Axis Of Incident Beam Research Articles

Multislit gamma camera for external beam radiotherapy assistance: Experimental proof of concept

The orthogonal computed tomography (OrthoCT) concept, based on orthogonal ray imaging, is a low-dose imaging technique currently under investigation to potentially aid in external-beam radiation therapy treatments. This technique involves detecting radiation scattered within the patient and emitted at approximately 90° from the direction of the incoming beam. This scattered radiation can be collected by a 1D-detector system with a multisliced collimator positioned perpendicular to the incident beam axis. Such a system holds promise for on-board imaging with the patient positioned and prepared for treatment, as well as for real-time treatment monitoring. In this study, a multi-slice OrthoCT detector prototype was developed and tested under in-beam irradiation. The system utilizes gadolinium orthosilicate scintillator crystals coupled to photomultiplier tubes and a collimator made of lead slices. Experimental measurements were conducted using a heterogeneous phantom made of acrylic with an air cavity inside. The phantom was irradiated with a TrueBeam linac operating at 6 MV in the flattening-filter-free mode. The findings of this study indicate that this innovative imaging technique is capable of providing morphological images of the phantom. This accomplishment is achieved without the need to rotate the X-ray source around the object to be irradiated, demonstrating the feasibility of such a system.

Rapid biomechanical imaging at low irradiation level via dual line-scanning Brillouin microscopy.

Brillouin microscopy is a technique for mechanical characterization of biological material without contact at high three-dimensional resolution. Here, we introduce dual line-scanning Brillouin microscopy (dLSBM), which improves acquisition speed and reduces irradiation dose by more than one order of magnitude with selective illumination and single-shot analysis of hundreds of points along the incident beam axis. Using tumor spheroids, we demonstrate the ability to capture the sample response to rapid mechanical perturbations as well as the spatially resolved evolution of the mechanical properties in growing spheroids.

Study of the Disperse Composition of Suspensions and Sputtered Substances by means of Small-Angle Light Scattering

Spatial distribution of the light scattered by a disperse system of particles depends on their sizes, shapes, positions, etc., which can be used for experimental determination of the parameters mentioned. For stochastic systems with the particles’ sizes exceeding the radiation wavelength, most of the scattered radiation concentrates near the incident beam axis. In this small-angle approximation, the scattering pattern is especially simple and regular, which enables to develop efficient procedures for the disperse system investigation. We describe the algorithm for determination of the mean particle radius in the system with lognormal distribution of the particle sizes and negligible multiple scattering. The algorithm’s performance is demonstrated on the practical example of the “fog” generated by a gasoline injector. The ways are discussed for further algorithm generalization and its extension to a non-parametric analysis of disperse systems with a priori unknown form of the particle sizes’ distribution.

Passage of 10-keV Electrons through a Bundle of Dielectric Channels

Passage of a 10-keV electron beam through a bundle of hollow polysulfone fiber with an inner channel diameter of 160 ± 60 µm was studied. The dependence of the electron beam that passed through the channels on the channel tilt angle with respect to the incident beam axis was measured. The fraction of electrons with an energy loss of less than 10% after passing the channels was also estimated.

Прохождение электронов с энергией 10 keV через массив диэлектрических каналов

The passage of 10 keV electron beam through a bundle of a hollow polysulfone fiber with an inner channel diameter of 160 ± 60 μm was studied. Dependence of a fraction of the electron beam transmitted through the channels on the tilt angle of the channels relative to the incident beam axis is measured. The fraction of electrons that experienced energy losses of less than 10% after passing through the channels was estimated.

Absolute measurements of the double differential electronic emission cross-sections of isolated pyrene molecule (C16H10) in interaction with keV protons

Electronic emission of an isolated pyrene molecule induced by 50 to 125 keV protons has been investigated. The corresponding absolute double differential cross-sections have been measured at an angle of 35° with respect to the incident proton beam axis. These measurements were compared to the simulations of emitted electrons’ spectra using a semi-classical trajectory Monte-Carlo (SCTMC) approach. The simulated SCTMC cross-sections show reasonably good agreement with the measured ones.

