Astrophysical and Atmospheric Neutrinos.

The low energy (<200 MeV) atmospheric neutrino flux is calculated by a three-dimensional Monte Carlo cascade in order to estimate the backgrounds for low energy astrophysical neutrino detections. We find that the atmospheric neutrino flux is a significant background and allows only a narrow neutrino astronomy window at 14-30 MeV for the continuous flux of neutrinos from relic supernova explosions. Our calculation includes the geomagnetic latitude dependence of detector site and the angular dependence of neutrino detection direction as an extension to the one-dimensional calculation for use in future neutrino oscillation experiments with atmospheric neutrinos. The atmospheric muon flux calculation, a by-product of this three-dimensional atmospheric neutrino calculation, agrees with the measurement at sea -level within 10% accuracy. We also calculate the medium energy (300 MeV (LESSTHEQ) E(,(nu)) (LESSTHEQ) 3 GeV) atmospheric neutrino flux by a one-dimensional Monte Carlo cascade in order to find the backgrounds for underground nucleon decay experiments and for neutrino oscillation tests with atmospheric neutrinos. Experiments at different latitudes have already confirmed both our atmospheric neutrino fluxes to about 20% accuracy and the gross features of the neutrino angular distribution. As well, we calculate the ultra-high energy (>1 TeV) astrophysical neutrino flux by a nearly model-independent analytic method to estimate the ultra-high energy neutrino flux from active astrophysical compact objects by comparison with the observed gamma ray flux at comparable energies. Applied to several known gamma ray sources, our calculation shows the neutrino flux from them to be only marginally detectable by DUMAND-size underwater detectors.