To search for neutrino oscillations in the mass range ∆m2 > 0.1 eV2 requires an intense source of well characterized neutrinos. Stopped pion decay from the 1.4 MW, 1.3 GeV , short duty-cycle SNS proton beam is such a source. Neutrinos from stopped pion decay have a well defined flux, well defined energy spectrum, and low backgrounds. The dominant decay scheme that produces neutrinos from a stopped pion source is
π + → µ+ νµ , τ = 26 ns
followed by
µ+ → e+ νµ νe , τ = 2.2 µs.
The neutrinos from stopped π − ’s are highly suppressed because the negative pions are absorbed in the surrounding target material. Thus, neutrinos from the π − decay chain are significantly depleted and can be estimated from the measured νµ , νµ , and νe flux. The νµ energy is monoenergetic (Eνµ = 29.8 MeV ), while the νµ and νe have known Michel energy distributions with an end-point energy of 52.8 MeV . With a simple beam-on timing cut, one can obtain a fairly pure νµ sample with only a 14% contamination of νµ and νe each. This remaining background is easily measured from the time distribution and subtracted.
The expected proton rate from the SNS of 2.2 × 1023 protons/yr, coupled with a yield of 0.12 neutrinos per proton, produces 2.8 × 1022 ν/yr for 100% operation efficiency. Furthermore, the SNS is planning a future upgrade that will deliver MW beams to two sources, making the interesting situation of multiple baselines with a single detector.
A key component of the neutrino oscillation measurement is the physical size of the stopped pion source, which adds an uncertainty to the neutrino path length. For the SNS, the compact liquid mercury target will contribute approximately 25 cm (FWHM), or ∼ 0.4% to the neutrino path length uncertainty.