The neutron, though an essential component of most stable nuclei, is itself unstable: After a mean lifetime of about 15 minutes, a free neutron decays into a proton, an electron, and an electron antineutrino. Every so often, a decaying neutron produces an energetic photon, too; usually, as qualitatively shown in the figure, the photon accompanies the creation of a high-speed electron, in a process called electron inner bremsstrahlung.
In 2006, the RDK experiment (not an abbreviation but a play on the words “radiative decay”), led by Jeffrey Nico of NIST in Gaithersburg, Maryland, found that out of every thousand neutron decays, 3.1 ± 0.3 produced a photon with energy between 15 keV and 340 keV, the range to which their detectors were sensitive. Now Nico and his colleagues have released the results of the RDK II experiment: They’ve cut their experimental uncertainty in half, added a second detector array to pick up photons with energies down to 0.4 keV, and made the first precision measurements of the shape of the radiative decay spectrum.
The measured spectrum was fully consistent with theoretical predictions. Indeed, it would have been astonishing if it hadn’t been. Electron inner bremsstrahlung is governed by quantum electrodynamics, a well-understood theory that’s been thoroughly tested, though never before in this particular way.
But pure QED calculations treat the proton and neutron as point particles rather than composite particles made up of quarks. Perturbation theory predicts that the baryons’ internal structure and nonzero size should alter the radiative decay spectrum by about 1%. If RDK’s accuracy can be increased by a further factor of five, it should be sensitive to those effects. (M. J. Bales et al., Phys. Rev. Lett. 116, 242501, 2016.)
