Carrier-free measurements of natural 10Be/9Be ratios at low energies

At ETH Zurich a new method to measure directly the natural ¹⁰Be/⁹Be ratio of a sample with the compact (0.6 MV) AMS system Tandy has been developed, which allows us to use standard sample preparation methods and AMS techniques. Our results show that carrier-free measurements with compact AMS machines represent a fast, less expensive and more precise alternative for applications where only the relative variation of the ¹⁰Be/⁹Be ratio is needed. Increased source efficiency for (artificial) low carrier samples has been found. If this effect could be exploited for natural samples, it would open the field for other applications like in situ ¹⁰Be-dating of quartz with compact, low energy AMS systems.     

Competitive 10Be measurements below 1 MeV with the upgraded ETH–TANDY AMS facility

Competitive ¹⁰Be measurements at energies as low as 0.75 MeV are now possible with the compact ETH AMS system TANDY. In this paper we describe and discuss the modifications that led to the significantly improved performance for ¹⁰Be at the 0.6 MV accelerator. Results from the first routine measurement show that ¹⁰Be on the upgraded TANDY is now fully competitive with larger AMS systems with respect to background and measurement precision. The total efficiency for ¹⁰Be is comparable to our large 6 MV Tandem system and thus sufficient for the full range of applications in the Earth and Environmental Sciences.     

Status and research programs of the multinuclide accelerator mass spectrometry system at the University of Tsukuba

We present the current status and research programs of a multinuclide accelerator mass spectrometry (AMS) system on the 12UD Pelletron tandem accelerator at the University of Tsukuba (Tsukuba AMS system), Japan. A maximum terminal voltage of 12 MV is available for the AMS system. The Tsukuba AMS system can measure environmental levels of long-lived radioisotopes of ¹⁴C, ²⁶Al, ³⁶Cl and ¹²⁹I by employing a molecular pilot beam. Recently, enhancements in AMS techniques and equipment, including sample preparation, the ion source and the data acquisition system, have improved the performance of ³⁶Cl-AMS. The standard deviation of fluctuations is typically ±2%, and the machine background level for the ³⁶Cl/Cl ratio is lower than 1 × 10^?15 with a halite sample. We have measured over 500 samples in 1 year, including samples for earth and environmental sciences and nuclear safety research.     

The French accelerator mass spectrometry facility ASTER: Improved performance and developments

Following the installation and acceptance test of the French 5 MV AMS facility ASTER, the focus has been on improving the capability for routine measurements of ¹⁰Be and ²⁶Al. Quality assurance has been established by the introduction of traceable AMS standards for each nuclide, by self-monitoring through participation in round-robin exercises and proficiency testing, and by surveillance of long- and short-time variability of blank and reference materials. A background level of 3 × 10^?14 makes ASTER well-suited for measuring ⁴¹Ca/⁴⁰Ca in the10^?12 region, which is sufficient for a wide range of applications. Routine AMS measurements of volatile elements like ³⁶Cl and ¹²⁹I will most likely become feasible in the very near future as the result of significant improvements in the ion source design.     

AMS measurement technique after 30 years: Possibilities and limitations of low energy systems

A brief summary of the development of AMS over the last 30 years is given and major development steps for the measurement technique are described. With the appearance of compact low energy AMS systems 10 years ago, a new category of AMS instruments has been introduced. This has resulted in a boom of new AMS facilities with more than 20 installations over the last 5 years. But low energy AMS is not limited to radiocarbon only and there is a great potential for ¹⁰Be, ²⁶Al, ¹²⁹I and actinides measurements at compact AMS systems. The latest developments towards the low energy limit of AMS resulted in two new types of systems, the NEC Single Stage AMS and ETH MICADAS operating with terminal voltages of about 200 kV only. These systems have enormous impact, not only on the use of AMS in biomedical research but also on radiocarbon dating. Precision limits of radiocarbon dating will be discussed for the MICADAS type instruments.     

