Radiative Decay of the Free Neutron

QED predicts that beta decay of the neutron into a proton, electron  
and antineutrino should be accompanied by a continuous spectrum of  
soft photons. While this inner bremsstrahlung branch has been  
previously measured in nuclear beta and electron capture decay, it  
has never been observed in free neutron decay. The radiative decay  
mode of free neutrons, measuring photons in coincidence with both the  
emitted electron and proton has been observed for the first time. A  
branching ratio of (3.13 ± 0.34) × 10^-3(68 per cent level of  
confidence) in the energy region between 15 and 340 keV was  
determined, where the uncertainty is dominated by systematic effects.  
The value is consistent with the predictions of QED and chiral  
perturbation theory;  the characteristic energy spectrum of the  
radiated photons, which differs from the uncorrelated background  
spectrum, is also consistent with the calculated spectrum. This  
result may provide opportunities for more detailed investigations of  
the weak interaction processes involved in neutron beta decay.