MeV Ion Research Articles

Ion beam induced fluorescence imaging in biological systems

Imaging fluorescence generated by MeV ions in biological systems such as cells and tissue sections requires a high resolution beam (<100nm), a sensitive detection system and a fluorescent probe that has a high quantum efficiency and low bleaching rate. For cutting edge applications in bioimaging, the fluorescence imaging technique needs to break the optical diffraction limit allowing for sub-cellular structure to be visualized, leading to a better understanding of cellular function. In a nuclear microprobe this resolution requirement can be readily achieved utilizing low beam current techniques such as Scanning Transmission Ion Microscopy (STIM). In recent times, we have been able to extend this capability to fluorescence imaging through the development of a new high efficiency fluorescence detection system, and through the use of new novel fluorescent probes that are resistant to ion beam damage (bleaching). In this paper we demonstrate ion beam induced fluorescence imaging in several biological samples, highlighting the advantages and challenges associated with using this technique.

A gas ionisation Direct-STIM detector for MeV ion microscopy

Direct-Scanning Transmission Ion Microscopy (Direct-STIM) is a powerful technique that yields structural information in sub-cellular whole cell imaging. Usually, a Si p-i-n diode is used in Direct-STIM measurements as a detector. In order to overcome the detrimental effects of radiation damage which appears as a broadening in the energy resolution, we have developed a gas ionisation detector for use with a focused ion beam. The design is based on the ETH Frisch grid-less off-axis Geiger–Müller geometry. It is developed for use in a MeV ion microscope with a standard Oxford Microbeams triplet lens and scanning system. The design has a large available solid angle for other detectors (e.g. proton induced fluorescence). Here we report the performance for imaging ReNcells VM with μm resolution where energy resolutions of <24keV fwhm could be achieved for 1MeV protons using isobutane gas.

Kelvin probe characterization of buried graphitic microchannels in single-crystal diamond

In this work, we present an investigation by Kelvin Probe Microscopy (KPM) of buried graphitic microchannels fabricated in single-crystal diamond by direct MeV ion microbeam writing. Metal deposition of variable-thickness masks was adopted to implant channels with emerging endpoints and high temperature annealing was performed in order to induce the graphitization of the highly-damaged buried region. When an electrical current was flowing through the biased buried channel, the structure was clearly evidenced by KPM maps of the electrical potential of the surface region overlying the channel at increasing distances from the grounded electrode. The KPM profiling shows regions of opposite contrast located at different distances from the endpoints of the channel. This effect is attributed to the different electrical conduction properties of the surface and of the buried graphitic layer. The model adopted to interpret these KPM maps and profiles proved to be suitable for the electronic characterization of buried conductive channels, providing a non-invasive method to measure the local resistivity with a micrometer resolution. The results demonstrate the potential of the technique as a powerful diagnostic tool to monitor the functionality of all-carbon graphite/diamond devices to be fabricated by MeV ion beam lithography.

Electroluminescence from a diamond device with ion-beam-micromachined buried graphitic electrodes

Focused MeV ion microbeams are suitable tools for the direct writing of conductive graphitic channels buried in an insulating diamond bulk, as demonstrated in previous works with the fabrication of multi-electrode ionizing radiation detectors and cellular biosensors. In this work we investigate the suitability of the fabrication method for the electrical excitation of color centers in diamond. Differently from photoluminescence, electroluminescence requires an electrical current flowing through the diamond sub-gap states for the excitation of the color centers. With this purpose, buried graphitic electrodes with a spacing of 10μm were fabricated in the bulk of a detector-grade CVD single-crystal diamond sample using a scanning 1.8MeV He+ micro-beam. The current flowing in the gap region between the electrodes upon the application of a 450V bias voltage was exploited as the excitation pump for the electroluminescence of different types of color centers localized in the above-mentioned gap. The bright light emission was spatially mapped using a confocal optical microscopy setup. The spectral analysis of electroluminescence revealed the emission from neutrally-charged nitrogen-vacancy centers (NV0, λZPL=575nm), as well as from cluster crystal dislocations (A-band, λ=400–500nm). Moreover, an electroluminescence signal with appealing spectral features (sharp emission at room temperature, low phonon sidebands) from He-related defects was detected (λZPL=536.3nm, λZPL=560.5nm); a low and broad peak around λ=740nm was also observed and tentatively ascribed to Si-V or GR1 centers. These results pose interesting future perspectives for the fabrication of electrically-stimulated single-photon emitters in diamond for applications in quantum optics and quantum cryptography.

