Patent Application
20070194256
The present subject matter relates to an optimized radiation shielding material, shields comprising the material and to structures incorporating such shields. Protection from types of various types of radiation including, for example forms of radiation expected to be encountered in space related or high altitude.
Many fields benefit from radiation shielding, including space related or high-altitude applications. Prior art radiation shielding materials include Boron, Tungsten, Titanium, Tantalum, Gadolinium, Hafnium, Osmium, Platinum, Gold, Silver, or Palladium or some combination of these.
Earlier materials, such as RAD-COAT™ or RAD-PAK™, either placed a coating directly on the semiconductor die or package or utilized shields made of a gold plated Tungsten-Copper alloy. Foil shielding, however, when incorporated into the walls of space vehicles, are prone to mechanical failure during use, as the adhesive bond between the metal and the organic adhesive of the composite is susceptible to thermal expansion mismatch between the metal and the adhesive. Further the bonds between dissimilar materials are not as robust as bonds between like materials. In the invention, having the matrix material in the shield either match or be chemically compatible with the composite construction of the spacecraft mitigates this. The matrix adhesive in the shield can bond chemically with the adhesive in the walls of the spacecraft, forming a truly mechanically integrated structure.
There is a need for a radiation shield that can be an integral part of the space craft construction that will not compromise the mechanical performance of the spacecraft; which is compatible with the assembly processes associated with said construction; which is easy to fabricate and which can address the radiation shielding needs of the application.
Briefly stated, in accordance with embodiments of the present invention, a radiation shielding material comprising a mixture of Tungsten and of a neutron attenuator. Gadolinium and Boron are preferred neutron attenuators. The Gadolinium may be passivated to facilitate its handling. A nano-layer coating may comprise silica or alumina formed by atomic deposition. The passivation layer reduces the chemical activity of the filler while retaining the maximum attenuation performance of the pure metal. A shield comprises the Tungsten-passivated Gadolinium mixture in a binder such as epoxy which may be partially or wholly cured. A structure may be constructed that is integral with the shield.
The drawings consist of Drawing 1, FIGS. 1 and 2 and Graphs 1-4.
Embodiments of this invention exhibit, through the use of modeling, improved attenuation of Black Body X-rays, while also providing significant thermal neutron attenuation through the selection a filler comprising two metals when compared with similar shields fabricated with the use of one filler, e.g., Tungsten.
Embodiments of the invention specifically address the issues of shielding Blackbody X-ray radiation in the 1 keV to 10 keV range in Aerospace structures such as planes, interceptors, UAVs, spacecraft, missiles and satellites. Besides X-ray shielding, the material addresses 1) Thermal issues associated with high temperature in space, and more specifically that short intense thermal shock caused by burst of Black Body X-rays and 2) mechanical strength required for high stress environments of space.
The embodiments comprise a radiation shield filler composition providing both high Z and effective neutron attenuation. Boron and Gadolinium are effective neutron attenuators in that their attenuation performance is materially higher than other metals. Tungsten is more dense. One form of the material is a composite which is comprised of Tungsten and Gadolinium in a binder material using resins. The Gadolinium may be passivated. A radiation-shield composition according Atomic deposition may be used to form a nano-layer of silica or alumina. The passivation layer reduces the chemical activity of the filler while retaining the maximum attenuation performance of the pure metal. Other compositions also exist and the optimum percentage of tungsten and gadolinium depends on the energy of X-rays. Boron may alternatively be used as a neutron attenuator. Boron has lower density,
In embodiments of the invention the resin in the Tungsten composite acts as an integral low Z material so that no separate absorber is required. Other fillers can be substituted for a portion of the Tungsten for different environments, such as Gadolinium or Boron for neutron shielding. Further, thickness variations can be made to optimize the spatial shielding efficiency (and weight). Powder mixtures and powder gradients are possible which provide the best overall reduction of the various forms of radiation. The lack of sharp interfaces eliminates the thermal spikes that can occur at these locations.
Embodiments of the invention in one form comprise a filler to block the radiation. The filler comprises metals in a binder, preferably an organic resin, such as epoxy or cyanate ester. The proportion of metals in the filler may be selected based on the radiation shielding performance and environment for such species as X-ray, neutron, gamma and cosmic rays. Radiation performance may be optimized for each type of radiation environmental requirement and is proportional to radiation attenuation. Additionally, the formulation may employ additives such as fumed silica and various solvents to facilitate rheological modifications during processing.
