Blattnig, S. R.

Norbury, J. W.

Slaba, T. C.

NASA Technical Reports Server

Radiation Models for Exposure
Analyses in Deep Space
T.C. Slaba1, S.R. Blattnig1, J.W. Norbury1
1 NASA Langley Research Center, Hampton, VA
NASA Advisory Council
April 7, 2015
Washington, D.C.
https://ntrs.nasa.gov/search.jsp?R=20160006955 2019-08-31T02:32:57+00:00Z
2Outline
• Exposure analysis overview
• Galactic cosmic ray environment and models
• Radiation transport through shielding 
• Projecting exposures for mission analysis and vehicle design
• Summary
3Exposure Analysis Overview
Exposure &
Biological response
Shielding 
models
Environment
models Physics models
nasa.gov/sites/default/files/14-271.jpg
nasa.gov/centers/johnson/slsd/
about/divisions/hacd/hrp/about-
space-radiation.html
humanresearchroadmap.
nasa.gov/evidence/report
s/Carcinogenesis.pdf
4Galactic Cosmic Ray Environment
• The galactic cosmic ray (GCR) environment is omnipresent in space and 
fluctuates between solar minimum and solar maximum on an approximate 
11 year cycle
– Exposures differ by approximately a factor of 2 between nominal solar extremes
– Broad spectrum of particles (most of the periodic table) and energies (many orders of magnitude)
– Difficult to shield against due to high energy and complexity of field
Relative abundance of elements in the 1977 solar 
minimum GCR environment, normalized to neon
GCR flux at solar minimum and solar maximum
5Galactic Cosmic Ray Model Description
• The Badhwar O'Neill (BON) galactic cosmic ray model(1) is used at NASA as 
input into radiation transport codes for
– vehicle design, mission analysis, astronaut risk analysis 
– other models used as well (discussed in later slides)
• The BON model has had several revisions(2-5); all of them are based on the 
same fundamental framework
– Model equations are solved to describe particle transport through solar system
– Solar activity is described by a single parameter (Φ) related to observed sunspot numbers
6Galactic Cosmic Ray Model Description
• The Badhwar O'Neill (BON) galactic cosmic ray model(1) is used at NASA as 
input into radiation transport codes for
– vehicle design, mission analysis, astronaut risk analysis 
– other models used as well (discussed in later slides)
• The BON model has had several revisions(2-5); all of them are based on the 
same fundamental framework
– Model equations are solved to describe particle transport through solar system
– Solar activity is described by a single parameter (Φ) related to observed sunspot numbers
• GCR spectrum outside the solar system is the 
boundary condition for the model (solid lines)
- Referred to as the local interstellar spectrum (LIS)
- Nearly constant over time
7Galactic Cosmic Ray Model Description
• The Badhwar O'Neill (BON) galactic cosmic ray model(1) is used at NASA as 
input into radiation transport codes for
– vehicle design, mission analysis, astronaut risk analysis 
– other models used as well (discussed in later slides)
• The BON model has had several revisions(2-5); all of them are based on the 
same fundamental framework
– Model equations are solved to describe particle transport through solar system
– Solar activity is described by a single parameter (Φ) related to observed sunspot numbers
• GCR spectrum outside the solar system is the 
boundary condition for the model (solid lines)
- Referred to as the local interstellar spectrum (LIS)
- Nearly constant over time
• GCR spectrum is attenuated near Earth and 
affected by solar activity level 
- Dashed lines show model spectra near Earth during 
solar minimum (Φ = 475)
8Galactic Cosmic Ray Model Description
• The Badhwar O'Neill (BON) galactic cosmic ray model(1) is used at NASA as 
input into radiation transport codes for
– vehicle design, mission analysis, astronaut risk analysis 
– other models used as well (discussed in later slides)
• The BON model has had several revisions(2-5); all of them are based on the 
same fundamental framework
– Model equations are solved to describe particle transport through solar system
– Solar activity is described by a single parameter (Φ) related to observed sunspot numbers
• GCR spectrum outside the solar system is the 
boundary condition for the model (solid lines)
- Referred to as the local interstellar spectrum (LIS)
- Nearly constant over time
• GCR spectrum is attenuated near Earth and 
affected by solar activity level 
- Dashed lines show model spectra near Earth during 
solar minimum (Φ = 475)
• GCR spectrum is more heavily attenuated during solar 
maximum
- Dashed lines show model spectra near Earth during 
solar minimum (Φ = 1100)
9Galactic Cosmic Ray Model Development
• GCR models are developed and validated by using measurements 
supported by Science Mission Directorate and others over the past 40 years
– Short duration, high energy, balloon and satellite measurements 
– Low energy, continuous measurements from ACE/CRIS (most of the available measurements)
– Current gap in measurement database for continuous, high energy measurements(6,7)
– Collaboration with AMS-II will begin to fill this important gap
Name Flight Time Ions (Z) Energy (GeV/n) Data pts.
