MASTER's THESIS Space Radiation Analysis

2009:107 MASTER'S THESIS

Space Radiation Analysis - Radiation Effects and Particle Interaction outside Earth Magnetosphere using GRAS and GEANT4

Lisandro Martinez

Luleå University of Technology Master Thesis, Continuation Courses Space Science and Technology Department of Space Science, Kiruna

2009:107 - ISSN: 1653-0187 - ISRN: LTU-PB-EX--09/107--SE

Space Radiation Analysis: Radiation Effects and Particle Interaction outside Earth Magnetosphere using GRAS and GEANT4

Master’s Thesis For the degree of Master of Science in Space Science and Technology

Lisandro M. Martinez Luleå University of Technology Cranfield University June 2009

Supervisor: Johnny Ejemalm Luleå University of Technology

June 12, 2009 MASTER’S THESIS

ABSTRACT

Detailed analyses of galactic cosmic rays (GCR), solar proton events (SPE), and solar fluence effects have been conducted using SPENVIS and CREME96 data files for particle flux outside the Earth’s magnetosphere. The simulation was conducted using GRAS, a European Space Agency (ESA) software based on GEANT4. Dose, dose equivalent and equivalent dose have been calculated as well as secondary particle effects and GCR energy spectrum.

The results are based on geometrical models created to represent the International Space Station (ISS) structure and the TransHab structure. The physics models used are included in GEANT4 and validation was conducted to validate the data. The Bertini cascade model was used to simulate the hadronic reactions as well as the GRAS standard electromagnetic package to simulate the electromagnetic effects.

The calculated total dose effects, equivalent dose and dose equivalent indicate the risk and effects that space radiation could have on the crew, large amounts of radiation are expected to be obtained by the crew according to the results. The shielding comparison between ISS and TransHab indicate that a tradeoff between the two will have to be made, since the first has a higher protective ratio compared to the TransHab; on the other hand the second one is more flexible and could eventually become a larger structure. The GCRs effects upon the structure are found to be comparable to experimental data.

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Acknowledgements

The work presented in this thesis could not have been done without the help, support, and participation of many people as well as institutions.

First, I would like to thank my advisor, Dr. Jennifer Kingston, for her constant support during the development of this thesis. Her interest in the subject as well as her good will was always reassuring during this work.

I am grateful to the Space Master consortium for their support, not only during this work but also for the past year and a half of outstanding education, especially to the staff in Kiruna who have been very helpful. To the ErasmusMundus scholarship, that financed my dual MSc education.

I would like to thank all the staff from Cranfield University, specially Dr. Steve Hobbs and Dr. Peter Roberts, who always found time for advice and guidance in the difficult area of space science. To my colleagues who by intellectual and personal support made this work possible.

I want to mention the help and support from the developers of GRAS and GEANT4 projects, who always answered my questions, specially Giovanni Santin from ESA/ESTEC and John Allison from The University of Manchester

I want to thank my family, my future wife, and my friends who always supported me during this gratifying and challenging time.

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Table of Contents Abstract...... i

Acknowledgements...... ii

Table of Figures...... v

Table of Tables ...... vii

List of Acronyms ...... viii

1. INTRODUCTION ...... 1

1.1. OBJECTIVES ...... 2 1.2. TRAJECTORIES...... 2 1.3. Propulsion…………………………………………………………… .…………………………………………………………. 5 1.4. Constrains…………………………………………………………………………………………………………………..……. 8 1.5. Mars Transit Vehicle ………………………………………………………………………………………………………….9 1.6. Physiological Risks ...... 10 1.6.1. Artificial Gravity ...... 10 1.6.2. Radiation ...... 11

2. SPACE PHYSICS AND RADIATION...... 14

2.1. Space Radiation ...... 14 2.1.1. The Sun and the Solar Wind ...... 14 2.1.2. Galactic Cosmic Rays ...... 17 2.1.3. Solar Proton Events ...... 18 2.2. Radioactivity and Radiation Protection ...... 19 2.2.1. Charged Particles ...... 20 2.2.2. X and γ Radiation ...... 20 2.2.3. Neutron ...... 20 2.3. Energy Absorption ...... 20 2.4. Radiation Effects ...... 22 2.4.1. Deterministic ...... 22 2.4.2. Stochastic Effects ...... 23 3. GEOMETRy AND SOFTWARE DEVELOPMENT ...... 25 3.1. GEANT4 ...... 25 3.2. GRAS ...... 28 3.3. Geometry ...... 30 3.3.1. ISS Model ...... 31 3.3.2. TransHab ...... 32 4. SOFTWARE VALIDATION ...... 37

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4.1. Stopping Power ...... 37 4.2. Total Dose ...... 43 4.3. Hadrons, Electromagnetic and Ion Validation ...... 44 4.4. Astronaut Phantom Test ...... 44 4.5. Validation Results ...... 46

5. SIMULATION OF THE INTERPLANETARY RADIATION ENVIRONMENT ...... 48 5.1. Data Normalization ...... 48 5.2. Computational Parameters ...... 51 5.3. Average Total Dose ...... 52 5.4. GCR Particle Fluence and Energy Spectra ...... 57 5.5. Equivalent Dose ...... 61

6. CONCLUSION ...... 63

7. RECOMMENDATIONS...... 66 7.1. Limitations...... 66 7.2. Further Work ...... 67

8. BIBLIOGRAPHY ...... 70

Appendix A ...... 73 Appendix B...... 74 Appendix C ...... 77 Appendix D ...... 80

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Table of Figure Figure 1: Mars mission trajectories...... 3 Figure 2: Hemisphere and the Interstellar Medium ...... 15

Figure 3: Sun Spot numbers for the latest five cycles...... 16

Figure 4: JPL‐91 Solar Proton Fluence from SPENVIS ...... 17

Figure 5: Solar Min GCR flux outside Earth's Magnetosphere ...... 18

Figure 6: SPE flux worst day case...... 19

Figure 7: Particle flux and energy provided to GRAS to perform particle analysis ...... 29

Figure 8: ISS structure representation with ICRU sphere ...... 32

Figure 9: ISS module structure ...... 33

Figure 10: TransHab layer structure composition...... 35

Figure 11: shows the layer structure of the TransHab...... 36

Figure 12 to Figure 21: Validation Stopping Power ...... 38 to 42

Figure 22: Solar Proton Fluence ...... 43

Figure 23: Dose analysis geometry ...... 44

Figure 24: Spenvis solar proton fluence ...... 45

Figure 25: Alpha particles energy distribution...... 46

Figure 26: Representation of radiation SPEs and Solar Fluence flux simulation...... 49

Figure 27: GCRs Isotropic flux representation ...... 50

Figure 28: GCRs particle flux and interaction with TransHab structure...... 51

Figure 29: Average dose equivalent for ISS and TransHab models...... 55

Figure 30: SPEs effects on ICRU sphere Dose Equivalent values...... 57

Figure 31a: GCRs particle fluence inside ISS model ...... 58

Figure 31b: GCRs particle fluence inside the TransHab model ...... 58

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Figure 32: H energy spectra entering the ISS model during Solar Max...... 59

Figure 33: H energy spectra entering the TransHab model during Solar Max...... 60

Figure 34: GCR and SPE equivalent dose effects in human tissue ...... 61

Figure A1: Solar Max particle flux outside Earth´s magnetosphere from CREME96...... 75

Figure A2: Solar Min particle flux outside Earth´s magnetosphere from CREME96 ...... 75

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Table of Tables

Table 1: Opposition and Conjunction class mission characteristics ...... 3

Table 2: Mars Conjunction Class Mission Opportunities ...... 5

Table 3: Propulsion system comparison ...... 6

Table 4: Mission Constrains ...... 8

Table 5: Mass budget for the transit Mars s/c ...... 9

Table 6: Radiation in the Space Environment...... 12

Table 7: Limits of exposure to sunlight in space ...... 12

Table 8: Organ Dose limits ...... 13

Table 9: Radiation weighting factors ...... 21

Table 10: Ten year human radiation dose limits ...... 24

Table 11: Indicates the different Geant4 physics models ...... 26

Table 12: Physics model used for the simulation ...... 27

Table 13: Indicates the ISS structure composition...... 31

Table 14: Indicates material composition, thickness and quantity for the TransHab structure ...... 34

Table 15: Total dose results from GRAS and SPENVIS ...... 43

Table 16: Validation result from Mulassis and GRAS...... 44

Table 17: Results of astronaut phantom test with proton radiation ...... 45

Table 18: Results of astronaut phantom test with alpha particle interactions...... 46

Table 19 a and b: Total average dose in Solar Max and Solar Min ...... 53

Table 20: Total average dose by Solar Fluence...... 53

Table 21: SPE average total dose effect during worst day case scenario ...... 54

Table 22: ISS Columbus module radiation environment ...... 55

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List of Acronyms

AG: Antigravity

ALARA: As Low As Reasonably Achievable

ASTAR: stopping power and range tables for helium ions

BERT: Bertini Cascade Model

BNTR: Bimodal Nuclear Thermal Rocket

CERN: European Organization for Nuclear Research

CHIPS: Chiral Invariant Phase Space

CPU: Central Processing Unit

CREME96: Cosmic Ray Effects on Micro Electronics

DOSTEL: DOSimetry TELescopes

ESA: European Space Agency

GCR: Galactic Cosmic Rays

GDML: Geometry Description Markup Language

GEANT4: toolkit for the simulation of the passage of particles through matter

GNC: Guidance Navigation and Control

GRAS: Geant4 Radiation Analysis for Space

HEP: High Energy Parameterized

ICRU: International Commission on Radiation Units and Measurements

IMF: Interplanetary Magnetic Field

IMLEO: Initial Mass in Low Earth Orbit

INFI: Instituto Natzionale di Fisica Nuclear)

