Nuclear Recoil Following Neutrino Emission from Beryllium 7

The nuclear recoil spectrum of the electron capture isotope Be⁷ was measured from nearly monolayer deposits of the isotope on lithium fluoride and wolfram surfaces. The recoil spectrum was measured with an electrostatic analyzer having a two percent resolution. The spectra exhibited a peak at the high energy end of the spectrum consistent with the hypothesis of single neutrino emission in Be⁷. The maximum recoil ion energy observed was 55.9±{}1.0 electron volts.