Primary Ion Beam Research Articles

Stigmatic imaging of secondary ions in MeV-SIMS spectrometry by linear Time-of-Flight mass spectrometer and the TimePix detector

Secondary ion mass spectrometry (SIMS), based on primary ions within the MeV energy domain, also known as MeV-SIMS, is a subject of increasing scientific interest. The main drive for the interest in the development of MeV-SIMS is the ability to desorb high yields of large non-fragmented organic molecular ions from the sample surface. This makes MeV-SIMS particulary useful in imaging of biological tissues.Imaging methods based on scanning a focused primary ion beam are associated with demanding focusing of the heavy energetic ions. As an alternative, stigmatic imaging mode has been studied here, applying point-to-point imaging characteristics of secondary ions in the linear Time-Of-Flight mass spectrometer. In stigmatic imaging approaches, spatial resolution is independent of the focussed spot size of the ionising primary ion beam, but instead dependant on the ability of the ion optics to project an image of the ion distributions removed from the surface onto a position sensitive ion detector.

A NanoSIMS 50 L Investigation into Improving the Precision and Accuracy of the 235U/238U Ratio Determination by Using the Molecular 235U16O and 238U16O Secondary Ions

A NanoSIMS 50 L was used to study the relationship between the 235U/238U atomic and 235U16O/238U16O molecular uranium isotope ratios determined from a variety of uranium compounds (UO2, UO2F2, UO3, UO2(NO3)2·6(H2O), and UF4) and silicates (NIST-610 glass and the Plesovice zircon reference materials, both containing µg/g uranium). Because there is typically a greater abundance of 235U16O+ and 238U16O+ molecular secondary ions than 235U+ and 238U+ atomic ions when uranium-bearing materials are sputtered with an oxygen primary ion beam, the goal was to understand whether use of 235U16O/238U16O has the potential for improved accuracy and precision when compared to the 235U/238U ratio. The UO2 and silicate reference materials showed the greatest potential for improved accuracy and precision through use of the 235U16O/238U16O ratio as compared to the 235U/238U ratio. For the UO2, which was investigated at a variety of primary beam currents, and the silicate reference materials, which were only investigated using a single primary beam current, this improvement was especially pronounced at low 235U+ count rates. In contrast, comparison of the 235U16O/238U16O ratio versus the 235U/238U ratio from the other uranium compounds clearly indicates that the 235U16O/238U16O ratio results in worse precision and accuracy. This behavior is based on the observation that the atomic (235U+ and 238U+) to molecular (235U16O+ and 238U16O+) secondary ion production rates remain internally consistent within the UO2 and silicate reference materials, whereas it is highly variable in the other uranium compounds. Efforts to understand the origin of this behavior suggest that irregular sample surface topography, and/or molecular interferences arising from the manner in which the UO2F2, UO3, UO2(NO3)2·6(H2O), and UF4 were prepared, may be a major contributing factor to the inconsistent relationship between the observed atomic and molecular secondary ion yields. Overall, the results suggest that for certain bulk compositions, use of the 235U16O/238U16O may be a viable approach to improving the precision and accuracy in situations where a relatively low 235U+ count rate is expected.

0.12–0.54 MeV C60 ion impacts on a poly(methyl methacrylate) target: Characterization through emission properties of negative secondary ions

The impacts of C60 ions on a poly(methyl methacrylate) target in the energy range of 0.12–0.54 MeV were characterized by analysis of negative secondary ion (N-SI) counting data measured using a time-of-flight secondary ion mass spectrometer combined with pulsed primary C60 ion beams in an event-by-event counting mode. Analysis of the counting data was performed to obtain probability distributions of emitted N-SI numbers for one impact and the impact energy dependence of relative emission yields for selected N-SIs species. These emission properties obtained could be qualitatively explained in terms of the characteristics of the emission processes and ion track structures reported for C60 impacts.

