Small Beam Diameter Research Articles

A new positron source with high flux and excellent electron-optical properties

Positron annihilation spectroscopy is a well established research tool to study the surface and bulk electron distributions of solids and liquids. These are extracted from the energy and angular distributions of the two 511keV X-rays, produced during the annihilation of a thermal positron and an electron from the sample. Positron investigations and monitoring, however are currently not used in an industrial environment due to the lack of a sufficiently intense positron sources to record distribution functions with good statistics within minutes. Most positron spectrometers have radioactive sources which produce only modest intensities (106e+/s). An improvement by at least a factor of 100 is needed to become viable for on-line positron metrology. We propose to combine several technologies to generate a positron beam with good electron-optical properties, such as a small divergence angle and small beam diameter and a flux of 108e+/s or more. Positrons from a 10Ci beta source will be moderated with a stack of 12 layers of tungsten meshes. The thermalized positrons will be accelerated into a deflection focusing analyzer (DFA) which focuses the positrons into a small (1–2mm2) area of a second moderator. A rare gas solid will be used to thermalize the positrons once more. The moderating area forms the small emitter source for a accelerating gun that generate a beam of mono-energetic positrons of any desired energy.

Gamma Knife 3‐D dose distribution near the area of tissue inhomogeneities by normoxic gel dosimetry

The accuracy of the Leksell GammaPlan, the dose planning system of the Gamma Knife Model-B, was evaluated near tissue inhomogeneities, using the gel dosimetry method. The lack of electronic equilibrium around the small-diameter gamma beams can cause dose calculation errors in the neighborhood of an air-tissue interface. An experiment was designed to investigate the effects of inhomogeneity near the paranosal sinuses cavities. The homogeneous phantom was a spherical glass balloon of 16 cm diameter, filled with MAGIC gel; i.e., the normoxic polymer gel. Two hollow PVC balls of 2 cm radius, filled with N2 gas, represented the air cavities inside the inhomogeneous phantom. For dose calibration purposes, 100 ml gel-containing vials were irradiated at predefined doses, and then scanned in a MR unit. Linearity was observed between the delivered dose and the reciprocal of the T2 relaxation time constant of the gel. Dose distributions are the results of a single shot of irradiation, obtained by collimating all 201 cobalt sources to a known target in the phantom. Both phantoms were irradiated at the same dose level at the same coordinates. Stereotactic frames and fiducial markers were attached to the phantoms prior to MR scanning. The dose distribution predicted by the Gamma Knife planning system was compared with that of the gel dosimetry. As expected, for the homogeneous phantom the isodose diameters measured by the gel dosimetry and the GammaPlan differed by 5% at most. However, with the inhomogeneous phantom, the dose maps in the axial, coronal and sagittal planes were spatially different. The diameters of the 50% isodose curves differed 43% in the X axis and 32% in the Y axis for the Z =90 mm axial plane; by 44% in the X axis and 24% in the Z axis for the Y=90 mm coronal plane; and by 32% in the Z axis and 42% in the Y axis for the X=92 mm sagittal plane. The lack of ability of the GammaPlan to predict the rapid dose fall off, due to the air cavities behind or near the lesion led to an overestimation of the dose that was actually delivered. Clinically, this can result in underdosing of lesions near tissue inhomogeneities in patients under treatment.

Application of adjoint Monte Carlo to accelerate simulations of mono‐directional beams in treatment planning for Boron Neutron Capture Therapy