Thermoelectric Figures of Merit of Zn4Sb3 and Zrnisn-based Half-heusler Compounds Influenced by Mev Ion-beam Bombardments

Semiconducting β-Zn4Sb3 and ZrNiSn-based half-Heusler compound thin films with applications as thermoelectric (TE) materials were prepared using ion beam assisted deposition (IBAD). High-purity solid zinc (Zn) and antimony (Sb) were evaporated by electron beam to grow the β-Zn4Sb3 thin film while high-purity zirconium (Zr) powder and nickel (Ni) tin (Sn) powders were evaporated by electron beam to grow the ZrNiSn-based half-Heusler compound thin film. Rutherford backscattering spectrometry (RBS) was used to analyze the composition of the thin films. The grown thin films were subjected to 5 MeV Si ions bombardment for generation of nanostructures in the films. We measured the thermal conductivity, Seebeck coefficient, and electrical conductivity of these two systems before and after 5 MeV Si ions beam bombardment. The two material systems have been identified as promising TE materials for the application of thermal-to-electrical energy conversion, but the efficiency still limits their applications. The electronic energy deposited due to ionization in the track of MeV ion beam couldcause localized crystallization. The nanostructures produced by MeV ion beam can cause significant change in both the electrical and the thermal conductivity of thin films, thereby improving the efficiency. We used the 3ω-method (3rd harmonic) measurement system to measure the cross-plane thermal conductivity, the van der Pauw measurement system to measure the electrical conductivity, and the Seebeck-coefficient measurement system to measure the cross-plane Seebeck coefficient. The thermoelectric figures of merit of the two material systems were then derived by calculations using the measurement results. The MeV ion-beam bombardment was found to decrease the thermal conductivity of thin films and increase the efficiency of thermal-to-electrical energy conversion.

Novel nanoscale precipitate structures in ion-irradiated Cu1−xVx alloys: “Cherry-pit” formation

Abstract

MeV ion beam lithography of biocompatible halogenated Parylenes using aperture masks

Parylenes are poly(p-xylylene) polymers that are widely used as moisture barriers and in biomedicine because of their good biocompatibility. We have investigated MeV ion beam lithography using 16O+ ions for writing defined patterns in Parylene-C, which is evaluated as a coating material for the Cochlear Implant (CI) electrode array, a neuroprosthesis to treat some forms of deafness. Parylene-C and -F on silicon and glass substrates as well as 50μm thick PTFE were irradiated to different fluences (1×1013-1×1016 1MeV 16O+ ions cm−2) through aperture masks under high vacuum and a low pressure (<10−3mbar) oxygen atmosphere. Biocompatibility of the irradiated and unirradiated surfaces was tested by cell-counting to determine the proliferation of murine spiral ganglion cells. The results reveal that an oxygen ion beam can be used to pattern Parylene-C and -F without using a liquid solvent developer in a similar manner to PTFE but with a ∼25× smaller removal rate. Biocompatibility tests showed no difference in cell adhesion between irradiated and unirradiated areas or ion fluence dependence. Coating the Parylene surface with an adhesion-promoting protein mixture had a much greater effect on cell proliferation.

Time-of-flight mass spectrometry (TOFMS): From niche to mainstream

World War II electronic advances suggested the use of time-of-flight for mass measurements, leading to the development of a commercial TOFMS instrument by Bendix. This instrument used electron impact ionization, but the subsequent application of fission fragment bombardment (PDMS), coupled to TOFMS, was responsible for significant advances in the mass spectrometry of large biomolecules. Bombardment by accelerated MeV or keV ions, again coupled to TOFMS, was found to be equally successful. Although these TOF methods were the only ones initially capable of measuring the masses of really large biomolecules, most mass spectrometrists were still wedded to sector/quadrupole instruments for high/low-end mass measurements. The “coup de grace” for TOF (as generally believed) was the discovery of a suitable matrix (glycerol), which enabled the use of sector/quadrupole instruments for measurements of large biomolecules (“fabulous FAB”). Ironically, it was the discovery of suitable matrices for laser excitation that revived TOFMS, aided by the additional accuracy provided by the reflecting geometry, and the subsequent development of orthogonal injection, as well as the coupling to liquid chromatography. Enormous technical advances in TOF were driven by the invention of MALDI and facilitated by the parallel advances in digital electronics. All of mass spectrometry has benefitted from these technical advances, but TOF most of all because of the need for high speed acquisition and large data storage capacity. Thus TOF is now in the mainstream of biological applications of mass spectrometry.