ISP: Specific Impulse

ISS: International Space Station

JPL: Jet Propulsion Laboratory

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LEO: Low Earth Orbit

LEP: Low Energy Parameterized

MPD: Magneto Plasma Dynamics

MULASSIS: MUlti‐LAyered Shielding Simulation Software

NASA: National Aeronautics and Space Administration

NCRP: National Council of Radiation Protection

NEP: Nuclear Electric Propulsion

NERVA: Nuclear Engine Rocket Vehicle Application

NIST: National Institute of Standards and Technology

NTR: Nuclear Thermal Rocket

PSTAR: stopping power and range tables for protons

QGS: Quark Gluon String

S/C: Spacecraft

SPE: Solar Proton Events

SEP: Solar Electric Propulsion

SPENVIS: Space Environment Information System

TEPC: Tissue Equivalent Proportional Counter

TMI: Trans Mars Insertion

TPS: Thermal Protection System

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Chapter 1

1. Introduction

Throughout the years human space exploration has been an important part of the evolution of society, since the beginning of the space programs man and women have had the opportunity to experience life in space as we move into the 21st century. The future holds new mission and difficult challenges that should be addressed in advance in order to provide continuous support to the evolution of space exploration; missions to the Moon and Mars are becoming a reality pushing today’s technological limits one step forward.

Scientist and engineers should study and understand the dangers of the upcoming journeys and explore to the deepest extent the hazardous situation that will make the voyage a safe one. The space environment pose a threat to astronauts, the Mars transit times for a mission will expose the crew to the longest periods for which humans have ever been in space, pushing their bodies and minds to the limit. Different physical issues should be addressed previously to the beginning of these exiting trips, long exposure to zero gravity, radiation, and psychological issues are the most significant ones.

Current propulsion technology imposes a setback to the progress of deep space exploration; while rocket technology needs improvements new development such as

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nuclear propulsion are still under investigation. The most significant work has been done by NASA in the early years of the human space program in which the problematic of using a different propulsion technology (i.e. nuclear) have been addressed; the selection of the propulsion system has a significant impact on the mission changing transit and surface times in Mars making it a crucial and critical decision to be made in the upcoming years.

The piloted Mars interplanetary transfer orbit (Earth‐Mars) will be a high energy transfer orbit utilizing a fast trajectory to limit the exposure of the crew to radiation and zero gravity effects, where same procedure will be applied to the return transfer orbit (Mars‐Earth). The limiting factors for the transfer time are the entry velocities, in which case a decrease in fly time of one of the legs leads to a higher entry velocity. Therefore, tradeoffs between time and performance will have to be taken into consideration in the design phase.

A common concern throughout the literature indicates the necessity of having the crew the least amount of time in space due to the fast deconditioning that the astronauts will suffer during flight. This is why important attention should be paid to the tradeoff during trajectory analysis.

1.1. Objectives

The work done in this thesis tries to addresses the current issues in deep space exploration, as mentioned in the previous section there are many areas, which need review and a deeper understanding before taking this endeavor. Currently, there are many efforts among the scientific community to tackle some of these problems; this is why the work done in this paper intends to explore the effects of radiation exposure to humans inside different structures in deep space conditions.

1.2. Trajectories

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There are two main trajectory types for human mission to Mars, conjunction class and opposition class (figure 1). Both of this trajectories have direct implication on the time

zspent in the red planet upon arrival, the first one, conjunction class, proposes a longer stay in the planet while opposition a longer flight time.

Figure 1: Mars mission trajectories. (Drake, 2007)

According to the NASA Exploration Blue Print Data Book (Drake, 2007), mission times ranges between 365‐661 days for opposition class and 892‐945 days for conjunction class with surface stay times ranging from 30 to 596 days accordingly. Each of these mission architectures has advantages and disadvantages that will be fundamental to the decision of the final mission planning. Some of these differences are expressed in table 1.

Table 1: Opposition and Conjunction class mission characteristics

Opposition (Short Stay) Conjunction (Long Stay)

Transit Vehicle Larger vehicle to The use of advance propulsion accommodate the crew enhances the mission implies larger mass architecture decreasing the requiring advance mass and shortening the trans propulsion for a reasonable mars injection (TMI) mass at Earth orbit

Trajectory Venus Swing‐by, distance Direct from Sun ~0.7 AU

Departure and Larger delta V and large Shorter delta V throughout the Arrival Velocities propulsive requirements mission

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Human Health Long zero‐g space mission Zero‐g gravity exposure similar and longer radiation to ISS experience and less time exposure to the crew. exposure to the radiation Venus Swing‐by increases environment the exposure to radiation

Abort Mission Propulsive abort capabilities Free return abort capability, Trajectory two‐year free return, and three‐year free return.

In addition to the normal consideration for such a trip, abort mission considerations shall be made. There are many options ranging from free return mission to propulsive transfer times that will affect the flight trajectory around Mars. The selection of the abort mission characteristic will depend on the orbit energy and delta v capabilities at Mars and the Earth. Of course the selection of the abort mission will commit the crew to that specific orbit, therefore in‐depth trade studies should be made to provide the best option for the crew in case a problem arises during the Trans Mars Insertion (TMI).

Due to the implication of a human mission to Mars and to the fact that the space transit times between transfer orbits are not very different, a longer surface stay should be intended for the first mission, decreasing the human exposure to the space environment until a full understanding of the implications of long exposure to zero‐g and radiation.

According to ESA and NASA exploration programs 2030 is the decade in which a human mission to Mars will take place. Therefore, according to the conjunction class transfer orbits specific dates will be available for departure during that time. Table 2 shows different opportunities obtained from a NASA Technical Memorandum (Young, 1984). The dates are in Julian date format.

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Table 2: Mars Conjunction Class Mission Opportunities (NASA, 1984)

Mission Earth‐ Leave Arrive Leave Arrive Outbound Mars Inbound Total Mars Earth Mars Mars Earth Trip Time Stopover Trip Mission Year Opposition J.D. (Days) Time Time Time J.D. 2460000 (Days) (Days) (Days) 2460000 (EMOS)

2031 2989 2860 3142 3642 3858 282 500 216 998 (.1107) (.1165) (.0826) (.1202)

2033 3775 3706 3906 4456 4656 200 550 200 950 (.1016) (.1111) (.0996) (.1013)

2035 4584 4508 4712 5242 5512 204 530 270 1004 (.1091) (.0876) (.1202) (.1129)

2037 5382 5290 5646 5986 6276 356 340 290 986 (.1363) (.0947) (.1046) (.0957)

2040 6157 6052 6392 6732 7036 340 340 304 964 (.1193) (.0833) (.0922) (.0968)

2042 6920 6812 7130 7470 7802 318 340 332 990 (.1060) (.0836) (.0870) (.1127)

2044 7688 7568 7874 8214 8564 306 340 360 996 (.1003) (.0940) (.0866) (.1318)

1.3. Propulsion

There are many different propulsion systems that could be potentially developed to perform the transfer trajectory to Mars. The development of new technologies is a requirement for the completion of this mission and it will be an advantage for the crew due to the shorter transit times given by new technologies. Griffin et al (B. Griffin, 2004) provides a quick comparison table (table 3) that gives a rough idea of the advantages and disadvantages of each propulsion system.

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Table 3: Propulsion system comparison (B. Griffin, 2004)

Propulsion Description Advantages Disadvantages Options

Chemical Conventional cryogenic rocket ‐Mature technology ‐Low performance engines. ‐High thrust, short burn times leads to high IMLEO Insulated tanks with vapour‐ ‐Ballistic interplanetary except for cooled shields to reduce boil transfers facilitate conjunction profile off. Start T/W 0.1 to 0.25 implementing artificial gravity with long transfer Isp ~ 460s times ‐Cryogenic with hydrogen, low density, needs heat leak control ‐Expendable system

Chemical/Aeroc Same as chemical except ‐Reduces IMLEO by replacing ‐Performance still apture aerocapture used for MOI. one major manoeuvre with marginal for "hard Large aeroshell needed aerocapture year" opportunities requiring either intact launch ‐Aerocapture risk: or in‐space assembly. Lander TPS/thermal, GN&C may capture separately to ‐Mars Vhp limited to simplify configuration. ~ 6 Km/s for safe aerocapture ‐Expendable system

NTR Nuclear thermal rocket ‐Known technology ‐Nuclear costs and engine, hydrogen propellant, ‐Twice the Isp of chemical risks Isp ~ 900s. propulsion reduces IMLEO ‐Engine test protocols Usually drop tanks utilized for and sensitivity to not resolved (how to each major manoeuvre. opportunity contain radioactive Insulated tanks as ‐High thrust, short burn times products) above; start T/W <= 0.1 to ‐Ballistic interplanetary ‐Cryogenic with transfers facilitate hydrogen, low

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reduce nuclear engine size. implementing artificial gravity density, needs heat leak control ‐Expendable system

SEP Large (multi‐megawatt) solar ‐Known technology with flight ‐Large size may electric propulsion system, experience in small size require more space performs all major ‐High Isp reduces IMLEO and assembly than other manoeuvres. sensitivity options Isp typically 3000s; ‐No hydrogen propellant ‐High‐power electric MPD or comparable thrusters. ‐Reusable system thrusters not mature Achievable power‐to‐ mass ratio may not permit opposition‐ class profiles

NEP Large (multi‐megawatt) ‐Known technology (no space ‐Nuclear costs and nuclear electric propulsion experience or experimental risks system, probably Brayton or prototypes except ‐Large size may liquid metal Rankine power thermoelectric and require more space generation performs all major thermionic conversion) assembly than other manoeuvres. ‐High Isp reduces IMLEO options Isp typically 3000s; ‐No hydrogen propellant ‐High‐power electric MPD or comparable thrusters. ‐Potentially reusable system. thrusters and space configuration power conversion not mature ‐Achievable power‐ to‐mass ratio may not permit opposition‐ class profiles

During the past decade many different propulsion systems have been considered for this mission; each of whom has pros and cons as mentioned in table 3, therefore in order to fulfill all the mission requirements extensive research is needed in this area.

