{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"0790ee3eed3cae3137ca2886314c8e409308a6e7413b22e6efe71ecbb95fd5f5","created":"2026-10-03T07:18:08Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"e45783bfc6d384da45ab19b2c1f09f68575fcb839883f93068f60d1121e773a5","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"value","assisted_by":[],"external_id":"nuc.stellar-reaction-rates.c12-alpha-gamma","kind":"well-posed","literature_status":"open","n":"1","parents":[],"plain":"When helium burns in a star, carbon-12 captures a helium nucleus to become oxygen-16 at a rate nobody has measured at the relevant energy. This one rate fixes the carbon-to-oxygen ratio of stars and the masses of the black holes they leave.","posed_since":"","precise":"Determine the total astrophysical S-factor $S_{\\mathrm{tot}}(E_{\\mathrm{cm}}=300\\,\\mathrm{keV})$ of $\\mathrm{12C}(\\alpha,\\gamma)\\mathrm{16O}$ (E1 and E2 ground-state transitions plus cascades), at the Gamow peak for $T \\sim 2e8\\,\\mathrm{K}$, with total uncertainty below 10 percent. Direct data stop near $E_{\\mathrm{cm}} \\sim 1\\,\\mathrm{MeV}$, so the answer requires either direct data closer to 300 keV or an R-matrix extrapolation whose subthreshold-state inputs (1- at 7.12 MeV and 2+ at 6.92 MeV in 16O) are fixed to the needed accuracy.","problem_ref":null,"references":"","settled_by":"A direct measurement below $E_{\\mathrm{cm}} \\sim 0.8\\ \\mathrm{MeV}$ at an underground or high-intensity facility combined with a global $R$-matrix fit that reduces $S_{\\mathrm{tot}}(300\\ \\mathrm{keV})$ to under 10 percent uncertainty.","status_note":"The 2017 R-matrix evaluation (deBoer et al.) remains the reference; no direct measurement has yet reached the Gamow window.","title":"The $12\\mathrm{C}(\\alpha,\\gamma)16\\mathrm{O}$ rate at helium-burning energies to 10 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