UNIVERSITY OF CINCINNATI
Date: 30-Sep-2010
I, Brandon Michael De Graaf , hereby submit this original work as part of the requirements for the degree of: Master of Science in Nuclear & Radiological Engineering It is entitled: Determining the Effect of Shielding for an Eye Exposed to Secondary
Particles Produced by Galactic Cosmic Rays using MCNPX Modeling
Student Signature: Brandon Michael De Graaf
This work and its defense approved by: Committee Chair: Henry Spitz, PhD Henry Spitz, PhD
10/29/2010 1,124 Determining the Effect of Shielding for an Eye Exposed to Secondary Particles Produced by Galactic Cosmic Rays using MCNPX Modeling
A thesis submitted to the Graduate School of the
University of Cincinnati
in partial fulfillment of the requirements for the degree of
Masters of Science in Nuclear Engineering
of the College of Engineering and Applied Science
by
Brandon de Graaf
B.S. Mechanical Engineering
October 2010
Committee Chair: Henry Spitz, Ph. D.
Committee Members: Samuel Glover, Ph. D; Bingjing Su, Ph. D; Thomas Huston, Ph. D. Abstract
This thesis sets out to analyze the use of shielding to protect the lens of the eye from secondary particles produced by Galactic Cosmic Rays interacting with the structure of spacecrafts. The specific aim is to develop a model that will utilize a shield to reduce frontal dosage to an astronaut’s lens by 20%. In the MCNPX model, a proton beam will pass through an aluminum structure to create the secondary particles. Dosage to the lens will then be determined after the various shielding materials are placed between the eye and the source.
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This Page is Intentionally Left Blank
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Preface/Acknowledgement
I want to thank the nuclear engineering staff at the University of Cincinnati, including Dr. Spitz, Dr. Su, and Dr. Glover, for their constant support and guidance throughout my undergraduate and graduate career. Without them this degree would not have been possible.
I also wanted to thank Dr. Maldonado for introducing me to the field and inspiring my growth as a nuclear engineer.
Lastly, I am dedicating this thesis in memory of Dr. Christenson. His passion for nuclear engineering resonated in all of his students and I am lucky to have enjoyed my time with him as my professor.
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1. Table of Contents
Page
Abstract iii
Preface/Acknowledgement v
List of Figures viii
List of Tables ix
Chapter
1. Introduction 1
Galactic Environment 1 Radiation Fundamentals 3 Radiation Protection 5 Effects on the Eye 6
2. Experiment 9
Methodology 9 MCNPX 11 Model Description 11 Shield Materials 14
3. Results 17
Secondary Particles Produced after Aluminum 17 Flux to the Eye Lens 17 Dose in the Eye Lens 18 Dose Reduction 19
4. Discussion 20
Breakdown of Results 20 Best Shield 21 Explanation for Higher Flux after Shield Materials 23 Benchmarking/ Realistic Results 23 Front Exposure vs. Back Exposure 24 Importance of the Results 25 Recommendations for Future Work 25
5. Conclusion 26
vi
Bibliography 27
Appendices
Appendix A: GCR Spectrum 29
Appendix B: Energy Spectrum of Secondary Particles after the Aluminum Wall 30
Appendix C: Front vs. Back Exposure 32
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List of Figures
Figure Page
1 The Three principal sources of space radiation 1
2 Production of secondary particles in aluminum 4
3 Dimensions (in millimeters) of the human eye 7
4 GCR spectra at the 1997 Solar Minimum and 1990 Solar Maximum 10
5 Diagram of geometry for MCNPX model 11
6 MCNPX model with eye blow up 12
7 Flux in the Eye Lens by secondary particle 17
8 Dose to the Eye Lens by secondary particle 18
9 MCNPX model of exposure from a beam through the back of the head 24
10 1997 GCR Spectrum used in MCNPX model 29
11a Energy Spectra by particle 30
11b Energy Spectra by particle 31
11 Front vs. Back Dose to the Eye Lens by secondary particle 32
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List of Tables
Table Page
1 Non-Shield Material Description 13
2 Shield Material Description 14
3 Thicknesses Required for 500 MeV Electron and 10000 MeV Proton
– Selected Materials 15
4 Thicknesses Required for 500 MeV Electron and 10000 MeV Proton
– Unselected Materials 16
5 Secondary Particles Produced after Aluminum Block 17
6 Lens Flux in particle/cm^2/proton After Each Shield Type 18
7 Lens Dose in MeV/g/proton After Each Shield Type 19
8 % Change in Total Dose as Compared to the No Shield Case 19
9 Dose in MeV/g/cm^2/month 22
10 Lens Dose in MeV/g/proton for Front and Back Exposure 32
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1. Introduction
1.1 Galactic Environment
The galactic environment consists of two main sources: 1) Galactic Cosmic Rays (GCR) and 2)
Solar Particle Events (SPE). Both sources include a dominant contribution of protons along with
high energy (E) and high atomic number (Z) particles often referred to as HZEs (Nelson 2003).
