Precision Measurement of the Radiative $\Beta$ Decay of the Free Neutron

Understand

The standard model predicts that, in addition to a proton, an electron, and an antineutrino, a continuous spectrum of photons is emitted in the $\beta$ decay of the free neutron.

• We report on the RDK II experiment which measured the photon spectrum using two different detector arrays.
• An annular array of bismuth germanium oxide scintillators detected photons from 14 to 782~keV.
• The spectral shape was consistent with theory, and we determined a branching ratio of 0.00335 $\pm$ 0.00005 [stat] $\pm$ 0.00015 [syst].