Amorphous Targets Research Articles

Polarized and unpolarized positron sources for electron–positron colliders

A two-step positron facility using a multi-GeV electron beam is under study for electron–positron colliders as ILC or the SuperB factory. In the first step, the electron beam impinges on an oriented tungsten crystal. Channelling radiation provides a large number of γ-rays and consequently creates a large number of pairs in an amorphous target placed after the crystal. Such a method has been successfully tested at CERN and KEK and is now operating at KEK. As a second step and as an upgrade of the system, the electron beam, instead of being sent to the crystal, collides with a circularly polarized laser beam, creating circularly polarized Compton backscattered γ-rays, which impinge on a thin amorphous target where they generate longitudinally polarized positrons. Simulation results are provided for the two steps and the choice of the parameters is discussed for ILC and for the SuperB factory. Particularly, the choice of an ERL instead of a classical linac is recommended for the ILC.

Multiple scattering of relativistic electrons and positrons in thin tungsten crystals

As noted long ago, multiple scattering of relativistic charged particles in a thin crystal differs significantly from that in an amorphous target of the same thickness [1], which in its turn, influences the spectral-angular and polarization characteristics of gamma-radiation during channeling [2], parametric x-ray radiation [3], and other processes. Results [4] also demonstrate the importance of correct allowance for an average squared angle of multiple scattering 2 θ when solving the problem of angular distributions of soft photons radiated in crystals by relativistic particles. The aim of this work is investigation of scattering of electrons and positrons with energy of 500 MeV propagating through tungsten thin crystals in the channeling regime in the vicinity of the and directions for a model of binary collisions. Angular distributions obtained in computer experiments are given, and the average squared angle of multiple scattering 2 θ is compared with that given by the theory of multiple scattering of relativistic particles in an amorphous target and a thin crystal. The results obtained confirm a significant increase in the average squared angle of multiple scattering for angles of radiation incidence relative to atomic chains [1] lying in the interval from one to three Lindhard angles theoretically predicted by the model of multiple scattering on continuous potentials of atomic chains, but are more exact due to independence of the choice of a potential chain model.

Reduction of the energy loss of swift molecular ions in solids due to vicinage effects in the charge state

We have calculated the energy loss of swift O{sub 2}{sup +}, N{sub 2}{sup +}, and C{sub n}{sup +} (n=2-60) molecular ions moving through an amorphous carbon target. The dielectric formalism is used to evaluate the vicinage effects in the energy loss of the atomic ions that form the molecular projectile, but we take into account that the charge state of these atomic ions is affected by their correlated motion through the target and by the screened Coulomb potential between them. When vicinage effects in the charge state are taken into account, the Coulomb repulsion is weakened, leading to a reduction in the interatomic separations ({approx}3% for N{sub 2}{sup +} and {approx}9% for C{sub 60}{sup +}, both having similar velocities). These charge state effects can be neglected for diatomic molecular ions, but they give rise to a reduction of {approx}8% in the vicinage effects in the energy loss of larger molecular ions with cage like geometrical structures, like C{sub n}{sup +} (n=20,60) projectiles.

Experimental and theoretical characterization of the [formula omitted] clusters produced by 337 nm UV laser

The structures and abundances of C n = 2 - 16 - clusters produced from amorphous and graphite targets by 337 nm UV laser are analyzed. A systematic search for the more stable conformers at the DFT/B3LYP level of calculation is presented. Two new series and some new structures of previously reported series of negatively charged carbon clusters are proposed: linear (new members), monocyclic (new members), fused ring and non-planar series. The cluster stability analysis indicates higher abundances for the n = 2, 4, 6, 8 and 12 clusters in the negative spectra, in agreement with the experimental observations.

Experimental study of photon beam polarimetry based on nuclear [formula omitted] pair production in an amorphous target

The degree of linear polarization of a coherent bremsstrahlung (CB) was measured using an azimuthal asymmetry of incoherent e + e - pair production. The measurement was carried out in the γ -2 photon beam line of the Yerevan synchrotron, using a 2.55 GeV electron beam and a diamond radiator oriented to position the primary coherent peak in the interval 0.9–1.1 GeV. The polarization at the peak was measured to be 0.56 ± 0.06 , in good agreement with the value computed from CB spectral shape analysis.

