Richard L. Kiefer

Issues in protection from galactic cosmic rays

Radiation risks to astronauts depend on the microscopic fluctuations of energy absorption events in specific tissues. These fluctuations depend not only on the space environment but also on the modifications of that environment by the shielding provided by structures surrounding the astronauts and the attenuation characteristics of the astronauts body. The effects of attenuation within the shield and body depends on the tissue biological response to these microscopic fluctuations. In the absence of an accepted method for estimating astronaut risk, we examined the attenuation characteristics using conventional linear energy transfer (LET)-dependent quality factors (as one means of representing relative biological effectiveness, RBE) and a track-structure repair model to fit cell transformation (and inactivation) data in the C3H10 T1/2 mouse cell system obtained for various ion beams. Although the usual aluminum spacecraft shield is effective in reducing dose equivalent with increasing shield thickness, cell transformation rates are increased for thin aluminum shields. Clearly, the exact nature of the biological response to LET and track width is critical to evaluation of biological protection factors provided by a shield design. A significant fraction of biological injury results from the LET region above 100 keV/µm. Uncertainty in nuclear cross-sections results in a factor of 2–3 in the transmitted LET spectrum beyond depths of 15 g/cm2, but even greater uncertainty is due to the combined effects of uncertainty in biological response and nuclear parameters. Clearly, these uncertainties must be reduced before the shield design can be finalised.

Modified polymeric materials for durability in the atomic oxygen space environment

Organometallic compounds have been incorporated into organic polymers to improve their durability to the environment of the low earth orbit (LEO), particularly their resistance to erosion by atomic oxygen (AO). Bis(triphenyltin) oxide (BTO) was added to a thermoplastic polyetherimide, Ultem, and exposed on the Mir space station. The addition of the BTO to Ultem significantly reduced the mass loss in LEO. Aluminum acetylacetonate was added to a thermoset, PMDA-ODA polyimide. that is currently deployed on the International Space Station. Two films are placed in the ram direction exposed to AO and space radiation. Three films are placed in the wake direction and are exposed to space radiation but not AO. The doped films show superior resistance to AO.

Shielding against galactic cosmic rays

Ions of galactic origin are modified but not attenuated by the presence of shielding materials. Indeed, the number of particles and the absorbed energy behind most shield materials increases as a function of shield thickness. The modification of the galactic cosmic ray composition upon interaction with shielding is the only effective means of providing astronaut protection. This modification is intimately connected with the shield transport properties and is a strong function of shield composition. The systematic behavior of the shield properties in terms of microscopic energy absorption events will be discussed. The shield effectiveness is examined with respect to conventional protection practice and in terms of a biological endpoint: the efficiency for reduction of the probability of transformation of shielded C3H10T1/2 mouse cells. The relative advantage of developing new shielding technologies is discussed in terms of a shield performance as related to biological effect and the resulting uncertainty in estimating astronaut risk.

The effects of atomic oxygen on polymer films containing bis(triphenyltin) oxide

A tin-containing compound, bis(triphenyltin) oxide (BTO), was added to two polymeric materials in an effort to reduce the erosion of the materials in the presence of atomic oxygen. The BTO was added to a solution of a thermoplastic polyetherimide, and to the amic acid solution of a polyimide thermoset before curing. The films of these materials were tested in the laboratory by exposing them to atomic oxygen from a plasma generator. Subsequently, they were exposed to the Low Earth Orbit (LEO) on two Space Shuttle flights. In all cases, the BTO-containing materials exhibited significantly less mass loss than the corresponding pure polymer. Analysis by X-ray photoelectron spectroscopy showed that after exposure to atomic oxygen, the BTO-containing films had a predominance of tin oxide on their surfaces. This could serve as a barrier to resist further erosion.

The effects of UV radiation on several high-performance polyimide composites

Three high-performance polyimides were exposed at 177°C to ultraviolet radiation at three different intensities and for three different times so that the product of intensity and time was a constant. Intensities of 1, 2 and 3 suns, and times of 500, 250 and 167h were used. One sun is the power in space at one earth-sun distance and is 0.135J s -1 cm -2 . The polyimides were LARC-8515, PETI-5 and K3B. Samples were analysed by XPS, TGA, TMA and DMA. Thermal analyses by TGA, TMA and DMA, which measure bulk properties, showed essentially no difference between samples exposed to heat and UV radiation, and control samples. Surface analysis by XPS showed an apparent decrease in carbonyl on the surface of some exposed samples. This could be correlated for the PETI-5 and LARC-8515 samples to surface contamination by a silicone-containing material.