This paper deals with the application of the adjoint transport theory in order to optimize Monte Carlo based radiotherapy treatment planning. The technique is applied to Boron Neutron Capture Therapy where most often mixed beams of neutrons and gammas are involved. In normal forward Monte Carlo simulations the particles start at a source and lose energy as they travel towards the region of interest, i.e., the designated point of detection. Conversely, with adjoint Monte Carlo simulations, the so-called adjoint particles start at the region of interest and gain energy as they travel towards the source where they are detected. In this respect, the particles travel backwards and the real source and real detector become the adjoint detector and adjoint source, respectively. At the adjoint detector, an adjoint function is obtained with which numerically the same result, e.g., dose or flux in the tumor, can be derived as with forward Monte Carlo. In many cases, the adjoint method is more efficient and by that is much quicker when, for example, the response in the tumor or organ at risk for many locations and orientations of the treatment beam around the patient is required. However, a problem occurs when the treatment beam is mono-directional as the probability of detecting adjoint Monte Carlo particles traversing the beam exit (detector plane in adjoint mode) in the negative direction of the incident beam is zero. This problem is addressed here and solved first with the use of next event estimators and second with the application of a Legendre expansion technique of the angular adjoint function. In the first approach, adjoint particles are tracked deterministically through a tube to a (adjoint) point detector far away from the geometric model. The adjoint particles will traverse the disk shaped entrance of this tube (the beam exit in the actual geometry) perpendicularly. This method is slow whenever many events are involved that are not contributing to the point detector, e.g., neutrons in a scattering medium. In the second approach, adjoint particles that traverse an adjoint shaped detector plane are used to estimate the Legendre coefficients for expansion of the angular adjoint function. This provides an estimate of the adjoint function for the direction normal to the detector plane. In a realistic head model, as described in this paper, which is surrounded by 1020 mono-directional neutron/gamma beams and from which the best ones are to be selected, the example calculates the neutron and gamma fluxes in ten tumors and ten organs at risk. For small diameter beams (5 cm), and with comparable relative errors, forward Monte Carlo is seen to be 1.5 times faster than the adjoint Monte Carlo techniques. For larger diameter neutron beams (10 and 15 cm), the Legendre technique is found to be 6 and 20 times faster, respectively. In the case of gammas alone, for the 10 and 15 cm diam beams, both adjoint Monte Carlo Legendre and point detector techniques are respectively 2 and 3 times faster than forward Monte Carlo.

Extraction of small-diameter beams from single-component plasmas

A nondestructive technique is described to extract small-diameter beams from single-component plasmas confined in a Penning-Malmberg trap following radial compression using a rotating electric field. Pulsed beams with Gaussian radial profiles and diameters as small as 50μm are extracted from electron plasmas initially 2mm in diameter. A simple theory for the beam diameter predicts 4λD (full width to 1∕e), where λD is the Debye length, in good agreement with experimental measurements on electron plasmas. Applications and extensions of this technique to create bright, finely focused beams of positrons and other scarce particles are discussed.

Production of a uniform cellular injury by raster scanning of cells for the study of laser bioeffects

Efforts to understand laser bioeffects in cells and tissues have been hindered by a nonuniform cellular response of the specimen, resulting in graded biochemical effects. In addition, the small beam diameters of commonly used lasers limit the number of cells expressing a response to numbers inadequate for the study of biochemical effects. For a limited emission power, expansion of the beam diameter reduces the irradiance, thus requiring longer exposure durations to produce a cellular response. Cultured human retinal epithelial cells were exposed as a single spot ("tophat" exposure) from a carbon dioxide (CO(2)) laser operating at 10.6 microm or scanned with a raster system and compared with thermal injury produced with heated saline for short periods (1-9 s) at relatively high temperature (55-70 degrees C). Cell viability and induction of the 70 kDa heat shock protein were evaluated as indicators of the cellular response. Initial attempts to use a tophat (uniform energy distribution) exposure resulted in a nonuniform cellular response (and nonuniform energy distribution) due to diffraction effects from the 2-mm selection aperture. However, raster scanning for appropriate times with the CO(2) laser yielded uniform cell viability and heat shock protein synthesis that were comparable to dipping cells in heated saline. Because scanning results in a homogeneous exposure of cells, the described scanning technique may be applied to studies of cellular responses to other lasers to evaluate photochemical and photomechanical effects.

Quantitative trace element analysis with sub-micron lateral resolution

In recent years many nuclear microprobes have developed to sophisticated tools for elemental analysis with high resolutions down to about 1μm micron. The application to trace element analysis is mainly in the field of biological and medical research. Numerous successful studies on microscopic scale structures, e.g. cells, lead to the demand for higher spatial resolution or lower detection limits. Therefore, several labs started new efforts for sub-micron resolutions, sometimes intending 100nm.The Leipzig microprobe laboratory LIPSION has recently improved its analytical capabilities. We are now able to perform quantitative trace element analysis with sub-micron spatial resolution (beam diameter 0.5μm at 120pA). As an example we give the trace element distribution in neuromelanin (intracellular pigment of neurons). Furthermore, when the scan size is reduced from cellular level, i.e. about 50μm, to sub-cellular level of about 10μm, the beam diameter can further be reduced by choosing smaller object diaphragms. The unavoidable reduction in beam current will not affect the mapping sensitivity unless the accumulated charge per spatial resolution is not decreased. The smallest beam diameter with analytical capabilities for elemental analysis we achieved thus far was about 300nm in diameter. It enables an outstanding microPIXE resolution. However, some difficulties appeared in high-resolution work, which limited the acquisition time to less than 30min.