In another form, the filler further comprises an adhesive so that a shield may be affixed to another structure without application of a separate adhesive layer. The adhesive may comprise the binder. An exemplary adhesive binder is epoxy which has been partially, e.g., 60%, cured.
Another embodiment comprises a shield comprising the filler. The shield by be heated to a temperature greater than Tg, a glass transition temperature, and shaped. The shape will be retained when the temperature decreases below Tg. Further, a structure integrally including the shield may also be provided.
Embodiments of the invention can use a broader array of fillers than the foil approach. While Tungsten is the preferred shielding material for many radiation environments, Tungsten in sheet and foil is very difficult to work with because of its brittleness. Joining is equally difficult because if its refractoriness, poor solder-ability and oxidation resistance. Embodiments of the invention address this problem by using powder fillers. The powder, combined with a resin matrix is easier to shape and process but still yields high enough density to provide effective radiation shielding.
Embodiments of the invention also permit the incorporation of various sensor devices within its structure to enable real time monitoring of the spacecraft health for such parameters as temperature, radiation, and pressure. This refers to the ability of embodiments of the invention to perform as a “smart composite.”
The shielding material is embedded in a flexible fiber reinforced carbon based matrix so that it can it can be applied as a B-stage to allow placement in non-planar structures and be co-processed with the overall assembly of an aerospace vehicles skin. The process allows for brittle materials that have good shielding properties to be used. The processing is compatible with composites technology and uses adhesives that are chemically and mechanically compatible with the composite. Since it bonds with the composite, it forms an integral part of the structure to yield excellent adhesion at the attachment surfaces.
Dispersing the powders in a polymer eliminates the potential for residual thermal stresses between the fiber reinforced composite and the high modulus metal coating with dissimilar coefficients of thermal expansion. These stresses can unbalance the structure or cause de-lamination of foil technologies.
This technology provides for great application and manufacturing flexibility. A flat substrate will allow the coating to be doctor bladed on at the required thickness. Coatings on curved and complex surfaces can be achieved, either by fabricating a flexible tile or stenciling the material into a location within the composite structure. With the proper choice of adhesive, the material can be cured into the composite, resulting in strong interface adhesion. This will produce a laminate of powder and uncured resin in a controlled thickness sheet that can be stacked for greater attenuation. Layers can be built up with a mix of shielding materials to further improve attenuation performance.
Another configuration option includes fabrication of a decal that can be placed in a transfer carrier. The resin can be ‘B’ staged for easy attachment to the structural base. Co-curing or a separate cure is then possible. Producing the material in a sheet allows the use of pressure to consolidate the powder for greater compaction and a higher final density/efficiency. A sample composition of the material is provided below in table 1.
As will be seen with respect to FIG. 1, attenuation of radiation at each of a number of radiation levels is improved with a Gadolinium concentration of 20%-40% compared to proportions outside this range. This range provides for optimization of thermal neutron absorption and of X-ray attenuation. A further preferred range is 23%-37%. However, much higher proportions of Gadolinium may be provided. The Gadolinium and Tungsten need not be provided in separate layers, but may be provided as a mixture of powders.
Radiation Optimization.
The actual percentages of materials for selected applications may be based on modeling the intended environment. For the natural space environment, a low Z, high Z material with a final low Z material is optimum for proton and electron shielding. The final low Z material absorbs the secondary particles generated from the high Z materials. This is optimized in embodiments of the invention compound using finite particles intermixed with High Z material and Low Z resin binders.
For Black Body radiation, ratios of the high Z material can be optimized for the shielding certain blackbody X-rays which is a function of the actual blackbody temperature.
As an example, the optimum amount of Gadolinium to reduce the blackbody photon radiation varies with the blackbody temperature. FIG. 1 show the graph of dose rate after shielding using various amounts of Gd with the total amount of Gd/W composite shielding equal to 100 mils. This can be equated to the percentage of Gd in the compound structure as shown in FIG. 2.
Dose from Other Materials
Different materials were evaluated for shielding properties in combination with the tungsten composite (density of 13.2 gm/cm3)
20 mils of material was added to 80 mils of W composite with a 13.2 gm/cm3 density. As can be seen by comparing Al to None (only 80 mil of the W composite, no additional material), Al provides very little attenuation for any of the black body spectra. Osmium (Os) is the best theoretically as the most dense material, but is not a practical choice due to its cost. As can be seen, other materials can be used and still meet the required dose reduction and may have other optimal properties like thermal conductivity or melting temperature.
