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  • American Institute of Physics (AIP)  (9,330)
  • 1990-1994  (9,330)
  • 1991  (9,330)
  • 1
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A quadrupole mass spectrometer (QMS) utilizing the second stable zone in Mathieu's stability diagram was developed. The system performance was experimentally compared with the conventional QMS using the first stable zone. The peak shapes obtained were sharper and the tail components arising from adjacent peaks were less than those of the conventional QMS, although the analyzable range was limited to lower mass and the attenuation in sensitivity was about 1/50. This improved QMS is usable in detecting very small peaks adjoining large ones.
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  • 2
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A versatile electron-impact time-of-flight mass spectrometer (EI-TOFMS) in a pulsed molecular beam has been constructed. The instrument contains a fast rise time, square pulse generator, biased at high voltage, which simplifies the implementation of EI-TOFMS. Power field effect transistors are used as the active switching element, which afford compact size and simplified circuit construction. The effect of the shape of the repelling pulse on the flight time and space focusing condition is estimated by numerical calculation. Preliminary test results are provided for pulsed molecular beams of ammonia and hydrogen iodide.
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  • 3
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The construction and the fundamental studies of a kilohertz-range flash x-ray generator having a triode in conjunction with an extremely hot cathode are described. This generator consisted of the following components: a constant-high voltage power supply, an energy storage condenser of 100 nF, a constant high-voltage power supply for regulating an initial grid voltage of −1.6 kV, a grid pulser, and an x-ray tube. The x-ray tube was of an enclosed-triode type and consisted of the following major parts: an anode rod made of copper, a plane anode tip (target) made of tungsten, a focusing electrode made of iron, a hot cathode (filament) made of tungsten, a grid made from tungsten wire, and a glass tube body. The energy storage condenser was charged from 50 to 70 kV, and the electric charges in the condenser were discharged repetitively to the x-ray tube by the grid electrode driven by the grid pulser. The temperature of the filament was about 2000 K, and the cathode current was primarily controlled by the grid voltage and its value was less than 1.2 A. The pulse widths were about 1 μs and the maximum repetitive frequency was about 2.0 kHz. The x-ray intensity was 19.7 nC/kg at 0.5 m per pulse with a peak grid voltage of 1.0 kV and a condenser charged voltage of 70 kV.
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  • 4
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A time-of-flight spectrometer has been specially designed for measuring the masses of ions produced by low-energy positrons interacting with organic molecules in a Penning trap. To make the flight times insensitive to the starting positions of the ions in the trap, acceleration was done using a potential that varied as the square of the distance of the ion from the detector. The containment of the positrons in the Penning trap for extended time periods effected very long collision paths between the positrons and the molecules. The entire length of the Penning trap source could be sampled. These features produced high sensitivity, enabling the measurement of ionization processes with small cross sections. The spectrometer is useful for mass spectrometry of ions produced by processes other than positron interaction. Both large and small volume sources can be accommodated.
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  • 5
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    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 2138-2141 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A spectroscopic plasma tomography measurement system has been developed and operated on a field-reversed configuration (FRC) plasma machine with the use of five photocollector arrays. Each photocollector array that is located azimuthally around the plasma column consists of ten optical fibers and a pinhole. Two-dimensional (2D) and time-resolved visible emission profiles can be reconstructed numerically from the fifty-channel projection data. The use of five multichannel visible monochromators makes it possible to obtain a result that has explicit physical meanings. A computer simulation has been performed to demonstrate the potential to reconstruct the 2D profile without the assumptions about plasma rotation or symmetry. The first experimental result for visible bremsstrahlung emission profiles of the FRC plasma is presented.
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  • 6
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A high proton yield beam was produced in a semicylindrical multicusp plasma generator. The dimension of the cylinder is 34 cm in diameter and 129 cm in length, and the beam extraction area is 8 cm in width and 120 cm in length. A strongly linked magnetic field configuration was utilized to confine the primary electrons in the cylinder and prevent them from flowing into the beam extraction region. As a temporary test hydrogen ion beams were extracted from the central region of the plasma generator. Ion species in the beam were measured by a Doppler-shifted spectroscopy of Balmer-alpha light and found to be 95% at a beam current density of 175 mA/cm2 at the filling pressure of 0.4 Pa.
