Helium Beam Research Articles

3‐D Nanofabrication of Silicon and Nanostructure Fine‐Tuning via Helium Ion Implantation (Adv. Mater. Interfaces 10/2022)

Fine-Tuning of Nanostructures via Helium Ion Implantation In article number 2101643, Xiaolei Wen, Renwei Mao, and Huan Hu invent an approach for producing 3D silicon nanostructures with super small roughness based on substrate swelling caused by implantation of focused helium ion beams. Furthermore, mimicking the leaning tower of Pisa, they implant on the bottom of the readily made nanostructures to fine-tune the distance into sub 5-nm as well as the tilting angles.

Characterization of MOSFET Dosimeters for Alpha Particle Therapy

Alpha particle therapy, such as diffusing alpha-emitters radiation therapy (DaRT) and targeted alpha-particle therapy (TAT), exploits the short range and high linear energy transfer (LET) of alpha particles to destroy cancer cells locally with minimal damage to surrounding healthy cells. Dosimetry for DaRT and TAT is challenging, as their radiation sources produce mixed radiation fields of <inline-formula> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula> particles, <inline-formula> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> particles, and <inline-formula> <tex-math notation="LaTeX">$\gamma $ </tex-math></inline-formula> rays. There is currently no dosimeter for real-time <b><i>in vivo</i></b> dosimetry of DaRT or TAT. Metal-oxide-semiconductor field-effect transistors (MOSFETs) have features that are ideal for this scenario. They can be read out in real-time nondestructively, store information permanently, and their sensitivity to radiation can be varied by changing the bias on the gate during irradiation. Moreover, owing to their compactness, MOSFETs can fit into fine-gauge needle applicators, such as those used to carry radioactive seeds into the tumor. This study characterized the response of MOSFETs designed at the Centre for Medical and Radiation Physics, University of Wollongong. Irradiations were performed with a mono-energetic helium ion beam (He^2&#x002B;) of 5.5 MeV and an Americium-241 (²⁴¹Am) source, for MOSFETs with three different gate oxide thicknesses (0.55, 0.68, and 1.0 <inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula>). The results showed that the response was linear with alpha dose up to 25 Gy. Also, it was found that a gate bias of between 15 and 60 V would optimize the sensitivity to alpha particles with energy of 5.5 MeV.

Measurements of charge transfer and target-electron-loss cross sections forH+,D+, andHe+impact on lithium at low energies

Experimental cross section measurements for charge transfer and for electron loss from lithium via charge transfer and direct ionization are reported for collisions between light ions and lithium atoms. Beams of protons, deuterium ions, and helium ions intersect a lithium atom beam, and collision cross sections are inferred by measuring the number of lithium ions produced in the collisions. The collision energy ranges studied are 0.13--3.5 keV for proton impact, 0.13--0.83 keV for deuterium ion impact, and 0.28--3 keV for helium ion impact. At proton collision energies below 0.7 keV our charge transfer cross sections are in excellent agreement with the theoretical predictions, resolving a long-standing discrepancy between theory and experiment in this energy range. Cross sections for proton and for deuterium ion impact on lithium are measured to be identical when scaled to the same collision velocity, indicating that quantum effects are not significant in the energy range studied. Comparison between our cross section measurements for lithium electron loss due to helium ion impact and previously measured charge transfer cross sections show that ionization is a negligible process in ${\mathrm{He}}^{+}+\mathrm{Li}$ collisions at energies below 3 keV. Undulations in the measured collision cross sections at low energy are observed for both proton and helium ion impact and are compared to molecular orbital close-coupling calculations.

