Delta Electron Database for the Calculation of Astronaut Dose Rate and Cell Damage from Space Radiation
Stephen Emerson and Jane H. MacGibbon, University of North Florida
Possible
Biological
Effects
of
Radiation
Biological Effects of Radiation
The Space Radiation Problem
Space radiation, including the
radiation on other planets and
moons, is significantly more
dangerous than radiation on
Earth. Space radiation has three
main components: galactic
cosmic radiation; solar particles;
and the Van Allen radiation belts.
Space radiation particles are
mainly highly energetic ions
(hydrogen to iron), photons and
electrons. Space radiation research
focuses on predicting the amount
of radiation astronauts will
receive during space travel and
the long-term consequences of
exposure to space radiation.
The Problem of Bio-Radiation
The long and short term effects
of radiation
Effects of Radiation on Cells
Linear
Energy Transfer (LET)
Effects of Space Radiation on the Body
The most likely event starts with
space radiation impacting the
shielding of the space shuttle or
station. As the radiation travels
through the spacecraft shielding it
may undergo nuclear fragmentation
producing a different distribution of
particle species and energy. The
radiation may then travel through
the astronaut causing ionization
and/or other energy deposition.
When ionizing radiation interacts
with the body, DNA damage can
occur. Ionizing radiation can cause
DNA damage by directly striking
the DNA molecule or it can create
free radicals in the cell which then
attack the DNA molecule.
• Radiation Carcinogenesis
Fluorescent markers indicate
the presence of double-strand
breaks in DNA caused by low
levels of x-ray radiation.
• Radiation Mutagenesis
DNA being damaged by
radiation
• Radiation Induced Cataracts
dE
Z
D
z 2  (  )(1   )
dx
A


2
2
2

1 
 log  2me c  
where;  (  ) 
2 
 I (1  me )


M



• Chromosomal Damage
Delta Electron Ionization Processes
The Van Allen belts are just one of the
radiation hazards astronauts must be protected
from during space travel.
on the body are fairly well
When understood.
ionizing radiation interacts with a target atom, the amount of energy
transmitted
unit length to the target by the radiation is known as the
However science has still
yet toperfind
linear energy transfer (LET). LET can be useful in determining the
accurate methods and models
to predict
the potential biological effects due to radiation.
radiation dose
and subsequent
In generalthe
the biological
effects are greater for high LET radiation because
amount and type of radiation
body will
more energy is deposited in each cell.
receive when exposed to radiation. The
• Heavy Ions are high LET radiation
focus of this research is to help further
• Photons and X-Rays are low LET radiation
provide accurate modeling of radiation
energy transfer.
Once the DNA has been damaged, the
cell will do one of three things. The
DNA damage may cause the cell to die,
(apoptosis); the cell may repair the
DNA correctly and the cell will
continue to operate normally; or the cell
may incorrectly repair the DNA
damage leaving the cell with a
permanent error, potentially causing
cancer and other permanent long term
effects. Several of these effects
include:
The Bethe-Bloch Equation is a
good approximation for the
electron energy loss (stopping
power) due to ionization.





C
2

 