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  • 7
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    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 2146-2153 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A ring of accurately placed and matched printed circuit coils is placed at the center of the DIII-D tokamak vacuum vessel, aligned magnetically with the toroidal field and used to measure the nonaxisymmetric magnetic field of each of the 18 poloidal field coils. [Toroidal and poloidal variations can be of mode n and m, respectively, with helical variations of form cos(nφ−mθ).] From the error fields, it is computed that the most irregular poloidal field coil is one of the outer vertical field coils having a shift of 1.9 ±0.2 cm (compared to a diameter of 482 cm), and having an ellipticity 1.0027. All the 18 poloidal field coils together make a resonant, helical, radial error field for m=2, n=1 of about 1.4 × 10−4 of the toroidal field.
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  • 8
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    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 2154-2158 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A compact supersonic molecular beam source was developed that has a liquid nitrogen trap to cryogenically pump the gas and is small enough in size to install in conventional molecular beam epitaxy (MBE) systems. With the source, well-collimated molecular beams of N2O and NO2 whose flux densities are ∼1016 molecules cm−2 s−1 are generated. The effectiveness of the source was proved in the preparation of oxide superconducting thin films by MBE with an NO2 beam where as-grown superconducting films were obtained in high vacuum of ≤10−5 Pa.
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  • 9
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We report an NMR probe that we use in ultra low temperature experiments in a top-loading dilution refrigerator. The probe is thermally anchored to the 1.2 K pumped 4He pot and is thermally isolated from the sample located inside a Kel-F cup containing liquid 3He. The probe is adapted from a standard double resonance probe, using air trimmer capacitors, a λ/4 cable, a homemade saddle coil, and metal film resistors. The double resonance design breaks the probe's tuning ranges into two bands. The high frequency band tunes from 100 to 150 MHz covering all the nuclei with large gyromagnetic ratios, 3H, 19F, 1H, 3He. The low frequency band tunes for 17 to 54 MHz covering all nuclei with gyromagnetic ratios between 133Cs and 119Sn. The air trimmer capacitors have an open structure that allows efficient pumping and the metal film resistors are used to reduce the Q of the probe. This probe has been successfully used for a wide range of nuclei including 1H, 19F, 3He, 119Sn, 117Sn, 115Sn, 11B, 13C, 29Si, 27Al, and 2H.
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  • 10
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    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 2163-2166 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Important information about physical characteristics of clays can be inferred from the distribution of the orientations of straight lines in photographs of cross sections of its samples. This article describes the development of a semiautomated system for clay analysis that is based on the application of computer vision techniques. Due to the inherent complexity of these techniques, two strategies have been used in order to reduce the execution time: an effective technique, the binary Hough transform, is applied for detection of straight lines and the transputer is used to enhance the system processing power. The article includes the characterization of the clay analysis application, the description of the binary Hough transform for detection of straight line segments, the presentation of the semiautomated system, and a complete illustrative example.
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  • 11
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    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 2205-2212 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A theoretical analysis of standing waves in a pipe with flow was conducted aimed toward the development of a flowmeter based on a measurement of phase difference between two points on the circumference of a pipe separated axially by an integral multiple of half sound wavelength. Effects of nonzero Mach number, variable ratios of upstream and downstream running waves, choice of location of the sensing pressure taps, and the incorrect spacings were examined numerically. Calculations were also made for steam flow where the maximum Mach number is much smaller than in cold air flows. For an industrial application where steam flow velocities are below 50 m/s, a very accurate sensor is shown to be possible. Experimental data were obtained by using an active aeroacoustic source over the range of 50 m/s to zero in an air flow. Experimental phase plots obtained from several coast-down tests have been compared with analytical results. The agreement with theory is excellent when the spacings are near Nλ/2 and the sensing locations are near the pressure antinodes. Otherwise, some zero shifts are introduced. Theory predicts that for steam flow much better results can be expected because the maximum Mach number is much smaller. Effects due to the (1−M2) factor would be imperceptible for steam flow.
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  • 12
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    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 2218-2227 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Commercial carbon composition resistors have been used as gauges to make dynamic stress measurements in homogeneous and heterogeneous reactive and inert materials. Initial loading was provided by plane wave lenses or shaped charge jets. A series of gas gun and aquarium experiments has been conducted to characterize the behavior of the gauges. Calibrations up to about 14 GPa for nominal 470 Ω resistors and 17 GPa for nominal 4700 Ω resistors are presented. The accuracy of the carbon resistor gauges is limited by response time considerations when submicrosecond rise times are encountered, and there is hysteresis during release. The gauge-to-gauge reproducibility appears to be adequate, and they survive in situations where no other stress transducer has been successfully used, such as in reacting beds of large-particle gun propellants.
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  • 13
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A novel method and instrument capable of either monitoring continuously a condensation process or estimating the thickness of a liquid layer in the μm range are presented. The underlying physical principle is a variant of the photopyroelectric technique in which the changes in the thermal wave propagation due to the presence of the liquid layer are sensed by the pyroelectric sensor. The theoretical model developed supports the experimental results obtained.
