Estimates of SPE Radiation Exposures on Mars for Female Astronauts in Hemispherical Habitats

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1 Estimates of SPE Radiation Exposures on Mars for Female Astronauts in Hemispherical Habitats Lawrence W. Townsend 1, Mahmoud PourArsalan 2 and Michael I. Hall 3 University of Tennessee, Knoxville, Tennessee, , USA Radiation exposure estimates for female crew members within simple hemispherical habitats constructed of aluminum on the surface of Mars are made for representative radiation environments consisting of solar particle event protons. Females, because of their generally smaller physical stature, have less body self-shielding and are expected to receive larger doses than males for the same incident radiation environments. In this work we use the BRYNTRN radiation transport code, originally developed at NASA Langley Research Center, and the Computerized Anatomical Female human geometry model to estimate exposures for aluminum shield areal densities corresponding to those provided by a spacesuit, a surface lander, and a permanent habitat. Comparisons of the predicted organ exposures with current NASA Permissible Exposure Limits are presented and discussed. Nomenclature BFO = blood forming organ BRYNTRN = BaRYoN TRaNsport CAF = computerized anatomical female CAM = computerized anatomical man cgy = centigray csv = centisievert D = absorbed dose E = effective dose GCR = galactic cosmic ray H = dose equivalent H T = organ dose equivalent HZETRN = High Z and Energy TRaNsport ICRP = International Commission on Radiological Protection J/kg = joules/kilogram NCRP = National Council on Radiation Protection and Measurements PEL = permissible exposure limit Q = quality factor RBE = relative biological effectiveness SPE = solar particle event T = tissue = tissue weighting factor w T I. Introduction T some point in the future, human space exploration may include crews visiting the surface of Mars for A extended periods of time. A major concern in thinking about such missions is exposure of crews to harmful space radiation environments consisting of the ever present galactic cosmic ray (GCR) background, and sporadic, large solar energetic particle events (SPEs). An earlier study suggests that the highest radiation exposures are likely 1 Condra Professor of Nuclear Engineering, Department of Nuclear Engineering. 2 Graduate Research Assistant, Department of Nuclear Engineering. 3 Graduate Research Assistant, Department of Nuclear Engineering. 1

2 to occur during the interplanetary transits from Earth to Mars. 1 However, significant exposures could also be received during lengthy stays on the Martian surface. The exposure rates on the surface, however, should be lower because of the shielding provided by the overlying atmosphere of Mars. 2 In recent work, we focused on estimating the radiation exposures from the current galactic cosmic ray (GCR) environment, which is one of the most intense of the space era. 3 That study suggested that GCR exposures on the surface of Mars would not exceed current NASA Permissible Exposure Limits (PELs) 4 for radiation. Other prior work, carried out nearly two decades ago, 2,5 suggested that the blood forming organ (BFO) dose from a combined solar minimum galactic cosmic ray (GCR) environment and a large solar particle event (SPE), for an astronaut at the mean elevation of Mars, would not exceed the 30-day radiation exposure limit (25 rem or 25 csv in modern units) recommended at that time by the National Council on Radiation Protection and Measurements (NCRP) 6 for the BFO. Those calculations used an older GCR environmental model, 7 earlier versions of the HZETRN and BRYNTRN space radiation transport codes, 8,9 and substituted a simple 5-cm water depth approximation for the actual bone marrow or BFO body self-shielding distribution, rather than using a realistic human geometry model, such as the computerized anatomical man (CAM) 10 or female (CAF) 11 models. However, those studies did include the variation in Mars atmospheric shield thickness (particle path length through the atmosphere) as the arriving particle trajectories changed relative to the local zenith. In addition, the 2π shadow shielding provided by the planet s mass was also accounted for. Radiation exposure estimates were made for both Mars CO 2 atmosphere density models, the warm low density model (16 g cm -2 ) and the cold high density model (22 g cm -2 ) areal densities. 