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Abstract

A kinematically-complete 2-body final state measurement of Compton scattering of 662-keV photons is presened, where both scattered photon energy and electron recoil energy are measured versus photon scattering angle, theta gamma. Passive collimation of the photon beam is avoided; each recoiling electron triggers a photon-scattering event providing active beam collimation. Recoiling electrons have low energies at small theta gamma, impairing electron detection efficiency. Examining the recoiling-electron energy spectra in coincidence with high-resolution gammas indicates a 1"x 1"NaI detector is superior to a 1"x1" NE-102 plastic scintillator as the active scattering material, for efficient recoil-electron detection. Electron efficiencies versus theta gamma are measured by comparing e-Y coincident yield with the relativistically-correct Klein-Nishina predictions, indicating the detection-efficiency for recoil-electrons is near 100% at theta gamma >30 degrees. Scattered-photon energy pulses and recoil-electron energy pulses are summed electronically to produce an invariant peak at 662 keV, reducing systematic errors in coincident-yield extraction, In addition, E gamma spectra are taken at several theta gamma to provide an experimental value for electron mass; an easier measurement than the Millikan oil drop experiment, but with similar predictive consequences.

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