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Precision Measurement of Radiative Neutron Decay Branching Ratio and Energy Spectrum


Although neutron decay is typically considered as a three-body process, in the radiative correction it is always accompanied by inner-bremsstrahlung (IB) soft photons, n e− + p + νe + γ.


While IB has been measured in nuclear beta decay and electron capture decays, it has only recently been observed in free neutron beta decay. In 2006, we reported the first observation of this radiative decay mode in the journal Nature. The experiment was completed at the NG-6 fundamental physics end station. Since that time, we upgraded the apparatus to enable us to make a precision measurement (≈1%) of both the branching ratio and energy spectrum of the decay photons. The experiment operated by detecting an electron in prompt coincidence with a photon followed by a delayed proton. The requirement that there be a triple coincidence provided a powerful suppression of background events.  This was critical because the very low rate of these events was insufficient to make it measurable above the large rate of random coincidences.  A beam of cold neutrons passed through the bore of a superconducting solenoid. Decay electrons and protons were guided out of the beam by the magnetic field and detected by a silicon detector. The primary improvement was increasing the solid angle of photon detection by constructing a 12-element annular BGO detector that surrounded the decay region of the cold neutron beam (Figure 1). The new detector allowed us to  upgrade the apparatus (Figure 2), and we completed the data acquisition in November of 2009. The photon detector performed very well as did a second detector consisting of bare photodiodes. This detector allowed us to lower the energy detection threshold to about 200 eV, significantly lower than the 15 keV from the first run. In 2012 with published a paper on operation of the detector. In addition, we published papers on anomalous behavior of avalanche photodiodes in high magnetic fields and at low temperatures and on the response of large area avalanche photodiodes to low energy x rays.

In this second run of the experiment, we expect to measure the radiative decay branching ratio to a 1% total uncertainty. We have completed an initial analysis of all the data. We are currently in the process of refining the analysis and studying the systematic effects. Statistically speaking, we should be able to reach our goal of
approximately 1% and are optimistic that we will be able to quantify the systematics at a similar level of uncertainty. This experiment represents an important exploration to future precision radiative decay experiments below 1 % uncertainty.

Photograph Captions:
Figure 1: Photon detection is done by twelve BGO crystals viewed by avalanche photodiodes forming an annular ring around the neutron beam.

Figure 2: Photograph of the apparatus assembled on the NG-6 beam line. The superconducting solenoid is in the center and the beam line is seen entering near the bottom.
Neutron Decay Branching
Photographs by: Neutron Interactions and Dosimetry Group

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