 signature. After sample dissolution and addition of sufficient amounts of stable chlorine carrier the produced chlorine-36 and thus the original chlorine-35 of the sample can be determined using AMS. We have developed and applied the method for analysis of chlorine in steel samples. The chlorine content of steel is of interest to nuclear industry, precisely because of above mentioned high neutron capture cross section for chlorine-35, which leads to accumulation of chlorine-36 as long-term nuclear waste. The samples were irradiated at the TRIGA Mark II reactor of the Atominstitut in Vienna and the 36 Cl-AMS setup at the Vienna Environmental Research Accelerator (VERA) was used for 36 Cl/Cl analysis. [ABSTRACT FROM AUTHOR]

Laser photodetachment is the process when the extra electron of a negative ion is removed by means of laser radiation. This can happen only if the photon energy is larger than the electron affinity of the ion. The process can be used in mass spectrometry to selectively suppress unwanted isobars, provided that the electron affinity of the unwanted isobar is lower than that of the isobar under investigation. At the Ion Laser InterAction Setup (ILIAS) at the University of Vienna laser photodetachment of negative atomic and molecular ions is studied and its applicability for selective isobar suppression in accelerator mass spectrometry (AMS) is evaluated. The setup provides mass separated beams of negative ions with energies up to 30 keV. Negative ions are produced in a Middleton type cesium sputter ion source, mass selected and overlapped with a strong continuous wave laser beam. In order to extend the interaction time of ions and laser, the ion beam is decelerated to thermal energies in a gas-filled radio frequency quadrupole cooler. For an appropriate choice of the photon energy, unwanted isobars are neutralized while the isobar of interest is unaffected and remains negatively charged. A description of the ILIAS setup and results from the commissioning phase of the RFQ cooler are presented. Up to 8% ion beam transmission could be achieved after a recent redesign of the extraction system. Furthermore first results of photodetachment experiments of 63 Cu − within the RFQ cooler are presented. [ABSTRACT FROM AUTHOR]

Radioisotopes produced in natural or anthropogenic fission are widely used for tracer studies of environmental processes, in nuclear forensics, and are important for nuclear waste disposal. Besides the well-known 137 Cs, the longer-lived sister isotope 135 Cs ( T 1/2 = 2.3 Myr) is also produced, and the combined measurement of the two isotopes would allow for assessment of contaminating sources. The insufficient suppression of the stable isobar 135 Ba presently prevents AMS measurements down to expected natural levels of 135 Cs/ 133 Cs ≈ 10 −11 . Via the difference in electron affinities between Cs and Ba further isobar suppression should be achievable after the installation of the Ion-Laser-Interaction System (ILIAS) at VERA. We present a preparatory study on the performance of the 3 MV VERA AMS facility for 135 Cs concerning ion formation, transmission and detection. Since the usual Cs sputtering would obscure the 135 Cs/ 133 Cs ratio of a sample, Rb sputtering was successfully applied and tested also for various other typical AMS elements. Partial suppression of 135 Ba is possible with the extraction of Cs − and negative Cs-fluorides. Cs − currents of several 10 nA were extracted over hours from mg amounts of Cs 2 SO 4 material. The transmission to various charge states was tested with gas (Ar, He) and foil stripping. Experiments showed that no suppression in the detection system is possible at high beam energies with the VERA facility. For this reason, gas stripping to low charge states (2 + , 3 + ) with transmissions up to 30% is favorable to guarantee optimal beam transport to the detector. In the present setup, utilizing a simple Bragg-type detector, the blank 135 Cs/ 133 Cs ratios from chemically pure samples are determined by the 135 Ba background to a value of (4.0 ± 1.3)·10 −9 . [ABSTRACT FROM AUTHOR]

A Centre for Nuclear and Accelerator Technologies (CENTA) has been established at the Comenius University in Bratislava comprising of a tandem laboratory designed for Ion Beam Analysis (IBA), Ion Beam Modification (IBM) of materials and Accelerator Mass Spectrometry (AMS). The main equipment of the laboratory, i.e. Alphatross and MC-SNICS ion sources, 3 MV Pelletron tandem accelerator, and analyzers of accelerated ions are described. Optimization of ion beam characteristics for different ion sources with gas and solid targets, for transmission of accelerated ions with different energy and charge state, for different parameters of the high-energy ion analyzers, as well as first AMS results are presented. The scientific program of the CENTA will be devoted mainly to nuclear, environmental, life and material sciences. [ABSTRACT FROM AUTHOR]

