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(11) | EP 1 120 795 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Laminated lightweight radiation shielding materials |
| (57) Laminated materials or structures that provide radiation shielding for spacecraft
components in an orbiting spacecraft against electron and proton radiation. The laminated
materials or structures have three laminated layers. The first layer (11) is made
of one or more materials having a low Z/A (atomic number/atomic weight) ratio that
preferentially attenuates electron and proton radiation, although they may generate
Bremsstrahlung radiation in the process. The second layer (12) is made of one or more
materials with a high Z2/A (atomic number squared/atomic weight) ratio that preferentially attenuate Bremsstrahlung
radiation, although the materials may generate photo-electrons in the process. The
third layer (13) is made of one or more materials having a low Z/A (atomic number/atomic
weight) ratio that attenuate photo-electrons emitted from the second layer, as well
as electrons and protons that pass through the first and second layers. |
BACKGROUND
[0001] The present invention relates generally to radiation shielding materials, and more particularly, to improved laminated lightweight radiation shielding materials.
[0002] The prevailing conventional solution for shielding electronic enclosures is to increase wall thickness, typically aluminum, as necessary to achieve the required shielding capability. Physical Sciences Inc. and Composite Optics, Inc. have produced prototypes of laminated lightweight shielding material. Also, shielding materials are disclosed in US Patent No. 5,324,952 and US Patent No. 4,833,334. US Patent No. 5,324,952 discloses shielding that shields against neutrons, while US Patent No. 4,833,334 discloses shielding that shields against x-rays. Neither patent is particularly relevant to the present invention.
[0003] A technical paper written by B. D. Spieth et al., entitled "Shielding Electronics Behind Composite Structures", Dec. 1998, mentions the use of a three-layer composite structure, with tantalum as the center layer, for radiation shielding in various orbits. The Spieth et al. paper discloses the use of a metal center layer and shielding in geosynchronous orbit. This paper also indicates that the advantage of the three layer structure is to minimize warpage. This paper does not disclose or suggest anything about material selection (such as polymer outer layers or a high atomic number/low modulus center layer) to optimize the shielding effectiveness. This paper also does not disclose or suggest anything regarding minimizing the risk of delamination due to thermal expansion mismatch between the outer layers and the metal center layer.
[0004] Accordingly, it would be advantageous to have laminated lightweight radiation shielding materials, such as may be used in a spacecraft environment, that overcome limitations of conventional materials, and provide the opportunity for significant mass savings and/or increased reliability.
SUMMARY OF THE INVENTION
[0005] According to the invention there is provided a lightweight radiation shielding structure (10), comprising: a first layer (11) comprised of one or more materials having low atomic number/atomic weight ratio that preferentially attenuates electron and proton radiation, while possibly generating Bremsstrahlung radiation in the process; a second layer (12) comprised of one or more materials with high atomic number squared/atomic weight ratio that preferentially attenuates Bremsstrahlung radiation, while possibly generating photo-electrons in the process; and a third layer (13) comprised of one or more materials having low atomic number/atomic weight ratio that attenuates photo-electrons emitted from the second layer, as well as electrons and protons that get through the first and second layers.
[0006] The present invention provides for a laminated material or structure that provides radiation shielding for spacecraft components in orbit against electron and proton radiation. Relative to a material that is currently used by the assignee of the present invention, the laminated material provides equivalent radiation shielding for spacecraft components in orbit against electron and proton radiation at a greatly reduced mass. Alternatively, the laminated material may be configured to provide greatly increased radiation shielding at equivalent mass.
[0007] The laminated material or structure comprises three laminated layers. The first layer is made of one or more materials having a low Z/A (atomic number/atomic weight) ratio that preferentially attenuate electron and proton radiation, although they may generate Bremsstrahlung radiation in the process. The second layer is made of one or more materials with a high Z2/A (atomic number squared/atomic weight) ratio that preferentially attenuate Bremsstrahlung radiation, although they may generate photo-electrons in the process. The third layer is made of one or more materials having a low Z/A (atomic number/atomic weight) ratio that attenuate photo-electrons emitted from the second layer, as well as electrons and protons that pass through the first and second layers.