The separation between the two involves the energy spectrum and the origin of the sources; GCR originate from the galaxy, while SPE originate from our Sun. A third source is also found in
Low Earth Orbit (LEO) and involves trapped electrons and protons in the Earth’s geomagnetic field, also known as Earth’s radiation belts.
Fig 1: The three principal sources of space radiation: (1) galactic cosmic rays, (2) solar particle events, (3) trapped radiation in the Earth’s Radiation Belts. All three sources are affected by the Earth’s magnetic field. (E.R Benton, E.V. Benton 2001)
Galactic Cosmic Rays can be thought of as the background radiation in space, as they are constantly being imposed on the spacecraft. The particles come from unknown sources in the galaxy, but its constituents are known and include 87% protons, 12% helium ion, and 1% heavier
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ions. The intensity of GCR varies with the 11-year solar cycle. The solar cycle refers to the
change in solar activity over the course of time. Solar winds attenuate GCR, meaning energy is
removed through interactions. These solar winds, which are created from highly ionized gas
emitted by the sun, are strongest during maximum solar activity. Thus GCR are weakest during
solar maximums and strongest during solar minimums. Additionally, GCR are strongest near the
Earth’s Poles. This is because GCR consists of charged particles and the magnetic attraction
concentrates these particles near Earth’s poles. Current shielding techniques involve adding
more equipment or structural material to the areas used most by astronauts. However, radiation
shielding is usually considered at a later part in the spacecraft design, using less than optimum
ways (Wilson et al. 2001). This means full protection from these particles is impractical due to
large amount of material required, and thus large amount of weight. Therefore current research
involves overhauling the design thought process and including lighter materials to protect
astronauts.
Solar Particle Events refer to particles released during rare but intense solar flares (E.R. Benton,
E.V. Benton 2001). The particles released consist mostly of protons, with a small contribution from helium ions and heavier particles (NCRP 98 1994). Solar flares are infrequent and unpredictable, but occur during the active period of the solar cycle. Yet, not every solar flare produces particles of radiation concern, thus planning for radiation protection from this source is not an exact science. Current techniques involve moving astronauts to a more shielded area in
the spacecraft when the SPE occur. These areas may be designed specifically with extra
shielding, or involve rooms where there is enough equipment to act as a shield from the particles.
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However, beyond these techniques it would be impractical to build a vessel with enough shielding for an event that might occur and only temporarily lasts.
Trapped particles are charged particles that cannot escape the Earth’s magnetic field due to the intensity of the attraction. These trapped particles form a belt around the earth known as Earths radiation belt, also sometimes called the Van Allen belt. The belt consists of two distinct regions known as the inner and outer belts. The inner belt extends to about 2.8 times the Earth’s radius, while the outer extends from 2.8 to 12 times the Earth’s radii (NCRP 98 1994). These belts are not uniformly distributed and can change intensities with solar winds. In addition, intensity of the outer belt is much greater than the intensity of the inner. For radiation protection concerns, protons are the dominant charged particles. The major control to prevent doses is adjusting the vessel altitude. Additionally, mission plans will avoid high intensity areas in the belts and will alert astronauts if the vessel is about to temporarily enter a high intense region.
For intergalactic travel GCR will account for the largest contribution of dose since it is a constant source for the astronauts. In addition, approximately half of the expected exposures on the
International Space Station will be from GCR (Wilson et al. 2001). While SPE can cause more damage, they are infrequent and current shielding techniques will suffice. Therefore, this thesis will focus only on GCR and its respective secondary particles.
Radiation Fundamentals
Heavy charged particles, such as protons, lose energy by interacting with materials. The primary form of interaction occurs through the Coulomb force between the charged particles and the
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atomic electrons or nuclei. This interaction removes tightly bound electrons from their orbits,
causing the atoms to become charged or ionized. Another form of interaction is called a nuclear reaction, which occurs through the nuclear force with the nuclei. In this interaction, a charged particle will cause a neutron to be ejected from the nuclei. Generally, the probability of this reaction is low; however in the case of high-energy particles, such as those found in GCR, the reaction becomes more probable.
When the charged protons from GCR interact with the aluminum space vessel wall, secondary particles are created and pass into the environment inside the space vessel. This process can be referred to as target fragmentation. Depending on the kinetic energy of the primary charged particles, the nuclear interaction can follow a number of different channels, producing two or more secondary particles (E.R Benton, E.V. Benton 2001). Furthermore, when charged protons interact with the human body, ionizations cause the biological changes in cells. These changed cells will either die due to a significant change, or remain intact as “damaged cells”.
