Ray Scattering Research Articles

Analysing collimator structure effects in head-scatter calculationsfor IMRT class fields using scatter raytracing

The frequent blocking of the irradiated volume in intensity modulatedradiation therapy (IMRT) makes the head-scatter fraction of the incidentphoton fluence more significant than that in conventional therapy with openfields. On the other hand, certain collimator configurations block scatterphotons directed to a given observation point while allowing primary photonsto be transmitted. The `anomalous blocking' makes the primary field a poorindicator of the scatter fluence. Since large MU-to-cGy ratios in IMRT canmagnify head-scatter uncertainties, it becomes necessary to accurately modelboth the effective scatter source and the collimator structure that limits thescatter reaching the irradiated volume. First we obtain a dual-source model,using a Taylor series expansion to derive the effective scatter sourcedistribution from the data measured for the Elekta SL20 linac equipped with amulti-leaf collimator (MLC). Then, using a raytracing algorithm, we calculatethe transmission of scatter rays from the effective scatter source plane topoints in the patient plane. The method can account for the anomalous blockingof scatter by the MLC leaves and the back-up diaphragms. For a variety ofcollimator settings tested, the calculations agree with measurements to anaccuracy of 0.002Ψ10×10, where Ψ10×10 is the total(primary + scatter) photon fluence of an open 10×10 cm2 field forthe same MU delivered. Although the significance of collimator structure inIMRT depends strongly on fields shapes employed for the delivery, potentialcumulative errors on the order of a few per cent can be avoided in fluencecalculations if the proposed method is used.

Irregular scattering of acoustic rays by vortices.

The scattering of high-frequency sound wave, under geometrical acoustic approximation, by three stationary vortices in two dimensions is investigated. For a sufficiently high Mach number of the vortex flow, the scattering of sound rays becomes irregular, displaying a new example of chaotic scattering for a time-reversal breaking system. The fractal dimension, as well as the unstable and stable manifolds of the scattering dynamics, is presented.

Zuoz holds another successful summer school

This was the sixth Zuoz school in a series that has become well established in the neutron scattering diary. This year's school on “Complementarity Between Neutron and Synchrotron X–ray Scattering” drew approximately 110 people from 18 different countries (including as far away as Australia) to participate in the school and enjoy the beautiful surroundings of the Engadine valley from August 9–13, 1998. The meeting opened with a tour-de-force overview of the relevant cross sections by W. Fischer (PSI), and went on the first day to concentrate on surfaces, films and multilayers with talks by S. Sinha (Argonne) and D. McMorrow (RisG) and then a theoretical talk by M. Altarelli (ESRF), who introduced some of the capabilities of the latest synchrotrons, addressing interesting subjects such as orbital ordering in the transition-metal oxides. Powder diffrac with synchro(Griffith Univ., Australia), respectively. Magnetic correlations in lowdimensional systems was discussed by H. Braun (PSI) and W. Stirling (U. Liverpool, UK), covering magnetic neutron and X–ray scattering. G. Schiitz (Wiirzburg) introduced magnetic dichroism and the interesting idea to use it as a microscope to image domains. Turning to soft matter (polymers), D. Richter (KFA, Jiilich) showed what information inelastic scattering can bring to the subject. Returning to magnetism, W. Buyers (Chalk River, Canada) discussed magnetic neutron inelastic scattering, and V. Geshkenbein (ETH, Zurich) continued on the same general subject with an account of vortices in the superconducting state. The school also contained two clear lectures on the optics of synchrotrons and neutrons by world experts A. Freund (ESRF) and P. Boni (PSI), respectively. Refocusing on the physical problems as well as the techniques, Hans Ott (ETH, Zurich), in an evening lecture, gave the participants a walk through “hot minding us of the equal importance of topics” in condensed matter research, all the way from quantum dots to nonFermi liquids and on to the interface between biology and physics.

Plasticity in aerogels

When gently stressed, aerogels show an elastic response. However it was found that under isostatic pressure aerogels display an irreversible shrinkage which may be attributed to plastic behaviour. As a consequence of this plastic shrinkage it is possible to densify and modify the elastic properties of aerogels at room temperature. The structural evolution is followed by Small Angle X ray Scattering and the increase of the connectivity is revealed by the evolution of the elastic properties of the material. The SAXS data show that the densification mechanism is different from that obtained by sintering at high temperature. The densification mechanism induces a textural change at the periphery of the constitutive clusters but not inside, conversely to a sintering effect. We also show that the elasticity of the material is strongly influenced by this structural transformation. The power law evolution of the elastic modulus as a function of the density, usually observed on as-prepared and sintered aerogels, is not valid for compressed material.

