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Review of Scientific Instruments

Issue 8 • Date Aug 1989

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Displaying Results 1 - 25 of 46
  • Variable‐period electrostatic and magnetostatic undulator designs for generating polarized soft x rays at PEP

    Page(s): 2571 - 2578
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    It can been shown that the generation of soft x rays on high energy storage rings such as PEP is most effectively performed with low‐field, variable‐period insertion devices, in contrast to the fixed‐period devices represented by conventional transverse magnetostatic (TM) technology. Moreover, variable‐period undulators of arbitrary field strength can be shown to be preferable to variable‐gap devices under all circumstances where the generation of excessive superfluous power is undesirable, or where the available range of tuning with a single device must be maximized. In this article, basic arguments for these claims are presented, and an electrostatic implementation of a variable‐period polarizing undulator is described. Some attractive properties of a recently designed, higher‐field magnetostatic analog of the electrostatic device are also mentioned. View full abstract»

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  • On the phase‐space description of synchrotron x‐ray beams

    Page(s): 2579 - 2585
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    Simple graphic methods for illustrating the transformation of x‐ray beams by optical elements such as mirrors, monochromators, and slits are very helpful tools when optimizing synchrotron radiation experiments. They provide guidelines for matching the optical components to the source and to the experiment, and for checking the physical meaning of the more detailed results obtained by subsequent ray‐tracing calculations. Phase‐space diagrams have been used extensively to describe synchrotron radiation experiments by plotting an angular variable against a positional one with the possibility to add the energy in a third dimension. For neutron scattering, the traditional method consists in parallel representations, in reciprocal and in real space, that can also be considered as another kind of phase‐space diagram. In this paper, we show the specific advantages and the complementary character of the above diagrams. The representation of several optical elements and their effect on beam transformation is given first individually and then for some arrangements of two or more beam‐defining devices. Finally, a beamline is described and optimized as a demonstration of the usefulness and the limits of these graphic methods. View full abstract»

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  • A virtual phase CCD detector for synchrotron radiation applications

    Page(s): 2586 - 2591
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    A two‐dimensional charge coupled device (CCD) detector, based on the Texas Instruments ‘‘virtual phase’’ CCD, has been developed for synchrotron radiation applications. Simultaneous near‐edge and multilayer scattering experiments have been carried out with the detector on an energy‐dispersive synchrotron beamline. The detector was used in an optical mode where the CCD element is coupled to a phosphor screen by a pair of focusing and demagnifying lenses. We report on the performance of the detector in this mode. View full abstract»

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  • An amplified tunable picosecond dye laser based on an active‐passive mode‐locked Nd:YAG laser

    Page(s): 2592 - 2595
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    An amplified tunable picosecond dye laser has been constructed which is pumped by an active‐passive mode‐locked Nd:YAG laser. The laser is tunable from 555 nm to ≫700 nm. It provides a maximum of 2‐mJ pulses with pulse widths of ∼17 ps (FWHM). The tuning range can be extended by frequency doubling and/or mixing with the residual 1064‐nm pulses. The dye laser is very stable and relatively inexpensive to construct. View full abstract»

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  • Wide‐band acousto‐optic light modulator for frequency domain fluorometry and phosphorimetry

    Page(s): 2596 - 2600
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    Multifrequency‐phase and modulation fluorometry allows for accurate analysis of fluorescence decay in the frequency domain. Essential to these frequency domain methods is a high‐frequency modulation of the light source. Techniques for generating wide‐band modulation of light are currently limited to the use of Pockel’s cells and intrinsically modulated sources such as mode‐locked lasers and synchrotron radiation. We present a method that employs two acousto‐optic modulators in series for use with cw light sources. This modulator system gives two orders of magnitude more intensity output than the Pockel’s cell modulator and requires less than one‐tenth of the rf driving power. In addition, the Pockel’s cell system is limited to modulation frequencies less than 250 MHz, whereas the particular implementation discussed here gives a quasicontinuous distribution of modulation frequencies from dc to 320 MHz. To obtain this range of frequencies, acoustic standing waves are set up simultaneously in each modulator, and the desired modulation frequency is achieved by choosing the proper combination of the two standing‐wave frequencies. Light modulation is obtained at twice each of the individual standing‐wave frequencies and at the sum and difference of twice the two acoustic frequencies. Data are presented to illustrate the use of this system for the measurement of phosphorescence as well as fluorescence decay. View full abstract»

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  • Rapid‐scanning spectrometer based on fiberoptics applied to flames