P+ Al + P
n
e-
Fig 2: Production of secondary particles in aluminum
- 4 -
Absorbed dose is one measure for radiation and refers to the amount of energy deposited per unit mass. The unit of absorbed dose is MeV per gram, which is referred to as a Gray (Gy). With each interaction that occurs between the radioactive particle and target, energy is removed and deposited in the target. In terms of biological damage, a Gy from one radioactive particle, such as a proton, does not produce the same damage as another radioactive particle, such as an electron. For this reason, weighting factors can be applied to calculate what is called an equivalent dose. In this study, all values are reported as absorbed dose, therefore no weighting factors were applied to the results.
Placing a thick material between a radioactive source and a human being is one of the techniques used to reduce the amount of absorbed dose received. This technique is called shielding and it implies an alteration of the radiations through interactions with intervening materials by which the intensity of the radiations is decreased (Wilson et al. 2001). A good shield is characterized as a material that will maximize the number of electrons per unit mass, maximize the nuclear reaction cross sections per unit mass, and minimize the production of secondary particles.
1.2 Radiation Protection
Radiation Protection has always been a concern for NASA, however the relatively recent trend towards long-term missions has now started a research initiative to ensure the safety of the astronauts. Such missions, as living on the International Space Station and future human exploration of other planets, are dependent on said research. However, protecting astronauts involves many uncertainties, including: a complete definition of the space radiation environment, predicting SPE events, and an understanding of material interactions with such high-energy
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particles. However, the goal of radiation protection in space is to eventually enable long-term, or
even permanent, human presence in space without incurring unacceptable health risks due to the
unavoidable exposure to ionizing space radiation (Schimmerling 2003).
Three critical organs are the focus of current NASA radiation protection initiatives, including the
skin, ocular lens, and blood forming organs (Tripathi et al. 2001). The limits set for career
exposure to these organs are based on not increasing the actuarial probability of cancer by more
than 3% above current probabilities for unexposed populations. However, present exposure
limits only involve the LEO atmosphere and no limits have been set for deep space exploration
(Tripathi et al. 2001). The National Council on Radiation Protection (NCRP) has started the
initiative to develop these limits.
With respect to GCR, the most important biological effect is the mutation and transformation of cells to the point at which self-repair does not occur. However, data on these effects for humans
is incomplete due to the lack of actual human exposure to such high-energy particles. The most
relevant epidemiological data on human exposure comes from World War II and the atomic
bombs. Certain ground based research facilities are able to reproduce the high-energy particles
locate in space by the use of a proton beam, but no human tests have been conducted due to the
risks of exposure. Additionally, human exposures have been tracked using instruments worn by
astronauts and a handful of space experiments have been performed, but the low number of
samples does not provide the statistical robustness required to confidently define the risks. This
being said, there is still ground that needs covered and to truly protect astronauts much more
research needs to be performed.
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1.3 Effects on the Eye
The main biological concern for radiation interacting in the eye involves cataracts in the lens. A cataract is defined as an abnormality of the eye, characterized by opacity of the lens. When radiation damages cells within the lens, those cells remain in the eye. These damaged cells will then form opaqueness in the lens, thus causing cataracts. Radiation effects do not necessarily cause blindness, except in extremely high doses. Blindness usually occurs due to the physical damage of other components in the eye. However, these components are insensitive to the radiation effects and therefore not a concern for radiation protection initiatives.
Fig 3: Dimensions (in millimeters) of the human eye,
reproduced from Worgul, 1982. (NCRP 98 1994)
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The cause of opacity in the lens has been researched, but no definitive reason has been found. In a healthy human eye, new fiber cells are constantly manufactured to replace old ones (Blinding
Flashes 2004). Fiber cells are the transparent cells that allow light to pass through the eye and be interpreted by the brain as images. It is these cells that become opaque, however this effect might occur years after the actual exposure. Current research has shown that radiation upsets the process of creating the fiber cells, however because of the time delay, it difficult to pinpoint the actual cause (Blinding Flashes 2004).
NASA has been performing research related to astronaut lens exposures and cataracts, studying all astronauts in the program. The study shows epidemiological data linking an increased risk of cataracts for astronauts with higher lens doses (>8 mSv) of space radiation relative to those who have had lower doses (Cucinotta et al. 2001). In fact, at least 48 astronauts have suffered from cataracts, of which 35 participated in high dose missions.
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2. Experiment
2.1 Methodology
The approach of this experiment is to see if glasses worn by astronauts can reduce dose, thus minimizing shielding materials and cost. Throughout this experiment, the practicality of the shield will be acknowledged, including lens thickness and clarity. The study is also focusing on the environment inside the space vessel due to the fact that current techniques involve adding shielding to the outside of the vessel.
