The Title Goes Here



Estimates of Radiation Exposures for human crews in deep space from the 15 January 2005 Solar Energetic Particle Event using the Earth-Moon-Mars Radiation Environment Module.

PourArsalan Mahmoud and Lawrence W. Townsend

Department of Nuclear Engineering, 215 Pasqua Engineering Bldg., University of Tennessee, Knoxville, TN 37996-2300; ltownsen@tennessee.edu

Nathan A. Schwadron and Kamen Kozarev

Department of Astronomy, 275 Commonwealth Avenue, Boston University, Boston, MA 01760

Maher A. Dayeh and Mihir I. Desai

Southwest Research Institute, 6220 Culebra Rd., San Antonio, TX 78238-5166

Total number of pages 22

Total number of tables 1

Total number of figures 10

Abstract. The Earth-Moon-Mars Radiation Environment Module (EMMREM) is a numerical model for characterizing the time-dependent radiation environment in the Earth-Moon-Mars and Interplanetary space environments. In this work we demonstrate the capabilities of the module for performing analyses of time-dependent exposures from solar energetic particle events near Earth, Moon and Mars by calculating time dependent dose rates, dose equivalent rates, and accumulated dose and accumulated dose equivalents for surrogates of the skin and the Blood Forming Organs (BFO) of crew members shielded by as much as 10 g/cm2 of aluminum shielding for the January 15, 2005 SEP event. The motivation for the development of EMMREM is the need to better understand the radiation hazards in deep space and near Earth and other planetary bodies, in support of possible future space exploration by manned and unmanned spacecraft. Characterizing the radiation environment for different locations on and close to Earth for solar energetic particle (SEP) events is fairly well developed. However, estimating the probable radiation environment near Mars and other locations throughout the Solar system is not currently supported for SEP events. Such capability is critical for future human exploration of the Moon and Mars in the upcoming decades. The calculated doses for the Skin and BFO surrogates are compared with the NASA’s short term Permissible Exposure Limits (PELs).

Key words: Organ doses, SEP events and EMMREM.

1. Introduction

Risks to flight crews and instruments from the Solar Energetic Particle (SEP) and Galactic Cosmic Rays (GCR) environments are a major concern in planning for long-duration manned missions. The Earth-Moon-Mars Radiation Environment Module (EMMREM) is a recently-developed numerical model for characterizing the time-dependent radiation environment in the Earth-Moon-Mars and interplanetary space environments1, 2. The motivation for this effort is the need for better understanding of the radiation hazards in deep space and near Earth and other planetary bodies, in support of possible future space exploration by manned and unmanned spacecraft.

The backbone of EMMREM Module includes a 3D parallelized energetic particle transport numerical simulation code (EPREM) and a parallelized version of the BRYNTRN space radiation transport code initially developed at NASA Langley Research Center3, 4. EPREM solves for the particle distribution function by including the effects of pitch-angle scattering, adiabatic focusing and cooling, convection, and stochastic acceleration. The modular design of EPREM allows for the interplanetary magnetic field to be changed easily. EPREM has been designed to incorporate perpendicular transport of particles and includes a solver for cross-field diffusion and particle drifts. It can propagate a radiation environment measured at one location in the solar system, to any other desired location in the solar system. More detail information listed elsewhere5. BRYNTRN is used to calculate time dependent dose rates and dose equivalent rates for different amounts of aluminum shielding and depths of water for the ambient solar particle event environment that is output from EPREM. The calculations can be carried out in near real time using EMMREM Module.

Characterizing the radiation environment for different locations on and close to Earth for Solar Energetic Particle (SEP) events is fairly well developed. However, estimating the radiation environment near the Moon, Mars, and other locations throughout the Solar system is not currently supported for SEP events. Such capability is critical for future human exploration of the Moon and Mars in the upcoming decades.

The prediction of the radiation environment through the inner heliosphere requires an

understanding of how evolving disturbances generate energetic particles, and how the energetic particles subsequently propagate and evolve through the inner heliosphere. The main challenge in developing EMMREM was to develop flexible interfaces between observations made in the space science community and the EPREM and BRYNTRN modules. Currently an initial version of the EMMREM system has been developed. Herein we present only a brief description of the system. Details of the EMMREM framework are presented elsewhere2. Using the initial version of the EMMREM framework, we performed realistic simulations with observations from the January 15, 2005 solar energetic particle event (SEP) as part of a program of module testing and as an example of the EMMREM module capabilities. Herein we present and discuss EMMREM predictions of dose, dose rates, dose equivalent and dose equivalent rates throughout the January 15, 2005 event, for observers near Earth, Moon and Mars, for various aluminum shield thicknesses representative of actual space radiation shielding scenarios.