Changing the Material Density and Thickness
For optimum material properties the amount of the high Z material used in this case Tungsten can be reduced and still provide an effective shield. A Tungsten composite with a density of 11.4 gm/cm3 is shown for dose rate at two thicknesses (60 mils and 100 mils), two fluences (0.1 and 0.5 cal/cm2) and 3 black body temperatures (3, 5 and 8 keV). Table 3 and 4 show the results of these calculations.
Typical commercial parts will survive up to dose rates of 1E8 rad/sec, therefore 60 mils of the 11.4 gm/cm3 W composite should be sufficient for a 0.1 cal/cm2 fluence threat while additional shielding would be required for 0.5 cal/cm2 threats for the hotter 8 keV Blackbody spectrum.
This enables use of commercially available semiconductor parts in new defense systems.
Using known dose rate susceptible parts would allow for less shielding. For a large number of parts with unknown susceptibility a thicker shield would be required.
Configuration Comparisons
Summary:
Several examples of material configurations for radiation shielding can be seen in Table 5.
Modeling Performance Comparisons
Configuration 1: 95 mils Tungsten/5 mils Gadolinium/Organic Resin Composite,
Configuration 2: 80 mils Tungsten/20 mils Gadolinium/Organic Resin Composite.
Modeling performance for X-ray shielding attenuation is seen below. Using 3 KeV blackbody X-ray. Using nominal 100 mil (0.100″) thickness for both composite structures, these models both show eleven orders of magnitude attenuation in Dose Deposition through the sample as seen in Graphs 1 and 2.
The differences between configuration three and four are the use of a single filler versus blended fillers. Where a single filler system might be effective at blocking one radiation species, such as X rays, blended fillers provide the advantage of shielding multiple radiation species, for example, in the case of configuration 4, both X rays and neutrons.
Note that the models use a 70% of theoretical density for Tungsten. For both configuration 3 and configuration 4, there is almost complete attenuation.
In terms of application, embodiments of the invention offer advantages over current technologies. Complex shapes are difficult to conform to with brittle and stiff foils. Embodiments of the invention is designed to be process flexible, as it can be applied as a paste, a stencil or B-stage to allow placement in non-planar structures and be co-processed with the overall assembly. The processing is compatible with composites technology as it uses adhesives that are chemically and mechanically compatible with the composite. Since it bonds with the composite, it forms an integral part of the structure to yield excellent adhesion at the attachment surfaces. A comparison of radiation blocking technologies can be seen in Table 6.
Radiation Modeling Evaluation of Shielding Material
Shielding material: The shielding material is 63 mil thick (0.16 cm) and is comprised of 80% W and 20% Gd in a composite. The overall density is 11.4 gm/cm3. Also compared was a pure W in a composite also with a density of 11.4 gm/cm3. Additionally POSS (Polyhedral oligomeric silsesquioxane) with a single Gd atom was modeled. POSS has 8 Si atoms with 12 O atoms and one Gd atom per molecule. The molecular weight of POSS is 605 gm/mole.
Assumptions:
THTK 3.0 was used to model x-ray attenuation of 4 different blackbody temperatures between 3 and 10 keV. The pulse shape was 10 ns with a 30 ns FWHM. The X-ray total peak fluence was 0.1 cal/cm2. FIGS. 1 and 2 shows two attenuation curves (3 keV and 10 keV). Because THTK can't model compounds, the modeling was performed using two layers 50.4 mils of W composite with a density modified to 11.4 gm/cm3 and 12.6 mils of Gd. For POSS we assume 60 mils of silica and 3 mils of Gd. Thermal neutrons where modeled using straightforward cross-sections and Gd density.
Blackbody X-rays
Table 7 shows the attenuation for 4 Black Body temperatures for three different materials all 63 mils thick. As can be seen, the Space Micro W—Gd shield has roughly a 2× improvement over pure W for the low temperature spectrum and a 3× improvement for the 10 keV spectrum. The Space Micro W—Gd material has between 7 to 4 orders of better shielding performance than POSS-Gd for Black body X-rays.
Simulated Environments and Analysis
SMI has provided predictive modeling of the candidates for radiation attenuation, using the Testable Hardware Toolkit (THTk), provided by the Defense Threat Reduction Agency (DTRA). The shielding properties of the samples were simulated using DTRA's Testable Hardware Toolkit (THTk) software. The Testable Hardware Toolkit (THTk) is a nuclear survivability analysis and modeling tool.5
Neutron Shielding Performance
Background: High-energy neutrons (a few MeV) undergo inelastic reactions, which produce recoils and secondary particles. Neutrons with energies in the keV range interact with atoms primarily through elastic collisions, and recoil atoms. Thermal neutrons are in the eV range.
