  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 Si and Ge crystals using analytic models produced since the
1960's and 70's to describe channeling and blocking effects. We used data
obtained to avoid channeling in the implantation of dopants in Si crystals to
test our models.
