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Precision Measurement of the Branching Ratio and Energy Spectrum of Radiative Neutron Decay


Neutron beta-decay is typically considered as a three-body process, but it is always accompanied by inner-bremsstrahlung soft photons, n → e + p + νe + γ. Measuring this branch of the decay that includes the photon permits test of theoretical calculations and corrections.


While inner-bremsstrahlung 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 at the NIST Center for Neutron Research. Since that time, we upgraded the apparatus to enable us to make a precision measurement of 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 was a significant upgrade to 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 500 eV, significantly lower than the 15 keV from the first run. In 2012 we published a paper on operation of the detector. In addition, papers were published 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 few percent total uncertainty. We have completed an initial analysis of all the data. We are currently in the process of refining the analysis and finalizing the systematic effects. In terms of the statistical precision, we should be able to reach our goal of approximately 1% and are optimistic that we will be able to quantify the systematics at similar levels 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 Physics Group

End Date:


Lead Organizational Unit:



Matthew Bales
Technical University Munich

Herbert Breuer
University of Maryland

Timothy E. Chupp
University of Michigan

Benjamin O'Neill
Arizona State University


Neutron Physics Group

M. Scott Dewey
Thomas R. Gentile
H. Pieter Mumm
Jeffrey S. Nico
Alan K. Thompson
Kevin J. Coakley (NIST - Boulder)


Jeffrey S. Nico