Axis Of Incident Beam Research Articles

Energy, angular and spatial distributions of primary electrons inside photoconducting materials for digital mammography: Monte Carlo simulation studies

Materials such as a-Se, a-As2Se3, GaSe, GaAs, Ge, CdTe, CdZnTe, Cd0.8Zn0.2Te, ZnTe, PbO, TlBr, PbI2 and HgI2 are potential candidates as photoconductors in direct detectors for digital mammography. The x-ray induced primary electrons inside a photoconductor's bulk comprise the initial signal that propagates and forms the final signal (image) on the detector's electrodes. An already developed model for a-Se has been properly extended to simulate the primary electron production in the materials mentioned. Primary electron characteristics, such as their energy, angular and spatial distributions that strongly influence the characteristics of the final image, were studied for both monoenergetic and polyenergetic x-ray spectra in the mammographic energy range. The characteristic feature in the electron energy distributions for PbI2 and HgI2 is the atomic deexcitation peaks, whereas for the rest of the materials their shape can also be influenced by the electrons produced from primary photons. The electrons have a small tendency to be forward ejected whereas they prefer to be ejected perpendicular (θ = π/2) to the incident beam's axis and at two lobes around φ = 0 and φ = π. At practical mammographic energies (15–40 keV) a-Se, a-As2Se3 and Ge have the minimum azimuthal uniformity whereas CdZnTe, Cd0.8Zn0.2Te and CdTe the maximum one. The spatial distributions for a-Se, a-As2Se3, GaSe, GaAs, Ge, PbO and TlBr are almost independent of the polyenergetic spectrum, while those for CdTe, CdZnTe, Cd0.8Zn0.2Te, ZnTe, PbI2 and HgI2 have a spectrum dependence. In the practical mammographic energy range and at this primitive stage of primary electron production, a-Se has the best inherent spatial resolution as compared to the rest of the photoconductors. PbO has the minimum bulk space in which electrons can be produced whereas CdTe has the maximum one.

Impact of large-angle scattering on diffusively backscattered halos

We discuss the impact of large-angle scattering events in highly forward-scattering media on the spatial distribution of the diffusively reflected light. We show that, even for highly forward-scattering media, the reflected light near the incident beam axis is strongly dependent on the small number of large-angle scattering events. Reliable modeling of near-axis reflection thus requires accurate knowledge of the scattering phase function's behavior at large angles.

TH-C-T-617-02: Effect of Range Modulation On Neutron Dose Equivalent Exposures Around a Passive Scattering Proton Therapy Treatment Nozzle

Purpose: The purpose of our study was to examine the influence of range modulation on the neutron dose equivalent exposures outside the treatment volume around a large-field passively scattered proton therapy treatment nozzle. Method and Materials: In this study, the neutron dose equivalent spectra per proton (H(E/p)) and total neutron dose equivalent per therapeutic absorbed dose (H/D) were calculated using Monte Carlo simulations of the neutron fluence and the energy dependent neutron fluence-to-dose equivalent conversion factor for NCRP 38. (H(E)/p) and H/D were calculated at 54 locations around a passively scattered proton therapy treatment nozzle for varying amounts of range modulation. Results: As the step thickness of the range modulator wheel increased from 1.0 to 12.0 cm, the peak values of H(E)/p increased from 1⋅10−17 to 2⋅10−17 mSv/Gy at 50 cm from isocenter along the beam's central axis. In general, H/D increased with increasing range modulation at all locations studied, and the maximum H/D exposures shifted upstream of isocenter and away from the end of the nozzle. Conclusion: Several important findings can be summarized from the presented work. First, with increasing thickness of the RMW step, the high-energy peak in the H(E)/p spectra shifted to higher energies. Second, at 90° with respect to the proton beam axis, the high-energy peak occurs at substantially lower neutron energies. Also, the H/D values around the treatment nozzle increased as the modulation of the beam increased. Finally, the H/D values change significantly with distance from isocenter and angle with respect to the incident beam axis, due in part to the effects of the nozzle components on the neutron fluence downstream of the end of the nozzle.

A forward magnetic spectrometer system for high-energy heavy-ion experiments

A small aperture magnetic spectrometer has been built to study hadron production in 197 Au+ 197 Au collisions at the AGS energy of 11.6A GeV/ c. It operates in the forward angular range from 6 to 30° with respect to the incident beam axis and covers the mid-rapidity region for heavy particles such as protons. The detector components of the spectrometer system include two time projection chambers, four drift chamber modules and a time-of-flight scintillation counter wall. A few new technologies are implemented in the design of the system to achieve the performance goals. The spectrometer has proved to function properly under the high particle-density environment encountered in experiments with the heavy-ion colliding system. The achieved momentum resolution is 1.3% in r.m.s. for pions at 1 GeV/ c and 1.6% for protons at the same momentum. With the time-of-flight resolution of 76 ps in r.m.s., the particle identification momentum limit extends to 4 GeV/ c for pions, 3 GeV/ c for kaons, 5 GeV/ c for protons, and 4.5 GeV/ c for anti-protons. The tracking efficiency stays above 86% for tracks up to 5 GeV/ c with as many as 10 tracks in the spectrometer aperture.