Micro electron beam welding

Electron Beam Welding is a well-established bonding method in macro applications. The properties of this method make it very promising for Microsystems, the very small possible beam diameter predestine this technology for the realisation of smallest spot or line welds of metal and also of non-metal materials with locally concentrated energy input into the component. The wearless and inertia-free beam allows highly dynamic joining processes. Where even smallest particles of dust can influence the results, the processing inside a vacuum chamber protect the components to a high degree from contamination and oxide formation during the welding cycle. In this study, the process is downscaled by modifying a scanning electron microscope (SEM), which operates according to the same principle as an electron beam welding machine. This creates the possibility to observe and weld the components with the same device. The necessary modifications are investigated, as well as the possibilities to expand the device, which makes it a promising tool for the flexible production of hybrid Microsystems. The presented investigations and results are acquired within the project SFB 440 “Assembly of Hybrid Microsystems”, financed by the “German Society for Research” (DFG).

Therapeutic step and shoot proton beam spot-scanning with a multi-leaf collimator: a Monte Carlo study

In step and shoot spot-scanning, a small-diameter proton beam is magnetically swept and varied in energy in order to cover the tumour. Initial estimates of the beam size indicate that additional collimating hardware will be needed for lower energy proton beams in order to achieve a clinically acceptable lateral dose falloff at the edge of the proton beam. In this report, we present dosimetric data from Monte Carlo simulations with a model of a simple multileaf collimator which indicate that such a device may be used to improve the lateral dose falloff. The dosimetric quantities relevant to the clinical usefulness of the device are studied, including lateral penumbra, leaf transmission and scalloping effect. Multileaf collimation is compared with a differential spot-weighting technique of sharpening the lateral dose falloff.

High-power CO2 laser-MIG hybrid welding for increased gap tolerance. Hybrid weldability of thick steel plates with a square groove

High power enhancement of laser processors has recently led to laser welding being used for the joining of thick steel plates. For example, the literature includes a study on the practical application of a 45 kW CO2 laser for the welding of thick steel plates,1 although few such applications have thus far been documented. The limited application of lasers to welding of thick steel plates is attributable to the narrow gap tolerance provided and the small laser beam diameter that leads to unacceptable welding defects, such as underfills, melt-through, and porosities.

Welding of AZ31B Magnesium Alloy by YAG Laser/ TIG Arc Hybrid Welding Process

Recently, magnesium alloy is in the limelight as ECological material with high ability of recycling and lightweight property. Especially, because of outstanding properties as a structural material, the magnesium alloy is in great demand. Under these circumstances, the technical researches and developments of welding process for magnesium alloy welding have great significance to expand industrial application of magnesium. In magnesium alloy welding, arc welding process is generally used. However, heat input by the welding arc affects the magnesium alloy plates, and as the result it makes wide heat affected zone and large distortion of the welded joint. As one of the possible means to improve quality of the welded joint, application of laser welding process may be recommended. However, the low boiling point of magnesium generates some weld defects, such as porosity and solidification cracking. Furthermore, the small laser beam diameter is very sensitive to edge preparation in butt joint. Accordingly, application of laser/ arc hybrid welding process to magnesium alloy welding was proposed. The laser/ arc hybrid welding process is a new process combining the laser beam and the arc as welding heat source. The laser beam and arc influence and assist one another. By application of hybrid welding, synergistic effects are achievable, and disadvantage of the respective processes can be compensated. In this study, welding of AZ31B magnesium alloy thin plates using YAG laser/ TIG arc hybrid welding process was investigated. In order to confirm the properties of the welded joints, tensile testing was performed.

