NOTICE: Due to a lapse in annual appropriations, most of this website is not being updated. Learn more.
Form submissions will still be accepted but will not receive responses at this time. Sections of this site for programs using non-appropriated funds (such as NVLAP) or those that are excepted from the shutdown (such as CHIPS and NVD) will continue to be updated.
An official website of the United States government
Here’s how you know
Official websites use .gov
A .gov website belongs to an official government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.
Observation of the Radiative Decay Mode of the Free Neutron
Published
Author(s)
Jeffrey S. Nico, Maynard S. Dewey, Thomas R. Gentile, Hans P. Mumm, Alan K. Thompson, B Fisher, I Kremsky, Fred E. Wietfeldt, T E. Chupp, R Cooper, E Beise, K G. Kiriluk, J Byrne, Kevin J. Coakley
Abstract
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.
Nico, J.
, Dewey, M.
, Gentile, T.
, Mumm, H.
, Thompson, A.
, Fisher, B.
, Kremsky, I.
, Wietfeldt, F.
, Chupp, T.
, Cooper, R.
, Beise, E.
, Kiriluk, K.
, Byrne, J.
and Coakley, K.
(2006),
Observation of the Radiative Decay Mode of the Free Neutron, Nature, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=841194
(Accessed October 8, 2025)