Observations of this SEP event at 1 AU were obtained from the Space Environment Monitor (SEM) subsystem on board the Geostationary Operational Environmental Satellite (GOES-11). SEM provides magnetic field, energetic particle, and soft X-ray data for the ambient environment. Reported here are the hourly averaged proton intensities measured by the energetic particle sensor (SEM/EPS) at 6 different energy ranges between 4 and 500 MeV. These proton data were obtained from the Space Physics Interactive Data Resource (SPIDR: ) of the Space Weather Prediction Center within the National Oceanic and Atmospheric Administration (NOAA).

The outline of the paper is as follows. In the next section, a brief overview of the EMMREM module framework is presented. This is followed by a discussion of methods used herein to estimate radiation doses and dose equivalents from the incident SEP event radiation environment. Results of radiation exposure estimates from EMMREM for the January 15, 2005 Solar Energetic Particle (SEP) event are then presented and discussed. Finally, the paper concludes with a summary of the work presented

2. Operational Overview of the EMMREM Module

Figure 1 displays a schematic of the present EMMREM framework. The entire system is controlled by a number of bash shell and Perl scripts. The EMMREM Module package is a UNIX based system that can be run on a single PC with LINUX OS. We use UBUNTU V8.04 as our preferred choice of the LINUX distributions.

The INPUT PARSER converts the external EMMREM format data (from GOES, ACE, other spacecraft, artificial SEP data, etc.) into internal EMMREM format data (distribution

function time series). Heliocentric positions of the relevant bodies for the time range of a particular run are generated by a command utilizing the CSPICE library of the NASA

SPICE Toolkit (), and the data for those are generated by the NASA NAIF group in the form of kernel files.

The Energetic Particle Radiation Environment Module (EPREM), which is a 3-D kinetic numerical simulation of solar energetic particles transport throughout the inner heliosphere, solves for the propagation and acceleration of energetic particles in the evolving magnetic fields of the inner heliosphere using inputs based on observations from satellites. It is a parallelized code written in C/C++. EPREM OUTPUT includes observer time series of distribution function spectra for the various observer positions. The OUTPUT PARSER then converts observer outputs from distribution function time series into flux time series for several energies for input into BRYNTRN6.

The BaRYoN TRaNsport Module (BRYNTRN) is a deterministic, coupled proton-neutron space radiation transport model that transports incident protons and their secondary products (neutrons, protons, deuterons, tritons, hellions, and alphas). In this work, the spectra are transported through aluminum spacecraft shielding and then through an additional quantity of water simulating human tissue4. BRYNTRN is written in FORTRAN 77 and was parallelized for this work. Its output contains dose and dose equivalent time series for different shielding depths and materials (aluminum and water in this case).

[pic]

Figure 1 Present EMMREM Framework

3. Calculation Methods for Radiation Dose and Dose Equivalent

Figure 2, shown below, displays the outputs of the Output Parser for the incident proton spectra for the January 15, 2005 SEP event. These are transported through an aluminum shield and the simulated human geometry using BRYNTRN. For this work, the incident proton spectra and their reaction products (1n, 1p, 2H, 3H, 3He and 4He) are transported through as much as 10 g/cm2 of aluminum and then through an additional 10 g/cm2 of water which simulates body soft tissue. Since actual human geometry model files are not yet incorporated into the EMMREM framework, we use 1 g/cm2 of water as a reasonable substitute for the actual body self-shielding associated with skin and eye exposures, and 10 g/cm2 as a reasonable substitute for the blood forming organs (BFO) 7, 8, 9, 10.

These organ self-shielding substitutes were initially selected for use in order to facilitate testing the ability of the EMMREM framework to carry out near real-time calculations, for updated particle flux intervals as small as five minutes, during an actual event. They will be replaced in the future by realistic body organ self-shielding distributions obtained from actual human geometry models, such as the computerized anatomical man (CAM) model11. The transported particle fluxes are converted to dose and dose equivalent using the methods described in Wilson4.

Absorbed dose (or simply dose) is the primary physical quantity used in radiation protection and dosimetry. It is defined in ICRU Report 5112 as the mean energy dE imparted by ionizing radiation to matter of mass dm. The dose unit is in Grays (1 Gray = 1 joule/kg).

