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The targets that make the neutrons wear out quickly.","posed_since":"","precise":"Design an accelerator-driven source giving epithermal neutron flux above $10^9\\,\\mathrm{per}\\,\\mathrm{cm}^2\\,\\mathrm{per}\\,\\mathrm{s}$ at the beam port with fast-neutron and photon dose per epithermal neutron below the IAEA-recommended limits (of order $2 \\times 10^{-11}$ $\\mathrm{rad}\\ \\mathrm{cm}^{2}$ each), using $\\mathrm{Li}-7(p,n)$, $\\mathrm{Be}-9(p,n)$ or $(d,n)$ reactions, with target lifetime of at least hundreds of treatment hours under proton-induced blistering and heat loads of order $10^{11}\\,\\mathrm{erg}/\\mathrm{s}$. An answer is a source with measured spectrum, flux and target lifetime meeting all criteria.","problem_ref":"ac2fbbf8a9ddeec402c9c5aad060fa656f51d59c4d76411f312454c244cc8352","references":"","settled_by":"Measured beam-port spectra and dose components plus target endurance tests on an operating clinical source.","status_note":"An accelerator-based neutron capture system was approved in Japan in 2020 for head and neck cancer, while target lifetime and beam quality at lithium targets remain under development.","title":"Accelerator neutron sources meeting epithermal beam 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