  The paper concerns the crystal based collimation suggested to upgrade the
Large Hadron Collider collimation system. The issue of collimation efficiency
dependence on the muscut angle characterizing nonparallelity of the channeling
planes and crystal surface is mainly addressed. It is shown for the first time
that even the preferable positive miscut could severely deteriorate the
channeling collimation efficiency in the crystal collimation UA9 experiment. We
demonstrate that the positive miscut influence can increase the nuclear
reaction rate in the perfectly aligned crystal collimator by a factor of 4.5.
We also discuss the possible miscut influence on the future LHC crystal
collimation system performance as well as suggest simple estimates for the beam
diffusion step, average impact parameter of particle collisions with the
collimator and angular divergence of the colliding particle beam portion.