12 In this work we revisit the issue of radiation exposures on the surface of Mars, specifically for possible large solar energetic particle events, comparable to ones previously observed in the modern space era (since the late 1950 s). The transport of the incident SPE proton radiation spectra through the CO 2 atmosphere is carried out using the BRYNTRN space radiation transport code developed at NASA Langley Research Center. 10 Also, a realistic body self shielding distribution is used to estimate organ doses and dose equivalents, which are in turn used to calculate the organ doses and effective dose, which are the relevant radiation protection quantities for comparing to the present limits, as specified in the NASA Permissible Exposure Limits (PELs). 4 These newer calculations also employ the quality factors specified in Publication 60 of the International Commission on Radiological Protection (ICRP), 13 which replaced the previously-used values from ICRP Publication The outline of the paper is as follows. In section II, the scenarios to be analyzed are described. Section III presents details of the methods used to obtain the radiation exposure estimates for these scenarios for comparison with the appropriate radiation limits. Section IV presents the calculated results and discusses them. Finally, section V summarizes the work and provides concluding remarks. II. Description of Mars Surface Scenarios Assume that a female crewmember is located at ground 300 level at the mean elevation of the 250 surface of Mars in the center of a hemispherical structure composed of 200 aluminum. This location inside the 150 hemisphere was selected because it is the location within the structure 100 where the radiation exposures are the highest. The female human geometry was selected for the study, 50 0 rather than a male geometry, which is more commonly used, because females generally have smaller Angle from Zenith, degrees physical stature resulting in less body self-shielding and are therefore more likely to receive larger doses 16 g/cm2 22 g/cm2 than males, for the same incident environments and external shielding. Figure 1. Mars atmosphere path length versus arrival angle from the zenith for both warm low-density (16 g cm -2 ) and cold highdensity (22 g cm -2 ) atmosphere models 2 Pathlength, g/sq. cm.

3 In addition, radiation limits for females are slightly lower than for males of the same age, due to the increased radiosensitivities of some organs. 4 As was done for the recent study of GCR exposures, 3 three different areal densities for the aluminum hemisphere are investigated: (1) 0.3 g cm -2, which is comparable to the protection provided by a spacesuit; (2) 5 g cm -2, which is comparable to protection provided by a surface lander; and (3) 40 g cm -2, which is comparable to that provided by a permanent habitat. The Mars atmosphere composition is assumed to be pure CO 2. Both low-density (16 g cm -2 ) and highdensity (22 g cm -2 ) models 12 were used. These areal densities are seen by those particles traveling vertically through the Mars atmosphere starting from the mean surface elevation, and represent the thinnest shielding provided by the overlying atmosphere. Since the incoming SPE radiation is isotropic, the atmosphere shielding thicknesses (path lengths) are greater for particles arriving at angles greater than zero, with respect to the local zenith. The results presented herein account for this by averaging the dose for all arrival angles from 0 to 87 degrees. At an arrival angle of 87 Proton Fluence, per sq.cm. 1.E+12 1.E+11 1.E+10 1.E+09 1.E+08 1.E+07 1.E+06 1.E Energy, MeV Oct-89 Sep-89 Aug-72 Figure 2. Proton fluence spectra for the August 1972, September 1989 and October 1989 events obtained from the Weibull parameterizations of Xaps os et al [16]. degrees from the zenith (3 degrees above the local horizon) the atmosphere areal densities are ~295 g cm -2 for the high-density model and ~230 g cm -2 for the low-density model. As the arrival angle approaches the horizon, the areal densities increase dramatically, exceeding 460 g cm -2 for both models. At these depths, contributions to the organ exposures are trivial and can be ignored. Figure 1 displays the atmosphere path lengths in units of g cm -2 as a function of arrival angle measured from the local zenith. Note that the path length increases slowly with increasing angle out to ~ 45 degrees, and then increases much more rapidly as the angle from the zenith increases beyond 80 degrees. These path lengths include the effects of the curvature of the surface of Mars. The incident radiation environments are Weibull parameterizations of the measured SPE proton spectra observed in the August 1972, September 1989 and October 1989 events. 