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Nuclear thermal propulsion is a possible solution as pointed out by Borowski in his paper (Borowski, et al., 2001), emphasizes the extensive research which was conducted during the 60’s and 70’s in the NERVA program (Nuclear Engine for Rocket Vehicle Application) leading to a few certain answers. On the other hand, nuclear electric propulsion (NEP) has a large advantage over the NTP since it does not use LH2 propellant, simplifying the issues of storing cryogenic liquids. According to research performed in NTP and in BNTR the amount of propellant (LH2) needed for the entire mission is 50 t (tons), using the data obtained from the NASA Exploration Blueprint Data Book (Drake, 2007) the expected total mission mass for a long stay (conjunction class transfer) in Mars ranges between 400 and 700 t.

Nuclear propulsion shielding provided by (Borowski, et al., 2001) mentions a minimum required 2.84 kg/MWt of reactor power. Therefore for a 335 MW reactor proposed by (Gandini, 2003), the total nuclear reactor shielding is

1.4. Constrains

Table 4 provides final constrains for a typical mission.

Table 4: Mission Constrains

Parameters Constrain Crew size 6 Orbit Transfer Conjunction (Long Stay) Total Mission to Mars (days) 892‐1004 Abort Mission Scenario 2 years free return trajectory Total Mission Mass (mt) 400‐700 % Vehicle Mass 31% Radiation GCR and SPE studies Zero‐Gravity Partial AG with exercise Earth Departure June 29th 2035 Mars Arrival January 19th 2036 Mars Departure July 12th 2037 Earth Arrival March 29th 2038

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1.5. Mars Transit Vehicle

According to the NASA Exploration Blueprint (S. Hoffman, 1997), the transit spacecraft should provide supporting capabilities for a crew of six for up to 200‐day transitions to and from Mars, provide zero‐g countermeasure and deep space radiation protection, return propulsion stage integrated with transit system, and provide return to Earth abort capability for up to 30 hours post Trans Mars Injection (TMI) (Drake, 2007). Table 5 provides a table indicating an ideal mass budget for the transit Martian spacecraft.

Table 5: Mass budget for the transit Mars s/c (Drake, 2007)

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1.6. Physiological Risks

During the mission to Mars astronauts will be exposed to a harsh environment that will affect different areas of their bodies. Long exposure to microgravity will generate bone loss, cardiovascular alterations, muscle atrophy and neurovestibular disturbances (Drake, 2007); while exposure to radiation will generate short term issues (physiological effects) such as cataracts, acute radiation sickness and damages to central nervous system and long term issues such as cancer, hereditary effects and neurological disorders.

In order to get the most out of the crew and to accomplish the mission objectives, the astronauts should be protected against the physiological effects imposed by space exploration. In many of the proposed architectures for the human mission to Mars the leading method for solving the zero‐g issue is the artificial gravity generation; where for radiation mitigation the proposed technique is to use shielding in the s/c structure; the main difficulty with this is to find materials that will provide crew protection for the broad range of environmental effects caused by space radiation.

1.6.1. Artificial Gravity

W. Von Braun and C. Clarke introduced the idea of artificial gravity many years ago; it provides a countermeasure and acts as a mitigation technique addressing a series of physiological risks during human space flight. This device should maintain the crew’s level of physical performance without affecting psychological aspects created by the constant transition between gravity scenarios.

Artificial gravity should provide benefits and eliminate the effects associated with the zero‐g environment using s/c spinning (entirely or partially). Many different investigators indicate that a combination between AG, exercise, and a strict diet is likely to be the optimal mitigation technique for certain health risks faced by the crew (C. Allen, 2003).

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In the NASA Guidelines and Capabilities for Designing Human Missions (NASA, 2003) report some AG design considerations are addressed.

Intermittent AG Continuous AG

Short arm centrifuge: During S/C rotation or crew compartment

this approach crews are exposed rotation: This may affect the to AG during their sleep time or nervous system with the constant

during power exercise times changes of sensor stimulation introducing a number of g induced by the rotation. Coriolis

transitions between the desired forces created by this rotation will level of g and the zero‐g give the feeling of motion; this environment. may cause nausea and vomiting during the entire mission. Also long duration AG might affect how subjects readapt to the Mars‐ Earth environment.

1.6.2. Radiation

Radiation is the greatest risk for the astronauts’ health; during spaceflight the crew will be exposed to two different types of radiation, ionizing and none ionizing. The radiation environment during a mission to the red planet will fluctuate considerably over location and time; this means that a crew will require different protection techniques over the course of their flight. Table 6 shows a table from a NASA case study (C. Allen, 2003) that indicates the different types of radiation, their frequencies and their source; this table helps understand the main issues and provides guidelines for designing countermeasure techniques.

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Table 6: Radiation in the Space Environment. (C. Allen, 2003)

Ionizing radiation is a particular concern among flight physicians since it damages the genome as it travels through the cell’s nucleus. “It also affects the genome by producing free radicals and by transducing signals between adjacent cells within the body.” (C. Allen, 2003)

Currently there are constrains imposed by the National Council on Radiation Protection and Measurements (NCRP) on crew radiation exposure, this are based on past studies of similar earth jobs and LEO astronauts exposure; this should only be use as a guideline for the time being but future consideration and new constrains should be provided.

Non Ionizing radiation

Table 7: Limits of exposure to sunlight in space (C. Allen, 2003)

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Ionizing Radiation

Table 8: Organ Dose limits (C. Allen, 2003)

One of the mitigation techniques proposed by the NASA Radiation Shielding Materials Workshop (NASA , 2000) are hydrogen‐based liquids such as liquid hydrogen and recycled water, these could be implemented in the spacecraft structure to protect the crew against space radiation.

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Chapter 2

2. Space Physics and Radiation 2.1. Space Radiation

The space radiation environment outside Earth’s Magnetosphere consist of Galactic Cosmic Rays (GCRs), Solar Proton Events (SPE) and Solar Fluence. The variation of these phenomenon are directly linked to the solar activity, the Sun’s changes and its cycles pose a direct effect on the flux and time variation of the space radiation environment. Whilst at solar maximum a higher number of sun spots are present and a higher probability of the occurrence of an SPEs the GCRs’ flux decreases accordingly and inversely to the solar wind strength, on the other hand, when the sun is at solar minimum the opposite happens, the decrease of the intensity in the solar wind allows the increase in the GCR’s flux.

2.1.1. The Sun and the Solar Wind

The Sun is a class 2 star at the center of the Solar System which accounts for 99.8 % of its mass; the distance to the Earth is about ~150 million km or 1AU, and like most stars the sun is made of 74 % Hydrogen which accounts for 92% of its volume, 24 % Helium as well as other quantities such as oxygen, carbon, and neon to name a few. The

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solar wind is a continuous flow of ionized plasma from the Sun’s corona, it consist mainly of H with a small percentage of helium. The solar wind is a supersonic flow with speeds around 400 km/s; therefore the plasma travels from the Sun to the Earth in approximately four days, this plasma extend to the Heliosphere decreasing intensity as it gets farther and away from the Sun (ERSMARK, 2006).

Figure 2: Heliospehre and the Interstellar Medium (Eastman, 1990)

Due to the rotation of the Sun the fields form a spiral effect and the plasma flows rapidly away carrying the frozen‐in magnetic field.

Many aspects of the Sun behavior are periodical and follow an 11 year cycle which affects the interplanetary medium, during times of high activity there may be release of energy and matter in the form of solar flares and coronal mass ejections which impose a threat to spacecrafts. A fair indication of sun’s activity is the number of sunspots, which can be observed from earth and are an indication of the phases of the cycle; during solar minimum the Sun’s surface is almost spotless, the number o spots increases reaching the maximum value during solar max. Figure 3 indicates the number of sunspots per cycle.

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Figure 3: Sun Spot numbers for the latest five cycles (2009)

The variation of the solar cycles affects the solar wind and the Interplanetary Magnetic Field (IMF) exerting a large influence to the dynamics of the Solar System making a fundamental effect in any model or simulation.

