3D Acoustic Wave Sparsely Activated Localization Microscopy With Phase Change Contrast Agents

OBJECTIVE

The aim of this study is to demonstrate 3-dimensional (3D) acoustic wave sparsely activated localization microscopy (AWSALM) of microvascular flow in vivo using phase change contrast agents (PCCAs).

MATERIALS AND METHODS

Three-dimensional AWSALM using acoustically activable PCCAs was evaluated on a crossed tube microflow phantom, the kidney of New Zealand White rabbits, and the brain of C57BL/6J mice through intact skull. A mixture of C 3 F 8 and C 4 F 10 low-boiling-point fluorocarbon gas was used to generate PCCAs with an appropriate activation pressure. A multiplexed 8-MHz matrix array connected to a 256-channel ultrasound research platform was used for transmitting activation and imaging ultrasound pulses and recording echoes. The in vitro and in vivo echo data were subsequently beamformed and processed using a set of customized algorithms for generating 3D super-resolution ultrasound images through localizing and tracking activated contrast agents.

RESULTS

With 3D AWSALM, the acoustic activation of PCCAs can be controlled both spatially and temporally, enabling contrast on demand and capable of revealing 3D microvascular connectivity. The spatial resolution of the 3D AWSALM images measured using Fourier shell correlation is 64 μm, presenting a 9-time improvement compared with the point spread function and 1.5 times compared with half the wavelength. Compared with the microbubble-based approach, more signals were localized in the microvasculature at similar concentrations while retaining sparsity and longer tracks in larger vessels. Transcranial imaging was demonstrated as a proof of principle of PCCA activation in the mouse brain with 3D AWSALM.

CONCLUSIONS

Three-dimensional AWSALM generates volumetric ultrasound super-resolution microvascular images in vivo with spatiotemporal selectivity and enhanced microvascular penetration.

Broad Elevation Projection Super-Resolution Ultrasound (BEP-SRUS) Imaging With a 1-D Unfocused Linear Array

Super-resolution ultrasound (SRUS) through localizing spatially isolated microbubbles (MBs) has been demonstrated to overcome the wave diffraction limit and reveal the microvascular structure and flow information at the microscopic scale. However, 3-D SRUS imaging remains a challenge due to the fabrication and computational complexity of 2-D matrix array probes. Inspired by X-ray radiography which can present information within a volume in a single projection image with much simpler hardware than X-ray computerized tomography (CT), this study investigates the feasibility of broad elevation projection super-resolution (BEP-SR) ultrasound using a 1-D unfocused linear array. Both simulation and in vitro experiments were conducted on 3-D microvessel phantoms. In vivo demonstration was done on the Rabbit kidney. Data from a 1-D linear array with and without an elevational focus were synthesized by summing up row signals acquired from a 2-D matrix array with and without delays. A full 3-D reconstruction was also generated as the reference, using the same data of the 2-D matrix array but without summing row signals. Results show that using an unfocused 1-D array probe, BEP-SR can capture significantly more information within a volume in both vascular structure and flow velocity than the conventional 1-D elevational-focused probe. Compared with the 2-D projection image of the full 3-D SRUS results using the 2-D array probe with the same aperture size, the 2-D projection SRUS image of BEP-SR has similar volume coverage, using 32 folds fewer independent elements. This study demonstrates BEP-SR's ability of high-resolution imaging of microvascular structures and flow velocity within a 3-D volume at significantly reduced costs. The proposed BEP method could significantly benefit the clinical translation of the SRUS imaging technique by making it more affordable and repeatable.

Fast 3D Super-Resolution Ultrasound With Adaptive Weight-Based Beamforming

OBJECTIVE

Super-resolution ultrasound (SRUS) imaging through localising and tracking sparse microbubbles has been shown to reveal microvascular structure and flow beyond the wave diffraction limit. Most SRUS studies use standard delay and sum (DAS) beamforming, where high side lobes and broad main lobes make isolation and localisation of densely distributed bubbles challenging, particularly in 3D due to the typically small aperture of matrix array probes.

METHOD

This study aimed to improve 3D SRUS by implementing a new fast 3D coherence beamformer based on channel signal variance. Two additional fast coherence beamformers, that have been implemented in 2D were implemented in 3D for the first time as comparison: a nonlinear beamformer with p-th root compression and a coherence factor beamformer. The 3D coherence beamformers, together with DAS, were compared in computer simulation, on a microflow phantom and in vivo.