ACE/CRIS Satellite 1998-present 5-28 0.05 – 0.5 8288
AMS STS-91 1998 1, 2 0.1 – 200 58
ATIC-2 Balloon 2002 1, 2, 6, 8, 10,…,14, 26 4.6 – 103 55
BESS Balloon 1997-2000, 2002 1, 2 0.2 – 22 300
CAPRICE Balloon 1994, 1998 1, 2 0.15 – 350 93
CREAM-II Balloon 2005 6-8, 10, 12, 14, 26 18 – 103 42
HEAO-3 Satellite 1979 4-28 0.62 – 35 331
IMAX Balloon 1992 1, 2 0.18 – 208 56
IMP-8 Satellite 1974 6, 8, 10, 12, 14 0.05 – 1 53
LEAP Balloon 1987 1, 2 0.18 – 80 41
MASS Balloon 1991 1, 2 1.6 – 100 41
PAMELA Satellite 2006-2009 1, 2 0.08 – 103 472
TRACER Balloon 2003 8, 10, 12,…,20, 26 0.8 – 103 55
Lezniak Balloon 1974 4-14, 16, 20, 26 0.35 – 52 131
Minagawa Balloon 1975 26, 28 1.3 – 10 16
Muller STS-51 1985 6, 8, 10, 12, 14 50 – 103 16
Simon Balloon 1976 5-8 2.5 – 103 46
82% of 
available 
data
10
• Recent work has significantly reduced model uncertainties by taking a more rigorous 
approach to model calibration and validation – resulted in BON2014(1)
– Determined measurements (energies) most important for exposure quantities behind shielding(6)
– Model parameters calibrated using optimization methods with an emphasis on higher energies(1,7)
– Comprehensive validation metrics applied to quantify model uncertainty(1,7)
– Previous efforts focused more heavily on lower energy ACE/CRIS measurements
Fraction of available 
measurements in each 
energy bin
• Significant need for continuous, time-resolved (e.g. 
monthly) high energy measurements to further reduce 
model uncertainties(7)
– AMS-II collaboration will begin to fill this important gap
• Energy region above 0.5 GeV/n accounts for 95% of 
exposure(6)
• Most of the measurements have been taken below 0.5 
GeV/n(7)
Galactic Cosmic Ray Model Development
11
International Models and Comparisons
• Nymmik (MSU) has developed a semi-empirical 
model(8,9)
– Used by Russian Space Agency and others (DLR, ESA)
– Official update has not been provided recently
• Matthia et al. (DLR) recently developed a 
simplified form of Nymmik’s model(10)
– Shown to be reasonably accurate(7,10)
• GCR models tend to agree reasonably well at highest energies where effects of solar 
modulation are less pronounced
– Most important for exposure quantities behind shielding(6)
• Continuous, time-resolved (e.g. monthly) measurements at high energies needed to 
further reduce uncertainties
– Most important gap is high energy proton and alpha data
– AMS-II collaboration will begin to fill gap
12
International Model Comparisons
• Human exposure quantities behind shielding are in good agreement if 
updated galactic ray models are used
– Effective dose computed as weighted sum of tissue exposures in detailed human model
– BON2014 and Matthia are within 10% of each other, on average, over past 40 years
– Models tend to agree well at higher energies where impacts of solar activity are reduced
13
Radiation Transport
• Radiation is modified as it passes through shielding and tissue
– Modifications due to atomic and nuclear interactions
• Radiation transport codes are used to describe these processes 
– Galactic cosmic ray model provides the boundary condition
– Atomic and nuclear interaction parameters are generated by separate models 
– Shielding model for realistic vehicles is also required (and has some uncertainty)
• NASA's radiation transport code is HZETRN(11-15)
– Highly efficient compared to Monte Carlo methods (seconds vs. days or longer)
– Efficiency needed to support vehicle design, engineering, and optimization activities
– Extensive verification against Monte Carlo and validation against space flight 
measurements
Radiation 
beam
Shield
Spectrum of 
particles and 
energies 
14
Transport Code Comparisons
• Comparisons against state-of-the-art Monte Carlo codes shown below(16)
– HZETRN agrees with Monte Carlo to the extent they agree with each other(13,15-17)
– Differences in nuclear interaction models still present and highlights need for further model 
development and experimental measurements(15,16,18-20)
56Fe solar minimum galactic 
cosmic ray spectrum
Tissue sphere with radius 15 g/cm2
surrounded by 20 g/cm2 of aluminum
15
Integrated Model Validation - ISS
