Showing papers on "Beam (structure) published in 1981"

Electron beam dose calculations.

Abstract: Electron beam dose distributions in the presence of inhomogeneous tissue are calculated by an algorithm that sums the dose distribution of individual pencil beams. The off-axis dependence of the pencil beam dose distribution is described by the Fermi-Eyges theory of thick-target multiple Coulomb scattering. Measured square-field depth-dose data serve as input for the calculations. Air gap corrections are incorporated and use data from 'in-air' measurements in the penumbra of the beam. The effective depth, used to evaluate depth-dose, and the sigma of the off-axis Gaussian spread against depth are calculated by recursion relations from a CT data matrix for the material underlying individual pencil beams. The correlation of CT number with relative linear stopping power and relative linear scattering power for various tissues is shown. The results of calculations are verified by comparison with measurements in a 17 MeV electron beam from the Therac 20 linear accelerator. Calculated isodose lines agree nominally to within 2 mm of measurements in a water phantom. Similar agreement is observed in cork slabs simulating lung. Calculations beneath a bone substitute illustrate a weakness in the calculation. Finally a case of carcinoma in the maxillary antrum is studied. The theory suggests an alternative method for the calculation of depth-dose of rectangular fields.

Free-Vibration Analysis of Rotating Beams by a Variable-Order Finite-Element Method

Abstract: The free vibration of rotating beams is analyzed by means of a finite-element method of variable order. This method entails displacement functions that are a complete power series of a variable number of terms. The terms are arranged so that the generalized coordinates are composed of displacements and slopes at the element extremities and, additionally, displacements at certain points within the element. The displacement is assumed to be analytic within an element and thus can be approximated to any degree of accuracy desired by a complete power series. Numerical results are presented for uniform beams with zero and nonzero hub radii, tapered beams, and a nonuniform beam with discontinuities. Since the present method reduces to a conventional beam finite-element method for a cubic displacement function, the results are compared and found to be superior to the conventional results in terms of accuracy for a given number of degrees of freedom. Indeed, essentially exact eigenvalues and eigenvectors are obtained with this technique, which is far more rapidly convergent than other approaches in the literature.

Numerical results from large deflection beam and frame problems analysed by means of elliptic integrals

Abstract: Numerical evaluations of elliptic integral solutions of some large deflection beam and frame problems are presented. The values are given in tabular form with up to six significant figures. The numerical technique used for evaluating the elliptic integrals is described.

Method for laser depilation

Abstract: The roots of human hairs of a patient are devitalized using high intensity, short duration pulses of light having wavelengths with respect to which the skin of the patient is non-absorbative and the hair of the patient is relatively absorbative. A narrow, focused beam of the light is aimed at the epidermis of the patient adjacent the hair such that an extension of the beam intersects the hair root at an angle relative to the skin surface. A short pulse passes through the skin and is absorbed in the hair root, destroying its blood supply. Apparatus for practicing the method employs a manually controlled two-axis positioning system supporting the focusing system that is connected to a laser light source by a flexible fiber optic bundle. A shutter selectively positionable in the optical path allows a low intensity beam to be produced for aiming and the shutter is removed from the optical path for the pulse period to produce the high energy beam.

A theory of rheed

Abstract: Elastic Reflection High Energy Electron Diffraction (RHEED) by solid surfaces is studied theoretically. First, the problem of finding the electron reflection and transmission coefficients of a crystal slab is formally solved. Following this, it is shown how the formal solution may be used in a practical computation of the diffracted beam intensities. These two results are applied to a study of high energy (20 keV) electron diffraction by the Ag(001) surface. Rocking curves are computed to illustrate the dependence of the reflection coefficients on the glancing angle of the incident beam, the incident beam azimuth being in the [110] direction. The curves are shown to have several features in common with a typical set of LEED I - V plots: primary Bragg peaks, secondary Bragg peaks and resonance peaks are all present. The dependence of the reflection coefficients on the deviation of the incident beam azimuth from the [110] direction is also described. Additional computations are made to illustrate the sensitivity of the RHEED pattern to the details of the surface structure: the relative heights of the peaks in the rocking curves are shown to be quite sensitive to the spacing of the topmost atomic layers.