Displacements of optical vortices in Laguerre–Gaussian beams diffracted by a soft-edge screen

Based on the earlier approach (Bekshaev et al. 2016, 2017), we numerically analyze the edge diffraction of circular Laguerre–Gaussian (LG) beams carrying optical vortices (OV) with allowance for the non-sharp edge character. The edge blurring is modeled by the smooth variation of the screen transparency within a rectilinear band; the band half-width being a quantitative measure of the edge softness. It is shown that the edge blurring reduces the intensity of the diffraction fringes (ripple structure) and smoothens the spiral-like trajectories of OV cores in the diffraction beam cross section. Accordingly, the absolute values of the OV displacements decrease, while the trajectories’ discontinuities (“jumps”) become less articulate and disappear. The edge-softness action is more noticeable just behind the screen (about 10% of the Rayleigh length of the incident LG beam) and/or under conditions of weak diffraction perturbation (when the screen edge is far enough from the incident beam axis). The results obtained demonstrate the possibility of the smooth and purposeful adjustment of the OV positions within the diffracted beam, which can be useful in the OV metrology and for the micromanipulation techniques.

Charge dissipation and self focusing limit in high current density ion beam transport through a micro glass capillary

Charge dissipation and self focusing limit in high current density ion beam transport through micro glass capillaries are investigated. It is demonstrated that ion beams can be guided efficiently (capillary exit diameters; straight capillary (SC): 860 µm, tapered capillary (TC): 500 µm, 300 µm, 200 µm, 110 µm, 45 µm and 20 µm) with minimal loss of beam current, and thereby transport of ion beams (J ~ 600 Am−2) become possible, over a tilt angle of 5° from the incident beam axis. The investigation reveals the existence of a lower limit on the capillary size, below which Coulomb repulsion of the beam’s space charge dominates over the inward radial forces of the charges that get smeared on the capillary inner wall, and the beam can no longer self-focus, although the beam may be transported. Beam size compression at the capillary exit by as much as 81% could be achieved. A theoretical model validates the result and the self focusing factor tends to 1 for smaller capillary outlet. Charge dissipation is evaluated during a hysteresis cycle of beam current with ion energy, and it is found that maximum charge dissipation occurs for the 300 µm capillary outlet (~6 × 1013 electronic charge). A nonlinear behaviour of the beam spot diameter with the capillary outlet is observed. Particle in cell (PIC) simulations are performed to interpret results.

Nonlinear frequency doubling characteristics of asymmetric vortices of tunable, broad orbital angular momentum spectrum

We report on a simple experimental scheme to generate and control the orbital angular momentum (OAM) spectrum of the asymmetric vortex beams in a nonlinear frequency conversion process. Using a spiral phase plate (SPP) and adjusting the transverse shift of the SPP with respect to the incident Gaussian beam axis, we have transformed the symmetric (intensity distribution) optical vortex of order l into an asymmetric vortex beam of measured broad spectrum of OAM modes of orders l, l − 1, l − 2, …, 0 (Gaussian mode). While the position of the SPP determines the distribution of the OAM modes, we have also observed that the modal distribution of the vortex beam changes with the shift of the SPP of all orders and finally results in a Gaussian beam (l = 0). Using single-pass frequency doubling of the asymmetric vortices, we have transferred the pump OAM spectra, l, l − 1, l − 2, …, 0, into the broad spectra of higher order OAM modes, 2l, 2l − 1, 2l − 2, …, 0 at green wavelength, owing to OAM conservation in nonlinear processes. We also observed an increase in single-pass conversion efficiency with the increase in asymmetry of the pump vortices producing a higher power vortex beam of mixed OAM modes at a new wavelength than that of the pure OAM mode.

