Gamma rays — the spectrum meets the nucleus

At the top of the dial the photons come from nuclear transitions and antimatter. Sealed: the electron's rest energy m_e·c² computes to 510.99 keV (m_e measured by CODATA; c, h, e exact by definition): https://narrowhighway.com/s/2e10f3cd833c0da8ed1973eeb517c9de779e8956cd7d04eeee423aab280c4455 — so when an electron meets a positron, two 511 keV photons fly apart back-to-back, and a PET scanner draws the body by catching those pairs. Cobalt-60's 1.17/1.33 MeV lines, cosmic gamma bursts — the wavelengths are shorter than nuclei. Here the spectrum walk hands off to the nuclear fleet: decay chains and half-lives verify in-domain (NUCLEAR_VERIFY), and the boundary stays honest — which isotope emits what is measured nature, never derivation.