Clean Beam Research Articles

PHYSICSOPPORTUNITIES ATNEUTRINOFACTORIES

▪ Abstract This article describes the physics capabilities of a future facility that would provide extremely intense and clean neutrino beams. This facility, called a neutrino factory, consists of a muon storage ring with long straight sections, pointed at experimental areas both near and far. Given the expected intensity of muons stored in the rings, this facility could give the lepton sector what decades of precision kaon and B experiments have given the quark sector. One would measure with unprecedented precision the parameters of the leptonic mixing matrix, explore the neutrino mass pattern, and ultimately search for CP violation in the leptonic sector. In addition, the high fluxes at a neutrino factory would open up a whole new field in precision neutrino scattering physics close to the ring itself.

Applications of radioactive ion beams to solid-state physics

Radioactive atoms have been used in solid-state physics and in material science for many decades. Besides their classical application as tracer for diffusion studies, nuclear techniques such as Mofibauer spectroscopy, perturbed angular correlation, β-NMR, and emission channeling have used nuclear properties (via hyperfine interactions or emitted α- or β-particles) to gain microscopical information on the structural and dynamical properties of solids. During the last decade, the availability of many different radioactive isotopes as a clean ion beam at ISOL facilities like ISOLDE/CERN has triggered a new era involving methods sensitive for the optical and electronic properties of solids, especially in the field of semiconductor physics. Extremely sensitive spectroscopic techniques like deep-level transient spectroscopy (DLTS), photoluminescence (PL), and Hall effect gain a new quality by using radioactive isotopes: Due to their decay the chemical origin of an observed electronic and optical behavior of a specific defect or dopant can be unambiguously identified. This review will briefly introduce the experimental techniques used and browse through the ongoing experiments in solid-state physics using radioactive ion beams demonstrating the wide variety of problems under study involving bulk properties, surfaces and interfaces in many different systems like semiconductors, superconductors, magnetic systems, metals and ceramics.

An active collimator system for a high-energy photon beam

Active collimators are designed to both collimate the tagged photon beam and suppress the background of secondary photons. These secondary photons are generated by interaction processes in the collimator medium. They may reach the hadron target with a lower energy compared to the energy determined by the tagging system. Accordingly, those events are tagged with a wrong energy and have to be rejected. By means of a Monte Carlo simulation we investigated the effect and the performance of an active collimator system and developed an optimal design of the collimators to prepare a clean high-energy photon beam. The resulting set-up ensures a precise determination of the tagging efficiency and of the photon flux. The active collimators are a necessary tool to correctly determine photoproduction cross-sections. Results of test measurements at ELSA in Bonn, and MAMI B in Mainz are discussed. Special interest is given to the use of the active collimators in the experiment to test the GDH sum rule at ELSA up to 3.2 GeV .

Experimental Determination of the 236U(nth, f) Cross Section

The 236U(nth, f) cross section was measured for the first time at the high-flux reactor of the Institut Laue-Langevin in Grenoble, yielding a value of (0.3 ± 1.0) mb. This means that σf ≤ 1.3 mb, which is about two orders of magnitude lower than previously adopted. This result was obtained by combining a highly enriched 236U sample with a very clean neutron beam and assuming that the Westcott factor gf = 1 for 236U(n, f) with cold neutrons. The new value is compatible with the latest fission resonance data and with the subthreshold character of the thermal neutron induced fission of 236U.

Broad-area diode-laser system for a rubidium Bose-Einstein condensation experiment

We report on master-oscillator power amplification using a broad-area laser diode (BAL) emitting at a wave- length of λ = 780 nm. The master oscillator is an injection- locked single-mode diode laser delivering a seeding beam of 35 mW, which is amplified in double pass through the BAL up to 410 mW. After beam shaping and spatial filtering by a single-mode fibre we obtain a clean Gaussian beam with a maximum power of 160 mW. There is no detectable contri- bution of the BAL eigenmodes in the spectrum of the output light. This laser system is employed for operation of a 87 Rb magneto-optical trap (MOT) and for near-resonant absorption imaging in a Bose−Einstein condensation experiment.

