Browsing by Author "McGirl, Natalie"
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Item Crew Radiation Exposure Estimates from GCR and SPE Environments During a Hypothetical Mars Mission(46th International Conference on Environmental Systems, 2016-07-10) McGirl, Natalie; Pawel, A.J.; Schappel, Daniel; Shamblin, Jacob; Younkin, Timothy; Townsend, LawrenceSome recent analyses of potential radiation exposures to crews on the surface of Mars focused solely on exposures from extremely large solar particle events (SPE). These analyses estimated radiation doses to critical organs, as well as effective doses, and compared them to crew permissible exposure limits. Contributions to crew exposures from galactic cosmic rays (GCR) were not addressed. In this work we present results for crew exposure estimates from GCR particles, as well as SPE protons. We assume a 200-day transit between Earth and Mars, a 500-day stay on the surface of Mars, and a 200-day transit from Mars back to Earth. For the transit phases of the mission we assume that the crew is protected by a spacecraft having 20 g cm-2 aluminum or 40 g cm-2 aluminum shielding. For the stay on the surface of Mars, we assume a habitat shielded by 40 g cm-2 aluminum within the Martian atmosphere at altitudes of 0 km and -7 km – the former corresponding to the mean surface elevation, and the latter to the depth of the Hellas Impact Basin. The mission is assumed to begin 450 days prior to solar maximum and to end 450 days after solar maximum. The maximum for solar cycle 23 is used and assumed to have occurred on May 15, 2000. Radiation exposure estimates are obtained utilizing the Online Tool for the Assessment of Radiation in Space (OLTARIS) computational tool developed at NASA Langley Research Center. For the SPE contributions, the maximum SPE proton fluences that can be tolerated without exceeding career radiation limits are estimated, after accounting for the contribution to the total mission effective dose from the chronic GCR background exposures.Item Radiation Exposure Estimates for Deep Space Missions Revisited(47th International Conference on Environmental Systems, 2017-07-16) Townsend, Lawrence; de Wet, Wouter; Zaman, Fahad; McGirl, Natalie; Heilbronn, Lawrence; Moussa, HannaVarious studies of potential exposures to the space radiation environment for astronauts on missions beyond low-Earth orbit (LEO) have been carried out and the results published in conferences and journals. These studies involved estimating radiation exposures from galactic cosmic rays and/or solar energetic particle events for missions to the moon, Mars and beyond. Estimating potential risks usually involved comparing the calculated doses and effective doses to NASA limits published in NASA Standard 3001. These radiation limits consist of: (1) short-term limits imposed to prevent early effects such as nausea, vomiting and lethality; (2) non-cancer lifetime limits are imposed to prevent late term degenerative effects in the lens of the eye, central nervous system, cardiovascular system; and (3) career effective dose limits to prevent a 3% excess risk of exposure induced death (REID) from cancer at the 95% confidence limit. Specific effective dose values for the career effective dose limits depend on the particulars of the specific mission, including the gender of the crew member, the expected environments, and the length of the mission. Many previous studies compared the estimates of effective doses for mission crew members to a table in NASA Standard 3001 that listed values for the 3% excess cancer risk. These values did not include the 95% confidence limits, which would have reduced those effective doses by an approximate factor of 3. Hence, some studies suggested that the estimated effective doses would be below the career limits, when in fact they would exceed those limits. In this work, we revisit some of these deep space mission analyses and compare the estimated doses and effective doses to the correct limits which include the 95% confidence limits.Item Secondary Neutron Yields Produced by Thick-Target Aluminum Interactions(46th International Conference on Environmental Systems, 2016-07-10) McGirl, Natalie; Castellanos, Luis; Srikrishna, Ashwin; Heilbronn, LawrenceIn May 2015, neutron production from thick-target shielding experiments were conducted at the NASA Space Radiation Laboratory (NSRL) at Brookhaven National Laboratory (BNL). Secondary neutron yields produced by 0.4- and 1.0-AGeV iron ions impinging upon a 30 g/cm^2 aluminum target were measured with liquid scintillators at 10, 30, 45, 60, 80, and 135 degrees off the beam axis. Neutral and charged particle events in the liquid scintillators were first separated using thin plastic “veto” detectors. Neutron and gamma ray signal contributions in the neutral events were then separated with pulse shape discrimination. Finally, neutron energy spectra were produced using the time-of-flight technique. Experimental results will be compared with transport model calculations using codes such as MCNP and PHITS. Future experiments in 2016 and 2017 will include the use of a shadowbar system, front and back thick targets, and a variety of projectile species and energies. The shadowbar system will consist of two iron bars, one 1-meter long and the other 2-meters long, for use in a full background characterization, while the front and back targets are composed of high-density polyethylene or aluminum with thicknesses between 20 and 60 g/cm^2. Finally, proton, helium, carbon, silicon, and iron projectiles will be utilized at beam energies of 0.4, 0.75, 1.5, and 2.5 AGeV. This systematic study on secondary neutron production from a variety of thick targets by several high-energy ion species will be incorporated into the uncertainty analysis for NASA-developed transport codes. Ultimately, these measurements will help determine optimal shielding thicknesses for future space applications.