Experimental techniques for deflection and radiation studies with bent crystals

What happens when a high energy charged particle crosses an amorphous material? It loses energy by ionization and its trajectory is affected by the multiple Coulomb scattering, being these phenomena originated by uncorrelated collisions with the atoms. If the atoms of the target were distributed according to an ordered scheme, the uncorrelated collisions would turn into a coherent interaction with the whole atomic structure. This is the case of an aligned crystal that, depending on the orientation, is seen as a set of atomic planes or strings by the impinging particles. Planes and strings produce potential wells able to confine the charged particles in a transversal region of the crystal, in the so called channeling condition, so that, bending the crystal, particles are forced to follow the curvature, being deflected. This simple and powerful idea, dating 1979, is at the basis of many theoretical and experimental studies that have proven its effectiveness, described the possible applications and optimized the deflection performances. The contribution of this thesis locates itself in this field as an attempt to provide a picture of the experimental techniques and the analysis procedures developed to investigate bent crystals in the last years. In this period, bent crystal physics has witnessed a tremendous progress characterized by the increase of performances and the discovery of new phenomena. The driving forces of this process have been essentially three: bent crystals have been identified as a possible solution of the LHC collimation problem, bringing considerable resources to their research field; new bending techniques exploiting secondary deformations have been implemented and, finally, the single particle track reconstruction has become the core of the crystal testing procedures, making the measurements faster and more precise. This thesis is a bridge across two complementary fields, that is the experimental techniques applied in the crystal study and the description of the observed phenomena from the phenomenological point of view as well as from the microscopic theoretical one, resulting in a complete overview on the bent crystal physics. The bent crystals features are presented in the very beginning of the thesis, with the key elements of the research field described in the first chapter before going to an overview on the most important applications to show how bent crystals can be exploited in accelerator physics. The last section of the chapter focuses on the microscopic bent crystal behaviour to describe the physics behind the effects induced by the crystal on the charged particles crossing it. The second chapter describes the state of the art of the bent crystals characterization and test. The first section presents the experimental setup, from the inspiring concepts to the description of the single components and procedures. The setup basic idea is that the combination of a silicon microstrip tracking system with a multi-stage goniometer allows to measure the relative alignment between the beam and the crystal as well as the crystal deflection angle. The second section is dedicated to the analysis methods used to characterize the single crystal behaviour in terms of the channeling and the volume reflection phenomena. The described procedures are applied in a wide range of beam conditions from 13 GeV to 400 GeV of energy with positive and negative particles; all the data have been acquired at CERN on different beamlines during the past three years. The third chapter explores a different bent crystal aspect, that is the radiation emission. Coherent interaction with atoms, in fact, does not only mean deflection capabilities but also quasi-periodic trajectories that, in turn, produce an enhancement of the emitted electromagnetic radiation. The description of the theoretical bases of this phenomenon both in channeling and volume reflection is presented in the first section of the chapter, while the second one is dedicated to the setup and the results obtained with a 120 GeV/c positron beam. The last chapter goes back to the bent crystals deflection issue; going beyond channelling and volume reflection, it shows innovative bent crystals deflection schemes, developed in the last years in order to enhance the efficiency and the angular acceptance provided by channeling. This result can be achieved creating a sequence of aligned crystals to increase the deflection angle induced by volume reflection as shown in the first section or exploiting the interaction with a crystalline axis that, as described in the second section, originates new phenomena such as the multi volume reflection in one crystal (MVROC) and the axial channeling able to deflect both positive and negative particles with high efficiency.