Precision Measurement of the Radiative β Decay of the Free Neutron

Area of Science:

• Nuclear Physics
• Particle Physics

Background:

• The standard model of particle physics predicts photon emission during free neutron beta decay.
• Understanding this rare decay provides insights into fundamental interactions.

Purpose of the Study:

• To precisely measure the photon energy spectrum from neutron radiative decay.
• To test the spectral shape predicted by the standard model.
• To determine the branching ratio of this decay process.

Main Methods:

• Utilized the RDK II experiment with two distinct detector arrays.
• Employed bismuth germanium oxide scintillators for high-energy photons (14–782 keV).
• Used large area avalanche photodiodes for low-energy photons (0.4–14 keV).

Main Results:

• Photon spectral shapes were consistent with standard model predictions across both detector arrays.
• Determined a branching ratio of 0.00335±0.00005(stat)±0.00015(syst) using scintillators.
• Obtained a branching ratio of 0.00582±0.00023(stat)±0.00062(syst) using photodiodes.

Conclusions:

• The study provides the first precision test of the photon energy spectrum shape in neutron radiative decay.
• Substantially improved branching ratio determinations over a wide energy range were achieved.
• Results validate theoretical predictions for this fundamental particle physics process.