Volume 76, Issue 5, May 2005
- optics; atoms and molecules; spectroscopy; photon detectors
- particle sources, optics and acceleration; particle detectors
- nuclear physics, fusion and plasmas
- microscopy and imaging
- condensed matter; materials
- biology and medicine
- gravity; geophysics; astronomy and astrophysics
- electronics; electromagnetic technology; microwaves
- thermometry; thermal diffusivity; acoustic; photothermal and photoacoustic
- general instruments
Index of content:
- OPTICS; ATOMS AND MOLECULES; SPECTROSCOPY; PHOTON DETECTORS
76(2005); http://dx.doi.org/10.1063/1.1896947View Description Hide Description
To measure the 3D characteristic of biological nonsmooth surface, a multipoint focus-detecting method based on the light splitting effect of crossed grating was developed. The surface profile is obtained by analyzing the variation in spot size on it rather than in light intensity used in conventional confocal method. Thus the influence of surfacereflectivity variation could be reduced. The system is simple and the lateral resolution could be adjusted by changing the moving step of the stage on which the sample lies. Currently the axial uncertainty is about . The profile of fish scales was measured and the uncertainties were analyzed.
76(2005); http://dx.doi.org/10.1063/1.1896685View Description Hide Description
Details of a new approach for performing metastable de-excitation spectroscopy are given. A beam of metastable helium atoms, produced in a hollow cathode dc discharge, is collimated and subsequently focused using Doppler cooling of the transition at , forming an intense probe of up to . The large distance between source and sample means that the beam is relatively free of UV photons and metastable atoms, removing the need for quench lamps and chopper wheels. As well as providing a clean high intensity source, the well defined nature of the beam is a necessary step towards using more sophisticated laser-cooling techniques with the ultimate aim of producing a metastable helium microscope. MDS and UPS spectra from Si(111) are shown.
76(2005); http://dx.doi.org/10.1063/1.1897666View Description Hide Description
This work shows a method to characterize nearly flat reflective surfaces. Slope error on the mirror surface under the total reflection condition of x rays is measured by perfect crystal rocking curve profile widening. The experiment was performed using a conventional x-ray tube and a simple optical setup. The sensitivity was sufficient to detect at least a slope error of , typical quality for synchrotron mirrors. It is shown that this technique can also be used to characterize roughness and curvature. The method proposed here is an alternative to the traditional techniques for surface characterization and has the advantages of simplicity, versatility and sensitivity, and the synchrotron mirror quality can be evaluated under similar conditions as those for the designed operation.
76(2005); http://dx.doi.org/10.1063/1.1896525View Description Hide Description
We introduce a neural-network-based filter devised to extend the dynamic range of pyroelectricdetectors which otherwise would only be useful for medium-to-high energy measurements. To accomplish this task, we trained a multilayer perceptron through the back-propagation method using the theoretical signal derived from the detector equivalent electric circuit. We tested the performance of the neural-network filter both numerically and experimentally. In the former case we recovered theoretical signals corrupted with white and impulse noise and compared the results with those obtained through the use of standard filtering methods. In the latter case, we applied the filter to measure pulses from a Nd:YAG laser whose energy was below the detector noise-equivalent energy. With this processing technique in a standard PC we have been able to accurately measure laser energy values as low as one-tenth the detector’s noise-equivalent energy and at repetition rate.
Discharge flow tube coupled to time-of-flight mass spectrometry detection for kinetic measurements of interstellar and atmospheric interests76(2005); http://dx.doi.org/10.1063/1.1898727View Description Hide Description
An apparatus has been developed to study the kinetics of radical reactions of interstellar and atmospheric interests. It consists of a fast flow reactor in which atoms and radicals are initially produced by a microwave discharge. The gas mixture is ionized either using electron ionization or using single photonionization (at 10.2 eV) or resonance enhanced multiphoton ionization and is detected using time-of-flight mass spectrometry. In order to prove the reliability of the apparatus to perform kinetic measurements, the well-known reaction of H atoms with -but-2-ene has been investigated at room temperature and found in very good agreement with previous measurements reported in the literature.