This model is not designed to recreate the environment in the spacecraft or even recreate the dose experienced by astronauts. Instead, the model is designed to test the change in absorbed dose between an unshielded and shielded case. The goal is to see if astronauts would have dose to their lens reduced by wearing glasses. In this regard, the most simplified worst-case scenario is a beam of particles aimed directly at the front of the eye. Since GCR consist mostly of protons, the source will be modeled as a proton beam. The energy and spectrum will be based on average GCR activity during the 1997 solar minimum. As stated before, GCR are strongest during a solar minimum, therefore it is conservative to use data during this period. The following figure shows the spectrum used in the model:
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Fig 4: GCR spectra at the 1997 Solar Minimum (full lines)
and 1990 Solar Maximum (dashed lines) according
to Badhwar et al.
The model of the eye is kept simple. The outer eye, or sclera, the vitreous humor, and the lens will be modeled separately as each has their own unique composition and density. Other components of the eye are radio-insensitive and not a concern in this study; therefore will not be modeled. A simple spherical model will be used for the eye, while an ellipse will be used for the lens.
Secondary particles are created by passing the proton beam through an aluminum section that represents the vessel wall. The expected secondary particles include secondary protons, neutrons, pions, heavy particles, and gamma rays. However, only neutrons, photon, electrons and protons will be tracked in the model.
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2.2 MCNPX
MCNPX is a general-purpose Monte Carlo radiation transport code designed to track many
particle types over broad ranges of energies (Pelowitz 2008). The title MCNPX stands for
Monte Carlo N-Particle eXtended. The code was first released in 1997, but the development
began as early as 1994 through an extension of the MCNP4B and LAHET 2.8 codes. This
transport code is controlled by Los Alamos, who has been developing Monte Carlo transport codes for nearly sixty years (Pelowitz 2008). Models in MCNPX are three dimensional and time dependent. The code also utilizes the latest cross-section libraries or uses physics models when particle energies are above the maximum energies in the cross-section libraries, making the code truly versatile.
2.3 Model Description
A simple diagram of the geometry is provided below:
Source Vessel Wall Shield Eye (protons)
5cm x 5cm 5cm x 5cm 2.415cm 2.365cm x 6 g/cm^3 (2.22cm) x 0.3cm 1cm x 0.42cm
Fig 5: Diagram of geometry for MCNPX model
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Fig 6: MCNPX model with eye blow up
The proton beam is located on the left side of the aluminum block at its center. All of the protons are concentrated into a zero radius beam forcing the particles to start from the same position. This position is set directly in line and perpendicular to the lens of the eye, providing the best probability for all particles to enter the lens.
The aluminum block representing the vessel wall is a rectangle 5 cm wide by 5 cm tall and has a density thickness 6 g/cm2, which correlates to a thickness of 2.22 cm. The typical aluminum structure in the space program ranges from 2 to 10 g/cm3, therefore the median value was chosen for the model (NCRP 98 1994). The shield is also a rectangle 5 cm wide by 5 cm tall, but is 0.3 cm thick. It is placed 30 cm from the aluminum block, 1 cm from the eye, and has a comparable
- 12 - thickness to most safety glasses, providing a realistic model. The outer eye structure is a sphere with a diameter 2.415 cm, while the inner eye structure is a sphere with a diameter of 2.365 cm, making the outer eye structure 0.05 cm thick. The lens of the eye is an ellipse of 1 cm tall and radius of 0.42 cm. All materials other than the shield materials are described in the following table:
Table 1: Non-Shield Material Description
Density Weight g/cm^3 Fraction Description Standard Aluminum, Aluminum 2.7 Al – 1 represents the vessel wall
H - 0.1050 Film of C - 0.2560 tissue N - 0.0270 containing O - 0.6020 Outer Eye eye fluid, 1.03 Na - 0.0010 (Sclera) often P - 0.0020 referred to S - 0.0030 as the "white Cl - 0.0020 of the eye" K - 0.0020
H - 0.1080 Transparent C - 0.0410 jelly filling N - 0.0110 Eye Fluid the space 1.03 O - 0.8320 (Vitreous Humor) between the Na - 0.0030 lens and the S - 0.0010 retina Cl - 0.0040
H - 0.0960 Transparent, C - 0.1950 biconvex, N - 0.0570 semi-solid O - 0.6460 Eye Lens 1.07 body in the Na - 0.0010 eye and is P - 0.0010 devoid of a S - 0.0030 blood supply Cl - 0.0010
Biological material composition provided by ICRU report 44
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2.4 Shield Materials
The shield material tested in the model include:
Table 2: Shield Material Description
Density Weight g/cm^3 Fraction Description
O - 0.156453 Common N - 0.080866 type of Glass, Lead 6.22 Ti - 0.008092 radiation As - 0.002651 protection Pb - 0.751938 lenses
Borosilicate B - 0.040066 glass, O - 0.539559 contains Na - 0.028191 Glass, Pyrex 2.23 boron which Al - 0.011644 may act as a Si - 0.377220 neutron K - 0.003321 absorber H - 0.080538 Glass Lucite (Plexiglass) 1.19 C - 0.599848 alternative O - 0.319614 Structural polymer H - 0.040821 recently C - 0.749991 Polyetherimide 1.27 researched N - 0.047272 as potential O - 0.161993 shield material H - 0.143716 Most widely Polyethylene 0.93 C - 0.856284 used plastic
Data Provided by NIST
The materials chosen for this experiment were selected to encompass the characteristics of a good shield. Shield materials were also chosen based on electron and proton ranges. Insight into which materials would theoretically act as the best shield is shown in the following table:
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Table 3: Thicknesses Required for 500 MeV Electron and 10000 MeV Proton – Selected Materials 500 MeV 10000 MeV Electron Proton Glass, Lead 0.16' 36.2' Glass, Pyrex 1' 80' Lucite (Plexiglass) 2.3' 134' Polyetherimide ** 143' Polyethylene 3' 159' Data Provided by NIST ** Data for electron range were not included for these materials by NIST
Additionally, the selection process ensured that a spectrum of densities and electron/proton ranges were picked to test the variation of the response to shielding from the secondary particles.