A study of ion implantation into crystalline germanium

We report an experimental and simulation study for introducing dopant ions into (1 0 0) germanium in the energy range from 5 to 40 keV. The successful calibration of our Monte Carlo ion implantation simulator for amorphous and crystalline Ge targets is demonstrated by comparison of predicted boron profiles with SIMS data. The implantation profile in Ge is shallower than in Si for a given energy due to the larger nuclear and electronic stopping power of Ge atoms. The generated point defects and amorphized regions in the crystal are calculated by a modified Kinchin–Pease model. We found that the higher displacement energy in Ge, the stronger backscattering effect, and the smaller energy transfer from the ion to the primary recoil of a cascade are mainly responsible for the significantly reduced implantation damage in Ge. The point responses for boron implantations in Si and Ge are compared with each other. Finally the simulation results for the formation of interdigitated Ge-on-Si PIN-photodiodes are presented.

Surface structuring by multiple ion beams

We study surface patterns produced by low energy ion sputtering of amorphous targets based on a generalized version of the damped Kuramoto-Sivashinsky equation with the focus on the simultaneous incidence of four identical ion beams. This setup allows us to quantitatively investigate the relevance of anisotropic ion-induced effective surface diffusion in theory and in experiments. We find that hexagonal, square, and superstructure patterns can develop depending on the strength of that anisotropy. Squares and superstructures serve as clear indicators for the presence of ion-induced effective surface diffusion. We also discuss the possibility of simulating multiple ion beam incidence by stepwise rotation of the target.

Comparative study of silicon and germanium sputtering by 1–20 keV Ar ions

Sputtering of amorphous Si and Ge targets by 1–20keV Ar ions has been studied using the binary-collision simulation. Special attention was given to the angular distribution of sputtered atoms; namely, the energy dependence of the exponent n in the function cosnθ approximating the angular distribution (θ is the polar ejection angle). It has been shown that at all incident energies the value of n for Ge is much higher than that for Si, which is in contrast with analytical predictions. The reasons for this discrepancy are discussed in detail. In addition, the simulated values of the sputtering yield are also considered and compared with the data from the literature.

Charge state of C10 and C5 energetic cluster ions in amorphous carbon targets: simulations

We present here detailed simulations of the interaction of energeticC10 and C5 clusters at the energies of 1, 2, and 4 MeV per carbon atom with an amorphous carbontarget. The spatial evolution of the cluster components is simulated accounting for bothscattering and Coulomb explosion. The former is calculated by means of the Monte Carlomethod while the latter is computed by means of molecular dynamics. The charge state ofthe individual cluster components is calculated as a function of penetration depth, and isdetermined by the competition between electron ionization and recombination. Theresults of calculations of the effect of the neighbouring cluster components on thesuppression of the values of the charge state are presented and compared to theexperimental values of Brunelle et al. Charge state suppression calculations for the2 MeV/C clustersfor both C10 and C5 agree well with the experimental results for penetration depths of less than about 500 and250 Å respectively, assuming the intracluster Coulomb potential is screened by four target valence electrons.At 4 MeV/C the results are similar although less screening is required; a possible explanation is theinability of the plasma to completely screen the higher velocity projectiles. The1 MeV/C calculated results however differ in their behaviour from the2 and 4 MeV/C cases.

Focused ion beam sculpting curved shape cavities in crystalline and amorphous targets

This work demonstrates accurate sculpting of predetermined micron-scale, curved shapes in initially planar solids. Using a 20keV focused Ga+ ion beam, various features are sputtered including hemispheres, parabolas, and sinusoidal wave forms having dimensions from 1to30μm. Ion sculpting is accomplished by varying the dose at different points within individual scans. The doses calculated per point account for the material-specific, angle-dependent sputter yield, Y(θ), the beam current, and the ion beam spatial distribution. Several target materials are sculpted using this technique. These include semiconductors that are made amorphous or disordered by the high-energy beam and metals that remain crystalline with ion exposure. For several target materials, curved feature shapes closely match desired geometries with milled depths within 5% of intended values. Deposition of sputtered material and reflection of ions from sloped surfaces are important factors in feature depth and profile evolution. Materials that are subject to severe effects of redeposition (e.g., C and Si) require additional dose in certain regions in order to achieve desired geometries. The angle-dependent sputter yields of Si, C, Au, Al, W, SiC, and Al2O3 are reported. This includes normal incidence values, Y(0°), and Yamamura parameters f and Σ.