The electromagnetic ion beam instability upstream of the Earth's bow shock

Abstract: The linear theory of the electromagnetic ion beam instability for arbitrary angles of propagation has been studied. The parameters considered in the theory are typical of the solar wind upstream of the earth's bow shock when a 'reflected' proton beam is present. Maximum growth occurs for propagation parallel to the ambient magnetic field B, but this instability also displays significant growth at wave-vectors oblique to B. Oblique, unstable modes seem to be the likely source of the compressive magnetic fluctuations recently observed in conjunction with the 'diffuse' ion population. An energetic ion beam does not directly give rise to linear growth of either ion acoustic or whistler mode instabilities.

Early-age Thermal Crack Control in Concrete

Abstract: Early-age thermal cracking occurs when the restrained thermal contraction strain exceeds the tensile strain capacity of concrete. The restraint to thermal movement is the product of the coefficient of thermal expansion of the concrete, the temperature fall from a peak level during cement hydration, and a restraint factor. This report discusses each of these factors in turn, and shows how early-age thermal strains can be reduced by taking appropriate measures during the design and construction of reinforced concrete elements. The concrete temperature rise is shown to depend on a number of factors: cement content, type of aggregate, possible use of cement replacement materials, as well as ambient temperature, formwork used and section thickness. Restraint is a function of the construction sequence and the constraint of neighbouring elements. If early-age thermal cracking cannot be prevented, crack width can be controlled with reinforcement. Equations for the control of crack widths are developed, and while they are based on present requirements, they differ in two important respects. First, the concept of a restraint factor is introduced, then it is suggested that the crack widths are only controlled within an effective zone around the reinforcement and not necessarily throughout the section thickness. (TRRL) (TRRL)

A Fast X-Ray Absorption Spectrometer for Use with Synchrotron Radiation

Abstract: A quasi-parallel and polychromatic beam of synchrotron radiation is focused and dispersed by a curved crystal, so that the energy of each ray of the focused beam varies as a function of convergence angle through the focus. The specimen is placed at the focus. By measuring the X-ray intensity distribution across the beam behind the focus, in the presence and absence of the specimen, the absorption spectra of Cu and Ni metal foils were obtained. Using an X-ray film as the detector, a spectrum from a Cu foil was obtained in 0.1 seconds when the SPEAR storage ring at Stanford was operated at 3.1 GeV and 80 mA. The energy resolution is approximately 2.0 eV and the energy range of the spectrum is approximately 1 keV.

Film dosimetry of megavoltage photon beams: A practical method of isodensity‐to‐isodose curve conversion

Abstract: The central problems of photon beam film dosimetry are the dependence of film response upon photon energy, processing conditions, and film plane orientation. We have overcome these problems by accurately fitting the depth‐dependent sensitometric curve of Kodak XV‐2 film (exposed parallel to beam axis) to the equation OD(D,d) =OD s (1 − exp {−α0[1+β(d−d m )]D}) where OD(D,d) is the optical density for dose D at depth d. OD s , α0, and β are constants characteristic of the film and beam energy but are independent of field size. Only central axis depth dose data for a single field are required to determine their values. A computer program based upon this equation has been written which successfully generates single field isodose curves from film data for a variety of field sizes (including wedged fields) with an accuracy of ±3%. Data are presented for 60Co, and 4 and 10 MV x rays.

Edge Focusing

Abstract: Beam transport matrix elements describing the linearly falling fringe field of a combined function bending magnet are expanded in powers of the fringe field length by iteratively solving the integral form of Hill's equation. The method is applicable to any linear optical element with variable focusing strength along the reference orbit. Results for the vertical and horizontal focal lengths agree with previous calculations for a zero gradient magnet and an added correction to the dispersion is found for this case. A correction to the fringe field gradient peculiar to a combined-function magnet with strong edge focusing is also found. The influence of edge focusing on the tunes and chromaticities of the NSLS rings is described. The improved chromaticity calculation for the booster was of particular interest since this ring has bending magnets with poletips shaped to achieve small positive chromaticities.

Parametric interaction and spatial collapse of beam-driven Langmuir waves in the solar wind

Abstract: Observations are presented of the parametric decay and spatial collapse of Langmuir waves driven by an electron beam streaming into the solar wind from the Jovian bow shock. Long wavelength Langmuir waves upstream of the bow shock are effectively converted into short wavelength waves no longer in resonance with the beam. The conversion is shown to be the result of a nonlinear interaction involving the beam-driven pump, a sideband emission, and a low level of ion-acoustic turbulence. The beam-driven Langmuir wave emission breaks up into a complex sideband structure with both positive and negative Doppler shifts. In some cases, the sideband emission consists of isolated wave packets with very short duration bursts, which are very intense and are thought to consist of envelope solitons which have collapsed to spatial scales of only a few Debye lengths.