Ion irradiated graphite exposed to fusion-relevant deuterium plasma

Graphite samples were irradiated with 5MeV carbon ions to simulate the damage caused by collision cascades from neutron irradiation in a fusion environment. The ion irradiated graphite samples were then exposed to a deuterium plasma in the linear plasma device, MAGPIE, for a total ion fluence of ∼1×1024ionsm−2. Raman and near edge X-ray absorption fine structure (NEXAFS) spectroscopy were used to characterize modifications to the graphitic structure. Ion irradiation was observed to decrease the graphitic content and induce disorder in the graphite. Subsequent plasma exposure decreased the graphitic content further. Structural and surface chemistry changes were observed to be greatest for the sample irradiated with the greatest fluence of MeV ions. D retention was measured using elastic recoil detection analysis and showed that ion irradiation increased the amount of retained deuterium in graphite by a factor of four.

Effects of MeV ion irradiation on structural and optical properties of SnO2–ZnO nanocomposites prepared by carbothermal evaporation

Nanocomposite thin films of SnO2–ZnO were prepared by carbothermal evaporation of mixture of Sn and ZnO without using any catalyst. The nanocomposite thin films were irradiated with 8MeV Si3+ ions at room temperature to fluence varying from 1×1014 to 1×1015ions/cm2. Effects of MeV ion irradiation on the structural and optical properties of SnO2–ZnO nanocomposites were studied using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) with energy dispersive X-ray spectroscopy, Photoluminescence spectroscopy (PL) and Raman spectroscopy. XRD studies revealed the presence of Sn, SnO2 and ZnO nanostructures in the as-deposited films. FESEM studies on the irradiated samples revealed the formation of ZnO nanosheets. A tentative growth mechanism underlying the formation of ZnO nanosheets is proposed. Ion irradiation resulted in a significant enhancement in the UV and defect emissions from ZnO and SnO2 nanostructures in the nanocomposite.

An analytical model for the mechanical deformation of locally graphitized diamond

We propose an analytical model to describe the mechanical deformation of single-crystal diamond following the local sub-superficial graphitization obtained by laser beams or MeV ion microbeam implantation. In this case, a local mass–density variation is generated at specific depths within the irradiated micrometric regions, which in turn leads to swelling effects and the development of corresponding mechanical stresses. Our model describes the constrained expansion of the locally damaged material and correctly predicts the surface deformation, as verified by comparing analytical results with experimental profilometry data and Finite Element simulations. The model can be adopted to easily evaluate the stress and strain fields in locally graphitized diamond in the design of microfabrication processes involving the use of focused ion/laser beams, for example to predict the potential formation of cracks, or to evaluate the influence of stress on the properties of opto-mechanical devices.

Surface morphology and deuterium retention of tungsten after low- and high-flux deuterium plasma exposure

The surface morphology and deuterium retention were investigated of polycrystalline tungsten targets that were exposed to deuterium plasmas at widely varying conditions. By changing only one parameter at a time, the isolated effects of flux, time and pre-damaging on surface modifications and deuterium retention were studied. The sample exposed to low-flux plasma (1020 m−2s−1) is mostly smooth with only a few areas containing very large blisters (50–500 µm). The samples exposed to high-flux plasmas (1024 m−2s−1) show large numbers of smaller blisters (1–10 µm) and in addition even smaller protrusions (<750 nm). The size of the blisters and their density strongly increase with fluence. Pre-damaging tungsten with MeV ions leads to less blisters but to more protrusions. In addition to these (sub-)micrometer-sized structures, all samples show formation of nanostructures. Comparison of a low-flux and high-flux sample exposed to similar fluence showed that the variation in morphology is dominated by the flux differences. It is shown that the blisters and protrusions originate in inter- and intra-granular cavities, respectively. The depth of the cavities underneath the surface correlates well with the depth distributions of the retained deuterium. Trapping of significant amounts of deuterium therefore seems to take place in and/or close to these cavities and gives rise to an additional peak in the thermal desorption spectrum at 700 K.

