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.     

Depth-profiling of implanted 28Si by (α,α) and (α,p0) reactions

Silicon nanocrystals enclosed in thin films (Si quantum dots or Si QDs) are regarded to be the cornerstone of future developments in new memory, photovoltaic and optoelectronic products. One way to synthesize these Si QDs is ion implantation in SiO₂ layers followed by thermal annealing post-treatment.Depth-profiling of these implanted Si ions can be performed by reactions induced by α-particles on ²⁸Si. Indeed, for high incident energy, nuclear levels of ³²S and ³¹P can be reached, and cross-sections for (α,α) and (α,p₀) reactions are more intense. This can help to increase the signal for surface silicon, and therefore make distinguishing more easy between implanted Si and Si coming from the SiO₂, even for low fluences.In this work, (α,α) and (α,p₀) reactions are applied to study depth distributions of 70 keV ²⁸Si⁺ ions implanted in 200 nm SiO₂ layers with fluences of 1 × 10¹⁷ and 2 × 10¹⁷ cm^?2. Analysis is performed above E_R = 3864 keV to take advantage of resonances in both (α,α) and (α,p₀) cross-sections. We show how (α,p₀) reactions can complement results provided by resonant backscattering measurements in this complex case.     

Comparison of detector systems for the separation of 36Cl and 36S with a 3-MV tandem

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 × 50 mm²) as stop detector. We optimized the TOF setup by using silicon nitride foils to reduce scattering tails in the energy spectra.At 3 MV terminal voltage, corresponding to a particle energy of 24 MeV of ³⁶Cl⁷⁺, we achieved a ³⁶S⁷⁺-suppression of 21,500 (50% ³⁶Cl-detector-efficiency).     

Improved 10Be and 26Al-AMS with a 5 MV spectrometer

Detector and ion source changes have increased Be and Al count rates and reduced measurement background at SUERC. Low energy 16 MeV ²⁶Al³⁺ ions can be separated from interferences by adopting thin silicon nitride membrane detector windows. In contrast, a thick Havar detector window is used to preferentially slow boron ions for simplified ¹⁰Be vs. ¹⁰B separation without an additional gas cell.     

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.     

Tailoring the spring constant of Si nanorod structures using swift heavy ion irradiation

This study reports a post-deposition technique of engineering the mechanical properties of cantilever-like silicon nanorods by using swift heavy ion irradiation. Slanted silicon nanorods grown by glancing angle deposition technique on a patterned Si(1 0 0) substrate are irradiated by 100 MeV Ag⁺⁸ ions at a fluence of 10¹⁴ ions cm^?2. The average spring constant (k) of the nanorods determined by force–distance spectroscopy reduces to 65.6 ± 20.8 Nm^?1 post-irradiation as compared to 174.2 ± 26.5 Nm^?1 for pristine nanorods. Scanning electron micrographs show bending of the Si nanorods after irradiation. Micro-Raman and high-resolution transmission electron microscope studies on pristine and irradiated Si nanorods confirm the transformation of nanocrystalline regions present in pristine nanorods to amorphous phase on irradiation. This structural transformation and bending of the nanorods are responsible for the observed changes in the mechanical properties post-irradiation. The technique offers a simpler possibility of tailoring mechanical properties of nanostructures post-deposition by ion irradiation.     

Regular surface patterns by local swelling induced by He implantation into silicon through nanosphere lithography masks

Nanopatterning of silicon surfaces by means of He⁺ ion implantation through self-organized colloidal masks is reported for the first time. Nanosphere lithography (NSL) masks with mask openings of 46–230 nm width were deposited on Si(1 0 0) wafers. He⁺ ions were implanted through these masks in order to induce a local cavity formation and Si surface swelling. The surface morphology and the subsurface structure were studied using atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (XTEM), respectively, as a function of mask and implantation parameters. It is demonstrated that regular arrays of both individual hillocks and trough-like circular rings can be generated.     

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.     

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.     

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.     

Development of a TOF-ERDA measurement system for analysis of light elements using a He beam

A time-of-flight ERDA (TOF-ERDA) measurement system has been developed for the analysis of light elements. He ions are used for the incident beam, and recoil light ions are detected with the system. The system consists of a time detector and a silicon detector, and energy and velocity of recoil ion are measured simultaneously. The depth resolution of 21.6 ± 2.2 nm (FWHM) has been obtained by an ERDA measurement of a thin carbon layer onto a silicon wafer using a 5.7 MeV He beam. The mass resolution is better than 1 for elements up to oxygen. Maximum detectable depth of carbon in a PET film is about 650 nm. An ERDA measurement of implanted carbon in a silicon wafer has been demonstrated.     