RESULTS

Simulation results demonstrated that all three adaptive weight-based beamformers can narrow the main lobe, suppress the side lobes, while maintaining the weaker scatter signals. Improved 3D SRUS images of microflow phantom and a rabbit kidney within a 3-second acquisition were obtained using the adaptive weight-based beamformers, when compared with DAS.

CONCLUSION

The adaptive weight-based 3D beamformers can improve the SRUS and the proposed variance-based beamformer performs best in simulations and experiments.

SIGNIFICANCE

Fast 3D SRUS would significantly enhance the potential utility of this emerging imaging modality in a broad range of biomedical applications.

Fast and Selective Super-Resolution Ultrasound In Vivo With Acoustically Activated Nanodroplets

Perfusion by the microcirculation is key to the development, maintenance and pathology of tissue. Its measurement with high spatiotemporal resolution is consequently valuable but remains a challenge in deep tissue. Ultrasound Localization Microscopy (ULM) provides very high spatiotemporal resolution but the use of microbubbles requires low contrast agent concentrations, a long acquisition time, and gives little control over the spatial and temporal distribution of the microbubbles. The present study is the first to demonstrate Acoustic Wave Sparsely-Activated Localization Microscopy (AWSALM) and fast-AWSALM for in vivo super-resolution ultrasound imaging, offering contrast on demand and vascular selectivity. Three different formulations of acoustically activatable contrast agents were used. We demonstrate their use with ultrasound mechanical indices well within recommended safety limits to enable fast on-demand sparse activation and destruction at very high agent concentrations. We produce super-localization maps of the rabbit renal vasculature with acquisition times between 5.5 s and 0.25 s, and a 4-fold improvement in spatial resolution. We present the unique selectivity of AWSALM in visualizing specific vascular branches and downstream microvasculature, and we show super-localized kidney structures in systole (0.25 s) and diastole (0.25 s) with fast-AWSALM outperforming microbubble based ULM. In conclusion, we demonstrate the feasibility of fast and selective imaging of microvascular dynamics in vivo with subwavelength resolution using ultrasound and acoustically activatable nanodroplet contrast agents.

Design and Construction of a Low-Frequency Ultrasound Acquisition Device for 2-D Brain Imaging Using Full-Waveform Inversion

The main techniques used to image the brain and obtain structural data are magnetic resonance imaging and X-ray computed tomography. These techniques produce images with high spatial resolution, but with the disadvantage of requiring very large equipment with special installation needs. In addition, X-ray tomography uses ionizing radiation, which limits their use. Ultrasound imaging is a safe technology that is delivered using compact and mobile devices. However, conventional ultrasound reconstruction techniques have failed to obtain images of the brain because of, fundamentally, the presence of the skull and the distortion that it produces on ultrasound. Recent studies have indicated that full-waveform inversion, a computational technique originally from Earth science, has the potential to generate accurate 3-D images of the brain. This technology can overcome the limitations of conventional ultrasound imaging, but a prototype for transcranial applications does not yet exist. Here, we investigate different designs of an annular array of ultrasound transducers to optimize the number of elements and rotations needed to conduct transcranial imaging with full-waveform inversion. This device uses small-diameter, low-frequency transducers that readily propagate ultrasound through the skull with good signal-to-noise ratios. It also incorporates the use of rotations to produce a high-density coverage of the target and acquire redundant traces that are beneficial for full-waveform inversion. We have built a ring of 40 transducers to illustrate that this design is capable of reconstructing images of the brain, retrieving its anatomy and acoustic properties with millimeter resolution. Laboratory results reveal the ability of this device to successfully image a 2.5-D brain- and skull-mimicking phantom using full-waveform inversion. To our knowledge, this is the first prototype ever used for transcranial-like imaging. The importance of these findings and their implications for the design of a 3-D reconstruction system with possible clinical applications are discussed.