The compositions of all the materials were also checked to ensure neutron activation was not a concern.
Other materials considered for this study included sheet glass, polysulfone, and polycarbonate.
As shown in the table below, the polysulfone range is comparable to polyetherimide, just as the polycarbonate range is comparable to lucite, but both require a slightly larger thickness.
Therefore, these materials were eliminated as potential shields. For sheet glass specifically, it had a comparable range to pyrex. However, sheet glass has a slightly higher content of sodium, which can be activated by neutrons (0.02% to 0.04% as compared to pyrex glass). Additionally, the boron content in pyrex has a potential to serve as a neutron absorber, therefore pyrex was chosen over sheet glass.
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Table 4: Thicknesses Required for 500
MeV Electron and 10000 MeV Proton – Unselected Materials 500 MeV 10000 MeV Electron Proton Glass, Sheet 1' 79' Polysulfone ** 144.5' Polycarbonate 2.4' 136' Data Provided by NIST ** Data for electron range were not included for these materials by NIST
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3. Results
3.1 Secondary Particles Produced after Aluminum
Table 5: Secondary Particles Produced after Aluminum Block Mean particle/cm^2/ Max Energy proton +/- (MeV) Neutron 3.43E-03 0.0024 22900 Photon 2.36E-03 0.0028 8300 Electron 3.61E-05 0.0209 5200 Proton 3.46E-02 0.0002 50000
3.2 Flux in the Eye Lens
Neutron Flux in Lens Photon Flux in Lens
1.00E-02 4.50E-03 9.00E-03 4.00E-03 8.00E-03 3.50E-03 7.00E-03 3.00E-03 6.00E-03 2.50E-03 5.00E-03 2.00E-03 4.00E-03 3.00E-03 1.50E-03 2.00E-03 1.00E-03 1.00E-03 5.00E-04 0.00E+00 0.00E+00 Flux (particles/cm^2/proton) Flux (particles/cm^2/proton) d x e e d d x e e d e n e e id yr le ea cit len y L u y Shiel Lucite h s, Pyr L s, P s s, No s lyet No Shiel olyeth Glass, Lea Gla Po Gla Glas P Polyetherimid Polyetherim Shield Type Shield Type
Electron Flux in Lens Proton Flux in Lens
6.00E-05 7.480E-01 7.460E-01 5.00E-05 7.440E-01 4.00E-05 7.420E-01 3.00E-05 7.400E-01
2.00E-05 7.380E-01 7.360E-01 1.00E-05 7.340E-01 Flux (particles/cm^2/proton) 0.00E+00 Flux (particles/cm^2/proton) 7.320E-01 e d d x e e d d x e d a re te el n e cite le hiel L uci imid hi Pyre u S s, , Py L er L thy o s therimi N la No S lass, Lea ye lass lyeth lye G G Polyethyleno G Glass, Pol P Po Shield Type Shield Type
Fig 7: Flux in the Eye Lens by secondary particle - 17 -
Table 6: Lens Flux in particles/cm^2/proton by Particle After Each Shield Type Neutron +/- Photon +/- Electron +/- Proton +/- No Shield 5.28E-03 0.0004 3.12E-03 0.0004 3.36E-05 0.0037 7.465E-01 0.0000 Glass, Lead 9.31E-03 0.0005 4.14E-03 0.0007 5.53E-05 0.0050 7.378E-01 0.0000 Glass, Pyrex 6.37E-03 0.0019 3.48E-03 0.0021 3.73E-05 0.0172 7.419E-01 0.0000 Lucite 5.92E-03 0.0007 3.34E-03 0.0007 3.48E-05 0.0061 7.437E-01 0.0000 Polyethylene 5.80E-03 0.0004 3.12E-03 0.0004 3.28E-05 0.0037 7.442E-01 0.0000 Polyetherimide 5.99E-03 0.0018 3.33E-03 0.0020 3.44E-05 0.0171 7.435E-01 0.0000
- indicates lowest value
3.3 Dose to the Eye Lens
Neutron Lens Dose Photon Lens Dose
2.50E-03 3.50E-04 3.00E-04 2.00E-03 2.50E-04 1.50E-03 2.00E-04
1.00E-03 1.50E-04 1.00E-04 5.00E-04 5.00E-05 Dose (MeV/g /proton) /proton) (MeV/g Dose Dose (MeV/gDose /proton) 0.00E+00 0.00E+00 e e d d x e ld d x e el e e a i ea len en mid L Pyr y Le ucite yl ri s, Lucite Shi , Pyre L s eth s eth the No Sh ss, ly etherimid No lass, s y Gla o y la lye Gla P G G Pol Pol Po Shield Type Shield Type
Electron Lens Dose Proton Lens Dose