About a contactless transmission of 10 keV electrons through tapering microchannels

The possibility of increasing the current density of a beam of fast electrons passes through glass cone-shaped channels was demonstrated in [1]. But the fraction of the electrons that passed through the conical channels without loss of the initial energy was not cleared up. Measurements of X-ray spectra generated by transmitted electrons in copper target mounted in vicinity of capillary output were performed for a detailed study of the contactless passage of 10 keV electrons through conical capillaries. All the measurements were made at different tilting angles with respect to the incident beam axis. It is shown that a significant part of electrons retains its initial energy even at the angles exceeding geometric opening of the capillary.

Dual-channel quantitative phase microscopy based on a single cube beamsplitter interferometer

Quantitative phase microscopy, as a non-destructive and non-invasive measurement technique, can indirectly reflect three-dimensional (3D) morphology and optical properties of transparent microstructure object by measuring phase information. In recent years, this kind of technique has been widely used to detect and investigate the characteristics of biological cells and it has become more and more important in the field of modern biomedical and life science. In this paper, only by using a single cube beamsplitter interferometer, a simple single-shot dual-channel quantitative phase microscopic measurement technique is demonstrated for 3D quantitative phase imaging of biological cells. In the proposed method, a conventional non-polarized cube beamsplitter is the most pivotal element. Unlike its traditional application method, the cube beamsplitter is tilted in a nonconventional configuration and the illumination beam is only incident on the left (or right) half of the cube beamsplitter (just the one side of central semi-reflecting layer), and a very small angle is introduced between the central semi-reflecting layer and the optical axis of incident beam. Based on the light splitting characteristic of the cube beamsplitter, two replicas of incident beam are generated. These two generated replicas (transmission beam and reflection beam) are of symmetry with respect to each other, and they will encounter and form interference when the direction of the incident beam meets a certain condition. Adjust the sample to a suitable position and make it only contact one half of incident beam, and the modulated beam will be seen as the object beam and the remaining clean half of incident beam as the reference beam. When the interference phenomenon occurs, two interference channels with a relative π (rad) phase-shift in one interferogram are acquired simultaneously only using one digital camera, and the higher spatial frequency of interference fringes can be achieved by adjusting a relatively big angle between the central semi-reflecting layer and the optical axis of incident beam. Because of the off-axis interference mode, we only need to record one interferogram to gain the continuous phase information and avoid using complex phase-shift techniques. At the same time, this proposed method is of simple structure and easy to operate due to using less ordinary off-the-shelf optical elements. All these simplify the structure of the system and reduce the cost of the system as much as possible. Finally, the phase information of paramecium is successfully obtained from different interference channels respectively. Furthermore, according to the characteristic of π (rad) phase-shift, we also realize the calibration and determination of ultimate precise phase information of sample by using the method of averaging between these two channels. The experimental results show that our proposed method is suitable for 3D surface morphology measurement of small transparent samples.

Singular skeleton evolution and topological reactions in edge-diffracted circular optical-vortex beams

Edge diffraction of a circular optical vortex (OV) beam transforms its singular structure: a multicharged axial OV splits into a set of single-charged ones that form the ‘singular skeleton’ of the diffracted beam. The OV positions in the beam cross section depend on the propagation distance as well as on the edge position with respect to the incident beam axis, and the OV cores describe regular trajectories when one or both change. The trajectories are not always continuous and may be accompanied with topological reactions, including emergence of new singularities, their interaction and annihilation. Based on the Kirchhoff-Fresnel integral, we consider the singular skeleton behavior in diffracted Kummer beams and Laguerre-Gaussian beams with topological charges 2 and 3. We reveal the nature of the trajectories’ discontinuities and other topological events in the singular skeleton evolution that appear to be highly sensitive to the incident beam properties and diffraction geometry. Conditions for the OV trajectory discontinuities and mechanisms of their realization are discussed. Conclusions based on the numerical calculations are supported by the asymptotic analytical model of the OV beam diffraction. The results can be useful in the OV metrology and for the OV beam’s diagnostics.

