Unobtanium Radiation Shielding?

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? 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?