Sputter Ion Source Research Articles

Unimolecular processes in diatomic carbon anions at high rotational excitation

On the millisecond to second time scale, stored beams of diatomic carbon anions C2− from a sputter ion source feature unimolecular decay of yet unexplained origin by electron emission and fragmentation. To account for the magnitude and time dependence of the experimental rates, levels with high rotational and vibrational excitation are modeled for the lowest electronic states of C2−, also including the lowest quartet potential. Energies, spontaneous radiative decay rates (including spin-forbidden quartet-level decay), and tunneling dissociation rates are determined for a large number of highly excited C2− levels and their population in sputter-type ion sources is considered. For the quartet levels, the stability against autodetachment is addressed and recently calculated rates of rotationally assisted autodetachment are applied. Nonadiabatic vibrational autodetachment rates of high vibrational levels in the doublet C2− ground potential are also calculated. The results are combined to model the experimental unimolecular decay signals. Comparison of the modeled to the experimental rates measured at the cryogenic storage ring facility CSR gives strong evidence that C2− ions in quasistable levels of the quartet electronic states are the so far unidentified source of unimolecular decay. Published by the American Physical Society 2024

Recent experiments on photo-assisted negative ion production in caesium sputter ion source at the JYFL Pelletron facility

We have measured the effect of a 15 W, 445 nm laser on Br− beam current produced by a SNICS ion source by systematically varying the ion source parameters, most importantly the target holder material, and temperatures of the ioniser and Cs oven. The target holder material has no significant effect on the observed photoenhancement of the beam current. The evidence shows that the laser allows reaching higher beam currents at low ioniser temperatures compared to running the source without the laser but at higher ioniser temperature. We have observed a priming effect, i.e. applying a number of laser pulses at certain ion source settings causes the beam current measured without the laser to increase significantly and remain high after ceasing the laser pulsing. These observations suggest that the photo-assisted effect is related to changes of the cathode caesium coverage. Finally, we report the results of our first attempts to sustain a stable beam current at the elevated level over several hours, achieved through active control of the laser power.

IMPROVING IONPLUS MICADAS PERFORMANCE WITH RECESSED GRAPHITE

ABSTRACT Cathodes with recessed sample surfaces have several benefits in cesium sputter ion sources, including higher output, more efficient use of sample material, and improved focusing of the extracted ion beam. However, the Ionplus MICADAS uses cathodes with a graphite surface that is essentially flush with the sample holder. To evaluate the performance of recessed graphite with the MICADAS and determine the optimal surface depth, we tested four different depths, including the standard (flush) pressing method, 0.5 mm, 1.0 mm, and 1.5 mm. We found that recessed depths of 1.0 and 1.5 mm resulted in 20% higher ion beam current compared to the standard method under the same source conditions. The results are consistent with the beam produced from the recessed targets being more narrowly focused with a lower emittance, resulting in better transmission through the accelerator. Small graphite samples (200 µg C) with recessed surfaces produced higher currents for longer, leading to a 2–3× increase in sample ionization efficiency. Additionally, there was some evidence that isotopic ratio measurements of recessed samples were more stable over time. Overall, samples recessed to 1 mm depth offered numerous advantages over the standard pressing method and we have subsequently started pressing all MICADAS graphite using this approach.

Underground nuclear astrophysics: Status and recent results from Felsenkeller laboratory

For almost three decades it has been known that the study of astro-physically important nuclear reactions between stable nuclei requires the use of low-background, underground accelerator laboratories. The Felsenkeller shallow-underground laboratory in Dresden, shielded by a 45 m thick rock cover, hosts a 5 MV Pelletron ion accelerator with an external sputter ion source (mainly able to provide carbon and oxygen beams) and an internal radio-frequency ion source (providing proton and alpha beams). The reduced muon, neutron and gamma-ray background achieved both with natural and active shielding situate the laboratory well in line with deep underground accelerator labs worldwide and allows highly sensitive nuclear reaction experiments. Currently, measurements affecting the solar fusion and Big Bang nucleosynthesis are ongoing. In addition to in-house research by HZDR and TU Dresden, the lab is an open facility for scientific users worldwide, with beam time applications reviewed by an independent science advisory board. Furthermore, EU-supported transnational access is available via the ChETEC- INFRA network for nuclear astrophysics. A brief introduction to underground nuclear astrophysics, status of the Felsenkeller shallow-underground laboratory and some preliminary results are discussed.

