First Measurement of the Neutron $\ensuremath{\beta}$ Asymmetry with Ultracold Neutrons

First Measurement of the Neutron $β$ Asymmetry with Ultracold Neutrons

R. W. Pattie, Jr. et al. (UCNA Collaboration)

Phys. Rev. Lett. 102, 012301 – Published 5 January 2009

Abstract

We report the first measurement of an angular correlation parameter in neutron $β$ decay using polarized ultracold neutrons (UCN). We utilize UCN with energies below about 200 neV, which we guide and store for $\sim 30 s$ in a Cu decay volume. The interaction of the neutron magnetic dipole moment with a static 7 T field external to the decay volume provides a 420 neV potential energy barrier to the spin state parallel to the field, polarizing the UCN before they pass through an adiabatic fast passage spin flipper and enter a decay volume, situated within a 1 T field in a $2 \times 2 π$ solenoidal spectrometer. We determine a value for the $β$ -asymmetry parameter $A_{0} = - 0.1138 \pm 0.0046 \pm 0.0021$ .