76(2005); http://dx.doi.org/10.1063/1.1899483View Description Hide Description
We developed a laser interferometer allowing parts per million resolution to be achieved in fringe division for the measurement of the Si (220) lattice spacing by combined x-ray and optical interferometry. With the aim of obtaining measurement uncertainty, we pursued the study of a troublesome noise in interferometer operation and eventually identified its origin in a minute interferometer optical feedback. We investigated mathematically the feedback effect and verified predictions experimentally. Imperfect isolation and light recycling having been identified and subsequently experimentally proved, we took remedial steps in order to improve measurement capabilities. This investigation has brought into light and has made it possible to quantify an interferometernoise up to now overlooked.
76(2005); http://dx.doi.org/10.1063/1.1899404View Description Hide Description
A velocity slice imaging method is developed for measuring the angular distribution of fragment negative ions arising from dissociative electron attachment (DEA) to molecules. A low energy pulsed electron gun, a pulsed field ion extraction, and a two-dimensional position sensitive detector consisting of microchannel plates and a wedge-and-strip anode are used for this purpose. Detection and storage of each ion separately for its position and flight time allows analysis of the data offline for any given time slice, without resorting to pulsing the detector bias. The performance of the system is evaluated by measuring the angular distribution of from and comparing it with existing data obtained using conventional technique. The capability of this technique in obtaining forward and backward angular distribution data is shown to have helped in resolving one of the existing problems in the electron scattering on .
76(2005); http://dx.doi.org/10.1063/1.1914780View Description Hide Description
We describe the implementation of a cylindrical T-shaped alkali-vapor cell for laser spectroscopy in the presence of a longitudinal electric field. The two windows are used as two electrodes of the high-voltage assembly, which is made possible by a metallic coating which entirely covers the inner and outer sides of the windows except for a central area to let the laser beams in and out of the cell. This allows very efficient application of the electric field, up to in a rather dense superheated vapor, even when significant photoemission takes place at the windows during pulsed laser irradiation. The body of the cell is made of sapphire or alumina ceramic to prevent large currents resulting from surface conduction observed in cesiated glass cells. The technique used to attach the monocrystalline sapphire windows to the cell body causes minimal stress birefringence in the windows. In addition, reflection losses at the windows can be made very small. The vapor cell operates with no buffer gas and has no magnetic part. The use of this kind of cell has resulted in an improvement of the signal-to-noise ratio in the measurement of parity violation in cesium vapor underway at ENS, Paris. The technique can be applied to other situations where a brazed assembly would give rise to unacceptably large birefringence in the windows.
76(2005); http://dx.doi.org/10.1063/1.1896945View Description Hide Description
We present details of a spin-echo spectrometer, designed to make possible a wide range of new surface dynamics measurements. The apparatus operates at beamenergy of , sufficiently high to enable processes such as surface Bragg diffraction and permit inelastic and quasielastic scattering measurements at up to momentum transfers of about . We describe the requirements for the machine, details of the major components used to fulfil these requirements, and the performance of the overall spectrometer. The machine can access a Fourier time range of , and yields a resolution of for inelastic spectrum reconstruction, although under favorable circumstances quasielastic broadenings as narrow as can be resolved, allowing correspondingly slower processes to be studied.
Portable 543 nm length standard and magnetic-induced zero-crossing shift on length standard transition76(2005); http://dx.doi.org/10.1063/1.1898216View Description Hide Description
This study reports on the development of a portable iodine-stabilized 543 nm (green) length-standard laser, which is low cost and robust. All the needed optics and electronics were integrated into one box which measured 36 cm in length, 30 cm in width, and 15 cm in height. Simple circuits for third harmonic locking are presented and the laser’s frequency instability could be as small as , normalized to one hertz bandwidth. The influence of the magnetic field to the locking point of the 543 nm length-standard transitions was investigated.
76(2005); http://dx.doi.org/10.1063/1.1914785View Description Hide Description
We describe a type of phase and frequency detector employing both an analog phase detector and a digital phase and frequency detector. The analog and digital detectors are mutually exclusive so that only one of them is active at any given time, resulting in a phase detector with both the broad capture range of digital circuits and the high speed and low noise of analog mixers. The detector has been used for phase locking the diode lasers generating the sequence of Raman pulses in an atom interferometer. The rms phase error of the phase lock is about 100 mrad in a 5 Hz–10 MHz bandwidth. The limit set on the interferometer phase resolution by the residual phase noise is 1.1 mrad. Since the digital circuitry is implemented with a programmable logic device the detector can be easily adapted to other experiments requiring frequency/phase stabilization of lasers sources.