Ultrafast 3-D Ultrasound Imaging Using Row-Column Array-Specific Frame-Multiply-and-Sum Beamforming

Row-column arrays have been shown to be able to generate 3-D ultrafast ultrasound images with an order of magnitude less independent electronic channels than traditional 2-D matrix arrays. Unfortunately, row-column array images suffer from major imaging artifacts due to high sidelobes, particularly when operating at high frame rates. This article proposes a row-column-specific beamforming technique, for orthogonal plane-wave transmissions, row-column-specific frame multiply and sum (RC-FMAS), that exploits the incoherent nature of certain row-column array artifacts. A series of volumetric images is produced using row or column transmissions of 3-D plane waves. The voxelwise geometric mean of the beamformed volumetric images from each row and column pair is taken prior to compounding, which drastically reduces the incoherent imaging artifacts in the resulting image compared to traditional coherent compounding. The effectiveness of this technique was demonstrated in silico and in vitro, and the results show a significant reduction in sidelobe level with over 16-dB improvement in sidelobe to main-lobe energy ratio. Significantly improved contrast was demonstrated with contrast ratio increased by ~10 dB and generalized contrast-to-noise ratio increased by 158% when using the proposed new method compared to the existing delay and sum during in vitro studies. The new technique allowed for higher quality 3-D imaging while maintaining high frame rate potential.

Volumetric Flow Estimation in a Coronary Artery Phantom Using High-Frame-Rate Contrast-Enhanced Ultrasound, Speckle Decorrelation, and Doppler Flow Direction Detection

The coronary flow reserve (CFR), relating to the volumetric flow rate, is an effective functional parameter to assess the stenosis in the left anterior descending (LAD) coronary artery. We have recently proposed to use high-frame-rate (HFR) contrast-enhanced ultrasound (CEUS) to estimate the volumetric flow rate using ultrasound (US) speckle decorrelation (SDC) without any assumptions about the velocity profile. However, this method still has challenges in imaging deep and small vessels, such as LAD. In this study, we proposed to address the challenges and demonstrate the feasibility of volumetric flow rate measurement in a coronary mimicking phantom with pulsatile flow using a 1-D array cardiac probe, vector Doppler, and an optimal probe rotation/tilting for flow direction detection. Both simulations and in vitro experiments were conducted to validate the proposed method. It is shown that in-plane velocities estimated by vector Doppler under a 10° probe tilting resulted in smaller percentage error (+5.2%) in flow rate estimates than that in US imaging velocimetry (-20.2%) although their relative standard deviations were very close, being 2.6 and 2.8 ml/min, respectively. The flow rate estimated by SDC without direction detection had an error higher than 70%. A 10° tilting of the probe had the best results in flow rate estimation compared to the 5° or 15° tilting. Realistic global motions in the LAD increased the flow rate estimation error from 5.2% to 14.2%. It is concluded that it is feasible to measure the volumetric flow rate in a coronary artery flow phantom with a conventional cardiac probe, using HFR acquisition, Doppler, and SDC analysis. Potentially, this technique could also be applied to investigate the volumetric flow rate in other small vessels similar to the LAD.

Ion tracks in silicon formed by much lower energy deposition than the track formation threshold

Damaged regions of cylindrical shapes called ion tracks, typically in nano-meters wide and tens micro-meters long, are formed along the ion trajectories in many insulators, when high energy ions in the electronic stopping regime are injected. In most cases, the ion tracks were assumed as consequences of dense electronic energy deposition from the high energy ions, except some cases where the synergy effect with the nuclear energy deposition plays an important role. In crystalline Si (c-Si), no tracks have been observed with any monomer ions up to GeV. Tracks are formed in c-Si under 40 MeV fullerene (C₆₀) cluster ion irradiation, which provides much higher energy deposition than monomer ions. The track diameter decreases with decreasing the ion energy until they disappear at an extrapolated value of ~ 17 MeV. However, here we report the track formation of 10 nm in diameter under C₆₀ ion irradiation of 6 MeV, i.e., much lower than the extrapolated threshold. The diameters of 10 nm were comparable to those under 40 MeV C₆₀ irradiation. Furthermore, the tracks formed by 6 MeV C₆₀ irradiation consisted of damaged crystalline, while those formed by 40 MeV C₆₀ irradiation were amorphous. The track formation was observed down to 1 MeV and probably lower with decreasing the track diameters. The track lengths were much shorter than those expected from the drop of S_e below the threshold. These track formations at such low energies cannot be explained by the conventional purely electronic energy deposition mechanism, indicating another origin, e.g., the synergy effect between the electronic and nuclear energy depositions, or dual transitions of transient melting and boiling.