Progress from ASDEX Upgrade experiments in preparing the physics basis of ITER operation and DEMO scenario development

An overview of recent results obtained at the tokamak ASDEX Upgrade (AUG) is given. A work flow for predictive profile modelling of AUG discharges was established which is able to reproduce experimental H-mode plasma profiles based on engineering parameters only. In the plasma center, theoretical predictions on plasma current redistribution by a dynamo effect were confirmed experimentally. For core transport, the stabilizing effect of fast ion distributions on turbulent transport is shown to be important to explain the core isotope effect and improves the description of hollow low-Z impurity profiles. The L–H power threshold of hydrogen plasmas is not affected by small helium admixtures and it increases continuously from the deuterium to the hydrogen level when the hydrogen concentration is raised from 0 to 100%. One focus of recent campaigns was the search for a fusion relevant integrated plasma scenario without large edge localised modes (ELMs). Results from six different ELM-free confinement regimes are compared with respect to reactor relevance: ELM suppression by magnetic perturbation coils could be attributed to toroidally asymmetric turbulent fluctuations in the vicinity of the separatrix. Stable improved confinement mode plasma phases with a detached inner divertor were obtained using a feedback control of the plasma β. The enhanced D α H-mode regime was extended to higher heating power by feedback controlled radiative cooling with argon. The quasi-coherent exhaust regime was developed into an integrated scenario at high heating power and energy confinement, with a detached divertor and without large ELMs. Small ELMs close to the separatrix lead to peeling-ballooning stability and quasi continuous power exhaust. Helium beam density fluctuation measurements confirm that transport close to the separatrix is important to achieve the different ELM-free regimes. Based on separatrix plasma parameters and interchange-drift-Alfvén turbulence, an analytic model was derived that reproduces the experimentally found important operational boundaries of the density limit and between L- and H-mode confinement. Feedback control for the X-point radiator (XPR) position was established as an important element for divertor detachment control. Stable and detached ELM-free phases with H-mode confinement quality were obtained when the XPR was moved 10 cm above the X-point. Investigations of the plasma in the future flexible snow-flake divertor of AUG by means of first SOLPS-ITER simulations with drifts activated predict beneficial detachment properties and the activation of an additional strike point by the drifts.

Collisional radiative model for the evaluation of the thermal helium beam diagnostic at ASDEX upgrade

The thermal helium beam diagnostic at ASDEX Upgrade is used to infer the electron density n e and temperature T e in the scrape-off layer and the pedestal region from the emission of visible lines of the locally injected helium. The link between n e and T e and the emission is provided by a collisional radiative model, which delivers the evolution of the populations of the relevant excited states as the He atoms travel through the plasma. A computationally efficient method with just three effective states is shown to provide a good approximation of the population dynamics. It removes an artificial rise of T e at the plasma edge when using a simple static model. Furthermore, the re-absorption of the vacuum ultra-violet resonance lines has been introduced as an additional excitation mechanism being mainly important in the region close to the injection point. This extra excitation leads to a much better fit of the measured line ratios in this region for larger puff rates.

Measurements on Five Characterizing Properties of Graphene Oxide and Reduced Graphene Oxide Foils

Micrometric foils of graphene oxide are prepared in the laboratory starting from water dispersed solution of graphene oxide at low concentration. Measurements on five physical properties have been performed in the foils of graphene oxide and reduced graphene oxide. The reduction is obtained thermally, using IR and UV laser irradiations and MeV helium ion beam irradiations at different fluence. The parameters such as the atomic composition, the mass density, the surface roughness, the wetting ability, and the electrical conductivity are investigated. This last parameter assumes different values for the pristine graphene oxide foils and for the reduced ones. Comparison of experimental measurements with the literature data and the results and discussion are presented.

Hexamethyldisiloxane coating by plasma to create a superhydrophobic surface for fabric masks

This study evaluated the effect of hexamethyldisiloxane (HMDSO) plasma on the fabrics surface to produce masks. Samples of 400-thread count cotton fabric were used. The fabrics was covered with a layer of HMDSO employing plasma enhanced chemical vapor deposition. Optical Microscopy, helium ion beam microscopy (HIM) and atomic force microscopy (AFM) were used for topographic analysis. Fourier transformed infrared spectroscopy technique was used to analyze the chemical modifications on surface. The level of hydrophobicity was evaluated using contact angle measurements. To evaluate the effect of washing cycle on the plasma coating, a protocol proposed by the World Health Organization was used. All experiments were performed in triplicate. For the statistical analysis, the Mann–Whitney, Kruskal–Wallis and Dunn tests were used. AFM showed that, after HMDSO plasma coating, a layer of small granules agglomerated on the original surface of the fabric was visualized. The fabric, which initially had a 0° contact angle value, presented angles of 120° after the plasma coating. It was concluded that the use of cotton fabric coated with HMDSO plasma proved to be adequate for the manufacture of protective masks, since the coating made the surface hydrophobic and this property is maintained even after washing cycles.