Forces of a Single-Beam Gradient-Force Optical Trap on Dielectric Spheroidal Particles in the Geometric-Optics Regime

We developed a computational code to calculate the forces of a single-beam gradient-force optical trap exerted on dielectric ellipsoidal particles in the geometric-optics regime. Using this code, the axial and the transverse trapping stability of spheroidal particles, the semi-major axis of which is perpendicular (type-A) and parallel (type-B) to an incident beam axis, was evaluated and the effects of the nonspherical geometry of the particles were analyzed. As the fractional deformation ratio increased, the axial trapping stability improved for type-B particles, whereas it degraded for type-A particles. It can, therefore, be concluded that type-B particles can be trapped more stably than type-A particles. It was also observed that the axial trapping stability can be improved by the use of a TEM01*-mode beam instead of a TEM00-mode beam.

Transducer misalignment effects in beam reflection from elastic structures

In two-transducer, fluid-coupled ultrasonic reflection measurements phase matched to guided modes of elastic solid structures, the effects of misalignment (i.e., differences) between the receiver angle and the incident beam angle on the receiver voltage have been studied. The received voltage is typically due to contributions from the specular reflection and any of the several possible guided wave modes excited by the incident wave field. It is found that misalignment leads to changes in the relative amplitudes of the various contributions. Further, the more highly collimated the beam (or the contributor), the more pronounced are the effects. It is shown that the signal maximum is not a reliable indicator of receiver alignment. These conclusions are based on measurements and on calculations that have been performed at incident angles selected both close to, or far from, phase-matching angles to guided wave modes on plates, curved surfaces, and cylindrical shells. Receiver voltage coordinate scans have been performed with receiver angles misaligned from the incident beam axis by 1 to 4 degrees. The receiver voltage versus scan parameter in planar and curved structures is calculated by employing complex transducer points to synthesize two-dimensional, sheet-beam transducer fields with Gaussian apertures, spectral analysis to formulate the beam-structure interaction problem, and asymptotic methods to evaluate the resulting spectral integrals. The model predictions are generally in good agreement with the experiments.

Direct determination of impact-parameter-dependent stopping powers for million-electron-volt He ions penetrating Er-doped GaAs.

We have directly determined the impact-parameter-dependent stopping powers for 2.0- and 2.5-MeV He ions passing through GaAs single crystals. The points reside in the preparation of the single-crystal sample with a dopant of a heavy element located in some definite interstitial site at definite depth and in synthesis of a Monte Carlo program to simulate accurately the ion trajectories. Er-doped homoepitaxial GaAs layers grown by molecular-beam epitaxy and by metal-organic chemical-vapor deposition were used for this purpose. As previously reported, fine single-crystal clusters of ErAs are formed in the GaAs host and Er takes the position exactly equivalent to the tetrahedral interstitial site. The present Monte Carlo simulation has revealed the fact that some definite impact-parameter region dominates the backscattering Er peak position and this region shifts continuously by tilting the incident beam axis slightly from a major crystal axis. The results obtained are compared with the Oen-Robinson [Nucl. Instrum. Methods 132, 647 (1976)] model and other theoretical predictions and clearly show that inner-shell excitations and ionizations contribute significantly to the stopping power even for large impact parameters. \textcopyright{} 1996 The American Physical Society.

Monte Carlo calculations of fast neutrons distribution inside graphite

Both energy and spatial distributions of fast neutrons inside infinite slabs of graphite (density = 1.72 gm · cm −3) are calculated. The blocks were irradiated (on the surface) with parallel and normal incident neutrons of 15 MeV with a beam diameter of 6 cm. Using Monte-Carlo technique, fluxes at different locations inside the medium, along and away from the incident beam's axis (radial position of the detector) were calculated. The calculations were performed for graphite slabs of different thicknesses ranging from 5 to 90 cm. Comparisons with some measurements are presented. The variation of the fast neutron build up factor with the depth inside the graphite block was investigated. Its dependence on the block thickness was also studied.

A re-emitted positron energy spectrometer

The development of an ultra-high vacuum apparatus for re-emitted positron energy spectroscopy is described. Positrons from a 22Na source are moderated by a 6 mu m thick polycrystalline tungsten foil in transmission geometry and transported by electrostatic optics to the sample, which is held at 45 degrees to the incident beam axis. Positrons re-emitted from the sample into a 2 degrees angular cone about the surface normal pass through an entrance slit in a second set of electrostatic lenses and are transported to a concentric hemispherical energy analyser. Re-emitted positron energy spectra are accumulated using a computer-controlled multi-channel scaler whose ramp voltage output drives the hemispherical analyser control unit. Examples of measurements and an evaluation of system performance are presented and future applications of the instrument are discussed.