The data used for the simulations was obtained using the JPL model from SPENVIS (Space Environment Effects and Information System); SPENVIS is a web interactive tool that provides scientists and engineers the required information on space environment, it has the capabilities of generating orbits and allowing the study of radiation effects on the spacecraft (Belgian Institute for Space Aeronomy, European Space Agency, 2009). The data used by the JPL‐91 model consists of continuous records of daily average fluxes, and the model assumes that there were no significant proton fluence events during these periods. The JPL model uses a 7 year period of the active sun in order to consider for the statistical events occurring during that period. Figure 4 shows a plot of the results obtained from SPENVIS

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Figure 4: JPL‐91 Solar Proton Fluence from SPENVIS at near Earth Interplanetary conditions

2.1.2. Galactic Cosmic Rays

The dominant flux of GCRs is believe to come from supernova events, its thought that Fermi acceleration is the cause of particles accelerating near the speed of light. 90% protons, 9% alpha particles, and the remaining 1% of various ions compose the GCR flux. These particles are trapped inside the galactic magnetic field and travel around the galaxy various times before reaching the Solar System. (ERSMARK, 2006)

The GCRs’ energy varies with quantity, more particles are in the lower energy of the spectrum whether high energetic particles have very small flux, and therefore the analysis performed in this investigation was truncated to 102 GeV energies due to the low flux of the remaining particles. Figure 5 shows the data obtained from the CREME96 (Cosmic Ray Effects on Micro‐Electronics) Model for GCRs flux outside Earth's magnetosphere. The CREME96 model allows the creation of numerical representation of

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the radiation environment in near‐Earth orbits making it an important tool for this study (NRL, 2007).

Figure 5: Solar Min GCR flux outside Earth's Magnetosphere

The charge particles within the GCRs are affected by the electromagnetic environment present in the Solar System (IMF); the IMF affects the flux of particles depending of its intensity, for instance when the Sun is in the solar maximum period it produces a much stronger IMF decreasing the GCR particle flux. The opposite happens when the sun is at solar minimum, this modulation is very important for human mission design since affects directly the s/c environment.

2.1.3. Solar Proton Events

Solar Proton Events are related to the solar maximum period and closely associated to the solar‐flare events; protons and ions, which interact with the

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interplanetary medium to travel through the Solar System, compose the particle flux. It is thought that the acceleration of the particles is done by the recombination of the Sun's magnetic field with its surface producing the flux indicated in figure 6.

Figure 6: SPE flux worst day case

The analysis performed in this thesis was completed using the particle flux representing the worst day scenario using CREME96; figure 10 shows the energy spectrum of this event. The main issue when studying the SPEs effects on humans in interplanetary space is that the lack of protection from the earth's magnetosphere, this is an unavoidable problem which has harmful consequences during the entire mission.

2.2. Radioactivity and Radiation Protection

In 1896 Becquerel discovered that some naturally occurring elements were radioactive by observing the blackening of photographic films in the vicinity of Uranium (Martin A., 1996). This radiation process and transformation is called radioactive decay

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that usually is manifested in the form of charged particles and gamma rays. These types of radiation interact with matter producing different effects.

2.2.1. Charged Particles

Alpha and beta particles loose energy by interacting with the electrons in the medium causing them to raise their energy levels (i.e. excitation) or by separate them from the atom (i.e. ionization). A very important effect when the charged particles are slowed rapidly is the emission of X‐rays. (Martin A., 1996)

2.2.2. X and γ Radiation

There are three main processes for which this radiation interacts with matter: photoelectric effect, Compton scattering, and pair‐production (Martin A., 1996). In the first one the total energy is transferred to the electron that is kicked out of the atom. The second one, only a partial amount of the incidence energy is lost during the particle interaction with the electron, then the incidence particles continues with lower energy. The pair production is the resulting case in which gamma rays may interact with an intense electric field producing an electron‐positron interaction (Martin A., 1996).

2.2.3. Neutron

Since the neutron is an uncharged particle it cannot produce ionization, ultimately neutrons transfer their energy to charged particles that will produce ionization.

2.3. Energy Absorption

Absorbed Dose: “is a measure of energy deposition in any medium by any type of ionizing radiation.” (Martin A., 1996) The unit is the gray [Gy] that represents the energy deposited in a volume by the mass of that volume [J/kg]

Dose Equivalent: In order to evaluate the risk of the absorbed dose many characteristics should be taken into consideration such as particle types, species, and

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energy. The dose equivalent (ICRP 92, 2003) can be calculated as the sum of the energy deposition in combination with the radiation quality factor Q.

=()

Where

=1 <10 /0.32−2.2 10, 100/300 >100 / and L is the Linear Energy Transfer in water. (ERSMARK, 2006)

Equivalent Dose: Since biological systems do not react similarly to different particles it is important that these are separated and treated independently depending on the produced damage. For example 0.05 Gy of fast neutron can produce as much damage as 1 Gy of γ radiation (Martin A., 1996); these should be taken into account if it is intended to provide an accurate view of the total biological effective dose. That is why the absorbed dose of each type of radiation is multiplied by a radiation weighting factor (Wr) “which reflects the ability of the particular type of radiation to cause damage” (Martin A., 1996). This value depends on the ionization level caused by the radiation. Table 9 shows the value of wr for the different types of radiation.

Table 9: Radiation weighting factors

wr values Type of radiation wr X‐rays, gamma rays and electron 1 Protons 5 Thermal Neutrons 5 Fast Neutrons 5 to 20 Alpha particles 20

2.4. Radiation Effects

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High energy radiation is harmful to the human body, and there are many different ways to which radiation can enter the human body, the most relevant been the circulatory system, respiratory system and digestive system. Relevant to this research, space radiation poses a harmful environment to the astronauts since this type of radiation has enough energy to ionize water that is the main constituent of cells; this effect will lead to changes in the molecules which will affect the construction and function of different cells. These effects are exposed by humans as medical symptoms such as radiation sickness, cataracts, or cancer in the longer term.

The changes induced by radiation effect can be summarized in two types, Deterministic and Stochastic effects. The deterministic effects are those in which the exposure above a certain threshold energy increase in severity as the dose is increased such as acute radiation effects and cataracts; and the stochastic effects are those which the probability of occurrence is proportional to the dose increase such as cancer and hereditary effects. (Martin A., 1996)

2.4.1. Deterministic

Acute radiation: these effects occur sometime after the exposure to a large dose of radiation in a short period of time. The main effect of the acute radiation is the prevention or delay of the cell division as well as killing the cells inducing depletion in cell population. Depending of the dose received the main effects of this type of radiation are observed in bone marrow, gastrointestinal and neuromuscular damage; the absorption of a dose above 1 Gy gives rise to radiation sickness and its side effects such as nausea and vomiting only a few hours after the exposure. On the other hand, if a dose of 2 Gy or larger is absorbed could lead to dead a few days after the exposure.

Cataracts: the effects of cataracts induced by radiation could develop many years after the dose was absorbed. The main effect is the visual impairment due to the appearance of opacities in the lens eye

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2.4.2. Stochastic Effects

Cancer: The damage to the cell's control system performed by radiation causes the rapid proliferation of cells in a body organ; this is transmitted to the daughter cells creating a population of abnormal bodies damaging the normal cells. The appearance of cancer after a dose exposure could vary between 5 to 30 years and it is not clear yet how does it happens since it cannot be differentiated from other types of cancer. The ICRP estimates that the exposure to radiation of 1 Sv1 would have a 10 % result of cancer in an all age population; these leads to the possibility of estimating fatal cancer risk utilizing the risk factor given by the ICRP (0.05 Sv‐1).

Risk = Risk Factor (Sv‐1) x Dose (Sv)

therefore a 10 mSv dose

Risk= 0.05 Sv‐1 x 0.01 Sv= 5x10‐4

Or saying that 10 mSv radiation dose has a 0.05% chance of fatal cancer.

Hereditary Effects: this effects are created as a result of the damages in the reproductive cells leading to genetic mutations in the genetic material of the cells (Martin A., 1996), the values of the hereditary effects are unknown and the genetic effects could be passed by the irradiation of the male or female with low doses of around 3 x 10‐2 Sv.

Clearly the radiation effects cannot be overlooked when analyzing the possibility of exploring the depths of the solar system, this brief description of the effects and consequences show that large efforts should be placed in the development of radiation protection for astronauts. The development of preventive techniques such as medical or mechanical techniques should be studied before astronauts are exposed to long journeys outside the earth magnetosphere. High energy particles such as the ones included in the

1 Sievert (Sv) it is a radiation unit desined to represent biological radiation effects rather than physical aspects, it is equivalent to 1 J/Kg

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GCRs impose a challenging situation since they are very harmful and could lead to large dose rates absorption by the astronauts. Therefore low atomic number materials in combination to high density aluminum shelters are considered as a possible solution for this problem.

As a consequence of the lack of information there are uncertainties of the possible effects that humans would suffer during this journey, currently, the dose rate limits used by NASA and recommended by the NCRP are based on the 3% lifetime excess risk of cancer mortality, and takes into consideration the age of first exposure and the sex (Stanford, 1999). The career limits depend on age, sex and organ exposed and the values are shown in table 10.