The Evolution of Lateral Dose Distributions of Helium Ion Beams in Air: From Measurement and Modeling to Their Impact on Treatment Planning

To start clinical trials with the first clinical treatment planning system supporting raster-scanned helium ion therapy, a comprehensive database of beam characteristics and parameters was required for treatment room-specific beam physics modeling at the Heidelberg Ion-Beam Therapy Center (HIT). At six different positions in the air gap along the beam axis, lateral beam profiles were systematically measured for 14 initial beam energies covering the full range of available energies at HIT. The 2D-array of liquid-filled ionization chambers OCTAVIUS from PTW was irradiated by a pencil beam focused at the central axis. With a full geometric representation of HIT’s monitoring chambers and beamline elements in FLUKA, our Monte Carlo beam model matches the measured lateral beam profiles. A second set of measurements with the detector placed in a water tank was used to validate the adjustments of the initial beam parameters assumed in the FLUKA simulation. With a deviation between simulated and measured profiles below ±0.8 mm for all investigated beam energies, the simulated profiles build part of the database for the first clinical treatment planning system for helium ions. The evolution of beamwidth was also compared to similar simulations of the clinically available proton and carbon beam. This allows a choice of treatment modality based on quantitative estimates of the physical beam properties. Finally, we investigated the influence of beamwidth variation on patient treatment plans in order to estimate the relevance and necessary precision limits for lateral beam width models.

Ion-beam Modification of the Local Luminescent Properties of Hexagonal Boron Nitride

Hexagonal boron nitride is a promising material of modern optoelectronics. Point defects in this material can serve as single-photon sources. In this paper we investigate the modification of the luminescent properties of hexagonal boron nitride by means of local irradiation with focused gallium and helium ion beams. It is demonstrated that the intensity of band-to-band cathodoluminescence monotonically decreases with increasing ion fluence for both gallium and helium. The luminescence band of about 2 eV may become more intense after exposure to He ions with certain ion fluence. The effect of complete quenching of luminescence after gallium irradiation is used to estimate the diffusion length of excess charge carriers. Keywords: point defects, cathodoluminescence, scanning helium ion microscope, excess charge carriers.

Ионно-лучевая модификация локальных люминесцентных свойств гексагонального нитрида бора

Hexagonal boron nitride is a promising material of modern optoelectronics. Point defects in this material can serve as single-photon sources. In this paper we investigate the modification of the luminescent properties of hexagonal boron nitride by means of local irradiation with focused gallium and helium ion beams. It is demonstrated that the intensity of band-to-band cathodoluminescence monotonically decreases with increasing ion fluence for both gallium and helium. The luminescence band of about 2 eV may become more intense after exposure to He ions with certain ion fluence. The effect of complete quenching of luminescence after gallium irradiation is used to estimate the diffusion length of excess charge carriers.