16 These spectra are depicted in Fig. 2. III. Computational Methods The incident SPE proton spectrum is transported through the Mars pure CO 2 atmosphere (up to 300 g cm -2 ), then through the appropriate hemispherical aluminum shielding thickness, and finally through the body self-shielding for the CAF organ of interest, using the BRYNTRN space radiation transport code developed at NASA Langley Research Center. The BRYNTRN code transports all incident protons and their nuclear reaction secondary products (protons, neutrons, deuterons, tritons, helions and alphas). The transport code also includes stopping powers to account for energy loss due to excitation and ionization by the charged particles traversing the medium resulting from collisions with the orbital electrons of the atoms and molecules in the target medium. The CAF model body self-shielding distributions in combination with the BRYNTRN results are then used to calculate the dose and dose equivalent estimates to critical organs. The CAF body organ self-shielding distributions are obtained using 968 rays uniformly covering the 4π solid angle surrounding a particular site in an organ. Doses and organ dose equivalents are calculated for the bladder, bone marrow, breast, colon, esophagus, eye lens, heart, kidney, liver, lung, ovary, pancreas, skin, stomach, and thyroid. For organs such as the skin and bone marrow, which are distributed throughout the body, doses and dose equivalents are obtained by averaging over more than 33 anatomical locations for each organ. For large organs, such as the bladder, breast, kidney, etc, averages over at least 10 sites in the organ are made. For localized organs, such as the eye lens, only one site is used. Organ doses (D) are in units of centigray (cgy), 3

4 where 1cGy = 0.01 Gy = 1 rad and 1 Gy = 1 J/kg. Organ dose equivalents (H) are in centisievert (csv) where 1cSv= 0.01 Sv = 1 rem and 1Sv = 1 J/kg. The effective dose (E) units are also csv. The organ doses are calculated by folding the dose as a function of depth in water (tissue surrogate) obtained from the HZETRN transport code with the body self-shielding distribution for the organ of interest. In a similar fashion, the organ dose equivalents, which represent the product of dose with a quality factor, Q (H = QD), are also calculated by folding the body self-shielding distribution for the organ of interest with the dose equivalent as a function of depth in water obtained from BRYNTRN. Once the organ dose equivalents are calculated for each organ, the effective dose is calculated using E = w H (1) T T T where H T is the organ dose equivalent for the specific organ/tissue denoted by T and the tissue weighting factors w T are the proportionate detriment of the organ when the whole body is irradiated. 17 The values for w T used in this work are listed in Table 5.1 of Ref. 17. IV. Results A. Effective Dose The results of the effective dose calculations for female crew members, as a function of path length, in units of areal density, through the warm, low-density, CO 2 atmosphere, are displayed in Figures 3 through 5 for each of the three events and different hemispherical habitats at the mean elevation on Mars. The total effective dose values are presented in Table 1. The values for the warm, low-density atmosphere model are obtained by averaging the effective doses presented in Figures 3 through 5 over all arrival angles from the zenith (0 degrees) to 87 degrees (near the local horizon) in increments of 1 degree. The values for the cold, high-density atmosphere are obtained using the same averaging procedure with the effective dose versus path length results for that atmosphere. The results presented in Table 1 include the effects of the 2π solid angle shadow shielding provided by the planet s bulk. In recent work, 3 the annual effective doses for female crewmembers at the mean elevation on Mars, for the most recent solar minimum period (2009), were estimated to be ~24-26 csv, varying little between the two atmosphere models or with Effective Dose, csv Mars Pathlength, g per cm sq. 0.3 g/cm^2 Al 5 g/cm^2 Al 40 g/cm^2 Al Figure 3. Effective dose in units of csv as a function of Mars atmosphere areal density (path length) for different hemispherical aluminum habitat thicknesses for the August 1972 event. Effective Dose, csv Mars Pathlength, g per cm sq. 0.3 g/cm^2 Al 5 g/cm^2 Al 40 g/cm^2 Al Figure 4. Effective dose in units of csv as a function of Mars atmosphere areal density (path length) for different hemispherical aluminum habitat thicknesses for the September 1989 event. 4