Highlights: [•] Long-term memory effect in negative ion sources investigated for chlorine isotopes. [•] Interlaboratory comparison of four up-to date negative ion sources. [•] Ion source improvement at DREAMS for minimization of long-term memory effect. [•] Long-term memory effect is the limitation for precise AMS data of volatile elements. [•] Findings to be considered for samples with highly variable ratios of 36Cl/Cl &129I/I. [Copyright &y& Elsevier]

Abstract: Recent progress with compact ionization chambers has opened new possibilities for isobar suppression in accelerator mass spectrometry (AMS). Separation of 36Cl (t 1/2 =0.30Ma) at natural isotopic levels from its stable isobar 36S became feasible at particle energies of 24MeV, which are also accessible for medium-sized tandem accelerators with 3MV terminal voltage like VERA (Vienna Environmental Research Accelerator). Investigations with an ionization chamber revealed how physics favors isobar separation even at energies below the maximum of the Bragg curve. The strong energy focusing effect at high energy losses reduces energy straggling significantly and isobar separation steadily increases up to almost full energy loss. With an optimized detection setup, sulfur suppression factors of 2×104 have been achieved. Refraining from the additional use of degrader foils has the benefit of high transmission to the detector (∼16%), but requires a low sulfur output from the ion source. Therefore several backing materials have been screened for sulfur content. The dependence of the sulfur output on the AgCl sample size has been investigated as well. Precision and accuracy have been thoroughly assessed over the last two years. Since drifts in the spectra are efficiently corrected by monitoring the position of the 36S peak, the reproducibility for high ratio samples (36Cl/Cl>10−12) is better than 2%. Our blank value of 36Cl/Cl≈(5±5)×10−16 is competitive to other labs. 36Cl has become a routine AMS-isotope at VERA. Recently we also explored novel techniques for additional sulfur suppression already in the ion source. While results with a small gas reaction cell in front of the sputter target were discouraging, a decrease in the sulfur/chlorine ratio by one order of magnitude was achieved by directing 300mW continuous wave laser beam at 445nm towards the cathode in the ion source. [Copyright &y& Elsevier]

Abstract: Accelerator mass spectrometry (AMS) of 36Cl (t 1/2 =0.30Ma) at natural isotopic concentrations requires high particle energies for the separation from the stable isobar 36S and was so far the exclusive domain of tandem accelerators with at least 5MV terminal voltage. Using terminal foil stripping and a detection setup consisting of a split-anode ionization chamber and an additional energy signal from a silicon strip detector, a 36S suppression of >104 at 3MV terminal voltage was achieved. To further increase the 36S suppression energy loss straggling in various counter gases (C4H10, Ar–CH4 and C4H10–Ar) and the effect of “energy focusing” below the maximum of the Bragg curve was investigated. The comparison of experimental data with simulations and published data yielded interesting insights into the physics underlying the detectors. Energy loss, energy straggling and angular scattering determine the 36S suppression. In addition, we improved ion source conditions, target backing materials and the cathode design with respect to sulfur output and cross contamination. These changes allow higher currents during measurement (35Cl− current≈5μA) and also increased the reproducibility. An injector to detector efficiency for 36Cl ions of 8% (16% stripping yield for the 7+ charge state in the accelerator, 50% 36Cl detection efficiency) was achieved, which can favorably be compared to other facilities. The memory effect in our ion source was also thoroughly investigated. Currently our measured blank value is 36Cl/Cl≈3×10−15 when samples with a ratio of 10−11 are used in the same sample wheel and 36Cl/Cl≈5×10−16 if measured together with samples with a ratio of 10−12 or below. This is in good agreement with the lowest so far published isotope ratios around 5×10−16 and demonstrates that 3MV tandems can achieve the same sensitivity for 36Cl as larger machines. [Copyright &y& Elsevier]

Abstract: The possibility of detecting 36Cl for geological exposure dating has been explored for several years at VERA (the Vienna Environmental Research Accelerator). First results on real samples were obtained with an ionization chamber (developed at the ETH/PSI, Zürich, Switzerland) with two anodes. To improve the suppression of 36S, we equipped the ionization chamber with an exit window and added a Time-of-Flight (TOF) system with a double-sided silicon strip detector (50×50mm2) as stop detector. We optimized the TOF setup by using silicon nitride foils to reduce scattering tails in the energy spectra. At 3MV terminal voltage, corresponding to a particle energy of 24MeV of 36Cl7+, we achieved a 36S7+-suppression of 21,500 (50% 36Cl-detector-efficiency). [Copyright &y& Elsevier]

