Showing papers on "Shields published in 1996"

Mechanised Shield Tunnelling

Abstract: This book is translated from German. The development potential of the mechanised shield tunnelling method is far greater than that of the shotcrete method and other traditional tunnelling machines because it is a safer, more effective method of tunnel construction. This book considers the planning of the construction method and design of the machine in harmony with the geological and hydrological conditions, settlement requirements and safety considerations. The technological developments in Germany and world-wide are presented. The book includes chapters on tunnel lining, tailskin seals, grouting and injection procedures, open-faced shields, compressed air shields, slurry shields, earth pressure balance shields, combined shields and special configuration shields and special construction methods.

Transmission through shields of quasi-monoenergetic neutrons generated by 43- and 68-MeV protons-I : Concrete shielding experiment and calculation for practical application

Abstract: The energy spectra of neutrons that penetrate 25- to 200-cm-thick concrete shields are measured using 40- and 65-MeV quasi-monoenergetic neutron sources at the 90-MeV AVF cyclotron of the Takasaki Ion Accelerator Facility for Advanced Radiation Application at the Japan Atomic Energy Research Institute.

Some observations about shielding extremely low-frequency magnetic fields by finite width shields

Abstract: Shielding of extremely low-frequency magnetic fields is limited by fields penetrating through imperfect material and leakage around the edges of shields which do not completely enclose the shielded region The latter effect, more specifically the effect of source geometry and shield type on leakage, is the subject of this work Particular attention is paid to elementary cylindrical dipole sources since the dominant fields of complex two-dimensional (2-D) sources can be represented as those of dipole sources When possible, insight is gained from a simple analytic model for shielding by semi-infinite and finite width perfect electric and perfect magnetic shields In cases for which the simple model is not valid, insight is gained from numerical results It has been found that source orientation and shield type are critical issues and can cause differences in shielding effectiveness in excess of 20 dB

Laser safety features of eye shields

Abstract: Background and Objective A number of lasers are available for cutaneous periorbital surgery, yet not all eye shields are appropriate for all applications We tested a variety of commercially available eye shields to assess their safety features Study Design/Materials and Methods Six commercially available eye protectors were studied A focused laser was incident upon the shield, and the intensity and exposure duration required for visible damage to the shield were measured We then measured the temperature on the underside of the eye shield during exposure from the laser Time-dependent temperature measurements were made with a type-T thermocouple fixed to the eye shield with silicon grease Results Thermal response curves and rates of warming for each of the six eye shields were generated Plastic shields showed significant thermal damage with most of the lasers tested The metallic shields warmed more slowly and to a lesser degree Conclusion Overall, the metallic eye shields had the most acceptable safety profile Many of the plastic shields exhibited significant thermal damage, and therefore we discourage their use in periorbital laser surgery © 1996 Wiley-Liss, Inc

Magnetic field attenuation of nonlinear shields

Abstract: The analysis of shielding performance of planar shields against near field sources is carried out in the time domain to account for the nonlinear behavior of ferromagnetic materials used in low frequency applications. To this end, the Schelkunoff approach for shielding problems has been reformulated in the time domain introducing the transient wave impedances which relates transient electric and magnetic field components and appear in the integrodifferential boundary conditions. The final equation system is solved by means of a numerical procedure based on the finite element method. The obtained results are compared with analytical and measured data in different configurations.

Magnetostatic simulations for design of superconducting magnetic shields

Abstract: A magnetostatic approach to numerical simulations of magnetic field attenuation by superconducting shields is demonstrated on simple geometries. Comparisons to published measurements and analytic calculations show that results are accurate for simulation of a shield in the shape of a cylindrical tube. The capabilities of the method are shown by simulations which close the cylinder with end caps having access ports or gaps. With end caps having cylindrical ports, the simulated attenuation transits smoothly between the analytical results for semi-infinite tubes of the two radii. Radial gaps between solid end caps and the cylinder allow little flux leakage for time-varying fields, but significant leakage for static fields.