The dose needed to achieve a given level of non-acute (long term) biological risk (mainly for cancer indication or mortality) is different for different types of radiations. To account for this difference, dose is multiplied by a unitless constant called the quality factor (Q) defined in ICRP Report 6013. Q is defined as a function of LET in water (a tissue surrogate). The product of dose (D) and quality factor (Q) is called dose equivalent (H). If the dose is in Gray (Gy), dose equivalent is in Sievert (Sv). Dose equivalent values for particular organs are not compared directly to the NASA Permissible Exposure Limits promulgated in NASA_STD_3001_Vol 114. Instead, they are used to compute Effective Dose (E), which is a weighted average of organ dose equivalents over a variety of organs (more than 15) when the whole body is irradiated. Career limits are given in terms of the effective dose, which are cumulative for the entire career or lifetime of the exposed individual. Since the focus of this work is to demonstrate the near real-time capabilities of the EMMREM module to estimate dose, calculations of effective dose are beyond the scope of the current work, but will be included in future revisions to the EMMREM framework.

The results of the absorbed dose simulations are compared with the Permissible Exposure Limits (PELs), shown in Table 1, which are used by NASA for human activities in space14. The limits are expressed in units of centiGray-Equivalent, which are obtained from the absorbed dose (D) as

D(cGy-Eq.) = D(cGy) × RBE (1)

Where RBE (Relative Biological Effectiveness) is a multiplicative factor applied to account for the ability of some types of radiations to produce more biological damage than others for the same dose. For SEP protons the RBE is assumed to be 1.5, as recommended by the NCRP15. RBE is defined as

[pic] (2)

Where D the dose of radiation of a particular type (protons or alphas for example) necessary to produce some biological end point (e.g., chromosome aberrations, radiation sickness) and DX is the dose of a reference radiation (usually x-rays or gamma rays) needed to produce that same effect. RBE, as defined by Equation (2) is a function not only of LET, but also of particle type, dose rate, dose levels, and the particular biological effect, such as acute radiation syndrome (radiation sickness), etc., being investigated.

Table 1 Permissible Exposure Limits (PELs) for short-term or career non-cancer Effects taken

from NASA_STD_3001_Vol 114

|Organ |30 days limit |1 Year Limit |Career Limit (cGy-Eq) |

| |(cGy-Eq) |(cGy-Eq) | |

|Lens* |100 |200 |400 |

|Skin |150 |300 |400 |

|BFO |25 |50 |NA |

|Heart** |25 |50 |100 |

|CNS*** |50 |100 |150 |

|CNS*** (Z≥10) |- |10 |25 |

*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.

[pic]

Figure 2 Proton flux vs. time for the January 15, 2005 SEP event.

4. Results

Results for observers near Earth, Earth’s moon, and Mars for the specified aluminum and water layers are presented. The proton energies for the input solar particle event data range from 4 to 500 MeV. Assumed aluminum shielding real densities, which simulate shielding

thicknesses used for manned space missions are:

0.3 g/cm2 - nominal spacesuit

1.0 g/cm2 - thick spacesuit

5.0 g/cm2 - nominal spacecraft

10.0 g/cm2 - SEP storm shelter

The calculated results for different observers presented herein help us to understand the possible variations in severity of space radiation exposures during the event at different locations in the inner heliosphere.

Figures 3 through 6 display dose and dose equivalent results for the Earth observer. Since the Moon is very near Earth relative to the Sun, the radiation environments for the Earth and the Moon observers are very similar. Therefore, estimates of the skin and BFO exposures are essentially identical (differences are much less than 1%). Thus, only results for the Earth observer are presented at 0.95 AU.

The predicted rates of skin and BFO dose and dose equivalent for the Earth observer, as expected, rise and fall in the same pattern as the SEP fluxes as a function of time. In addition, as the thickness of the aluminum shielding increases, the exposures are reduced, also as expected. The accumulated skin and BFO dose and dose equivalent results show increases for the higher exposure rates, and then level off trends as the exposure rates decrease.

As seen in Figure 3, the BFO dose limits are exceeded for aluminum shield areal densities corresponding to spacesuit in early afternoon on day 20 (20 January). The BFO 30 days dose limits are also exceeded for aluminum shield areal densities corresponding to nominal spacecraft thicknesses on the morning of day 24(24 January) The BFO doses, however, are not large enough to cause any acute radiation syndrome (radiation sickness) effects. The dose levels for the skin and BFO are below their respective limits if the crewmembers are inside the SEP storm shelter.