    Page(s): 2601 - 2605
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    A rapid‐scanning optical spectrometer based on fiberoptic probes, used to measure radical emission in low‐pressure premixed laminar flames, is presented. Emission spectra and intensity profiles as a function of height above the burner have been recorded for OH, CH, and C2 in an ethylene/oxygen/nitrogen flame (14% C2 H4 , 64% O2 , 22% N2 ), stabilized at 5 Torr on a sintered stainless‐steel burner. The emitted light was guided from the flame reactor to the spectrometer by fused silica optical fibers. Rapid scanning (100 scans/s) eliminates the effects of turbulence and temporal signal fluctuations, as 104 –105 scans over a preset spectral window are collected and signal averaged within a few minutes. The technique is discussed and compared to classical methods in flame spectroscopy. View full abstract»

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  • A combustion facility for high‐pressure flame studies by spectroscopic methods

    Page(s): 2606 - 2609
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    We have developed a high‐pressure combustion facility that includes the following: (1) a pressure vessel constructed from standard stainless‐steel piping components; (2) an internal x‐y burner translation system employing inexpensive stepper motors and translation mechanisms; (3) two flat‐flame burners; and (4) an electronic gas delivery system, which is interfaced to a microcomputer. The facility is designed for study of high‐pressure flames by spectroscopic methods and should also aid in the development of spectroscopic tools for high‐pressure combustion environments. View full abstract»

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  • Light scattering apparatus for angular dependent studies of anisotropic materials

    Page(s): 2610 - 2613
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    An improved method for light scattering from acoustic phonons in small, low‐symmetry solids is described. This instrumentation greatly facilitates the determination of elastic constants in highly anisotropic materials. Elastic scattering can be greatly reduced so that weaker phonon modes can be observed. Errors associated with the determination of the polarization directions of an anisotropic material are minimized by considering crystal optics. View full abstract»

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  • Picosecond time‐resolved circular dichroism spectroscopy: experimental details and applications

    Page(s): 2614 - 2627
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    An experimental apparatus for measuring time‐resolved circular dichroism (CD) with picosecond resolution is described. The time resolution of the device is determined by the laser pulse width, not the modulation frequency of an electro‐optic or piezo‐optic crystal as is the case for commercial CD spectrometers. The time‐dependent CD signal can be monitored over the wavelength range from 280 to 850 nm. The data from single wavelength studies can be pieced together to generate time dependent spectra. The experimental approach is compared to previous time‐resolved CD techniques which, for technical reasons, have been limited to, at most, nanosecond resolution. The picosecond apparatus is shown to be free of many of the polarization artifacts present in these previous devices. The utility of this technique is demonstrated by examining result on the time dependent near‐UV CD of myoglobin following the photoelimination of coordinated CO from carbonmonoxymyoglobin. View full abstract»

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  • A 77‐K cold stage for Raman microprobes and optical microscopy

    Page(s): 2628 - 2630
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    A 77‐K cold stage has been developed for spectroscopic measurements in an optical microscope. This stage eliminates fogging of the optical windows observed in an earlier design by adding vacuum jackets around the liquid nitrogen transfer lines and by maximizing the sample‐to‐environment distance without sacrificing optical imaging power. Increased maneuverability of the cold stage is achieved by using flexible stainless‐steel liquid‐nitrogen transfer lines. In our application, the sample in the cold stage is illuminated with a focused laser beam and vibrational resonance Raman scattering spectra are recorded. We have used this Raman microprobe system to measure the Raman scattering from a variety of individual rod photoreceptor cells. This work has provided new information about the mechanism of wavelength regulation in color vision. View full abstract»

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  • A new detection scheme for Fourier transform‐ion cyclotron resonance spectrometry in Penning traps

    Page(s): 2631 - 2634
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    A new detection scheme for Fourier transform‐ion cyclotron resonance (FT‐ICR) studies is proposed and initial experimental results are presented. While in conventional FT‐ICR, the difference of the transient signals from two opposite electrodes is recorded and Fourier transformed, it is also possible to use the sum of these signals. In this way resonances are observed at the harmonic of the reduced cyclotron frequency 2 ν+ as well as ν+- and ν+-, the later being important for accurate mass measurements. The experiments were performed with a Penning trap in an electromagnet. View full abstract»

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  • An in situ cell for transmission EXAFS measurements on catalytic samples

    Page(s): 2635 - 2638
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    An in situ cell suitable for transmission EXAFS measurements on catalytic samples is described. The cell can be used for catalyst pretreatments in various atmospheres (including H2, H2S, O2 and CO) in a temperature range upto 700 K. The sample is heated by conducting heat from an external heater to the sample. During measurement the samples can be cooled down to 77 K by conducting heat from the sample to an external liquid nitrogen container. During the pretreatment and the measurement a waterflow through the body of the cell keeps certain crucial parts from overheating or icing up. To avoid radiation leaks in powdery samples these samples are pressed in a selfsupporting wafer and held in a disk‐shaped sampleholder. Tests by various catalytic groups have proven the suitability of the design. View full abstract»