The new 300 kV multi-element AMS system at the TONO Geoscience Center, Japan Atomic Energy Agency

A new AMS system (HVEE 4103Bo-AMS) was installed at Tono Geoscience Center, Japan Atomic Energy Agency in November 2019 and AMS experiments started in 2020. The system consists of a SO-110C Cs sputtering ion source, a low-energy spectrometer with an ESA and an injection magnet, a 300 kV tandem accelerator, a high energy spectrometer with two analyzing magnets and an ESA, and a gas ionization chamber for rare isotope measurement. The system is designed for high-sensitivity detection of 10Be, 14C, 26Al and 129I. A comparison with our 5 MV Pelletron is also given in this paper.

Photo-enhanced O−, H− and Br− ion production in caesium sputter negative ion source—no evidence for resonant ion pair production

It has been proposed that the negative ion yield of a caesium sputter ion source could be enhanced by promoting neutral caesium atoms to electronically excited 7p states supporting resonant ion pair production. We have tested this hypothesis by illuminating the cathode of a caesium sputter ion source with an adjustable wavelength laser and measuring its effect on the extracted beam currents of O−, H− and Br− anions. The laser exposure causes the beam currents to increase but the effect is independent of the wavelength in the range of 440–460 nm, which leads us to conclude that there is no evidence for resonant ion pair production. The photon-induced beam current enhancement scales with the applied laser power and, depending on the ion source conditions, can more than double the extracted beam current. We present a qualitative explanation for the observed effect. The model, based on photoelectron emission and subsequent increase of the caesium sputtering rate, thus liberating negative ions from the cathode, is supported by the data demonstrating that the caesium sputter ion source can produce Br− beams without thermal surface ionization as a source of Cs+ ions, i.e. in external laser-driven mode without heating the surface ionizer.

Sputter ion source cathode pit reconstruction by stylus profilometer, optical techniques and X-ray micro computed tomography

In Caesium sputter ion sources of negative ions, SNICS, an accurate reconstruction of pit profiles is important to assess the efficiency of cathode material utilization to increase the output of negative ions. Highly accurate pit profile measurements in sputter cathodes are difficult to perform for geometries where the depth is large compared to the diameter. In this paper, optical confocal microscopy, stylus profilometry and X-ray micro-computer tomography are used successfully to reliably reconstruct such high aspect ratio cathode pit geometries. Simulations of Cs+ beam trajectories and calculated cross-sectional current density distributions are consistent with the observed pit profiles. Experimental results demonstrate the practicability of the proposed measurement techniques for SNICS cathodes and other structures with complex surface topographies.

Preliminary results of positive ion mass spectrometry based on a 2.45 GHz ECR ion source and a non-metallic gas target

Positive-ion mass spectrometry (PIMS) is a positive-to-negative ion conversion metrology that just operated on the opposite of traditional accelerator mass spectrometry (AMS) on high-precision radiocarbon dating. With a higher efficient simple structure plasma ion sources instead of AMS sputter ion sources, lower ion energy (tens keV) in place of MeV, and limited footprint facility, PIMS will be a powerful competitor to AMS in the future. To precisely measure 14C, interference of molecular isobars (such as 12CH2, 13CH) will be eliminated with higher charged ions (charge state ≥ 3; molecular cations 12CH2 2+ is metastable, can be eliminated by higher stripping pressure.), nitrogen will be eliminated inside the charge-exchange cell (CXC) through charge exchanging with target gas. To demonstrate the probability of PIMS with a 2.45 GHz ECR ion source, an experiment was launched at Peking University (PKU). A compact permanent magnet 2.45 GHz ECR ion source is designed to produce carbon ions from CO2. A CXC unit was installed after the extraction system and ethylene is chosen as charge exchange gas. Preliminary experiments prove that more than hundreds of microamp of C2+ beam can be produced with this ECR source. By injecting ethylene into CXC, several microamp C- beam was detected and the C2+ ions charge exchange efficiency to C- ions can reach close to 5%.

Effects of Cs+ and Arn+ ion bombardment on the damage of graphite crystals

Intercalation mechanisms and diffusion or segregation phenomena in graphitic materials play a crucial role in different applied science fields. The investigation of such phenomena is usually accomplished through depth profiling experiments. Ar-GCIBs (Argon- Gas Cluster Ion Beams) are commonly adopted for in-depth concentration profiling of organic or soft materials; on the other hand, cesium ions are in general more suitable for the sputtering of inorganics. During such experiments, the beam-target interaction could alter chemistry and structure of the material. In this work, we define the optimal conditions in terms of both sputtering ion source and energy to preserve the crystal features. HOPG was used as a model system to compare morphological, physical, and chemical effects induced by different Arn+ clusters, and ultra-low energy Cs+ beam during ToF-SIMS (Time of Flight Secondary Ion Mass Spectrometry) depth profiling experiments. We demonstrated, through in-situ AFM (Atomic Force Microscopy) analysis, that the monoatomic Cs+ beam alters to a lower extent the HOPG structure. On the contrary, Ar-GCIBs strongly modify the graphite surface basal plane and underlying layers. However, HOPG crystals treated with the cesium monoatomic source undergo a chemistry modification leading to the formation of graphite oxide (GOx) together with the presence of hydrogen, and cesium adducts.

