Showing papers on "Shields published in 1994"

Test and performance of a BGO Compton-suppression shield for GAMMASPHERE

Abstract: Bismuth germanate (BGO) Compton-suppresion shields have been constructed to surround the Ge detectors of the GAMMASPHERE array. A shield consists of six hexagonal tapered BGO elements, each coupled to two 1-in. × 1-in. photomultiplier tubes. In addition, a cylindrical BGO detector is placed behind the Ge detector to intercept the forward scattered γ-rays. One hundred ten such shields are planned for the GAMMASPHERE array. Procedures for measuring the performance of these shields have been developed. Large (70%) Ge detectors when used with these shields give a peak-to-total ratio of better than 0.60. To date 85 shields have been tested and approved for use in GAMMASPHERE.

A new approximating model for gamma-ray buildup factors of stratified shields

Abstract: A new approximate expression for gamma-ray buildup factors of multilayered shields is proposed. The expression is formulated based on the vector form and considers the gamma-ray energy spectrum directly. It treats the gamma-ray transmission by a transmission matrix and the backscattering by an albedo matrix. Its capability of reproducing the buildup factors for multilayered shields is demonstrate by using double-layered shields composed of two materials of water, iron, and lead at 1 and 10 MeV. The data of three-layered shields of these materials are also very well reproduced. The mechanism of the density effect arising, which appears in the buildup factor for a point isotropic source, is clearly interpreted by the current method to be a geometrical effect. A correction factor for incorporating the density effect into the current expression is derived. The modified expression is successfully applied to buildup factors for a 0.5-MeV point isotropic source for two-layered shields of water and iron.

Key differences in material requirement and technology for large electromagnetic shields, with respect to other large-scale applications of high-T c superconductors

Abstract: The main specifications for the use of high-Tc superconductors in the realization of extended surfaces to be used for the fabrication of electromagnetic shields are illustrated. In particular, the differences are stressed between the specifications required by this application and those required by other large-scale applications, namely electrotechnical machinery and magnetic levitation. From these premises, the approach to the fabrication technology of shields is briefly discussed, which also shows important differences with respect to other applications. The analysis is supported by examples of the solutions which are being developed and adopted in the world and in Torino.

A novel method for scattered radiation compensation in X-ray imaging systems, using partially transparent shields (PTS)

Abstract: A new method for scattered radiation compensation in 2D projection radiographic imaging is presented. By comparing the detected signal under a small partially transparent shield (e.g. aluminum disk or strip), positioned between the X-ray source and the object being imaged, with the detected signal near the border of the shadow of the shield, the radiation scatter signal in the location of the shield is calculated. For polychromatic X-radiation, calibration with two known basis materials allows accurate calculation of the radiation scatter. Shields are positioned at several locations between object and source. By an interpolation technique, the radiation scatter in every region of interest can be calculated. The radiation scatter image is then subtracted from the original image of the object. The method needs only one exposure of the object. The primary radiation signals at the locations of the shields have undergone an extra drop, but the information about the object under the shields is not lost. One could restore the image by compensating for the effect of the shields on the primary signal. The theory of PTS is presented and results of experiments are given. >

Stationary induction electrical equipment

Abstract: PURPOSE: To reduce stray loss of a stationary induction electrical equipment, which is caused by leakage magnetic fluxes, and to prevent a local temperature rise in the electrical equipment by a method wherein a large quantity of the leakage magnetic fluxes from a winding are effectively shielded with electromagnetic shields. CONSTITUTION: The end parts of electromagnetic shields 9 and 10 are respectively made to project from the end parts of core clamping fitments 7 and 8 and leakage magnetic fluxes shielded with the shields 9 and 10 are prevented from intruding into the fitments 7 and 8 by creeping. COPYRIGHT: (C)1995,JPO

