Day #1: the hard part is in, now I find out if it’s right

I’m building DOSEFIELD, a 1D model that estimates how much radiation an
astronaut absorbs on a deep-space mission — and how much shielding actually
helps. Cosmic rays in, dose-equivalent out.

What’s already solid: the stopping-power core matches NIST PSTAR reference data
to within 1.6%, and behind 10 g/cm² of aluminium at solar minimum the model
gives 1.96 mSv/day — about 0.70 Sv over a 360-day Mars cruise. That’s 17% past
NASA’s 600 mSv career limit. A round trip to Mars quietly blows through the
radiation an astronaut is allowed for their entire career. That’s the reason
this project exists.

But the first version only tracked primary particles, and it showed: my
aluminium, polyethylene and water curves were sitting almost on top of each
other. That’s physically wrong — polyethylene should shield noticeably better,
because hydrogen is good at breaking up heavy ions. The missing piece is nuclear
fragmentation.

So this week I implemented it — real fragmentation cross sections
(Bradt–Peters), not a fudge factor tuned to match NASA’s measured value. The
material difference has to come out of the physics: hydrogen has the shortest
nuclear interaction length per g/cm², so it should win on its own. No parameter
is fit to the answer.

Now I’m validating it, and I’m writing the test down BEFORE the result so I
can’t quietly move the goalposts. If the physics is right, I should see four
things: the curves separate (polyethylene below aluminium), aluminium flatten
out at depth, the mean quality factor drop from 4.82 toward ~3.5–4, and the dose
move closer to the real MSL/RAD cruise measurement. If they don’t, the model is
wrong and I’ll say so.

Numbers in the next entry. Either it works or it doesn’t — that’s the whole
point of logging the test first.

What it still isn’t: 1D, simplified fragmentation, not a replacement for NASA’s
HZETRN. Every value traces to a cited source, and the validation suite re-runs
on every change.