1.20E-04 2.3720
1.00E-04 2.3700 2.3680 8.00E-05 2.3660 6.00E-05 2.3640 4.00E-05 2.3620 2.00E-05 2.3600 Dose (MeV/gDose /proton) Dose (MeV/g /proton) /proton) (MeV/g Dose 0.00E+00 2.3580 e d e e d x e d e id a re el it e mid Lea c im L ucite i Pyrex r Py L s, Lu ss, the No Shield la ss, No Shi las ass, G l G Gla Polyethylen G Polyethylen Polyether Polye Shield Type Shield Type
Fig 8: Dose to the Eye Lens by secondary particle
- 18 -
Table 7: Lens Dose in MeV/g/proton by Particle After Each Shield Type Neutron +/- Photon +/- Electron +/- Proton +/- No Shield 1.15E-03 0.0045 2.41E-04 0.0047 6.73E-05 0.0033 2.3693 0.0000 Glass, Lead 2.07E-03 0.0080 3.32E-04 0.0080 1.12E-04 0.0046 2.3629 0.0000 Glass, Pyrex 1.40E-03 0.0275 3.00E-04 0.2120 7.57E-05 0.0156 2.3682 0.0005 Lucite 1.33E-03 0.0101 2.78E-04 0.0077 7.03E-05 0.0057 2.3699 0.0002 Polyethylene 1.28E-03 0.0048 2.65E-04 0.0065 6.61E-05 0.0033 2.3697 0.0000 Polyetherimide 1.32E-03 0.0263 2.75E-04 0.0205 6.97E-05 0.0154 2.3693 0.0001
- indicates lowest value
3.4 Dose Reduction
Table 8: % Change in Dose Compared to the No Shield Case Glass, Lead -0.225 Glass, Pyrex -0.036 Lucite 0.035 Polyethylene 0.024 Polyetherimide 0.007
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4. Discussion
4.1 Breakdown of Results
The secondary particles analyzed include neutrons, photons, electrons, and secondary protons.
An F2 tally was used to measure the flux and energy spectrum of each secondary particle crossing the backside surface of the aluminum. The unit of flux is reported as particle/cm^2/proton, meaning the value is normalized by the number of particles in the beam.
Table 5 shows that secondary protons have the largest flux at 3.46 E-02, followed by neutrons at
2.36 E-03, then photons at 3.43 E-03, and lastly electrons at 3.61 E-05. This table also shows the maximum energy of each particle in MeV as measured by MCNPX. Neutrons have a max energy at 22,900, photons have a max energy at 8,300, electrons have a max energy at 5,200, and finally protons have a max energy at 50,000. These maximum energies indicate that the shields must stop some very high-energy particles. In fact most radiobiological studies for human exposure only include particles with energies up to 100 MeV. Further analysis of the energy spectrum for each secondary particle can be found in Appendix B. These plots are produced from MCNPX and show the spectrum of energies based on user defined bins. In all plots, the peak falls in the lower energy region showing that the majority of the particles are only a portion of the max energy. However, even with being a fraction of the maximum energy, these particle energies are still significantly higher than energies seen on earth.
After the shield materials were placed in the model, the secondary particle flux was measured in the lens of the eye. An F4 tally, or volume tally, was used, reporting flux in units of particles/cm^2/proton. With the application of each shield, the flux is increased for neutrons, photons, and electrons. On the other hand, for protons the flux is reduced with the utilization of each shield. These results justify the physical correctness of the model. With each shield, the
- 20 -
flux will increase for neutrons, photons, and electrons because tertiary particles are created
through interactions between the photons/secondary particles and the shield material. A detailed explanation of why more particles are produced after the shield is used will be discussed later in section 4.3. Additionally, proton flux will reduce because the shield materials absorb protons.