Magnetic shielding using high-temperature superconductors

Abstract: Magnetic shields of various high-temperature superconductors, YBa2Cu3O7−x (YBCO), YBa2Cu3O7−x-Ag composites (random inclusions as well as non-random coatings) and Bi2Sr2Ca2Cu3Ox (BSCCO) were prepared by uniaxial as well as isostatic compression with various dimensions. The shielding properties were measured at 77 K for dc and ac magnetic fields in the range of frequencies from 100 Hz to 10 kHz. The critical penetration field (CPF), defined as the value of the applied magnetic field at which a detectable field was observed inside the cylinder, varied from cylinder to cylinder and also with the ageing of the cylinders in the case of YBCO shields. The highest value of CPF was 16 G at 77 K for YBCO shield prepared by isostatic compression. Even though the stability of BSCCO shields with respect to ageing is good, the CPF values are very low compared to those for YBCO. Detailed studies were performed in the case of YBCO shields. The CPF decreased as a function of time over a period of 90 days. The CPF decreased as the frequency of the applied field was increased. The wave form of the field inside the pot for a sinusoidal applied field was highly distorted and showed the presence of higher harmonics with appreciable amplitude. The wave form was Fourier-analysed to yield the field inside the shield along with the harmonics. The shields with Ag addition seem to give better performance at high fields.

Response of externally excited coaxial cables with wire braided shields

Abstract: The frequency responses of coaxial cables employing wire braided shields, excited by external fields, is studied using an algorithm based on a distributed circuit model, which uses both electric and magnetic field parameters to model the external field coupling. The simplicity and fast speed of the model enable computer-aided analysis of externally induced noise in cable interconnects to be carried out in a computationally efficient manner. A general CAD algorithm is developed, based on this model, and it is applied to the study of the response of cables over a ground plane. The algorithm is then used to study the effects of different braid constructions on the responses of cables excited by external fields. Responses of cables with optimum braid designs in their shields are studied in detail. The model uses the experimental values of the leakage parameters which are measured separately using a standard triaxial text fixture.

High Tc superconductor magnetic shields and method of making same

Abstract: High Tc superconducting magnetic shields are provided, together with a method of fabricating such shields, wherein the shields exhibit very high critical applied magnetic field values of at least about 50 Gauss at 77K. In fabrication procedures, a particulate superconducting ceramic oxide (24) (e.g., thallium 2223) is placed within a uniaxial die assembly (10) and subjected to compression while the die is heated via an external heating jacket (26). After formation of a self-sustaining body (24a), the die (10) is additionally heated via the jacket (26). External heating of the die (10) with the superconducting material therein reduces internal stresses within the shield body.

Development of the superconducting magnet system for a bioreactor

Abstract: A superconducting magnet for a bioreactor system was developed in order to determine the biotechnical and biomedical responses of microorganisms to a high magnetic field. The magnet has a horizontal bore, and the field strength at the center is 7 tesla at an operating current of 120 Amperes. The room-temperature bore diameter is 160 mm. Persistent current mode and detachable power leads are adopted. A cryocooler was installed to cool the radiation shields, which enable continuous operation for 6 months without the need to resupply liquid helium. Magnetic shields cover the cryostat to minimize the leakage field. Two reactors are installed inside and outside of the bore. Magnetic shields also cover the outside reactor in order to reduce the strength of the magnetic fields to less than the geomagnetic level. The temperatures of the reactors are controllable, ranging from 283K to 343K with /spl plusmn/0.1K. >

Magnetic shields made of high Tc superconducting ceramic Tl2Ba2Ca2Cu3O10

Abstract: Magnetic shields were fabricated with high temperature ceramic superconductor Tl2Ba2Ca2Cu3O10. The attenuation rate of this kind of device is 120–180 dB in a dc field at 77 K. The critical shielding field is 70–80 Gauss at 77 K. The frequency characteristic of these devices was investigated from 0 to 2 KHz.

High-Tc Magnetic Shields Prepared by a Low Pressure Plasma Spray

Abstract: Excellent shielding performance can be expected in a superconducting magnetic shield because the magnetic flux are completely removed from inside by Meissner effect. Thus, superconductors can be used as a shield for magnetic noise during very weak magnetic measurements, such as required for biomagnetics. Discovery of the high-Tc superconducting material brought about a complete change in the application of superconductors. Because the operation of superconductor at the liquid nitrogen temperature has become possible, large superconducting magnetic shields have become realistic.

Melt-casting hearth

Abstract: PURPOSE: To obtain a casting having high quantity and little porous holes by disposing plural radiating directional shields at a constant interval in a heating zone and reducing scattered heat radiation into a cooling zone. CONSTITUTION: A die 6 in the inner part of a housing 2 is heated with heating elements 3a, 3b. The die 6 is placed on a cooling disk 5 and the cooling disk 5 is descended with an elevator device 16. Plural annular radiating directional shields 10 are vertically disposed to the longitudinal axis 13 of the die 6. These shields 10 obstruct the heat radiations S1 , S2 and the heat ray is not reached to the portion B of the die 6 positioned in the cooling zone 11 and the temp. difference between a solid phase zone and a liquid phase zone, is increased. In this way, controlled solidified structure or crystallized structure can be prepared.

