Improved 36Cl AMS at 5 MV

Small changes to our ion source and gas ionization detector have significantly improved Cl measurement by reducing source memory and increasing interference rejection. Gas stripped low energy 30 MeV ³⁶Cl⁵⁺ ions are still efficiently transported to the detector but ³⁶S vs. ³⁶Cl separation is improved by an order of magnitude. Accordingly ³⁶Cl/Cl background is <10^?15 before additional interference correction that is also newly automated. ³⁵Cl^? currents have increased to 30 μA and ³⁶Cl/Cl inter-cathode repeatability is 3%.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Optimization of aluminum thickness for absorption of undesired Ti K X-rays in the measurement of low energy brachytherapy source strength

The contribution of Ti K X-rays to total air kerma strength for low energy brachytherapy sources (¹²⁵I and ¹⁰³Pd) are calculated for different source-to-aperture distances of an indigenously designed free air ionization chamber. For 30 cm source-to-aperture distance, calculated contribution of Ti K X-rays is 4%. The Ti K X-rays can be eliminated by a relatively thin aluminum filter, but the primary photons emitted by the source will also be attenuated. This effect should be compensated by applying a suitable correction factor. The uncertainty in the attenuation correction factor has been also calculated for different thicknesses of aluminum by a Monte Carlo uncertainty analysis algorithm programmed in FORTRAN. The results show that the optimum thickness of the aluminum absorber is 100 μm, for which the contribution of Ti K X-rays in air kerma strength is reduced to less than one hundredth of the uncertainty in the correction factor, ensuring that the uncertainty in the air kerma strength will be mainly due to the uncertainty in the correction factor. The calculated uncertainties are 1.7 × 10^?3, and 3.4 × 10^?3 for ¹²⁵I and ¹⁰³Pd sources, respectively.     

Effect of <200 keV proton radiation on electric properties of silicon solar cells at 77 K

The change in electric properties of back-field silicon solar cells was investigated under the irradiation of protons with the energies less than 200 keV at 77 K. Experimental results showed that the short circuit current, maximum output power and open circuit voltage decrease to different extent with increasing the fluence and energy of protons. Under the 120 keV proton irradiation for the fluence of 1 × 10¹⁶ cm⁻², a large amount of radiation-induced defects with the energy level H1 +0.47 eV were formed. In terms of analyzing the time dependence of electric properties, the performance lifetime of the silicon cells under the exposure of <200 keV protons was predicted.     

Evaluation of gamma-ray and neutron energy for area monitoring system in the IFMIF/EVEDA accelerator building

The engineering validation of the IFMIF/EVEDA prototype accelerator, up to 9 MeV by supplying the deuteron beam of 125 mA, will be performed at the BA site in Rokkasho. A design of this area monitoring system, comprising of Si semiconductors and ionization chambers for covering wide energy spectrum of gamma-rays and ³He counters for neutrons, is now in progress. To establish an applicability of this monitoring system, photon and neutron energies have to be suppressed to the detector ranges of 1.5 MeV and 15 MeV, respectively. For this purpose, the reduction of neutron and photon energies throughout shield of water in a beam dump and concrete layer is evaluated by PHITS code, using the experimental data of neutron source spectra. In this article, a similar model using the beam dump structure and the position with a degree of leaning for concrete wall in the accelerator vault is used, and their energy reduction including the air is evaluated. It is found that the neutron and photon flux are decreased by 10⁴-order by employing the local shields using concrete and polyethylene around beam dump, and the photon energy can be suppressed in the low energy.     

Damage accumulation in gallium nitride irradiated with various energetic heavy ions

In this work a study of damage production in gallium nitride via elastic collision process (nuclear energy deposition) and inelastic collision process (electronic energy deposition) using various heavy ions is presented. Ordinary low-energy heavy ions (Fe⁺ and Mo⁺ ions of 110 keV), swift heavy ions (²⁰⁸Pb²⁷⁺ ions of 1.1 MeV/u) and slow highly-charged heavy ions (Xeⁿ⁺ ions of 180 keV) were employed in the irradiation. Damage accumulation in the GaN crystal films as a function of ion fluence and temperature was studied with RBS-channeling technique, Raman scattering technique, scanning electron microscopy (SEM) and transmission electron microscopy (TEM).For ordinary low-energy heavy ion irradiation, the temperature dependence of damage production is moderate up to about 413 K resulting in amorphization of the damaged layer. Enhanced dynamic annealing of defects dominates at higher temperatures. Correlation of amorphization with material decomposition and nitrogen bubble formation was found. In the irradiation of swift heavy ions, rapid damage accumulation and efficient erosion of the irradiated layer occur at a rather low value of electronic energy deposition (about 1.3 keV/nm³), which also varies with irradiation temperature. In the irradiation of slow highly-charged heavy ions (SHCI), enhanced amorphization and surface erosion due to potential energy deposition of SHCI was found. It is indicated that damage production in GaN is remarkably more sensitive to electronic energy loss via excitation and ionization than to nuclear energy loss via elastic collisions.     

Effect of keV ion irradiation on mechanical properties of hydrogenated amorphous silicon

The susceptibility of mechanical properties of hydrogenated amorphous silicon (a-Si:H) to the implantation-enhanced disorder has been studied with the aim to extend the application field of this material in the technology of micro-electromechanical systems. Effect of keV ion irradiation on the elastic modulus, E, of hardness, H, and of root-mean-squared roughness to silicon ion implantation has been determined. The mechanical properties were evaluated by nanoindentation testing. E of 119 GPa and H of 12.3 GPa were determined for the as-prepared a-Si:H film. The implantation of silicon ions leads to a decrease in E and H, evaluated for a series of the implantation fluences in the range of 1.0 × 10¹³–5.0 × 10¹⁶ cm^?2. Surface smoothing has been observed at high fluences and low ion energy of 18 keV, suggesting that ion beam may be used as a tool to reduce the roughness of the a-Si:H surface, while keeping intact the mechanical properties inside the film. The conducted experiments show that it is possible to prepare a-Si:H films with hardness and smoothness comparable to crystalline silicon.     