Characterization of [formula omitted] clusters produced by electronic sputtering

The structures and abundances of C n = 2 – 16 + clusters produced by electronic sputtering from the impact of 252Cf fission fragments on CO ice and from the UV laser desorption of amorphous carbon and graphite targets are analyzed. A systematic search for the more stable conformers at the DFT/B3LYP level of calculation and their contribution to the mass spectra is reported. A new methodology for a proper taxonomic description of the cluster isomers is proposed. Besides the known linear and monocyclic clusters series, new members of these series and two new series are described: the bi-cyclic and/or tri-cyclic and the rhombic core series.

Nonlinear Ripple Dynamics on Amorphous Surfaces Patterned by Ion Beam Sputtering

Erosion by ion-beam sputtering (IBS) of amorphous targets at off-normal incidence frequently produces a (nanometric) rippled surface pattern, strongly resembling macroscopic ripples on aeolian sand dunes. A suitable generalization of continuum descriptions of the latter allows us to describe theoretically for the first time the main nonlinear features of ripple dynamics by IBS, namely, wavelength coarsening and nonuniform translation velocity, that agree with similar results in experiments and discrete models. These properties are seen to be the anisotropic counterparts of in-plane ordering and (interrupted) pattern coarsening in IBS experiments on rotating substrates and at normal incidence.

50 × 50 μ m pixel magnetic focus field emitter array image sensor with high-gain avalanche rushing amorphous photoconductor target

A 50×50μm pixel field emitter array image sensor with a highly sensitive avalanche-mode photoconductive target and a scanning electron focusing system consisting of permanent magnets were fabricated and tested as a step toward the development of ultrahigh-sensitivity compact image sensors for high-definition television cameras. The experimental results revealed that the magnetic focusing conditions of the prototype sensor were in accordance with those estimated from the simulation, and the prototype sensor could obtain both enough resolution for its pixel size and high sensitivity by focusing the electrons emitted from the field emitter array onto the highly sensitive photoconductive target.

Range and longitudinal range straggling with various stopping power formulations for oxygen ions in Si and SiO2

AbstractThe results on comparison of different energy‐loss formulations as well as calculations of the range and range straggling have been presented for the ion–target system where oxygen ions are incident on amorphous silicon and silicon dioxide targets. Applying stopping‐power expressions by Montenegro et al., Öztürk et al., Wang and He, and Gümüş and Köksal, we have numerically calculated range and range straggling within a transport‐theory formalism. An interesting result is that the Montenegro et al. formula produces results better than its corrected version; the Öztürk et al. relation, despite the range and range straggling calculations indicating that the produced results by using the Öztürk et al. formulation are better than the Montenegro et al. formula when compared with experiment. The results with respect to the range straggling of oxygen ions in silicon dioxide show that the deviation from the experiment for the Öztürk et al. formulation is 42% at 200 keV, being the smallest deviation among the SRIM program and the other techniques. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Summary of experimental studies, at CERN, on a positron source using crystal effects

A new kind of positron sources for future linear colliders, where the converter is an aligned tungsten crystal, oriented on the 〈111〉-axis, has been studied at CERN in the WA103 experiment with tertiary electron beams from the SPS. In such sources the photons resulting from channeling radiation and coherent bremsstrahlung create the e+e− pairs.Electron beams, of 6 and 10GeV, were impinging on different kinds of targets: a 4mm thick crystal, a 8mm thick crystal and a compound target made of 4mm crystal followed by 4mm amorphous disk. An amorphous tungsten target 20mm thick was also used for the sake of comparison with the 8mm crystal and to check the ability of the detection system to provide the correct track reconstruction. The charged particles coming out from the target were detected in a drift chamber immersed partially in a magnetic field. The reconstruction of the particle trajectories provided the energy and angular spectrum of the positrons in a rather wide energy range (up to 150MeV) and angular domain (up to 30°). The experimental approach presented in this article provides a full description of this kind of source. A presentation of the measured positron distribution in momentum space (longitudinal versus transverse) is given to allow an easy determination of the available yield for a given momentum acceptance. Results on photons, measured downstream of the positron detector, are also presented. A significant enhancement of photon and positron production is clearly observed. This enhancement, for a 10GeV incident beam, is of 4 for the 4mm thick crystal and larger than 2 for the 8mm thick crystal. Another important result concerns the validation of the simulations for the crystals, for which a quite good agreement was met between the simulations and the experiment, for positrons as well as for photons. These results are presented after a short presentation of the experimental setup and of the track reconstruction procedure.