Porous iron pellets for AMS 14C analysis of small samples down to ultra-microscale size (10–25 μgC)

We developed the use of a porous iron pellet as a catalyst for AMS ¹⁴C analysis of small samples down to ultra-microscale size (10–25 μgC). It resulted in increased and more stable beam currents through our HVEE 4130 ¹⁴C AMS system, which depend smoothly on the sample size. We find that both the expected decrease of oxalic acid standards and increase of backgrounds with decreasing sample size, due to increasing influence of contamination, are reproducible. Using a mass-dependent background correction for dead (1.0 ± 0.4 μgC) and modern (0.25 ± 0.10 μgC) contamination, we obtain reliable results for small samples down to 10 μgC and possibly smaller. Due to our low graphitization yield for ultra-small samples (increases from 40% to 80% on average with sample size), we measured graphite standards as small as 3 μgC. The standard deviation of the corrected activity is about 5% for a 10-μgC HOxII standard.Here we report the iron pellet technique, which is new to the best of our knowledge. It is generally applicable for AMS ¹⁴C laboratories that want to measure small samples down to ultra-microscale size. As an illustrative test-case, we analyze ¹⁴C data for IAEA-C5, C7 and C8 samples with masses ranging from 15 to 300 μgC.     

Small-mass graphite preparation by sealed tube zinc reduction method for AMS 14C measurements

Previous work has demonstrated that the sealed tube Zn reduction method for converting CO₂ to graphite for AMS ¹⁴C measurements produces targets that can be measured with high precision and low background for samples of about 1 mg C down to approximately 0.1 mg C at the Keck Carbon Cycle AMS facility at the University of California, Irvine (KCCAMS). Now a modified method has been developed to prepare small-mass samples ranging from 0.015 to 0.1 mg C. In this modified method, the volume of the sealed reactor tube is reduced to ～1.9 cm³, and the amounts of Zn and TiH₂ reagents are reduced proportionally. The amount of Fe catalyst used remains the same to ensure a long lasting current in the AMS. Small-mass samples prepared by this method generally yield ¹²C⁺¹ currents of about 0.5 μA per 1 μg C. An in situ simultaneous AMS δ¹³C measurement allows for correction of both graphitization and machine-induced isotopic fractionation, and is a prerequisite for high precision and accurate measurements using the Zn reducing method. Corrections for modern-carbon and dead-carbon background components are applied to samples based on small-mass samples of a ¹⁴C free material and of a modern standard covering the sample size range. It was discovered during additional investigation into lowering the modern-carbon background component that baking assembled reactor tubes at 300 °C for 1 h prior to use resulted in significantly lower modern-carbon background values. The accuracy and precision of small-mass samples prepared by this method are size dependent, but is usually ±10–15‰ for the smallest samples (0.015–0.02 mg C), based on duplicate measurements of primary and secondary standards.     

Comparison of 41Ca analysis on 0.5 MV and 5 MV-AMS systems

A comparative study was made between the compact AMS system at the PSI/ETH Laboratory of Ion Beam Physics in Zurich with 0.5 MV terminal voltage and the 5 MV-AMS system at the Scottish Universities Environmental Research Centre (SUERC), Glasgow. Overall 34 urinary samples with ⁴¹Ca/⁴⁰Ca ratios in the range from 4 × 10^?11 to 3 × 10^?10 were processed to CaF₂ and aliquots of the same material were measured on both instruments.Measurements on the compact AMS system were performed in charge state 3+ achieving a transmission of 4% at 1.7 MeV beam energy. Under these conditions a suppression of the interference ⁴¹K is virtually impossible. However, samples with an excess of potassium can be identified by a shift of the ⁴¹Ca/⁴¹K peak in the ΔE ? E histogram of the gas ionization detector employed and a criterion for data rejection can be defined. An overall precision of ～4% and a ⁴¹Ca/⁴⁰Ca background level of 5 × 10^?12 have been reached.For studies with higher demands on the detection limit AMS systems like the one at SUERC are attractive: in charge state 5+ and using a gas stripper beam energy of 27 MeV, a transmission of 5%, a ⁴¹K suppression factor of ～500 and a ⁴¹Ca/⁴⁰Ca background level of 3 × 10^?14 are achieved.We demonstrate that both systems are well suited for large-scale ⁴¹Ca biomedical applications.     