Surface passivation of GaAs by ultra-thin cubic GaN layer

Attempts were made to passivate the GaAs (001) surface by a pseudomorphic ultra-thin cubic GaN layer formed by a nitrogen radical (N-radical) or nitrogen plasma irradiation technique. Reflection high-energy electron diffraction (RHEED) pattern observations and detailed X-ray photoelectron spectroscopy (XPS) analysis have shown that ultra-thin cubic GaN layer on GaAs (001) surface with desirable surface stoichiometry can be realized with the optimization of surface nitridation process parameters. The passivation effects, characterized by ultra-high vacuum photoluminescence (UHV PL) analysis, revealed strong enhancement in band-edge PL intensity of GaAs after passivation as large as a factor of 10 when compared with the as-grown clean molecular beam epitaxy (MBE) GaAs surface.

MARS: Inverted time-of-flight backscattering spectrometer at SINQ

Very recently, MARS entered the realization stage as the first second generation instrument at SINQ, Villigen, Switzerland. Its fundamental design (distance from source, pulse repetition rate and analyzer material) is very similar to that of IRIS (ISIS, UK), but there are some distinct differences mainly to increase the intensity and to adapt it to SINQ: The whole guide will be supermirror coated ( m=2) and five choppers will provide a very clean beam at the sample. In addition, all analyzer banks will be covered with mica crystals pointing to single 3He-detectors (a total of 12 banks and detectors). This arrangement enables a very flexible adaptation of the secondary to the primary energy resolution – which is depending on the incident energy – by adjusting the analyzer's 2 Θ-angle. A by-product of this discrete number of analyzer banks (in contrast to IRIS' continuous bank) is the possibility to place additional diffraction detectors at various angles between the analyzer banks and hence cover a wider d-range. MARS is planned to become operational in 2001.

Laser beam combining and cleanup by stimulated Brillouin scattering in a multimode optical fiber

A new technique for combining low-power laser beams has been demonstrated by use of semiconductor diode lasers. The technique, which is appropriate for any single-longitudinal-mode laser, is based on stimulated Brillouin scattering (SBS) in long multimode optical fibers. It produces a clean Gaussian-like beam that corresponds to the fundamental fiber mode, irrespective of the profile of the pump. Coherent as well as incoherent combining was demonstrated, and conversion slope efficiencies as high as 67% and 83% were shown to be achievable for the single-pass and ring-cavity SBS geometries, respectively.

Radioactive ion beams and techniques for solid state research

In this paper we review the most recent and new applications of solid state characterization techniques using radioactive ion beams. For such type of research, high yields of chemically clean ion beams of radioactive isotopes are needed which are provided by the on-line coupling of high resolution isotope separators to particle accelerators, such as the Isotope Separator On-line (ISOLDE) facility at CERN. These new experiments are performed by an increasing number of solid state groups. They combine nuclear spectroscopic techniques such as Mössbauer, Perturbed Angular Correlations (PAC) and Emission Channeling with the traditional non-radioactive techniques like Deep Level Transient Spectroscopy (DLTS) and Hall Effect measurements. Recently isotopes of elements, not available before, were successfully used in new PAC experiments, and the first Photoluminescence (PL) measurements, where the element transmutation plays the essential role on the PL peak identification, have been performed. The scope of applications of radioactive ion beams for research in solid state physics will be enlarged in the near future, with the installation at ISOLDE of a post-accelerator device providing radioactive beams with energies ranging from a few keV up to a few MeV.

Quantification of mean energy and photon contamination for accurate dosimetry of high-energy electron beams

The scientific background of the standard procedure for determination of the mean electron energy at the phantom surface from the half-value depth has been studied. The influence of energy, angular spread and range straggling on the shape of the depth dose distribution and the and ranges is described using the simple Gaussian range straggling model. The relation between the and ranges is derived in terms of the variance of the range straggling distribution. By describing the mean energy imparted by the electrons both as a surface integral over the incident energy fluence and as a volume integral over the associated absorbed dose distribution, the relation between and different range concepts, such as and the maximum dose and the surface dose related mean energy deposition ranges, and , is analysed. In particular the influence of multiple electron scatter and phantom generated bremsstrahlung on is derived. A simple analytical expression is derived for the ratio of the incident electron energy to the half-value depth. Also, an analytical expression is derived for the maximum energy deposition in monoenergetic plane-parallel electron beams in water for energies between 2 and 50 MeV. Simple linear relations describing the relative absorbed dose and mass ionization at the depth of the practical range deposited by the bremsstrahlung photons generated in the phantom are derived as a function of the incident electron energy. With these relations and a measurement of the extrapolated photon background at , the treatment head generated bremsstrahlung distribution can be determined. The identification of this photon contamination allows an accurate calculation of the absorbed dose in electron beams with a high bremsstrahlung contamination by accounting for the difference in stopping power ratios between a clean electron beam and the photon contamination. The absorbed dose determined using ionization chambers in heavily photon contaminated (10%) electron beams may be too low - by as much as 1.5% - without correction.