Study of slowing down and thermalization of externally injected ion beams in electron cyclotron resonance ion source plasmas

Electron cyclotron resonance ion source based charge breeder is a promising way to produce rare isotope beams with a high charge state, based on capture and thermalisation of the primary ion beam by the ECR plasma. A simulation technique based on a Monte Carlo collision operator is developed to study the capture of externally injected ion beams in ion source plasmas. To illustrate the utility of the method, we have studied the dynamics of Ar1+ ions in oxygen plasma. Evolution of statistically averaged quantities such as the beam size, average directional velocity, and rms velocity is studied for various beam parameters. It is observed that the number of undamped particles and the steady state beam size increase with the increase in beam emittance. The simulation result indicates that the capture and thermalisation of the ion beam are affected by the kinetic energy and emittance of the beam.Electron cyclotron resonance ion source based charge breeder is a promising way to produce rare isotope beams with a high charge state, based on capture and thermalisation of the primary ion beam by the ECR plasma. A simulation technique based on a Monte Carlo collision operator is developed to study the capture of externally injected ion beams in ion source plasmas. To illustrate the utility of the method, we have studied the dynamics of Ar1+ ions in oxygen plasma. Evolution of statistically averaged quantities such as the beam size, average directional velocity, and rms velocity is studied for various beam parameters. It is observed that the number of undamped particles and the steady state beam size increase with the increase in beam emittance. The simulation result indicates that the capture and thermalisation of the ion beam are affected by the kinetic energy and emittance of the beam.

The Crab Pulsar: Origin of the Crab Nebula’s Radio Pairs

Abstract Previously, we constructed a model—essentially a plausibility argument—in which the Crab Pulsar produces a spatially separated ion dominated and pair plasma dominated, magnetically striped relativistic wind, with the ion wind’s kinetic energy and electromagnetic Poynting fluxes being comparable. In this paper, the polar cap ion–photon pair production of that model is replaced with pair production by ion curvature synchrotron photons. The first primary ion curvature photons, and, contrary to conventional wisdom, also the first primary electron curvature photons, do not immediately convert into pairs. The primary beam particles continue to accelerate, and the actual photons that convert into pairs, which then short out the parallel electric field and terminate the acceleration, are produced by the further accelerated, higher energy particles. Simple estimates of the ensuing pair production cascade give pair multiplicities—the number of pairs per primary beam particle—of M ± ≈ 6–8 × 104, comparable to standard calculations, but much less than the 3 × 106 value deduced by Rees and Gunn in order to sustain the Crab Nebula’s N ± ≈ 1051 radio-emitting pairs against adiabatic expansion energy losses. Using a simple spin-down evolution model for the pulsar’s rotation frequency, the time-integrated pair cascade production driven by the primary ion beam can produce the N ± ≈ 1051 radio pairs, whereas the primary electron beam produces about an order of magnitude fewer pairs.

Defect sizing, separation, and substrate effects in ion-irradiated monolayer two-dimensional materials

Precise and scalable defect engineering of 2D nanomaterials is acutely sought-after in contemporary materials science. Here we present defect engineering in monolayer graphene and molybdenum disulfide (MoS$_2$) by irradiation with noble gas ions at 30 keV. Two ion species of different masses were used in a gas field ion source microscope: helium (He$^+$) and neon (Ne$^+$). A detailed study of the introduced defect sizes and resulting inter-defect distance with escalating ion dose was performed using Raman spectroscopy. Expanding on existing models, we found that the average defect size is considerably smaller for supported than freestanding graphene and that the rate of defect production is larger. We conclude that secondary atoms from the substrate play a significant role in defect production, creating smaller defects relative to those created by the primary ion beam. Furthermore, a similar model was also applied to supported MoS$_2$, another promising member of the 2D material family. Defect yields for both ions were obtained for MoS$_2$, demonstrating their different interaction with the material and facilitating comparison with other irradiation conditions in the literature.

