Multilayer Shield Research Articles

A sheet impedance approximation for electrically thick multilayered shields

This paper describes an extension of the sheet impedance concept to treat inhomogeneous or multilayered shields that may be thick in terms of material shield wavelengths. For shields with magnetic materials, a simple relation between the equivalent electric and magnetic currents representing the shield is obtained. This allows the magnetic current to be treated as a dependent unknown and the electric current to be found as the solution of a single surface integral equation shown to be a perturbation of that for a perfect electric conducting (PEC) surface. By using the proper interior equivalent problem, it is shown that the method produces accurate and stable results for shielding by a rectangular box.

Investigation on Magnetic Shielding for ELF Magnetic Field from Three-Phase Electric Power Line

Recently it is necessary to reduce ELF (Extremely Low Frequency) magnetic field, which arises from the electric power line in the building. The magnetic shielding with ferromagnetic material is effective in the reduction of ELF magnetic field from the three-phase electric power line. Then, the current of the electric power line fluctuates in proportion to use load, season and time.Therefore, we experimentally measured the magnetic shielding effectiveness with current fluctuation. Also, we numerically calculated them using the finite element method reflected the nonlinearity of the magnetization characteristics. The magnetic material selected thin plate of the permalloy. Characteristics of single-layer shielding and multi-layer shielding were examined.It was confirmed that the shielding effectiveness increased with the increase in the current and that it decreases from the threshold. As a result, it was proven that the shielding property was able to estimate by the finite element method analysis considering magnetization curve.

Cost effective honeycomb and multi-layer insulation debris shields for unmanned spacecraft

Ways to improve the tolerance of unmanned spacecraft to hypervelocity impact are presented. Two new honeycomb and multi-layer insulation (MLI) shields were defined: (1) double honeycomb, and (2) enhanced or toughened MLI (with additional Kevlar 310 and/or Betacloth layers). Following hypervelocity impact testing, a new ballistic limit threshold was defined, based on rear facesheet perforation and witness plate damage characteristics. At 12 km/s, the ballistic limit of single honeycomb was 0.58 mm (aluminium sphere), rising to 0.91 mm for double honeycomb, 1.00 mm for double honeycomb with MLI and 1.17 mm for double honeycomb with toughened MLI. A damage equation, based on the modified Cour-Palais equation with ESA constants, was compared with the data and found to be conservative. The impact angle exponent was increased in order to reduce the equation under-prediction for the oblique incidence data. An equivalent rear wall thickness was defined in order to distinguish between shield types above 7 km/s. The spacecraft survivability analysis showed that the double honeycomb and toughened MLI significantly reduced the number of perforating particles over the baseline single honeycomb design. The mass increase of these shields is approximately 1.2 kg/m 2 for double honeycomb and 0.8 kg/m 2 for toughened MLI.

Design of multilayered cylindrical shields using a genetic algorithm

The design of infinitely long multilayered cylindrical shields with circular cross-section are considered and a method based on the genetic approach is proposed. An analytical method for the calculation of the shielding effectiveness of a cylindrical shield, consisting of homogeneous layers is presented for the case of an obliquely incident plane wave. By making use of this method, a genetic algorithm is implemented for the design of multilayered cylindrical shields in order to achieve a prespecified shielding effectiveness for a given band of frequencies or a range of angles of incidence.

Homogeneous and Multilayered Shields for Neutrons and Gamma-Rays

Measurements were carried-out to compare the attenuation properties of homogeneous shields and shields of two layers and three layers for fast neutrons and total gamma rays. These were performed by measuring the fast neutron and total gamma-ray spectra behind homogeneous shields of magnetite–limonite, ilmenite–ilmenite and magnetite–magnetite concretes. The two layers assembly consists of iron and one of the above mentioned concretes, while the three layers shield consists of water, iron and one of the previously mentioned concretes. All measurements were carried-out using a neutron-gamma spectrometer with stilbene scintillator coupled to a fast photomultiplier tube. Separation between pulses of recoil protons and recoil electrons was achieved by a pulse shape discrimination technique. The obtained data are presented in the form of neutron and gamma spectra measured behind homogeneous shields and shields of two layers and three layers. The spectra measured behind homogeneous shields were used to derive the total macroscopic cross-sections for neutrons and the total attenuation coefficients for total gamma rays.