Stark shift and field ionization of arsenic donors in 28Si-silicon-on-insulator structures

We develop an efficient back gate for silicon-on-insulator (SOI) devices operating at cryogenic temperatures and measure the quadratic hyperfine Stark shift parameter of arsenic donors in isotopically purified 28Si-SOI layers using such structures. The back gate is implemented using MeV ion implantation through the SOI layer forming a metallic electrode in the handle wafer, enabling large and uniform electric fields up to 2 V/μm to be applied across the SOI layer. Utilizing this structure, we measure the Stark shift parameters of arsenic donors embedded in the 28Si-SOI layer and find a contact hyperfine Stark parameter of ηa = −1.9 ± 0.7 × 10−3 μm2/V2. We also demonstrate electric-field driven dopant ionization in the SOI device layer, measured by electron spin resonance.

Effect of bombardment with helium ions on the molecular-topological structure and elemental composition of the surface of polytetrafluoroethylene

Irradiation of a polytetrafluoroethylene (PTFE) film with accelerated (1–5 MeV) helium ions at a fluence of 1015 ion/cm2 has been studied. The efficiency of carbonization, defined as a decrease in the fluorine content and an increase in the carbon content on the surface of the polymer, increases with the incident ion energy. A characteristic feature of the topological structure of PTFE is the presence of four high-meltingpoint crystalline modifications, the “branching points” of the pseudo-network of the amorphous matrix block, in addition to a low-melting-point modification with the melting onset temperature of 13°C. After bombardment with 3–5 MeV ions, only two crystalline blocks remain in the polymer and a cluster block, which was not present in the unirradiated polymer, appears. The overall mass fraction of the crystalline structure in the irradiated polymer (0.48) is below that in the initial polymer (0.66), indicating amorphization of its structure.

MeV ion beams generated by intense pulsed laser monitored by Silicon Carbide detectors

The high energy ions produced with intense pulsed laser were analyzed with Silicon Carbide detectors. In order to realize high performances and radiation resistant detectors, high quality and thick epitaxial layer were grown on a substrate and a Schottky diodes were then realized. These detectors were employed to probe the plasma generated with a 300 ps laser at intensity of 1016 W/cm2 operating at Prague Asterix Laser System Laboratory. They show a fast response and a high sensitivity to high energy ions. Metallic and polymeric thin films were irradiated and the produced plasmas were monitored in forward and backward directions. The analysis of the time-of-flight spectra evidences the emission of protons and ions at different energies. The spectra were deconvolved with a shifted Maxwell Boltzmann distribution. In our experimental conditions we detected protons in the energy range 1.2 – 3.0 MeV and heavy ions between 1.0 MeV up to 40 MeV depending on the target and the laser energy. The results were compared with the ones obtained by Thompson Parabola Spectrometer.

Measurements and 3D Monte Carlo simulation of MeV ion transmission through conical glass capillaries

To investigate possible beam guiding and current density enhancement properties for conical glass capillaries for MeV ions, transmission measurements with well collimated ion beams of 1MeV He+ and 71MeV Xe19+ were done. No guiding effect was observed and only decent enhancement factors ε between 1 and 2 were measured and attributed to scattered particles. A 3D Monte Carlo simulation which is based on SRIM code was developed. The simulation reproduces the experimental results very well. This shows that the transmission properties of capillaries for MeV ion beams are governed by simple elastic scattering of the ions in the glass of the capillary.