The influence of polyatomic primary ion chemistry on matrix effects in secondary ion mass spectrometry analysis

The application of mass spectrometry imaging techniques to determine two- (2D) and three- (3D) dimensional chemical distribution ideally provides uniform, high sensitivity to multiple components and reliable quantification. These criteria are typically not met due to variations in sensitivity due to the chemistry of the analyte and surrounding surface chemistry. Here we explore the influence of projectile beam chemistry and sample chemistry in time-of-flight secondary ion mass spectrometry (TOF-SIMS). To the authors' knowledge this is the first time the combined effects of projectile chemistry and sample environment on the quantitative determination of mixed samples have been systematically studied. Secondary ion yields of lipid and amino acid mixtures were measured under 20 keV C60 , Arn , and (H2 O)n cluster ion bombardment (n= 2000 or 4000) using TOF-SIMS. Ion suppression/enhancement effects were studied in dry sample films and in trehalose and water ice matrices. The extent of the matrix effects and the secondary ion yield were found to depend on the chemistry of the primary ion beam and (for C60 , Arn ) on the nature of the sample matrix. Under (H2 O)n bombardment the sample matrix had negligible effect on the analysis. Compared with C60 and Arn , water-containing cluster projectiles enhanced the sensitivity of TOF-SIMS determination of the chosen analytes and reduced the effect of signal suppression/enhancement in multicomponent samples and in different sample matrices. One possible explanation for this is that the (H2 O)4000 projectile initiates on impact a nanoscale matrix environment that is very similar to that in frozen-hydrated samples in terms of the resulting ionisation effects. The competition between analytes for protons and the effect of the sample matrix are reduced with water-containing cluster projectiles. These chemically reactive projectile beams have improved characteristics for quantitative chemical imaging by TOF-SIMS compared with their non-reactive counterparts.

A-Crater-within-a-Crater Approach for Secondary Ion Mass Spectrometry Evaluation of the Quality of Interfaces of Multilayer Devices.

Further development and optimization of modern optoelectronic devices requires fast and reliable procedures that may evaluate the quality of interfaces. For thick multilayer devices, mixing effect may significantly prevent proper interpretation of secondary ion mass spectrometry depth profiles especially if a region of interest is located far from the sample surface. In this work, we present how to overcome this problem with a so-called a-crater-within-a-crater approach. In this notion, a high energetic primary ion beam is used to rapidly remove most of the material forming a large crater. Then, the energy is significantly reduced and a new smaller crater is formed at the bottom of the previous one. Close to the region of interest, the impact energy is decreased to 150 eV and thus an interface can be analyzed with minimal mixing effect and thus its quality can be adequately assessed. Usefulness of this approach is tested on an epitaxial structure of a triple-junction solar cell and reliable information about the structure imperfection has been obtained: p and n dopants in the tunnel junction overlapped, deteriorating the operation of the device.

Cluster ion beam generation from a wetted needle emitter for organic secondary ion mass spectrometry (organic SIMS) using a protic ionic liquid, propylammonium nitrate.

Propylammonium nitrate (PAN, [C3 H7 NH3 ][NO3 ]), a protic ionic liquid, has an active proton in its molecular structure, so that it can promote protonation. In addition, PAN has high aggregability, so that it tends to form large aggregated (or cluster) ions. These features will be desirable for cluster ion beams in secondary ion mass spectrometry (SIMS) of organic materials. The aggregability may enable us to generate a cluster ion beam from a needle tip wetted with PAN by electrospray in vacuum. For these reasons, cluster ion beam generation was investigated using an externally wetted needle emitter. A sharpened glass rod was used as a needle emitter. PAN was electrosprayed in vacuum using the needle emitter to generate a cluster ion beam. Beam characteristics were investigated with an apparatus for measuring transient responses of a beam current. SIMS experiments were also performed using the cluster ion beam as a primary ion beam; arginine and polyethylene glycol (PEG300) were analyzed. A stable cluster ion beam was generated from the needle emitter wetted with PAN. The ion beam consisted of mixed cluster ions whose m/z ranged from about 180 to 5000 or higher. The cluster ion beam successfully produced protonated molecules [M + H]+ (M denotes arginine and PEG molecules) with relatively little fragmentation. Adduct ions [M + C3 H7 NH3 ]+ formed by propylammonium-attachment reaction were also detected for PEG. It has been demonstrated that a needle emitter wetted with PAN can generate a cluster ion beam that includes massive cluster ions. The cluster ion beam proved to be helpful in producing molecular secondary ions and suitable for a primary ion beam in organic SIMS.