The other tally used with the eye model measured dose within the lens, based on the contribution from each secondary particle. The energy deposition tally, or F6 tally, was used. This tally reports the dose for a volume in units of MeV/g/proton, also known as the Gray/proton. As shown in the results, the no shield case proved to have the lowest deposited dose from neutrons and photons. Additionally, polyethylene demonstrated shielding the best against deposited dose from electrons. Finally, the lead glass provided the best protection from protons and provided
the lowest total deposited dose.
4.2 Best Shield
The data provides a mixed view for determining a decisive best shield. By looking at solely
Figure 7, all materials increase flux for neutrons, protons, and electrons, making it appear that
adding any materials may increase dose. Conversely, the same figure shows that adding shield
materials reduces the dose from protons. Alternatively, by looking solely at Figure 8, the no
shield case appears to have the lowest overall dose. This suggests that having no shield may be
the best method to reducing dose. Ultimately, the key in the analysis lies in the proton dose.
Upon further analysis, the dose from this particle is a factor of three to five times higher in
magnitude than all other particles and is therefore the largest contributor of dose in the lens. This
means that the best shield is the material that reduces proton dose the most.
- 21 -
Of the materials tested, the lead shield resulted in the lowest proton dose. The magnitude is
about 0.007 Gy/proton less than using no shield. While this value is small, it is also the
difference in magnitude per source proton. If the dose were to be extrapolated, encompassing
dose acquired over an entire long-term mission, then this small difference would have a large
effect. The only concern with using lead glass is that it also produces the largest amount of
tertiary particles compared to the other tested materials, as shown in Figure 7. However, the
magnitude of dose increase from neutrons, photons, and electrons combined is about 0.001
Gy/proton. This means that the magnitude of the decrease in dose from protons is larger than the magnitude of the increase in dose from all the tertiary particles combined. A comparison of the net dose change between lead and the other materials shows that lead will have the lowest total dose of all materials, even taking into account the production of tertiary particles. The following table provides a clear picture of the above statement:
Table 9: Dose in MeV/g/cm^2/month No Shield 135.32 Glass, Lead 135.01 Glass, Pyrex 135.27 Lucite 135.36 Polyethylene 135.35 Polyetherimide 135.33
Assuming a constant flux of 2 particles/cm^2/day, calculated from Figure 4
Additionally, in terms of biological damage, the concern for protons is justified. As stated before, individual particles may cause more damage than others. Radiation weighting factors provided by the NRC and ICRP Publication 92 show the Relative Biological Effect (RBE) of specific particles. Both protons and neutrons have a larger weighting factor than photons and electrons, indicating protons and neutrons have a larger biological effect (Cember and Johnson,
2009). Thus, a higher importance must be placed on reducing proton interaction in the lens.
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4.3 Explanation for Higher Dose after Shield Materials
There are two main explanations for a higher dose when using the shield material. First, the particle flux is increased after the shield due to the production of tertiary particles. These particles stem from interactions with the materials, including some rare interactions associated with high-energy particles. More neutrons are created through a process called photonuclear interaction. Additionally, photons are created from multiple processes including: bremsstrahlung, neutron interactions, electron x-rays, and electron positron annihilation. Lastly, electrons are created through processes such as: auger electrons from both electrons and proton vacancies, Compton recoil, photoelectric effect, and pair production. The second reason for higher dose involves the fact that the shield material will slow down any of the higher energy particles it cannot stop, thus increasing the probability of energy deposition. Alternatively, if no material were used, the high-energy particles would just pass through the lens without depositing any energy. Therefore, the combination of a higher flux and a higher probability for energy deposition leads to a higher dose in the lens when a shield is used.
4.4 Benchmarking/ Realistic Results
This study considered a worst-case scenario for frontal exposure; a concentrated beam of particles aimed directly at the eye lens. This is a conservative approach, however it is not a realistic phenomenon that occurs in the space vessels. For this reason, there are no actual cases to benchmark the dose measurements as calculated by the model. However, if any of the materials were to shield the lens in this scenario, then the same results would apply to the actual environment. Additionally, if a realistic model approach were to be used, then the effect of the eyewear shields would not have been seen as easily and limits could not have been tested as
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thoroughly. Therefore, these results do provide a successful indication of how the shield
materials will respond if worn as eyeglasses.
4.5 Front Exposure vs. Back Exposure
Within this study, a simple analysis was performed on dose accumulated from a beam of protons
that entered the rear of the lens after passing through an unshielded human head. The purpose of
this analysis was to determine if the only concern for dose to the lens came from frontal
exposure. The setup was similar to model used in this study, a concentrated proton beam was
aimed directly at a lens, is passed through an aluminum block, and the lens was placed the same
distance from the source. However, in this analysis, the beam passes through a smaller block of
air, then skull, and finally brain before entering the eye. The following figure shows the model:
Skull Brain
Fig 9: MCNPX model of exposure from a beam through the back of the head
Results, which can be found in Appendix C, show that doses from neutrons and photons were worse, while the doses from electrons and protons were better. Yet, as stated before, protons contribute the most to dose. By analysis, the dose from protons entering the back of the lens is
31% lower than dose from protons entering the front of the eye. Thus a beam of particles that passes through the eye from the exposed front is actually worse, in terms of dose, then a beam entering the back of the head into the lens.