Analysis and application of heavy isotopes in the environment

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 ～150 amu) 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, ^{239,240,241,242,244}Pu), 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.     

Development of a preparation system for the radiocarbon analysis of organic carbon in carbonaceous aerosols in China

Carbonaceous aerosols comprising a large fraction of elemental carbon (EC) and organic carbon (OC) are considered to affect both global climate and human health. Radiocarbon measurements have been proved to be a useful isotopic tracer for distinguishing contemporary and fossil emissions. An optimized system of a two-step thermal preparation system for radiocarbon (¹⁴C) measurement of OC/TC is firstly established in China. In this system, OC/TC are converted into carbon dioxide under a pure oxygen flow at 340 °C/650 °C and then reduced to graphite for AMS target using the method of zinc reduction. Afterwards, radiocarbon measurements of the targets performed by the NEC Compact AMS System at the Institute of Heavy Ion Physics, Peking University. The measured results for estimated reference martial including HOx I, HOx II and IAEA-C6 are consistent with internationally accepted values. The radiocarbon-based source appointment of carbonaceous aerosols in China would be much more convenient and faster with the preparation system developed in this work.     

Fluorides or hydrides? 41Ca performance at VERA’s 3-MV AMS facility

Recent improvements in isobaric suppression for medium-mass isotopes, e.g. ⁴¹Ca, offer new possibilities for tandem accelerators with terminal voltages of 3 MV or lower; i.e. when dealing with particle energies ?1 MeV/amu. In particular, detection of ⁴¹Ca requires sufficient discrimination of the stable isobar ⁴¹K. We explored the limits of ⁴¹Ca detection at our 3-MV AMS facility by means of different types of particle detectors: The ΔTOF method, which is based on the different flight-time of isobars after passing a thick absorber foil. The second method makes use of a new type of compact ionization chamber: ⁴¹K and ⁴¹Ca are separated in energy due to their different energy loss in the detector entrance foil and the detector gas, which is measured via a segmented anode.At VERA we measured ⁴¹Ca/Ca ratios below 10^?13 for commercial CaF₂ material serving as blank samples. CaH₂ sputter targets, with the extraction of ${\text{CaH}}_3^{-}$, yielded background ratios as low as ⁴¹Ca/Ca = 1 × 10^?15. The typical measurement precision at VERA for ⁴¹Ca measurements was between 2% and 5%. These results demonstrate that AMS facilities based on 3-MV tandems have reached the sensitivity level of larger AMS facilities for a wide range of applications, with the advantage of high overall efficiency and sample throughput.     

On the measurement of 10Be on the 1 MV compact AMS system at the Centro Nacional de Aceleradores (Spain)

In this work we present the most recent improvements performed by our group on ¹⁰Be measurements on the 1 MV AMS system recently set up at the CNA (Centro Nacional de Aceleradores), in Seville (Spain). Our efforts have been focused on the study of the viability of our system for BeO and BeF^? measurements. To achieve this, different standard materials have been measured to demonstrate the reliability of the system for BeO measurements in a wide ¹⁰Be/⁹Be atomic ratio range and several environmental samples have been studied both at the 1 MV AMS CNA facility and at the 6 MV AMS ETH-PSI facility of Zurich to validate our measurements. The results show a good agreement between laboratories. New experiments also have been carried out selecting 1+ and 2+ charge states at the exit of the accelerator and inserting Si₃N_3.1 foils with different thicknesses to separate ¹⁰Be from its isobar, ¹⁰B. The influence of each foil on the overall transmission (detected ¹⁰Be compared to BeO^? injected into the accelerator) and background level was also assessed. In addition some tests were also done to assess the viability of BeF₂ and BaBeF₄ measurements at our system. Several metal matrices and cathode preparation procedures for BeO samples were investigated to maximize current and cathode lifetime.     