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  • 14
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A modified centrifuge has been developed. There are four advantages to this apparatus: (i) The presence of centrifugal acceleration permits the formation of the boundary between the two phases of a fluid system more quickly after phase separation, especially for viscous fluids such as polymer blends. (ii) The rotor is able to accommodate three samples. (iii) The formation of homogeneous solutions in the one phase region can be achieved within the constant temperature rotor which can be controlled to ±0.003 °C up to ≈300 °C. (iv) The phase separation temperature can be measured as a function of centrifugal acceleration up to 1500 gravity. By using the centrifugal apparatus, the coexistence curves of polystyrene in methylcyclohexane were measured. The results in terms of critical exponents and preexponential factors were in excellent agreement with the best literature values and thus confirmed the reliability of our apparatus.
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  • 15
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The hardware and software designs for fast measurement of dielectric properties of different substances over a wide range of characteristic times (10−5–10−10 s) by means of time domain dielectric spectroscopy (TDS) are presented. The characteristic feature of the described installation is the nonuniform time sampling of measured signals, which enables one to cover a wide range of characteristic times with a single measurement. Comparative results for an aqueous solution of lysozyme obtained on the traditional apparatus and on the described one are presented.
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  • 16
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    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 2266-2270 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: In this article we describe how nonlinear electrical transmission lines can be used to produce bursts of very short-time voltage pulses (solitons). The relevant parameters and relations for designing suitable transmission lines are given. The number of solitons and their amplitudes are given in a diagram as functions of the (normalized) amplitude and the width of the rectangular input pulse. Several examples of constructed transmission lines (including some very fast structures) are described and solitons produced by them are shown.
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  • 17
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: In this article we describe a simple SQUID magnetometer system useful for the magnetic characterization of high temperature superconductors. The system employs a commercial rf SQUID probe with associated electronics located in a 4He gas exchange cryostat. The system is equipped with a copper coil dc magnet for fields up to 50 Gauss. The cryostat is surrounded by a μ-metal shield that keeps the remanent field to less than 20 mGauss. The apparatus has been used extensively to measure both the superconducting transition temperature and width as well as the relative quantities of magnetic flux expulsion and exclusion. It has therefore served as an important first step in the evaluation of sample quality.
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  • 18
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    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 2281-2287 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A data acquisition system designed for magnetic resonance instrumentation is presented. Its performance may be suitable for a broad set of applications where repetitive and noisy signals must be time-averaged in order to increase the signal-to-noise ratio. The system is built from easily available, low cost parts on a standard board for the PC-AT (16-bit) bus. 12-bit data acquisition and 32-bit time averaging take place in the memory of the PC under control of its CPU. The sustained data rate, including on-line signal averaging, can be as high as 250 kHz (500 kHz with interlaced data acquisitions). Timing is accomplished by on-board software programmable hardware timers. The CPU is then free for other tasks during the time, if any, between acquisition scans. Thus on-line display of the time-averaged data and their FT spectrum may be performed without impairing the timing accuracy, which is limited by the 10-MHz board clock to 100 ns. All functions of the interface are controlled by machine-language routines, callable from within quickbasic programs.
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  • 19
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A simple and elegant technique is outlined in which a specially designed crucible may be used for noble metal evaporation in a pendant-drop type electron beam evaporator with virtually no modifications.
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  • 20
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A novel synchronous real-time analog sampling method for obtaining the sum and difference interferograms in double modulation Fourier transform infrared absorption experiments is described, and the application of this sampling methodology to polarization-modulation FTIR measurements of thin films at metal surfaces is demonstrated. A quadratic approximation of the background signal is used to calculate the difference interferogram. The demodulation of a test waveform with the real-time sampling electronics reveals how the bandwidth limitations of previous double modulation experiments on FTIR interferometers that employed lock-in amplifiers have been eliminated.
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  • 21
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: An experimental apparatus designed to study structural and electronic properties of surfaces is described. Ultraviolet photoemission (UPS) and metastable deexcitation spectroscopies (MDS) have been implemented in order to obtain information on electronic states in the valence-band region with different surface sensitivity. The supersonic metastable beam source is fully described and characterized by time-of-flight measurements while the characteristics of the apparatus have been tested in the study of the Ag(110) surface, clean and following the dissociative chemisorption of oxygen. Exploiting the surface sensitivity of MDS, measurements as function of the oxygen coverage allow a direct correlation between work function increase and the population of the s-p bands of the substrate near the Fermi edge. Oxygen-induced states around 7.5 eV, difficult to observe in UPS because of strong contributions from the d bands of the substrate, are also detected.