Finite element analysis of i-beam power efficiencies of grounded shields

Abstract: Power emciencies of grounded shields for structural I-beams in the steel industry are studied. Shielding is necessary in such environment due to the high consumption of stray power losses originating from the induction of eddy currents in I-beams. The objectives of eddy current shielding is to minimize stray power losses and improve power emciencies. In this study, grounded shields are considered. The finite element method is used in this study in conjunction with the asymptotic boundary conditions for rectangular boundaries which are efficient and effective for problems with large aspect ratios.

Micro-structuring of workpieces by cluster ions - using beam shields and apertures

Abstract: The method for micro-structuring of given regions of a workpiece by a beam of cluster ions with chosen energy and chosen ratio of neutral particles to the charge of cluster ions is based on material removal by the beam action. It is characterised by the following facts: (a) micro-structures are introduced into the workpiece (2), which consists of a very brittle and hard material, or are separated from it by at least one radiation operation with the use of a directed beam (1) of cluster ions provided with a predetermined shield (3); and (b) by means of further shields and/or apertures and successive radiation operations, which take place at reduced material removal rate, the micro-structures are brought to their final dimensions and surface quality. Micro-structuring is carried out with the use of a beam of cluster ions produced by condensation of gases. Fixed and/or moving (rotating) shields and apertures are used to produce required micro-structures. USE/ADVANTAGE - For micro-structuring of workpieces made of metallic and non-metallic materials. Surfaces of very brittle and hard materials (cobalt-samarium, tungsten, diamond etc.) can be provided with micro-structures through material removal.

Influence of grains priority orientation on magnetic shielding-trapping properties of BPSCCO superconducting magnetic shields

Abstract: The influence of priority orientation of the Bi-Pb-Sr-Ca-Cu-O superconducting grains for three types of the superconducting magnetic shields on shielded and trapped magnetic flux densities, time decay of the trapped magnetic flux density and apparent pinning potential is shown. A suitable modification of the preparation conditions of the shields with wall-thickness of about 5 mm led to shielded and trapped magnetic flux densities of ∽ 40 mT at the temperature of 77 K.

Magnetic Fields inside Internal Magnetic Shields for TV Tubes

Abstract: The shape of an internal magnetic shield in a TV tube determines in a complex way to what extent the influence of the earth's magnetic field on the TV image is reduced. Conformal mapping and finite element calculations of the field distribution in models of shields were performed to clarify these effects. In a two-dimensional model of a shield, the mislanding that occurs outside the shield can be more than compensated for by the field distribution inside the shield. In an axially symmetric three-dimensional model, compensation is weaker, because the fields inside the shield are lower.

Process and device used to renovate railroad track installations

Abstract: The invention provides for renovation of railroad track installations through the use of a sliding shield driver (9) which in the work area secures the trench (6) formed by the removal of the old track foundation and, as the renovation work advances, travels in the direction of the old track to be renovated. The sliding shield driver (9) has lateral shields (15) securing the trench walls as well as ground shields (13) resting on the trench bottom; these shields are linked to the base frame (10) of the shield driver via hydraulic shield cylinders. Preferably, the shield driver consists of a header frame and a trailer frame with trailer shields (20) pivotably attached to the lateral shields (15) at the trailer frame (19). The lateral shields with the trailer shields are of such a length as to secure the trench walls over the entire work area.

Overvoltage protecting device

Abstract: not available for EP0674816Abstract of corresponding document: US5594614PCT No. PCT/DE93/01158 Sec. 371 Date Aug. 25, 1995 Sec. 102(e) Date Aug. 25, 1995 PCT Filed Dec. 2, 1993 PCT Pub. No. WO94/14216 PCT Pub. Date Jun. 23, 1994A surge arrester (1) for high voltage has a porcelain housing (2) with safety shields (3, 4, 7, 8). In the central area along the longitudinal axis of the surge arrester (1), safety shields (3) with a larger diameter may alternate with safety shields (4) having a smaller diameter. The surge arrester (1) has blow-out apertures (5) at its ends, and gas under high pressure flows out of these apertures in the event of a-short-circuit current. To protect the safety shields from damage, the safety shields (7, 8) that are closest to the blow-out apertures (5) are constructed with smaller diameters than the other safety shields.