Absolute intensities and perpendicular temperatures of supersonic beams of polyatomic gases

Abstract: The centre-line intensity I(0)(s−1sr−1) of a supersonic beam source is fully described by the peaking factor κ=π(I(0)/N), with N (s−1) the flow rate. For a skimmerless source in a cryoexpansion chamber we find κ = 2.07, 1.48, 1.47, 1.20 and 1.17 for Ar, O2, N2, CO2 and CH4, respectively. These results are in fair agreement with numerical solutions of the flow field downstream of the sonic plane. The shielding effect of the skimmer is conveniently described by mapping the perpendicular distribution, which cools geometrically, into a constant virtual source distribution in the nozzle plane. We use a bimodal distribution to analyze the beam profile measurements. For Ar and Kr the radius R1 of the narrow virtual source is given by R1/zref=4.4(Ξ/100)0.28 with zref=0.802Rn, Rn being the nozzle radius, Ξ=3.189((γ−1)/γ) 1 2 n 0 z ref (C 6 /kT 0 ) 1 3 the source parameter, n0 and T0 the number density and the temperature of the reservoir, respectively. These results are in good agreement with a “last collision surface” extension of the thermal conduction model by Habets. For O2 and N2 we find R1/zref=10.1(Ξ/100)0.42 with zref= 0.598Rn. By scaling the “last collision surface” from the Ar and Kr data for the case of diatomics we find fair agreement with a modified thermal conduction model by Klots. The broad virtual source is very important for describing the shielding effect of the skimmer. The radius R2 is in the range 4⩽R1/R2⩽8 and its population consists of 30% ( γ= 5 3 ) to 50% ( γ= 7 5 ) of all particles. For Ξ⩽100 th e loss of intensity is less than 10% if we use the design rules Rs=20Rn for monoatomic and Rs=40Rn for diatomic gases, with Rs the radius of the circular skimmer opening.

Measured statistics of laser-light scattering in atmospheric turbulence

Abstract: The statistical properties associated with the intensity fluctuations of a He–Ne laser beam propagating through extended clear-air turbulence over a homogeneous range were measured and compared with existing theoretical models. The statistics were determined by measuring the first five normalized moments of the beam intensity over varying distances from 183 m to 3 km. The beam geometry was approximately that of a spherical wave. Calculated values of the refractive-index-structure parameter Cn2 varied from a low of approximately 10−14 m−2/3 to a high value of the order of 10−10 m−2/3. Thus the data obtained correspond to many conditions of turbulence from weak to strong turbulence in the saturation regime. We found that the data generally support the lognormal model under conditions of weak turbulence and the negative-exponential distribution in the superstrong-turbulence regime. The K distribution fits some of, but not all, the data in the saturation regime. For conditions of strong turbulence before the superstrong regime, none of the above statistical models fits the data well. This is due primarily to the fact that a looping effect appeared in the data whereby the third- and higher-order normalized moments appear to follow a looped curve in the vicinity of their maximum values when they are plotted as functions of the second normalized moment. None of the theoretical curves for the normalized moments of the above-named distributions exhibits this looping effect (since they are essentially one-parameter distributions), and therefore they have limited applicability with respect to our data.

The exponential edge-gradient effect in x-ray computed tomography.

Abstract: The exponential edge-gradient effect must arise in any x-ray transmission CT scanner whenever long sharp edges of high contrast are encountered. The effect is non-linear and is due to the interaction of the exponential law of x-ray attenuation and the finite width of the scanning beam in the x-y plane. The error induced in the projection values is proved to be always negative. While the most common effect is lucent streaks emerging from single straight edges, it is demonstrated that dense streaks from pairs of edges are possible. It is shown that an exact correction of the error is possible only under very special (and rather unrealistic) circumstances in which an infinite number of samples per beam width are available and all thin rays making up the beam can be considered parallel. As a practical matter, nevertheless, increased sample density is highly desirable in making good approximate corrections; this is demonstrated with simulated scans. Two classes of approximate correction algorithms are described and their effectiveness evaluated on simulated CT phantom scans. One such algorithm is also shown to work well with a real scan of a physical phantom on a machine that provides approximately four samples per beam width.

