Further studies of the OMNIS supernova neutrino observatory: optimisation of detector configuration and possible extension to solar neutrinos

The basic design principles and earlier origins of OMNIS (Observatory for Multiflavour Neutrino Interactions from Supernovae) have been described in a previous paper [Astropart. Phys. 8 (1997) 27]. Its purpose would be to record a large number of mu and tau neutrinos from a supernova burst, complementing other world detectors which observe mainly electron antineutrinos. This would enable a cosmologically significant neutrino mass to be measured or definitely excluded by the arrival time profile. The detector is based on neutral current excitation of lead and iron target nuclei followed by release of neutrons. Further studies by Fuller et al. [Phys. Rev. D59 (1999) 085005] have shown that a distinctive two-neutron signal will result from an MSW transition between νμ,τ and νe in the supernova, thus adding further physics capability to OMNIS. In this paper we summarise the published neutron production estimates for different targets, with and without mixing, and discuss the results of simulations of a range of target/detector configurations, with the objective of optimising the single and double neutron signals from a given target mass. Discussions are included of the choice of neutron detection method, and the effect of neutron and gamma backgrounds. It is further proposed that OMNIS detector modules might be designed to include real-time solar neutrino spectroscopy using the nuclear excitation principle devised by Raghavan [Phys. Rev. Lett. 78 (1997) 3618].