Managing Space Radiation Risk in the New Era of Space Exploration (2008)

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D Glossary and Acronyms absorbed dose (D): Average amount of energy imparted by ionizing particles to a unit mass of irradiated material in a volume sufficiently small to disregard variations in the radiation field but sufficiently large to average over statistical fluctuations in energy deposition, and where energy imparted is the difference between energy entering the volume and energy leaving the volume. The same dose has different consequences depending on the type of radiation delivered. Unit: gray (Gy), equivalent to 1 J/kg. ACE: Advanced Composition Explorer acute effects: short-term biological effects of exposure to radiation, including headaches, dizziness, nausea, and illness that can range from mild to fatal ALARA (As Low As Reasonably Achievable): A safety principle, as well as a regulatory requirement, that em- phasizes keeping doses of and exposure to radiation as low as possible using reasonable methods, and not treating dose limits as “tolerance values”; defined at NASA as limiting radiation exposure to a level that will result in an estimated risk below the limit of the 95 percent confidence level. albedo: secondary radiation produced by interactions of galactic cosmic rays and high-energy solar protons with matter in the atmosphere or on the surface alpha particle: An energetic charged nucleus consisting of two protons and two neutrons. This particle is identi- cal to the 4He nucleus. apoptosis: A specific mode of cell death (also known as programmed cell death) that can be triggered by exposure to radiation, especially in cells of lymphoid/myeloid or epithelial lineage. Extensive apoptosis contributes to the hematopoietic and gastrointestinal symptoms seen in acute radiation syndrome. Ares V/Heavy Lift Launch Vehicle: the Constellation system vehicle that will deliver cargo from Earth to low Earth orbit 111 

112 MANAGING SPACE RADIATION RISK IN THE NEW ERA OF SPACE EXPLORATION ascent stage: The pressurized upper stage of the Lunar Lander in which the crew pilots the lander from lunar orbit to the lunar surface and return. The ascent stage takes off from the descent stage, leaving the latter behind on the surface. AU: astronomical unit AX-2: NASA Ames Research Center Experimental Suit 2, designed during the Apollo program as a lunar surface hard suit to bend at the waist and rotate in the torso so that the crew member can reach down to the ground with one hand. Fabricated from fiberglass. AX-5: NASA Ames Research Center Experimental Suit 5, designed during the Space Station Advanced Develop- ment program to provide a durable hard suit for extended operations in zero gravity. Fabricated from numerically milled aluminum forgings. BEIR: Committee on Biological Effects of Ionizing Radiation BEVALAC: An early, high-energy synchrotron accelerator constructed in the 1950s at Lawrence Berkeley National Laboratory and used to discover the antiproton. Closed in 1992. biological end point: effect or response being assessed, e.g., cancer, cataracts bipolar device: a type of semiconductor whose operation is based on both majority and minority carriers BNL: Brookhaven National Laboratory BRYNTRN: a computer code for simulating baryon transport carbon composite: A structural material that can substitute for aluminum and other metals in the construc- tion of many parts of a spacecraft, notably the pressure vessel shell. Composite may incorporate boron, epoxy, p ­ olyethylene, hydrogen, or other materials that enhance radiation shielding properties. cargo habitat: a crew habitat that the Lunar Lander carries for delivery to the Lunar Outpost as a key part of the “Outpost-first” strategy CHMO: chief health and medical officer chronic effects: long-term effects of exposure to radiation; includes cancer, cataracts, and nervous system damage CI: confidence interval CME: coronal mass ejection, an explosion of plasma released from the atmosphere (or corona) of the Sun CNS: central nervous system computerized anatomical male/female: a model of human geometry used to evaluate radiation doses at various points inside the body Constellation system: the complete ensemble of launch vehicles, flight vehicles, ground support, support services, and lunar and planetary surface systems associated with the Vision for Space Exploration 