Table 10: Ten year human radiation dose limits (Bernabeu, 2007)

Age at exposure (yr) Female Male 25 0.4 0.7 35 0.6 1 45 0.9 1.5 55 1.7 3

The crewmembers of a mission to Mars will be exposed to ionizing radiation inside the Earth's magnetosphere when traveling through the radiation belts and to Gars during the interplanetary journey. Inevitably, crew members will be exposed to large radiation doses and therefore new regulations should be stated to include interplanetary exploration; in any case, the radiation protection should be made following the ALARA (As Low As Reasonably Practical) concept in order to provide the maximum possible shielding.

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Chapter 3

3. Geometry and Software Development 3.1. GEANT4

“Geant4 is a toolkit for the simulation of the passage of particles through matter. Its areas of application include high energy, nuclear and accelerator physics, as well as studies in medical and space science.” (GEANT4‐CERN, 2009) It is a Monte Carlo simulation particle transport software developed by CERN, ESA, and many universities around the world; it’s a C++ open source software that provides solutions to particle interaction with matter including visual analysis of the geometry. This is the reason why the software has become an important tool in any type of radiation analysis including human radiation protection.

Monte Carlo simulation is a numerical solution based on statistical parameter and random number; it provides the possibility to study the motion of particles in a material calculating the depth of the particle interaction. The depth or traveled distance of the particle is called steps and these can be varied in the simulation. During a simulation the step size should be chosen in order to provide an accurate result without increasing the

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computational time, therefore a trade‐off between times and accuracy is imposed by the particle interaction resolution.

During the simulation the structure geometry was constructed using GDML (Geometry Description Markup Language), an XML software that provide physical properties to the Geant4 software. The computational time imposed to the software is subject to the complexity of the model and the number of materials for which the particle should be studied.

In order to be able to study and model different situations a pre‐set of physics models are included in GEANT4, the different physic lists include electromagnetic and hadronic models with different accuracies and computational requirements. (GEANT4‐ CERN, 2007) This Models are shown in table 11

Table 11: Indicates the different Geant4 physics models (GEANT4‐CERN, 2009)

Geant4 Physical Model Model Name Purpose and Particle Energy Level Chiral Invariant Phase Space Model for stopped particles Up to 151 MeV (CHIPS) and low energy gammas Precompound model for low energy protons 25‐100MeV Bertini and Binary cascade cascade models for particles below 10GeV models Quark Gluon String (QGS) model for particles above 20GeV High Precision Neutron model for low energy neutrons 20MeV Low Energy Parameterized protons 730MeV (LEP) High Energy Parameterized models for high energy 67GeV, 400GeV (HEP) protons

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The simulations performed against the spacecraft models different physics models where applied depending on the type of particle and energy levels:

Table 12: Physics model used for the simulation

Particle Energy Range Physics Model Protons <10 GeV QGSP_BERT_HP and Electromagnetic Standard Protons >10 GeV and <100 GeV LHEP Ions <10 GeV QGSP_BERT

QGSP_BERT: “QGSP is the basic physics list applying the quark gluon string model for high energy interactions of protons, neutrons, pions, and Kaons and nuclei. The high energy interaction creates an exited nucleus, which is passed to the precompound model modeling the nuclear de‐excitation.”

QGSP_BERT: “is like QGSP, but using Geant4 Bertini cascade for primary protons, neutrons, pions and Kaons below ~10GeV. In comparison to experimental data we find improved agreement to data compared to QGSP which uses the low energy parameterized (LEP) model for all particles at these energies. The Bertini model produces more secondary neutrons and protons than the LEP model, yielding a better agreement to experimental data.” (GEANT4‐CERN, 2008)

QGSP_BERT_HP: “This list is similar to QGSP_BERT and in addition uses the data driven high precision neutron package (NeutronHP) to transport neutrons below 20 MeV down to thermal energies.” (GEANT4‐CERN, 2008)

LHEP: “This is the main LHEP based physics list, using exclusively parameterized modeling. In addition to the above, standard electromagnetic physics options are used.

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This physics list is in use in high energy experiments for shower simulations.” (GEANT4‐ CERN, 2008)

The selected physical model where selected in accordance to GEANT4 physics list reference catalog published in (GEANT4‐CERN, 2008)

3.2. GRAS

Gras Is the GENAT4 Radiation Analysis for Space (GRAS) software (Santin, 2005), it’s a 3‐D modeler that provides the opportunity to perform different analysis using the same radiation parameters; the results provided by GRAS are: particle fluence, path length, charge deposition, equivalent dose, total dose, and dose equivalent. The use of the different type analysis provides the user the necessary tools to explore the effects of radiation exposure using different geometrical models as well as space radiation types; in this research space radiation in the form of high energy particles was simulated in a virtual world created with GDML tools.

GRAS provides a versatile User Interface (UI) which can be utilize via command line or by the use of macro files, either way the program allows to change the different parameters of the simulation such as geometrical models, radiation types, physical models and model parameters; the object oriented approach of the system gives the user an easy access to the macro files which contain all the information. In order to provide an accurate simulation results GRAS allows to change or modify the parameter in order to match different models, therefore geometry, physics models, step limits, particle generation, visualization, and results can be modified to match the desired analysis characteristics.

During this research Equivalent Dose Analysis was performed to the ICRU Sphere in order to match physical characteristics of the human body, the software provides a weighting factor for the equivalent dose analysis based on the ICRP 60 (ICRP 60, 1991) and ICRP 92 (ICRP 92, 2003) which can be choose by the user. The model applies the

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weighting factor to the incident primary particle and then this values are propagated to secondary particles created during the interactions, boundaries are selected by the user to indicate the point whether the secondary particles are created or not. Fluence analysis is possible, and as with the other parameters, surfaces should be defined in order for the program to indicate the flux of primary and secondary particles.

Another important feature of the program is the possibility to provide the particle flux vs. energy in list form allowing the possibility to use the outputs from SPENVIS and CREAME96. Figure 7 shows how this feature was used for GCRs protons.

Figure 7: Particle flux and energy provided to GRAS to perform particle analysis

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3.3. Geometry

In order to be able to perform the different simulations and to obtain meaningful results geometries with a certain level of accuracy had to be created, since the results are very much affected by the accuracy and size of the world and geometric models, a trade‐ off had to be made in order to finish the simulation. GRAS allows a series of software products to be use in order to upload the physical objects; in this case, GDML was use for convenience. For all of the simulations a four meter diameter world sphere was created to provide a vacuum environment where the physical models could be placed for simulation purposes.

During the simulation different models and radiation types have been used providing a fundamental ground of comparison between the models.

The main idea of the research conducted is to provide a comparison between different types of material configuration and radiation types in order to get an idea of the dangers and effects to which humans will be subjected during a deep space mission to distant planets. Two different types of models have been constructed, one representing the current ISS‐Columbus model geometry, scaled down to meet the software constrains, and the other one a construction of the TransHab structure proposed by different engineers and scientist at NASA (NASA, 2003).

In order to provide a human body representation inside the different structures an ICRU Sphere detector has been place at the center of the different geometries. The ICRU Sphere is a tissue equivalent representation, it's a 30 cm diameter sphere with density of 1 g/cm3 with a mass composition of 76.2% O, 11.1% C, 10.1% H, 2.6% N, the detector was placed inside the sphere at a depth of 10 mm which is the recommended value for strong penetrating radiation (Shani, 2001).

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GDML was the main application used to provide the geometric model to GRAS and GEANT4, it consist of a file which it uses a specific syntax to construct the physical model, due to the time constrain of the thesis the development of the models took a large amount of time and therefore the models are simple in structure and materials (See Appendix D for details in the geometry file). Both models, the ISS and TransHab are 3‐D cylinder shape geometries with one ICRU sphere placed in the interior of the detector. The particle showers were conducted by GRAS and GEANT4 simulating different scenarios, the GCRs where modeled as a isotropic flow around the world sphere providing meaningful statistical results, a trade‐off had to be made in order to account for CPU running time and performance, to simulate the SPE and the Solar Fluence a directional particle flow was used to shine the entire ICRU sphere during the simulation.

3.3.1. ISS Model

Different layers of aluminum, Kevlar, and Nextel compose the model representing the ISS materials. The characteristics are shown in the following table.

Table 13: Indicates the ISS structure composition

S/C Al Kevlar Nextel Al hull Cylinder 2.57 mm 5.6 mm 5.3 mm 4.8 mm Aft/front 2.57 mm 5.6 mm 5.3 mm 4.8 mm

The main structure values are:

S/C length = 50 cm

S/C Radius = 50 cm

Figure 8 shows the physical model constructed by GDML and represented using GRAS

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Figure 8: ISS structure representation with ICRU sphere

In figure 8 it’s possible to see the different layers of the structure, this model represents a first approximation of the simulation process of deep space exploration, larger models with higher detail and accuracy should be created to provide better simulation results; during the current investigation a simple model was used due to time constrains and computational limitation. Figure 9 shows a representation of the ISS structure with the ICRU sphere placed inside (side removed to see interior), the blue lines represent protons from the GCRs isotropic shower, as this particles strike the surface produce secondary particles if the energy level are high enough or are stopped by the shielding.

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Figure 9: ISS module structure shown with front side removed and GCRs isotropic particle shower

3.3.2. TransHab

The TransHab model has been recommended many times by different authors, scientists and engineers since it provides a simple, reliable, economic as well as advance inflatable structure to accommodate a crew of astronauts during exploration missions. The main structure of the inflatable TransHab is made of low atomic number materials in order to restrict the formation of secondary particles and provide shielding of highly energetic particles. The TransHab structure is still under investigation and current developments are being studied in order to demonstrate its feasibility.

