  High precision measurements of isospin $T=1/2$ decays in the neutron and
nuclei provide strong model-independent constraints on extensions to the
standard model of particle physics. A measurement of the $\beta$-asymmetry in
$^{19}$Ne decay between the initial nuclear spin and the direction of the
emitted positron is presented which establishes this decay as the most
precisely characterized nuclear mirror and fixes the Fermi-to-Gamow-Teller
mixing ratio to $\rho = 1.6014(+21/-28)_{sys}(8)_{stat}$. This is consistent
with the previous, most precise measurement, produces a value of the CKM
unitarity parameter $V_{ud}$ in agreement with the nuclear mirror, neutron and
superallowed $\beta$-decay data sets, shows no evidence for second class
currents, and can be effectively used with neutron decay data to place a limits
on exotic tensor couplings.
