Bent Silicon Crystal Research Articles

ON THE POSSIBILITY OF USING STRAIGHT AND BENT CRYSTALS FOR CHARGED PARTICLE BEAM STEERING AT THE FUTURE MULTIFUNCTIONAL ACCELERATOR COMPLEX OF NSC KIPT

The scattering of electrons and positrons with energy of 500 MeV on straight and bent silicon crystals has been considered using numerical simulations. The selected particle energy corresponds to parameters that can be achieved at the future multifunctional accelerator complex of the NSC “Kharkiv Institute of Physics and Technology”. It is shown that bent crystals, at certain orientations, allow deflecting charged particles at angles significantly exceeding the critical angle of axial channeling. It is also shown that experimental investigation of the effect of reducing inco- herent scattering of high-energy positrons when orienting the crystal relative to the direction of particle incidence near one of the main crystal axes is of interest.

Simulation of deflection and photon emission of ultra-relativistic electrons and positrons in a quasi-mosaic bent silicon crystal

Abstract 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.

Experiment on Highly Efficient Deflection of a 1-GeV Proton Beam by a Bent Crystal at the PNPI Synchrocyclotron

The deflection of a 1-GeV proton beam by a bent silicon crystal 1 mm long by an angle of (3.0 ± 0.1) mrad with an efficiency of (32 ± 3)% for end-face capture into the channeling regime has been observed for the first time in the presented experiment. The developed crystal deflector makes it possible to increase the beam deflection angle and can be used to produce low-intensity beams at intermediate energies.

Monitoring of the heavy-ion beam distribution using poly- and monochromatic x-ray fluorescence imaging.

In this work, the first proof of the principal of an in situ diagnostics of the heavy-ion beam intensity distribution in irradiation of solid targets is proposed. In this scheme, x-ray fluorescence that occurs in the interaction of heavy-ions with target atoms is used for imaging purposes. The x-ray conversion to optical radiation and a transport-system was developed, and its first test was performed in experiments at the Universal Linear Accelerator in Darmstadt, Germany. The Au-beam intensity distribution on thin foils and Cu-mesh targets was imaged using multiple x-ray pinholes (polychromatic imaging) and 2D monochromatic imaging of Cu Kα radiation by using a toroidally bent silicon crystal. The presented results are of importance for application in experiments on the investigation of the equation of states of high energy density matter using high intensity GeV/u heavy-ion beams of ≥1010 particles/100ns.

Topography of bent crystals with microradian resolution in one dimension.

Optimum performance in x-ray imaging and spectroscopy of plasmas with bent crystals is achievable only when the crystal reflects the x rays theoretically perfectly across its entire surface. However, typical thin quartz (101̄1) crystal samples kept flat by direct attachment to a flat substrate reflect 8keV x rays differently across their surface, on a scale comparable to the ideal rocking curve. Additional processing improves the uniformity. Irradiation of flat crystals with collimated, monochromatic x rays in rocking curve topography shows such problems directly, with microradian resolution. Nonuniform x-ray reflection is more difficult to document for strongly bent crystals because, then, monochromatic, collimated x rays satisfy the Bragg condition only along a narrow stripe that may be too narrow to resolve with the available cameras. However, it can be resolved with a knife edge that moves through the reflected x rays with the necessary spatial precision as demonstrated here for a bent silicon crystal. This shows qualitatively similar imperfections in the reflection as flat quartz and as the bent quartz analyzers reported on previously with lower resolution.

Feasibility of crystal-assisted collimation in the CERN accelerator complex

Bent silicon crystals mounted on high-accuracy angular actuators were installed in the CERN Super Proton Synchrotron (SPS) and extensively tested to assess the feasibility of crystal-assisted collimation in circular hadron colliders. The adopted layout was exploited and regularly upgraded for about a decade by the UA9 Collaboration. The investigations provided the compelling evidence of a strong reduction of beam losses induced by nuclear inelastic interactions in the aligned crystals in comparison with amorphous orientation. A conceptually similar device, installed in the betatron cleaning insertion of CERN Large Hadron Collider (LHC), was operated through the complete acceleration and storage cycle and demonstrated a large reduction of the background leaking from the collimation region and radiated into the cold sections of the accelerator and the experimental detectors. The implemented layout and the relevant results of the beam tests performed in the SPS and in the LHC with stored proton and ion beams are extensively discussed.

Tronox site in Thann inaugurates sulfuric acid recycling unit

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.