Neutron shielding calculations are dependent on the energies of the neutrons. Thermal neutrons (<0.025 eV) can be captured while fast neutrons require moderation or slowing down through multiple interactions with atoms.
Thermal Neutron Shielding Calculations
Gd-155 and Gd-157 make up 14.8% and 15.65% of natural Gd and have a thermal neutron cross-section of 60 000 and 255 000 barns respectively. We assumed the average Gd composition cross-section of 48,890 which is less assuming that a natural composition Gd was used.
Space micro W—Gd material has a density of 3.5E21 Gd atoms/cm3 while the POSS with one Gd atom has a density of Gd of 1.2E21 Gd atoms m3. Therefore it can be expected that the thermal neutron shielding effectiveness of the Space Micro material is about twice as effective at shielding thermal neutrons then the Gd loaded POSS.
Exposure to thermal neutrons creates a (n, gamma) reaction converting the isotopes to stable even-numbered isotopes. Gamma emissions are up to 8.5 MeV on 157Gd and up to 7.9 MeV for 155Gd. Additionally internal conversion electrons (Auger electrons) in the range of 41 keV and below are produced.
Our assumptions for shielding effectiveness are based on thermal neutron shielding. The input values are shown in table 8
For this back of the envelope calculation we assumed that the reduction in neutron flux would be proportional to: Gd cross-section*thickness of the shield*proportion of Gd*the number of atoms or
S*T*%*N (1)
The result is a four-fold reduction in thermal neutron flux using the above equation.
Comparison of Shielding Effectiveness to POSS
Polyhedral Oligomeric Silsesquioxanes or POSS allows for one or more metals to be added for shielding purposes. Spratt describes the effectiveness of thermal neutron shielding using POSS. Using TART and a weight fraction of 0.5 Gd: POSS, 1 mm of shielding reduced the thermal neutron flux to 32% of the incident dose. 1.5 mm (˜60 mils) of material would reduce the thermal neutron flux to 20% of the incident dose. This is comparable to the reduction calculated for the Space Micro shield using 20% Gd loading. Spratt used a more exact calculation using TART. Based on the 2 to one density ratio of Gd in the Space Micro shield material versus the POSS material, it is quite possible that the Space Micro material would actually have a 2× higher attenuation if a more exact TART calculation was performed.
Shielding Comparisons
Table 7 shows the attenuation for 4 Black Body temperatures for three different materials all 63 mils thick. As can be seen, the Space Micro W—Gd shield has roughly a 2× improvement over pure W for the low temperature spectrum and a 3× improvement for the 10 keV spectrum. The Space Micro W—Gd material has between 7 to 4 orders of better shielding performance then POSS-Gd for Black body X-rays.
Graph 3 and Graph 4 show the Blackbody attenuation curves for the Space Micro W—Gd shielding material for a 3 keV and 10 keV spectrum. The sharp rise at 50 mils is the boundary between the W and Gd material. The actual material is intermixed so there would be no inhomogeneity in the dose deposition profile.
Embodiments of the invention may be optimized for a material for the attenuation of Blackbody X-rays as well as thermal neutrons, thus shielding high speed aerospace systems from pulsed, man-made weapon X-ray radiation. The material is both strong and thin and provides the radiation shielding while withstanding high thermal and mechanical shock from flight and incident radiation. Further, Embodiments of the invention are to be employed within a composite structure in space related or high altitude (exoatmospheric) applications.
US Patent References:
Other References:
1. Space Micro Inc., MDA04-212 Proposal, “Radiation shielding”, June 2004
2. P. Layton, “Radiation Shielding of COTS Electronics”, Internal Correspondence, 2005
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This application is a Continuation in Part of and claims priority from U.S. patent application Ser. No. 11/431,474 filed May 10, 2006, the contents of which are hereby incorporated by reference in their entirety. This application also claims priority from provisional patent applications 60/789,252, filed Apr. 6, 2006, 60/835,711, filed Aug. 7, 2006, the disclosures of which are incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| 60679537 | May 2005 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11431474 | May 2006 | US |
| Child | 11784600 | Apr 2007 | US |
