Fission-Antineutrino Interaction with Protons

The positron kinetic-energy spectrum from the antineutrino absorption reaction $p({\overline{v}}_e,{\beta }^{+})n$ was measured by using a 3.2-liter gadolinium-loaded decalin-based liquid-scintillator target placed between two cylindrical NaI crystals 29 cm in diameter and 7.6 cm thick. The crystals were set to see 0.51-MeV ${\beta }^{+}$ annihilation radiation. A prompt triple coincidence of all three detectors followed by the delayed capture of the neutron is the signature for the reaction. The experiment was performed at one of the powerful fission reactors of the Atomic Energy Commission's Savannah River Plant. The data were obtained during 2484 h of operation at the rate of 0.187 events per hour, with a signal-to-background ratio of approximtely 3/1. The antineutrino spectrum from ${\mathrm{U}}^{235}$ fission gave evidence for a higher energy tail than previously known, implying the presence of half-lives in the several-millisecond range. The cross section per fission antineutrino for the inverse beta decay of the proton was measured to be (0.94±0.13) × ${10}^{-43\mathrm{}}$ ${\mathrm{cm}}^2$, a value consistent with the two-component theory of the neutrino. The measured antineutrino spectrum indicates an expected cross section for the reaction $d({\overline{\nu }}_e,{\beta }^{+})2n$ of (4.2±1.4) × ${10}^{-45\mathrm{}}$ ${\mathrm{cm}}^2$/ (fission ${\overline{\nu }}_e$) and a cross section of (8.4±2.8) × ${10}^{-46\mathrm{}}$ ${\mathrm{cm}}^2$/ (fission ${\overline{\nu }}_e$) for the production of >1.5-MeV recoil electrons by the elastic scattering reaction ${\overline{\nu }}_e+e^{-}$.