Channeling efficiency reduction in high dose neutron irradiated silicon crystals for high energy and high intensity beam collimation and extraction

The channeling process in bent silicon crystals are used since '70s to manipulate beams of high energy particles. During the last decade, several studies and experiments carried out by the UA9 Collaboration at CERN demonstrated the possibility to use bent crystals for beam collimation, extraction, focusing and splitting in particle accelerators. These crystals are subject to deterioration due to the interaction of the particles with the crystal lattice, degrading the beam steering performance. For this reason, robustness tests are crucial to estimate their reliability and operational lifetime. A ∼8% of reduction in channeling efficiency on crystals irradiated with 2.5·1021/cm2 thermal neutrons was measured and reported in this manuscript. Extrapolations to possible operational scenarios in high energy accelerators are also discussed.

Investigation on radiation generated by sub-GeV electrons in ultrashort silicon and germanium bent crystals

We report on the measurements of the spectra of gamma radiation generated by 855 MeV electrons in bent silicon and germanium crystals at MAMI (MAinzer MIkrotron). The crystals were 15 upmu text {m} thick along the beam direction to ensure high deflection efficiency. Their (111) crystalline planes were bent by means of a piezo-actuated mechanical holder, which allowed to remotely change the crystal curvature. In such a way it was possible to investigate the radiation emitted under planar channeling and volume reflection as a function of the curvature of the crystalline planes. We showed that, using volume reflection, intense gamma radiation can be produced – with intensity comparable to that obtained in channeling but with higher angular acceptance. We studied the trade-off between radiation intensity and angular acceptance at different values of the crystal curvature. The measurements of radiation spectra have been carried out for the first time in bent germanium crystals. In particular, the intensity of radiation in the germanium crystal is higher than in the silicon one due to the higher atomic number, which is important for the development of the X-ray and gamma radiation sources based on higher-Z deformed crystals, such as crystalline undulators.

Observation of strong reduction of multiple scattering for channeled particles in bent crystals

Strong reduction of multiple scattering for channeled particles has been observed in an experiment on the deflection of a 180 GeV/c π+-meson beam by bent silicon crystals. The RMS deflections due to multiple scattering for the channeled particles were about six times smaller than for non-channeled ones. It was shown that the approach suggested recently for the description of multiple scattering for channeled particles using the experimental data for random crystal orientations gives fair agreement with the experiment.

Channeling efficiency in a target-crystal assembly

In view of possible future fixed target experiments requiring precisely steered charged particle beams, the UA9 Collaboration has undertaken experimental studies of the use of bent silicon crystals for this purpose. The channeling efficiency of positively charged particles inside the crystalline lattice has been investigated in detail for a setup with a tungsten target installed in front of the crystal. Due to multiple Coulomb scattering inside the target, the channeling efficiency was observed to be reduced by a factor of about 6.1 for a 180 GeV/c quasi-parallel hadron beam. The yield of nuclear interaction secondaries as an estimation of the additional machine background is also discussed.

Angular asymmetry of the nuclear interaction probability of high energy particles in short bent crystals

The rate of inelastic nuclear interactions in a short bent silicon crystal was precisely measured for the first time using a 180 GeV/c positive hadron beam produced in the North Experimental Area of the CERN SPS. An angular asymmetry dependence on the crystal orientation in the vicinity of the planar channeling minimum has been observed. For the inspected crystal, this probability is about sim 20% larger than in the amorphous case because of the atomic density increase along the particle trajectories in the angular range of volume reflection, whose dimension is determined by the crystal bending angle. Instead, for the opposite angular orientation with respect to the planar channeling, there is a smaller probability excess of sim 4%.

Channeling and radiation experiments at SLAC

Since 2014, a SLAC-Aarhus-Ferrara-CalPoly collaboration augmented by members of ANL and MIT has performed electron and positron channeling experiments using bent silicon crystals at the SLAC End Station A Test Beam as well as the FACET accelerator test facility. These experiments have revealed a remarkable channeling efficiency of about 24% under our conditions. Volume reflection is even more efficient with almost the whole beam taking part in the reflection process. A positron experiment demonstrated quasi-channeling oscillations for the first time at high beam energy. In our most recent experiment we measured the spectrum of gamma radiation for crystal orientations covering channeling and volume reflection. This series of experiments supports the development of more advanced crystalline devices capable e.g. of producing narrow-band gamma rays with electron beams or studying the interaction of the electrons with the wakefields generated in the crystal at high beam intensity.

