Free Antineutrino Absorption Cross Section. I. Measurement of the Free Antineutrino Absorption Cross Section by Protons

Abstract

The cross section for the reaction $p(\overline{\nu }, {\beta }^{+})n$ was measured using antineutrinos ($\overline{\nu }$) from a powerful fission reactor at the Savannah River Plant of the United States Atomic Energy Commission. Target protons were provided by a 1.4×${10}^3$ liter liquid scintillation detector in which the scintillator solution (triethylbenzene, terphyenyl, and POPOP) was loaded with a cadmium compound (cadmium octoate) to allow the detection of the reaction by means of the delayed coincidence technique. The first pulse of the pair was caused by the slowing down and annihilation of the positron (${\beta }^{+}$), the second by the capture of the neutron ($n$) in cadmium following its moderation by the scintillator protons. A second giant scintillation detector without cadmium loading was used above the first to provide an anticoincidence signal against events induced by cosmic rays. The antineutrino signal was related to the reactor by means of runs taken while the reactor was on and off. Reactor radiations other than antineutrinos were ruled out as the cause of the signal by a differential shielding experiment. The signal rate was 36±4 events/hr and the signal-to-noise ratio was $\frac{1}{5}$, where half the noise was correlated and cosmic-ray associated and about half was due to non-reactor-associated accidental coincidences. The cross section per fission $\overline{\nu }$ (assuming 6.1 $\overline{\nu }$ per fission) for the inverse beta decay of the proton was measured to be $(11\mathrm{}\pm 2.6)\times {10}^{-44\mathrm{}}{\mathrm{cm}}^2/\overline{\nu }$ or (6.7±1.5)×${10}^{-43\mathrm{}}$ ${\mathrm{cm}}^2$/fission. These values are consistent with prediction based on the two-component theory of the neutrino.