Measuring the Parity-Violating Neutron Spin Rotation in Helium: The Neutron Spin Rotation Experiment

We have performed a precision measurement of the parity-violating neutron spin rotation in helium due to the nucleon-nucleon weak interaction at the NIST Center for Neutron Research. The measurement employed a beam of low energy neutrons passing through a liquid helium target system located between a neutron polarizer--analyzer pair. The parity-violating spin rotation magnitude was determined from measured count asymmetries in the analyzer. The expected parity-violating spin rotation of order $10^{-6}$ rad placed severe constraints on the apparatus design. In particular, isolation of the parity-odd component of the spin rotation from the much larger Larmour precession required use of a nonmagnetic target system that allowed movement of the target helium upstream or downstream of a vertical precession coil, which enabled us to take asymmetry differences to subtract background rotations. We describe the design and performance of the apparatus used for this experiment.