  Pristine stars with masses between ~140 and 260 M_sun are theoretically
predicted to die as pair-instability supernovae. These very massive progenitors
could come from Pop III stars in the early universe. We model the light curves
and spectra of pair-instability supernovae over a range of masses and envelope
structures. At redshifts of reionization z >= 6, we calculate the rates and
detectability of pair-instability and core collapse supernovae, and show that
with the James Webb Space Telescope, it is possible to determine the
contribution of Pop III and Pop II stars toward reionization by constraining
the stellar initial mass function at that epoch using these supernovae. We also
find the rates of Type Ia supernovae, and show that they are not rare during
reionization, and can be used to probe the mass function at 4-8 M_sun. If the
budget of ionizing photons was dominated by contributions from top-heavy Pop
III stars, we predict that the bright end of the galaxy luminosity function
will be contaminated by pair-instability supernovae.
