Radiation hardiness is a critical issue for materials used in long-duration space flight. Molecular level modeling using parallel software and high-end computing facilities at Ames Research Center allows the developer of space materials to simulate radiation damages to structural, shielding, and electronic materials.
The development and testing of novel materials and structure concepts for future exploration applications require the simulation of space environment effects, including the effect of space radiation. Frequently, radiation hardiness of the materials used plays a large role in determining the useful lifetime of the satellite, robot, or the lunar base. Simulation and modeling of the radiation hardiness of space materials enable the developer to arrive at a better choice of materials, reducing the design cycle time, and improving safety. The development of radiation hardened materials requires a complete simulation of the damage process under a variety of different space radiation environments such as on the lunar and Mars surfaces and during interplanetary travel. The Ames modeling tool, together with the supercomputers dedicated to support NASA’s missions, allow the developer faster design turnaround time and less expensive testing than the experimental facility at Brookhaven. Furthermore, the Brookhaven facility operates using one type of charged particle beam at a time and does not reproduce the mixed charged particles environments in space. The modeling tool, on the other hand, can providing more complete testing by using a more realistic simulation of the mixed radiation environment in space.
The state-of-the-art high-end computing facilities at Ames and parallelized physics and chemistry software enable the full simulation of radiation damage. Such simulation enables the developer to link the dosimetry of space radiation with the nature and severity of the damage. The simulation provides the developer with a less expensive tool and faster turn-around-time in the design cycle of radiation hardened materials. The Ames simulation software of radiation damage is unique in the following aspects:
Beginning-to-End Simulation: Most simulation tools of radiation damage deals with the initial, high energy event. In our model, the simulation begins with the impact of high energy charged particles on the material and tracks the damage by following the energy deposition and the production of secondary species down to the eventual thermalization of the energetic particles.
Dissociative ionization: While ionization by high energy charged particles is always included in radiation damage models, the simultaneous process of ionization and bond breaking (dissociative ionization) is often neglected. However, radicals produced from bond breaking can also damage the material, and in the Ames model they are properly accounted for. Figure 2 presents the probability of producing an H radical, the C6H5+ ion, and an electron when an electron impacts on benzene, the building block of the phenolic material that is a candidate of ablative TPS.
Right: Figure 1. Space radiation environment during a solar storm.
Chemical reactions: Because chemical reactions generally occur toward the end of the damage process when most of the high energy charged particles are nearly thermalized, they are neglected in most radiation damage models. However, chemical reactions can also produce significant damages. A complete simulation of radiation damage must include chemical reactions and this is done in the Ames model. Figure 3 shows the reaction probability of C2H with H2. C2H is a radical produced when charged particles react with polyethylene, a common shielding material.
Below: Figure 2. Production probability of Hand C6H5+ by electron impact on benzene.
The hazards of space radiation pose significant problems in a long duration space flight and they must be included in the design consideration. In particular, the radiation hardiness of the materials used in structure, shielding, and electronics equipments plays a large role in determining the useful lifetime of the satellite, probe, or robot. Simulation and modeling of the radiation hardiness of space materials enable the developer in deriving a better choice of materials, reducing the design cycle time, and improving safety.
Space radiation is more energetic and more complex than terrestrial radiation. It is composed of many different particles. The component of space radiation that causes the greatest damage is the highly-charged, energetic heavy ions present in Galactic Cosmic Rays. These highly charged ions have an energy range 100 MeV - 1 GeV. According to the International Commission on Radiation Units and Measurements (ICRU Report 31, 1979), each charged particle with 100 MeV energy generates more than 5 ´ 106 electrons in its track, with most of electrons are in the 1 - 20 eV range. These secondary electrons will play havoc with the electronic equipments on board. If not properly shielded, on-board computers, sensors, and other electronic equipments will function in an unpredictable manner.
Right: Figure 3. Reaction probability of C2H with H2.
The high energy protons emitted during a solar storm (Figure 1) is another source of lethal space radiation. While they are less energetic than the heavy ions from Galactic Cosmic Rays, their large number density during a storm may also cause severe damage to space materials.
The space radiation environment is difficult to reproduce in the laboratory. The only available test bed is at Brookhaven National Laboratory using the high energy charged particle beams. Modeling and simulation, on the other hand, provide a viable alternative during the design cycle.