The Radiative Decay Mode of the Free Neutron

The beta-decay of the neutron has been extremely important in  
understanding the framework of the Standard Model of physics.  The  
neutron is an important laboratory to verify predictions from this  
and other new, exotic theories.  The well-established theory of  
quantum electrodynamics predicts that neutron beta-decay, consisting  
of an emitted proton, electron, and anti-neutrino, can be accompanied  
by high-energy photons.  The first observation of this rare decay  
mode was made by measuring the proton, electron, and photon in  
coincidence.  A high magnetic field from a superconducting magnet was  
used to guide the charged particles to a silicon surface-barrier  
detector.  The photons, being unaffected by magnetic fields, require  
a large area detector capable of operating in the cryogenic, high  
magnetic field environment of the superconducting magnet.  An  
inorganic, bismuth germanate scintillating crystal coupled to an  
avalanche photodiode served as a novel photon detector.  Furthermore,  
an electrostatic mirror was used to change the available phase space  
for decays by reflecting the low energy protons towards the surface- 
barrier detector that were initially emitted in the wrong direction  
for detection.  Extensive Monte Carlo simulation was used to predict  
the decay rate versus applied mirror voltage.  A branching ratio of  
(3.13±0.34)x10-3 was measured, consistent with the theoretical  
prediction of 2.85x10-3 for photons of energy 15 keV to 340 keV.   
Also described is work to improve the overall uncertainty to the 1%  
level.  In order to increase the number of events collected, the  
design and testing of a new 12-element detector is described.