3-D Super-Resolution Ultrasound Imaging With a 2-D Sparse Array

High-frame-rate 3-D ultrasound imaging technology combined with super-resolution processing method can visualize 3-D microvascular structures by overcoming the diffraction-limited resolution in every spatial direction. However, 3-D super-resolution ultrasound imaging using a full 2-D array requires a system with a large number of independent channels, the design of which might be impractical due to the high cost, complexity, and volume of data produced. In this study, a 2-D sparse array was designed and fabricated with 512 elements chosen from a density-tapered 2-D spiral layout. High-frame-rate volumetric imaging was performed using two synchronized ULA-OP 256 research scanners. Volumetric images were constructed by coherently compounding nine-angle plane waves acquired at a pulse repetition frequency of 4500 Hz. Localization-based 3-D super-resolution images of two touching subwavelength tubes were generated from 6000 volumes acquired in 12 s. Finally, this work demonstrates the feasibility of 3-D super-resolution imaging and super-resolved velocity mapping using a customized 2-D sparse array transducer.

Motion Artifacts and Correction in Multipulse High-Frame Rate Contrast-Enhanced Ultrasound

High-frame-rate (HFR) ultrasound (US) imaging and contrast-enhanced US (CEUS) are often implemented using multipulse transmissions, to enhance image quality. Multipulse approaches, however, suffer from degradation in the presence of motion, especially when coherent compounding and CEUS are combined. In this paper, we investigate this effect on the intensity of HFR CEUS in deep tissue imaging using simulations and in vivo contrast echocardiography (CE). The simulation results show that the motion artifact is much higher when the flow is in an axial direction than a lateral direction. Using a pulse repetition frequency suitable for cardiac imaging, a motion of 35 cm/s can cause as much as 28.5 dB decrease in image intensity, where compounding can contribute up to 18.7 dB of intensity decrease (11 angles). These motion effects are also demonstrated for in vivo cardiac HFR CE, where the large velocities of both the myocardium and the blood are present. Intensity reductions of 10.4 dB are readily visible in the chamber. Finally, we demonstrate how performing motion-correction before pulse inversion compounding greatly reduces such motion artifact and improve image signal-to-noise ratio and contrast.

High-Frame-Rate Contrast Echocardiography Using Diverging Waves: Initial In Vitro and In Vivo Evaluation

Contrast echocardiography (CE) ultrasound with microbubble contrast agents has significantly advanced our capability for assessment of cardiac function, including myocardium perfusion quantification. However, in standard CE techniques obtained with line by line scanning, the frame rate and image quality are limited. Recent research has shown significant frame-rate improvement in noncontrast cardiac imaging. In this work, we present and initially evaluate, both in vitro and in vivo, a high-frame-rate (HFR) CE imaging system using diverging waves and pulse inversion sequence. An imaging frame rate of 5500 frames/s before and 250 frames/s after compounding is achieved. A destruction-replenishment sequence has also been developed. The developed HFR CE is compared with standard CE in vitro on a phantom and then in vivo on a sheep heart. The image signal-to-noise ratio and contrast between the myocardium and the chamber are evaluated. The results show up to 13.4-dB improvement in contrast for HFR CE over standard CE when compared at the same display frame rate even when the average spatial acoustic pressure in HFR CE is 36% lower than the standard CE. It is also found that when coherent compounding is used, the HFR CE image intensity can be significantly modulated by the flow motion in the chamber.

Fully Automatic Myocardial Segmentation of Contrast Echocardiography Sequence Using Random Forests Guided by Shape Model

