Radiativeβdecay of the free neutron

Abstract

The theory of quantum electrodynamics predicts that the β decay of the neutron into a proton, electron, and antineutrino is accompanied by a continuous spectrum of emitted photons described as inner bremsstrahlung. While this phenomenon has been observed in nuclear β decay and electron-capture decay for many years, it has only been recently observed in free-neutron decay. We present a detailed discussion of an experiment in which the radiative decay mode of the free neutron was observed. In this experiment, the branching ratio for this rare decay was determined by recording photons that were correlated with both the electron and proton emitted in neutron decay. We determined the branching ratio for photons with energy between 15 and 340 keV to be (3.09 ± 0.32) × 10 −3 (68% level of confidence), where the uncertainty is dominated by systematic effects. This value for the branching ratio is consistent with theoretical predictions. The characteristic energy spectrum of the radiated photons, which differs from the uncorrelated background spectrum, is also consistent with the theoretical spectrum.

Electromagnetic signals from the decay of free neutrons in the first hours of neutron star mergers

“…The decay electrons may power a synchrotron emission if a magnetic field is present. We approximate the energy of the decay electron to be constant by weight-averaging the narrow emitted electron's energy spectrum (Cooper et al 2010). We find E e ≈ 250keV, which corresponds to an electron Lorentz factor of γ e ≈ 1.5.…”

Precision Measurement of the RadiativeβDecay of the Free Neutron

“…An experiment in 2002 placed a limit on the branching ratio for this process [14], and in 2006 the RDK I collaboration reported the first definitive observation of radiative decay [15, 16]. The RDK II experiment improved upon its predecessor by reducing statistical uncertainties through the use of additional photon detectors, improving the understanding of systematic uncertainties through detailed energy response studies of the detectors, and significantly extending the detectable photon energy range to between 0.4 keV and the 782 keV photon energy endpoint.…”

“…The RDK II experiment improved upon its predecessor by reducing statistical uncertainties through the use of additional photon detectors, improving the understanding of systematic uncertainties through detailed energy response studies of the detectors, and significantly extending the detectable photon energy range to between 0.4 keV and the 782 keV photon energy endpoint. Further information on the RDK II apparatus can be found in other references by the collaboration [16–19]. …”

“…The reactor-produced cold neutron beam was guided to the experiment as in RDK I [16], but with increased neutron collimation to decrease backgrounds and systematic uncertainties in decay locations. Using a calibrated 6 Li-foil neutron flux monitor [24, 25] mounted downstream of the detection region, the typical neutron rate was determined to be 1.1×10 8 per second.…”

“…In addition, the theoretical value quoted in Refs. [15, 16] did not include a Coulomb correction to the outgoing decay electron [21, 22]; this should be the largest effect at order α 2 where α is the fine structure constant. Incorporating both of these changes would not alter the value of the previously measured branching ratio, 0.00309 ± 0.00032, but would adjust the RDK I energy range to 19 keV – 313 keV instead of 15 keV – 340 keV and result in a corrected theoretical branching ratio of 0.00259.…”