Analysis of radiation emitted by electrons and positrons through multiple volume reflection in a bent silicon crystal

When a charged particle crosses a crystal with a small angle with respect to a crystallographic axis (or plane), it suffers a series of correlated collisions (i.e. coherent interactions) with the neighbouring atoms in the same row (plane) [1]: this is a result of the influence of the crystal lattice on the motion of the charged particle [2]. In this conditions a charged particle moves in the potential generated by several axes or planes: a positive particle can be captured inside the interaxial (interplanar) potential wells, while a negative particle oscillates around the atomic planes (or axes). In other words the particle is channeled between or around the planes (axes).
If the crystal is bent, a channeled particle follows the direction of the bent atomic planes (or axes), thus bending from its initial direction by an angle equal to the bending angle of the crystal itself [3]. For this reason bent crystals can be used for extraction or collimation of particles from the circulating beam in an particle accelerator.
Over the years, by studying bent crystals, other effects were discovered such as volume reflection (VR), consisting in the deflection of ’quasi-channeled’ particles in a direction opposite to that of crystal bending [4]. The VR angular deflection is fixed by the intrinsic crystal properties and is typically smaller than for channeling. Nevertheless, this effect shows a larger angular acceptance, whichmakes it very interesting fromthe point of view of the beamextraction and collimation. With the aim of increasing the VR angular deflection, two different mechanisms have been developed in recent years: multi volume reflection in a sequence of crystals [5] andmultiple volume reflection inside one crystal (MVROC) [6].
In this thesis we focus our attention on the last effect. MVROC may occur under nearaxis alignment with the particle beam: in this case VR from several lying of planes sharing the same axis sum up leading to significant deflection increase. In chapter 4 the procedure followed to evaluate the mean deflection angle and the efficiency for charged particles crossing the crystal in MVROC geometry is presented.