E/Q and ME/Q2 contributions to machine background in sequential injection radiocarbon AMS

Routine radiocarbon machine background values in the range of 10^?16 (corresponding to ～65–70 ka in the radiocarbon timescale) are routinely achieved at the sequential injection AMS system of CEDAD, University of Salento, Lecce, Italy. Although these background values are adequate for dating purposes and well below the sample-processing induced contamination, in recent times efforts have been made to identify the factors limiting the maximum achievable sensitivity of the AMS spectrometer and eventually extend the measured background below the current limits. Different possible sources of machine background have been thus investigated. Dedicated experiments allowed to quantitatively estimate the contributions of both ME/Q² and E/Q interferences to machine background and the corresponding attenuation factor of the AMS spectrometer. In particular our results indicate that the possibility to gate the ¹⁴C detector with the sequential injection of the three carbon masses results in a significant improvement of sensitivity mainly because of the reduction of the contributions to machine background of the ambiguities resulting from the injection of ¹²C and ¹³C negative beams. A comparison of the results obtained at CEDAD with those obtained in similar experiments with different experimental set-up are presented.     

Nanoscale metal-silicide films prepared by surfactant sputtering and analyzed by RBS

Surfactant sputtering has been applied to modify the surface structure of Si substrates and to produce ultrathin metal-silicide films with nickel and platinum surfactants, utilizing the steady state coverage of a Si-substrate surface with surfactant atoms simultaneously during sputter erosion by combined ion irradiation and surfactant atom deposition. Si (1 0 0) substrates were eroded using 5 keV Xe-ions and 10–30 keV Ar ions at incident angles of 65° and 70° with fluences of up to 2 × 10¹⁸/cm² under continuous sputter deposition of platinum and nickel from targets irradiated simultaneously by the same ion beam. These surfactant atoms form metal-silicides in the surface near region and strongly modify the substrate sputter yield and the surface nanostructure. Atomic force microscopy and scanning electron microscopy were carried out to observe a transition of surface topography from ripple to relief patterns, granular patterns or smooth surfaces. The Si and metal sputter yield as function of the steady state metal coverage were determined by combination of Rutherford-backscattering spectroscopy (RBS) and profilometry. The composition and the depth distributions of metal-silicide films were analyzed via high-resolution RBS and transmission electron microscopy. We show that RBS results in comparison with SRIM and TRIDYN sputter yield simulations allows us to identify the silicide surface structure on the nanometer scale.     

Boron suppression with a gas ionization chamber at very low energies (E < 1 MeV)

Efficient boron suppression for precise ¹⁰Be measurements with AMS is crucial. The performance of ΔE ? E_res gas ionization chambers is also very important for isobar suppression at low beam energies (<1 MeV). A boron suppression of 6–7 orders of magnitude is achievable with the ETH ΔE ? E_res gas ionization in the standard operational mode (readout of ΔE and E_res electrodes). Some physical effects such as pulse height defect or the energy focusing effect within a ΔE section will be discussed, emphasizing ¹⁰Be measurements below 1 MeV. Additionally the potential of silicon nitride membranes as passive energy degraders in front of the detector is demonstrated.     

Direct measurements of small 14C samples after oxidation in quartz tubes

Small ¹⁴C samples gain importance in environmental research and for dating purposes. However, throughput of such samples is limited by the preparation of graphite targets for accelerator mass spectrometry (AMS) measurements. In our approach, oxidation of samples with copper oxide in quartz tubes was applied to form CO₂ which was measured directly with the gas ion source of the small AMS facility MICADAS. The presented method was designed to meet the requirements for fast and easy handling of small samples (<100 μg carbon). As combustion byproducts are likely to interfere with ionisation processes in the gas ion source, we additionally investigated the effects of several gases on C^? currents.     

Highly sensitive AMS measurements of 53Mn

Our AMS system, with the gas-filled detector system GAMS, has been optimized for measurements with ⁵³Mn. A high sensitivity has been achieved. A newly installed cesium sputter ion source yields an improved emittance, and thus a higher mass resolution. By the extraction of the manganese molecule MnF^? instead of MnO^? we can suppress the isobaric chromium background in the ion source by more than a factor of three. The GAMS system achieves an isobaric suppression factor of about 3 × 10⁸. Measurements on blank samples yielded upper limits for the ⁵³Mn/⁵⁵Mn ratios of 7 × 10^?15.     

