Radiation shielding - is it necessary in the Outer Solar System?

Radiation shielding is always considered in all projects of space
settlements that I've ever seen. Sometimes it covers just the "population
sector" of the settlement, leaving agricultural and industrial parts
unprotected; sometimes it covers the whole thing. However, it always impose
severe engineering and architectural constraints in the project. After all,
shields - and I'm talking about *passive* shields, those that would block
radiation using a very thick wall - are *very* massive, and you must take
that into account when you're designing a rotating space city. Several tons
of rock or water per square meter of outer hull...

However, the most dangerous kind of radiation in space is the one coming
from the Sun - a solar flare strong enough can, indeed, hit a human being
with a lethal dose of radiation. Cosmic radiation, on the other hand, is
rather constant and somewhat much less harmful: being exposed to it brings
as much risk of getting cancer as smoking, according to a recent article
that I read in "Scientific American". Therefore, is radiation shielding a
necessity in the Outer Solar System? I think that the solar radiation would
be so feeble beyond Jupiter's orbit that "light" space colonies, having
nothing but an atmosphere-containing hull, would be perfectly viable. Does
anyone have data on how far from the sun solar radiation in space would be
harmless?

Dr. Omni
ICQ # 53853815
http://www.gold.com.br/~lsc