Measurement of the Beta-Neutrino Correlation of Sodium-21 using Shakeoff   Electrons
  The beta-neutrino correlation coefficient, $a_{\beta\nu}$, is measured in sodium-21 by detecting the time-of-flight of the recoil nucleus detected in coincidence with the atomic electrons shaken off in beta decay. The sample of sodium-21 is confined in a magneto-optic trap. High detection efficiency allows low trap density, which suppresses the photoassociation of molecular sodium, which can cause a large systematic error. Suppressing the fraction of trapped atoms in the excited state using a dark trap also reduces the photoassociation process, and data taken with this technique are consistent. The largest remaining systematic uncertainties come from the measurement of the position and size of the atom trap, and the subtraction of background. We find $a_{\beta\nu}$ =0.5502(60), in agreement with the Standard Model prediction of $a_{\beta\nu}$ =0.553(2), and disagreeing with a previous measurement which was susceptible to an error introduced by the presence of molecular sodium. 
P.A. Vetter, J.R. Abo-Shaeer, S.J. Freedman, and R. Maruyama