Thermoelectric properties of Zn4Sb3/CeFe(4−)Co Sb12 nano-layered superlattices modified by MeV Si ion beam

We prepared multilayers of superlattice thin film system with 50 periodic alternating nano-layers of semiconducting half-Heusler β-Zn4Sb3 and skutterudite CeFe2Co2Sb12 compound thin films using ion beam assisted deposition (IBAD) with Au layers deposited on both sides as metal contacts. The deposited multilayer thin films have alternating layers about 5 nm thick. The total thickness of the multilayer system is 275 nm. The superlattices were then bombarded by 5 MeV Si ion at six different fluences to form nano-cluster structures. The film thicknesses and composition were monitored by Rutherford backscattering spectrometry (RBS) before and after MeV ion bombardment. We have measured the thermoelectric efficiency, Figure of Merit ZT, of the fabricated device by measuring the cross plane thermal conductivity by the 3rd harmonic (3ω) method, the cross plane Seebeck coefficient, and the electrical conductivity using the van der Pauw method before and after the MeV ion bombardments. We reached the remarkable thermoelectric Figure of Merit results at optimal fluences.

The variable extension of Saturn׳s electron radiation belts

Contrary to the permanent MeV ion belts which are relatively stable in intensity over both short and long time scales and are modulated by a single Galactic Cosmic Ray driven source, the electron belts of Saturn appear to be much more complex in both structure and temporal evolution. In order to understand the responses of this system to the different factors that may control it (internal or external/solar sources) we study its long-term, temporal evolution. We achieve that by tracking the equatorial distance of the belts׳ outer boundary, using MIMI/LEMMS energetic charged particle observations over a period of more than 7years. This boundary is defined at the distance that a selected count rate level is measured in a LEMMS channel that has the properties of an omnidirectional, integral energy detector. Simulated solar wind moments, energetic neutral atom (ENA) observations and solar irradiance data are used to support the analysis. In many cases, correlations of the different datasets are weak, suggesting that the electron belts are modulated in time scales that are much shorter than the sampling of the electron belt boundary (typically every 10–30days). Still, we find several cases of persistent, long term and strong perturbations in the system that appear to have corresponding disturbances in the extension of the electron belts, even on such long time scales. From the analysis of those intervals we believe that we have established a solid link with the planetary ring current as the primary source of the electron belts of Saturn. This is concluded mostly on the basis of an extended ring current decay in 2011 (inferred through ENA observations), coinciding with a similar, unusual drop in the electron belt extension (and intensity). This means that the electron belts should reflect also the modulation of the ring current. We suggest that possible sources of long term modulation are both the solar UV irradiance of the Saturnian thermosphere and the solar wind. The former appears to acquire large values and exhibit significant changes during the period of the electron belt/ring current decay, while an approximate 2-week periodicity (characteristic for the case of two solar wind streams per solar rotation) appears in the electron belts in the form of recurrent expansions or intensifications. The latter finding indicates that external forcing by the solar wind can have a considerable impact on the planet׳s electron belts, contrary to the expectations that Saturn׳s magnetospheric dynamics are rotationally dominated and internally driven.

Simulation of MeV ion transmission through glass micro-capillaries

A Monte-Carlo simulation of MeV ion transmission through glass micro-capillaries had successfully reproduced experimental energy and angular distributions obtained for 71 MeV Xe19+ ions on the IRRSUD beam line of GANIL as well as for MeV protons at ETH facility. The transmitted beam is composed of two parts: a core beam, due to direct transmission through the capillary, and a diffuse halo around, experiencing a large energy loss. The present calculation shows that the halo part is due to elastic scattering by the capillary wall.

Deuterium retention and near-surface modification of ion-irradiated diamond exposed to fusion-relevant plasma

Chemical vapour deposited diamond was irradiated with 5 MeV carbon ions to simulate the damage caused by collision cascades from neutron irradiation in a fusion environment. Ion-irradiated samples were then exposed to a deuterium plasma in MAGPIE with ion flux of ∼1.3 × 1021 ions m−2 s−1. Raman and near edge x-ray absorption fine structure (NEXAFS) spectroscopy were used to characterize the degree of disorder and sp2-bonding induced by the ion irradiation. The signals of sp2-bonded and disordered carbon were observed to decrease after exposure to the deuterium plasma, although sharp Raman peaks indicative of vacancy and interstitial defects induced by the MeV ions were less affected. Recovery of a diamond-like surface after plasma exposure was evident in the NEXAFS spectra. Elastic recoil detection analysis showed that the ion-damaged diamond retained more deuterium than diamond exposed only to deuterium plasma. For the case of unirradiated samples, diamond retained more deuterium than graphite. However, for the case of the ion-irradiated samples, diamond exhibited less deuterium retention than graphite.