5 aluminum shielding areal density. Although it is highly unlikely that solar particle events of the magnitude assumed in this work would occur during solar minimum, combining the GCR estimate of 26 csv with the results from this work should provide a reasonable upper bound on effective dose for a one year mission on the Martian surface. For a NASA astronaut, the career radiation exposure is limited to not exceed 3 percent risk of exposure induced death (REID) from a fatal cancer. NASA uses a statistical assessment of uncertainties to ensure that this risk limit is not exceeded at a 95 percent confidence level. Table Effective Dose, csv Mars Pathlength, g per cm sq. 0.3 g/cm^2 Al 5 g/cm^2 Al 40 g/cm^2 Al Figure 5. Effective dose in units of csv as a function of Mars atmosphere areal density (path length) for different hemispherical aluminum habitat thicknesses for the October 1989 event. Table 1. Calculated Effective Doses (in csv) for Female Crew Members on the Surface of Mars for the Three Solar Particle Events and Assumed Aluminum Shield Areal Densities. Solar Particle Event 2 lists the career permissible exposure limits for effective dose (in units of csv) for a female crewmember as a function of age at first exposure. Adding the 26 csv maximum contribution from GCR, obtained in Ref. 3 to the entries in Table 1, and comparing these results to the PELs listed in Table 2, we note that the career effective dose limit is not exceeded for any of the three SPEs, shielded by the atmosphere of Mars and as little as 0.3 g cm -2 of aluminum. The effective dose estimate for the October 1989 SPE, shielded by the warm, lowdensity atmosphere and 0.3 g cm -2 of aluminum is closest to the 25 y old female limit (31 csv exposure versus the 37 csv) but the limit is not close to being exceeded. Note also from Table 1 that a habitat with an aluminum shield equivalent of 40 g cm -2 will reduce the effective doses from any of these solar events to a relatively insignificant level. Finally, we note that the effective dose estimates for the October 1989 event are at ~ 3 times larger than the values from either of the other two events. B. Organ Doses Table 3 lists the permissible exposure limits for shortterm or non-cancer effects. Since we are concerned with preventing any short term effects, comparisons of organ Effective Dose (csv) 0.3 g cm -2 Aluminum 5 g cm -2 Aluminum 40 g cm -2 Aluminum Low-Density High-Density Low-Density High-Density Low-Density August September October High-Density Table 2. NASA Career Permissible Exposure Limits (PELs) for Female Astronauts for a One Year Mission. 4 Age (years) Effective Dose (csv)

6 doses will be made with the 30 days limits. The limits are expressed in units of centigray-equivalent. These are obtained from the absorbed dose (D) using D ( cgy Eq) = D ( cgy) RBE (2) where the absorbed dose is the mean energy imparted (absorbed) per unit mass (1 Gy = 1 J/kg), and the RBE (Relative Biological Effectiveness) is a multiplicative factor that is applied to account for the ability of some types of radiations to produce more biological damage than others for the same dose. For SPE protons an RBE = 1.5 is used, as recommended by the NCRP. 18 Table 3. Permissible exposure limits for short-term or career non-cancer effects. 4 Organ 30 day limit (cgy-eq) 1 Year Limit (cgy-eq) Career (cgy-eq) Lens* Skin BFO NA Heart** CNS*** CNS*** (Z 10) *Lens limits are intended to prevent early (< 5 yr) severe cataracts (e.g., from a solar particle event). **Heart doses calculated as average over heart muscle and adjacent arteries. ***CNS (central nervous system) limits should be calculated at the hippocampus. Table 4. Calculated Organ Doses for Female Astronauts in cgy-equivalent at the Mean Altitude on Mars for the October 1989, September 1989 and August 1972 Solar Particle Events. Skin Eye Bone Marrow (BFO) Density Model Central Nervous System (CNS) Event Low High Low High Low High Low High Low High 0.3 g cm -2 Aluminum Shield 10/ / < 1 8/ < 1 < 1 < 1 < 1 < 1 5 g cm -2 Aluminum Shield 10/ / < 1 8/ < 1 < 1 < 1 < 1 < 1 < 1 40 g cm -2 Aluminum Shield 10/89 < 1 < < 1 < 1 < 1 < 1 < 1 < 1 < 1 9/89 < 1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 8/72 < 1 < 1 <1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 Heart 6