Note also that the skin 30 days dose limits are not exceeded for any of the aluminum shield thicknesses during this event for the Earth observer.

We now turn our attention to the results for the Mars observer, which are displayed in Figures 7 through 10. As was the case for the Earth observer, the BFO 30 days dose limits are also exceeded for this event for the Mars observer. For the aluminum shield areal density corresponding to the nominal spacesuit the BFO 30 days dose limits are also exceeded by mid-day 20 (20 January). The BFO limits are also exceeded for this event at Mars for the aluminum shield areal density corresponding to the nominal spacecraft on the morning of day 24 (24 January) (Figure 7). Note also from Figure 9 that the skin 30 days dose limits are exceeded for an aluminum shield areal density corresponding to the nominal spacesuit on the morning of day 21(21 January).

Note that these computational results could be different and the conclusions drawn from them altered, if actual human geometries were used instead of the simple surrogates used herein. That analysis, however, will have to wait until a future date when the human geometry module has been incorporated into EMMREM. Nevertheless, the ability of the EMMREM framework to carry out these types of calculations has been clearly demonstrated.

The peak Gray-Equivalent Rates and accumulated Gray Equivalents in centiGray-equivalent for Skin and BFO for Earth and Mars observers for the October 2003 were compared with the August 1972 peak Gray Equivalent Rates for an aluminum shielding depth of 1 g/cm2 for validating the calculated results of the EMMREM. This comparison is made in order to give a perspective on the severity of the 2003 event compared to one of the highest exposure events of the human space era. A comparative analysis of the August 1972 event to the Halloween event suggests that cumulative doses and peak dose rates for both Earth/Moon and Mars observers significantly larger for the 1972 event. Details of the comparison for validation of EMMREM

calculated results are written elsewhere16 .

[pic]

Figure 3 BFO Gray Equivalent rates (bottom panel) and accumulated BFO Gray

Equivalent (top panel) at Earth during the January 15, 2005 event. The aluminum (Al)

and water (H2O) depths in g/cm2 are displayed in the legend in the bottom panel.

[pic]

Figure 4 BFO Dose Equivalent rates (bottom panel) and accumulated BFO Dose

Equivalent (top panel) at Earth during the January 15, 2005 event. The aluminum (Al)

and water (H2O) thicknesses in g/cm2 are displayed in the legend in the bottom panel.

[pic]

Figure 5 Skin Gray Equivalent rates (bottom panel) and accumulated skin Gray Equivalent

(top panel) at Earth during the January 15, 2005 event. The aluminum (Al) and water (H2O)

thicknesses in g/cm2 are displayed in the legend in the bottom panel.

[pic]

Figure 6 Skin Dose Equivalent rates (bottom panel) and accumulated Skin Dose Equivalent

(top panel) at Earth during the January 15, 2005 event. The aluminum (Al) and water (H2O)

thicknesses in g/cm2 are displayed in the legend in the bottom panel.

[pic]

Figure 7 BFO Gray Equivalent rates (bottom panel) and accumulated BFO Gray Equivalent

(top panel) at Mars during the January 15, 2005 event. The aluminum (Al) and water (H2O)

thicknesses in g/cm2 are displayed in the legend in the bottom panel.

[pic]

Figure 8 BFO Dose Equivalent rates (bottom panel) and accumulated BFO Dose Equivalent

(top panel) at Mars during the January 15, 2005 events. The aluminum (Al) and water (H2O)

thicknesses in g/cm2 are displayed in the legend in the bottom panel.

[pic]

Figure 9 Skin Gray Equivalent rates (bottom panel) and accumulated skin Gray Equivalent

(top panel) at Mars during the January 15, 2005 event. The aluminum (Al) and water (H2O)

thickneses in g/cm2 are displayed in the legend in the bottom panel. Note that the skin doses are

less than the 30 days limit of 150 cGy-Equivalent for all aluminum shielding areal densities.

[pic]

Figure 10 Skin Dose Equivalent rates (bottom panel) and accumulated Skin Dose Equivalent

(top panel) at Mars during the January 15, 2005 event. The aluminum (Al) and water (H2O)

thicknesses in g/cm2 are displayed in the legend in the bottom panel.