Matrix-assisted production of actinide molecular anions for AMS

The multiple probable oxidation states of actinide elements can lead to their molecular anions to result in complicated yield patterns from a Cs+ sputter ion source. For actinides AMS, two methods can be practiced in order to concentrate the yields into just one or two anion species. The present common practice has been to avoid excessive oxygen reactions in the sputter targets, so that the mono-oxide anions can be produced with good or applicable ionization efficiencies. However, this method requires the typical 860-type Cs+ sputter ion sources to be operated with relatively high Cs+ fluxes, a condition that not all such sources in use can be operated with lasting stability. On the contrary, with low Cs+ fluxes, good or applicable ionization efficiencies can also be achieved for producing element-specific polyatomic oxide, oxyfluoride, or fluoride anions of actinides, if the sputter targets are aided with dominantly oxidizing or fluorinating matrices. This paper describes a systematic study of such phenomena using ZnO and FeF3 to form the respective oxidizing and fluorinating matrices. It was found that using the chosen set of polyatomic fluoride anions and the FeOxHy + FeF3 sputter targets, not only all rare actinides could be measured with low backgrounds matching those using mono-oxides, but also with the available ionization efficiencies fully utilized within a shorter time. A few additional merits are also noted.

Initial tests of 26Al fluoride target matrix on MILEA AMS system

In this study, the possibility of using a superhalogenide ion AlF4- extracted from Na3AlF6 based target materials for 26Al/27Al measurements by Accelerator Mass Spectrometry (AMS) was investigated. Determination of 26Al by usual mass spectrometric methods is generally hampered by the presence of the isobar 26Mg. However, the AMS method enables significant suppression of this interference, particularly by negative ion formation. After initial current tests in a caesium sputtering ion source of the Tandetron system, performance of the fluoride materials was analysed on the MILEA AMS system at ETH Zürich. The AlF4- current was used to evaluate the performance of the respective samples. Additives as PbF2 were tested to increase the extracted ion currents and samples containing MgF2 were used to investigate the presence of isobaric ions. Subsequently, the ionization efficiency of AlF4- was determined by recording the 27Al2+ current on the high energy side and the signals of 26Al2+ were investigated with the gas ionization detector.

The relative yields of trace-element-carbide anions from a Cs+-sputtered graphite target and their use for evaluating the long-term ion source memory in AMS

Using a solid graphite sample the relative yields of trace-element-carbide molecular anions, containing between 1 and 8 carbon atoms (MC1−8−), from a Cs+ sputter ion source were surveyed for elements (M) across the periodic table. Except for gaseous and radioactive atoms, all elements are present in the graphite at some trace levels, so this investigation could readily be done using AMS by counting unambiguously a selected Mq+ ion, with charge state q ≥ 3, from a MCn− ion beam injected into the accelerator. MC1,2− are found to be the most prolific carbide negative ions for the majority of elements, but Cs is clearly exceptional as CsC4,6,8− are the dominant ones instead. These results are consistent with past observations. Based on the same MC1−8− survey data, the relative elemental abundances in the graphite sample were also assessed. These abundances demonstrated a means for evaluating the basic long-term ion source memory background for any rare atom searches by AMS.

Experimental evidence on photo-assisted O− ion production from Al2O3 cathode in cesium sputter negative ion source

The production of negative ions in cesium sputter ion sources is generally considered to be a pure surface process. It has been recently proposed that ion pair production could explain the higher-than-expected beam currents extracted from these ion sources, therefore opening the door for laser-assisted enhancement of the negative ion yield. We have tested this hypothesis by measuring the effect of various pulsed diode lasers on the O− beam current produced from Al2O3 cathode of a cesium sputter ion source. It is expected that the ion pair production of O− requires populating the 5d electronic states of neutral cesium, thus implying that the process should be provoked only with specific wavelengths. Our experimental results provide evidence for the existence of a wavelength-dependent photo-assisted effect but cast doubt on its alleged resonant nature as the prompt enhancement of beam current can be observed with laser wavelengths exceeding a threshold photon energy. The beam current transients observed during the laser pulses suggest that the magnitude and longevity of the beam current enhancement depends on the cesium balance on the cathode surface. We conclude that the photo-assisted negative ion production could be of practical importance as it can more than double the extracted beam current under certain operational settings of the ion source.

