Martha Clowdsley

Design of Two RadWorks Storm Shelters for Solar Particle Event Shielding

In order to enable long-duration human exploration beyond low-Earth orbit, the risks associated with exposure of astronaut crews to space radiation must be mitigated with practical and affordable solutions. The space radiation environment beyond the magnetosphere is primarily a combination of two types of radiation: galactic cosmic rays (GCR) and solar particle events (SPE). While mitigating GCR exposure remains an open issue, reducing astronaut exposure to SPEs is achievable through material shielding because they are made up primarily of medium-energy protons. In order to ensure astronaut safety for long durations beyond low-Earth orbit, SPE radiation exposure must be mitigated. However, the increasingly demanding spacecraft propulsive performance for these ambitious missions requires minimal mass and volume radiation shielding solutions which leverage available multi-functional habitat structures and logistics as much as possible. This paper describes the efforts of NASAs RadWorks Advanced Exploration Systems (AES) Project to design two minimal mass SPE radiation shelter concepts leveraging available resources: one based upon reconfiguring habitat interiors to create a centralized protection area and one based upon augmenting individual crew quarters with waterwalls and logistics. Discussion items include the design features of the concepts, a radiation analysis of their implementations, an assessment of the parasitic mass of each concept, and the result of a human in the loop evaluation performed to drive out design and operational issues.

Numerical Uncertainty Quantification for Radiation Analysis Tools

Recently a new emphasis has been placed on engineering applications of space radiation analyses and thus a systematic effort of Verification, Validation and Uncertainty Quantification (VV&UQ) of the tools commonly used for radiation analysis for vehicle design and mission planning has begun. There are two sources of uncertainty in geometric discretization addressed in this paper that need to be quantified in order to understand the total uncertainty in estimating space radiation exposures. One source of uncertainty is in ray tracing, as the number of rays increase the associated uncertainty decreases, but the computational expense increases. Thus, a cost benefit analysis optimizing computational time versus uncertainty is needed and is addressed in this paper. The second source of uncertainty results from the interpolation over the dose vs. depth curves that is needed to determine the radiation exposure. The question, then, is what is the number of thicknesses that is needed to get an accurate result. So convergence testing is performed to quantify the uncertainty associated with interpolating over different shield thickness spatial grids.

Using spectral shape and predictor fluence to evaluate temporal dependence of exposures from solar particle events

Real-time estimation of exposure levels has been considered in NASAs operational strategies and structural capability for the protection of astronauts from exposure to large solar particle events (SPEs). The temporal profile of organ dose rates is also important for the analysis of dose-rate-dependent biological responses and the optimization of radiation shielding and future mission planning. A realistic temporal estimation of exposure profiles relies on: (1) the complete energy spectrum of SPE that defines the boundary condition for radiation transport simulation; (2) the radiation transport simulation with detailed shielding and body geometry models that determines particle transmission at each critical body organ; and (3) the assessment of organ dosimetric quantities and biological risks by applying the corresponding response models. This paper introduces a process of rapidly estimating temporal exposures to SPEs by implementing the distributions of the organ doses and the spectral-shape characterization of the major SPEs. Simultaneously, the unconditional probability exceeding the NASA 30-day limit of a blood-forming organ dose is estimated by taking into account the variability of detailed spectra of SPEs for a given predictor fluence. These temporal evaluations of SPEs can be applied to the development of real-time guidance and protection system on improving mitigation of adverse effects during space missions.

21 st Century Lunar Exploration: Advanced Radiation Exposure Assessment

On January 14, 2004 President George W Bush outlined a new vision for NASA that has humans venturing back to the moon by 2020. With this ambitious goal, new tools and models have been developed to help define and predict the amount of space radiation astronauts will be exposed to during transit and habitation on the moon. A representative scenario is used that includes a trajectory from LEO to a Lunar Base, and simplified CAD models for the transit and habitat structures. For this study galactic cosmic rays, solar proton events, and trapped electron and proton environments are simulated using new dynamic environment models to generate energetic electron, and light and heavy ion fluences. Detailed calculations are presented to assess the human exposure for transit segments and surface stays.

Thick-target yields of secondary ions and neutrons for validation of radiation transport codes

This paper presents the yields of secondary light ions and neutrons produced from bombardments of thick targets of aluminum by protons, helium, and iron ions. In March 2016, ion beams of 400- and 800-MeV protons, 400-AMeV helium, and 400- and 800-AMeV iron bombarded aluminum targets of 20, 40, and 60 g/cm2 thickness at the NASA Space Radiation Laboratory (NSRL) at Brookhaven National Laboratory (BNL) over the course of 100 hours. An additional aluminum target of thickness 60 g/cm2 was placed 3.5 meters downstream to model the increased secondary particle yields in an enclosed space. Surveys of neutrons and light ions were taken with organic liquid scintillators at six angles between 10° and 135° off beam axis, with further light ion measurements taken with pairs of sodium iodide (NaI) detector arrays at two positions between 10° and 30°. Utilizing the organic scintillators, neutron events were isolated with pulse shape discrimination, while charged particles were discerned by comparing energy deposited in the detector and time of flight. Energy spectra were then obtained by using time-of-flight analysis techniques. Light ion spectra were generated using NaI detector array data by comparison of energy deposition between the NaI detectors pairs. These results will be compared to simulations of the experiment calculated by MCNP6. This experiment is a part of a multi-year project to supplement the amount of measured data available for validation and verification of radiation transport computer codes used in quantifying radiation exposure and assessing cancer risk incurred during manned space flight missions.