[0008] The unique layered structure of the present invention sequentially attenuates electrons, protons, and secondarily generated radiation. The present invention minimizes mass while maximizing shielding and providing an optimum ratio range of shielding effectiveness per unit mass. The design of the laminated material takes into account the effects of thermal expansion mismatch between the layers, thus minimizing the risks of delamination and warpage. The present invention uses commonly available materials.
[0009] The present invention is lower in mass than aluminum, for example. The present invention does not require special formulations of materials, such as the Physical Sciences design mentioned in the Background section. The present invention uses high atomic number material in the center for maximum efficacy. The present invention preferably uses a symmetrical design to minimize warpage, unlike the Composite Optics design mentioned in the Background section.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawing figures, wherein like reference numerals designate like structural elements, and in which:
Fig. 1 illustrates an exemplary laminated lightweight radiation shielding material or structure in accordance with the principles of the present invention; and
Fig. 2 is a cross-sectional view of an exemplary complex-shaped shielding structure whose layers conform to contours of the complex shape.
DETAILED DESCRIPTION
[0011] Referring to the drawing figures, Fig. 1 illustrates an exemplary laminated lightweight radiation shielding material 10 or structure 10 in accordance with the principles of the present invention. The laminated lightweight radiation shielding material 10 or structure 10 comprises three layers 11, 12, 13. The first layer 11 comprises one or more materials having a low Z/A (atomic number/atomic weight) ratio, such as boron, graphite, aramid, or other polymer-based fiber-reinforced non-metallic matrix composite materials. The materials forming the first layer 11 may be fabricated from commercially available composite materials, with no modification. The first layer may be comprised of sub-layers of differing low Z/A materials. The first layer 11 preferentially attenuates electron and proton radiation, but may generate Bremsstrahlung (secondary) radiation in the process.
[0012] The second layer 12 comprises one or more materials with a high Z2/A (atomic number squared/atomic weight) ratio, such as gold, lead, silver, titanium, tantalum, or tungsten. Thus, the materials forming the second layer 12 may be fabricated from commercially available metal foils, for example. The second layer may be comprised of fiber-reinforced metallic or non-metallic matrix composite where the fibers have been coated with high Z2/A material, such as tantalum or tungsten. The second layer may be comprised of sub-layers of differing high Z2/A materials, although other materials may be between the sub-layers of high Z2/A materials to bond them together. The second layer 12 preferentially attenuates Bremsstrahlung radiation, but may generate photo-electrons in the process. It is preferred that the material forming the second layer 12 have low tensile modulus and low tensile strength, or a thermal expansion that approximates that of the first and third layers 11, 13 to minimize interlaminar stresses.
[0013] The third layer 13 comprises one or more materials having a low Z/A (atomic number/atomic weight) ratio. The third layer 13 may be made of the same material as the first layer. The third layer may be comprised of sub-layers of differing low Z/A materials. The third layer 13 attenuates photo-electrons emitted from the second layer 12, as well as electrons and protons that get through the first and second layers. The thickness and materials properties of the third layer 13 are preferably, but not necessarily, the same as the thickness and materials properties of the first layer 11 to minimize warpage of the laminated lightweight radiation shielding material 10 or structure 10. Thermal expansion of the first and third layers 11, 13 may also be tailored to approximate that of the second layer to minimize warpage of the laminated material.
[0014] The laminated radiation shielding material 10 or structure 10 may be fabricated using a hot press, an oven, or an autoclave. Layers and sub-layers of the laminated radiation shielding material 10 may be co-cured together or secondarily bonded together. Fiber fabrics preimpregnated with uncured non-metallic matrix may be used in the first and third layers 11, 13. The uncured non-metallic matrix may be used as and adhesive to bond to the second layer 12 during a co-cure process. If metal foils are to be used in the second layer 12, the surface of the metal foils must be prepared for enhanced adhesion to the adjacent layers of the laminated material. This preparation may be through abrasion, chemical etching, and/or application of primer. Fiber fabrics preimpregnated with uncured non-metallic matrix may be used in the second layer 12. In which case, it would be advantageous to co-cure the three layers 11, 12, 13 together at the same time.
