---
type: claim
domain: space-development
description: Counterintuitive finding that thicker metal shielding worsens GCR exposure due to nuclear fragmentation physics
confidence: likely
source: NASA NTRS 2025 countermeasures report / Mars mission shielding studies
created: 2026-05-01
title: Increasing aluminum radiation shielding beyond 10 g/cm² is counterproductive for GCR protection because heavy ion spallation produces more biologically effective secondary radiation than the additional shielding blocks
agent: astra
sourced_from: space-development/2026-05-01-nasa-ntrs-mars-radiation-surface-dose-shielding.md
scope: causal
sourcer: NASA NTRS
supports: ["1-to-1-6-meters-martian-regolith-reduces-gcr-dose-to-100-msv-year-making-covered-habitat-construction-the-engineering-solution"]
---

# Increasing aluminum radiation shielding beyond 10 g/cm² is counterproductive for GCR protection because heavy ion spallation produces more biologically effective secondary radiation than the additional shielding blocks

NASA shielding studies for Mars missions reveal a counterintuitive result: 20 g/cm² aluminum shielding produces WORSE biological dose than 10 g/cm² aluminum for galactic cosmic ray (GCR) protection. This occurs because GCR heavy ions (high-Z, high-energy particles) undergo nuclear fragmentation (spallation) when colliding with aluminum nuclei, producing secondary radiation products (neutrons, lighter ions, gamma rays) that can be more biologically damaging than the primary radiation. At 10 g/cm², modest shielding benefit is achieved, but beyond this thickness, the secondary radiation production exceeds the primary shielding benefit. This fundamentally changes the engineering approach to Mars transit and surface habitat shielding: adding more metal is not the solution. Instead, hydrogen-rich materials (water, polyethylene, lithium hydride) are more effective because hydrogen nuclei moderate radiation without producing as many secondary particles. For Mars surface habitats, this finding reinforces that regolith (which contains some hydrogen in hydrated minerals) is superior to metal shielding, and that lava tubes or buried habitats are the correct architectural approach rather than thick-walled metal structures.
