Precision measurement of the radiative beta decay of the free neutron
Christopher D. Bass, Maynard S. Dewey, Thomas R. Gentile, Hans P. Mumm, Alan Keith Thompson, M J. Bales, R. Alarcon, E J. Beise, H Breuer, Jim Byrne, T E. Chupp, Kevin J. Coakley, R L. Cooper, B. O'Neill, F E. Wietfeldt
The theory of quantum electrodynamics predicts that a continuous spectrum of photons is emitted in the beta decay of the free neutron in addition to a proton, an electron, and an antineutrino. We report the first precision test of the shape of the photon energy spectrum from radiative decay and a substantially improved determination of the branching ratio. The photon spectrum was measured over three decades of energy using two different detectors arrays. An annular array of bismuth germanium oxide scintillators detected photons in the energy range of 14 keV to 782 keV. The spectral shape was consistent with theory and we determined a branching ratio of 0.00331 +/- 0.00005 [stat] +/- 0.00015 [syst]. A second detector consisting of large area avalanche photodiodes directly detected photons in the energy range of 0.4 keV to 14 keV. For this array, the spectral shape was consistent with theory and the branching ratio was determined to be 0.00567 +/- 0.00022 [stat] +/- 0.00061 [syst].