Low-frequency shielding effectiveness of inhomogeneous enclosures

Abstract: This paper presents a two dimensional analytical solution for electromagnetic shielding problems at frequencies below resonances. The application of the method to three dimensional shields yields good approximations, except for errors nearby edges which are not parallel to the magnetic field. The solution is valid for enclosures of arbitrary shape with any given combination of materials and wall thicknesses. Considering skin effects, the shielding effectiveness is derived by solving the Helmholtz equation for individual parts of the wall with different thicknesses and materials. The solution for the total shield is then found by applying Faraday's law in integral form. Three cases are discussed: heterogeneous enclosures consisting of several wall sections with different materials and wall thicknesses, coated, laminated and nested shields and multi-cavity enclosures. Different examples of combinations of non-magnetic and magnetic materials are discussed in detail. Finally, the potential of saving material and weight by using multiple shields is investigated.

Termination for a shielded cable

Abstract: A cable (10) has a plurality of conductors (12) with individual conductive shields (14) and one common shield (16) for all conductors, the outer end portions of which shields are terminated by being clampingly retained between tapered surfaces (48,52; 50,54) of the first, second and third termination rings (42,44,46).

Electromagnetic shields for thermotherapy

Abstract: The shields concern all short-wave or microwave electromagnetic applicators for thermotherapy in oncology, physical medicine, cosmetology, urology, veterinary medicine and pharmacokinetics, to produce improved heating efficiency and penetration, improved treatment comfort and lower electromagnetic risk to patient's critical organs and to operators. Shields envelopes applicators and parts of body regions producing EM enclosures of containment of radiation leakages. External hood shields block lateral and backward leakages. Regional shields block also leakages through the exposed tissues. Inner shields trap spurious applicators electric fields keeping them away from overheating the patient's adipose. The passive shields protect adjacent critical organs from the treatment fields. The regional shield (26) envelopes both applicator (3) and body region (2) encompassing target (1) heated with beam (10) through the air-gap (8) and establishing the EM enclosure (27) of containment of the EM energy diffused by (3), which blocks the dispersion of the lateral beam fraction (12) through (8) and of the beam fraction (11) that crosses target (1).

Approximate estimates of the characteristics of propellant ignition using radiant flux through shields with various properties

Abstract: An analytical solution to the problem of propellant ignition using radiant flux is presented in a conjugate statement. The solution generalizes and supplements the results obtained earlier. The cases of ignition through absolutely transparent and opaque shields are considered. Approximate formulas for estimation of the time and temperature of the chemical-reaction onset and the time and temperature of the loss of quasi-stationary equilibrium in different limiting cases are obtained. Estimates of the influence of heat transfer into the environment (by the conductive mechanism) on the ignition characteristics are given. A comparison of the ignition characteristics of the propellant and shields is made.

Numerical simulation of the interaction of dust particles of a comet with a spacecraft

Abstract: The paper deals with the physical and mathematical statement of problems that arise in estimating the safety of the shield design of the “Vega” spacecraft, which was chosen preliminarily under the guidance of R. Z. Sagdeev. Numerical results for failure of shields impacted by particles of the dusty atmosphere of Halley's comet are presented. The size of the failure zones in the shields are determined as a function of the particle size and density. The probability of an impact on the second shield of the particles penetrating the shield or resulting in spalling of solid fragments is estimated. The momentum transmitted by the particle flux to the solar batteries of the spacecraft is determined.

Method and device for working by energy beam

Abstract: PURPOSE: To freely machine an ultrafine pattern arbitrarily on the curved or slant face with a simple structure by positioning shields on the surface of an object to be machined or at a position away from the surface and transferring the pattern of the shields to the surface of the object. CONSTITUTION: Minute rod-like shields 50 are placed on an object W to be worked that has a shape of a base plate, and then are irradiated with an energy beam from the above for machining. By the irradiation with the kind of energy beam B properly selected, etching is performed on the object W to form projected lines 71. As the energy beam B, a high-speed atomic beam is used in which energy particles are free from electric charge and in which directivity is superior. An additional machining is also possible for forming a film 72 for example by changing the kind of gas and the energy.

