The theory of quantum electrodynamics (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. Recently, the photon energy spectrum and branching ratio for neutron radiative decay have been calculated using two approaches: a standard QED framework1,2,3 and heavy baryon chiral perturbation theory4 (an effective theory of hadrons based on the symmetries of quantum chromodynamics). The QED calculation treats the nucleons as point-like, whereas the latter approach includes the effect of nucleon structure in a systematic way. Here we observe the radiative decay mode of free neutrons, measuring photons in coincidence with both the emitted electron and proton. We determined 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, where the uncertainty is dominated by systematic effects. The value is consistent with the predictions of both theoretical approaches; 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.
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We thank S. Gardner of the University of Kentucky for discussions and her interest in this work. We acknowledge the support of the National Institute of Standards and Technology, US Department of Commerce, which provided the neutron facilities used in this work. This research was made possible in part by support from the National Science Foundation and a US Department of Energy interagency agreement.
Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.
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Nico, J., Dewey, M., Gentile, T. et al. Observation of the radiative decay mode of the free neutron. Nature 444, 1059–1062 (2006). https://doi.org/10.1038/nature05390