The theory of quantum electrodynamics 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 decay and electron capture decay, it has never been observed in free neutron decay. We report the first observation of this radiative decay mode of the neutron, measured by recording photons in coincidence with both the electron and proton emitted in neutron decay. Correlated background from external bremsstrahlung generated in the electron detector is estimated to contribute less than 3 % of the observed radiative decay event rate. The result of measurements of the dependence of the radiative decay rate on the available phase space of the decay is consistent with the expected behaviour. The characteristic energy spectrum of the radiated photons, which differs from the uncorrelated background spectrum, is also consistent with the calculated spectrum. We determine a branching ratio of (3.13 0.34) x 10-3 (68% level of confidence) in the energy region between 15 keV and 340 keV, where the value is dominated by systematic uncertainties. The result is consistent with theoretical predictions.
Pub Type: Journals
beta decay, cold neutron, neutron decay, quantum electrodynamics, radiative decay, weak interactions