Optimization of ferromagnetic shields for solenoidal SMES

Abstract: Ferromagnetic shields for solenoidal superconducting magnetic energy storages (SMES) are optimized with respect to the stray field or the mass of the shield. Two optimization strategies of zero order are compared: the deterministic downhill simplex method and the evolution strategy, a stochastic method. Due to the comparison a combination of both methods is tested. To guarantee short computation times the underlying nonlinear field analysis is based on an integral equation method.

Improved getter pump, particularly for a portable chemical analysis instrument

Abstract: An improved getter pump, particularly suitable for use in a vacuum device wherein it is necessary to minimize the volume taken up by the on-site pump and the heating power it requires, includes, around a known assembly of getter elements (1) mounted on a sheath (20) which houses a heating member (2), a plurality of tubular metallic shields (7, 8, 9) coaxial with one another and with said heating member, preferably in the number of three. Said tubular shields are secured at one closed end thereof to a base support (6), and at the other end they communicate with the space to be evacuated through two metallic nets (17, 18) at the two innermost shields (7) and (8), whereas the outermost shields (if they are more than two) of larger diameter are free at said end. The outermost net (18) is preferably more close-meshed and formed by thinner metallic wire with respect to the inner net (17).

Low-temperature space observatories: The effect of realistic surface properties

Abstract: Space-based infrared telescopes will be of great importance in the search for extrasolar planets, and in their subsequent investigation. Such telescopes benefit enormously from radiative cooling. We have investigated this cooling with the aid of a Monte-Carlo model of a cylindrical telescope tube shielded from the Sun by two flat plates. Our primary concern has been to establish to what extent radiative exchanges have to be modeled more accurately than has been the case up to now. We have shown that the equilibrium temperatures depend significantly on the directionality of the radiative properties of the materials coating the tube and shields. Also significant is the polarization of the radiation exchanged between the tube and shields, the change of radiative properties with wavelength and temperature, and the spacing between the shields and tube. There is a need for further measurements of infrared radiative properties at cryogenic temperatures, and we are planning to make some.

Nuclear Analysis for the Light Ion Fusion Power Reactor LIBRA-SP

Abstract: A 12 m thick zone of blanket tubes is needed for the LIBRA-SP chamber wall to be a lifetime component The roof should be located at 17 m from the target to be a lifetime component The depth of t

Process for manufacturing shields of different size for fluorescent lamps and shields produced through such a process

Abstract: A process for manufacturing shields of different size for fluorescent lamps consists in the steps of: providing a support for powders in the form of a metallic strip having a width (L) equivalent to about the circumference of the smallest shield to be manufactured; depositing on said support strip at least one track of getter material and at least another of mercury releasing materials in powdered form; and producing pieces of the support strip by cutting the same with parallel cuts having a pitch (D) and at an angle (α) with respect to the central axis (X-X) of the strip which are both varying according to the desired size of the shield.

System for protection driver from blinding by lights of on-coming automobile

Abstract: FIELD: transport engineering; antiblazing devices SUBSTANCE: system has driver's guard shield and headlight shields, all shields made in the form of optical polarizers Vectors of polarization of shields are turned around axis coinciding with direction of vehicle running through angle of approximately 45 degrees relative to vertical, for instance, to the right EFFECT: enhanced reliability of protection

Lithium Hydride Debris Shields for Plasma Radiation Sources

Abstract: : The use of lithium hydride (LiH) for both return current structures and initial debris shielding is explored and a general set of design criteria is derived. It is found that, particularly when the testing requirement is for argon K shell radiation, LiH shields can improve the expected fluence on a test object over present or recently proposed technologies. The concept rests upon a tradeoff between the proximity of a test object to the plasma radiation source (PRS) and the opacity of any shielding that is interposed. Because a larger mass of intervening shield material is possible with LiH, the expected debris velocities are 100 times or more slow than current measurements. A cold press technique for the fabrication of the shield and its electrical connection to a pulser is specified.

