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Magnetically Trapped Neutron Lifetime Experiment


The decay of the free neutron is the simplest nuclear beta decay and is the prototype for all charged current semi-leptonic weak interactions.  The decay parameters, the neutron lifetime in particular, provide essential inputs to investigations of the weak interaction.


A precise value for the neutron lifetime is required for several internal consistency tests of the SM including searches for right-handed currents and tests of the unitarity of the CKM mixing matrix.  Measurements of neutron decay coefficients provide information on the vector and axial-vector coupling constants gv and ga.  The neutron lifetime is also an essential parameter in the theory of Big Bang Nucleosynthesis. As well as in determinations of the expected neutrino flux from nuclear reactors.

At present there is a 4 σ discrepancy between determinations of the neutron lifetime coming from UCN bottles when compared to those coming from neutron beam techniques.  This discrepancy is not understood.  Thus it is essential to resolve this disagreement, a goal that can best be accomplished through measurements using systematically different techniques.  As beam-type experiments are limited by measurements of the neutron flux, and material bottle experiments are complicated by wall interactions, magnetic trapping techniques offer a powerful approach for both solving this discrepancy and improving the precision of the neutron lifetime.


Operating Apparatus
Operating Apparatus (Photograph by: Neutron Physics Group)

Lead Organizational Unit:



Neutron Interactions and Dosimetry Group

Hans (Pieter) Mumm

Alan K. Thompson

NIST Associates:

Paul R. Huffman
North Carolina State University

Karl Schelhammer
North Carolina State University