General Formalism for Calculating Gamma-Ray Buildup Factors in Multilayer Shields into MERCURE-6 Code

This study proposes a new method for calculating gamma-ray buildup factors in multilayer shields. The formula combines the buildup of single-layer shields with products and quotients. The tests have shown that the approximating formula reproduces the reference data of double layer shields very well for most cases. With the same parameters and with a new physical consideration that takes into account in a global way the degradation of the gamma-ray energy spectrum, the buildup factors of three and five layer shields were also very well reproduced. The new formula is incorporated in the new version of the MERCURE-6 point-kernel code.

Transform method for calculating low-frequency shielding effectiveness of planar linear multilayered shields

A two-dimensional analytical solution for quasistatic magnetic field shielding with planar infinite multilayered shields is presented. The magnetic field source is a system of long straight wires parallel to the shield, carrying sinusoidal currents. The analysis assumes that material media can be considered linear under the applied source fields. The spatial Fourier cosine and sine transforms are applied to the analytical expressions of the magnetic field intensity and flux density is obtained by solving the diffusion equation in each layer. Using transfer relations for every layer in terms of transformed variables allows one to obtain the shielded field, and thus the shielding effectiveness, with no need to determine the integration functions explicitly. The results obtained with both this approach and a finite-element computer code are in good agreement. The method seems to be also suited for the analysis of problems with more complex geometries and source distributions.

Flexible and deployable meteoroid/debris shielding for spacecraft

The penetration resistance of various flexible multi-layer shielding concepts has been assessed in hypervelocity impact (HVI) tests at NASA Johnson Space Center (JSC) and at Southwest Research Institute (SwRI). The shields tested consist of 3 to 4 spaced bumpers made of ceramic cloth (Nextel 1 1 Nextel is a trademark of 3M Corporation, Kevlar is a trademark of DuPont Corporation ) and a rear wall composed of high-strength cloth (Kevlar 1 ). Low-density polyurethane foam is used between bumper layers and rear wall to deploy the bumpers after the shield is delivered to orbit and maintain bumper standoff while on-orbit. Ballistic limit equations have been derived from the HVI test results. These equations are being used to size shielding for an inflatable module, which is planned for potential use on Space Station.

The optimum arrangement of the plates in a multi-layered shield

A normal impact of a three-dimensional rigid conical impactor penetrating into a layered shield is studied using a simplified model for an impactor–shield interaction. The shield consists of adjacent plates manufactured from one of two possible materials, and the total thickness of the plates manufactured from every material is given. It is found that advancing any plate inside a shield in the direction of penetration causes a monotone change in the ballistic limit velocity. A criterion for increasing or decreasing of the ballistic limit velocity, which depends on the properties of the materials of the layers in the shield, is determined. A maximum ballistic limit velocity is attained for a two-layered shield without alternating the plates manufactured from different materials.

A New Approximating Formula for Calculating Gamma-Ray Buildup Factors in Multilayer Shields

This study proposes a new approximating formula for calculating gamma-ray buildup factors in multilayer shields. The formula combines the buildup factors of single-layer shields with products and quotients. The feasibility of the formula for reproducing the buildup factors was tested by using point isotropic buildup factors calculated with the SN1D discrete ordinates code as reference data. The dose buildup factors of single-, double-, and multilayer shields composed of water, aluminum, iron, and lead were calculated for a spherical geometry in the energy range between 10 MeV and 40 keV and for total thicknesses of up to 30 mean free paths. The calculation of the buildup factors takes into account the bound electron effect of Compton scattering (incoherent scattering), the coherent scattering, the pair production, and the secondary sources of bremsstrahlung and fluorescence.The tests have shown that the approximating formula reproduces the reference data of double-layer shields very well for most cases. With the same parameters and with a new physical consideration that takes into account in a global way the degradation of the gamma-ray energy spectrum, the buildup factors of three- and five-layer shields were also very well reproduced.