Localization and migration of phase singularities in the edge-diffracted optical-vortex beams

When a circularly-symmetric light beam with optical vortex (OV) diffracts at an opaque screen with the sharp edge, the OV core is displaced from the beam axis and, in case of the m-charged incident OV, decomposed into |m| single-charged ones. By means of numerical simulations and based on examples of incident beams with topological charges |m| = 1, 2, 3 we show that, while the screen edge monotonously advances towards the beam axis, the OVs in the diffracted beam cross section move away from the incident beam axis along spiral-like trajectories. The trajectories contain fine structure details that reflect the nature and peculiar spatial configuration of the diffracting beam. For the Kummer beams’ diffraction, the trajectories contain self-crossings and regions of ‘backward’ rotation (loops); in the case of Laguerre–Gaussian beams, the trajectories are smoother. The numerical results are supported by analytical approximations and conform to experiments. The general shape of the trajectories and their local behavior show high sensitivity to the diffraction conditions (spatial structure of the diffracting beam, its disposition with respect to the screen edge, etc), which can be used in diverse metrological applications.

Spatial profile and singularities of the edge-diffracted beam with a multicharged optical vortex

We analyze the edge diffraction of circular Laguerre–Gaussian beams LG02 and LG03 carrying multicharged optical vortices (OVs), with special attention to spatial properties and evolution of the diffracted beam. The problem is considered numerically within the frame of paraxial approximation and the Fresnel diffraction theory. The diffracted beam evolution is interpreted on the basis of the incident beam symmetry breakdown and decomposition of a higher-order OV into a set of single-charged secondary OVs. Upon propagation, the separate OVs describe specific trajectories within the diffracted beam cross section but in the far field they are localized on the symmetry axis parallel to the screen edge. The OVs' positions in each cross section reflect information of the screen edge position with respect to the incident beam axis. If the incident OV is stopped by the screen, the diffracted beam possesses no OV just behind the screen but in the course of its further propagation, single-order OVs appear consecutively so that in the far field the number of OVs equals to the absolute topological charge of the incident OV. Possible applications for the remote measurements of small linear displacements are discussed.

Optical vortex generation by volume holographic elements with embedded phase singularity: Effects of misalignments

Based on the linear theory for optical vortex (OV) formation in volume holographic elements (HE) with embedded phase singularity (A. Bekshaev et al., Opt. Commun. 285 (2012) 4005), we analyze theoretically the OV-beams obtained when the incident Gaussian beam axis deviates from the optical axis of the HE. For different displacements of the incident beam with respect to the HE centre, the spa- tial characteristics of the diffracted beams and their evolution during the post-HE propagation are investigated numerically with allowance for the radiation extinction in the HE depth. A special attention is paid to behaviour of the beam centroid (centre of gravity) trajectory. The sensitivity of the generated OV-beam profile to the inci- dent beam misalignments can be used for the output beam shaping and control, in particular, for compensation of the OV-beam distortions associated with the light ex- tinction.

SU‐E‐T‐116: The Water Equivalence of Organic Liquid Scintillators for Proton Dosimetry

Organic liquid scintillators are currently under investigation for use in proton dosimetry. The purpose of this work is to evaluate the water equivalence of these materials as a preliminary step to identify scintillators that are well-suited to this purpose. Stopping powers were calculated for 0.001-1000 MeV protons in water, polystyrene, and two organic liquid scintillators: BC-531 and OptiPhase 'Hi-Safe' 3 at 0%, 25%, and 50% concentrations of water. Angular scatter was quantified by theta0, a characteristic multiple Coulomb scattering angle analogous to the standard deviation of a Gaussian distribution of proton angles relative to the incident beam axis. Theta0 was calculated as a function of depth over the range of 200 MeV protons in these materials. Collisional stopping power in BC-531 ranged from +44% to +1% ofthat in water. It remained within 6% from 2-600 MeV. OptiPhase ranged from +24% to -2%, with smaller deviations at increased water concentrations. At all concentrations, OptiPhase showed smaller deviations than polystyrene and BC-531 and remained within 1% of water from 2-600 MeV.Theta0 was very similar for all materials, with deviations from water of 5 milliradians or less over the majority of the proton range. BC-531 showed deviations of 10 milliradians or more in the last few millimeters of the range. OptiPhase showed smaller deviations than BC-531 or polystyrene, and these deviations decreased with increasing water concentration. OptiPhase was found to be more water equivalent than BC-531 or polystyrene in stopping power and angular scatter, and increased water concentration improved both quantities. Large deviations in stopping power were only found below 2 MeV for any material, where proton range is less than 0.1 millimeter. The deviations from water found in angular scatter were less significant, and probably too small to affect measurement.

