Solar-neutrino neutral-current detection methods in the Sudbury neutrino observatory

The Sudbury Neutrino Observatory will study the solar-neutrino problem through the detection of charged-current (CC), neutral-current (NC), and elastic-scattering (ES) interactions of solar neutrinos with heavy water. The measurement of the NC rate relative to the CC rate provides a nearly model-independent method of observing neutrino oscillations. The NC interaction breaks up the deuteron producing a neutron and a proton. The interaction rate in the original design is measured by observing Čerenkov light from showers produced by neutron-capture γ rays from the capture of the NC neutrons by a selected additive to the heavy water. These signals overlap the CC and ES signals, so that the measurement of the NC rate requires the subtraction of two signals obtained at different times. This paper describes our investigation of an alternate detection method in which the thermalized neutrons are captured by (n, α) or (n, p) reactions on light nuclei. The resulting charged-particle products are uniquely detected by scintillators or proportional counters, completely separating this NC signal from the CC and ES Čerenkov signals, thus simplifying its measurement, improving its significance, and allowing observation of otherwise unobservable short-term NC fluctuations. Although background rates for the new techniques have not yet been determined, the experimental advantages justify further development work.