A Mode Selector to Suppress Fluctuations in Laser Beam Geometry

Abstract: Our development of a gravitational wave detector requires a Michelson interferometer of extreme sensitivity capable of measuring 10-16 m (i.e. some 10-10 of a wavelength λ of the illuminating laser light). Even after painstaking alignment of the interferometer components, and after considerable improvement of the laser stability, noise contributions much in excess of this goal were observed, due partly to fluctuations of the laser beam geometry. The two most obvious types of geometric beam fluctuations are a lateral beam jitter and a pulsation in beam width; these lead to spurious interferometer signals if the interfering wavefronts are misaligned in their tilts or in their curvatures respectively. The geometry of the laser beam can be considerably stabilized by passing it through an optical resonator. The geometric beam fluctuations, as viewed from this resonator, can be described by a well-centred ground mode TEMoo, contaminated by transverse modes TEM mn , with amplitudes decreasing rapidly with the mo...

Crossed beam measurement of absolute cross sections: an alternative method and its application to the electron impact ionisation of He+

Abstract: A new experimental method is reported for the crossed beam measurement of absolute cross sections. It essentially consists in sweeping one of the beams across the other in a linear 'see-saw' motion. The method is applied to one case of electron-ion collisions. A description is given of an electron gun designed to realise a linear and distortionless motion of a ribbon electron beam. Measurements are reported regarding the electron impact ionisation of He+ in the energy range 55-74 eV.