Comparing Trash Disposal and Reuse Options for Deep Space Gateway and Mars Missions

Taking out the trash at NASA’s newly proposed Deep Space Gateway (DSG) will not be a trivial task. While not the most important aspect of planning this cislunar outpost, there are several options that should be carefully considered since they may affect the crew as well as mission mass and volume. This study extends an earlier one, which focused on waste disposal options for a Mars Transit Vehicle. In that study, gasifying and venting trash along the way was found to noticeably reduce propellant needs and launch mass, whereas keeping processed trash on board in the form of radiation shielding tiles would significantly lower the crew’s radiation dose during a solar particle event. Another favorable strategy was packing trash in a used logistics module for disposal. Since the DSG does not need much propulsion to maintain its orbit and Orion will be present with its own radiation storm shelter at the Gateway, the driving factors of the waste disposal trade study are different than for the Mars mission. Besides reviewing the propulsion and radiation shielding factors, potential drivers such as mass, power, volume, crew time, and human factors (e.g. smell) were studied. Disposal options for DSG include jettison of a used logistics module containing waste after every human stay, jettison of the same logistics module after several missions once it is full, regular disposal of trash via an airlock, or gasifying waste products for easier disposal or reuse. Conversely, a heat melt compactor device could be used to remove water and stabilize trash into tiles which could be more compactly stored on board and used as radiation shielding. Equivalent system mass analysis is used to tally the benefits and costs (mass, volume, power, crew time) of each case on an equivalent mass basis. Other more subjective factors are also discussed. Recommendations are made for DSG and Mars mission waste disposal.

Accelerator-based measurements relevant for shielding design in space

Experimental work on light charged ion production from thick target shielding began this past May at the NASA Space Radiation Laboratory (NSRL) at Brookhaven National Laboratory (BNL). This paper presents the measured secondary light charged ion and neutron yields produced by 0.4- and 2.5-GeV protons and 0.4- and 1.0-AGeV iron ions striking a 30 g/cm2 aluminum target. Neutron and light charged ion (protons, deuterons, and tritons) measurements were taken with liquid scintillators and sodium iodide (NaI) detectors positioned at seven locations between 10 and 135 degrees off the beam axis to best cover the angular distributions of secondary particles, as determined by MCNPX simulations. In the liquid scintillators, neutron-gamma separation was achieved with pulse shape discrimination, and particle species were identified and isolated by analyzing the total charge deposited in the detector versus particle time of flight (TOF). After isolation, the TOF technique was utilized to produce energy spectra for protons, deuterons, tritons, and neutrons at various locations. Additionally, the stopping powers of light charged ions were compared in NaI detector pairs to generate energy spectra for protons, deuterons, and tritons. Preliminary results demonstrated promising agreement with MCNPX Monte Carlo transport code simulations for protons, deuterons, tritons, and neutrons, despite the lack of a full background characterization and optimization of detector settings. Results are expected to improve over the next three years with an increase in beam time, inclusion of specific liquid scintillator detection efficiencies, and an investigation of the physics model parameters in MCNPX. Future experiments will include the use of both forward and back targets composed of high-density polyethylene or aluminum with thicknesses of 20, 40, and 60 g/cm2. Furthermore, proton, helium, carbon, silicon, and iron projectiles will be utilized at energies of 0.4, 0.75, 1.5, and 2.5 AGeV. Ultimately, these measurements will be incorporated in the uncertainty analysis for the engineering codes that NASA uses to develop optimal shielding thicknesses for spacecraft and space habitat design.

Parametric Shielding Strategies for Jupiter Magnetospheric Missions

Judicious shielding strategies incorporated in the initial spacecraft design phase for the purpose of minimizing deleterious effects to onboard systems in intense radiation environments will play a major role in ensuring overall mission success. In this paper, we present parametric shielding analyses for the three Jupiter Icy Moons, Callisto, Ganymede, and Europa, as a function of time in orbit at each moon, orbital inclination, and various thicknesses, for low- and high-Z shielding materials. Trapped electron and proton spectra using the GIRE (Galileo Interim Radiation Electron) environment model were generated and used as source terms to both deterministic and Monte Carlo high energy particle transport codes to compute absorbed dose as a function of thickness for aluminum, polyethylene, and tantalum. Extensive analyses are also presented for graded-Z materials. Radiation absorbed doses presented in this paper can be utilized by the system designer in the electronic parts selection process.

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.

Radiation Environment and Shield Modeling Validation for CEV Design

The International Space Station (ISS) and the earlier station MIR provided the proving ground for future human long-duration space activity. A recent European Space Agency study recommended “…Measurement campaigns on the ISS form the ideal tool for experimental validation of radiation environment models, of transport code algorithms, and reaction cross sections.” Indeed, prior measurements on Shuttle have provided vital information impacting both environment model and transport code development by requiring dynamic models of the Low Earth Orbit (LEO) environment. Recent studies using CAD models of the ISS 7A configuration with TLD (thermo-luminescent detector) area monitors demonstrated that computational dosimetry requires environmental models with accurate non-isotropic as well as dynamic behavior, detailed information on rack loading, and an accurate 6-degree-of-freedom description of the ISS trajectory. The ISS model is now configured for 11A and uses non-isotropic and dynamic geomagnetic transmission and trapped proton models. The ISS 11A is instrumented with both passive and active dosimetric devices. ISS 11A and LEO model validation is an important step in preparation of the design and validation of the Crew Exploration Vehicle (CEV) under the Constellation program.