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Precision measurement of the radiative decay of the free neutron

Thomas R. Gentile (Radiation Physics Division, NIST)

A continuous spectrum of photons is emitted in the decay of the free neutron. In the RDK II experiment radiative photons were detected in coincidence with the electrons and protons from neutron decay. The experiment was performed on the NG-6 fundamental physics beam line using two different photon detector arrays. An annular array of bismuth germanium oxide (BGO) scintillators detected photons with energies between 14 keV and 782 keV and an array of large area avalanche photodiodes (APDs) directly detected photons with energies betwen 0.4 keV and 14 keV. The experiment represents the first precision test of the shape of the photon energy spectrum from neutron radiative decay and a substantially improved determination of the branching ratio over a broad range of photon energies. The results test comparison of chiral perturbation theory and QED calculations and corrections, and the experiment provides a basis for possible future experiments and yielded greater understanding of the neutron lifetime apparatus currently on the NG-C beam line.
Various measurements were performed to develop and understand the photon detectors. We discovered previously unknown effects of magnetic fields on APDs at cryogenic temperatures. A model for the response of APDs to low energy X-rays was developed based on measurements at the NIST Synchrotron Ultraviolet Research Facility and the National Synchrotron Light Source at Brookhaven National Laboratory. Studies of the nonproportionality of BGO were performed with radioactive sources.

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