Comparative study of the interface roughness of Ag/Au and Cu/Au multilayers with scanning tunneling microscopy and x-ray diffraction

The interface roughness is a very important parameter determining the performances of multilayered structures. Ag/Au and Cu/Au textured multilayers have been produced by thermal evaporation in an ultrahigh vacuum chamber. After deposition of the Au, Ag, or Cu layer, the surface roughness at different sampling lengths was probed in situ by scanning tunneling microscopy. After completion of the multilayers, the samples were taken from the ultrahigh vacuum chamber and measured in an x-ray-diffraction unit. Low-angle as well as high-angle specular x-ray-diffraction curves were analyzed using a refinement program which takes into account layer thickness fluctuations, interface roughness, and crystalline disorder. The average interface roughness, obtained from fitting the x-ray-diffraction profiles, is then compared to the roughness of the film growth front, which was probed with the scanning tunneling microscope. The roughness obtained from the x-ray-diffraction profiles corresponds to a lateral sampling scale which is in good agreement with the average grain size within the layers, indicating that x rays scatter coherently over a volume which is relatively small. Our work also shows the complementarity of the scanning tunneling microscopy and x-ray-diffraction technique to probe interface roughness of multilayers.

Airborne Radiometric Calibration - an Australian Perspective

Airborne radiometric systems must be calibrated to present data devoid of artefacts peculiar to their acquisition. The IAEA(1991) model of the components of airborne radiation is outlined. This features: aircraft background radiation; cosmic-effect (related to survey altitude); Stripping Ratios which account for the spectral overlap of radiation from different radioelement sources; and height attenuation coefficients, which quantify the effect of Compton Scattering of gamma rays passing through the air. Resources of calibration pads and radiometric test range sites are listed. The facets of calibration procedure are discussed according to their application to the IAEA model. The resulting calibration parameters are used in a process to generate airborne radiometric system sensitivities by comparison of airborne and ground spectrometer data. This method can avoid errors in system sensitivity caused by environmental factors.

ASCA measurements of the grain-scattered X-ray halos of eclipsing massive X-ray binaries: VELA X-1 and Centaurus X-3

view Abstract Citations (34) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS ASCA Measurements of the Grain-scattered X-Ray Halos of Eclipsing Massive X-Ray Binaries: VELA X-1 and Centaurus X-3 Woo, Jonathan W. ; Clark, George W. ; Day, Charles S. R. ; Nagase, Fumiaki ; Takeshima, Toshiaki Abstract We have measured the decaying dust-scattered X-ray halo of Cen X-3 during its binary eclipse with the ASCA solid-state imaging spectrometer (SIS). The surface brightness profile (SBP) of the image in the low-energy band (0.5-3 keV) lies substantially above the point-spread function (PSF) of the X-ray telescope, while the SBP in the high-energy band (5-10 keV) exhibits no significant deviation. By contrast, the SBPs of Vela X-1 during its eclipse are consistent with the PSF in both the low- and high-energy bands -- strong evidence that a dust halo is indeed present in Cen X-3. Accordingly, we modeled the SBP of Cen X-3 taken from six consecutive time segments under the principal assumptions that the dust is distributed uniformly along a segment of the line of sight, the grains have a power-law size distribution, and the low-energy source flux was the same function of orbital phase before as during our observation. The best-fit set of parameters included a grain density value of 1.3 g/cu cm, substanially less than the density of 'astronomical silicate.' This result supports the idea that interstellar grains are 'fluffy' aggregates of smaller solid particles. We attribute the failure to detect a halo of Vela X-1 during its eclipse phase to extended strong circumsource absorption that probably occurred before the eclipse and allowed the halo to decay away before the observation began. Publication: The Astrophysical Journal Pub Date: November 1994 DOI: 10.1086/187620 Bibcode: 1994ApJ...436L...5W Keywords: Eclipsing Binary Stars; Halos; Massive Stars; Pulsars; X Ray Binaries; X Ray Scattering; Imaging Spectrometers; Interstellar Matter; Light Curve; Astrophysics; STARS: BINARIES: ECLIPSING; ISM: DUST; EXTINCTION; STARS: PULSARS: INDIVIDUAL NAME: VELA X-1; STARS: PULSARS: INDIVIDUAL NAME: CENTAURUS X-3; X-RAYS: STARS full text sources ADS | data products SIMBAD (3) HEASARC (1)

Are cosmic rays effective for ionization of protoplanetary disks?