Structural Steel Design

Abstract: 1. Introduction to Structural Steel Design. 1.1 Advantages of Steel as a Structural Material 1.2 Disadvantages of Steel as a Structural Material 1.3 Early uses of Iron and Steel 1.4 Steel Sections 1.5 Metric Units 1.6 Cold-Formed Light-Gage Steel Shapes 1.7 Steel-Strain Relationships in Structural Steel 1.8 Modern Structural Steels 1.9 Uses of High-Strength Steel 1.10 Measurement of Toughness 1.11 Jumbo Sections 1.12 Lamellar Tearing 1.13 Furnishing of Structural Steel 1.14 The Work of the Structural Designer 1.15 Responsibilities of the Structural Designer 1.16 Economical Design of Steel Members 1.17 Failure of Structures 1.18 Handling and Shipping Structural Steel 1.19 Calculation Accuracy 1.20 Computers and Structural Design 2. Specifications, Loads, and Methods of Design. 2.1 Specifications and Building Codes 2.2 Loads 2.3 Dead Loads 2.4 Live Loads 2.5 Environmental Loads 2.6 Loads and Resistance Factor Design (LRFD) and Allowable Design (ASD) 2.7 Normal Strengths 2.8 Two Methods for Doing the Same Thing 2.9 Shading 2.10 Computation of Loads for LRFD and ASD 2.11 Computing Combined Loads with LRFD Expressions 2.12 Computing Combined Loads with ASD Expressions 2.13 Discussion of Sizes of Load Factors and Safety Factors 2.14 Author's Comment 2.15 Problems 3. Analysis of Tension Members. 3.1 Introduction 3.2 Nominal Strengths of Tension 3.3 Net Areas 3.4 Effect of Staggered Holes 3.5 Effective Net Areas 3.6 Connecting Elements for Tension Members 3.7 Block Shear 3.8 Problems 4. Design of Tension Members. 4.1 Selection of Sections 4.2 Built-Up Tension Members 4.3 Rods and Bars 4.4 Pin-Connected Members 4.5 Designs for Fatigue Loads 4.6 Problems 5. Introduction to Axially Loaded Compression Members. 5.1 General 5.2 Residual 5.3 Sections used for columns 5.4 Development of Column Formulas 5.5 The Euler Formula 5.6 End Restraint and Effective Length of Columns 5.7 Stiffened and Unstiffened Elements 5.8 Long, Short, and Intermediate 5.9 Column Formulas 5.10 Maximum Slenderness Ratios 5.11 Example Problems 5.12 Problems 6. Design of Axially Loaded Tension Members. 6.1 Introduction 6.2 AISC Design Tables 6.3 Column Splices 6.4 Built-Up Columns 6.5 Built- Up Columns with Components in Contact with each other 6.6 Connection Requirements for Built-Up Columns Whose Components are in Contact with Each other 6.7 Built-Up Columns with Components not in Contact with Each Other 6.8 Introductory Remarks Concerning Flexural-Torsional Buckling of Compression Members 6.9 Single-Angle Compression Members 6.10 Sections Containing Slender Elements 6.11 Problems 7. Design of Axially Loaded Compression Members Continued. 7.1 Further Discussion of Effective Lengths 7.2 Frames Meeting Alignment Chart Assumptions 7.3 Frames not meeting Alignments Chart Assumptions 7.4 Stiffness-Reduction Factors 7.5 Columns Leaning on Each Other for In-Plane Design 7.6 Base Plates for Concentrically Loaded Columns 7.7 Problems 8. Introduction to Beams. 8.1 Types of Beans 8.2 Sections used as Beams 8.3 Bending Stresses 8.4 Plastic Hinges 8.5 Elastic Design 8.6 The Plastic Modulus 8.7 Theory of Plastic Analysis 8.8 The Collapse Mechanism 8.9 The virtual-Work Method 8.10 Location of Plastic Hinge for Uniform Loadings 8.11 Continuous Beams 8.12 Building Frames 8.13 Problems 9. Design of Beams for Moments. 9.1 Introduction 9.2 Yielding Behavior-Full Plastic Moment, Zone 1 9.3 Design of Beams, Zone 1 9.4 Lateral Support of Beams 9.5 Introduction to Inelastic Buckling, Zone 2 9.6 Moments Capacities, Zone 2 9.7 Elastic Buckling Zone 3 9.8 Design Charts 9.9 Noncompact Sections 9.10 Problems 10. Design of Beams--Miscellaneous Topics. (Shear, Deflection, ect.) 10.1 Design of Continuous Beams 10.2 Shear 10.3 Deflections 10.4 Webs and Flanges with Concentrated Loads 10.5 Unsymmetrical Bending 10.6 Design of Purlins 10.7 The Shear Center 10.8 Beam-Bearing Plates 10.9 Problems 11. Bending and Axial Force. 11.1 Occurrence 11.2 Members Subject to Bending and Axial Tension 11.3 First-Order and Second-Order Moments for Members Subject To Axial Compression and bending 11.4 Magnification Factors 11.5 Moment Modification or C Factors 11.6 Review of beam-Columns in braced Frames 11.7 Design of Beam-Columns --Braced or Unbraced 11.8 Review of Beam-Columns in Unbraced Frames 11.9 Problems 12. Bolted Connections. 