Extremely high-mobility two dimensional electron gas: Evaluation of scattering mechanisms

We report on the characterization of selectively doped GaAs/AlGaAs heterostructures, grown by an extremely clean molecular beam epitaxy system, which exhibit a Hall mobility of a two dimensional electron gas exceeding 10×106 cm2/Vs for a wide range of undoped spacer layer thickness (50–100 nm). A maximum electron mobility of 14.4×106 cm2/Vs was measured at 0.1 K in a structure with a 68 nm spacer thickness and an areal carrier density of 2.4×1011 cm−2. This is the highest electron mobility ever reported, leading to a momentum relaxation mean-free path of ∼120 μm. We present experiments that enable us to distinguish between the main scattering mechanisms. We find that scattering due to background impurities limits electron mobility in our best samples, suggesting that further improvement in structure quality is possible.

CLEAN and WIPE

Clean is a matching pursuit process which has two weak points: in situations of astrophysical inter- est, the \clean map is not stable in the corresponding object representation space, and the \atoms of this rep- resentation (translated versions of the \clean beam) are not well suited for reconstructing the boundaries of the structuring entities of the object at the related resolution level. As a result, Clean must be interrupted before the best possible t is reached. How does Wipe wipe Clean clean? First, by introducing a global regularization prin- ciple based on the concept of resolution; and second, by conducting the matching pursuit process at the level of the high-resolution basis functions of the object space.

Reflective Optics for the ESRF Beamline ID 26

The ESRF beamline ID26 is dedicated to XAFS studies on ultra-diluted samples in the energy range 2.5-25 KeV using the excitation of X-ray fluorescence. The source consists of three phasable planar undulators, inserted in an high β section of the storage ring. The optical design, based on grazing incidence mirrors, aims at obtaining a highly stable clean beam with efficient harmonic rejection together with small vertical and horizontal focal spot sizes. The heat load associated with unwanted high energies is dump in the less critical component, i.e. the first mirror. The latter is a flat Si mirror which acts as a heat sink, introduces a cut-off at high energy and steers the beam out of the brehmsstrahlung emission cone of the straight section. Horizontal and vertical focusing are performed separately by a couple of piezoelectric (PZT) mirrors in a classical Kirkpatrick-Baez configuration. To overcome the limitation due to the maximum length of the commercially available PZT ceramics (150 mm), several plates have been assembled side to side thus allowing a much wider angular acceptance. A wavefront sensor will provide in-situ monitoring of the mirrors shape. Ray-tracing simulations (using the computer code Shadow) show that theoretical focal spot sizes as small as 106x8 μm 2 FWHM could in principle be obtained.

The new prompt gamma-activation analysis facility at Budapest

Prompt gamma-activation analysis (PGAA) is an important complementary technique to conventional instrumental activation analysis that can be successfully used in a number of cases when INAA is not applicable. Therefore, a PGAA facility has been constructed at the recently refurbished and upgraded Budapest Research Reactor. It occupies the end position of a new curved themal guide of 30 m length and 2.5×10 cm2 cross section which provides a clean beam of low energy neutrons. The sophisticated HPGe-BGO γ-ray spectrometer system can be operated in Compton-suppression and pair-spectrometer modes simultaneously. The octal splitting of the main BGO improves efficient pair mode operation when coincidences between pairs of opposite segments and the HPGe detector are required separately. Gamma-gamma coincidence measurements will also be possible when the new multiparameter data acquisition system is completed. One of the main tasks at the new facility will be the accumulation of new spectroscopic data for detector calibration and standardisation, as well as for the construction of a more accurate prompt γ-ray library for the chemical elements. Various applications are planned, such as the determination of hydrogen in fullerenes and of toxic trace elements in environmental samples.

Coherence of the two-level atom bichromatic beam splitter

The recently proposed two-level atom bichromatic beam splitter has been shown theoretically to give exceptionally clean beam splitting in momentum space. In this paper, we investigate the first-order coherence property of this beam splitter, which is an important test of its usefulness in interferometric applications. We find that the beam splitter exhibits maximum fringe visibility of 0.3, which may be increased significantly through conditioning on certain variables. We then propose a type of atom interferometer based on this system, which has many desirable properties when compared with existing versions.