view Abstract Citations (26) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Are Cosmic Rays Effective for Ionization of Protoplanetary Disks? Dolginov, Arkady Z. ; Stepinski, Tomasz F. Abstract The principal uncertainty in studying the magnetic properties of protoplanetary disks concerns the ionization levels of the disk's gas. The low gas temperature precludes thermal ionization, leaving cosmic rays as the dominant source of ionization. It has been shown that the resulting electrical conductivity is just high enough for a MHD dynamo to produce contemporaneously a magnetic field in most of the extended parts of a turbulent protoplanetary disk. Here we argue that the effectiveness of cosmic rays to ionize the bulk of the gas is impaired by the influence of the generated magnetic field on the propagation of cosmic rays within a disk. Cosmic rays scatter on magnetic inhomogeneities, and, as a result, their penetration depth decreases to only a fraction of the disk half-thickness, resulting in a severe depletion of free charge from the midplane regions of a disk. That, in turn, undercuts the dynamo mechanism, so extended parts of the disk are free from a dynamically significant magnetic field. We also point out that any additional, even small, in situ source of ionization, such as radioactive Al-26, may again make a dynamo a viable regeneration process capable of producing a dynamically important magnetic field. Publication: The Astrophysical Journal Pub Date: May 1994 DOI: 10.1086/174146 Bibcode: 1994ApJ...427..377D Keywords: Accretion Disks; Cosmic Rays; Magnetohydrodynamic Waves; Spatial Resolution; Star Formation; Stellar Magnetic Fields; T Tauri Stars; Coriolis Effect; Electrical Resistivity; Molecular Clouds; Planetary Magnetospheres; Astrophysics; ISM: COSMIC RAYS; MAGNETOHYDRODYNAMICS: MHD; STARS: FORMATION; STARS: MAGNETIC FIELDS full text sources ADS |

Development of polyurethane engineering thermoplastics. II. Structure and properties

AbstractRigid, tough thermoplastic polyurethanes were synthesized by a two–step technique. These materials presented a complex structure consisting of a continuous, rigid, amorphous, or semicrystalline matrix with a dispersed soft phase. The influence of soft–segment content upon rigidity modulus, heat–distortion temperature, and fracture behavior has been deter‐mined on model amorphous polyurethanes. The influence of crystallinity upon final prop‐erties has been studied on two materials: a preparation of ours and a commercial product. Various degrees of crystallinity have been induced in the samples by annealing. Phase structure was characterized by differential scanning calorimetry and wide–angle X–ray scat‐tering. Some of the polyurethane samples assayed in this paper exhibited similar properties to those reported in respect of more conventional engineering thermoplastics, such as ABS or nylon. © 1993 John Wiley & Sons, Inc.

The scattered solar X-ray background of the ROSAT PSPC

view Abstract Citations (82) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Scattered Solar X-Ray Background of the ROSAT PSPC Snowden, S. L. ; Freyberg, M. J. Abstract A model for accurately reproducing the light curves and spectra of the scattered solar X-ray background (SB) affecting ROSAT XRT/PSPC pointed observations is presented and demonstrated. This procedure, the modeling and subtraction of the SB, is vital for analysis of all observations of extended X-ray objects and the soft X-ray diffuse background where noncosmic background constituents must be precisely known. At the orbital altitude of ROSAT, about 550 km, and the zenith angles at which the XRT/PSPC observers, 97 deg or less, the scattering is dominated by atomic oxygen with Thomson-scattered X-rays in the 1/4 keV band and oxygen K-alpha fluorescently scattered X-rays at 0.54 keV. This produces field-of-view-integrated minimum count rates in the 0.1-1.0 keV band of about 0.25 counts/s during dayside observations with excursions to about 40 counts/s or more in particularly bad geometries. Typical cosmic background count rates in the same band range from 3 to 12 counts/s, demonstrating the need for the procedure described here. Publication: The Astrophysical Journal Pub Date: February 1993 DOI: 10.1086/172289 Bibcode: 1993ApJ...404..403S Keywords: Background Radiation; Rosat Mission; Solar X-Rays; X Ray Scattering; Absorption Cross Sections; Light Curve; Radiative Transfer; Stellar Models; Thomson Scattering; Solar Physics; ARTIFICIAL SATELLITES; SPACE PROBES; COSMOLOGY: DIFFUSE RADIATION; INSTRUMENTATION: SPECTROGRAPHS; SUN: X-RAYS; GAMMA RAYS full text sources ADS | data products SIMBAD (2)

An apparatus for studying sputter deposition with x rays

While electron-based structural probes have been extensively applied to low-pressure film growth techniques such as evaporation, similar studies on sputter deposition have been hindered by the relatively high gas pressure (1–10 mT) inherent to the technique. Because x rays scatter more weakly than electrons the absorption in the ambient is not significant and diffraction from the growing film can be observed if a sufficiently bright source of x rays is used. We have developed a chamber which combines features of an ultrahigh-vacuum sputter-deposition system with the goniometric adjustments and the x-ray windows of a scattering chamber. A description of its design and recent results are presented.

