  We present a new hybrid code for large volume, high resolution simulations of
cosmic reionization, which utilizes a N-body algorithm for dark matter,
physically motivated prescriptions for baryons and star formation, and an
adaptive ray tracing algorithm for radiative transfer of ionizing photons. Two
test simulations each with 3 billion particles and 400 million rays in a 50
Mpc/h box have been run to give initial results. Halos are resolved down to
virial temperatures of 10^4 K for the redshift range of interest in order to
robustly model star formation and clumping factors. This is essential to
correctly account for ionization and recombination processes. We find that the
halos and sources are strongly biased with respect to the underlying dark
matter, re-enforcing the requirement of large simulation boxes to minimize
cosmic variance and to obtain a qualitatively correct picture of reionization.
We model the stellar initial mass function (IMF), by following the spatially
dependent gas metallicity evolution, and distinguish between the first
generation, Population III (PopIII) stars and the second generation, Population
II (PopII) stars. The PopIII stars with a top-heavy IMF produce an order of
magnitude more ionizing photons at high redshifts z>10, resulting in a more
extended reionization. In our simulations, complete overlap of HII regions
occurrs at z~6.5 and the computed mass and volume weighted residual HI
fractions at 5<z<6.5 are both in good agreement with high redshift quasar
absorption measurements from SDSS. The values for the Thomson optical depth are
consistent within 1-sigma of the current best-fit value from third-year WMAP.