- PARTICLE SOURCES, OPTICS AND ACCELERATION; PARTICLE DETECTORS
76(2005); http://dx.doi.org/10.1063/1.1889465View Description Hide Description
Space-based ion mass spectrometers and neutral atom imagers often utilize a thin foil for generation of secondary electrons and employ a coincidence measurement between detection of the secondary electrons and detection of the primary ion or neutral atom, allowing unambiguous detection of the particle in a large noise background and determination of properties of the particle using time-of-flight measurement. We demonstrate a simple and straightforward method for laboratory calibration and in situ quantification and monitoring of the absolute detection probabilities of the detectors and the absolute detection efficiency of the detector subsystem without knowledge of the incident particle flux.
A refocusing modified velocity map imaging electron/ion spectrometer adapted to synchrotron radiation studies76(2005); http://dx.doi.org/10.1063/1.1900646View Description Hide Description
We present a modified velocity map imaging (VMI) spectrometer to be used in angle-resolved molecular photoionization studies in the gas phase with synchrotron radiation (SR) in the VUV/soft x-ray range. The main modifications as compared to the original design of Eppink and Parker [A. T. J. B. Eppink and D. H. Parker, Rev. Sci. Instrum.68, 3477 (1997)] are an open repeller which allows the VMI spectrometer to be coupled to an independent dispersive electrostatic analyzer for combined operation in coincidence mode experiments, and the introduction of a coupled double Einzel lens in the flight tube in order to collect the full solid angle for higher kinetic energy particles. The length and position of the lenses have been optimized by a genetic algorithm to obtain the maximum kinetic energy possible without compromising the energy resolution. Ray-tracing simulations and SR experiments show that the lenses can increase the kinetic energy bandwidth by a factor of up to 2.5. Furthermore, a remarkable improvement in the radial focusing of the particles’ momenta can be achieved when the lens array is operated in optimum fashion. The accuracy in the determination of the angular parameters, already satisfactory in the original VMI design, is not compromised by the lens operation. Experimentally, we succeeded in collecting electrons with kinetic energy and 6% relative energy resolution with a detector of effective diameter, despite the larger ionization volume given by the SR as compared to laser multiphoton experiments. We predict that, by changing to a detector diameter of and reducing the focal length by a third, particles with energies up to could be collected by applying to the repeller electrode.
76(2005); http://dx.doi.org/10.1063/1.1899309View Description Hide Description
Electron bunches with large energy and angle spread are not easy to be analyzed with conventional spectrometers. In this article, a device for the detection of high energy electrons is presented. This detector, based on the traces left by electrons on a stack of dosimetric films, together with an original numerical algorithm for traces deconvolution, is able to characterize both angularly and spectrally (up to some mega-electron-volts) a broad-spectrum electron bunch. A numerical test was successfully performed with a virtual electron beam, which was in turn reconstructed using a Montecarlo code (based on the CERN library GEANT4). Due to its simplicity and small size, the spatial high energy electron beamanalyzer (SHEEBA) detector is particularly suitable to be used in laser plasma acceleration experiments.
Ion beams of carbon clusters and multiply charged fullerenes produced with electron cyclotron resonance ion sources76(2005); http://dx.doi.org/10.1063/1.1900663View Description Hide Description
The production of carbon cluster ions by injecting fullerene vapor into different electron cyclotron resonance (ECR) ion sources (MONO1001/ and ) is described. The extracted ion mass spectra show a bimodal distribution, well known from collisions between electrons or ions and fullerenes. In addition to small carbon clusters and even-numbered fullerene ions, odd-numbered clusters with are detected with low intensities. In particular, we have analyzed the mass spectra as a function of the rf power applied to the ECR plasma. Optimum power values are found for the production of individual carbon cluster ions, which increase with decreasing cluster size. Whereas at low power the production of fullerene ions dominates, the intensity of the very small carbon clusters is found to strongly increase with the injected power. This difference in the power dependence is used to vary and to determine the beam composition for ions with identical mass/charge ratios.
