The standard model predicts that, in addition to a proton, an electron, and an antineutrino, a continuous spectrum of photons is emitted in the β 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 ± 0.00005[stat] ± 0.00015[syst]. A second detector array of large area avalanche photodiodes directly detected photons from 0.4 to 14 keV. For this array, the spectral shape was consistent with theory, and the branching ratio was determined to be 0.00582 ± 0.00023[stat] ± 0.00062[syst]. We report the first precision test of the shape of the photon energy spectrum from neutron radiative decay and a substantially improved determination of the branching ratio over a broad range of photon energies.

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Published in Physical Review Letters, v. 116, issue 24, 242501, p. 1-6.

© 2016 American Physical Society

The copyright holder has granted permission for posting the article here.

The authors of this article are collectively known as RDK II Collaboration.

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This research was made possible in part by support from the National Science Foundation (Grants No. PHY-0969654, No. PHY-1205266, No. PHY-1205393, No. PHY-1306547, and No. PHY-1505196) and the U.S. Department of Energy (Grant No. DE-FG02-96ER40989 and an interagency agreement).

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