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  • Time‐of‐flight laser mass spectrometer for gas analysis in a collisional regime

    Page(s): 2639 - 2641
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    We have built and tested a new type of time‐of‐flight mass spectrometer for measurements of fragmentation processes in a collisional regime. Preliminary test results are given for triethylamine. View full abstract»

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  • High q‐resolution electron gun for low energy electron diffraction

    Page(s): 2642 - 2645
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    We present a new unipotential electron gun design for low energy electron diffraction. The gun has excellent spatial resolution. With a 170‐mm gun‐to‐detector working distance, it produces a spot between 50 and 114 μm in diameter for energies between 500 and 100 eV, respectively. These specifications make it ideal for high q‐resolution studies of surface defects. The unipotential design offers independent beam energy and current control without refocusing, and operates at beam currents as high as 25 nA (near the space‐charge limit). The operating current of this gun is 10–40 times higher than existing guns with similar spatial properties. View full abstract»

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  • Analysis of the energy consumption of LaB6 cathodes in SEM applications

    Page(s): 2646 - 2649
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    The power required to heat a lanthanum hexiboride cathode for use in scanning electron microscopes (SEM) has been calculated and found to be in close agreement with experimental measurements. The analysis indicates that direct heating of the LaB6 by the heater current, and efficient heat transfer to the cathode from the heated supports, results in low power consumption. The operating temperature of the cathode support material must be carefully designed to avoid poisoning of the LaB6 by excessive metal evaporation and deposition on the cathode. The LaB6 cathode developed here is well suited to the small heater power supply of the SEM and improves the picture quality over the traditional tungsten filament cathodes. View full abstract»

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  • New method for directly monitoring the electron beam intensity profile in a scanning electron microscope

    Page(s): 2650 - 2652
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    A new method for directly monitoring the electron beam intensity profile in a scanning electron microscope is proposed. This method employs phase lock‐in technique which electronically differentiates the integrated current collected, while scanning the rocked electron beam across a knife edge and directly obtains the one‐dimensional intensity distribution of the electron beam probe. The method can be employed for beam currents as low as 30 pA with the spatial resolution accuracy of ±125 Å. Electron beam diameters can be measured at higher accuracies due to the inherent improvement in the S/N ratio provided by this method. Electron beam aberrations can also be directly observed by studying the intensity profiles of the probe cross section. View full abstract»

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  • A simple alkali metal ion gun

    Page(s): 2653 - 2656
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    An easily constructed alkali‐metal‐ion gun with a large metal‐zeolite reservoir is described. The gun provides ion‐beam currents from 100 eV to 5 keV in excess of 10 μA with a wide range of focal lengths. The ion trajectories of the extraction, acceleration, and focusing have been computed using the program simion. A versatile three‐electrode extraction geometry that provides ion trajectories which emulate the planar diode is described. View full abstract»

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  • Angular‐resolved ion‐beam sputtering apparatus for large‐area deposition

    Page(s): 2657 - 2665
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    A new apparatus to utilize the directional nature of ion‐beam sputter deposition is presented. Employment of an angular setting slit together with translational substrate scanning enables large‐area deposition, keeping the angular settings unchanged. The merits of this apparatus were demonstrated step by step on a typical example: CdTe obliquely deposited films known for the anomolous photovoltaic effect. View full abstract»

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  • Apparatus for codeposition and layered deposition of reactive metals and volatile organic compounds with control of stoichiometry and film thickness

    Page(s): 2666 - 2672
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    An effective method is reported for the simultaneous or sequential physical vapor deposition of reactive alkali metals and volatile macrocyclic complexants which react to form thin stoichiometric films of alkalides and electrides. These thermally unstable and highly reactive films, typically between 300 and 1500 Å thick, are deposited under high vacuum onto a quartz substrate which can be cooled to as low as -130 °C. Quantitative optical absorption measurements are made on these films in situ. Acquisition of the optical data and control of sample deposition rates, oven temperatures, and film thicknesses are accomplished with a computer. View full abstract»

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  • Determination of the local current density by means of tangential Thomson scattering: Experimental setup, feasibility test, and preliminary observations