Имитационное облучение ДУО стали 16Cr – 4Al – 2W – 0,3Ti – 0,3Y2O3, перспективной для термоядерных реакторов, в установке Плазменный фокус “Вихрь”

A study of the radiation-thermal resistance of ferritic steel 16Cr – 4Al – 2W – 0.3Ti – 0.3Y2O3 was made. This ODS (oxide dispersion strengthened) steel is perspective for fusion applications. The “Vikhr” Plasma Focus installation was used to introduse of powerful pulsed flows of helium ions and helium plasma. The power density of a beam of fast helium ions and high-temperature helium plasma flows was ~ 108 and 107 W/cm2 at exposure times of ~ 50 and 100 ns, respectively. The number of pulses N varied in the range from 10 to 30. The rate of evaporation and radiaсtive sputtering changed slightly with an increase in the number of pulses of energy flows acting on the material and amounted to h ≈ 0.01 – 0.02 μm/puls. The irradiated surface after repeated melting under the action of a pulsed radiation-thermal load with powerful energy flows acquired a wave-like character with inclusions of dispersed micro particles of the second phase, containing mainly yttrium, oxygen, aluminum, iron, and titanium. At the same time, in contrast to the refractory metals (W, Mo, Ti) earlier under similar radiation conditions studied, no micro- and macro cracks were formed on the surface of the material facing the plasma. “Vikhr” Plasma Focus setup proved to be an effective tool for simulation testing of candidate materials with magnetic and inertial plasma confinement.

Thermal rectification on asymmetric suspended graphene nanomesh devices

Graphene-based thermal rectification was investigated by measuring the thermal transport properties of asymmetric suspended graphene nanomesh devices. A sub-10 nm periodic nanopore phononic crystal structure was successfully patterned on the half area of the suspended graphene ribbon by helium ion beam milling technology. The ‘differential thermal leakage’ method was developed for thermal transport measurement without disturbance from the leakage of electron current through the suspended graphene bridge. A thermal rectification ratio of up to 60% was observed in a typical device with a nanopore pitch of 20 nm. By increasing the nanopore pitch in a particular range, the thermal rectification ratio showed an increment. However, this ratio was degraded by increasing the environmental temperature. This experiment suggests a promising way to develop a high-performance thermal rectifier by using a phononic crystal to introduce asymmetry on homogeneous material.

OL97 - Real-time positron emission imaging for range verification in helium beam radiotherapy

OL97 - Real-time positron emission imaging for range verification in helium beam radiotherapy

Dual-layer spectral CT for proton, helium, and carbon ion beam therapy planning of brain tumors.

Pretreatment computed tomography (CT) imaging is an essential component of the particle therapy treatment planning chain. Treatment planning and optimization with charged particles require accurate and precise estimations of ion beam range in tissues, characterized by the stopping power ratio (SPR). Reduction of range uncertainties arising from conventional CT‐number‐to‐SPR conversion based on single‐energy CT (SECT) imaging is of importance for improving clinical practice. Here, the application of a novel imaging and computational methodology using dual‐layer spectral CT (DLCT) was performed toward refining patient‐specific SPR estimates.A workflow for DLCT‐based treatment planning was devised to evaluate SPR prediction for proton, helium, and carbon ion beam therapy planning in the brain. DLCT‐ and SECT‐based SPR predictions were compared in homogeneous and heterogeneous anatomical regions. This study included eight patients scanned for diagnostic purposes with a DLCT scanner. For each patient, four different treatment plans were created, simulating tumors in different parts of the brain.For homogeneous anatomical regions, mean SPR differences of about 1% between the DLCT‐ and SECT‐based approaches were found. In plans of heterogeneous anatomies, relative (absolute) proton range shifts of 0.6% (0.4 mm) in the mean and up to 4.4% (2.1 mm) at the distal fall‐off were observed. In the investigated cohort, 12% of the evaluated organs‐at‐risk (OARs) presented differences in mean or maximum dose of more than 0.5 Gy (RBE) and up to 6.8 Gy (RBE) over the entire treatment. Range shifts and dose differences in OARs between DLCT and SECT in helium and carbon ion treatment plans were similar to protons.In the majority of investigated cases (75th percentile), SECT‐ and DLCT‐based range estimations were within 0.6 mm. Nonetheless, the magnitude of patient‐specific range deviations between SECT and DLCT was clinically relevant in heterogeneous anatomical sites, suggesting further study in larger, more diverse cohorts. Results indicate that patients with brain tumors may benefit from DLCT‐based treatment planning.

