The Conservation of Momentum in the Beta-Decay ofY90

Monolayer sources of yttrium (90) are formed by evaporation in a vacuum of ${10}^{-7\mathrm{}}$ mm of Hg. The momentum of individual recoil ions is measured by timing their flight in a field free space. The positively charged Zr (90) recoils appear to lose about 6-electron volts energy as they escape from the surface. If a 6-electron volt correction is made to the observed recoil spectra, there is good agreement regarding shape and absolute amplitude with those recoil spectra calculated on the assumption of a neutrino-electron angular correlation function of ${(1-\beta cos\theta )}^2$. For this function the most probable angle between the neutrino and the electron is ∼135°. These results are in agreement, within experimental error, with earlier data obtained with ${\mathrm{P}}^{32}$. The exact limits of error due to chemical effects at the surface cannot yet be assigned, so it is not possible to make a definite selection of the form of the neutrino-electron angular correlation function.