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The task is to find the physical or chemical process responsible.","posed_since":"2014","precise":"Identify the mechanism by which mean dose rates above roughly $4000\\ \\mathrm{rad}/\\mathrm{s}$ ($1\\ \\mathrm{rad} = 100\\ \\mathrm{erg}/\\mathrm{g}$), delivered in under $200\\ \\mathrm{ms}$, raise normal-tissue tolerance at fixed total dose (typically $1000\\ \\text{to}\\ 3000\\ \\mathrm{rad}$ single fraction) without a matching rise in tumour tolerance. Candidate classes: transient radiolytic oxygen depletion, inter-track radical-radical recombination altering peroxidation yields, differential iron and labile metal pools, immune and vascular responses. An answer is a quantitative model reproducing the measured sparing as a function of dose, dose rate, pulse structure and tissue oxygen tension.","problem_ref":null,"references":"","settled_by":"A model whose parameters are fixed by independent chemical and physiological measurements and that predicts, without refitting, the sparing measured across electron, proton and photon FLASH experiments in several tissues.","status_note":"As of 2025 oxygen depletion alone is widely judged insufficient and no consensus mechanism exists.","title":"Physical mechanism of normal-tissue sparing at ultra-high dose 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