  A comprehensive numerical investigation has been conducted on the angular
distribution and spectrum of radiation emitted by 855 MeV electron and positron
beams while traversing a 'quasi-mosaic' bent silicon (111) crystal. This
interaction of charged particles with a bent crystal gives rise to various
phenomena such as channeling, dechanneling, volume reflection, and volume
capture. The crystal's geometry, emittance of the collimated particle beams, as
well as their alignment with respect to the crystal, have been taken into
account as they are essential for an accurate quantitative description of the
processes. The simulations have been performed using a specialized relativistic
molecular dynamics module implemented in the MBN Explorer package. The angular
distribution of the particles after traversing the crystal has been calculated
for beams of different emittances as well as for different anticlastic
curvatures of the bent crystals. For the electron beam, the angular
distributions of the deflected particles and the spectrum of radiation obtained
in the simulations are compared with the experimental data collected at the
Mainz Microtron facility. For the positron beam such calculations have been
performed for the first time. We predict significant differences in the angular
distributions and the radiation spectra for positrons versus electrons.
