On Mar 4, 8:35 am, CharlesRCap...@[EMAIL PROTECTED]
wrote:
> One of the problems with using nuclear powerplants in your spacecraft
> is that they tend to kill the crews if you don't put them at a safe
> distance outside the hull or put adequate shielding between them and
> your crew. To a lesser degree you also need shielding for cosmic rays
> too.
There are also solar storm protons and protons trapped in planetary
radiation belts to worry about. Those expecting action may also be
concerned about the neutrons from nuclear detonations and protons from
particle beams.
> So the problem is that the only scientifically sound way I know of to
> protect crews from gamma radiation is either distance (putting your
> reactor outside your ship, far outside your ship) or physical
> shielding made up of heavy elements. (Lead, Gold, etc...)
Two points:
(1) The thickness of heavy elements needed to protect against typical
gamma rays produced by fission is not all that large. To stop 63% of
all 1 MeV gamma rays, you need 14 grams per square centimeter of lead,
or 140 kg per square meter. Double that for 86% protection, triple
for 95% protection, quadruple for 98% protection, and quintuple for
99.5% protection. The radiation protection of lead reaches its
minimum at 4 MeV gamma rays, with 23.8 grams per square centimeter
required for 63% protection (at higher energies, the protection
actually increases). In contrast, iron reaches its minimum at 8 MeV,
with 33.4 grams per square centimeter required for 63% protection -
the steel girders and decking and pressure hulls and fuel tanks will
provide a significant amount of radiation shielding against gamma
rays. (Data taken from
http://physics.nist.gov/PhysRefData/XrayMassCoef/cover.html)
(2) The big worry from fission reactors (as well as nuclear
detonations) is neutrons. The best shielding against neutrons is
light elements, with hydrogen by far the best neutron stopper.
Hydrogen is also preferred against proton and ion radiation, such as
that from cosmic rays, solar storms, planetary radiation belts, and
particle beams. Materials containing hydrogen, such as plastics,
waxes, or water, are often used as neutron stoppers. Similarly,
putting the propellant tanks (which are typically full of either
hydrogen or hydrogen rich materials) between the reactor and the crew
would provide a significant amount of protection from the neutrons.
Neutron radiation will be a greater concern with fast reactors than
moderated reactors (which include a significant amount of neutron
"shielding" already in the form of the moderator).
> Is there some sort of unobtanium material that could be used?
Not if it is made of normal matter.
> For
> example: I know that gamma radiation is stopped by hitting the
> nucleus of an atom (the more massive the nucleus the better, because
> it covers a larger area) so lots of heavy nuclei in depth is the best
> protection we can use today.
You are mixing up neutron and proton radiation with gamma rays. Gamma
rays are stopped by either (a) the photoelectric effect, where an
electron in an atom absorbs a gamma ray and acquires its energy, (b)
Compton scattering, where a gamma ray bounces off an electron, giving
the electron part of its energy, and (c) pair production, where a
gamma ray striking an electron bound to an atom is destroyed while
producing an electron and a positron. For pair production and Compton
scattering, you want as many electrons as possible between you and the
radiation source. Since the number of electrons is roughly
proportional to the nuclear mass, you get very roughly the same
protection for the same amount fo weight of material irregardless of
the material type. This is not true for photoelectric absorption,
where the tightly bound electrons of the core orbitals of heavy
elements are better at stopping higher energy x-rays and lower energy
gammas (and where you get complicated jumps in the absorption spectrum
as you pass shell ionization energies). However, the dominant method
of stopping gamma rays at nuclear energies (a few MeV) is Compton
scattering.
Now neutron radiation is stopped only by smacking into a nucleus. At
high energies (around 100 MeV or more) this is also the dominant
method of stopping protons (at lower energies, protons lose energy
primarily by ionizing the atoms they pass by). Therefore, you want
the most nuclear cross sectional area for a given amount of mass (I'm
ignoring the odd resonances for neutron capture and the enhanced
scattering cross sections for lower energy neutrons - which is not
really all that good of an approximation at fission and fusion
energies but works quite well for higher energies). As you can
imagine from the cube-square law, this means that light elements will
provide the most stopping cross sectional area, with hydrogen the best
of all. In addition, when a fission or fusion neutron smacks a
nucleus, it will bounce off while transferring part of its energy to
the nucleus. The closer the neutron is in mass to the nucleus it
hits, the more of its kinetic energy is given to the nucleus (this is
like a billiard ball transferring much of its energy to another
billiard ball during a collision, but bouncing off of a bowling ball
with most of its original energy). On average, a neutron will lose
half of its energy in each collision with a hydrogen nucleus, but only
a small fraction in a collision with anything heavier.
> What if (using nanotechnology) we were
> able to line up those lead atoms in precise little structures that
> create a solid wall (or a less porous net) of atoms rather than
> needing deep layers of the stuff to make sure that a nucleus is struck
> by the ray before it exits the material? Would this material be
> significantly lighter than solid slabs of lead of equivalent
> protectiveness?
Doesn't work. You can't just position atoms arbitrarily. If you get
them too close, their electrons repel each other and force the atoms
apart. They then move apart until they reach the proper distance
where they form chemical bonds. Chemical bonds favor the atoms in
certain orientations, meaning you will tend to get certain angles
between atoms. For simple compounds, this means you usually end up
with a crystal structure of ordered atoms, although the individual
crystals may be only a few nanometers across (this is what leads to
much modern "nanotechnology," just nanocrystalline bulk materials; you
can also get structures with short range order but no long range order
- we call these liquids or glasses). If atoms are not arranged at
their favored distances and angles, they will move until they are in
these "comfortable" positions and orientations.
Now, we do see some effects relating to radiation stopping due to the
arrangement of atoms in a material. Unfortunately, this works in the
wrong direction. It is called channeling, and occurs when a heavy
charged radiation particle moves down one of the directions where the
crystal atoms all line up in rows. This means that there are long
stretches where the particle can travel without encountering a nucleus
or the tightly bound core electrons surrounding the nucleus, enabling
that particle to travel much further than normal through matter of
that type.
In general, however, the interaction of radiation with a material is
with its atoms as individuals and collective effects are small. Thus,
it doesn't matter how you arrange the atoms, what matters is just how
many atoms you stick between you and the source (this is not true for
electrons at low energies, where collective behavior of the electrons
in the material have a significant effect, leading to significant
energy loss through the excitation of plasmons, for example. However,
low energy electrons are short ranged in matter and we don't need to
worry about them for this purpose).
> I also know that part of the problem with using thin sheets of
> material to stop gamma radiation is that the secondary particles that
> are produced when a nucleus is struck are actually worse than not
> stopping the gamma rays at all.
This is not a problem with gammas. They do produce a secondary
electron cascade, but this is short ranged in matter. It is a
significant concern with high energy protons and ions - when one of
these slams into a nucleus, they can fragment the nucleus into
particles, and can produce large numbers of pions or other mesons, or
even (at more than a couple GeV) other protons and neutrons and their
antiparticles. These secondary particles will also be at a high
energy and will travel some distance before colliding with another
nucleus, which may produce a further shower of particles. I was
playing with some simulations of radiation interaction with matter
(the GEANT4 code, available for free but it has a rather steep
learning curve and takes up a fair amount of hard disk space), and was
finding that the cascades produced by 1 GeV protons tended to have
several hundred MeV remaining after punching through a meter of water
shielding.
Luke
|