12.1 Introduction 12.2 Types of Bolts 12.3 History of High-Strength Bolts 12.4 Advantages of High-Strength Bolts 12.5 Sung Tight, Pretensioned, and Slip-Critical Bolts 12.6 Methods for Fully Pretensioning High-Strength Bolts 12.7 Slip-Resistant Connections and Bearing-Type Connections 12.8 Mixed Joints 12.9 Sizes of Bolt Holes 12.10 Load Transfer and types of Joints 12.11 Failure of Bolted Joints 12.12 Spacing and Edge Distance of Bolts 12.13 Bearing-Type Connections-Loads passing through Center of Gravity of Connections 12.14 Slip-Critical Connections-Loads Passing Through Center of Gravity of Connections 12.15 Problems 13. Eccentrically Loaded Bolted Connections and Historical Notes on Rivets. 13.1 Bolts Subject to Eccentric Shear 13.2 Bolts Subject to Shear and Tension (Bearing Type Connections) 13.3 Bolts Subject to Shear and Tension (Slip-Critical Connections) 13.4 Tension Loads on Bolted Joints 13.5 Prying Action 13.6 Historical Notes on Rivets 13.7 Types of rivets 13.8 Strength of Riveted Connections-Rivets in Shear and Bearing 13.9 Problems 14. Welded Connections. 14.1 General 14.2 Advantages of Welding 14.3 American Welding Society 14.4 Types of Welding 14.5 Prequalified Welding 14.6 Welding Inspection 14.7 Classification of Welding 14.8 Welding Symbols 14.9 Groove Welds 14.10 Fillet Welds 14.11 Strength of Welds 14.12 AISC Requirements 14.13 Design of Simple Fillet Welds 14.14 Design of Connections for Members with Both Longitudinal and Transverse Fillet Welds 14.15 Some Miscellaneous Comments 14.16 Design of Fillet Welds for Truss Members 14.17 Plug and Slot Welds 14.18 Shear and Torsion 14.19 Shear and Bending 14.20 Full-Penetration and Partial-Penetration Groove Welds 14.21 Problems 15. Building Connections. 15.1 Selection of Type of Fastener 15.2 Types of Beam Connections 15.3 Standard Bolted Beam Connections 15.4 AISC Manual Standard Connection Tables 15.5 Designs of Standard Bolted Framed Connections 15.6 Designs of Standard Welded Framed Connections 15.7 Single-Plate or Shear Tab Framing Connections 15.8 End-Plate Shear Connections 15.9 Designs of Welded Seated Beam Connections 15.10 Stiffened Seated Bean Connections 15.11 Design Of moments Resisting FR Moment Connections 15.12 Column Web Stiffeners 15.13 Problems 16. Composite Beams. 16.1 Composite Construction 16.2 Advantages of Composite Construction 16.3 Discussion of Shoring 16.4 Effective Flange Widths 16.5 Shear Transfer 16.6 Partially Composite Beams 16.7 Strength of Shear Connectors 16.8 Number, Spacing, and Cover Requirements for Shear Connectors 16.9 Moment Capacity of Composite Sections 16.10 Deflections 16.11 Design of Composite Sections 16.12 Continuous Composite Sections 16.13 Design of Concrete-Encased Sections 16.14 Problems 17. Composite Columns. 17.1 Introduction 17.2 Advantages of Composite Construction 17.3 Disadvantages of Composite Columns 17.4 Lateral Bracing 17.5 Specifications for Composite Columns 17.6 Axial Design Strengths of Composite Columns 17.7 Shear Strength of Composite Columns 17.8 LRFD Tables 17.9 Loads Transfer at Footings and Other Connections 17.10 Tensile Strength of Composite Columns 17.11 Axial Load and Bending 17.12 Problem 18. Cover-Plated Beams and Built-Up Girders 18.1 Cover-Plated Beams 18.2 Built-Up Girders 18.3 Built-Up Girder Proportions 18.4 Tension Field Action 18.5 Design of Stiffeners 18.6 Problems 19. Design of Steel Buildings. 19.1 Introduction to Low-Rise Buildings 19.2 Types of Steel Frames Used for Buildings 19.3 Common Types of Floor Construction 19.4 Concrete Slabs on Open-Web Steel Joists 19.5 One-Way and Two-Way Reinforced Concrete Slabs 19.6 Composite Floors 19.7 Concrete-Pan Floors 19.8 Steel- Decking Floors 19.9 Flat Slabs 19.10 Precast Concrete Floors 19.11 Types of Roof Construction 19.12 Exterior Walls and Interior Partitions 19.13 Fireproofing of Structural Steel 19.14 Introduction to High-Rise Buildings 19.15 Discussion of Lateral Forces 19.16 Type of Lateral Bracing 19.17 Analysis of buildings with Diagonal Wind Bracing for Lateral Forces 19.18 Moment-Resisting Joints 19.19 Design of Buildings for Gravity Loads 19.20 Selection of Members Appendix A. Derivation of the Euler Formula. Appendix B. Slender Compression Elements. Appendix C. Flexural-Torsion Buckling of Compression Members. Appendix D. Moment-Resisting Column Base Plates. Appendix E. Ponding. Glossary. Index.