High-Resolution 3D Imaging Microscopy Using Hard X-Rays

AbstractThe key strength of hard x-ray full-field microscopy is the large penetration depth of hard x-rays into matter, which allows one to image the interior of opaque objects. Combined with tomographic techniques, the three-dimensional inner structure of an object can be reconstructed without the need for difficult and destructive sample preparation. Projection microscopy and microtomography are now routinely available at synchrotron radiation sources. The resolution of these techniques is limited by that of the detector to 1 µm or slightly less. X-ray images and tomograms at higher spatial resolution can be obtained by x-ray optical magnification, for example, by using parabolic refractive x-ray lenses as a magnifying optic. Combining magnifying x-ray imaging with tomography allows one to reconstruct the three-dimensional structure of an object, such as a microprocessor chip, with resolution well below 1 µm. In x-ray scanning microscopy, the sample is scanned through a small-diameter beam. The great advantage of scanning microscopy is that x-ray analytical techniques such as fluorescence analysis, diffraction, and absorption spectroscopy can be used as contrast mechanisms in the microscope. In combination with tomography, fluorescence analysis makes it possible to reconstruct the distribution of different chemical elements inside an object (fluorescence microtomography), while combining absorption spectroscopy with tomography yields the distribution of different oxidation states of atomic species.

Measurement of a broad range of Gaussian laser beam diameters using various periodic and aperiodic rulings

A novel analytical approach is proposed for an accurate estimation of a broad range of Gaussian laser beam diameters. The new approach, based on the Fourier transform and the convolution operations, is used to study the performance of most recently proposed periodic and aperiodic rulings. Further, for applications that require extremely small beam diameter measurements, a new Ronchi-like ruling is proposed; while for applications requiring large beam diameters measurements, various aperiodic non-Ronchi rulings are proposed. Furthermore, for spot-size measurement applications, which range from very small to very large beam diameters, a new single aperiodic exponential ruling is proposed. This new ruling eliminates the necessity of using a large number of rulings for measuring a broad range of beam diameters.

A simple empirical method for high-quality electron microprobe analysis of fluorine at trace levels in Fe-bearing minerals and glasses

We present a new, high-quality electron microprobe method for analyzing F in Fe-bearing minerals and glasses down to trace levels ( K α peak onto the “shoulder” of the Fe L α 1 peak that arises when using synthetic multi-layered diffraction crystals. It also achieves high precision and accuracy while maintaining low detection limits and a small beam diameter. Analytical conditions for F using the new method and a synthetic W/Si diffraction crystal are: 10 kV accelerating voltage, 180 nA beam current, 8 μ m beam diameter, and 400 second total peak count time. An iterative beam exposure method is employed to minimize potential damage to the sample due to beam heating. The method presented here can be used to address problems related to degassing of F at active volcanoes, the reservoirs of F in the mantle, and the partitioning of F between minerals and silicate liquid.

Accurate measurement of small Gaussian laser beam diameters using various rulings

A new analytical approach based on Fourier transform and convolution operations for measuring very small Gaussian laser beam diameters is proposed. In addition, a comparative study of various recently reported rulings is presented. A general rule of thumb approach to select a ruling for measuring small beam diameters is demonstrated. Finally, a new Ronchi ruling capable of measuring extremely small beam radius is proposed.

Recent developments in the study of presolar grains

Studies of presolar grains have provided a wealth of data that are relevant to astrophysics. Their isotopic ratios are especially informative for understanding stellar nucleosynthesis and mixing in stars and stellar ejecta. In the past few years, isotopic ratios of heavy elements in single presolar grains have been analyzed with resonant ionization mass spectrometry. Molybdenum, Zr, and Ba isotopic patterns of SiC grains of a supernova origin agree with patterns predicted for a neutron burst. A new type of ion microprobe, the NanoSIMS, with its high sensitivity and small beam diameter, makes isotopic analyses of submicron grains possible.

Characterization of a polycapillary lens for use in micro‐XANES experiments

AbstractPolycapillary lenses permit x‐rays to be focused down to relatively small (<50 µm) beam diameters. A polycapillary device, suitable for focusing of synchrotron radiation, was characterized at beam line L of HASYLAB (Hamburg, Germany). Beam parameters such as beam size, transmission efficiency and gain in flux density were determined as a function of the energy of the primary monochromatic beam entering the polycapillary. At a distance of 4 mm between the lens tip and the sample, the smallest beam diameters were recorded: 30 µm at 7 keV, decreasing to 8 µm at 21 keV. In the energy range from 7 to 15 keV, the transmission efficiency varied between 30 and 45%, while the maximum gain in flux density was calculated to be around 3500. When the energy was changed from 7 to 30 keV, a vertical translation of only 7–8 µm was observed, indicating that this optic is highly suitable for use in µ‐XANES investigations. To illustrate this point, the investigation of paper corrosion due to iron gall inks and the speciation of Ni in environmental particles is described. Copyright © 2003 John Wiley & Sons, Ltd.