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4.6 Importance of the Results
The findings of this study show that, at the tested thickness, dose to the lens will be most effectively reduced when using lead glass as an eyeglass shielding device as compared to the other tested materials. However, the reduction in dose is only 0.22%, not the 20% goal as proposed in the hypothesis. Additionally, these results show that shielding made of lucite, polyetherimide, or polyethylene inside the vessel at the tested thickness is not recommended as it will cause higher doses to the crew. Lastly, practically speaking, shielding for the lens will rely heavily on shielding placed in the structure of the spacecraft to reduce the flux of secondary particles entering the inside environment. Reducing dose from frontal exposure by 0.3% will help, but will not significantly reduce cataracts in astronauts. Additionally, protecting from exposure to the back of the head with other possible materials will affect the comfort of the astronauts. Therefore, there maybe no type of shield that can be worn inside the vessel to significantly reduce dose to the lens.
4.7 Recommendation for Future Work
Recommendations for future work would include confirming these results with a physical experiment and potentially testing more materials. Additionally, using a more exact model for
the space vessel wall to test these conditions for specific spacecraft, like the ISS. The effect of
using exact materials, such as aluminum alloy, and using the exact thickness could significantly
change the secondary particle environment in the space vessel and thus change the results of this
study. Lastly, another potential study could be the use of eyewear as shielding in combination with shielding on the outside of the vessel.
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5. Conclusion
Out of the tested materials, lead glass is the most effective shield for frontal exposure of
secondary particles produced from GCR. When using this shield as an eyeglass shielding
device, dosage is reduced by 0.22%.
Lucite, polyetherimide, and polyethylene cause an increase in dose as compared to the no
shield case. This is due to the large increase of dose by the production of tertiary particles
and the small reduction in proton dose.
Exposure to the lens through front of the eye from secondary particles produced from
GCR is worse than exposure to the lens through the back of the head.
Practically speaking, shielding for the lens will most likely rely heavily on shielding
placed in the structure of the spacecraft to reduce the flux of secondary particles entering
the inside environment.
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Appendix A – GCR Spectrum
Spectrum per Badwhar et al.
Spectrum Used in Model
GCR Environment for Protons - 1997 Solar Minimum
1.0E+10
1.0E+08
1.0E+06
1.0E+04 particles/cm^2-
1.0E+02 MeV/year)
1.0E+00
1.0E-02
Particle Fluence (# 1.0E-04 1.0E-02 1.0E+00 1.0E+02 1.0E+04 Energy (MeV/amu)
Fig 10: 1997 GCR Spectrum used in MCNPX model
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Appendix B – Energy Spectrum of Secondary Particles after the Aluminum Wall
Energy Spectra for Neutrons
Energy Spectra for Photons
Fig 11a: Energy Spectra by particle
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Appendix B – Energy Spectrum of Secondary Particles after the Aluminum Wall – Cont’d
Energy Spectra for Electrons
Energy Spectra for Protons
Fig 11b: Energy Spectra by particle
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Appendix C – Front vs. Back Exposure
Neutron Front vs. Back Dose Photon Front vs. Back Dose
3.00E-03 6.00E-04
2.50E-03 5.00E-04
2.00E-03 4.00E-04 Front Front 1.50E-03 3.00E-04 Back Back 1.00E-03 2.00E-04 Dose (MeV/g/proton) (MeV/g/proton) Dose Dose (MeV/g/proton)Dose 5.00E-04 1.00E-04
0.00E+00 0.00E+00 Neutron Photon
Electron Front vs. Back Dose Proton Front vs. Back Dose
6.80E-05 2.50 6.60E-05 2.00 6.40E-05
6.20E-05 1.50 Front Front 6.00E-05 Back Back 5.80E-05 1.00 5.60E-05 Dose (MeV/g/proton) (MeV/g/proton) Dose (MeV/g/proton) Dose 0.50 5.40E-05
5.20E-05 0.00 Electron Proton
Fig 11: Front vs. Back Dose to the Eye Lens by secondary particle
Table 10: Lens Dose in MeV/g/proton for Front and Back Exposure Front Back MeV/g/proton +/- MeV/g/proton +/- Neutron 1.15E-03 0.0045 2.62E-03 0.0054 Photon 2.41E-04 0.0047 5.05E-04 0.0100 Electron 6.73E-05 0.0033 5.77E-05 0.0045 Proton 2.37 0.0000 1.64 0.0001
- lower value
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