• Efficiency of NASA’s transport code has allowed 
detailed validation studies to be performed using 
minute-by-minute active dosimetry(21)
– Allows rigorous statistical analyses to be performed
– Helped identify deficiencies in geometry models, 
geomagnetic field models, and high energy nuclear 
physics
• For astronaut risk assessment, end-to-end model results are normalized to area dosimeters on 
the International Space Station (ISS)
– Cancer risk models require more detailed information than area dosimeters provide
– Normalization procedures ensure cancer risk estimates are consistent with available dosimetry
– Normalized end-to-end model uncertainty is within 15%
• Direct model evaluation (without normalization) is used in validation and uncertainty 
quantification efforts
– Direct model evaluation needs to be accurate when dosimetry is unavailable (e.g. projections)
– Integrated model uncertainties for a recent ISS analysis(21) ranged from 10% - 50% and includes 
uncertainties associated with
o GCR and geomagnetic field models
o Nuclear physics and transport codes
o Shielding mass distribution of the ISS
o Dosimeter response
16
Integrated Model Validation – MSL/RAD
• Comparisons between Mars Science Laboratory/Radiation Assessment 
Detector (MSL/RAD) and NASA models are ongoing
– Cruise dose measurements and models show reasonable agreement(22)
– Surface dose measurements and models show reasonable agreement(23)
• Comparisons between NASA models and MSL/RAD surface measurements 
of particle fluxes have been made(24)
– Provides a more rigorous test of models than just dose / dose eq. comparisons
– Surface flux measurements show good agreement for some ions and reveals poor 
agreement for others
– Highlights the need for continued model development and additional measurements 
Dose 
(µGy/day)
Dose Eq. 
(µSv/day) Avg. Q
BON2011/HZETRN 0.445 1.80 4.05
MSL/RAD 0.481+0.08 1.84+0.33 3.82+0.25
MSL/RAD cruise exposure rates compared to BON2011/HZETRN model(22)
17
Mission Planning and Shield Design
• Past studies utilized static, representative environments for design analysis
– e.g. 1977 solar minimum used as a “design case”
– Solar modulation parameter, Φ = 1100 MV, used as a representative solar maximum 
• Current studies can now utilize a more robust probabilistic approach, 
allowing estimates to be provided within a certain confidence level
– Solar activity predictions are notoriously unreliable(25-27)
– Analysis not tied to specific solar activity or time period
• Plot at left considers variation in effective 
dose due to past solar activity as represented 
by BON2014
– For this shield configuration, effective dose values at 
solar minimum vary by +6% over the past 265 years 
• The data shown in the plot can be further 
analyzed to quantify probability of exceeding 
a given exposure value
– e.g. 2.5% probability of exceeding 369 mSv/year for this 
shielding configuration
– This type of analysis does not account for dramatic 
changes in future solar activity and other uncertainties 
(e.g. radiobiology)
18
Projecting Solar Activity
• Limiting case galactic cosmic ray environment would occur in the absence of 
solar activity and solar magnetic field
– Same as the environment outside the solar system, which is nearly constant over time
– In this environment, human exposures are ~2x larger than deepest solar minimum seen 
over past 265 years (unlikely to occur)
– Historical analyses(25,26,28) indicates solar magnetic field will not completely disappear 
during grand solar minimum (Dalton grand minimum occurred ~1800-1830)
• Model predictions of future solar activity 
(even near term) are uncertain
– Predictions (made near end of cycle 23) of peak 
activity for cycle 24 varied by factor of 5 (27)
– Longer term solar activity (beyond a solar cycle) 
may be intrinsically unpredictable(25,26)
– Probability of grand solar minimum in next 30 years 
estimated to be <10%(29)
– Maybe equally likely to see a strong solar maximum 
in next 30 years(29)
– For future mission planning, uncertainties in solar 
activity predictions and possible schedule slips 
make it difficult to plan for a specific solar level (e.g. 