Boron Containing Polyimides for Aerospace Radiation Shielding

In interplanetary travel and high altitude flight, humans will be exposed to high energy charged particles from solar flares and galactic cosmic rays. These particles lose energy in a material by Coulomb interactions and nuclear collisions. In nuclear collisions, large amounts of energy are transferred and secondary particles are formed from both the projectile and the struck nucleus. A significant portion of these particles are neutrons which can only lose energy by collisions or reactions with a nucleus. Hydrogen-containing materials, such as polymers, are most effective in reducing the neutron energy. When reduced to very low energies, neutrons have a high probability of reacting with a nucleus. Such reactions are dangerous in the human body, and can cause electronic equipment failure. Low energy neutrons react particularly well with a stable isotope of boron, 10 B. To test structural materials which contain both hydrogen to reduce the energy of neutrons and boron to absorb neutrons of reduced energy, samples of two polyimides were made which contained varying amounts of either amorphous boron powder or boron carbide whiskers. The polymers used were a thermoset, PETI-5 from Imitec, and a thermoplastic, K3B from Fiberite. Both materials were made in pure form and with up to 20% by weight of the boron additives. The addition of boron in either form did not change the thermal properties of these materials significantly. However, the compressive yield strength and the tensile strength were both affected by the addition of the boron materials. A neutron absorption test using a PuBe thermal neutron source showed that a 0.5 cm thick sample of K3B containing 15% amorphous boron powder absorbed over 90% of the incident neutrons.

Boron-Containing Fibers for Neutron-Absorbing Fabrics

[Abstract] The feasibility of incorporating neutron absorbing elements into polymer fibers for fabric construction was studied. It was found that enriched 10 B4C can be successfully compounded, extruded and drawn into filled polypropylene fibers. Fibers composed of 20%wt. of boron carbide with the antioxidant additive BHT were woven into a fabric for neutron exposure testing. Tensile data shows that the use of BHT in the processing of polypropylene fibers decreases the degradation and increases the tensile strength. The presence of boron carbide in the polymer slightly decreases both the tensile strength and modulus while increasing the elongation. The dispersion and adhesion of the boron carbide filler particles in the polymer was verified by SEM and optical microscopy. The presence of enriched boron carbide at 20%wt. in 0.8mm (two layers) of fabric decreased the fluence of neutrons by 42%. The neutron shielding ability of a filled fiber woven into a fabric has been verified. A high shielding efficacy can be achieved in thin materials with the use of isotopically enriched elements.

Hot-Press Fabrication of Polymer Fiber/Matrix Composites for Multifunctional Radiation Shielding

Aliphatic polymers, such as polyethylene, are very effective for radiation shielding against high-energy heavy ions. These materials, however, are restricted in their applications for human exploration missions into space because of their limited mechanical and thermal properties. This study aims to investigate processing protocols, both conventional and electron-beam, in order to boost the mechanical and thermal performance of aliphatic materials including matrix resins, fibers, and composites.

Shielding Technology and the Bush Initiative - Shielding for Lunar and Martian Missions

§¶ # ** †† ‡‡ §§ ¶¶ ## *** Past space missions beyond the confines of Earth’s protective magnetic field have been of short duration, and protection from the effects of solar particle events was of primary concern. Aluminum alloy structures provided sufficient protection to meet requirements to prevent early radiation syndrome and were further successfully employed as protection against trapped protons in low Earth orbit (LEO). Aluminum alloys have been the materials of choice for the first 40 years of the space program. The extension of operations beyond LEO to enable routine access to other interesting regions of space will require protection from the hazards of the accumulated exposures of Galactic Cosmic Rays (GCR), and efficient fragmentation of GCR ions with minimal production of secondary particles in shielding materials is essential to protecting the astronauts. Aliphatic polymer composites are the most efficient structural materials exhibiting significantly improved radiation shielding properties, but they are limited in meeting the requirements of the service environment. Aromatic polymer composites developed mainly for high-speed aircraft applications have improved thermal-mechanical properties over aluminum alloys, as well as improved radiation shielding performance. Aliphatic/aromatic hybrid polymers are being developed to give a range of thermal-mechanical properties. The development of functionally graded structures will provide optimum solutions to multifunctional materials requirements in the near-term space program, with far-term utility.

E-Beam Processing of Polymer Matrix Composites for Multifunctional Radiation Shielding

Aliphatic polymers were identified as optimum radiation shielding polymeric materials for building multifunctional structural elements for in-space habitats. Conceptual damage tolerant configurations of polyolefins have been proposed, but many manufacturing issues relied on methods and materials which have sub-optimal radiation shielding characteristics (for example, epoxy matrix and adhesives). In the present approach, we shall investigate e-beam processing technologies for inclusion of high-strength aliphatic polymer reinforcement structures into a highly cross-linked polyolefin matrix. This paper reports the baseline thermo-mechanical properties of low density polyethylene and highly crystallized polyethylene.