Development of laboratory standards for AMS measurement of 237Np

A new method for the preparation of ²³⁷Np/²³⁸U laboratory standards for accelerator mass spectrometry (AMS) measurement was developed at China Institute of Atomic Energy (CIAE). ²³⁷Np was generated by β-decay (6.75 days) of ²³⁷U produced via fast neutron induced ²³⁸U(n,2n)²³⁷U reaction. The ⁵⁸Co(n,2n)⁵⁹Co and ⁹³Nb(n,2n)^92mNb reactions were used to monitor the integral incident neutron flux. The cross-section of ²³⁸U(n,2n)²³⁷U was determined by measuring the emission rate of 208.0 keV γ-rays from ²³⁷U decay with a calibrated HPGe γ-ray spectrometer at CIAE. In order to correct for the self-attenuation of 208.0 keV γ-ray in the U sample, a ¹⁷⁷Lu–U method was introduced. The new correction method provided effective and high quality measurements of the cross-section for the ²³⁸U(n,2n)²³⁷U reaction; and the ²³⁷Np/²³⁸U ratio. A series of AMS standards for ²³⁷Np/²³⁸U (10^?9–10^?13) can be obtained by successive dilution of the original standard sample.     

Challenges and opportunities in high-precision Be-10 measurements at CAMS

We determined the overall efficiency for ¹⁰Be of the high-intensity LLNL modified Middleton cesium sputter source in combination with the CAMS FN mass spectrometer. BeO^? ionization efficiency is >3%. Charge exchange efficiency including transmission through the tandem for 7.5 MeV Be⁺³ is ～34%, resulting in a total system efficiency of just over 1%. At this efficiency and with very low backgrounds, we estimate our detection limit to be ～1000 ¹⁰Be atoms. Cathodes prepared with only ～80 μg of ⁹Be show only an ～33% reduction in ⁹Be beam current compared to a sample with ～200 μg. These same samples, prepared from 07KNSTD1032 standard material, contained 1 × 10⁷ and 5 × 10^{6 10}Be atoms and exhibited similar ionization and total system efficiency. These results demonstrate the feasibility of pursuing applications that require precise measurement of samples with low ¹⁰Be concentrations and/or small sample size.     

14C analysis via intracavity optogalvanic spectroscopy

A new ultra sensitive laser-based analytical technique, intracavity optogalvanic spectroscopy (ICOGS), allowing extremely high sensitivity for detection of ¹⁴C-labeled carbon dioxide has recently been demonstrated. Capable of replacing accelerator mass spectrometers (AMS) for many applications, the technique quantifies zeptomoles of ¹⁴C in sub micromole CO₂ samples. Based on the specificity of narrow laser resonances coupled with the sensitivity provided by standing waves in an optical cavity, and detection via impedance variations, limits of detection near 10^{?15 14}C/¹²C ratios have been obtained with theoretical limits much lower. Using a 15 W ¹⁴CO₂ laser, a linear calibration with samples from 5 × 10^?15 to >1.5 × 10^?12 in ¹⁴C/¹²C ratios, as determined by AMS, was demonstrated. Calibration becomes non-linear over larger concentration ranges due to interactions between CO₂ and buffer gas, laser saturation effects and changes in equilibration time constants. The instrument is small (table top), low maintenance and can be coupled to GC or LC input. The method can also be applied to detection of other trace entities. Possible applications include microdosing studies in drug development, individualized sub-therapeutic tests of drug metabolism, carbon dating and real time monitoring of atmospheric radiocarbon.     

