final answer on radiation shielding vs. 1975 Summer Study

OK, thanks to some help from Chris Sandridge at NASA, and the OLTARIS
project, I now have some pretty reliable numbers on GCR attenuation by
thick shields.

We simulated a sphere of water, bathed by cosmic radiation in free space
(at the 1977 solar minimum), and measured the dose equivalent (which
takes into account the "quality" or biological effect of particles of
various energies) at a number of different thicknesses. Here's the
result, in tabular form:

kg/m^2 rem/yr
0 81.7199
250 30.1771
500 23.5447
1000 20.5608
2000 15.2931
3000 10.5715
4000 7.0058
5000 4.4928
6000 2.8040
7000 1.7133
8000 1.0277
9000 0.6065
10000 0.3525

So the left column is how much mass (in the form of water, in this case)
you have to pile up around your habitat, to get the dose equivalent in
the right column.

Other shielding materials could be slightly more or less efficient than
water, but aren't going to change the picture all that much. Long story
short, if you want Earth-like levels of background radiation, you need
about 10 tons per square meter of shielding.

So the 1975 study, which said 4500 kg/m^2 would suffice to get to 0.5
rem/yr, was off by about a factor of two -- but not the factor of 10
that I was previously thinking.

Unfortunately this clear answer came in too late to make it into High
Frontier v0.02, which I just released earlier today. The radiation
model in that version uses the more conservative attenuation curve.
I'll correct that in the next version to fit this OLTARIS simulation.

Best,
- Joe