Background reduction by a getter pump around the ionization volume of a Lamb-shift polarimeter and possible improvements of polarized ion sources76(2005); http://dx.doi.org/10.1063/1.1898923View Description Hide Description
The Köln–Jülich Lamb-shift polarimeter is used to measure the nuclear polarization of the hydrogen or deuterium beam produced with the atomic-beam source for the polarized target at the ANKE spectrometer at COSY-Jülich. The precision of the earlier results had been dominated by the recombination of atoms in the ionizer. Protons or deuterons from the dissociativeionization of unpolarized recombined or molecules had strongly contributed to the extracted ion beam. To suppress this effect, in the new ionizer a nonevaporable getter pump of about or pumping speed surrounds the ionization volume. It reduces the extracted current of unpolarized ions, produced from the recombined molecular gas, by a factor of about 20 compared with the earlier value, which reduces the error of the polarizationmeasurements to about 0.5%. Now the or molecules in the ionization volume predominantly are those which are contained in the incoming beam from the atomic beam source. This allows the measurement of the fraction of unpolarized molecules in the polarized atomic or beam. The improvement achieved is a valuable step toward the measurement of the nuclear polarization of a gas sample, extracted from the storage cell of the polarized internal gas target for the spectrometer ANKE in the COSY-Jülich storage ring with the Lamb-shift polarimeter. Furthermore, the results show that the polarization of proton or deuteron beams would be increased by the installation of such a pump around the ionization volume of atomic-beam ion sources with an electron-impact ionizer. For ECR ionizers the recombined or molecules would be absorbed, whereas the noble gases, used as buffer, are not pumped by the getter material.
76(2005); http://dx.doi.org/10.1063/1.1903625View Description Hide Description
A configuration for an untuned broadband rf cavity with a low-voltage standing wave ratio (VSWR) is proposed. Although an untuned broadband cavity is currently implemented by loading magnetic alloy (MA) cores, the VSWR of such a cavity is expected to be no less than approximately 2.0 in the operational frequencies sweeping by a factor of about 10. A type of rf cavity, “in-series multistation” cavity, described here can cover a much broader frequency range sweeping by a factor of 20, while keeping the VSWR value below 1.2. The system consists of multiple stations, each of which is loaded with low- high-permeability MA cores. A “bench” test circuit was built and successfully tested.
- NUCLEAR PHYSICS, FUSION AND PLASMAS
Development of cold isostatic pressing graphite module for a heat-resistant lower hybrid current drive antenna76(2005); http://dx.doi.org/10.1063/1.1891465View Description Hide Description
Development of a plasma facing module using Cold Isostatic Pressing Graphite (CIPG) was successfully achieved for use in driving a heat-resistant Lower Hybrid Current Drive (LHCD) antenna. A thin stainless film, a molybdenumfilm, and a copperfilm were overlaid on the CIPG materials, the CIPG surfaces were successfully coated with the Cu-layer by diffusion bonding method. This module whose length is , has four waveguides, and a water cooling channel. High rf-power long pulse operations with water cooling were successfully tested up to , a stationary temperature and vacuum pressure was performed. But the maximum rf transmission power was limited to for a plasma facing module using Carbon Fiber Composite (CFC) due to poor Cu-plating, a power density large by more than a factor 2.5 was achieved with the CIPG module. The rf power density which meets the requirement of the rf electric field of for a design of the LHCD antenna in International Thermonuclear Experimental Reactor (ITER) was successfully performed.
Least-squares wave-front reconstruction of Shack-Hartmann sensors and shearing interferometers using multigrid techniques76(2005); http://dx.doi.org/10.1063/1.1896622View Description Hide Description
This article details a multigrid algorithm that is suitable for least-squares wave-front reconstruction of Shack-Hartmann and shearing interferometerwave-front sensors. The algorithm detailed in this article is shown to scale with the number of subapertures in the same fashion as fast Fourier transform techniques, making it suitable for use in applications requiring a large number of subapertures and high Strehl ratio systems such as for high spatial frequency characterization of high-density plasmas,optics metrology, and multiconjugate and extreme adaptive optics systems.
76(2005); http://dx.doi.org/10.1063/1.1898163View Description Hide Description
Dense samples of nuclear spin polarized are utilized in high energy physics, neutron scattering, atomic physics, and magnetic resonance imaging. Metastability exchange optical pumping can rapidly produce high polarizations at low pressures (few mbar). We describe a polarized gas compressor system which accepts of gas polarized to 70% by a neodymiumdoped lanthanum magnesium hexaluminate (Nd:LMA) laser and compresses it into a target with final polarization of 55%. The spin relaxation rates of the system’s components have been measured using nuclear magnetic resonance and a model of the polarization loss based on the measured relaxation rates and the gas flow is in agreement with a polarizationmeasurement using neutron transmission.