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  • 22
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    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 1438-1444 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: An aperture-limited, differentially pumped environmental cell has been developed and installed in a JEOL 4000 EX electron microscope. With this environmental cell it is possible to observe, at high spatial resolution, the detailed changes in microstructure that occur during the interaction between a solid and a gaseous environment. The cell has been pressurized to 70 Torr with H2 gas for several hours with no adverse effects on the microscope performance. While the initial tests have been performed with hydrogen gas, the materials used to construct the environmental cell can tolerate corrosive environments. The key features of the environmental cell design along with the results from some applications are presented in this paper.
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  • 23
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A sensitive spectrometer, based on a dc superconducting quantum interference device, for the direct detection of low-frequency pulsed nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR), is described. The frequency response extends from about 10 to 200 kHz, and the recovery time after the magnetic pulse is removed is typically 50 μs. As examples, NMR spectra are shown from Pt and Cu metal powders in a magnetic field of 6 mT, and NQR spectra are shown from 2D in a tunneling methyl group and 14N in NH4ClO4.
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  • 24
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    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 1445-1452 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We describe the design of a probe for dynamic-angle spinning (DAS) NMR experiments, comprised of a spinning cylindrical sample holder whose axis may be reoriented rapidly between discrete directions within the bore of a superconducting magnet. This allows the refocusing of nuclear spin magnetization that evolves under anisotropic interactions such as chemical shift anisotropy and quadrupolar coupling, providing high resolution NMR spectra for quadrupolar nuclei in solid materials. The probe includes an axial air delivery system to bearing and drive jets which support and spin a rotor containing the sample. Axis reorientation is accomplished with a pulley attached to the probehead and coupled to a stepping motor outside of the magnet. The choice of motor and gear ratio is based on an analysis of the moments of inertia of the motor and load, the desired angular resolution, and simplicity of design. Control of angular accuracy and precision are discussed, as well as the efficiency of radiofrequency irradiation and detection. High resolution DAS spectra of oxygen-17 and aluminum-27 nuclei in polycrystalline minerals illustrate the experimental capabilities.
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  • 25
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    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 1464-1467 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Measurement of beam properties at the foci of high energy linacs is difficult due to the small size of the waists in proposed and existing accelerators (1 nm −2 μm). This article considers the use of bremsstrahlung radiation from thin foils to measure the size and phase space density these beams using nonimaging optics. The components of the system are described, and the ultimate resolution, evaluated theoretically for the case of the Final Focus Test Beam (FFTB) at Stanford Linear Accelerator Center, is a few nm.
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  • 26
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    Review of Scientific Instruments 62 (1991), S. 1468-1470 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A numerical method is presented of finding the rms width and the height of a Maxwellian transverse velocity distribution from an experimentally obtained beamlet profile formed by a finite-width slit. The results are also least-squares fit to polynomials that can be very conveniently used in actual data analyses.
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  • 27
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: An electrostatically guided positron beam which is variable in energy from 0 to 60 keV is described. A frozen inert gas (Ar, Kr, and Xe) is used as the positron moderator. The efficiency, ε, defined as the ratio of the number of slow positrons at the target to the number of positrons emitted by the source has been measured; the values obtained for Ar, Kr, and Xe were ε∼ 2.5×10−4, 1.6×10−4, and 1.1×10−4, respectively. We estimate a source/moderator efficiency of εm∼7.6×10−4, 4.7×10−4, and 3.0×10−4 for Ar, Kr, and Xe, respectively, which includes source effects, grid transmission, geometrical losses, and beam transport. These values are comparable to the best reported values for thin metal foil moderators. The measured εm is specific to the electrostatic system; significantly higher values for the same moderator should be attainable in magnetically guided beams, where higher transmission for moderators with large energy spreads is usually obtained.
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  • 28
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    Review of Scientific Instruments 62 (1991), S. 1471-1474 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A compact cylindrical reflection mirror analyzer for low energy ion scattering spectrometer has been designed and constructed to be used in a vacuum thin film deposition system for in situ analysis of fresh-deposited films. The design of the energy analyzer is similar to that of the ellipsoidal mirror analyzer, consisting of an electrostatic reflection mirror of the cylindrical sector type with a mean radius of curvature of 4.0 cm and a sector angle of 70°, a high pass energy filter, and two slits. The performance of this analyzer has been tested by the scattering of Li+ ions with energies of 606, 707, and 807 eV, respectively, on Al, Ti, Co, Cu, and stainless steel samples at angles from 125° to 155°. An average energy resolution of (1)/(56) has been obtained at an energy band of 10 eV and primary ion energy of 800 eV. Better resolution is expected with a narrower energy band and a higher primary energy. In situ monitoring of thin Cu film deposited on Si substrate in Ar+ ion sputtering process has been done to show the feasibility of this system.
