A GCR proton/nucleus hitting a high-Z metal like aluminium can shatter a target nucleus (spallation), releasing a shower of secondary neutrons and gamma rays that a dense metal barely slows — thicker aluminium makes more showers, so dose behind the shield can rise before it falls.
Hydrogen-rich low-Z material (polyethylene, water) has a proton mass close to the incoming nucleus, so elastic collisions transfer energy efficiently without triggering fragmentation — dose there only ever goes down as you add thickness.
dose_Al(t) = primary·e^(-t/6) + shower(t) (non-monotonic)
dose_poly(t) = primary·e^(-t/9) (monotonic ↓)
- Cyan dot — incoming primary GCR particle.
- Orange/purple burst — secondary neutrons/gammas from a spallation event.
This 2D companion draws the same beam from the side: particles travel left → right, strike the shaded shield band, and either stop cleanly, fragment into a secondary shower, or punch through to the crew zone on the right, exactly as computed by the probability model above — the 3D version renders the identical model as an orbitable scene.