Computational study of the optical trapping of ellipsoidal particles

Ellipsoidal dielectric particles may be trapped in a linearly polarized Gaussian beam such that they are harmonically bound with respect to each of their rotational and translational degrees of freedom. The ellipsoid belongs to the highest symmetry class for which this is possible. Typically, the longest axis of the ellipsoid aligns itself with the incident beam axis and the second longest with the polarization direction. We investigate this special property by evaluating the trap stiffness matrix for dielectric ellipsoids with aspect ratios (largest:smallest dimension) in the range 1--10, using the discrete dipole approximation. The results are interpreted using a simple phenomenological model and conclusions are drawn concerning optimization of the trap stiffness for specific applications.

Research on the optical field distribution and micro-fabrication model in convex-surface laser lithography

A system of convex-surface laser lithography with diode laser is established in this paper. Based on this system, a mathematical model of optical field distribution and lithography on the photoresist layer of convex-surface substrate with diode laser is presented. According to the lithography system and model, some numerical simulations are carried out. The simulation result shows that lithographic lines on convex-surface lithography are not symmetric about the optical axis of incident laser beam. Axis of lines at different vector radius on convex-surface substrate will offset from the wavefront normal of incident laser beam. The offset distance depends on the slopes of different equivalent slants. The simulative results of lithographic model agree well with the lithographic experimental data.

Material ejection and redeposition following atmospheric pressure near-field laser ablation on molecular solids

Near-field laser ablation (NF-LA) coupled with mass spectrometry (MS) is very promising for highly spatially resolved chemical analyses on various substrates at atmospheric pressure, for example, in materials and life science applications. Although nanoscale sample craters can be produced routinely, no molecular mass spectra of ablated material from craters of <or=1 microm diameter have ever been acquired by NF-LA MS at atmospheric pressure. Some of the pressing questions are thus how much of the ablated material is transported into the mass spectrometer and in what form. Therefore, material redeposition on the near-field tip's surface from laser ablation of molecular solids was characterized with scanning electron microscopy. The crater profiles were studied by scanning probe microscopy. The results shown in this study demonstrate that there could be as much as 70% of the ablated material deposited on the near-field tip's surface. The redeposited products were found to be confined to a height of approximately 50 microm, thus suggesting that most components inside near-field ablation plumes propagate about the same distance for both anthracene and tris(8-hydroxyquinolinato)aluminum. Nanoparticles ablated from craters of <or=1 microm diameter are clearly observed. Furthermore, observation of tips after ablation of an anthracene surface angled at 60 degrees with respect to a horizontal surface shows that the direction of the near-field ablation plume is neither in the direction of the surface normal nor towards the axis of incident laser beam but deflected further away from surface normal.

Focused ion beam processing of organic crystal (TMTSF)2PF6. A combined conducting probe atomic force microscopy and secondary ion mass spectrometry study

We study the behavior of the organic material (TMTSF)2PF6 (TMTSF: tetramethyl tetraselena fulvalene) under focused Ga+ ion beam impact using conducting probe atomic force microscopy (CP-AFM) and secondary ion mass spectrometry. The (TMTSF)2PF6 crystals are exposed to a 25 keV focused ion beam with the sample native surface both perpendicular and slightly tilted to the incident beam axis, where drastically different etching effects are observed. Moreover, very large ion implantation depth, as compared to conventional substrates, is obtained when the sample surface is set perpendicular to the beam axis, while the implantation depth is much smaller at the tilted angle. This phenomenon is discussed in relation with the sample structure anisotropy. Finally, the transport property modifications of the impact areas are investigated by CP-AFM and discussed in relation with both the crystalline structure and the polarizability of the matrix.