For the simulation, the structure was constructed following the paper by Badhwar et al. (Badhwar, 2001) that has a detailed description of the TransHab structure. Table 14 shows the geometric structure of the TransHab used during this simulation.

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Table 14: Indicates material composition, thickness and quantity for the TransHab structure

TransHab Structure Material Thickness # Plies Total Thickness Kevlar Weave 0.4318 cm 1 0.4318 cm Kevlar fdi 0.0508 cm 10 0.508 cm Polyurethane Foam 7.62 cm 4 30.48 cm Nextel 620 0.112 cm 12 1.344 cm Low Density 0.04242 cm 12 0.509 cm Polyethylene

The different materials are distributed along the structure to provide a homogeneous layout, figure 10 shows how the structure is composed,

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Figure 10: TransHab layer structure composition (Badhwar, 2001)

As it is possible to observe in the figure, many layers and many different materials are used, some of these materials were very difficult to reproduce and therefore not implemented in the simulation, one of them is the Atomic Oxygen protection (AO), another is the Multi Layer Insulation (MLI). Figure 15 shows the 3‐D structure representation done by GDML implemented by GRAS.

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Figure 11: shows the layer structure of the TransHab

Figure 11 clearly shows the different layers implemented for the radiation simulation, it is very difficult to see all the layers since the thickness of some of them are very small, the color coding used in the structure helps identify the different layers; the thick grey layers are the polyurethane foams that represent the thickest part of the structure.

The shielding properties of the two structures will be shown in the results section, it is important to point out that the main structural differences between the ISS and the TransHab are that the second one is lighter and more versatile providing a larger living quarters for the crew and lower mass implications to the launch system and the transfer vehicle. Weight will be an important and significant variable when designing the mission to Mars; this is the reason why the TransHab habitat has been considered for this analysis.

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Chapter 4

4. Software Validation

The software validation is an important step that clearly certifies the accuracy of the results; in this case the validation was performed by comparing the stopping power of different particles, total dose deposition, and a final comparison with GEANT4 MULASSIS tool simulating an astronaut phantom. The different models used by GEANT4 have been previously validated in (GEANT4‐CERN, 2007) and the results are available online.

4.1. Stopping Power

The stopping power validation was conducted by comparing simulation data using GRAS and NIST (National Institute of Standards and Technology) PSTAR and ASTAR web databases that indicate the stopping power for protons and helium ions. ASTAR and PSTAR generate the data from ICRU (International Commission of radiation Units) Report 49. As shown in the next figures, the data from the 3‐D simulation with GRAS are obtained very accurate and similar to the ones from the NIST database.

Protons

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Figure 12: Proton Stopping Power Al 1 MeV to 10000 MeV

Figure 13: Proton Stopping Power H 1 MeV to 10000 MeV

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Figure 14: Proton Stopping Power N 1 MeV to 10000 MeV

Figure 15: Proton Stopping Power CH4 1 MeV to 10000 MeV

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Figure 16: Proton Stopping Power H2O 1 MeV to 10000 MeV

Alpha (Helium Ion)

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Figure 17: Alpha Stopping Power Al 10 MeV to 1000 MeV

Figure 18: Alpha Stopping Power H 10 MeV to 1000 MeV

Figure 19: Alpha Stopping Power N 10 MeV to 1000 MeV

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Figure 20: Alpha Stopping Power CH4 10 MeV to 1000 MeV

Figure 21: Alpha Stopping Power H2O 10 MeV to 1000 MeV

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4.2. Total Dose

Proton

The total dose was calculated using GRAS‐2.3, a simulated proton beam was targeted to a specific material and the total dose was measured. The data was compared with the SPENVIS Multi‐Layered Shielding Simulation (Mulassis) where the targeted material was a slab of 100 mm thickness. The proton beam used was a solar proton data obtained from SPENVIS shown in figure 22.

Figure 22: Solar Proton Fluence

Results

Table 15: Total dose results from GRAS and SPENVIS

Material Dose GRAS [MeV] Dose SPENVIS [MeV] % Diff Aluminum 3.164 +/‐ 0.534 3.1401 +/‐ 0.565 0.76 Water 4.00457 +/‐ 1.051 4.359 +/‐ 1.463 8.13

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4.3. Hadron, Electromagnetic and Ion Validation

For this test different models were used in order to provide accurate validation of the physics scenario, in order to accomplish this, a comparison between MULASIS and GRAS was performed. For the validation, a physical model consisting of two layers of different materials (water, aluminum) were used, and the dose deposition in each layer was calculated.

Table 16: Validation result from Mulassis and GRAS

Material Order Thickness Dose Gras [MeV] Dose Mulassis [MeV] % Difference Al 0 5 mm 3.201 +/‐ 0.0983 3.283 +/‐ 0.1577 2.5

H2O 1 4 mm 1.1991 +/‐ 0.0442 1.1628 +/‐ 0.0388 3.12

4.4. Astronaut Phantom Test

Proton

In order to analyze the dose deposition in a phantom a comparison between Mulassis and GRAS was conducted. The geometry consisted of shielding material (Aluminum and water) and a target (water) simulating an astronaut.

Aluminum Water

Air

Proton Phantom Beam

Figure 23: Dose analysis geometry

The proton beam use was generated by SPENVIS representing solar proton emission at 1 AU, figure 24 represents the proton fluence

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Figure 24: Spenvis solar proton fluence

The results obtained are shown in the following table

Table 17: Results of astronaut phantom test with proton radiation

Dose Gras [MeV] Dose Mulassis [MeV] % Difference Astronaut 168.468 +/‐ 6.82 172.43 +/‐ 6.857 3.96 Phantom

Alpha

For Alpha particles a range of energies representing GCR fluence was use to validate the dose deposition of Helium Ions, the following figure illustrates the alpha particles flux.

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Figure 25: Alpha particles energy distribution

Table 18 shows the phantom dose after the simulation

Table 18: Results of astronaut phantom test with alpha particle interactions

Dose Gras [MeV] Dose Mulassis [MeV] % Difference Astronaut Phantom 0.7521 +/‐ 0.0797 0.71074 +/‐ 0.0784 5.81

4.5. Validation Results

According to the results obtained in the different tests it is important to mention that all the data analyzed was within ten percent difference from the reference data, meaning that the models to be used during this research are within realistic comparable parameters. The analysis conducted against the NIST data was preformed to demonstrate the accuracy level of the GRAS model as shown in figures 12 to 21, while the validation for total dose and the astronaut phantom were intended to demonstrate the capabilities of the software to reach the proposed results. The errors observed in the validation are induced mainly by software effects, while MULASSIS is a 2‐D model of space radiation

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GRAS is a 3‐D model and it operates based on a different structure, nevertheless, these results help to prove the correct operation of the proposed software.

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Chapter 5

5. Simulation of the Interplanetary Radiation Environment

A comparison between the different geometries and radiation characteristics are presented in this chapter, every one of the geometries have been exposed to different radiation types including GCRs, SPEs, and Solar Fluence; a complete analysis of the absorbed dose and secondary particle creation is included. The geometries, the space environment, and the radiation fluxes have been discussed in the previous chapters. Appendix B shows an example of the simulation output.

5.1. Data Normalization

The data used in this section is subjected to normalization due to the characteristics of the software, while the program provides absolute numerical results the statistical character of the simulation should be taken into consideration for every simulation and therefore the normalization is needed. Two different normalization approaches have been considered depending on the radiation source, for GCRs it was developed an isotropic flux inside a sphere normalization while for SPEs and Solar Fluence the cosine law normalization was used.

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The concept of data normalization is that the simulation results (Xr) should be rescaled in order to have a meaningful representation in the real world (Xs). Following (Sullivan, 1971) and (Santin, 2007) in which the following approach was used:

where Nr is the number of real events expected and Ns is the number of simulated events.

The approach used to normalize the data from the Solar Fluence and the SPEs was the cosine law approach; the radiation source implemented for the particle shower was a 30 cm circle at a fix distance from the detector with rays’ perpendicular to the source plane as shown in figure 26.

Figure 26: Representation of radiation SPEs and Solar Fluence flux simulation to the s/c structure (Santin, 2007)

This approach induces lost of signal, since the particle flux points straight into the detector, and the angle of incidence of the particles does not changes, in future simulations, when larger models are implemented, the possibility of placing several detectors inside the s/c structure will help increase the statistical accuracy of the results. On the other hand, the effect of simulating an isotropic flux of GCRs inside the world sphere has a larger impact on the results.

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Figure 27: GCRs Isotropic flux representation

Continuing the analysis from the previous equation, Nr, is the number of real events expected, it is affected by the external flux, source surface, and solid angle; then integrating over the 2π source sphere gives

While the source sphere area is S= 4 π R2 leaving

2 2 Nr=φ 4 π R were φ represents the particle flux.

Using this process of rescaling, the data presented in this section contain values representing real world parameters with the corresponding statistical errors.

Figure 28 show the GCRs particle interaction with the TransHab structure, it is possible to see the creation of secondary particles and the interaction of primaries with the structure shielding.

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Figure 28: GCRs particle flux and particle interaction with the TransHab structure, in the left image it is clearly visible the particle interaction with the structure, the image on the right shows a higher number of events during a simulation. The front side of the structure it’s invisible and does not affect the outcome of the simulation.