Accelerator mass spectrometry: a remarkable week in May 1977

During a week in May 1977, solutions were found to the forty-year old problem of radiocarbon dating using mass spectrometry, as well as for the generalized mass spectrometry of measuring the abundances of extremely rare radioactive isotopes, such as long lived 236U. These solutions will be described together with a historical introduction, which points out the inherent advantages of some of the equipment developed for nuclear physics research that led towards the new mass spectrometry. In addition, the further developments, in what later came to be called accelerator mass spectrometry or AMS, can now be seen to have been the need to develop a better understanding of the following problems. (1) The appropriate atomic and molecular ion properties in order to expedite the separation of isobars. (2) The properties of the newer versions of the caesium sputter negative ion source that was hoped would almost eliminate the ion source memory effect. (3) The use of an improved accuracy tandem accelerator high voltage control for analysing very low final ion fluxes. Finally, (4) there was the need to assess the possible low level radiocarbon and other backgrounds of the accelerators or devices used to assist the isobar separation, after the accelerators had been used previously for many years of nuclear reaction studies. These developments were needed in order to add atomic and molecular isobar separation to the mass spectrometry for the further development of accelerator mass spectrometry (AMS) with tandem accelerators. When the basic problems had been solved, the detection and the accurate measurement of important natural very rare long-lived radio-active isotopes, such as for the sub parts per trillion (14C/C < 10−12) of natural 14C, was facilitated after further research. It was shown that there are very low levels of the atomic isobar anion 14N− from the caesium solid sample sputter ion source and that the 14C could be detected in the presence of over 1010 times as many mass 14 molecular anion hydrides 13CH−. The first and second excited states of the N anion, N(1S)− & N(1D)–, were shown later to exist but with short enough lifetimes for them not to interfere with the detection of 14C− at well below the level of 10−15 of 12C. The residual 14C background for an MP tandem accelerator, previously used for nuclear reaction studies, as well as the ion source memory effect were also found to be smaller than about 14C/C ~ 10−15 after the accelerator voltage control problem had been solved. These aims were all achieved during a week in May 1977 at the University of Rochester, leading also to the promise of the further successful development of radiocarbon dating by ion counting and the natural abundance measurements of other rare long lived radioactive isotopes by mass spectroscopy with anions or cations. In addition, the ability to date much smaller quantities of carbon, by a factor near 1000, was a huge advantage of the new method, as expected. However, it took almost a further decade of research and development to equal the accuracy of the mature beta counting radiocarbon dating method. The elimination of molecular ions from the mass spectrometry was later shown to be the key to detecting very low levels of actinides where atomic isobars are usually absent. The development of AMS for many other long lived radioactive isotopes is still continuing, as is the gradually increasing understanding of the need for an accelerator in some but not in all cases.

Use of a hybrid sputtering ion source for the determination of the bio-derived carbon fraction in particulate matter

The development of analytical methods capable of discriminating between bio-derived and fossil-derived carbon in Particulate Matter (PM) is a crucial aspect in the general efforts of identifying effective strategies for health-risks mitigations. The large difference in the isotopic signature between bio-derived and fossil derived materials in term of the concentration of 14C the radioactive carbon isotope can be effectively used for this purpose. Indeed, though the detection and measurement of 14C by AMS (Accelerator Mass Spectrometry) is a well established methodology, the reduced amount of carbon in PM samples (typically in the μg range) requires the use of proper ion sources with a high efficiency. We present here the set-up developed at CEDAD-University of Salento as based on a gas-accepting hybrid ion source and an in-house designed gas handling interface. The performances of the system are also shown by presenting the firsts tests carried out on PM sampled on quartz filters in Lecce, Italy.

Comparison of two methods to determine 129I in charcoal cartridge samples by AMS

To investigate the short- and long-term accumulated emissions from a nuclear facility, charcoal air filter cartridges were exposed at a reactor site for seven days. The short-lived 131I adsorbed in these filters was measured by gamma counting immediately after the samples were retrieved. The very long-lived 129I was analyzed later by AMS. For this purpose, two methodologies were developed: an extraction method and direct analysis, i.e., inserting an aliquot of the filter carbon directly into the target piece. The former method used a system designed to avoid sample loss resulting from the high I2 species volatility. Iodine-125 was used as a tracer to monitor the efficiency of this extraction process. The direct method was based on our experience with charcoal in the analysis of 14C in Cs sputter ion sources. In this direct analysis, charcoal samples were first spiked with a known quantity of stable iodine (127I), in an excess over the quantity naturally present in the filter, and then pressed into targets. In this way, 129I concentrations from all the charcoal samples were quickly determined. In both methods, it was found that the atom concentration of 129I in these samples exceeded that of the 131I by 2–3 orders of magnitude. These studies demonstrate the effectiveness of the charcoal capture method as a diagnostic tool for monitoring the emissions from nuclear facilities. This approach may be extended for the detection of other long-lived radioactive isotopes, which would further expand the scope of such monitoring studies.