    Page(s): 2673 - 2679
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    The local current density is one of the most important and most difficult parameters to be determined in a thermonuclear plasma. An experimental setup to measure the local electron drift velocity and thus, the current density, by means of tangential Thomson scattering, is presented. The drift velocity of the plasma causes a small wavelength shift of the scattered spectrum, which will be proportional to the local current density if toroidal ion rotation can be neglected. The ratio between the drift velocity and the mean thermal electron velocity is small, typically ∼0.05. A 20‐channel high transmission polychromator has been constructed to observe the electron temperature and density within an accuracy of 1% and the drift velocity within 20% at a plasma density ne≊5×1019 m-3 and a laser energy of 7 J. A feasibility test on radial scattering shows that there is good agreement between the accuracy obtained from quantum statistics and observations, respectively. The high transmission and relatively high resolution of the polychromator enable the observation of irregularities in the velocity distribution which should be taken into account in the determination of the drift velocity. Preliminary observations in the tangential direction demonstrate the possibility to determine the drift velocity by a single laser shot. At ne ≊6×1019 m-3 and a laser energy of 3.5 J in the scattering volume a wavelength shift of -2.5±0.5 nm has been recorded corresponding to jϕ≊1×107 A/m2. View full abstract»

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  • Electron beam and magnetic field mapping techniques used to determine field errors in the ATF torsatron

    Page(s): 2680 - 2689
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    The beam from an electron gun was used to trace flux surfaces in the Advanced Toroidal Facility (ATF) torsatron. The ATF magnetic field was run steady state at 0.1 T, and the electron beam was detected optically with an image‐intensified, solid‐state camera when it impinged on a phosphor‐coated screen. Closed flux surfaces and islands at several low‐order resonances were observed. The largest island, located at the ι= 1/2 surface, was from 5 to 6 cm in width, and its presence implied the existence of magnetic field errors. To determine if these error fields could be traced to small misalignments of the magnetic coils, a device capable of accurately measuring the radial and vertical magnetic field components of individual coil sets was placed in the center of ATF. This device allowed for a determination of the precise location of each of the coils that make up the ATF coil set. No significant coil misalignments were found. A further investigation of the coil configuration led to the identification of dipole fields in the helical field coil leads as the source of the field errors. The techniques developed in making these measurements are described in the text. View full abstract»

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  • Boxcar photography

    Page(s): 2690 - 2696
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    A simple, inexpensive diagnostic has been developed for time‐resolved imaging of repetitive self‐luminescent phenomena. An electro‐optic birefringent ceramic shutter is employed to perform photographic sampling and has demonstrated time resolution of 60 μs. Photographic film was used for image detection in this system, and methods of image enhancement in the presence of background light from a finite‐contrast shutter are discussed. The system has been applied to the study of plasma evolution in the CDX plasma confinement device. View full abstract»

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  • Performance of a pneumatic hydrogen‐pellet injection system on the Joint European Torus

    Page(s): 2697 - 2700
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    A pneumatic‐based, hydrogen isotope pellet injector that was developed at the Oak Ridge National Laboratory (ORNL) has been used in recent plasma fueling experiments on the Joint European Torus (JET). The injector consists of three independent machine‐gun‐like mechanisms (nominal pellet sizes of 2.7, 4.0, and 6.0 mm in diameter) and features repetitive operation (1–5 Hz) for quasi‐steady‐state conditions (≫10 s). An extensive set of injector diagnostics permits evaluation of parameters for each pellet shot, including speed, mass, and integrity. Pellet speeds range from 1.0 to 1.5 km/s. Over 3700 pellets have been fired with the equipment at JET, with about 1500 pellets shot for plasma fueling experiments. In recent experiments, the system performance has been outstanding, including excellent reproducibility in pellet speed and mass, and a reliability of ≫98% in delivery of pellets to the plasma. View full abstract»

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  • A sensitive double beam laser interferometer for studying high‐frequency piezoelectric and electrostrictive strains

    Page(s): 2701 - 2705
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    A double beam laser interferometer is built up to study high‐frequency piezoelectric and electrostrictive strains. The system is capable of resolving a displacement of 10-2 Å using lock‐in detection and measuring the strain all the way to the piezoelectric resonance frequencies using a digital oscilloscope for detection. The interference of sample bending to the detected signal is effectively avoided. View full abstract»

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  • Torsional oscillator for internal friction data at 100 kHz

    Page(s): 2706 - 2710
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    We describe a method for measuring the internal friction of solids at low temperatures. Our design uses a composite oscillator vibrating in torsion at 100 kHz. The oscillator makes good thermal contact to the cryostat while maintaining a low background loss of Q-1≂2×10-6 at 2 K. The usefulness of the design is demonstrated by data for BaF2 and a‐SiO2 from 2 to 300 K. View full abstract»

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Aims & Scope

Review of Scientific Instruments, published by the American Institute of Physics, is devoted to scientific instruments, apparatus, and techniques.

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Editor
Albert T. Macrander
Argonne National Laboratory