APPENDIX D 113 CRaTER: Cosmic Ray Telescope for the Effects of Radiation CREAM96: Cosmic Ray Effects on Micro-Electronics (1996 revision), a computer code cross section (σ): Measure of the probability per unit particle fluence of observing a given end point. Unit: cm 2.. descent stage: The lower stage of the Lunar Lander that includes the descent and landing engines and propellant tanks to serve them. The crew ascending back to lunar orbit in the ascent stage leaves the descent stage behind on the lunar surface. descent stage habitat: in the descent stage, a pressurized crew habitat in which the crew would live during sortie missions deterministic process: process whereby a given event will occur whenever its dose threshold is exceeded DNA: deoxyribonucleic acid DOD: Department of Defense DOE: Department of Energy dose: The average energy deposited by radiation per unit mass of material. Total dose refers to a combination of both ionizing and non-ionizing dose. See NIEL. dose equivalent (H): Estimate of radiation risk that accounts for differences in the biological effectiveness of different types of charged particles that produce the absorbed dose. H = Q × D, where Q is a quality factor based on the type of radiation (Q = 1 for x-rays). NASA uses Q as specified in ICRP Publication 60 (ICRP, 1991). Unit: sievert (Sv), equivalent to 1 J/kg. EDS: Earth departure stage effective dose (E): Estimate of radiation risk given in ICRP Publication 60 (ICRP, 1991). It sums the individual effects of all types of radiation present over all of the individual types of tissue in the body. Unit: cSv. electron volt (eV): a unit of energy equivalent to 1.602 × 10–19 joules EMU: Extravehicular mobility unit, the space suit developed for space shuttle crews that also serves on the ISS. The EMU features a hard upper torso and soft lower torso, arms, and legs over the pressure bladder. The entire EMU except the helmets and boots is covered by the thermal micrometeoroid garment. ESP: energetic storm particle EVA: extravehicular activity excess risk: the probability of a certain effect on an individual who has been exposed to a given dose of radiation compared with the baseline probability of that effect extravehicular activity (EVA): activity that occurs when a crew member moves from a spacecraft or habitat to the vacuum of space in a space suit 

114 MANAGING SPACE RADIATION RISK IN THE NEW ERA OF SPACE EXPLORATION fluence, or particle fluence (F): Number of particles incident on a small sphere centered at a given point in space, divided by the cross-sectional area of that sphere. Mathematically, it is given as dN/da, where N is the number of particles and a is the cross-sectional area. Unit: m2. fluence rate (dF/dt): Change in fluence over a given small time interval, or the time derivative of the fluence. Unit: m2/s. flux (Φ): Term used historically by the nuclear community for fluence rate and also used for particle flux density, but deprecated by the ICRU convention to eliminate confusion between the terms “particle flux density” and “radiant flux.” See fluence rate. FSP: fission surface power galactic cosmic rays: Essentially isotropic distribution of highly energetic particles from within and beyond our Galaxy. These rays are made primarily of hydrogen and helium but contain traces of all the elements. GCR: galactic cosmic radiation GOES: Geostationary Operational Environmental Satellite HDPE: high-density polyethylene, defined as having a density greater than 0.94 g/cm 3 HEPAD: High Energy Proton and Alpha Detector HZE: high atomic number and energy HZETRN: a transport code developed specifically for high-charge, high-energy particles that is widely used for space radiation shielding and design calculations ISS: International Space Station kerma: Kinetic energy released in materials, the sum of the initial kinetic energies for all charged particles released by uncharged ionizing radiation in a small sample of material divided by the mass of the sample. Kerma is the same as dose when charged particle equilibrium exists. latchup: a condition in a semiconductor in which the device is transformed into an anomalous state that no longer responds to input signals LCVG: liquid cooling and ventilation garment LEND: Low Energy Neutron Detector LEO (low Earth orbit): the environment in which most recent space missions have been concentrated, where the magnetic field of Earth provides protection against much of the radiation that would be encountered on more distant exploration missions LET (linear energy transfer): Measure of the average local energy deposition per unit length of distance traveled in the material. Unit: keV/μm. LIS: local interstellar GCR spectrum 