Système de mesure de l'énergie d'électrons rétrodiffusés en fonction de l'angle d'émission

We present in this paper an experimental device for determining for an electron beam incident normal to a target, the angular and energetic backscattered electron distribution as a function of the emission angle. We use as detector a semiconductor diode cooled to a temperature of 100 K by liquid nitrogen. This detector can be moved around the target. The first results show that this device enables us to obtain the energy spectrum of electrons backscattered at different angles to the incident beam axis. From these measurements we have calculated the total angular and energy distributions ; the result is in good agreement with those obtained by other authors.

De-channelling contrast in annular dark-field STEM

The image contrast in high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) for crystalline specimens oriented so that the incident beam axis is parallel to a principal axis or plane of the crystal is considered to be produced by a process of channelling of the electrons along rows or planes of atoms plus a process of high-angle scattering by the atoms. Crystal defects or the termination of a crystal at a face can change the channelling conditions and hence give image contrast. It is shown that at MgO crystal faces and at dislocations in Al the de-channelling of the electron beam produces an expansion of the diffraction pattern and hence an increase of ADF image intensity. It is suggested that the HAADF STEM technique may be particularly valuable for the study of defects in thick crystals and of surface structures.

The Effect of Particle Shape on the Amplitude of scattered light for a sizing instrument

AbstractThe relationship between size and intensity of laser light scattered from coal particles, glass beads and calibration pinholes in the size ranges 10–140 m̈m, 17–240 m̈m and 5–200 m̈m respectively has been measured. The purposes were to determine the effect of shape of non‐spherical particles and beads by varying three parameters in the collection optics.These were: the angle, θc, between the axis of the collection optics and that of the incident laser beam; the aperture of the f/3.66 collection lens; and the diameter of the spatial filter on the photodetector. A spatial filter diameter of 0.5 mm was used on the detector as a compromise between image vignetting and reducing the size of the effective measuring volume of the instrument. Due to refraction, and particularly for diameters larger than about 80 m̈m, the beads scattered more light than the pinholes or the coal particles. A mask, introduced to limit the collection lens aperture to angles less than 5.3°, reduced the intensity collected from the beads, while the response curves for the pinholes and coal particles were unaffected. Non‐spherical beads larger than about 60 m̈m with aspect ratios up to 1.8 resulted in departure from the response curve, established from the calibration pinholes, by up to a factor of two. The aspect ratio was defined by the maximum and minimum projected lengths of the image of the scatterer. The response curves for beads and coal particles approached that of the pinholes as θc was changed from 2.5° to 1.4° and as the angular position of the mask limiting the aperture of the collection lens nearest to the incident beam axis, θi, was reduced from 2.9° to 0.9°. The best results were obtained for θi and θc equal to 0.9° and 1.4° respectively, for which the precision and accuracy were 9 m̈m and + 5 m̈m for nominally spherical glass beads and 5 m̈m and −1 m̈m for the coal particles for diameters up to 60 m̈m and aspect ratio up to 2.5.

Determination of surface geometry for beam expansion.

Some applications, for example, large area acoustic imaging, require ultrasonic beams of 8- to 12-cm diameter with a uniform intensity<ep;bal;j> distribution in a plane normal to the beam axis. Expansion of the far field of a 1-in.-diam disk-type transducer by means of reflection from a surface with a specific geometry has been investigated. Using a reflecting surface minimizes energy loss due to attenuation common in beam expansion by acoustic lenses. For an energy flux angle variation not exceeding 4 deg from the reflected beam axis, the reflecting surface geometry is calculated. It is found that the expanded beam cross section after reflection is quite uniform. Uniformity of the intensity distribution across the reflected beam depends on the angle between the incident beam axis and the axis of the reflecting surface.

Efficiency of light diffraction by cross-striated muscle fibers under stretch and during isometric contraction