Russian space agency activities on the problem of technogenic space debris

In this paper the main directions and the major results of the activities on the problem of technogenic near-earth space (NES) orbital debris are discussed. With regard to monitoring the NES debris environment, the following issues are considered: the catalogue of space objects which includes objects in the geostationary ring, orbital debris models, and ground- and space-based observations. For the protection of spacecraft and Space Station from debris particles multilayer and other shields are used, as well as avoidance manoeuvres. Important issues are the determination of the location of impacts and restoration of the station wall tightness. The BUFFER program has been developed for the risk assessment of impacts of orbital debris particles with the Space Station. Measures are taken to reduce technogenic pollution of NES which include those to prevent launch vehicles and spacecraft explosions. Special attention is placed on the safe utilization of the geostationary orbit. From the results of these studies regulatory documents are issued.

空隙層を含む反射・吸収効果を考慮した多層構造体による放射線遮蔽性能の向上

A new type of radiation shield structure with narrow gaps is proposed and its shielding property is shown to be improved by taking the effects of reflection and absorption before and behind the gaps into consideration.The present shield structure consists of the multi-layered structure of shield materials. The thickness of one unit shield layer is about one mean free path of the incident X-ray in the shielding material. Three basic elements are applied to the present shield structure as follows. The first element is to equip narrow gaps between the shield layers. The second element is to use scattering/absorbing composite materials. The third element is to enclose the outside of multi-layered shield structure with absorbing materials.In the present paper the basic concept for the present shield structure is proposed as the X-ray shielding design and simplified equations are derived by considering X-ray energy flow. Using the derived equations, shielding calculations are made as to a simple multi-layered structure model. Furthermore, the fundamental X-ray penetration experiments and calculations by the Monte Carlo code EGS 4 are performed in order to prove the improvement in shielding property due to the present multi-layered structure with gaps. From these investigations, we obtained the result that the present multi-layered structure with narrow gaps can improve effectively shielding property.

Description of Multilayered Gamma-Ray Exposure Buildup Factors up to 40 mfp by the Approximating Model

An approximating formula recently proposed by the authors for γ-ray buildup factors of multilayered shields was applied for very thick shields up to 40 mfp. For this purpose, modifications were made to the model and the fitting method to improve the data reproducibility. The previous model was expanded so that it included both the plane-normal and point isotropic geometries.The verification test of the modified model was made for three materials; water, iron and lead. The separately published data of double-layered shields for point isotropic buildup factors calculated by EGS4 from 0.1 MeV to 10 MeV were used as well as newly calculated data at 1 MeV for the plane-normal geometry, and data for the point isotropic geometry of triple-layered shields at 1and 10 MeV.The present formula generally shows a very good reproducibility of the multilayer buildup factors, even in case of very thick shielding problems. The observed error between the approximating description and the EGS4 data is 15% in the intermediate energy range, about 30% in the higher energy range, and 35% at 0.3 MeV. However, the error in the approximation reaches a factor of 4 in the worst case at 0.1 MeV.

Functional study of a system of magnetic multilayer shields

Functional study of a system of magnetic multilayer shields

Description of Multilayered Gamma-Ray Exposure Buildup Factors up to 40 mfp by the Approximating Model.

An approximating formula recently proposed by the authors for γ-ray buildup factors of multilayered shields was applied for very thick shields up to 40 mfp. For this purpose, modifications were made to the model and the fitting method to improve the data reproducibility. The previous model was expanded so that it included both the plane-normal and point isotropic geometries.The verification test of the modified model was made for three materials; water, iron and lead. The separately published data of double-layered shields for point isotropic buildup factors calculated by EGS4 from 0.1 MeV to 10 MeV were used as well as newly calculated data at 1 MeV for the plane-normal geometry, and data for the point isotropic geometry of triple-layered shields at 1and 10 MeV.The present formula generally shows a very good reproducibility of the multilayer buildup factors, even in case of very thick shielding problems. The observed error between the approximating description and the EGS4 data is 15% in the intermediate energy range, about 30% in the higher energy range, and 35% at 0.3 MeV. However, the error in the approximation reaches a factor of 4 in the worst case at 0.1 MeV.

