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Getting to ten would place each emission directly, with no image reconstruction.","posed_since":"2017","precise":"Determine whether a coincidence time resolution of 10 ps full width at half maximum is attainable in a detector of sufficient stopping power for 511 keV photons, which corresponds to a position uncertainty of about 1.5 mm along the line of response. Required chain: photon production statistics and rise time, light transport, photodetector transit-time spread, and electronics jitter. An answer is a demonstrated detector pair or a proof that a chain element forbids it.","problem_ref":null,"references":"","settled_by":"A bench measurement of 10 ps coincidence resolution between two detectors with 511 keV efficiency comparable to clinical crystals.","status_note":"Laboratory detector pairs reached the tens of ps range by the mid 2020s; clinical systems were at about 180 to 230 ps.","title":"Is 10 ps coincidence time resolution 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