Emission Theory of Amorphous Material Sputtering: the Dependence of the Sputtering Coefficient on the Angle of Primary Ion Beam Incidence

The dependences of the sputtering coefficient on the types of accelerated ions, their energy, and the angle of incidence on a target are calculated. For Ar–Si, Хе–С, and H–W systems, acceptable coincidence between the calculated and experimental data is obtained. Two mechanisms for secondary-particle ejection from solids are established; they determine the dependences of the base sputtering coefficient for the base on the energy and the angle of primary ion incidence on the target.

Benefits of NaCl addition for time-of-flight secondary ion mass spectrometry analysis including the discrimination of diacylglyceride and triacylglyceride ions.

Diacylglycerides (DAGs) and triacylglycerides (TAGs) are two important lipid classes present in all mammalian cells that share similar chemical structures but differ in biological function in cells and tissues. Differentiation of these two species during time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis is therefore important, but has been difficult due to the formation of DAG-like ions during the ionization process of TAGs. We investigated the use of salt adduct formation as a quick and simple method to determine the origin of the DAG-like ions in ToF-SIMS spectra. NaCl was added to lipid standards of a DAG and a TAG and differences in fragmentation patterns were identified. The salt was then applied to prepared tissue samples by spraying with a saturated solution of NaCl in methanol and samples were analysed with ToF-SIMS using a 40keV (CO2 )6k + primary ion beam. A 40Da peak shift was observed in the DAG spectrum that was not observed in the TAG spectrum ([M+H-H2 O]+ to [M+Na]+ ) while the isobaric [M-RCOO]+ peak did not shift allowing differentiation between the two species. Spraying NaCl on to tissue sections indicated that the DAG-like ions originated from TAGs. With the method described in this paper, simple addition of salt by spraying on the sample leads to better interpretation of complex mass spectra from biological tissue samples, discriminating DAG and TAG fragment peaks.

Development of a Vacuum Electrospray Droplet Ion Gun for Secondary Ion Mass Spectrometry.

Atmospheric pressure electrospray had been used in previous studies to generate massive water droplet ion beams, and the beams successfully achieved efficient desorption/ionization of biomolecules, low damage etching of polymers and nonselective etching of metal oxides. However, this droplet ion beam was not practical as a primary ion beam for surface analysis instruments because it required differential pumping and lacked adequate beam current and density. To improve the beam performance, we have proposed to use vacuum electrospray of aqueous solutions as a beam source, and developed a technique for producing a stable electrospray of aqueous solution in vacuum. We also designed a prototype of a vacuum electrospray droplet ion gun, and measured the beam properties. Finally, the applicability of this ion gun in secondary ion mass spectrometry is discussed.

ToF-SIMS analysis of an organic layer using toluene and its cluster ion beam projectiles generated by multiphoton ionization

Secondary ion mass spectrometry (SIMS) has become a powerful tool for a chemical imaging of surfaces. In particular, cluster ion beams have been widely implemented with SIMS using spatial distributions of molecules as an analysis and imaging method for a sample composed of organic molecules. In this study, we developed a home-built time-of-flight (ToF)-SIMS instrument and generated organic molecular cluster ion beams ionized by multiphoton ionization (MPI) as a primary ion beam. Further, the sample stage loaded with rhodamine 6G was bombarded with size-controlled toluene cluster ions to produce secondary ions. Mass spectra of rhodamine 6G were acquired as a function of the size of toluene cluster ions. We plotted the fragments-to-parent ion ratio against each size of cluster ion, and we found that an intact rhodamine 6G signal would be observed with less fragmentations using (Tol)n > 18+. We expect this combination of the organic molecular cluster ion beam generated by MPI and ToF-SIMS to greatly expand the field of surface analysis.

