High Efficiency Spin Flipper for the n3He Experiment

The n$^3$He experiment, constructed on the Fundamental Neutron Physics Beamline (FnPB) at the Spallation Neutron Source, is designed to measure the parity violating (PV) proton asymmetry $A_p$ in the capture reaction \begin{equation}\label{I01} n + ^3\!\mathrm{He} \,\longrightarrow \, ^3 \mathrm{H} + p + 765\, \mathrm{keV} \end{equation} The asymmetry has an estimated value $A_p \sim -1\times10^{-7}$ and is directly related to the weak isospin conserved couplings $h_\rho^{0}$ and $\omega_\rho^{0}$ which are of fundamental interest in the verification of the meson exchange model of low energy NN intereactions. Data production for the n$^3$He experiment began in February 2015 and is scheduled to continue thru December 2015 -- reaching a statistical sensitivity $\delta A_p\sim 10^{-8}$ or better. I will discuss the spin flipper which is designed using the theory of double cosine-theta coils, and capable of flipping neutron spins with an efficiency approaching its maximum value $\epsilon_{sf} = 1$. I will also discuss the theory of Spin Magnetic Resonance (SMR) and how it is employed by the spin flipper to flip 60 Hz pulses of cold neutrons over a range of wavelengths.