Measurement of submicron laser beam radii

Abstract: Three methods for direct measurement of the intensity distribution in laser beams focused by microscope optics to waists of submicron width are described and compared. They use scans of the beam waist with (1) a knife-edge, (2) a submicroscopic point fluorescent source, and (3) convolution scans generated by the photobleached pattern of the focused beam. An indirect photographic technique is also evaluated. The laser beam is found to propagate ideally down to a minimum size usually limited by the aberrations of the optics.

In situ vaporization of very low molecular weight resists using 1/2 nm diameter electron beams

Abstract: A technique is presented for in situ measurements of contrast curves for electron beam vaporizable resists. Using a 1/2 nm diam beam of 100 keV electrons, we have etched lines, holes and patterns in NaCl crystals at the 2 nm size scale. Troughs about 1.5 nm wide on 4.5 nm centers and 2 nm diam holes have been etched completely through NaCl crystals more than 30 nm thick.

Measurement of Atmospheric Turbulence Induced Intensity Fluctuations in a Laser Beam

Abstract: We report measurements of the statistical properties of the intensity fluctuations in a He/Ne laser beam which had propagated 1·125 km through the earth's atmosphere. The beam size was such that the beam propagated as a spherical wave, and atmospheric conditions such that most measurements were made in the ‘saturation’ region (β0 ∼ 1-4). We found (1) that the variance of the intensity was considerably larger than that previously reported for plane wave propagation, and (2) that the probability distribution of the intensity and the derived moments were close to the K-distribution and its moments. The origin and significance of the K-distribution in atmospheric propagation is considered. Photon counting techniques were used for all measurements.

Stability and beam divergence of multimode lasers with internal variable lenses.

Abstract: A resonator with an internal lens can be characterized by its equivalent g-parameters which give the regions of stability if the lens has variable focal length However, other properties are not given by this equivalence and have to be investigated by means of the resonator matrix method Beam diameter, divergence angle, number of transversal modes, positions of beam waists, stability, and the correlations of these parameters with position and focal length of the internal lens are discussed Regions of low sensitivity to variations of the focal length are evaluated and compared with experimental results For some special resonator configurations numerical results are presented

Versatile single-shot background-free pulse duration measurement technique, for pulses of subnanosecond to picosecond duration

Abstract: An extension of the noncollinear second-harmonic generation technique for pulse autocorrelation measurements is described. A diffraction grating is used to produce a tailored, expanded beam, with a differential time delay along its expanded axis. When this beam is combined with an inverted replica of itself at the frequency-doubling crystal, the monitored spatial profile of the generated second-harmonic beam gives directly the duration of the incident laser pulse. A time resolution of better than 1 ps is obtained at 500 nm, and a total measurement range of∼80 ps. The optical system here described enables the extension of the measurement range in a simple manner.

Photoelectric input apparatus

Abstract: A photoelectric touch input panel has a plurality of crossed light beams which are broken by an object, the position coordinates of which are identified as outputs. Two spaced apart beam surfaces are provided, and the interrelationship between the beams in the two planes is employed to distinguish between interrupting objects on the basis of their size, attitude, and velocity characteristics. The number of beams broken in any beam plane is counted to determine the relative size of the object, or to determine the center line of the interrupting object. The beams in any beam plane are selected in accordance with the relative significance of the various beams, and interrupted beams are pulsed more rapidly than non-interrupted beams. The light-emitting devices of the various beam planes are constructed as an integral unit. The control system of the touch input panel is adapted for use in monitoring the size, shape, and activity of objects within a space defined by plural beam planes, independently of the touch input panel.

Altitude dependent model of the auroral beam and beam-generated electrostatic noise

Abstract: A height dependent model of the auroral beam and ionosphere is used to investigate the electrostatic noise generated by the auroral beam. The beam pitch angle is assumed to evolve with the first adiabatic invariant conserved. The beam generates whistler noise over most of the field line and upper hybrid noise only at low altitudes. Only a very narrow range of beam source densities generates noise without producing such large power fluxes that nonlinear effects are important. The strongest power fluxes in the auroral arc are likely to occur in the altitude range of a few thousand kilometers.

Measurement of internal stress within bulk materials using neutron diffraction

Abstract: Neutron diffraction measures stress through the small changes in atomic lattice parameters caused by strain. The method is similar to X-ray diffraction with the vital difference that a thermal neutron beam penetrates several centimetres in most materials allowing measurements within bulk samples. Ways of achieving the high resolution necessary for quantitative measurements are described. Results are presented for the strains in a mild steel bar subjected to known elastic stresses, and on the internal stresses in a deformed bar.

Dynamic Behavior of Continuous Beams with Moving Loads

Abstract: An analytical method for determining eigenvalues and eigenfunctions of continuous beams with arbitrary boundary conditions is developed by using a general solution of a differential equation for the lateral vibration of the beam. This method can be applied to nonuniform cross section beams and calculated to higher eigenvalues very accurately. The dynamic response of a continuous beam traversed by a moving load with constant velocity is studied. The analysis is conducted by the method of modal analysis. Numerical examples are presented to illustrate the applicability of the analysis and to investigate the dynamic characteristics of continuous beams.