We recently suggested using short bent crystals as
primary collimators in a two stage cleaning system for
hadron colliders, with the aim of providing larger impact
parameters in the secondary bulk absorber, through
coherent beam-halo deflection. Tests with crystals a
few mm long, performed with 70 GeV proton beams at
IEHP in Protvino, showed a channeling efficiency
exceeding 85 %. We also observed disturbing phenomena
such as dechannelling at large impact angle, insufficient
bending induced by volume capture inside the crystal,
multiple scattering of non-channeled protons and, for the
first time, a proton flux reflected by the crystalline planes.
Indeed, protons with a tangent path to the curved planes
somewhere inside the crystal itself are deflected in the
opposite direction with respect to the channeled particles,
with an angle almost twice as large as the critical angle.
This effect, up to now only predicted by computer
simulations, produces a flux of particles in the wrong
direction with respect to the absorber, which may hamper
the collimation efficiency if neglected