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.     

Aerosol formation and hydrogen co-deposition by colliding ablation plasma plumes of lithium and lead

In a high-repetition inertial fusion reactor, along with pellet implosions, the interior of target chamber is to be exposed to high-energy, short pulses of X-ray, unburned DT and He ash particles and pellet debris. As a result, wall materials will be subjected to ablation, ejecting particles in the plasma state to collide with each other in the center of volume. The interaction dynamics of ablation plasmas of lithium and lead, candidate first wall materials, has been investigated in the deposited energy density range from 3 to 10 J/cm²/pulse at a repetition rate of 10 Hz, each 6 ns long. The plasma density and electron temperature of colliding ablation plumes have been found to vary from the order of 10⁸–10¹³ 1/cm³ and from 0.7 to 1.5 eV, respectively. The formation of aerosol in the form of droplet has been observed with diameters between 100 nm and 10 μm. Also, hydrogen co-deposition has been found to occur particularly for colliding plumes of lithium, resulting in the H/Li atomic ratio from 0.15 to 0.27 in the hydrogen partial pressure range from 10 to 50 Pa.     

Design and performance of main vacuum pumping system of SST-1 Tokamak

Steady-state Superconducting Tokamak (SST-1) was installed and it is commissioning for overall vacuum integrity, magnet systems functionality in terms of successful cool down to 4.5 K and charging up to 10 kA current was started from August 2012. Plasma operation of 100 kA current for more than 100 ms was also envisaged. It is comprised of vacuum vessel (VV) and cryostat (CST). Vacuum vessel, an ultra-high (UHV) vacuum chamber with net volume of 23 m³ was maintained at the base pressure of 6.3 × 10⁻⁷ mbar for plasma confinement. Cryostat, a high-vacuum (HV) chamber with empty volume 39 m³ housing superconducting magnet system, bubble thermal shields and hydraulics for these circuits, maintained at 1.3 × 10⁻⁵ mbar in order to provide suitable environment for these components. In order to achieve these ultimate vacuums, two numbers of turbo-molecular pumps (TMP) are installed in vacuum vessel while three numbers of turbo-molecular pumps are installed in cryostat. Initial pumping of both the chambers was carried out by using suitable Roots pumps. PXI based real time controlled system is used for remote operation of the complete pumping operation. In order to achieve UHV inside the vacuum vessel, it was baked at 150 °C for longer duration. Aluminum wire-seals were used for all non-circular demountable ports and a leak tightness < 1.0 × 10⁻⁹ mbar l/s were achieved.     

SUERC AMS ion detection

In a short time Be, C, Al, Cl, Ca and I accelerator mass spectrometry (AMS) have been established on the National Electrostatics Corporation (NEC) 5 MV pelletron system at the Scottish Universities Environmental Research Centre (SUERC). While summarising the present performance of the system, this report will focus on the details of ion detection, which sample materials are used and the analytical procedures employed for each individual species during routine analysis.All rare isotope detection is with a single flexible detector and ion event analysis system, but switching of analysed species typically requires a detector reconfiguration. Configurations for routine ¹⁰Be, ¹⁴C, ²⁶Al, ³⁶Cl, ⁴¹Ca and ¹²⁹I detection have been established and will be presented here. Notably, there has proven to be sufficient suppression of the isobaric interferences of ³⁶Cl and ⁴¹Ca in the 5+ charge state using an argon gas stripper at a terminal voltage of 5.0 MV to allow for routine analysis of these isotopes.     

High-speed processing with Cl2 cluster ion beam

Cluster ion beam processes can produce high rate sputtering with low damage compared with monomer ion beam processes. Cl₂ cluster ion beams with different size distributions were generated with controlling the ionization conditions. Size distributions were measured using the time-of-flight (TOF) method. Si substrates and SiO₂ films were irradiated with the Cl₂ cluster ions at acceleration energies of 10–30 keV and the etching ratio of Si/SiO₂ was investigated. The sputtering yield increased with acceleration energy and was a few thousand times higher than that of Ar monomer ions. The sputtering yield of Cl₂ cluster ions was about 4400 atoms/ion at 30 keV acceleration energy. The etching ratio of Si/SiO₂ was above eight at acceleration energies in the range 10–30 keV. Thus, SiO₂ can be used as a mask for irradiation with Cl₂ cluster ion beam, which is an advantage for semiconductor processing. In order to keep high sputtering yield and high etching ratio, the cluster size needs to be sufficiently large and size control is important.     

Determination of differential cross-sections for the natK(p, p0) and 39K(p, α0) reactions in the backscattering geometry

In the present work, new, differential cross-section values are presented for the ^natK(p, p₀) reaction in the energy range E_lab = 3000–5000 keV (with an energy step of 25 keV) and for detector angles between 140° and 170° (with an angular step of 10°). A qualitative discussion of the observed cross-section variations through the influence of strong, closely spaced resonances in the p + ³⁹K system is also presented. Information has also been extracted concerning the ³⁹K(p,α₀) reaction for E_lab = 4000–5000 keV in the same angular range. As a result, more than ～500 data points will soon be available to the scientific community through IBANDL (Ion Beam Analysis Nuclear Data Library – http://www-nds.iaea.org/ibandl/) and could thus be incorporated in widely used IBA algorithms (e.g. SIMNRA, WINDF, etc.) for potassium depth profiling at relatively high proton beam energies.