Abstract: Silverpoint drawings from the Renaissance are among the most precious and rarest treasures of graphical art. Our research group is particularly interested in the analysis of silverpoint drawings by Albrecht Dürer (1471–1528). A very sensitive and non-destructive analytical method, either spatially resolved synchrotron-radiation induced X-ray fluorescence (SY-XRF) or proton-induced X-ray emission (PIXE), is needed to determine the chemical composition of the very faint silver marks on such drawings. Dürer drawings from the collection of the Albertina, Vienna, were analyzed to amend existing data on Dürer drawings. For this purpose an external-beam PIXE setup was installed at the Vienna Environmental Research Accelerator (VERA). It allows to analyze a spot of ∼0.15mm on the object in air with 3MeV protons, and to detect the emitted X-rays that are characteristic for the chemical composition with very good sensitivity and without harming the precious objects. After successful measurements on artificial test samples, four original silverpoint drawings were investigated: two portraits from Albrecht Dürer’s very early period (self-portrait and portrait of his father) and two drawings from Dürer’s sketch book of his travel to the Netherlands 1520/21. [Copyright &y& Elsevier]

Abstract: The potential for measuring the radionuclides 41Ca and 55Fe was investigated with the 3MV tandem accelerator at VERA. Interestingly, up to now, no applications have been published for 55Fe using the technique of AMS. This is in strong contrast to 41Ca, which is routinely measured by medium and large tandem accelerators in various applications. Using CaF2 samples the quantification of 41Ca down to levels of a few 10−13 for the isotope ratio 41Ca/Ca has become possible with the use of the ΔTOF technique. Both nuclides, 41Ca and 55Fe were found to be of interest in nuclear astrophysics. A first application of 41Ca detection at VERA is the measurement of the 40Ca(n,γ)41Ca cross section at stellar temperatures. Similarly, of astrophysical interest is the production of 55Fe via neutron capture on 54Fe. To this end, different Ca and Fe blank and standard samples were investigated with the goal to establish an AMS method for 41Ca and 55Fe measurements. Indeed, low background levels for both radionuclides, 41Ca and 55Fe, were observed in these studies. The full separation power of VERA results in a very low detection limit, while still providing a high overall transmission. [Copyright &y& Elsevier]

Abstract: In 1997 a new archaeological site was discovered in the Peruvian tropical rain forest. The site is located in an area which has been occupied by the Chachapoya, a pre-Incan people, from about 800AD on. The site comprises a large funerary place with several mausoleums built in the cliffs next to the Laguna de los Condores. More than 200 human mummies and funerary bone-bundles together with numerous grave artefacts have been found there. Although the site has been ascribed to the Chachapoya, the mummification method used is very similar to the one applied by the Inca. As part of an ongoing multidisciplinary project to explore the history of this site and of the Chachapoya people, twenty-seven (27) 14C-AMS age determinations were performed. Samples, bones and textile wrappings as well as samples from a funerary bone bundle plus associated grave artefacts were dated. The 14C data show that the site originates from the Chachapoya pre-Inca period and that in addition, it was used as a funerary place during the subsequent Inca occupation era. The radiocarbon results indicate that the Chachapoya may have changed their burial tradition due to the colonization by the Inca. [Copyright &y& Elsevier]

Abstract: A filtration method for radiocarbon determination on particulate organic carbon (POC) in glacier ice is explored at VERA (Vienna Environmental Research Accelerator). Previous work on true samples has shown that laboratory contamination is the limiting factor. By systematic investigations on the origin of carbon contamination, we were able to reduce the background significantly. In two first measurements using artificially produced clean ice a blank level as low as 1.3μgC/kg ice could be achieved. These results were obtained by a simplified combustion scheme using valve-sealed vials. The combustion yield for organic materials was found to be 80% with a very low background typically below 1μgC. Outliers turned out to be the main obstacle when dealing with such minute carbon amounts. [Copyright &y& Elsevier]

Abstract: “The techniques and equipment developed for AMS studies are well suited for identifying exotic negative ions”. With this sentence begins a pioneering paper by Roy Middleton and Jeff Klein (M&K) on small doubly-charged negative carbon clusters [Nucl. Instr. and Meth. B 123 (1997) 532]. M&K were the first to utilize Accelerator Mass Spectrometry to prove the existence of these clusters and a number of other exotic molecules. We review M&K’s efforts and show how their work is being continued at other laboratories. The latest developments are: (1) the discovery of long-lived molecular hydrogen anions and (2) the unambiguous identification of the smallest doubly-charged negative molecule (LiF3)2−. In particular we show new experimental data for , and for (LiF3)2−, and we try to answer the question why M&K’s search for this di-anion was unsuccessful. [Copyright &y& Elsevier]