5. Conclusions

The ability of the EMMREM module to characterize the time dependent radiation environments, at various locations in the solar system, has been demonstrated for the January 2005 SEP event. In general it is capable of performing calculations in the Earth, Moon, Mars, and interplanetary space environment for any SEP historical event with reasonable results. The calculated results indicate that the BFO doses exceed the 30 days dose limits from NASA_STD_3001 Vol 114 for both the Earth and the Mars observers for nominal spacesuit and nominal spacecraft shield areal densities. For the Mars observer, the skin dose limit would have been exceeded for the nominal spacesuit shielding, but not for the nominal spacecraft shielding thickness. For both the Earth and Mars observers, crews inside a storm shelter with aluminum shield areal densities of 10 g/cm2 would have been adequately protected from this event.

6. Acknowledgements

Research support from the NASA LWS EMMREM project and NASA Grant No. NNX07AC14G is gratefully acknowledged.

7. References

1. Schwadron, N. A, et al.: Earth-Moon-Mars Radiation Environment Module (EMMREM). 2006 IEEE Aerospace Conference, Big Sky, MT, July 2006.

2. Schwadron, N.A., L.W. Townsend, K. Kozarev, M. A. Dayeh, F. Cucinotta, M. Desai, M. Golightly, D. Hassler, R. Hatcher, M. Y. Kim, A. Posner, M. PourArsalan, H. Spence, R. K. Squier: The Earth Moon Mars Radiation Environment Module Framework, Space Weather Journal, 2010.

3. Wilson, J. W., L. W. Townsend, S. Y. Chun, W. W. Buck, F. Khan, and F. A. Cucinotta, “BRYNTRN: A Baryon Transport Computer Code,” NASA Technical Memorandum 4037, June 1988.

4. Wilson, J. W., L. W. Townsend, W. Schimmerling, G. S. Khandelwal, F. Khan, J. E. Nealy, F. A. Cucinotta, L. C. Simonsen, J. L. Shinn, J. W. Norbury: Transport Methods and Interactions for Space Radiations. NASA Reference Publication 1257, December 1991.

5. Kozarev K., N. A. Schwadron, M. A. Dayeh, L. W. Townsend, M. I. Desai, and M. PourArsalan, Modeling the 2003 Halloween Events with EMMREM: Energetic Particles in the Inner heliosphere, Radial Gradients, and Coupling to MHD, Space Weather, 2010.

6. Hatcher, R.; Townsend, L. W.; Schwadron, N. A.;  Kozarev, K.: Status of Developing a Near Real-Time Capability for Estimating Space Radiation Exposure Using EMMREM. 39th International Conference on Environmental Systems (ICES), Savannah, GA, July 12-16, 2009.

7. Bier, S.G., L. W. Townsend, W. L.Maxson: New equivalent sphere approximation for BFO dose estimation: solar particle events. Advances in Space Research, 21(12), 1777, 1998.

8. Townsend, L. W., E. N. Zapp: Dose uncertainties for large solar particle events: input spectra variability and human geometry approximations. Radiation Measurements 30, 337, 1999.

9. Lin, ZW, Can the equivalent-sphere model approximate organ doses in space radiation environments? Nuclear Technology166(3), 273, 2009.

10. Zapp, Townsend, Cucinotta, Solar Particle Event Doses for Interplanetary crews Variations Due to Body Composition Modeling. The proceedings of ANS RPSD Topical Meeting, Spokane, WA Sept 17-21, 2000.

11. Billings M.P., W.R. Yucker, The computerized anatomical man (CAM) model, NASA Contractor Report No. CR-134043, 1973.

12. ICRU, Quantities and Units in Radiation Protection Dosimetry. Report No. 51, International Commission on Radiation Units & Measurements, 7910 Woodmont Avenue, Suite 400 Bethesda, MD 20814-3095, 1993.

13. ICRP, International Commission on Radiological Protection, Report 60. P.O Box 1046, Station B, 280 Starter Street, Ottawa, Ontario K1p559 Canada, 1991.

14. NASA, NASA Space Flight Human System Standard Volume 1: Crew Health, NASA- STD-3001, vol. 1, NASA Headquarters, Washington, DC, 2007.

15. NCRP, Radiation Protection Guidance for Activities in Low-Earth Orbit. Report No. 132, National Council on Radiation Protection and Measurements, Bethesda, MD, 2000.

16. PourArsalan, M., L. W. Townsend, N.A. Schwadron, K. Kozarev, M. A. Dayeh, and M. I. Desai (2010), Time dependent estimates of organ dose and dose equivalent rates for human crews in deep space from the 26 October 2003 solar energetic particle event (Halloween event) using the Earth‐Moon‐Mars Radiation - Environment Module, Space Weather, 8, XXXXXX, doi:10.1029/2009SW000533.

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