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  • 29
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    Review of Scientific Instruments 62 (1991), S. 1489-1493 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Neutron source strength and yield from field-reversed configurations have been measured in the FRX-C/LSM magnetic compression experiment using plastic scintillators, indium activation samples, and moderated rhodium activation counters. The calibration of these neutron detectors is complicated by the changing shape and position of the plasma and by the presence of the massive aluminum compression coils. The overall uncertainty in the neutron measurements is estimated to be 45%.
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  • 30
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: High bandwidth capacitive probes are useful tools for potentials measurements in the tokamak scrape off layer. An improved capacitive divider probe design with very high frequency bandwidth is shown. The gain of this system is between 0 and −8 dB for frequencies from 0.1 Hz to more than 100 MHz, with the −3 dB point at approximately 40 MHz. The probe structure, circuits, performance, and experimental results from the Phaedrus-T tokamak are given.
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  • 31
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    Review of Scientific Instruments 62 (1991), S. 1475-1480 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A potential modulation differentiation technique which uses heterodyne modulation is presented. The second derivative I‘(V) of the collected current I(V) is obtained by modulating the voltage V using two synchronized signals with angular frequencies Kω0 and (K − 1)ω0, K(very-much-greater-than)1, and detecting the component of the collected current with angular frequency ω0. This technique eliminates problems caused by stray capacitively coupled signals and harmonic distortions produced by the signal generator and the preamplifiers. It can be applied to techniques in which the desired signal is the second derivative of the collected current, such as retarding field Auger electron spectroscopy, appearance potential spectroscopy, and work-function measurements. Auger spectra obtained using a multigrid retarding field analyzer and the proposed modulation technique show that only three grids are needed rather than the four grids required in the conventional experimental setup. Furthermore, there is no need for resonant filters or neutralization (bridge) amplifiers, and the high dynamic range commercial lock-in amplifier can be replaced by a circuit based on a single chip.
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  • 32
    ISSN: 1089-7623
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    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: An electron diffraction apparatus is described that has been designed specifically for use with molecular beams containing small particles in the nanometer size range. A novel electron detection system has been developed, using linear charge coupled device imagers, which allows rapid parallel measurement of the diffraction signal in a direct electron counting mode. The apparatus also features a 100 kV electron gun and electron optics derived from a Philips EM 300 electron microscope. Details of the apparatus are presented and measurement results on small silver particles in a beam of helium carrier gas are also given; these are interpreted using an icosahedral structure for the small particles.
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  • 33
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    Review of Scientific Instruments 62 (1991), S. 1498-1503 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A sputtering source utilizing both microwave and dc planar magnetron plasmas is described. Microwave power is introduced into the plasma by a coaxial-type cavity. The magnetron target is placed on the open end of the inner conductor. This source produces a plasma which is well matched, stable, and can operate continuously at gas pressures from 3×10−4 to 2×10−2 Torr. Plasmas with densities greater than 1011 cm−3 are obtained at gas pressure of 10−4 Torr using a microwave power of 100 W. The deposition rate of Cu is more than 0.13 μm/min for an argon gas pressure of 3×10−4 Torr, and microwave and dc powers of 100 W, respectively. This new source has many potential uses such as sputtering, etching, and chemical vapor deposition.
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  • 34
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    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The presence of a high-frequency ((approximately-greater-than)6 MHz) oscillation in the discharge current and voltage of a plasma-focus device is found experimentally and is explained by using equivalent circuit analyses. The occurrence of such an oscillation is attributed to the excitation of the resonant oscillation in the discharge circuit. It is also shown that the high-frequency response of the driver circuit, which consists of a capacitor bank and cables, plays an important role in determining the high-frequency behavior of the discharge current and voltage.
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  • 35
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    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 2008-2010 
    ISSN: 1089-7623
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    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The design and construction of a flange-mounted ultra-high vacuum compatible fatigue apparatus using a piezoelectric actuator is described. This apparatus is capable of operation under ambient pressures down to 10−9 Torr. Push-pull fatigue tests can be performed under load, total strain, and plastic strain control at frequencies up to 100 Hz. Control of the fatigue tests and data acquisition are done using standard fatigue control electronics and software.
