Ultra-cold Neutron Lifetime Measurement at Los Alamos

Alexander Saunders

A new experiment is now under development at Los Alamos National Lab
that will use Ultra-cold neutrons (UCN) stored in a magnetic bottle to
measure the neutron lifetime.  Knowledge of the neutron lifetime,
taken with measurement of the neutron beta decay asymmetry, allows
extraction of the vector and axial vector weak coupling constants.
Recent measurements of the neutron lifetime have been limited by
neutron-wall interactions and by statistics.  Both limitations will be
overcome in the new experiment, which will use a multipole permanent
magnet trap to eliminate interactions between the stored neutrons and
the trap walls, and will use the new UCN source at LANSCE to supply
enough UCNs to make a 1% measurement of the neutron lifetime in a
small number of fills.  The magnetic trap will be closed on the bottom
and sides by a Halbach array of permanent magnets and on the top by
gravity.  Decay beta particles will be guided to gas wire chamber
detectors by the same externally applied field that maintains the
neutron polarization.  The experiment is now in the detailed design
stage, using internal funding support from Los Alamos Lab.
Commissioning of the new UCN source is now nearing completion.  The
UCN source performance and prospects are described in a separate talk
by T. Ito.