APPENDIX D 115 LLO: low lunar orbit LRV: lunar rover vehicle Lunar Lander: the Constellation system vehicle that will travel between the Orion and the surface of the Moon MARIE: Mars Radiation Environment Experiment NASA: National Aeronautics and Space Administration NCRP: National Council on Radiation Protection and Measurements NIEL: Non-ionizing energy loss, also called displacement kerma. The total kerma can be divided into an ionizing component and a displacement, or NIEL, component. NIH: National Institutes of Health NM: neutron monitor NOAA: National Oceanic and Atmospheric Administration Nowcasting: prediction of total doses and the future temporal evolution of the dose once a solar particle event has begun NRC: National Research Council NSF: National Science Foundation NSRL: NASA Space Radiation Laboratory Orion Crew Exploration Vehicle: The Constellation system vehicle that will carry passengers in low Earth orbit, or from low Earth orbit to the Moon or Mars, and then back to Earth. Often referred to as CEV; in this report referred to as the Orion crew module. PDF: probability density function PEL (permissible exposure limit): Maximum amount of radiation to which an astronaut may be exposed. For terrestrial workers, PELs are legal limits, defined by OSHA. NASA PELs are set by the chief health and medical officer. PLR: pressurized lunar rover PLSS: personal life support system PPS: Proton Prediction System Q: quality factor RAD: Radiation Assessment Detector 

116 MANAGING SPACE RADIATION RISK IN THE NEW ERA OF SPACE EXPLORATION RBE (relative biological effectiveness): Measure of the effectiveness of a specific type of radiation or particle in producing a specific biological outcome relative to the outcome with the same dose of gamma rays. RBE = Dγ /Drad of interest. regolith: Abundant loose material found on the surface of a moon or planet. Colloquially known as dirt. REID (risk of exposure induced death): Measure of risk used by NASA as a standard for radiation protection; reflects a calculation of the probability of death due to exposure to radiation in space. RFI: request for information RTG: radioisotope thermoelectric generator SAMPEX: Solar Anomalous and Magnetospheric Particle Explorer SEC: Space Environment Center secondary radiation: radiation that has been generated by the passage of a primary particle through a material SEE (single-event effect): a class of effects in which damage results from a single ionizing particle traversing a microelectronic device, rather than the accumulated impact of a large number of particles sievert (Sv): The SI unit of effective dose. It is the product of the absorbed dose and a radiation weighting factor. SOHO: Solar and Heliospheric Observatory Solar cycle: The roughly 11-year cycle of solar activity, as reflected, for example, by variation in the number of sunspots (see Figure 2-5). The extrema of this cycle are known as “solar maximum” and “solar minimum.” At solar maximum, the numbers of sunspots, flares, CMEs, and SPEs are high; at solar minimum, they are low. The intensity of galactic cosmic rays near Earth varies inversely with the solar-activity cycle, being highest at solar minimum and lowest at solar maximum (see Figure 2-4). solar flare: a burst of energy released from the atmosphere (or corona) of the Sun Space Radiation Analysis Group (SRAG): the radiation protection body of NASA responsible for radiation monitoring, projecting exposures, and ensuring adherence to principles of ALARA spallation: A high-energy nuclear reaction in which a high-atomic-number target nucleus is struck by a high- e ­ nergy, light particle (typically a proton); this causes the target nucleus to break up into many components, releasing many neutrons, protons, and higher Z particles. SPE (solar particle event): Large fluxes of energetic particles produced by the Sun that can last from a few hours to a few days. Signatures of solar energetic-particle events may include significant increases in types of electro- magnetic radiation such as radio waves, x-rays, and gamma rays. SRAG: Space Radiation Analysis Group STEREO: Solar-Terrestrial Relations Observatory 

APPENDIX D 117 stochastic process: process whereby the likelihood of the occurrence of a given event can be described by a probability distribution TEPC: tissue equivalent proportional counter TMG: thermal micrometeoroid garment TMI: Three Mile Island trapped radiation: Ionized particles held in place by Earth’s magnetic fields. Also known as the Van Allen belt. U.S. NRC: U.S. Nuclear Regulatory Commission USAF: United States Air Force Wind: a NASA spacecraft that observes the Sun and solar wind Z: atomic number, the number of protons in the nucleus of an atom