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  • 36
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    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 2011-2015 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: An apparatus is described for the measurement of vapor pressures of low volatility solids and liquids by a static method in the range 10−2–100 mm Hg and 30 °C–300 °C. The method consists in inserting between the pressure gauge (here a capacitance manometer) and the product to be tested, a membrane that is maintained in equipressure by a suitable control loop. Experimental problems related to this arrangement are discussed. A test of the apparatus with a known product (benzoic acid) has been performed. The results have been found to be in good accordance with data found in the literature.
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  • 37
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    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 2016-2018 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: In order to automate the driving of terrestrial vehicles it is necessary to know at any time and with the highest accuracy the values of the speed of the vehicle and of the distance covered by the vehicle; it is also necessary to know the direction of the motion relative to the ground. These values must be measured in the real-time domain and this involves that contactless and very low time-constant sensors must be used. On the other hand, the applications concerned ask for a miniaturization of the whole measurement system. For these reasons, sensors using the Doppler effect in the microwave range are particularly well convenient to these applications. We developed an X band microwave Doppler cinemometer (MDC) with a printed antenna that is built using microstrip technology.
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  • 38
    ISSN: 1089-7623
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    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We report the results of a system to test the behavior of the wires of multiwire proportional chambers (MWPC) exposed for 10 min to a uniform force field of about 10 G strength and variable in time in the frequency range 0–2000 Hz. This acceleration will be present when the chambers, mounted inside a cosmic ray spectrometer, are launched by the Space Shuttle towards the NASA Space Station "FREEDOM.'' We have reproduced these conditions in the laboratory by an electronic device and we have found that, after repeated cycles of this treatment, the wire performance remained unchanged. This system can also be used to control the tension of the wires of a MWPC during its normal testing procedure.
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  • 39
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    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 2019-2021 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A method for magnetic hysteresis loop measurement on small samples of high Tc superconductors is described, applicable to other types of magnetic materials as well. It is based on an unusual working regime of the phase-sensitive detector with a square wave mixer signal and controllable reference phase setting. The main advantages of the method are its sensitivity and the possibility of measuring in a wide frequency range. A drawback of the method is that it is fully correct only in the case of a symmetrical hysteresis loop.
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  • 40
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    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 2036-2037 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: This note deals with measurement of local average velocity of a particle cloud in an air suspension by cross correlation of two attenuated laser beams.
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  • 41
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    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 2026-2033 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A new method for automatic determination of acoustic impedance ratio, or z ratio, of a thin solid film deposited upon an AT-cut quartz crystal is described. The method uses frequency information from two piezoelectrically excitable resonant modes which are not harmonically related. This results in an improvement of accuracy for the quartz crystal microbalance (QCM) technique, requiring no prior knowledge of a material's z ratio. The method is quite general and requires no adjustable parameter specific to a particular quartz crystal design. Results of extensive testing on wide ranging materials including metals, dielectrics, codeposited alloys, and sequential layers is presented. Excellent agreement is found between the predicted values and the gravimetrically measured mass of deposited films.
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  • 42
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    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 2038-2039 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: This note describes the design and operational experience of a solenoid valve used for pulsed supersonic beam experiments at cryogenic temperatures.
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  • 43
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    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 2034-2035 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We present a simple heater for sample temperature modulation in a thermoreflectance spectrometer. The performance of the heater is evaluated by determining the band to band transition energies at the E1 and E1+Δ1 critical points of GaAs.
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  • 44
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    Notes: Two devices for rapid quenching of powders are described. One of the devices, based on conventional resistance heating, is designed for application at temperatures up to 1200 °C and inert atmospheres. The other apparatus uses an rf coil as heating source and can be used at higher temperatures. The estimated quenching speed for both devices, is about 5×104 K/s. Examples of experiments are given to illustrate the performance of the devices.
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  • 45
    ISSN: 1089-7623
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    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A new method is described for measuring the critical current density and transition temperature of a superconducting film without making contact to it or modifying it in any way. This technique is particularly well suited for use with high transition temperature oxide films which are notoriously irreproducible and sensitive to patterning. It consists of positioning a flat, multiturn coil near the film surface and driving the coil with an audio frequency sine-wave current. Induced shielding currents flow in the film. We have calculated the radial dependence of the induced currents and show that the induced current density is zero at the coil center, rises to a maximum near the mean radius of the drive coil, and then falls off rapidly as the radius continues to increase. A measurement of the critical current per length can be obtained by monitoring the development of odd harmonic voltage components across the coil as the drive current is increased. We find that this measure of nonlinearity in the coil–film system increases abruptly when the maximum induced current equals the critical current. The critical current obtained by this inductive measurement has been demonstrated to give nearly the same value as that obtained by a transport measurement. Other advantages to this inductive measurement approach include: sensitivity only to intergranular critical current, and an ability to accommodate large substrates. Finally, with only minor modification to the electronics used to measure Jc, the superconducting transition temperature of the sample may be measured as well.
