Space Radiation Shielding — 2D Attenuation Monte Carlo and Half-Value Layers

Exponential attenuation

For a narrow, mono-energetic photon beam the uncollided intensity falls as I(x) = I0 · exp(-μ x), with μ = (μ/ρ)·ρ. The distance to an interaction is exponentially distributed, s = -ln(u)/μ, which is exactly how each photon here is sampled. The half-value layer is HVL = ln 2 / μ.

Mass attenuation coefficients are rounded values in the style of the NIST XCOM tables (total, including coherent scattering); polyethylene is built from carbon and hydrogen by mass fraction. Treat them as approximate.

What this view adds

The 3D page shows the shield as an object; here you see each photon's random free path, the depth histogram of where photons were removed, and the measured transmission converging on the straight line on the log plot.

Simplifications

Real spacecraft radiation is mostly energetic protons and heavy ions plus secondary particles, whose shielding depends on nuclear fragmentation and ionisation stopping, where hydrogen-rich materials win per kilogram. This lab models only photons in a narrow beam: no scattering build-up, no secondary particles, no dose conversion. It teaches the exponential law and the role of density and energy, not a mission dose estimate.