Myocardial contrast echocardiography (MCE) is an imaging technique that assesses left ventricle function and myocardial perfusion for the detection of coronary artery diseases. Automatic MCE perfusion quantification is challenging and requires accurate segmentation of the myocardium from noisy and time-varying images. Random forests (RF) have been successfully applied to many medical image segmentation tasks. However, the pixel-wise RF classifier ignores contextual relationships between label outputs of individual pixels. RF which only utilizes local appearance features is also susceptible to data suffering from large intensity variations. In this paper, we demonstrate how to overcome the above limitations of classic RF by presenting a fully automatic segmentation pipeline for myocardial segmentation in full-cycle 2-D MCE data. Specifically, a statistical shape model is used to provide shape prior information that guide the RF segmentation in two ways. First, a novel shape model (SM) feature is incorporated into the RF framework to generate a more accurate RF probability map. Second, the shape model is fitted to the RF probability map to refine and constrain the final segmentation to plausible myocardial shapes. We further improve the performance by introducing a bounding box detection algorithm as a preprocessing step in the segmentation pipeline. Our approach on 2-D image is further extended to 2-D+t sequences which ensures temporal consistency in the final sequence segmentations. When evaluated on clinical MCE data sets, our proposed method achieves notable improvement in segmentation accuracy and outperforms other state-of-the-art methods, including the classic RF and its variants, active shape model and image registration.

Fast Nonlinear Ultrasound Propagation Simulation Using a Slowly Varying Envelope Approximation

Medical systems usually consider linear propagation of ultrasound, an approximation of reality. However, numerous studies have attempted to accurately simulate the nonlinear pressure wave distortion and to evaluate the contribution of harmonic frequencies. In such simulations, the computation time is very large, except for the method based on the angular spectrum scheme where the derivative order is reduced using the Fourier transform. However, the harmonic computation is usually limited to the second harmonic because of quasi-linear approximation. In this paper, a slowly varying envelope approximation (SVEA) is used in the Fourier domain to compute the entire nonlinear distortion induced, including high harmonics and nonlinear mixing frequencies. The simulation by SVEA is evaluated by comparison with other simulation tools. The obtained deviation and difference remain low enough to fully validate such an approximation. Moreover, the simulator is implemented on a GPU to obtain a very fast tool, where the full nonlinear distorted [Formula: see text] field is computed in less than 10 s.

An attempt to apply the inelastic thermal spike model to surface modifications of CaF2 induced by highly charged ions: comparison to swift heavy ions effects and extension to some others material

Surface damage appears on materials irradiated by highly charged ions (HCI). Since a direct link has been found between surface damage created by HCI with the one created by swift heavy ions (SHI), the inelastic thermal spike model (i-TS model) developed to explain track creation resulting from the electron excitation induced by SHI can also be applied to describe the response of materials under HCI which transfers its potential energy to electrons of the target. An experimental description of the appearance of the hillock-like nanoscale protrusions induced by SHI at the surface of CaF₂ is presented in comparison with track formation in bulk which shows that the only parameter on which we can be confident is the electronic energy loss threshold. Track size and electronic energy loss threshold resulting from SHI irradiation of CaF₂ is described by the i-TS model in a 2D geometry. Based on this description the i-TS model is extended to three dimensions to describe the potential threshold of appearance of protrusions by HCI in CaF₂ and to other crystalline materials (LiF, crystalline SiO₂, mica, LiNbO₃, SrTiO₃, ZnO, TiO₂, HOPG). The strength of the electron-phonon coupling and the depth in which the potential energy is deposited near the surface combined with the energy necessary to melt the material defines the classification of the material sensitivity. As done for SHI, the band gap of the material may play an important role in the determination of the depth in which the potential energy is deposited. Moreover larger is the initial potential energy and larger is the depth in which it is deposited.

Understanding and simulating the material behavior during multi-particle irradiations

A number of studies have suggested that the irradiation behavior and damage processes occurring during sequential and simultaneous particle irradiations can significantly differ. Currently, there is no definite answer as to why and when such differences are seen. Additionally, the conventional multi-particle irradiation facilities cannot correctly reproduce the complex irradiation scenarios experienced in a number of environments like space and nuclear reactors. Therefore, a better understanding of multi-particle irradiation problems and possible alternatives are needed. This study shows ionization induced thermal spike and defect recovery during sequential and simultaneous ion irradiation of amorphous silica. The simultaneous irradiation scenario is shown to be equivalent to multiple small sequential irradiation scenarios containing latent damage formation and recovery mechanisms. The results highlight the absence of any new damage mechanism and time-space correlation between various damage events during simultaneous irradiation of amorphous silica. This offers a new and convenient way to simulate and understand complex multi-particle irradiation problems.

