  Relativistic outflows with neutrons inevitably lead to inelastic collisions,
and resulting subphotospheric gamma rays may explain prompt emission of
gamma-ray bursts. In this model, hadronuclear, quasithermal neutrinos in the
10-100 GeV range should be generated, and they may even have a high-energy tail
by neutron-proton-converter or shock acceleration mechanisms. We demonstrate
the importance of dedicated searches with DeepCore+IceCube, though such
analyses have not been performed. Successful detections enable us to
discriminate among prompt emission mechanisms, probe the jet composition, and
see roles of relativistic neutrons as well as effects of cosmic-ray
acceleration.