Abstract: This paper describes various modifications implemented at the 40-samples MC-SNICS Cs sputter source used at VERA. The pneumatic sample changer was replaced with a stepper motor drive, and also the adjustment collar for the vertical target wheel position was motorized. These two modifications allow now an automatic scanning of the target position with respect to the Cs-sputter beam. It also opens the way to use sample wheels holding more than 40 sample positions while keeping the original dimensions of the wheel. In order to hold the negative ion output constant, the control computer now regulates the Cs-capillary heater while the Cs-oven heater power supply stays fixed. Another modification was the replacement of the original conical ionizer with a spherical one, involving the enlargement of the diameter of the Cs-focus lens. This modification almost doubled the source output. To adapt the new ion optical geometry, we mounted a shield electrode in front of the target. [Copyright &y& Elsevier]

Abstract: A novel method to determine small cross-sections of nuclear reactions of astrophysical interest was implemented at the Vienna Environmental Research Accelerator laboratory (VERA). This method combines inverse-kinematics reactions taking place inside the stripper of a tandem accelerator with the identification of the recoil products using the accelerator mass spectrometry (AMS) technique. As a first test we studied the 1H(12C,γ)13N reaction, which is the starting reaction of the stellar CNO cycle of hydrogen burning. The 13N recoil products as well as the interfering isobar 13C were separated from the main 12C beam by common AMS techniques. Measurements with 13N recoil ions of different charge states were performed at center-of-mass energies of 229 and 206keV, respectively. Notwithstanding the restrictions of VERA, the principle feasibility of the method could be demonstrated. [Copyright &y& Elsevier]

Abstract: The ΔTOF detector is a new approach for isobar separation at energies below 1MeV/amu. The specific energy loss after a degrader foil can be measured very precisely with time-of-flight (TOF). Small differences in the specific energy loss between isobars lead to clear separation even at low energies. The resolution is not deteriorated by angular scattering or charge state fluctuations. Highly homogeneous low stress silicon nitride foils are used as degrader foils which reduces energy-loss tails significantly. For 36Cl measurements at 0.5MeV/amu a reduction of 36S by 104 could be achieved. The proprieties of these foils for the ΔTOF detector are discussed and first AMS measurements of 36Cl at 0.5MeV/amu and 41Ca at 0.4MeV/amu with the 3MV tandem accelerator VERA are presented. [Copyright &y& Elsevier]

Abstract: The use of tandem accelerators for accelerator mass spectrometry (AMS) allows to literally “analyze” molecules. When a molecular ion with mass M and charge Q is injected at the low-energy side, it is efficiently broken up into its atomic constituents during the stripping process in the terminal. At the high-energy side the positively charged atomic ions are again analyzed by their mass-to-charge ratio and by their energy in the detector (and eventually by their nuclear charge, too). We show the usefulness of the AMS method by identifying unambiguously the doubly-charged negative molecule (43Ca19F4)2− for the first time. It considerably eases the task that the total mass M =119 is odd, so the di-anion is injected at the half-integer mass-to-charge ratio M/Q =59.5, where no singly charged ions can interfere. The full power of AMS is needed when we try to proof the existence of di-anions with an integer M/Q, e.g. (23Na35Cl3)2−, whose stability is of interest for atomic physics theory. [Copyright &y& Elsevier]

Free doubly-charged negative molecules are a challenge for both theory and experiment. Their stability depends on a very delicate balance between interatomic structure, electronic interaction and Coulomb repulsion. Detecting di-anions of mass M with conventional mass spectrometry always bears the risk of erroneously measuring contributions from singly charged negative ions near mass M/2, or from molecular fragments with different energy. Another possible source of errors are mass ambiguities, e.g. M(12C4)≈M(16O3).In this contribution we show how one can overcome the methodical limits of conventional mass spectrometry by using AMS techniques for the analysis of very rare doubly-charged negative molecules. As an example, a new class of di-anions will be presented, (OCn)2-, n⩾5, which were discovered recently by secondary ion mass spectrometry, and were analyzed in detail by AMS. [Copyright &y& Elsevier]

Abstract: VERA has been operated for 12years with one 40-cathode SNICS ion source. Since the number of isotopes investigated at VERA increased steadily, we decided to install a second ion source. Due to space restrictions, a new injection line had to be built and merged into the existing one. The major task was to build an electrostatic analyzer assembly to accommodate injection from two ion sources. [Copyright &y& Elsevier]