Open Type Magnetic Shields by High Tc Superconducting Cylinders

Abstract: We developed open type magnetic shields for biomagnetic measurement combining high Tc superconducting cylinders and high-permeability cylinders. The size was i.d.220mm × o.d.310mm × length 650mm. High Tc superconducting cylinders were fabricated by spray-coated BiSrCaCuO thick films on Ag substrates. High-permeability cylinders were made of Ni-based alloy. The magnetic shielding effects were measured by the attenuation ratio of the inside magnetic fields and the external magnetic fields which were applied to axial and transverse direction. The transverse magnetic shielding effect of the high Tc superconducting cylinders was 7 × l0-2. The axial magnetic shielding effect of the high-permeability cylinders was 2×10”2. The magnetic shielding effects of the combination were under 7×10-4 regardless of the direction of external magnetic fields. Open type magnetic shields are easy to put investigated objects and SQUID sensors.

Analysis of the influence of baffles and shields on the cooling of a digital transmission equipment

Abstract: In this work design criteria for cooling of electronic systems used in a digital transmission equipment are considered An experimental study using a simulated electronic equipment in which vertically oriented circuit boards are aligned to form vertical channels is carried out Resistors are used to simulate actual components The temperature of several components in the printed circuit boards are measured and the influence of the baffles and shields on the cooling effect are discussed It was observed that the use of the baffles reduce the temperature levels and the use of shields, although protecting the components from magnetic effects, cause an increase in the temperature levels

Investigation of Natural and Man-Made Radiation Effects on Crews on Long Duration Space Missions. Final report

Abstract: Over the past several years, NASA has studied a variety of mission scenarios designed to establish a permanent human presence on the surface of Mars. Nuclear electric propulsion (NEP) is one of the possible elements in this program. During the initial stages of vehicle design work, careful consideration must be given to not only the shielding requirements of natural space radiation, but to the shielding and configuration requirements of the on-board reactors. In this work, the radiation transport code MCNP has been used to make initial estimates of crew exposures to reactor radiation fields for a specific manned NEP vehicle design. In this design, three 25 MW(sub th), scaled SP-100-class reactors are shielded by three identical shields. Each shield has layers of beryllium, tungsten, and lithium hydride between the reactor and the crew compartment. Separate calculations are made of both the exiting neutron and gamma fluxes from the reactors during beginning-of-life, full-power operation. This data is then used as the source terms for particle transport in MCNP. The total gamma and neutron fluxes exiting the reactor shields are recorded and separate transport calculations are then performed for a 10 g/sq cm crew compartment aluminum thickness. Estimates of crew exposures have more » been assessed for various thicknesses of the shield tungsten and lithium hydride layers. A minimal tungsten thickness of 20 cm is required to shield the reactor photons below the 0.05 Sv/y man-made radiation limit. In addition to a 20-cm thick tungsten layer, a 40-cm thick lithium hydride layer is required to shield the reactor neutrons below the annual limit. If the tungsten layer is 30-cm thick, the lithium hydride layer should be at least 30-cm thick. These estimates do not take into account the photons generated by neutron interactions inside the shield because the MCNP neutron cross sections did not allow reliable estimates of photon production in these materials. « less

Compensation for scattered radiation in dual-energy imaging using partially transparent shields

Abstract: In this paper we present a method for the compensation for scattered radiation in dual-energy imaging. The method uses shields (PTS), partially transparent for x-radiation, positioned between the x-ray source and the detector. By comparing the detected signals under and beside the shields, and using data from the dual-energy calibration procedure with basis materials, the radiation scatter can be calculated at the location of the shields. In a first implementation of the technique many shields are positioned at regular distances in the imaging field. The radiation scatter in the entire image can then be calculated by interpolation techniques. A second implementation of the technique makes use of a robust convolution with a low-pass filter. The parameters of the convolution kernel are determined by PTS-measurements at a limited amount of well-chosen locations of the object. After substraction of the radiation scatter, the images can be compensated for the effect of the shields. We tested the method with images of an artificial polystyrene phantom and with humanoid chest images, made with a computed radiography system.