2
Z





E  projectile energy
re  classical electron radius
C  shell correction
M  projectile mass
me  electron rest mass
  projectile velocity
N A  Avagadro' s Number
  higher order correction
  density correction
 1
Z  atomic number of the target
(1 -  2 )
z  projectile charge
x  path length
A  atomic weight of the target
  mass density of the target
D  4 re2 me c 2 N A
I  average ionization potential
Delta Electron Ionization Processes
Total Cross Section
The total cross-section is the probability of an
incoming projectile ion ionizing an electron
from a target atom or molecule.
Singly Differential Cross
Section (SDCS)
The singly differential cross section is the
differential ionization probability with respect
to the outgoing electron energy.
Total cross-sections for the ionization
of molecular oxygen by protons.
Singly differential cross sections for the ionization of
several molecular species by 1 MeV protons.
Double Differential Cross
Section (DDCS)
The doubly differential cross
section (DDCS) is the
differential probability of an
incoming projectile ion
ionizing a target electron
expressed with respect to the
angle and energy of the
outgoing electron. The
DDCS gives the most
accurate 3-D look at the
ionization process. The
DDCS is more complicated
that the total cross-section
and SDCS and cannot be
modeled theoretically.
Direct Coulomb
Ionization
The binary encounter peak occurs at the
electron angle and energy given by the
kinematics for a classical collision
between the projectile ion and an orbital
electron of the target atom.
When an incoming ion interacts with
an atom, the most significant energy
exchange is the Coulomb interaction
between the charged projectile and
the outer electrons of the target atom.
If the impact parameter (the closest
distance of approach between the ion
and the target atom) is small, the
energy transferred to the target
electron is large and the electron can
be treated as quasi-free. If the impact
parameter is large, the energy transfer
is smaller and the influence of the
target nucleus must be considered.
DDCS for the ionization of
molecular oxygen by a 30 MeV O5+
oxygen ion. The electron energy at
the binary encounter peak scale with
Double differential cross sections for the
ionization of electrons at 20° from molecular
hydrogen by protons of various impact energies.
the electron angle as
Loss Electrons
Binary Encounter Peak
  cos 2  .
Loss electrons are classified as two types
depending on where they originate. The two
types are electron loss (of a projectile
electron) to the continuum (ELC) and
electron capture (of a target electron by the
projectile) to the continuum (ECC). Both
types are emitted with nearly zero momentum
in the frame of the moving projectile.
DDCS for ionization of water vapour by 0.3 MeV u-1
protons and helium ions. The loss electron contribution is
noticeable on the 15° He+ spectrum around 160 eV.
Auger Electron
Emission
Spontaneous decay of excited states or
inner-shell vacancies created in the iontarget collision can produce ionized
electrons with discrete energy levels.
Excited outer-shell target electrons produce
Auger electrons isotropically with low
energy. Auger electrons from the projectile
have high energy in the lab frame.
DDCS at 160º for ionization of argon gas by 100
keV protons and argon ions. Auger electrons from
the target can be seen at low energies
Methods, Measurements and Parameterization Techniques
The Delta Electron Database
Normalized Data Sets
VBA Code
Microsoft Visual Basic for Applications (VBA) is used to write
the code to manipulate the database and datasets. Several
different VBA modules were used to normalize the datasets.
Below is a snapshot of just one of the modules of VBA code.
Over 1200 data sets of experimentally measured DDCS have been compiled into our database. The data is essentially decades worth of
ionization research results. The data covers many different projectile species (hydrogen to uranium) and target species (including hydrocarbons
and water) and a wide range of projectile energies. Information on each dataset is stored in the Master Spreadsheet.
A large part of the database task was normalizing the individual data
sets to one common set of units so the data sets could be accurately
compared to one and another. This process included unit
conversion, mathematical conversion and in some cases energy
conversions. Above is one set of normalized data.
Goals and Further Research
The main long term goal of this research project is to use
this database in the modeling of astronaut exposure to
ionizing radiation. Currently there is no accurate method
to predict the short and long term effects of ionizing
radiation astronauts will receive during future space
endeavors including a return trip to the Moon and to Mars.
Current and future tasks include creating a user friendly
interface that allows researchers to select individual targets
and ionizing projectiles over various energy distributions
and by other projectile and target characteristics. The
selected data sets can be used directly in Monte Carlo radiation codes. We are also developing
methods to parameterize the data sets. The parameterized mathematical fits will be used in
analytical and numerical modeling applications. Using data sets obtained from experiments and
parameterizations directly derived from the experimental data should significantly improve the
accuracy of the modeling of radiation exposure. Currently the modeling relies on inaccurate
theoretical approximations to the DDCS or data from a limited range of projectile species.
Multi-variable parameterization will also allow the user predict radiation exposure for
projectiles, targets and energy ranges for which the cross-sections have not yet been measured.
This work was supported by funding from UNF Department of Chemistry and Physics and UNF Summer Research Awards