Enhancement of Intact Ion Yields in ToF-SIMS Using Molecular Cluster Ion Beams

Recently, water and some other molecules are applied to the massive cluster projectiles for SIMS to improve the secondary ion yields by chemical effects. The ionization probability should be influenced by the properties of the primary ion beams as well as the chemical nature of the sample surface. However, the mechanisms of ionization by the large cluster collision is still unclear. In this study, we have generated water, methanol and methane molecular cluster ion beams and investigated the secondary ion emission from amino acid thin films bombarded by the molecular cluster ion projectiles.

Analysis of residual activity at the FRIB linear accelerator

The Facility for Rare Isotope Beams (FRIB) is an accelerator facility being established at Michigan State University (MSU). The facility will utilize a broad range of primary ion beams from 16O to 238U with a beam power of up to 400 kW and energy of 200 MeV/u for 238U in its baseline configuration to produce rare isotopes. A possible facility upgrade will include an increase of the beam energy up to 400 MeV/u for 238U and addition of new light ion beams down to 3He and protons for Isotope Separation Online (ISOL) operations. The FRIB double-folded linear accelerator will accommodate several high-rate beam loss devices. The residual activity of these devices were analysed and results are presented in this work.

Application of ‘Storing Matter’ technique in SIMS depth profile analysis

Secondary ion mass spectrometry (SIMS) is a very useful technique for the analysis of layered systems. It is based on the primary ion beam sputtering of solids and mass analysis of the emitted secondary ions. A main limitation of this technique results from the direct quantitative analysis, since the ionization efficiency of a given atom is highly influenced by the neighbouring atoms at the surface. This phenomenon is known as the ‘matrix effect’.This problem can be partially solved by separation of the sputtering and ionization processes. The ‘Storing Matter’ technique involves deposition of the sputtered matter onto a rotating collector. Subsequently, ionisation occurs during the ion beam bombardment of the stored matter.In case of sputtering of a given layered structure, the stored matter deposit presents the sequence of the deposition process and the analysis of this deposit reflects the investigated structure.We analyse a set of on oxidized steel samples and oxidized steel covered by aluminium. These types of structures are known for strong matrix effects at the metal oxide interfaces. The results of the ‘Storing Mater’ analyses are compared with the results of the classical SIMS analysis.

Quantitative depth distribution analysis of elements in high alloy steel using MCs+-SIMS approach

Elemental composition analysis in alloy samples using secondary ion mass spectrometry (SIMS) suffers from matrix effects which hamper the analytical results. These matrix effects arise due to numerous factors like change in surface work function due to primary ion beam implantation, presence of reactive chemical species in the samples or on the sample surface, sample composition, chamber pressure etc. which changes the ionization probability of secondary ion species and hence produces non-linear effects in the production of secondary ions. In order to minimize these effects, Cs+ primary ion beam in conjunction with MCs+ (M being the element of interest) secondary ion monitoring (MCs+-SIMS) approach has been used. The MCs+-SIMS approach has been extensively used for quantitative depth distribution analysis in semiconductor samples but limited reports are available for the analysis of alloys sample. In the present work, the MCs+-SIMS approach has been utilized to determine the chemical composition and quantitative depth distribution of multiple elements (C, Si, Ni, Mn, Cr, Co, Mo and Fe) in high alloy steel samples. Quantitative depth distribution analysis revealed a non-homogenous distribution of elements on the steel surface which can be due to the presence of surface oxide layers. The minor and major constituents such as Cr, Mo; Mn, Ni; and Si; were determined with relative error of <2%, 6% and 5%, respectively. Surface distribution analyses of these elements were also carried out using both Cs+ and O2+ primary ion beams. An interesting and contrasting feature has been observed for surface distribution analysis carried out with MCs+ as compared to M+ secondary ion detection mode. It was observed that the surface distribution patterns of non-homogenous elements showed homogenous distribution patterns in case of MCs+ secondary ion monitoring approach. This anomaly can be attributed to the difference in formation mechanism of MCs+ and M+ secondary ions.