Next Evolutions in Particle Therapy: Spot-Scanning Hadron Arc (SHArc) Therapy

To develop and evaluate spot-scanning hadron arc (SHArc) therapy, the first arc delivery technique for both light and heavy ions, in different clinical scenarios.Robust treatment planning and biological optimization procedures were developed within a GPU-accelerated dose engine and treatment planning system, establishing a dedicated energy/spot/angle selection protocol for robust SHArc delivery. Initial tests were performed for three clinical particle beams (protons, helium, and carbon ions) on water-equivalent cylindrical phantoms for simple targets and clinical-like scenarios with an organ-at-risk (OAR) in proximity of the target to compare SHArc against reference conventional techniques using single and multi-field configurations. SHArc patient treatments were surveyed in various sites (head, H&N, thorax and pelvic region). Dosimetric and biological features were calculated and evaluated applying the modified microdosimetric kinetic model. To determine impact of LET variations within the tumor volume, a model for hypoxia-induced radio-resistance was developed and applied for particle therapy to investigate impact on effective dose.SHArc plans exhibited similar target coverage with unique treatment attributes and distributions compared to conventional planning, with carbon ions demonstrating the greatest potential for tumor control and normal tissue sparing among the arc techniques. All SHArc plans exhibited a low-dose bath outside the target volume with a reduced maximum dose in normal tissues compared to single and multi-field plans. Moreover, favorable LET distributions were possible with the SHArc approach with substantial increases in LETmax and volume containing high-LET components. Moreover, reductions of high-LET regions in normal tissues and OARs were observed compared to static treatment beam delivery.The first arc treatment delivery technique using helium and carbon ion beams is proposed. Evidence suggests that through arc delivery of high-LET particle beams, SHArc therapy may offer uniquely valuable clinical advantages both dosimetrical and biological. Robust SHArc treatments could potentially improve tumor control by overcoming tumor microenvironment resistance factors such as hypoxia induced radio-resistance, and reduce toxicity in critical structures by minimizing high-LET components.

Next Evolutions in Particle Therapy: Helium Ion Treatment Planning, Delivery and Clinical Implications of Biological Modeling

This work aims to investigate the implication of relevant biological parameters in clinical treatment planning for the first helium ion beam therapy with active beam delivery.Patient plan optimization was performed using the first clinical TPS commissioned for Helium ion therapy with active beam delivery. Biological calculations are coupled to the modified microdosimetric kinetic model (mMKM) for considering bio-effect specific to each tumor and mixed radiation field spectra. Several plans for patients previously treated with proton beams were re-optimized with helium ions. Comparisons of the resulting biological dose distributions between the ions for the same fractionation scheme were conducted. Sensitivity studies were performed with these plans with respect to the chosen (α/β)x for changes in β (high or low in its clinical range) or on the fractionation scheme. Physical and biological quantities such as physical dose, LET and BED were also compared after computation with the secondary dose engine FRoG.During treatment plan optimization, all clinical constraints were reached with the helium ions. Overall, results indicate better CTV conformity and sparing of surrounding healthy tissue. Robustness analysis on 2mm/3% do not show any disadvantages in setup/SPR inaccuracies of helium ions compared to protons. Investigation on the sensitivity studies suggest clinically relevant levels of RBE-related uncertainties with selected (α/β)x, thus calling for compromise between fractionation scheme and RBE sensitivity in treatment design. All plans were dosimetrically verified in water and found to be within clinical tolerances.Prior to the first patient treatment using helium ions with active beam scanning delivery, assessment of clinical parameters and feasibility of patient planning using the mMKM was successfully performed.