When light is diffracted by a single frog muscle fiber the intensities I kappa of the different orders kappa (kappa = 1,2,3) strongly depend on the angle between the axis of the incident beam and the fiber axis. Maximum intensity is not obtained with perpendicular incidence (omega = 0 degree) but at angles that can be calculated for each order number and sarcomere length using Bragg's formula. In analogy to techniques developed for x-ray structure analysis of mosaic crystals we have rotated the fiber around an axis perpendicular to the fiber axis and to the incident beam axis within an angular range delta omega = +/- 35 degrees and recorded the light intensities I kappa. Diffraction efficiencies defined as E kappa = integral of I kappa d omega were studied as a function of sarcomere length and during isometric contraction. The sarcomere length dependences of the efficiencies E kappa of the first three orders show characteristic trends. E1 increases with fiber stretch, E2 has a minimum at a sarcomere length near 2.8 micrometers, and E3 has a maximum near 2.5 micrometers. These trends as well as the observed efficiency ratios are in fairly good agreement with predictions by the intensity formula developed for x-ray structure analysis. During isometric contraction, the diffraction efficiencies of the fiber decrease, with the decreases becoming greater the higher the order number. These decreases might be caused by a longitudinal displacement of myofibrils of up to 0.4 micrometers. The efficiency of light diffraction strongly depends on the tonicity of the bathing fluid. Hypertonic (3/2 x normal) solution reduces E1 to less than half, hypotonic (2/3 x normal) solution increases E1 to almost twice the value obtained in normal Ringer's solution.

Excited levels of87Sr

In-beam gamma rays from the reaction 87Rb(p,n gamma )87Sr at an incident proton energy of 3.8 MeV were measured. Ten levels up to an excitation of 2698 keV were proposed, of which the levels at 1228.5, 1253.8, 1770.5, 2110.8 and 2169.3 keV are definitely confirmed from coincidence data. The branching ratios for the proposed are deduced and probable spin-parity assignments are suggested. Lifetimes or lower limits on the lifetimes are obtained for some of the levels by the Doppler-shift attenuation method after a careful centroid-shift analysis of the singles spectra taken at 0 degrees and 90 degrees to the incident beam axis. Values or limits for B(E2), B(M1) and B(E1) are obtained. The level structure is discussed in the light of recent experimental and theoretical investigations.

Effect of channeling on positron backscattering from crystals

The dependence of the positron backscattering counting rate on the orientation angle of KCl, KBr, and KI single crystals relative to the incident beam axis is investigated. In the case where the initial positron beam was coincident with the 〈100〉 crystallographic axis, a dip in yield for backscattered positrons was observed. In comparison with positron measurements the backscattered electrons show a peak in yield. The minimum for positrons and the maximum for electrons disappeared in crystals heated up to about 800 °K. [Russian text ignored]

Enhancement of optical receiver sensitivities by amplification of the carrier

The amplification before detection of the carrier of a modulated optical signal by a narrow-band quantum amplifier enhances the signal-to-noise ratio, particularly when the signal wave-front is distorted. A further improvement is obtained by using a combination of wide-band and narrow-band quantum amplifiers. The practical application of these schemes requires a degenerate regenerative ring-type amplifier capable of amplifying arbitrary transverse field configurations. Experiments show that such an amplifier with a gain of 24 dB and a bandwidth of 1 MHz is feasible. The incident beam axis can be displaced by as much as ten times the beam-waist radius without losing more than 4 dB in gain. Frequency modulation may be converted into amplitude modulation by the phase shift introduced in the carrier.

Laser beam power control device

A simple, reliable device for continuously varying the intensity and plane of polarization of laser beams over a wide range is described. It consists of a series of transparent dielectric plates set at the Brewster angle and capable of rotation about the incident beam axis. This device has been used to control the beam intensities of a continuous wave helium-neon laser at a few microwatts and of a giant pulse Q-switched ruby laser at 200 MW.

Enhancement of optical receiver sensitivities by amplification of the carrier

The amplification before detection of the carrier of a modulated optical signal by a narrow-band quantum amplifier enhances the signal-to-noise ratio, particularly when the signal wave-front is distorted. A further improvement is obtained by using a combination of wide-band and narrow-band quantum amplifiers. The practical application of these schemes requires a degenerate regenerative ring-type amplifier capable of amplifying arbitrary transverse field configurations. Experiments show that such an amplifier with a gain of 24 dB and a bandwidth of 1 MHz is feasible. The incident beam axis can be displaced by as much as ten times the beam-waist radius without losing more than 4 dB in gain. Frequency modulation may be converted into amplitude modulation by the phase shift introduced in the carrier.

A Spectrophotometer for Determining Optical Constants in the Vacuum Ultraviolet Region

A Seya-Namioka type vacuum spectrophotometer for determining the optical constants of solids from the reflectance measurements was constructed. The apparatus is equipped with a 1 meter concave grating of the blazing angle 3°6', 1440 grooves per mm, supplied by Bausch and Lomb, the Weissler spark as a light source, and the photomultiplier 1P21 coated with sodium-salicylate. It can be operated over the range 6 to 28 eV. The reflectance measurements can be made at the angles of incidence from 7.5° to 75°, in order to apply the Tousey-Šimon type analysis as well as the Kramers-Kronig type analysis. Reflectance measurements are made free from the polarization error, since the reflectometer assembly can be rotated ±90° around the incident beam axis. As examples of determining the optical constants, the results for Si and Ge are presented.