5.2. Computational Parameters

The simulation using GRAS and GEANT4 was performed generating primary particles from different sources containing the entire geometric models; with the GCRs isotropic simulation some of the particles miss to hit the detector or even the main structure. The particles, which do interact with the geometric models, are recorded and dose deposition calculated, also secondary particle creation and flux are tacked and presented in histogram form. Primary and secondary particles, which interact with the ICRU sphere, are detected by a 1 mm thick shell placed 10 mm inside the sphere.

A computational tradeoff had to be made due to the length of the simulation times; the high complexity of particle interaction with the models and detector imposed a limit to the extent of this research. Therefore, in order to obtain significant results without affecting the statistical accuracy, the different models have been placed in a relatively

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small virtual spherical world (2m radius) increasing the number of hits per number of events.

This simulation was performed using a MacBook Pro 2.4 GHz Intel Core 2 Duo taking a total amount of more than 700 CPU hours to obtain the final results, including set‐up, data validation, and simulations.

5.3. Average Total Dose

Using GRAS it’s possible to obtain the average total dose absorbed by the ICRU sphere inside the structure, the simulation results allow to identify the radiation effects of each of the different radiation sources used during the simulation, tables 19, 20 and 21 show the average total dose absorbed by the detector inside the ICRU sphere by primaries and secondary particles.

Table 19 a and b compare the different effects of the GCRs to the ICRU sphere inside different structures during very different solar conditions. The radiation effects during solar min conditions are expected to be higher since the flux of particles between 1.5 MeV to 10000 MeV increases considerably as shown in Appendix A. As expected, this effect is observed in the radiation dose effect to the detector inside the ICRU sphere, table 19b shows a noticeable increase in the radiation effects by particles constrained to 10000 MeV such as H and He, on the other hand, the effects of the high energy H is not very significant. This result is significant since it shows that the change of solar cycle has a partial impact on the s/c, this narrows the area of investigation/interest and focuses the attention to the lower part of the energy spectrum. On the first column of both tables it is possible to observe that the radiation effects by low energy hydrogen is significant, while the effects shown in the second column are negligible (high energy hydrogen).

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Table 19 a and b: Total average dose in Solar Max and Solar Min

GCR Average total dose during Solar Max Conditions

H (Z=1) H High energy He (Z=2) Fe(z=26) [µGy/d] Total [µGy/d] [µGy/d) (z=1) [µGy/d] [µGy/d]

ICRU sphere 75507.9 3.35x108 133819.58 819398.52 3.36x108 No Shielding

ISS Shielding 5.77 382.4 7.01 0 395.18

TransHab 152.677 811.073 91.166 0 1054.92

GCR Average total dose during Solar Min Conditions

H (Z=1) H High energy He (Z=2) Fe(z=26) [µGy/d] Total [µGy/d] [µGy/d) (z=1) [µGy/d] [µGy/d]

ISS Shielding 81.61 382.4 75.76.01 0 539.77

TransHab 188.86 817.949 91.166 0 1055.86

Table 20: Total average dose by Solar Fluence

Solar Fluence Average total dose

H (Z=1) [µGy/d)

ICRU sphere No 0.0758 Shielding

ISS Shielding 0.00457

TransHab 0.000125

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Table 21: SPE average total dose effect during worst day case scenario

SPE Average total dose

H (Z=1) [µGy/d) H High energy Total (Z=1) [µGy/d] [µGy/d]

ICRU sphere 0.148 1060.41 1060.56 No Shielding

ISS Shielding 0.002413 1322.92 1322.92

TransHab 0.000111 1649.1 1649.1

The simulation results, as observed in the previous tables, shows the effect by which the ICRU sphere detector is affected due to different types of radiation, all of the results provide a comparison between an isolated/unprotected ICRU sphere (without shielding), and a ICRU sphere inside the ISS module and the TransHab module. It is easy to see the decrease of the radiation effects to the detector by the radiation shower when it is placed inside the shielding material. Another interesting result is that the ISS type structure provides a higher radiation protection to the detector than the TransHab structure, the reason for this is the use of non‐metal materials in the lighter structure, while this may seem as a disadvantage in this analysis, the TransHab structure should be lighter and easy to transport than the metal structures. These results propose a tradeoff between the selections of the transport vehicle for the crew during long duration space missions.

Table 18 shows the results obtained by Ersmark et al and the radiation environment detected by DOSTEL and TEPC during solar max condition. The simulation results by (Ersmark, 2006) and the dose equivalent values observed by DOSTEL and TEPC (radiation dosimeters inside the Columbus module) shown in table 22 include all the ions present in the GCRs flux. On the work done for this thesis H, He and Fe are the only ones

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included; this has a direct effect on the results and might explain some of the discrepancies.

Table 22: ISS Columbus module radiation environment during Solar Max inside Earth Magnetosphere (Ersmark, 2006)

GCR Model µGy/d µSv/d

CREME96 63 123

SIREST 99 179

Dosimeter

DOSTEL 92 409

TEPC 91 337

According to the results obtained during the development of this work using CREME96 GCRs fluxes in conjunction with GRAS, the following results were obtained (figure 29)

Figure 29: Average dose equivalent for ISS and TransHab models outside Earth’s Magnetosphere using CREME96 GCRs’ model.

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Analyzing the actual facts obtained by the dosimeters, the simulation of the Columbus module (Eastman, 1990) and the results obtained in the current simulation, it is clear that the equivalent dose outside the magnetosphere is much higher than those obtained by the other simulations. The numerical value of this simulation are similar to the ones obtained by the dosimeters inside the Columbus Module, which is orbiting at LEO; this fundamental discrepancy between real values and the simulation can be interpreted as an elemental difference induce by the difficulties to simulate the space radiation environment as well as the accuracy of the models.

As mentioned before in this report, the GEANT4 physics models only permit H simulations to values of 100 GeV, limiting the possibility of using other elements up to such energy values; another issue is that the values obtained during the current simulation only utilize three types of species (H, He, Fe) during the GCRs radiation analysis having a direct effect on the outcome of the results. Therefore, comparing the real values obtained in the thesis it is certain to expected radiation levels in deep space mission of much higher values than the ones obtained by the current simulation.

According to the results obtained and shown in figure 30, high SPEs will have very important effects on the crew and should be taken into consideration during the analysis of this type of space missions, inducing a large impact on the safety of the crew. Clearly, as shown in the figure 30, the creation of secondary particles by the interaction of high energy protons and the shielding material will have a negative impact on the crew of astronauts. Therefore, a special shelter area with heavy shielding of about 20‐30 g/cm2 (Spillantini, 2007) is needed to reduce these effects.

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Figure 30: SPEs effects on ICRU sphere Dose Equivalent values

5.4. GCR Particle Fluence and Energy Spectra

The isotropic flux of the GCRs interacts with the s/c structure and with the ICRU detector; some of the particles strike the spacecraft, some of them penetrate and hit the detector and some others don’t even interact with the model. This isotropic flux effect intends to represents the real case in which a small number of GCRs actually have a real effect on the target. According to the simulation, a very small number of particles actually strike the detector inside the ICRU sphere, the flux of particles inside the s/c depend on the shielding and thickness of the material; this is why the ISS and TransHab models provide different results. Figure 31 shows the particle fluence in perspective to primary particle flux inside the ISS and TransHab models.

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Figure 31a: GCRs particle fluence inside the ISS model that strikes the detector

Figure 31b: GCRs particle fluence inside the TransHab model that strikes the detector

Figures 31a and 31b show that the higher flux inside the structure corresponds to the high energy H model (10GeV < H < 100 GeV), in both cases the effect of gamma radiation represents the maximum flux into the detector, while the other have less effect.

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It is clear that the TransHab structure allows a much larger amount of particles and radiation inside the structure having a negative effect on the crew. The effect which induces the flux increase in the TransHab structure is the gamma radiation, while alpha, electron, neutron and protons have a relative smaller increase; this is a positive result since, as shown in table 9, the later ones have a much higher effect on human tissue than the gamma radiation.

The particle flux inside each model has a particular energy spectrum that depends on the radiation type; figure 32 and figure 33 show the data obtained in the simulation which illustrate the energy distribution inside each one of the s/c.

Figure 32: H energy spectra entering the ISS model during Solar Max

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Figure 33: H energy spectra entering the TransHab model during Solar Max

These two figures clearly indicate that each of the species have a different effects on the target and emphasizes the importance of a well developed shielding structure in order to protect the crew. Both figures have the same outline, the GCR Z1 line indicates the GCR particle flux from CREME96 truncated between the energies of 10 MeV and 10000 MeV, electron, gamma, neutron, and protons are identified as the secondary particles with energy and flux inside the model.

A clear observation that can be made directly from the figures is the low or inexistent flux of protons at lower energies; this indicates that the lower energy protons are stopped by the structure. The stopping effect has a direct outcome that is the creation of secondary particles, this is why at lower energies, when more protons are stopped higher number of electron, neutron, and gamma radiation are present inside the structure. As

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the energy increases a decrease of secondary particles can be observed and a slight increase of protons becomes more dominant on the energy spectrum inside the environment. This indicates the necessity of providing material with a low secondary particle creation; even more important is that the secondary particles that are created should be x‐rays, γ‐rays, and electrons which have a lower effect on the human tissue.