[0015] End covers for existing enclosures may easily be made from the laminated radiation shielding material 10 or structure 10 that is fabricated in planar form. In addition, the shielding material 10 may be molded into complex shapes using appropriate tooling. In such cases, all three layers 11, 12, 13 of the shielding material 10 conform to the contours of the complex shape, as is illustrated in Fig. 2.
[0016] Thus, laminated lightweight radiation shielding materials or structures have been disclosed. It is to be understood that the described embodiments are merely illustrative of some of the many specific embodiments that represent applications of the principles of the present invention. Clearly, numerous and other arrangements can be readily devised by those skilled in the art without departing from the scope of the invention. Materials may be selected to accomplish the satisfaction of structural requirements, and/or thermal requirements, and/or radiation shielding requirements, or any combination of requirements.
1. A lightweight radiation shielding structure (10), comprising:
a first layer (11) comprised of one or more materials having low atomic number/atomic weight ratio that preferentially attenuates electron and proton radiation, while possibly generating Bremsstrahlung radiation in the process;
a second layer (12) comprised of one or more materials with high atomic number squared/atomic weight ratio that preferentially attenuates Bremsstrahlung radiation, while possibly generating photo-electrons in the process; and
a third layer (13) comprised of one or more materials having low atomic number/atomic weight ratio that attenuates photo-electrons emitted from the second layer, as well as electrons and protons that get through the first and second layers.
2. The structure recited in claim 1 wherein the first (11) and/or third (13) layer comprise
boron fiber-reinforced non-metallic matrix composite material.
3. The structure recited in claim 1 or 2 wherein the first (11) and/or third (13) layer
comprise graphite fiber-reinforced non-metallic matrix composite material.
4. The structure recited in any preceding claim wherein the first (11) and/or third (13)
layer comprise high strength, high modulus, and/or high thermal conductivity graphite
fiber-reinforced non-metallic matrix composite material.
5. The structure recited in any preceding claim wherein the first (11) and/or third (13)
layer comprise aramid fiber-reinforced non-metallic matrix composite material.
6. The structure recited in any preceding claim wherein the first (11) and/or third (13)
layer comprise a polymer-based fiber-reinforced non-metallic matrix composite material.
7. The structure recited in claim 1 wherein the first layer (11) comprises sub-layers
of differing low Z/A ratio materials.
8. The structure recited in any preceding claim wherein the second layer (12) comprises
gold.
9. The structure recited in any of claims 1 to 7 wherein the second layer (12) comprises
lead.
10. The structure recited in any of claims 1 to 7 wherein the second layer (12) comprises
silver.
11. The structure recited in any of claims 1 to 7 wherein the second layer (12) comprises
titanium.
12. The structure recited in any of claims 1 to 7 wherein the second layer (12) comprises
tantalum.
13. The structure recited in any of claims 1 to 7 wherein the second layer (12) comprises
tungsten.
14. The structure recited in any of claims 1 to 7 wherein the second layer (12) comprises
metal foil.
15. The structure recited in any of claims 1 to 7 wherein the second layer (12) has low
tensile modulus and low tensile strength to minimize interlaminar stresses.
16. The structure recited in any claim preceding wherein the second layer (12) comprises
sub-layers of differing high Z2/A ratio materials.
17. The structure recited in any of claims 1 to 7 wherein the second layer (12) comprises
fiber-reinforced metallic or non-metallic matrix composite whose fibers are coated
with high Z2/A material.
18. The structure recited in claim 17 wherein the high Z2/A material comprises tantalum or tungsten.
19. The structure recited in any preceding claim wherein the second layer (12) has a thermal
expansion that approximates that of the first and third layers (11,13) to minimize
interlaminar stresses.
20. The structure recited in any preceding claim wherein thermal expansion characteristics
of the first and third layers (11,13) is tailored to approximate that of the second
layer (12) to minimize interlaminar stresses.
21. The structure recited in any preceding claim wherein the thickness and material properties
of the third layer (13) are substantially the same as the thickness and material properties
of the first layer (11) to minimize warpage.
22. The structure recited in any preceding claim wherein the third layer comprises sub-layers
of low Z/A materials.