Low temperature formation of luminescent Si nanocrystals with combined process of excimer UV-light irradiation and RTA

Si ion implantation was widely used to synthesize specimens of SiO₂ containing supersaturated Si and subsequent high temperature annealing induces the formation of embedded luminescent Si nanocrystals. In this work, the potentialities of excimer UV-light (172 nm, 7.2 eV) irradiation and rapid thermal annealing (RTA) to achieve low temperature (below 1000 °C) formation of luminescent Si nanocrystals in SiO₂ have been investigated. The Si ions were introduced at acceleration energy of 180 keV to fluences of 7.5 × 10¹⁶ and 1.5 × 10¹⁷ ions/cm². The implanted samples were subsequently irradiated with an excimer-UV lamp for 2 h. After the process, the samples were rapidly thermal annealed at 1050 °C for 5 min before furnace annealing (FA) at 900 °C. Photoluminescence spectra were measured at various stages at the process. Effective visible photoluminescence is found to be observed even after FA at 900 °C, only for specimens treated with excimer-UV lamp and RTA, prior to a low temperature FA process. Based on our experimental results, we discuss the mechanism for the initial formation process of the luminescent Si nanocrystals in SiO₂, together with the effects with excimer lamp irradiation and RTA process on the luminescence.     

Early stage of the crystallization in amorphous Fe–Si layers: Formation and growth of metastable α-FeSi2

Crystallization processes of amorphous Fe–Si layers have been investigated using transmission electron microscopy (TEM). Si(1 1 1) substrates were irradiated with 120 keV Fe ions at ?150 °C to a fluence of 1.0 × 10¹⁷ cm². An Fe-rich amorphous layer embedded in an amorphous Si was formed in the as-irradiated sample. Plan-view TEM observations revealed that a part of the amorphous Fe–Si layer crystallized to the metastable α-FeSi₂ after thermal annealing at 350 °C for 8 h. The lattice parameter of c-axis decreased with thermal annealing. It was considered that the change in the lattice parameter originates from the decrease of the Fe occupancy at (0, 0, 1/2) and its equivalent positions in the unit cell of the metastable α-FeSi₂.     

High energy Si ions bombardment effects on the properties of nano-layers of SiO2/SiO2 + Ag

We grew 50 periodic SiO₂/SiO₂ + Ag multi-layers by electron beam deposition technique. The co-deposited SiO₂ + Ag layers are 7.26 nm, SiO₂ buffer layers are 4 nm, and total thickness of film was determined as 563 nm. We measured the thickness of the layers using in situ thickness monitoring during deposition, and optical interferometry afterwards. The concentration and distribution of Ag in SiO₂ were determined using Rutherford backscattering spectrometry (RBS). In order to calculate the dimensionless figure of merit, ZT, the electrical conductivity, thermal conductivity and the Seebeck coefficient of the layered structure were measured at room temperature before and after bombardment with 5 MeV Si ions. The energy of the Si ions was chosen such that the ions are stopped deep inside the silicon substrate and only electronic energy due to ionization is deposited in the layered structure. Optical absorption (OA) spectra were taken in the range 200–900 nm to monitor the Ag nanocluster formation in the thin layers.     

CdTe detector use for PIXE characterization of TbCoFe thin films

Peltier cooled CdTe detectors have good efficiency beyond the range of energies normally covered by Si(Li) detectors, the most common detectors in PIXE applications. An important advantage of CdTe detectors is the possibility of studying K X-rays lines instead the L X-rays lines in various cases since CdTe detectors present an energy efficiency plateau reaching 70 keV or more. The ITN CdTe useful energy range starts at K-K_α (3.312 keV) and goes up to 120 keV, just above the energy of the lowest γ-ray of the ¹⁹F(p, p’γ)¹⁹F reaction. In the new ITN HRHE-PIXE line, a CdTe detector is associated to a POLARIS microcalorimeter X-ray detector built by Vericold Technologies GmbH (an Oxford Instruments Group Company). The ITN POLARIS has a resolution of 15 eV at 1.486 keV (Al-K_α) and 24 eV at 10.550 keV (Pb-L_α1). In the present work, a TbCoFe thin film deposited on a Si substrate was analysed at the HRHE-PIXE system. The good efficiency of the CdTe detector at 45 keV (Tb-K_α), and the excellent resolution of POLARIS microcalorimeter at 6.403 keV (Fe-K_α), are presented and the new possibilities open to the IBA analysis of systems with traditionally overlapping X-rays and near mass elements are discussed.