129I AMS at 0.5 MV tandem accelerator

The ¹²⁹I measurement program has been established at the 0.5 MV ‘Tandy’ accelerator of the PSI/ETH Zürich AMS facility. This development was made possible by using a SiN window instead of Mylar one in a gas ionization detector. The setting up of the ¹²⁹I measurement at Tandy is simple, the acquired performance is stable and reliable, and the quality of results is equal to or better than at our larger EN-tandem. With this setup, high sample throughput, which is required in many ¹²⁹I studies, can be easily achieved. The measurements are performed in the +3 charge state. At this charge state the major difficulty in the ¹²⁹I⁺³ identification is caused by a highly abundant 43⁺¹ (m = 43, q = +1) molecule interference. This is a positive molecular ion, because its intensity reduces exponentially with an increase in gas stripper pressure. We conclude that this molecule is ²⁷Al¹⁶O⁺ (m/q = 43/1 = 129/3) and comes from the break-up of (Al₂O₃ + Al)^? (m = 129) precursor at the terminal: (Al₂O₃ + Al)^? → ²⁷Al¹⁶O⁺. The expected isobaric interferences ⁴³Ca⁺¹ and ⁸⁶Sr⁺², which also originate from the break-up of molecules in the stripper, were found to be low and do not disturb the ¹²⁹I⁺³ measurements. The best repeatable performance with our standard sample material was achieved at 0.14 μg/cm² Ar gas stripper pressure with machine blanks showing ～6 × 10^?14 normalized ¹²⁹I/I ratio and 9% transmission through the accelerator. However, high ²⁷Al¹⁶O⁺ molecular rates were observed from the user samples, and in order to destroy these molecules we had to increase the stripper pressure to ～0.22 μg/cm². This increase in the stripper pressure degraded the machine blank values to ～9 × 10^?14 and reduced transmission to 8%. Nevertheless, the achieved measurement conditions are sufficient for measurement of nearly all ¹²⁹I samples that have been submitted to PSI/ETH over the last few years.     

Rapid aqueous release of fission products from high burn-up LWR fuel: Experimental results and correlations with fission gas release

Studies of the rapid aqueous release of fission products from UO₂ and MOX fuel are of interest for the assessment of the safety of geological disposal of spent fuel, because of the associated potential contribution to dose in radiological safety assessment. Studies have shown that correlations between fission gas release (FGR) and the fraction rapidly leached of various long-lived fission products can provide a useful method to obtain some of this information. Previously, these studies have been limited largely to fuel with burn-up values below 50 MWd/kg U. Collaborative studies involving SKB, Studsvik, Nagra and PSI have provided new data on short-term release of ¹³⁷Cs and ¹²⁹I for a number of fuels irradiated to burn-ups of 50–75 MWd/kgU. In addition a method for analysis of leaching solutions for ⁷⁹Se was developed. The results of the studies show that the fractional release of ¹³⁷Cs is usually much lower than the FGR covering the entire range of burn-ups studied. Fractional ¹²⁹I releases are somewhat larger, but only in cases in which the fuel was forcibly extracted from the cladding. Despite the expected high degree of segregation of fission gas (and by association ¹³⁷Cs and ¹²⁹I) in the high burn-up rim, no evidence was found for a significant contribution to release from the rim region. The method for ⁷⁹Se analysis developed did not permit its detection. Nonetheless, based on the detection limit, the results suggest that ⁷⁹Se is not preferentially leached from spent fuel.     

Developments in micro-sample 14C AMS at the ANTARES AMS facility

We continue development of micro-sample radiocarbon sample preparation and AMS measurement at the ANTARES AMS facility. We routinely prepare samples containing 10–200 μg of carbon using an iron catalyst with an excess of hydrogen in ～2.5 mL graphitisation reactors. These use a tube furnace to heat the catalyst to 600 °C and a Peltier-cooled water trap. Samples containing just a few micrograms of carbon can be prepared. We describe progress with a 0.5 mL laser-heated ‘microfurnace’ we are developing for the rapid and efficient graphitisation of ～5 μg samples. Following operating experience with a prototype unit, work has commenced on the development of a second-generation device with the goal of fully automated operation with minimal introduction of extraneous carbon.Key to development of micro-sample ¹⁴C AMS is the ability to reliably handle the graphite/iron sample and to mount it in the ion source target holder. We have developed a target holder that permits the sample to be loaded in a 1 mm diameter recess and rear pressed, ensuring a high quality surface finish, at a reproducible depth. Additionally we have developed a method for systematically aligning the sample stage with the cesium beam following ion source servicing.     

36Cl exposure dating with a 3-MV tandem

³⁶Cl AMS measurements at natural isotopic concentrations have yet been performed only at tandem accelerators with 5 MV 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 3.5 MV.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.     

10Be and 26Al measurements at the Zurich 6 MV Tandem AMS facility

The setups for the ¹⁰Be and ²⁶Al AMS measurements at the Zurich 6 MV EN Tandem AMS facility are presented and data analysis procedures for both radionuclides are discussed. Performance parameters of the system are shown. In-house AMS standards are compared to commercially available AMS standards.