大出力ＣＯ２レーザ・ＭＩＧハイブリッド溶接特性と開先裕度拡大効果―ハイブリッド溶接による厚鋼板の突合せ溶接特性（第１報）―

There are few applications of laser welding to thick plates joining. One reason is attributed to narrow gap tolerance due to a small laser beam diameter leading to unacceptable seam defects such as underfill and burn through. Another is easy formation of weld imperfection such as porosity and cracking in a keyhole—type of deeply penetrated laser weld beads. Hybrid welding with a laser beam and an arc has been receiving considerable attention as one of the methods to suppress such defects and imperfection. This paper reports fundamental investigation results of hybrid welding with combined high power CO2 laser and Metal Inert Gas (MIG) arc, and especially hybrid welding results of thick plates with such a wide gap that the majority of a laser beam can pass through. The behavior of laser-induced and arc plasmas near the molten pool during the hybrid welding was observed with a black-and-white CCD camera. Penetration depth were confirmed to depend upon the distance between laser beam irradiation location and MIG wire target. The different depths were interpreted in terms of the interaction between laser-induced plasma and molten metal, melt flows, and so on. Moreover, under the formation conditions of the deepest weld beads, the wide gap groove was fully filled with the molten metal. The observation suggests that the recoil pressure due to the laser-induced evaporation should force the molten metal fed with a MIG wire to fill the leading gap and a fully penetrated keyhole was formed during hybrid welding. Consequently, a sound hybrid weld bead could be produced in joining the plates of 22 mm thickness with 4 mm gap, and moreover some weld beads indicated the best grade of the inspection test result according to JIS Z3104.

Wave Localization of Doubly Periodic Guided-mode Resonant Grating Filters

It is known that a doubly periodic guided-mode resonant grating (GMRG) filter has a broad angular selectivity with a narrow spectral bandwidth. This means that the doubly periodic GMRG filter operates for small beam diameter and grating area. This report describes the wave localization in the doubly periodic GMRG filter. We investigated the spread area of light waves in the waveguide layer and the accumulation of field energy by numerical simulation using the finite differential time domain (FDTD) method. Simulation results showed that, in the case of a doubly periodic GMRG filter with a Q factor of 600, the field energy is spread over an area 5 um in width, which corresponds to the expected value from the angular tolerance. And the magnitude of the field energy in the waveguide layer was Q factor times greater than the incident energy. On the other hand, a singly periodic GMRG filter with the same Q factor spread the field energy over an area 72 urn in width. This filter does not work for a small size structure or a small diameter light beam.

Determining source size from diode laser systems

Although hazard classification is performed at predefined distances, the maximum permissible exposure (MPE) for extended-source diode lasers decreases as the distance from the laser increases. The MPE depends on the apparent angular subtense of the source, which is simply the magnified image of the emitting diode. Small diameter beams require special consideration.

Microprocessing by intense pulsed electron beam

Generation of intense electron beams by superposing two discharges, namely, a low-pressure DC glow discharge and a high-current pulsed discharge at pressures and voltages very similar to that of the pseudospark gap devices, has been studied. The small beam diameter, high peak current, and short pulse length are the characteristics of the electron beam generated using this technique. This technique does not require high vacuum facilities, and applications such as microprocessing, X-ray generation, and preionization for high-power lasers are feasible. The electron beam current is in the range of 10-30 A. Microprocessing as drilling tenths of microns diameter holes in different materials such as copper, titanium, and tantalum has been performed. The microstructure of the irradiated materials has been investigated by means of scanning electron microscope (SEM) and atomic force microscope (AFM). Currently, the possibility of processing a matrix of holes is being considered. In this paper, preliminary study regarding the interaction of intense short electron beams with different metallic targets is presented. The electron generator consists of a fast filamentary discharge in low-pressure gases, and the filling gases such as helium, argon, or nitrogen at approximately 0.1 torr pressure are used. Some of the important characteristics of the device for drilling and micromachining are that the targets can be easily changed, and the beam can be deflected by means of a magnetic field. After the initial device characterization, several holes at different experimental conditions were drilled onto the target materials, and the interaction of the beam with these different targets was investigated by means of a scanning electron microscope.