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  • 46
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    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We describe a new two-dimensional multioptical-fiber array (MOFA) with 144 elements, its use with charge-coupled device (CCD) image detection to perform parallel processing for light intensity measurements, and its use with a concave aspherical grating and CCD image detection to perform parallel processing for spectrometry. Each 2-m fiber extending from the MOFA is individually sheathed in a flexible, opaque cover. The light signals can be measured simultaneously with cross-talk ranging from 0.4% to 0.03% of the peak signal, depending upon configuration. We characterize the instrument in both the intensity and the spectrometric configurations, and present techniques for optimizing performance.
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  • 47
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    Review of Scientific Instruments 62 (1991), S. 595-599 
    ISSN: 1089-7623
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    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: PAPA (precision analog photon address) cameras are photon counting imagers which employ microchannel plates for image intensification. They have been used extensively in astronomical speckle imaging. The PAPA camera can produce artifacts when light incident on its microchannel plate (MCP) image intensifier is highly concentrated. The effect is exacerbated by adjusting the strobe detection level too low, so that the camera accepts very small MCP pulses. The artifacts can occur even at low total count rates if the image has highly a concentrated bright spot. This paper describes how to optimize PAPA camera electronics, and describes six techniques which can avoid or minimize addressing errors: (1) use the hottest possible MCP so the recharge time is short; (2) use a single plate MCP for a short recharge time; (3) keep the local photon flux low on the MCP input face; (4) avoid setting the strobe channel lower detection threshold too low; (5) optimize the camera by proper electronic adjustment of the strobe channel; (6) build the camera with specially contoured masks for best optimization of the strobe. When the camera operates within these guidelines, it produces photon counting images with minimal errors.
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  • 48
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    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 600-608 
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    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A pump-and-probe spectrometer is described that employs a pump pulse at 308 nm, with up to 1.4 mJ in 250 fs, and a continuum probe pulse in the spectral range 360–750 nm, of 80-fs pulse duration. The corresponding laser system is based on a colliding-pulse-mode dye laser operating at 616 nm, and a fs dye laser amplifier pumped by a XeCl* excimer laser. The amplifier has been designed specifically for pump-and-probe experiments, by multiplexing with horizontally and vertically polarized pulses for the pump and probe beam, respectively. Near-diffraction-limited output beams are achieved using a novel, capillary prismatic dye amplifier cell. The reliability of the apparatus is demonstrated by examining the photodissociation of bis-(p-aminophenyl) disulfide. The absorption band of the resulting p-aminophenyl-thiyl radicals shows a biexponential frequency shift in the course of several ps. This behavior is correlated with that of dielectric relaxation of the solvent.
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  • 49
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    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 660-666 
    ISSN: 1089-7623
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    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We explore the feasibility of Auger electron spectroscopy as a remote sensing technique to map the composition of the sunlit surfaces of planetary bodies without atmospheres. Solar x rays eject photoelectrons from the planetary surface. The resulting ions relax by emission of fluorescence x rays or Auger electrons, with energies characteristic of the element which is ionized. The spectrum of Auger electrons and photoelectrons is computed for a variety of elements and for representative lunar rock types illuminated by soft x-ray line and continuum emission typical of solar long-lived coronal active regions. The Auger electron lines for O, Si, Mg, Al, Fe, and Ca in lunar rocks stand well above the continuum background from photoelectrons and backscattered interplanetary electrons, with typical line-to-continuum ratios of ∼20 to (approximately-greater-than)103. An ideal Auger electron spectroscopy instrument to map the elemental composition of planetary surfaces should have a combination of high energy resolution ((approximately-less-than)2 eV) and large geometric factor ((approximately-greater-than)0.1 cm2 sr). This technique could be applied to the Moon, Mercury, asteroids, comets, and perhaps the moons of outer planets.
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  • 50
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    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A highly sensitive capacitance-change technique, suitable for thin polymer films, has been developed for the measurement of the coefficient of thermal expansion (CTE) in the film-thickness direction. Using this technique, we have determined the thickness-direction CTE of Kapton polyimide to be about 81±4 ppm/°C, averaged over the temperature range 50–150 °C. This polymer was chosen because it is widely used in the electronics industry for the fabrication of flexible printed circuits. Two other techniques, a Fabry–Perot laser interferometric technique, and the more conventional but less sensitive thermomechanical analysis, were also investigated. The thickness-direction CTEs obtained from the three techniques were at least a factor of 2.4 greater than the in-plane CTE of about 34±2 ppm/°C. The in-plane CTE, found here via simple length measurements, was in good agreement with literature values. It is speculated that the CTE anisotropy observed might be due to the in-plane molecular orientation of the polymer chains.