The selection of portal aperture using interactively displayed eye sections

Portal apertures are often defined by wax-pencil lines drawn on simulator films. The simulator-film images, suffer degradation due to X ray absorption and scatter in tissues proximal to and distal to the intended target volume. We describe a method for avoiding that degradation by using a 3-D dataset to produce high-contrast section images with a user-defined orientation. The calculation of these Beam's Eye Section (BES) images requires a set of medium to high-resolution transverse section data (the 3-D dataset), as well as a medium- to high-performance computer workstation. These CT-like section images, providing more contrast and detail than a projection (that is, a simulator or Digitally Reconstructed Radiograph [DRRJ) image, should allow for more accurate selection of portal apertures. The BES image plane is oriented perpendicular to the central axis of a user-selected beam. Once the user has created a beam, the system allows the user to step along that beam with a new BES image produced at each step. Contours manually drawn on these BES images are ultimately projected onto a DRR image. The outermost segments of these sometimes crisscrossing contours (the union of all projected areas) accurately defines the portal aperture needed to encompass the target at all levels.

96. ソリッドタイマー (前置型受光部、約 0.5mm A1 当量) 使用による X 線画像への影響と対策 (第一報)(装置-2 インバータ式装置他)

96. ソリッドタイマー (前置型受光部、約 0.5mm A1 当量) 使用による X 線画像への影響と対策 (第一報)(装置-2 インバータ式装置他)

96. ソリッドタイマー (前置型受光部、約 0.5mAl 当量) 使用による X 線画像への影響と対策 (第一報)(第 19 回秋季学術大会一般研究発表予稿)

96. ソリッドタイマー (前置型受光部、約 0.5mAl 当量) 使用による X 線画像への影響と対策 (第一報)(第 19 回秋季学術大会一般研究発表予稿)

Neutron and Gamma Ray Scattering Measurements for Subsurface Geochemistry

Developed for the oil industry, well logging instrumentation based on electrical, acoustic, and nuclear measurements has been providing information about the localization and evaluation of hydrocarbon-bearing strata for petroleum geologists and engineers since 1927. This method of exploring properties of the earth's crust without taking physical samples is attracting a growing audience of geologists and geophysicists because of recent developments that permit nondestructive measurements of subsurface geochemistry. A combination of nuclear measurement techniques, which use gamma ray and neutron sources, can provide detailed information on rock composition of interest to both industry and academia.

53. SPECT 画像の吸収補正に対する散乱線の影響に関する検討(第 46 回総会学術大会会員研究発表予稿)

53. SPECT 画像の吸収補正に対する散乱線の影響に関する検討(第 46 回総会学術大会会員研究発表予稿)

54. RI 画像への散乱線混入状態の解析とその除去法(RI-1 散乱線補正)

54. RI 画像への散乱線混入状態の解析とその除去法(RI-1 散乱線補正)

54. RI 画像への散乱線混入状態の解析とその除去法(第 46 回総会学術大会会員研究発表予稿)

54. RI 画像への散乱線混入状態の解析とその除去法(第 46 回総会学術大会会員研究発表予稿)

53. SPECT 画像の吸収補正に対する散乱線の影響に関する検討(RI-1 散乱線補正)

53. SPECT 画像の吸収補正に対する散乱線の影響に関する検討(RI-1 散乱線補正)

Atomic Level Investigations of W-Si Multilayers by High Resolution Tem and X Ray Scattering

ABSTRACTWe have prepared multilayer films of W-Si with bilayer repeat spacing from approximately 1.5 nm to 9 nm and performed high resolution electron microscopy and low angle x-ray scattering on them. Average composition estimates as inferred from deposition conditions, x ray scattering and electron microscopy are compared. Determinations of the individual layer thickness ratios by electron microscopy and x ray scattering vary significantly from expectations as the bilayer thickness approaches 1.5 nm. Layer intermixing to increase as the bilayer thickness decreases. Composition profiles as inferred from the Cuk x ray profile are compared to those inferred from the high resolution electron micrographs. Visual observations from melectron microscopy are presented indicating that the interface roughness is rapidly damped in the W-Si multilayer system. Estimates of the layer uniformity are made from the high resolution images.