Damage profiling of Ar + sputtered Si(100) surface by medium energy ion scattering spectroscopy

To study the damage profile development due to ion bombardment as a function of the ion dose and the ion incidence angle effect on the damage profiles, MEIS spectra were taken from a clean Si surface and ion beam sputtered Si surfaces. MEIS analysis shows that the surface layer becomes amorphous initially and the amorphous surface layer gets thicker for a Si(100) surface bombarded with 3 keV Ar + ions. The damage profile of the Si(100) saturated at the ion dose of3 × 10 16ion/cm 2at the incidence angle of 35° from the surface normal. At the saturation ion dose, the depth of the damaged layer was 9.6 nm. The depth of the damaged layer was significantly reduced from 14.2 nm at the surface normal to 4.8 nm at the incidence angle of 80°. Though the depth of the damaged layer was minimized for the sputtering at the extremely glancing angle of 80°, the damaged layer did not disappear but remained as a very shallow surface layer. These observations were reproduce by computer simulations by the Marlowe code qualitatively.

Deposition of metal layers on carbon assisted with the same metal's ion radiation

The depth distribution of metals inserted into graphite by means of clean ion beam assisted deposition displays a dependence on the energy density deposited in the atomic collision cascade. Qualitative and quantitative element, phase and chemical analyses of (Cu, Co, Mo, W)/C structures have been conducted with Rutherford backscattering and X-ray photoelectron spectroscopy (XPS) methods. In experimental conditions which satisfy a “global” spike model chemical forces such as the heat of mixing, cohesive energy and probably other effects may be employed for the explanation of experimental data. XPS analysis shows that in the Mo/C system near 60% of metallic molybdenum and 40% of oxide MoO x ( x≈2) are formed while in the W/C system almost every atom of tungsten is oxidized.

Beam analysis system for Texas A&M K500 cyclotron

A beam analysis system has been designed to provide a dispersion of 19.3 cm per % ΔP P with an ultimate aberration limited resolution of ΔE E up to 1 2500 for the full emittance of beams from the Texas A&M K500 cyclotron. Both dispersion matched and slit limited solutions for beam delivered to the MDM spectrometer were obtained. A total of 175 degrees of bend, in opposite direction 88 and 87 degree segments, is used with an intermediate focus between the segments. The first half may be used as a lower resolution analysis system with the second half serving to remove slit scattered particles so that a very clean beam can be transported to the MDM spectrometer for zero degree inelastic scattering measurements. The system is designed to use seven small inexpensive n = 0 dipoles with eight quadrupoles to provide focusing. Techniques for tuning are developed and the stability of the solutions with small changes in focusing are explored.

GaN based III–V nitrides by molecular beam epitaxy

GaN and its allied compounds, AlGaN and InGaN, exhibited electrical and optical properties that are well suited for applications to green/blue/UV emitters and detectors, and high temperature electronics. While vacuum deposition of III–V nitrides is not new, only recently have noteworthy results become available. This is in part due to the use of electron cyclotron resonance (ECR) and other reactive nitrogen sources and clean molecular beam epitaxy (MBE) systems. Low-temperature deposition capability, controlled and hydrogen-free environment, and in situ diagnostic capabilities afforded by MBE make this approach attractive for producing novel structures for devices and understanding the growth pathways which are distinctly different from the conventional III–V compounds. Cubic and wurtzite phases of GaN have been grown with MBE on GaAs, Si, sapphire and SiC, 3C and 6H, substrates. The p-type Mg-doped films require no post-anneals for activation, albeit with lower mobilities than those of organometallic chemical vapor deposition (OMCVD) films. Band edge luminescence with negligible yellow emission has been obtained. Electron mobilities are as high as about 600 cm 2/V · s with n-type doping levels in the high 10 16 cm −3 in wurtzitic films with cubic phase exhibiting higher values.

Characteristics of the 50V(nth,p)50Ti reaction.

The $^{50}\mathrm{V}$(${\mathit{n}}_{\mathrm{th}}$,p${)}^{50}$Ti reaction has been studied at a very clean thermal neutron beam of the ILL's high flux reactor. An accurate cross section value of (0.71\ifmmode\pm\else\textpm\fi{}0.04) mb has been determined, as well as a value of (0.002 364\ifmmode\pm\else\textpm\fi{}0.000 008) u for the $^{50}\mathrm{\ensuremath{-}}^{50}$Ti mass difference. Possible applications in nuclear astrophysics and in reactor technology are pointed out.