plan mission for solar max)See reference (27) for discussion of predictions for solar cycle 24
19
Design Tools
• The models used at NASA to support mission planning and vehicle design 
have been integrated into a web-based framework
– OLTARIS: https://oltaris.nasa.gov
• BON and Matthia galactic cosmic ray models
• Solar particle events (SPE)
– Fits to historically significant events (i.e.1972, 1989) 
are available and can be scaled
– User-defined spectra are supported through commonly 
used fitting functions with few parameters 
• Environmental models can be evaluated in 
low Earth orbit, deep space, or on planetary 
surface
• Environmental models are integrated with 
physics/transport models, detailed human 
phantom models and various geometry 
options
20
Summary
• Radiation analysis tools used for ISS operations, mission planning, and 
vehicle design in deep space and planetary surfaces are rigorously 
developed and validated
– Measurements from SMD and others used directly for model development and validation
– International models are compared or utilized where possible or appropriate
– Model development and validation efforts are ongoing
• Based on recent (~30 years) solar activity and available measurements
– Uncertainty assessment of BON2014 galactic cosmic ray model is +20%
– Normalized end-to-end model uncertainties are within 15%
– End-to-end uncertainties if normalization is not used range from 10%-50%
– Additional measurements needed to further reduce uncertainties (e.g. AMS-II)
• Mission planning and vehicle design is moving towards more robust 
probabilistic approaches
– Allows exposure estimates to be provided within a specified probability
– Allows various sources of uncertainty (solar activity, shielding, radiobiology) to be 
rigorously accounted for in the analyses
21
References (I)
(1) O’Neill, P.M., Golge, S., Slaba, T.C., NASA TP 2015-218569, 2015.
(2) O’Neill, P.M. and Foster, C.C., NASA TP 2013-217376, 2013.
(3) O’Neill, P.M., IEEE Trans. Nuc. Sci. 57: 3148-3153, 2010.
(4) O’Neill, P.M., Adv. Space Res. 37: 1727-1733, 2006.
(5) Badhwar, G.D., O’Neill, P.M., Adv. Space Res. 17: 7-17, 1996.
(6) Slaba, T.C. and Blattnig, S.R., Space Weather 12: 217-224, 2014.
(7) Slaba, T.C., Xu, X., Blattnig, S.R., Norman, R.B., Space Weather 12: 233-245, 2014.
(8) Nymmik, R.A., Panasyuk, M.I., Suslov, A.A., Adv. Space Res. 17: 219-230, 1996.
(9) ISO 15390, 2004.
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22
References (II)
(16) Wilson, J.W., Slaba, T.C., Badavi, F.F., Reddell, B.D., Bahadori, A.A., Life Sc. Space Res. 4: 46-61, 2015.
(17) Slaba, T.C., Blattnig, S.R., Clowdsley, M.S., Rad. Res. 176: 827-841, 2011.
(18) Heinbockel, J.H., Slaba, T.C., Blattnig, S.R., Tripathi, R.K., Townsend, L.W., Handler, T., Gabriel, T.A., Pinsky, L.S., Reddell, B., Clowdsley, M.S., 
Singleterry, R.C., Norbury, J.W., Badavi, F.F., Aghara, S.K., Adv. Space Res. 47: 1079-1088, 2011.
(19) Heinbockel, J.H., Slaba, T.C., Tripathi, R.K., Blattnig, S.R., Norbury, J.W., Badavi, F.F., Townsend, L.W., Handler, T., Gabriel, T.A., Pinsky, L.S., Reddell, 
B., Aumann, A.R., Adv. Space Res. 47:1089-1105, 2011.
(20) Norbury, J.W. and Miller, J., Review of nuclear physics experimental data for space radiation. Health Phys. 103: 640-642, 2012.
(21) Slaba, T.C., Blattnig, S.R., Reddell, B., Bahadori, A., Norman, R.B., Badavi, F.F., Adv. Space Res. 52: 62-78, 2013.
(22) Zeitlin, C., Hassler, D.M., Cucinotta, F.A., Ehresmann, B., Wimmer-Schweingruber, R.F., Brinza, D.E., Kang, S., Weigle, G., Bottcher, S., Bohm, E., 
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Bohm, E., Burmeister, S., Guo, J., Kohler, J., Martin, C., Reitz, G., Posner, A., Gomez-Elvira, J., Harri, A., J. Geophys. Res. Planets 119: 1311-1321, 2014.
(24). Ehresmann, B., Zeitlin, C., Hassler, D.M., Wimmer-Schweingruber, R.F., Bohm, E., Bottcher ,S., Brinza, D.E., Burmeister, S., Guo, J., Kohler, J., Martin, 
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23
Backup
From ref (27): Pesnell, W.D. (NASA Goddard Space Flight Center) 
Solar Physics 281: 507, 2012.
• Maximum SSN predictions of various types of models for cycle 24. 
• The observed maximum (so far) is 82 (horizontal red line).
24
Backup
From ref (28): Solanki, S., Krivova, N., Science 334: 916-917, 2011.
Reprinted with permission from AAAS.
• Sunspot numbers in Maunder minimum and near ~1810 were zero


Radiation Models for Exposure Analyses in Deep Space

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