7 Table 4 displays the organ doses for the skin, eye lens, central nervous system (brain), heart, and blood forming organs (BFO), which we represent by the bone marrow distribution. The dose values in the table are in units of cgy-equivalent, obtained by multiplying the calculated organ doses by the multiplicative proton RBE factor of 1.5, as described above. Comparing the results presented in Table 4 with the limits listed in Table 3, we note that all of the calculated organ doses are well below their respective limits. Except for the bone marrow dose from the October 1989 event, which is less than 25% of the limit, all other doses are ~ 10% or less of the applicable limit for all three events and all three aluminum shield areal densities. Note also, that a habitat of 40 g cm -2 reduces the exposures from all three events to a trivial level of 1cGy-Eq or less. V. Conclusion Estimates of radiation exposures, from possible large solar energetic particle events comparable to ones previously observed in the modern space era (since the late 1950 s), for female astronauts on the surface of Mars have been presented and compared with NASA permissible exposure limits. The calculations use a Weibull parameterization of the incident proton spectrum to provide the input into the space radiation transport code BRYNTRN, developed at NASA Langley research Center. The incident SPE spectrum is transported through the CO 2 atmosphere of Mars, both for high-density and low-density models, and then through various hemispherical configurations of aluminum shielding representative of a space suit, surface lander, and a permanent habitat. Organ doses and effective doses for a female astronaut, located at the highest dose point within the hemisphere, are calculated for all six combinations of atmosphere models and hemispherical shielding configurations, using the CAF human geometry model to represent body self-shielding distributions for the various organs. The resulting organ doses and effective doses are found to be well below NASA permissible exposure limits. References 1 Townsend, L. W., Cucinotta, F. A. and Wilson, J. W., Interplanetary Crew Exposure Estimates for Galactic Cosmic Rays, Radiation Research, Vol. 129, No. 1, 1992, pp Simonsen, L. C., Nealy, J. E., Townsend, L. W., and Wilson, J. W., Space Radiation Dose Estimates on the Surface of Mars,, Journal of Spacecraft and Rockets, Vol. 27, No. 4, 1990, pp Townsend, L. W., PourArsalan, M., and Hall, M. I., Estimates of Radiation Exposures on Mars for Female Crews in Hemispherical Habitats IEEE Aerospace Conference [CD-ROM], IEEE, New York, NASA Space Flight Human System Standard Volume 1: Crew Health, NASA-STD-3001, vol. 1, Simonsen, L. C., Nealy, J. E., Townsend, L. W., and Wilson, J. W., Martian Regolith as Space Radiation Shielding, Journal of Spacecraft and Rockets, Vol. 28, No. 11, 1991, pp National Council on Radiation Protection and Measurements, Guidance on Radiation Received in Space Activities, Report No. 98, Bethesda, MD, July Adams, J. H., Silberberg, R. and Tsao, C. H., Cosmic Ray Effects on Microelectronics, Part I: The Near-Earth Particle Environment, NRL Memo Report 4506-Part I, Wilson, J. W., Townsend, L. W., and Badavi, F. F., Galactic HZE Propagation Through Earth s, Radiation Research, Vol. 109, No. 2, 1987, pp Wilson, J. W., Townsend, L. W., Chun, S. Y., Lamkin, S. L., Ganapol, B. D., Hong, B. A., Buck, W. W., Khan, F., Cucinotta, F., Nealy, J. E., BRYNTRN: A Baryon Transport Model, NASA TP 2887, Billings, M. P. and Yucker, W. R., The Computerized Anatomical Man (CAM) Model, NASA CR , Atwell, W. A., Zapp, N., and Badavi, F. F., Space Radiation Exposure Estimates to Female Astronauts Using the Computerized Anatomical Female Model, 2000 International Conference on Environmental Systems (ICES), SAE, Warendale, PA, Smith, R. E. and West, G. S., Space and Planetary Environment Criteria Guidelines for Use in Space Vehicle Development, 1982 Revision, Vol. 1, NASA TM-82478, Wilson, J. W., Badavi, F. F., Cucinotta, F. A., Shinn, J. L., Badhwar, G. D., Silberberg, R., Tsao, C. H., Townsend, L. W., and Tripathi, R. K., HZETRN: Description of a Free-Space Ion and Nucleon Transport and Shielding Computer Program. NASA TP 3495, Recommendations of the International Commission on Radiological Protection, ICRP Publication 60, Pergamon, New York, Recommendations of the International Commission on Radiological Protection, ICRP Publication 26, Pergamon, New York, January Xapsos, M. A., Barth, J. L., Stassinopoulos, E. G., Messenger, S. R., Walters, R. J., Summers, G. P., and Burke, E. A., Characterizing Solar Proton Energy Spectra for Radiation Effects Applications, IEEE Transactions on Nuclear Science, Vol. 46, No. 6, 1999, pp National Council on Radiation Protection and Measurements, Limitation of Exposure to Ionizing Radiation, Report No. 116, Bethesda, MD, March

8 18 National Council on Radiation Protection and Measurements, Radiation Protection Guidance for Activities in Low-Earth Orbit, Report No. 132, Bethesda, MD, December