Thomson's multitaper approach combined with coherent plane-wave compounding to reduce speckle in ultrasound imaging

In ultrasound imaging, the speckle pattern limits the image quality. Spatial and frequency compounding are commonly used to reduce speckle noise or improve the contrast. Although recent implementations can preserve a frame rate that is compatible with real-time imaging (e.g., synthetic aperture compounding), most classic compounding techniques are based on the coherent combination of several radiofrequency images from the same investigated area, which reduces the frame rate. Furthermore, Thomson's multitaper approach aims to smooth the speckle by incoherently combining the obtained B-mode images after applying different apodization windows on the same original data. With only one acquisition, the frame rate remains high, but the spatial resolution is decreased. To improve the resolution and contrast while reducing the speckle noise, this paper proposes combining the coherent plane-wave compounding technique (CPWC) with Thomson's multitaper method. The resulting multitaper coherent plane-wave compounding (MCPWC) takes advantage of coherent and incoherent approaches. Simulations and experimental results demonstrate that in terms of the signal-to-noise ratio, contrast, and resolution, the image quality is increased using plane wave emissions at approximately ten steering angles with three Thomson's tapers. Outside the focal area, the lateral resolution is improved by a factor of 2, and the contrast is increased by approximately 2dB compared with images obtained using a single focalization technique and Thomson's multitaper approach.

SAXS investigations of the morphology of swift heavy ion tracks in α-quartz

The morphology of swift heavy ion tracks in crystalline α-quartz was investigated using small angle x-ray scattering (SAXS), molecular dynamics (MD) simulations and transmission electron microscopy. Tracks were generated by irradiation with heavy ions with energies between 27 MeV and 2.2 GeV. The analysis of the SAXS data indicates a density change of the tracks of ~2 ± 1% compared to the surrounding quartz matrix for all irradiation conditions. The track radii only show a weak dependence on the electronic energy loss at values above 17 keV nm(-1), in contrast to values previously reported from Rutherford backscattering spectrometry measurements and expectations from the inelastic thermal spike model. The MD simulations are in good agreement at low energy losses, yet predict larger radii than SAXS at high ion energies. The observed discrepancies are discussed with respect to the formation of a defective halo around an amorphous track core, the existence of high stresses and/or the possible presence of a boiling phase in quartz predicted by the inelastic thermal spike model.

Ion-induced elongation of gold nanoparticles in silica by irradiation with Ag and Cu swift heavy ions: track radius and energy loss threshold

Systematic investigations of the energy loss threshold above which the irradiation-induced elongation of spherical Au nanoparticles occurs are reported. Silica films containing Au nanoparticles with average diameters of 15-80 nm embedded within a single plane were irradiated with 12-54 MeV Ag and 10-45 MeV Cu ions at 300 K and at normal incidence. We demonstrate that the efficiency of the ion-induced nanoparticle elongation increases linearly with the electronic energy transferred per ion track length unit from the energetic ions to the silica film. Ion beam shaping occurs above a threshold value of the specific electronic energy transfer. Three relevant regions are identified with respect to the original size of the Au nanoparticles. For 15 and 30 nm diameter particles, elongation occurs for electronic stopping power larger than 3.5 keV nm(-1). For Au nanoparticles with 40-50 nm diameter an electronic stopping power above 5.5 keV nm(-1) is required for elongation to be observed. Elongation of Au nanoparticles with 80 nm diameter is observed for electronic stopping between ∼ 7-8 keV nm(-1). For all combinations of ions and energies, the ion track temperature profiles are calculated within the framework of the thermal spike model. The correlation between experimental results and simulated data indicates a thermal origin of the increase in the elongation rate with increasing the track diameter.

Role of thermodynamics in the shape transformation of embedded metal nanoparticles induced by swift heavy-ion irradiation

Swift heavy-ion irradiation of elemental metal nanoparticles (NPs) embedded in amorphous SiO(2) induces a spherical to rodlike shape transformation with the direction of NP elongation aligned to that of the incident ion. Large, once-spherical NPs become progressively more rodlike while small NPs below a critical diameter do not elongate but dissolve in the matrix. We examine this shape transformation for ten metals under a common irradiation condition to achieve mechanistic insight into the transformation process. Subtle differences are apparent including the saturation of the elongated NP width at a minimum sustainable, metal-specific value. Elongated NPs of lesser width are unstable and subject to vaporization. Furthermore, we demonstrate the elongation process is governed by the formation of a molten ion-track in amorphous SiO(2) such that upon saturation the elongated NP width never exceeds the molten ion-track diameter.