Tests of a prototype magnetostrictive tuner for superconducting cavities

The Continuous Electron Beam Accelerator (CEBA) uses mechanical tuners at 2 K driven by room temperature stepping motors in a feedback loop to maintain cavity frequency at 1497 MHz. A modification of the existing system was designed, replacing a passive section of the mechanical tuner with a magnetostrictive tuning element consisting of nickel rod and an industrially supplied 0.25 T superconducting solenoid. This assembly was tested with several magnetic shield configurations designed to keep the stray flux at the niobium cavity below 1 /spl mu/T when the cavity was normal, to maintain cavity Q. Results of the tests, including change in cavity performance when the cavity was locally quenched near the end of the solenoid, showed that a multilayer shield of 6 mm of steel, with sheets of mu metal, niobium and mu metal spaced appropriately outside the thick steel, was effective in containing the flux, both remanent and current-driven, preventing any change in cavity Q upon cooldown or quench with an external heater near the end of the solenoid. Hysteresis attributed to the nickel magnetostrictive element was observed.

Dynamical cancellation of the environmental magnetic disturbing fields

A cubic Helmholtz-coil triaxial system, with a mean side of 2 m and a volume of uniform field of 0.2 m3, allows the compensation of both the main ambient magnetic field and the urban magnetic noise, in the frequency band of 0–10 Hz. The main ambient magnetic field can be cancelled, or controlled, by a stabilised power supply. The alternating disturbing fields are compensated by a triaxial magnetometric system. The triaxial sensor unit is placed near the homogenous volume of the Helmholtz system. The feed-back loops of the magnetometer channels are closed using special windings of the triaxial Helmholtz system. This configuration enables one to cancel out the simulated disturbances at a ratio of 1 × 102 − 5 × 102 and the industrial noise at a ratio of 1 × 102. Functional studies of the magnetometric sensors and magnetometric devices can be performed in the homogenous volume created by the triaxial Helmholtz system. A system of cylindrical multilayer shielding can be placed in the volume of the dynamically compensated field: thus the ratio of magnetid field cancellation becomes 104.

Determination of inductive shielding factors in double-layer structures

Our experiments show that multilayer inductive shielding performs better than a monolayer shielding of equivalent thickness. We developed an analytic expression for the shielding factor S of a two-layer inductive shielding, extendible to an arbitrary number of layers. The experimental data, obtained from a reduced scale model, fitted this expression well. We demonstrated that multilayer inductive shielding offers better performance for a better price than a traditional monolayer shielding.

A dedicated empirical formula for γ-ray buildup factors for a point isotropic source in stratified shields

A dedicated empirical formula for evaluating γ-ray buildup factors of double-layered shields for a point isotropic source has been proposed for the first time by fitting it to the data of Monte Carlo calculations using EGS4 code. The calculated results from the present formula were compared with those using other empirical formulae. The extension of the present formula in the application for multilayered shields was tested for a number of combinations of stratified shields. By using infinite-medium buildup factors of single materials, the present formula will deliver buildup factors of stratified shields without suffering boundary effect instead of the ones of transmission type.

A computational methodology and results for quasistatic multilayered magnetic shielding

A methodology for accurate calculations of shielding factors for quasistatic multilayered magnetic shields is described. "Transfer relations" for individual layers with specified magnetic permeabilities and electrical conductivities are spliced together. Specific transfer relations for four layer geometries (planar, cylindrical with transverse fields, cylindrical with axial fields, and spherical) and constraints at source and shielded surfaces for six source-shield configurations involving both externally applied fields and enclosed sources are developed. Limiting cases are extracted for magnetically permeable, nonconducting layers, and for thin, magnetically nonpermeable, conducting layers. Reciprocity conditions are identified for interchanged source and shielded regions in planar, transverse field cylindrical, and spherical geometries. Variations of magnetic field and flux density with position are shown for a specific planar example involving alternating layers of aluminum and steel, with the same total shield thickness occupied by either one or five layer pairs. Simulations with alternating layers of aluminum and steel for the four layer geometries are used to study the effects of material composition, number of layer pairs, and air gaps. An optimal number of layer pairs for a given total shield thickness is identified. Results from simulations where induced currents in the steel layers are neglected are compared with those for simulations with a realistic conductivity value for steel to assess the relative effects of flux shunting and induced current shielding.