  During the epoch of reionization, Ly-alpha photons emitted by the first stars
can couple the neutral hydrogen spin temperature to the kinetic gas
temperature, providing the opportunity to observe the gas in emission or
absorption in the 21-cm line. Given the bright foregrounds, it is of prime
importance to determine precisely the fluctuations signature of the signal, to
be able to extract it by its correlation power.
  LICORICE is a Monte-Carlo radiative transfer code, coupled to the dynamics
via an adaptative Tree-SPH code. We present here the Ly-alpha part of the
implementation, and validate it through three classical tests. Contrary to
previous works, we do not assume that P_alpha, the number of scatterings of
Ly-alpha photons per atom per second, is proportional to the Ly-alpha
background flux, but take into account the scatterings in the Ly-alpha line
wings. The latter have the effect to steepen the radial profile of P_alpha
around each source, and re-inforce the contrast of the fluctuations. In the
particular geometry of cosmic filaments of baryonic matter, Ly-alpha photons
are scattered out of the filament, and the large scale structure of P_alpha is
significantly anisotropic. This could have strong implications for the possible
detection of the 21-cm signal.