Low-energy bismuth primary ion beams for molecular ion detection in TOF-SIMS

Low-energy (sub-keV) bismuth primary ion beams were used for time-of-flight secondary ion mass spectrometry (ToF-SIMS) to investigate their suitability for detecting molecular ions in organic materials. In this study, a 2 keV Bi3+ (667 eV/atom) primary ion beam was compared with a 54 keV Bi3++ (18 keV/atom) beam. The 2 keV Bi3+ beam detected much less fragmentation for some organic materials compared with the 54 keV Bi3++ beam. Atomic level fragmentation was most notably suppressed by the 2 keV Bi3+ beam and deprotonated molecule [M-H+] was also significantly suppressed. As a result, intact molecule (M+) and, interestingly, bismuth-attached secondary ion [M+Bi]+ were observed instead of the protonated molecule [M+H]+ using the 2 keV Bi3+ beam. Results suggest that low-energy bismuth primary ion beams are potential candidates for molecular ion detection in ToF-SIMS.

Measurement of the 234U(n, f ) cross-section with quasi-monoenergetic beams in the keV and MeV range using a Micromegas detector assembly

The 234U neutron-induced fission cross-section has been measured at incident neutron energies of 452, 550, 651 keV and 7.5, 8.7, 10 MeV using the 7Li (p, n) and the 2H(d, n) reactions, respectively, relative to the 235U(n, f ) and 238U(n, f ) reference reactions. The measurement was performed at the neutron beam facility of the National Center for Scientific Research “Demokritos”, using a set-up based on Micromegas detectors. The active mass of the actinide samples and the corresponding impurities were determined via $ \alpha$ -spectroscopy using a surface barrier silicon detector. The neutron spectra intercepted by the actinide samples have been thoroughly studied by coupling the NeuSDesc and MCNP5 codes, taking into account the energy and angular straggling of the primary ion beams in the neutron source targets in addition to contributions from competing reactions (e.g. deuteron break-up) and neutron scattering in the surrounding materials. Auxiliary Monte Carlo simulations were performed making combined use of the FLUKA and GEF codes, focusing particularly on the determination of the fission fragment detection efficiency. The developed methodology and the final results are presented.

An investigation of the beam damage effect on in situ liquid secondary ion mass spectrometry analysis.

During in situ liquid secondary ion mass spectrometry (SIMS) analysis, the primary ion beam is normally scanned on a very small area to collect signals with high ion doses (1014 to 1016 ions/cm2 ). As a result, beam damage may become a concern when compared with the static limit of SIMS analysis, in which the dose is normally less than 1012 ions/cm2 . Therefore, a comparison of ion yields in in situ liquid SIMS analysis versus traditional static SIMS analysis of corresponding dry samples is of great interest. In this study, a dipalmitoylphosphatidylcholine (DPPC) liposome solution was used as a model system. Both liquid sample and dry sample were examined. Secondary ion yields using three primary ion species (Bi+ , Bi3+ and Bi3++ ) with various beam currents were investigated. Usable ion yields for both positive and negative characteristic signals (including molecular ions and characteristic fragment ions) were achievable based on optimized experimental conditions for in situ liquid SIMS analysis. The ion yield of the key DPPC molecular ion was comparable to that of traditional static SIMS, and unexpected low fragmentation was observed. The flexible structure of the liquid plays an important role for these observations. Therefore, beam damage may not be a concern in in situ liquid SIMS analysis if proper experimental conditions are used.