5.5. Equivalent Dose

As mentioned before the biological damage due to radiation depends on the different radiation types, therefore a 0.05Gy of fast neutrons can do as much biological damage as 1 Gy of gamma radiation (Martin A., 1996). Extending this idea, I have tried to investigate the equivalent dose effects of the simulated particle shower during solar max and solar minimum condition inside the main models.

Figure 34: GCR and SPE equivalent dose effects in human tissue inside the different modules. Light blue represents solar minimum conditions

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This comparison (figure 34) indicates the real effect that GCRs and SPEs have in humans (i.e. dose deposition), the figure on the left shows the outcome of GCRs during solar maximum and solar minimum, even though that the effect inside the ISS are about 30 percent less than inside the TransHab, in both cases the effects during solar minimum are double of those of solar maximum condition imposing a real danger to the crew and an operational limitation that should be taken into consideration when the journey to Mars is developed.

The SPE worst day effects are also very dangerous and at large, it affects the crew of astronauts during the journey, and since the SPEs are largely made of protons a high density shelter for emergency situations would reduce the effect of such an event. Therefore, when the spacecraft is designed, either a higher shielded area should be created for the living quarters of the crew or an emergency shelter used with similar capabilities to what the International Space Station crew currently utilizes. The main issue with the high energy GCRs is that there is no feasible or realistic shielding that could stop the damaging effect which they carry.

The results obtained in this simulation have larger values than the current recommended dose limits to radiation workers by the ICRP, demonstrating the need of new regulation that include the future of space exploration. Risks and conscience should be raised among the crews regarding this matter and efforts should be carried in order to implement new technologies to protect and increase the safety of the future deep space missions.

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Chapter 6

6. Conclusion

GRAS‐GEANT4 simulations of the proton, He, and Fe induced radiation inside the ISS/TransHab models have been calculated; this involves cosmic rays, solar proton events, and solar fluence. The effects of different particles under different circumstances of solar condition have been carried out to observed and understand radiation effects upon different models and conditions; the results obtained will help determine and serve as a starting point for future studies.

GEANT4 particle transport toolkit was used to simulate the effects of radiation inside heavily shielded spacecraft; the Bertini Cascade model (BERT) was implemented to simulate these conditions, the data validation was performed before starting the simulation process in combination with previous published data provided by the GEANT4 team. In this work, ISS and TransHab models were developed according to current information gathered from relevant publications mentioned in previous chapters. The initial approximation (simple model) used during this simulation intends to set a starting point for future, more detailed and realistic simulations incorporating user friendly design software to allow the development of more accurate environmental models. It is

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important to mention that the geometries used are an approximation of the outside structure of the spacecrafts and no interior materials or structure have been included, allowing a larger flux of particles into the detector. Also, none of the external structure of the spacecraft structure has been included, e.g. fuel tanks, solar panels, or propulsion mechanism which eventually will play an important role into the radiation analysis of the mission.

Galactic cosmic rays total dose and dose equivalent rates have been calculated (using GRAS) and compared to experimental data during solar maximum condition. The results obtained from the simulations have been found to be larger than the values observed from other simulations and experiments inside the Earth’s magnetosphere. These results coincide with the expected results and indicate the necessity to extend this research into a higher accuracy simulation to be able to provide realistic and comparable results for future space missions. The calculated dose rate from GCRs at solar max inside the ISS model is 395.18 µGy/d, while inside the TransHab structure is 1054 µGy/d. While SPEs worst day case effects inside the ISS is 1322 µGy/d, and is 1649.1 µGy/d inside the TransHab model. These results are comparable with previous simulations and demonstrated a correlation with respect of the previous differences between experimental and modeled data presented by (Ersmark, 2006). The discrepancy which can be found and can be expected during the simulation are due to model inaccuracies as well as the physics model energy limitation and the computational power restriction during this work.

Therefore, the results from this simulation do not represent an accurate model for radiation deposition and particle modeling for deep space missions, but it represents a good model to analyze the effect of high energy particles on different material and gives a first degree approximation to the radiation effects on humans. The observed results during the analysis indicate that the inflatable TransHab structure may not provide enough shielding to the crew as is and extra shielding should be expected. On the other

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hand the ISS model represents a better shielding option but a robust and inconvenient structure for human exploration; therefore a tradeoff between the two models should be developed in order to increase the safety of the astronauts.

The high number of secondary particle produced by the shielding material is an unavoidable effect of radiation with large negative effects inside the s/c environment largely increasing the equivalent dose effect on humans. The effects carried out by primary and secondary radiation effects may limit the exploration time and Earth‐Mars transition times imposing a large limitation to current regulations. Therefore for long mission outside the Earth magnetosphere higher radiation exposure may be unavoidable due to longer exposure to GCRs and potential SPEs with the need for new regulations and continuous research to predict and tackle the effects before embarking a crew into the journey.

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Chapter 7

7. Recommendations

This section will try to provide an understanding of the limitations of the simulation as well as what could be done to improve the results if the research is continued.

7.1. Limitations

Due to the characteristics of the Monte Carlo simulation and the physics models used to study the effects of radiation in the ICRU sphere the length of each simulation became extensive. The computer used for the simulations did not provide enough computational power to shorten the simulation time and increase the number of events; this affected the accuracy of the simulation and increased the error margins. Evidently this imposes a setback to the work done and should be addressed in future simulations of the same kind.

The GDML software was used to develop the different spacecraft geometries, this software was developed by CERN as an interface to link GEANT4 and the desired geometries; and the software is very powerful but slow and time consuming. Other applications should be used as an interface between more popular 3‐D drawing software's

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(CATIA, PRO‐E, AUTOCAD, etc.) with GEANT4; this will enable the possibility to create larger and highly accurate models. Due to the extensive time it took to develop simple approximation models there was no room to develop more detailed structures affecting the initial objectives of this research.

The space environment for which the s/c operation will be conducted it is non‐ isotropic and very difficult to model; the software implemented during this thesis only simulates a limited part of the space environment. The particle physics models are not available above energies of 10 GeV for most of the particles except for protons, which can be modeled up to 100 GeV. According to space physics observations, most of the radiation which will affect the spacecraft during the Earth‐Mars or Mars‐Earth journey will have energies between these values; obviously, this imposes a small setback on the work and carries on a side effect that increases the uncertainty of the results. More efforts should be place in developing high energy models in order to accommodate most of the particles to provide a higher accuracy model for space applications outside the earth's magnetosphere.

It is important to recognize these limitations in early stages of the research process since the effects will affect the entire procedure. These effects only affect negatively the results inducing unwanted errors and increasing the time of the simulations.

7.2. Further Work

The limitations presented in the previous section have a direct implication on the results obtained during this work; some of these effects can be addressed if the proper equipment/software is used. In order to obtain better statistical results of the particle flux, especially the GCRs, multi processor computers could be included. Cranfield university has an outstanding “super computer” that could be implemented in this research in order to accelerate simulation times increasing the number of simulated events. This will specially allow a better understanding of the particles interaction with the spacecraft structure.

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As indicated in the limitation’s section the implementation of complex spacecraft structures could have positive effects of the outcome of the research; in order to do this, better software should be implemented to develop the spacecraft structure. As a suggestion, CATIA or similar software could be employed to construct the structure with higher level of detail; the prototype could be exported as a STEP file and then converted to a standard form (such as GDML) using the help of the software developed by StepTools (Step Tools, 2009). Other software which could be utilized to create accurate models is FASTRAD (FASTRAD, 2009); this software was developed to link GDML software with CATIA style tools. Appendix D shows the GDML file for the TransHab structure.

The effects expected on humans are a very important outcome for this work; therefore the use of many detectors such as the ICRU spheres will be very useful. On the other hand, other models of humans could be eventually implemented in order to provide a realistic example of the different radiation effects. The work done by INFN (Instituto Nazionale di Fisica Nucleare) in developing a human like structures will be of great help in future studies, this module will provide the opportunity to study the radiation effects in different organs as well as the identification of which are the most affected areas in the human body by space radiation.

Studies of the structure combination between TransHab and high density aluminum shielding should be investigated in greater depth since it has been demonstrated to be the most flexible approach to future space structures. Further studies should demonstrate the feasibility of using this combination considering the difficulty of in‐situ testing of such a model. This is one of the reasons why accurate model of the space environment are needed, GEANT4 and GRAS are trying to address the main issues but much more effort should be placed in the constant evolution of the different software to include and provide a realistic approach to certain radiation environments.

As mentioned in the conclusion, this work does not have the means to represent actual events of space physics; it can be employed as a tool to interpret the space

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radiation effects in different condition as well as the study of the effects of the radiation environment on different structures. With the implementation of these previous recommendations the work conducted in future simulation could introduce new variables in the study of space physics, especially in the area of deep space exploration. More resources should be given to this area of research as well as more time dedicated to the development of space models in order to support the goals set by scientist and engineers to provide a safe environment for future space missions.

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Appendix A

The figure in the top shows the particle flux during solar max and the one in the bottom show the particle flux during solar minimum. It is possible to see the difference in the flux quantity in the regions between 1 MeV to 104 MeV. In the area above 104 MeV the slope of the particle flux is similar and this is the reason why it has a lower effect on the radiation analysis when the data is compared.

Figure A1: Solar Max particle flux outside Earth´s magnetosphere from CREME96

Figure A2: Solar Min particle flux outside Earth´s magnetosphere from CREME96