Relaxation of axially confined 400 GeV/c protons to planar channeling in a bent crystal

An investigation on the mechanism of relaxation of axially confined 400 GeV/c protons to planar channeling in a bent crystal was carried out at the extracted line H8 from CERN Super Proton Synchrotron. The experimental results were critically compared to computer simulations, showing a good agreement. We identified a necessary condition for the exploitation of axial confinement or its relaxation for particle beam manipulation in high-energy accelerators. We introduce the idea of using a short bent crystal, aligned with one of its main axis to the beam direction, as a beam steerer or a beam splitter with adjustable intensity in the field of particle accelerators. In particular, in the latter case, a complete relaxation from axial confinement to planar channeling takes place, resulting in beam splitting into the two strongest skew planar channels.An investigation on the mechanism of relaxation of axially confined 400 GeV/c protons to planar channeling in a bent crystal was carried out at the extracted line H8 from CERN Super Proton Synchrotron. The experimental results were critically compared to computer simulations, showing a good agreement. We identified a necessary condition for the exploitation of axial confinement or its relaxation for particle beam manipulation in high-energy accelerators. We introduce the idea of using a short bent crystal, aligned with one of its main axis to the beam direction, as a beam steerer or a beam splitter with adjustable intensity in the field of particle accelerators. In particular, in the latter case, a complete relaxation from axial confinement to planar channeling takes place, resulting in beam splitting into the two strongest skew planar channels

Fabrication and characterization of silicon bent crystals for channeling experiments

I cristalli rappresentano uno strumento unico per la manipolazione di particelle cariche ad alta energia mediante interazioni coerenti quali il channeling, grazie al forte campo elettrico generato tra i piani e gli assi atomici. Le applicazioni più interessanti riguardano la collimazione, l’estrazione, la rivelazione e l’assorbimento di fasci di particelle, oltre alla generazione di radiazione x e gamma monocromatica. L’effettiva realizzazione di queste applicazioni è rimasta in sospeso per molti anni a causa della mancanza di bersagli cristallini adatti. Il lavoro presentato in questa tesi è stato dedicato alla fabbricazione di vari cristalli per alcuni recenti esperimenti. Tecniche di fotolitografia, etching in soluzioni alcaline, e taglio meccanico sono state utilizzate per produrre cristalli a strip piegati utilizzando la curvatura anticlastica, e cristalli a lamina piegati utilizzando l’effetto quasi-mosaico. Queste tecniche sono state ulteriormente migliorate introducendo la finitura magnetoreologica, la quale ha permesso di realizzare cristalli con superficie ultrapiatta e aventi disallineamento tra superficie e piani cristallini al di sotto di 5 μrad, come richiesto per la collimazione. Supporti pieganti in titanio, compatibili con l’ultra-alto vuoto della linea di fascio di LHC, e nuovi sistemi di fissaggio sono stati progettati. L’utilizzo di wafer di silicio su isolante ha permesso di fabbricare cristalli di uno spessore ridotto fino a 15 μm, permettendo di effettuare esperimenti sulla deflessione di elettroni di energia attorno al GeV con cristalli curvi. Inoltre, state inoltre realizzate e caratterizzate membrane piatte di silicio, sia cristallino che amorfo, finalizzate a studi fondamentali di diffusione. I metodi per la caratterizzazione sono stati notevolmente migliorati. In particolare, il diffrattometro di raggi x ad alta risoluzione già in uso è stato migliorato nelle prestazioni introducendo un autocollimatore appositamente progettato, e utilizzato per misurare direttamente la curvatura principale, il disallineamento tra superfice e piani cristallini, la curvatura anticlastica e la torsione. Questi sviluppi nella fabbricazione, piegatura e caratterizzazione dei cristalli hanno portato alla produzione di un numero consistente di bersagli cristallini utilizzati con successo in vari esperimenti svolti presso i più importanti acceleratori mondiali. Tra questi, i cristalli quasi-mosaico sono stati utilizzati per osservare per la prima volta una efficiente deflessione di elettroni di energia inferiore al GeV presso i laboratori di MAMI e SLAC, e per studiare quantitativamente la radiazione emessa da questi elettroni. Un cristallo a strip multipla è stato installato nell’acceleratore SPS al CERN per futuri test di collimazione. Un dei cristalli a strip realizzati è stato installato in LHC e testato con successo con protoni all’energia record di 6.5 TeV.Crystals represent a unique tool for high-energy charged particles manipulation via coherent effects, thanks to the strong electrical field generated between atomic planes and axes. Most promising applications are particle beam collimation, extraction, detection and absorption, as well as generation of hard narrow-band x- and gamma-radiation. The implementation of these applications was pending for many years due to the lack of proper crystal target. The work presented in this thesis was devoted to the fabrication of several crystals for some recent experiments. Namely, the techniques of photolitography, wet etching, and mechanical dicing have been applied to fabricate both strip crystals exploiting anticlastic curvature and plates exploiting the quasi mosaic effect. Those techniques have been further enhanced by magnetorheological finishing, which allowed one to fabricate crystals with ultra-flat surface and miscut angle as small as 5 μrad, essential for the beam collimation. Titanium crystal holders, compatible with ultra-high vacuum requirements of the LHC and new clamping system have been also devised. The usage of silicon-on-insulator wafers permitted to fabricate crystals with a thickness of down to 15 μm, essential for the experiments on Gev-energy electron deflection by bent crystals at MAMI and SLAC. Flat crystalline and amorphous membranes for fundamental scattering experiments have been also fabricated and characterized. Crystal characterization methods have been considerably improved. In that number, high resolution x-ray diffractometer was enhanced with a custom made autocollimator and applied to directly measure principal crystal bending, miscut angle, anticlastic bending and torsion. These developments in crystal fabrication, characterization and bending resulted in production of a number of the crystal targets successfully used in experiments at the leading high-energy world laboratories. Namely the quasi-mosaic crystals were used to reach efficient steering and radiation emission of sub-GeV electrons at MAMI and SLAC. The multistrip crystal assembly was installed in the SPS for collimation test. The strip crystal was installed at the LHC and successfully tested in the recent experiment at the record 6.5 TeV proton energy

Manufacturing and characterization of bent silicon crystals for studies of coherent interactions with negatively charged particles beams

Efficient steering of GeV-energy negatively charged particle beams was demonstrated to be possible with a new generation of thin bent silicon crystals. Suitable crystals were produced at the Sensor Semiconductor Laboratory of Ferrara starting from Silicon On Insulator wafers, adopting proper revisitation of silicon micromachining techniques such as Low Pressure Chemical Vapor Deposition, photolithography and anisotropic chemical etching. Mechanical holders, which allow to properly bend the crystal and to reduce unwanted torsions, were employed. Crystallographic directions and crystal holder design were optimized in order to excite quasi-mosaic effect along (1 1 1) planes. Prior to exposing the crystal to particle beams, a full set of characterizations were performed. Infrared interferometry was used to measure crystal thickness with high accuracy. White-light interferometry was employed to characterize surface deformational state and its torsion. High-resolution X-rays diffraction was used to precisely measure crystal bending angle along the beam. Manufactured crystals were installed and tested at the MAMI MAinz MIcrotron to steer sub-GeV electrons, and at SLAC to deflect an electron beam in the 1 to 10 GeV energy range