Unobtanium Radiation Shielding?
Charles...@gmail.com
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. (Electromagnetic shielding protects against most everything else,
except stray masses that might punch holes in your nifty ship.)
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...)
So what can we do to protect our valiant spacers except sticking a lot
of massive lead around them? I remember a few years back there was a
company that was saying that they had a cloth-like material that was
better protection from radiation than current protective suits (which
are made of led impregnated plastics or something similar) but I don't
remember anything coming of that.
Is there some sort of unobtanium material that could be used? 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. 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?
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. So if we produce a thin nanotech
material that stops the gamma rays but produces these secondary
particles, can we back it with some other material (lighter than lead
or gold or whatever) that can stop the secondary particles at a lower
mass penalty than the main material?
Luke Campbell
> 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
Robert Martinu
> 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?
In a 3-dimensional matrix some nuclei are shadowed by others and can'T
contribute, if you reduce your protective layer to two dimensions, they
are either used to increase shielding or left at the factory, saving you
mass.
> 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. So if we produce a thin nanotech
> material that stops the gamma rays but produces these secondary
> particles, can we back it with some other material (lighter than lead
> or gold or whatever) that can stop the secondary particles at a lower
> mass penalty than the main material?
Materials that contain a lot of hydrogen are good at this.
That water you want to use for some of the cooling systems could double
as shielding mass, structures made from PE/PP-plastics are also quite
useable.
John Schilling
>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. (Electromagnetic shielding protects against most everything else,
>except stray masses that might punch holes in your nifty ship.)
>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...)
>So what can we do to protect our valiant spacers except sticking a lot
>of massive lead around them? I remember a few years back there was a
>company that was saying that they had a cloth-like material that was
>better protection from radiation than current protective suits (which
>are made of led impregnated plastics or something similar) but I don't
>remember anything coming of that.
>Is there some sort of unobtanium material that could be used?
I favor Thomasite, myself.
However, note that if the question starts with, "Is there some sort of
unobtanium...", the answer to the question is "No, there isn't". That's
what "unobtanium" means.
This being rec.arts.sf.science, we can talk about the consequences if
someone were to imagine up such a material as the One Impossible Thing
allowed in their SF story, or we can explain why it actually is an
impossible thing. Which would you prefer?
And note that when someone in real life says, "I've invented a much
better sort of radiation protection", odds are that somewhere in the
fine print is, "...so long as the radiation isn't hard gamma". For
that, there really isn't much you can do except put lots of charge
in the way, and charge tends to come with an irreduceable ammount of
mass attached.
Especially if you want *stable* radiation shielding. Reasons why
positronium or muonium are not ideal for this purpose, are left as
an exercise for the student...
--
*John Schilling * "Anything worth doing, *
*Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" *
*Chief Scientist & General Partner * -13th Rule of Acquisition *
*White Elephant Research, LLC * "There is no substitute *
*John.Sc...@alumni.usc.edu * for success" *
*661-951-9107 or 661-275-6795 * -58th Rule of Acquisition *
Charles...@gmail.com
> On Tue, 4 Mar 2008 08:35:26 -0800 (PST), CharlesRCap...@gmail.com wrote:
> This being rec.arts.sf.science, we can talk about the consequences if
> someone were to imagine up such a material as the One Impossible Thing
> allowed in their SF story, or we can explain why it actually is an
> impossible thing. Which would you prefer?
Well, there is a difference between unobtanium and handwavium. The
latter is impossible when considered with our current understanding,
the former is not impossible but no one can think of a way to do it.
Which may mean that it IS impossible, but it gives the author more
wiggle room. Handwavium must be strictly regulated lest a story become
unbelievable, and is proscribed for the so called "hard" science
fiction. Unobtanium has a lower "cost" in believability, particularly
if the author provides a good explanation and relies on a chain of
technology that is at least plausible if not probable. Now I'm sure
you already knew this, so I'm just stating it for the record.
Now as for the question you asked, I am looking for unobtanium (not
impossible just unobtainable with current tech and/or scientific
understanding) rather than handwavium (impossible given current
understanding of science, but allowable if the author gives a vague
enough explanation, a "hand wave" as it were, and makes the assumption
that the reader will play along) so in this case, the explanation of
why it really is impossible would be the preferred route.
If reduced mass radiation shielding moves from unobtanium to
handwavium, then I must thus discard it if I wish to remain "hard" in
my science fiction.
Now, I'm not expecting to reduce the mass needed for radiation
shielding to negligible status, though any reductions in mass would be
welcome. If for no other reason than I don't have to explain to the
reader that the hull is lined with lead, when I could explain that it
is lined with a high tech composite that works some small amount
better than just lead.
Luke did mention some things that help from a narrative standpoint, he
reminded me that the reaction mass of the ship is quite able to
provide protection from the engine/powerplant's radiation, which is
good since if you run out of reaction mass, then there isn't much need
to run the engine/powerplant. He also pointed out my confusion on the
differences between neutron and gamma radiation.
Robert hit dead on the point I was trying to get at, which is that how
much does random placement of atoms in a material "waste" the
protection of atoms that are shadowed? I'm guessing not much, but a
small amount. Perhaps a crystal form of a heavy metal might perform
better if its "tunnels" are arranged to be perpendicular to the angle
that must be protected. (Not necessarily arranged in a two dimensional
plane but perhaps a crystal structure designed to minimize the number
of shadowed atoms.) However I get the feeling by responses that this
method would be unlikely to generate any kind of reasonable reduction
in mass for the cost of manufacture.
So perhaps a composite material that takes advantage of the way that
neutron radiation causes byproducts. Perhaps a material made up of
alternating layers of heavy metals and hydrogen bearing materials
might be more effective on a mass basis than just masses of metals or
hydrogen alone. Actually I seem to remember something like that being
proposed in the past, but I don't remember the specifics of the
debate.
dwight...@gmail.com
Why do you need the shielding effect anyway? Is it just to protect
the crew and passengers? If so, why not just posit some sort of
advanced medicine?
Damien Valentine
shot, but you might find something helpful in the discussion there.