Channeling in direct dark matter detection I: channeling fraction in NaI
  (Tl) crystals
  The channeling of the ion recoiling after a collision with a WIMP changes the
ionization signal in direct detection experiments, producing a larger signal
than otherwise expected. We give estimates of the fraction of channeled
recoiling ions in NaI (Tl) crystals using analytic models produced since the
1960's and 70's to describe channeling and blocking effects. We find that the
channeling fraction of recoiling lattice nuclei is smaller than that of ions
that are injected into the crystal and that it is strongly temperature
dependent.