FLASH Dose-Rate Helium Ion Beams: First In Vitro Investigations

<h3>Purpose/Objective(s)</h3> To establish and investigate the impact of LET and O₂ concentration on biological response to ultra-high dose-rate (uHDR, FLASH) helium ion beams compared to standard dose-rate (SDR) irradiation. <h3>Materials/Methods</h3> Beam delivery settings for raster-scanned helium ions at both uHDR and SDR were tuned to achieve > 100 Gy/s and 0.15 Gy/s, respectively. For both SDR and uHDR, 8 Gy and 12 Gy plan optimization and calibration for 10 × 10mm² fields was performed to assess <b>in vitro</b> response at lower LET (4.5 keV/μm) and higher LET (16 keV/μm) conditions. Clonogenic assay and DNA damage response (γH2AX) of human lung epithelial cancer cells A549 cells were used as biological indicators for the cellular response to uHDR and SDR irradiation for specified oxygen concentrations levels (1%, 21%). <h3>Results</h3> Average dose-rates for both lower and higher LET were 193 Gy/s and 0.15 Gy/s for uHDR and SDR, respectively. Cell survival was higher and γH2AX, a surrogate for DNA damage response, was lower for cells irradiated with uHDR as compared to SDR at 1% O₂ for both dose and LET levels. This difference in cellular response was not seen at 21% O₂ where cell survival and DNA damage response were unchanged at studied dose-rates. <h3>Conclusion</h3> The first uHDR delivery of raster-scanned particle beams was achieved using helium ions, reaching FLASH-level dose-rates of > 100 Gy/s. Baseline oxygen levels play a pivotal role, irrespective of investigated LET, for observation of a sparing effect for helium ions.

Single nucleotide discrimination with sub-two nanometer monolayer graphene pore

The effective sensing area is the key point of solid-state nanopore single-molecule detection, which is determined by the diameter of the hole and the thickness of the membrane. High spatial resolution can be achieved by using single-atom thickness two-dimensional materials, such as single-layer graphene and molybdenum disulfide. Helium ion beam microscopy provides an effective technique for manufacturing small-sized nanopores. Here, we show that the precise fabrication of sub-2 nanometer nanopores has been successfully achieved on a single-layer graphene film, which provides the best spatial resolution in room-temperature ionic liquid-assisted single-molecule detection. Many graphene nanopores of different sizes have been fabricated, with a minimum pore diameter of 1.4 nm. The application of these graphene nanopores manufactured by HIM shows that single-layer graphene nanopores can distinguish different homopolymer single-stranded DNA lengths and types, and can even identify four single nucleotides.

Linearized spectrum correlation analysis for thermal helium beam diagnostics.

We introduce a new correlation analysis technique for thermal helium beam (THB) diagnostics. Instead of directly evaluating line ratios from fluctuating time series, we apply arithmetic operations to all available He I lines and construct time series with desired dependencies on the plasma parameters. By cross-correlating those quantities and by evaluating ensemble averages, uncorrelated noise contributions can be removed. Through the synthetic data analysis, we demonstrate that the proposed analysis technique is capable of providing the power spectral densities of meaningful plasma parameters, such as the electron density and the electron temperature, even under low-photon-count conditions. In addition, we have applied this analysis technique to the experimental THB data obtained at the ASDEX Upgrade tokamak and successfully resolved the electron density and temperature fluctuations up to 90 kHz in a reactor relevant high power scenario.

Patterning of Complex, Nanometer-Scale Features in Wide-Area Gold Nanoplasmonic Structures Using Helium Focused Ion Beam Milling.

Meeting the evolving demands of plasmonics research requires increasingly precise control over surface plasmon properties, which necessitates extremely fine nanopatterning, complex geometries, and/or long-range order. Nanoplasmonic metasurfaces are representative of a modern research area requiring intricate, high-fidelity features reproduced over areas of several free-space wavelengths, making them one of the most challenging fabrication problems in the field today. This work presents a systematic study of the helium focused ion beam milling of gold for nanoplasmonic metasurface applications, using as its example a nanoplasmonic metasurface based on an array of nanometer-scale plasmonic-wire-loaded subwavelength apertures in a gold film. At each step, the pattern variations are compared to simulation to predict the experimental outcome. Our results show that even in a practical fabrication environment, helium ion beam milling can be used to reliably pattern 10 nm features into gold with 1:5 aspect ratio in complex geometries over a wide area.