Swelling of SiO2 Quartz Induced by Energetic Heavy Ions

A pronounced swelling effect occurs when irradiating SiO2 quartz with heavy ions (F, S, Cu, Kr, Xe, Ta, and Pb) in the electronic energy loss regime. Using a profilometer, the out-of-plane swelling was measured by scanning over the border line between an irradiated and a virgin area of the sample surface. The step height varied between 20 and 300 nm depending on the fluence, the electronic energy loss and the total range of the ions. From complementary Rutherford backscattering experiments under channelling condition (RBS-C), the damage fraction and corresponding track radii were extracted. Normalising the step height per incoming ion and by the projected range, a critical energy loss of 1.8 ± 0.5 keV/nm was found which is in good agreement with the threshold observed by RBS-C. Swelling can be explained by the amorphisation induced along the ion trajectories. The experimental results in quartz are compared to swelling data obtained under similar irradiation conditions in LiNbO3

Nanopores in track-etched polymer membranes characterized by small-angle x-ray scattering

Nanochannels and nanowires with diameters ranging from 30 to 400 nm were produced by etching ion tracks in thin polyarylate and polycarbonate foils. The shape and the size distribution of dry and wet nanochannels, as well as of nanowires grown therein, were examined by small-angle x-ray scattering. The x-ray intensity as a function of the scattering vector exhibits pronounced oscillations showing that both the channels and the wires have a highly cylindrical geometry and a very narrow size distribution. UV exposure before chemical etching significantly improves the monodispersity of the nanopores. For fixed etching conditions, the scattering patterns provide evidence that the diameter of dry and water-filled channels as well as for embedded nanowires are identical, demonstrating that the pores in the polymer are completely filled.

Fine structure in swift heavy ion tracks in amorphous SiO2

We report on the observation of a fine structure in ion tracks in amorphous SiO2 using small angle x-ray scattering measurements. Tracks were generated by high energy ion irradiation with Au and Xe between 27 MeV and 1.43 GeV. In agreement with molecular dynamics simulations, the tracks consist of a core characterized by a significant density deficit compared to unirradiated material, surrounded by a high density shell. The structure is consistent with a frozen-in pressure wave originating from the center of the ion track as a result of a thermal spike.

Creation of nanohillocks on CaF2 surfaces by single slow highly charged ions

Upon impact on a solid surface, the potential energy stored in slow highly charged ions is primarily deposited into the electronic system of the target. By decelerating the projectile ions to kinetic energies as low as 150 x q eV, we find first unambiguous experimental evidence that potential energy alone is sufficient to cause permanent nanosized hillocks on the (111) surface of a CaF(2) single crystal. Our investigations reveal a surprisingly sharp and well-defined threshold of potential energy for hillock formation which can be linked to a solid-liquid phase transition.

Wetting on nanorough surfaces

We investigate the wetting properties of random nanostructured surfaces, with particular attention devoted to the phenomenon of contact angle hysteresis. For this purpose, solid substrates were initially tailored at a nanometric scale by using swift heavy ion irradiation which produced a random distribution of defects. We characterize the wetting properties of water on these heterogeneous surfaces by an average spreading parameter and by the contact angle hysteresis. For weak values of the areal density of defects phi(d), the hysteresis grows linearly with phi(d), indicating that the defects pin the contact line individually. However, at higher values of phi(d), collective pinning effects appear and the hysteresis decreases with increasing phi(d). We show that in the linear regime our experimental results are in good quantitative agreement with theoretical predictions for contact angle hysteresis induced by a single isolated defect on a solid surface.

Jetlike component in sputtering of LiF induced by swift heavy ions

Angular distributions of sputtered atoms from SiO2 and LiF single crystals were measured under the irradiation of 1 MeV/u swift heavy ions. In contrast to the almost isotropic distribution of SiO2, an additional jetlike component was observed for LiF. The total sputtering yield of SiO2 ( approximately 10(2) atoms/ion) can be reproduced by an extended inelastic thermal spike model, whereas the huge yield of LiF ( approximately 10(4) atoms/ion) needs a substantial decrease of the sublimation energy to be described by the model.