A photon beam polarimeter based on nuclear [formula omitted] pair production in an amorphous target

To directly measure the linear polarization of coherent bremsstrahlung (CB) photons with a peak energy of 1 GeV at the Yerevan synchrotron, a simple polarimeter is proposed based on a e + e - pair spectrometer equipped with two hodoscopes of scintillating counters positioned above and below of the median plane, configured to detect symmetric pairs produced in an amorphous target. Monte Carlo calculations show that the use of a vertical slit collimator at the entrance of the magnet improves both the analyzing power and the energy resolution. The values of the analyzing power for various atomic form factors of the target (the 20 μ m of aluminum) reach a 0.25–0.28 with expected yields of useful events on the level of a few Hz at nominal operating intensity.

Pulsed laser deposition of magnetic films by ablation of Co- and Fe-based amorphous alloys

Magnetic films were prepared by pulsed laser ablation of amorphous magnetic ribbons (composition Co67Cr7Fe4Si8B14, Fe73.5Nb3Cu1Si13.5B9, and Fe40Ni40B20) and permalloy foils (composition FexNi1-x with x=22,50). Depositions were performed in a vacuum of (2–4)×10-5 Pa by KrF excimer laser pulses at fluences of between 2 and 7 J cm2. Films were deposited on oxidized silicon wafers, placed 60–80 mm apart from the target. Films were analyzed by SEM, XRD, RBS, and EDS. Ferromagnetic resonance (FMR) spectra were studied at GHz frequencies. From RBS and EDS measurements it follows that the stoichiometry of the targets is preserved in the films to a large extent. The films deposited from amorphous targets remain amorphous. From FMR studies it follows that Fe and Fe-Ni rich films exhibit properties close to those of bulk alloys, having very low magnetization motion damping parameter (λ) of 7.0–7.8×107 rad/s, which are appropriate for fast magnetic sensors.

Angular dispersion of protons transmitted through thin gold films

AbstractThe angular distributions of protons travelling through thin polycrystalline gold targets (∼15 nm) with incident energies in the range of 4–10 keV have been measured. The results confirm previous experiments at higher energies showing deviations from theoretical predictions based on the standard multiple scattering theory. In order to prove that the effect of crystal structure is one of the main causes of these deviations we have performed numerical simulations. To simulate the polycrystalline structure in a realistic way, we have made an analysis of the target by means of transmission electron microscopy (TEM) techniques. Including these characteristics in the simulation, together with the effect of vibrations and crystal disorder we analyzed the corresponding angular distribution. To evaluate the role of channeling, we also measured angular distributions of protons in a 〈100〉 gold foil and made the corresponding numerical simulations. The results show the critical influence of the target structure in the angular spectra of transmitted ions. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

256×192 pixel field emitter array image sensor with high-gain avalanche rushing amorphous photoconductor target

A 256×192 pixel field emitter array (FEA) image sensor with a highly sensitive high-gain avalanche rushing amorphous photoconductor (HARP) target was fabricated and tested as a step toward the development of a practical image sensor for ultrahigh sensitivity and ultrahigh-definition compact TV cameras. The experimental results showed that the prototype had an adequate resolution corresponding to the spot size of the scanning electrons on the HARP target which was estimated by the simulation. The prototype also had a wide dynamic range and high sensitivity, demonstrating its potential as a next generation practical image sensor and offering new application of FEAs.

THE LANDAU–POMERANCHUK–MIGDAL EFFECT FOR AMORPHOUS AND CRYSTALLINE MATTER

Presented are the conclusions drawn from experimental investigations of the Landau–Pomeranchuk–Migdal (LPM) effect in amorphous and crystalline targets as well as the preceding and ensuing theoretical activities. In amorphous targets the phenomenon is by now well studied both experimentally and theoretically and is at a mature stage where extrapolations to other energies or materials are well founded. In crystalline targets, the effect is not as well studied as it is harder to deconvolute from other effects like e.g. multiple photon emission. Even in this case, though, a seemingly consistent picture of the suppression mechanism that alters the effective radiation length emerges. Due to the increased scattering, crystals might be used to explore hitherto untested regimes of the LPM effect.