Photon emission by volume reflected electrons in bent crystals

We present a quantitative experimental analysis of the radiation emission of 12.6 GeV electrons moving along and across the (111) planes of a strongly bent silicon crystal with a bending radius of 0.15 m. We have measured the radiation emitted during volume reflection, which has shown to strongly enhance the emission of low-energy photons, and a convincing agreement between simulations and experimental data is found.

Beam steering performance of bent silicon crystals irradiated with high-intensity and high-energy protons

Beam steering performance of bent silicon crystals irradiated with high-intensity and high-energy protons has been studied. In particular, crystals of the type used for collimation and extraction purposes in the Large Hadron Collider and the Super Proton Synchrotron at CERN have been irradiated at the HiRadMat CERN facility with 2.5 times 10^{13} 440 GeV/c protons, with a pulse length of 7.2 upmu s. The purpose is to study possible changes in bending angle and channeling efficiency due to thermo-mechanical stresses in case of accidental irradiation during accelerator operations. A comparison between measurements performed before and after the irradiation does not show any appreciable performance reduction in either crystal.

Septum shadowing by means of a bent crystal to reduce slow extraction beam loss

The flux of high-energy protons slow-extracted from the CERN Super Proton Synchrotron (SPS) is limited by the induced radioactivity caused by the beam loss intrinsic to the extraction process. Methods to substantially increase the efficiency of the extraction process are of great interest to fulfill requests for an increasing flux of 400 GeV protons to the present experiments, located in the North Area of the SPS, and also for potential future experiments with very high demanded protons on target. A crystal shadowing technique to significantly reduce the beam scattered and lost on the electrostatic extraction septum during the third-integer resonant slow extraction process has been developed and a prototype system tested with beam. The technique is based on the use of a thin, bent silicon crystal to coherently channel or volume reflect the portion of beam that would otherwise impinge the wire array of the electrostatic septum and instead eject it into the transfer line toward the production targets of the experiments. In this paper, the concept is described and applied to the SPS machine in order to specify the requirements of the prototype crystal shadowing system. Beam dynamics simulations of the prototype system are compared and benchmarked to the results obtained through beam tests, before being exploited to understand the characteristics of the present system and the potential performance reach of an optimized, future operational configuration. The remaining challenges faced to bring the system into operation, the optimization possibilities and other potential applications are discussed.

Commissioning and operation of the Cherenkov detector for proton Flux Measurement of the UA9 experiment

The UA9 Experiment at CERN-SPS investigates channeling processes in bent silicon crystals with the aim to manipulate hadron beams. Monitoring and characterization of channeled beams in the high energy accelerators environment ideally requires in-vacuum and radiation hard detectors. For this purpose the Cherenkov detector for proton Flux Measurement (CpFM) was designed and developed. It is based on thin fused silica bars in the beam pipe vacuum which intercept charged particles and generate Cherenkov light. The first version of the CpFM is installed since 2015 in the crystal-assisted collimation setup of the UA9 experiment. In this paper the procedures to make the detector operational and fully integrated in the UA9 setup are described. The most important standard operations of the detector are presented. They have been used to commission and characterize the detector, providing moreover the measurement of the integrated channeled beam profile and several functionality tests as the determination of the crystal bending angle. The calibration has been performed with Lead (Pb) and Xenon (Xe) beams and the results are applied to the flux measurement discussed here in detail.

Focusing of 180 GeV/c pions from a point-like source into a parallel beam by a bent silicon crystal

At large accelerators, bent crystals are employed to deflect weakly divergent proton beams at the stages of extraction and collimation. We demonstrate that a divergent particle beam may be efficiently deflected using a crystal with a focusing entry face. A 180 GeV/c pion beam with divergence near 0.060 mrad, which exceeds the Lindhard angle by a factor of 4, has been experimentally deflected by 0.25 mrad with efficiency near 22%. The proposed focusing crystal may serve as an element of a novel particle optics for secondary-particle beams in the TeV energy region.

Dechanneling of high energy particles in a long bent crystal

Experimental results on deflection of a 180 GeV/c π+-meson beam by a 23 mm long bent silicon crystal are analyzed to study the dechanneling process of particles due to multiple scattering. A new approach for the determination of contributions from atomic nuclei and electrons to the multiple scattering using the experimental data for random crystal orientations is suggested. The results of simulations performed using this approach, in which the contribution from atomic electrons is considered according to the prescription of Bethe, are in a good agreement with the experiment.

Extravillous trophoblast invasion is accelerated by WNT10B

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.