Ion-pair Ionization in CO2-fed Cesium Sputter Sources

ABSTRACTA collision-radiation model of the solid sample cesium sputter ion source led to the rediscovery of anion production by ion-pair production. The model revealed physical processes that may produce high current outputs from such sources and suggested new ways of obtaining high outputs at lower heat and conductive stress to the source. Primary among these solutions is the electron excitation of primary Cs0 recycled from the sample to provide states that efficiently create chosen anions. Here we look at how the processes might apply to gas-fed ion sources.

Background in γ-ray detectors and carbon beam tests in the Felsenkeller shallow-underground accelerator laboratory

The relevant interaction energies for astrophysical radiative capture reactions are very low, much below the repulsive Coulomb barrier. This leads to low cross sections, low counting rates in $\gamma$-ray detectors, and therefore the need to perform such experiments at ion accelerators placed in underground settings, shielded from cosmic rays. Here, the feasibility of such experiments in the new shallow-underground accelerator laboratory in tunnels VIII and IX of the Felsenkeller site in Dresden, Germany, is evaluated. To this end, the no-beam background in three different types of germanium detectors, i.e. a Euroball/Miniball triple cluster and two large monolithic detectors, is measured over periods of 26-66 days. The cosmic-ray induced background is found to be reduced by a factor of 500-2400, by the combined effects of, first, the 140 meters water equivalent overburden attenuating the cosmic muon flux by a factor of 40, and second, scintillation veto detectors gating out most of the remaining muon-induced effects. The new background data are compared to spectra taken with the same detectors at the Earth's surface and at other underground sites. Subsequently, the beam intensity from the cesium sputter ion source installed in Felsenkeller has been studied over periods of several hours. Based on the background and beam intensity data reported here, for the example of the $^{12}$C($\alpha$,$\gamma$)$^{16}$O reaction it is shown that highly sensitive experiments will be possible.

A direct-AMS multi-isotope survey of uranium ore concentrates

Uranium ore concentrate (UOC) is an important nuclear material of interest for Canada. A large scale analytical program is being led by the Directorate of Security and Safeguards (DSS) of the Canadian Nuclear Safety Commission (CNSC) to establish a reference dataset of UOCs that have passed through and/or are currently under Canadian regulatory control. Isotopic ratios are among the signatures being captured under the reference dataset. Accelerator Mass Spectrometry (AMS) has been used for the measurement and assessment of 236U/238U and 187Os/188Os, respectively. Furthermore, since UOCs have uranium typically concentrated to ≥70% by weight, a direct-AMS assay method is possible wherein the samples can be measured without time consuming chemical digestion and processing. Using this direct-AMS approach, several related ratios (231Pa/238U, 230Th/238U, 226Ra/238U) were also assessed within the data acquisition sequence used for measuring the 236U/238U ratios, and 185Re, 187Re and 187Os, 188Os, 191Ir and 193Ir in the sequence for the 187Os/188Os ratios. The sum of these results can be shown together in a “bar-code” pattern to strengthen the capability for UOC source identification. Unexpectedly large 236U/238U ratios (up to ≥1 × 10−7) have been found in several UOC samples. The 187Os/188Os ratio has also been shown for the first time to be a viable supplementary signature of UOCs. This work shows that the direct-AMS method has the potential to become an effective tool for nuclear forensics provenance assessment applications with UOCs. The implications of the results, and the need for further refinement of the sputter target preparation, as well as the Cs+ sputter ion source itself, are also discussed.

A Preliminary ion Source Background Study at Lalonde AMS

ABSTRACTObservation of the ion source generated background has been an area of focus during our routine analytical work. It is noted that the results of very-low-ratio samples are dependent upon the particular procedures for measurement using the present-day Cs+ sputter ion sources. When measured without excessive Cs+ fluxes and without interleafing with other higher-ratio samples and references, the accelerator mass spectrometry (AMS) sensitivity can be somewhat improved. In some cases, it appears possible to assess old radiocarbon (14C) samples to beyond the long-standing 60 kyr limit. A number of observational studies are made for the sole purpose of minimizing the final contamination to the rare isotopes that is generated within the ion source.