Defense Intelligence Reference Document Aneutronic Fusion Propulsion(1)

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The energy absorbed is strongly related to the amount of radiation damage done to the
tissue. On Earth, the magnetic field of the planet helps to shield people from most of
the effects of radiation from the sun, but cosmic radiation and terrestrial sources of
radiation (granite, potassium, radon gas) all contribute to an annual background dose
that everyone receives. The average annual dose of radiation in the United States is
about 2.5 mSv from background and another 1.0 mSv from other source, such as
dental x-rays, commercial jet flights, and radiopharmaceuticals. The total annual dose
in the United States is approximately 3.5 mSv (millisieverts) per person.
In space, away from the protection of the Earth's magnetic field, the radiation dose
increases substantially to about 250 mSv per year. The radiation dose in space is
continuous, and the effects of being in space for extended periods of time may be
cumulative. As a comparison, 2,000 mSv of radiation in an acute dose can cause
significant medical problems and 5,000 mSv is usually fatal. Leukemia and other forms
of cancer are possible for people exposed to chronic doses of radiation at the levels
encountered in space.
The logical conclusion would be to carry radiation shielding into space to protect the
astronauts. The problem is that shielding is typically heavy and expensive. Four types
of radiation must be shielded:
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1. Gamma Rays (y)
These are energetic forms of electromagnetic radiation (photons) and tend to
penetrate most materials. High-density metals, such as iron, lead, and
uranium are usually used to shield gamma rays.
2. Beta Particles(~+, ~-)
These are electrons or positrons, the antimatter counterpart to electrons.
They are emitted by the radioactive decay of certain isotopes and through
nuclear fission. Because these are charged particles, they are fairly easy to
stop with minimal shielding.
3. Neutrons (n)
These uncharged particles are generated by nuclear fission and fusion. They
may penetrate metals, yet they can be slowed down until they decay in light
materials that contain hydrogen or carbon. Typical shielding material
includes water, paraffin wax, and polyethylene blocks.
4. Heavy Charged Particles (p, a)
Ions are atoms that have one or more of their electrons stripped from their
outer orbital. Due to their positive electric charge, ions are generally easy to
stop within any kind of material, unless the ions are very energetic. Typical
ions include protons, which are ionized hydrogen atoms, and alpha particles,
which are ionized helium nuclei. Cosmic radiation includes heavy ions
emitted by exploding supernovae and may include ions as heavy as iron
nuclei at extremely high energy.
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