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  • 51
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    Review of Scientific Instruments 62 (1991), S. 2063-2068 
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    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Some alternative methods of tuning variable-field transverse insertion devices that longitudinally displace the jaws of otherwise ordinary undulator structures have been recently proposed. In this article we discuss some of the off-axis characteristics of such schemes that could limit their range of tunability and affect their possible modes of implementation on conventional storage rings. Based on the corresponding analysis, a new set of field integral constraints for facilitating the operation of such devices is derived.
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  • 52
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    Review of Scientific Instruments 62 (1991), S. 2069-2075 
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    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We describe an active low-frequency vibration isolation system based on an earlier instrument developed at the Joint Institute for Laboratory Astrophysics (JILA). Several modifications, which have led to improved transient response, temperature stability, and a greatly reduced size, are discussed. We also comment on how the performance of the system is limited by sensor noise, mechanical resonances, and cross couplings between the different degrees of freedom.
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  • 53
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    Notes: A high power N2 laser of the double-Blumlein type having a modified gas flow system, electrode configuration, and discharge geometry with minimum inductance is described. By incorporating a triggered-pressurized spark gap switch, arc-free operation was achieved for a wide E/P range. The device gives a peak power in excess of 700 kW with a FWHM of 3 ns and an efficiency of 0.51%, which is remarkably high for a pulsed nitrogen laser system. The dependence of output power on parameters such as operating pressure, voltage, and repetition rate are discussed.
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  • 54
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    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 2080-2082 
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    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A folded-path two-detector photopolarimeter for measuring all four Stokes parameters of light is described. The light beam, whose state of polarization is to be measured, strikes the two (windowless and reflective) detector surfaces at oblique incidence and with noncoincident planes of incidence. After reflection from the second detector, the beam is folded back on itself by a mirror and the feedback is interrupted periodically with a light chopper. This intermittent feedback causes square-wave modulation of the two detector outputs which provides a 4×1 current vector I. The unknown 4×1 Stokes vector S of incident light is determined by S=A−1 I, where A is a wavelength-dependent 4×4 instrument matrix which is fixed by calibration. Based on an explicit expression for det A, all potential singularities of A are investigated and are found to be readily avoidable. Optimum parameters are also specified that make A as far from singular as possible by maximizing its determinant.
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  • 55
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    Notes: The initiation of many physiologically important biochemical reactions is dependent on the binding of a molecule or molecules from solution to an appropriate solid surface such as a cell membrane. The quantitative study of dynamic interactions of molecules in solution with immobilized surfaces under physiologically relevant conditions presents a significant physical problem. Traditionally these measurements have been performed by discontinuous, nonequilibrium assays that cannot accurately model in vivo reaction mechanisms. Total internal reflection fluorescence spectroscopy (TIRFS) is an analytical technique that is based on the excitation of fluorescence by means of an exponentially decaying surface energy wave called the evanescent wave. The limited "penetration depth'' of the evanescent wave leads to excitation of fluorescent molecules at or near the surface. This phenomena makes TIRFS uniquely qualified for the dynamic fluorescence study of liquid-solid interface binding reactions. The instrument described here permits the powerful technique of TIRFS to be used by biomedical researchers in their investigation of biochemical interface reactions. The fully automated, menu driven instrument allows the real time dynamic measurement of binding of fluorescent-labeled molecules in solution at physiologically relevant concentrations to appropriate immobilized surfaces.The novel design of the total internal reflection element enables the study of binding from essentially opaque and highly scattering solutions such as whole blood. Measurements may be performed under static, stirred, or flow conditions. The presently described TIRFS instrument is capable of tracking a binding reaction with evanescent fields established by two identical reflecting light beams with different reflection angles. This instrumental feature allows the dynamic measurement not only of the primary binding event but also of fluctuations in the distance between the optical interface and the point of signal generation. Data from experiments involving fluorescently labeled molecules binding to cell monolayers as well as binding experiments performed in whole blood are shown to verify the instrument performance. Due to its extraordinary capabilities and its user friendly design, the developed TIRFS instrument opens up a large number of relevant but previously unmeasurable experimental systems for investigation.
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  • 56
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    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The temperature determined by a single-shot coherent anti-Stokes Raman spectroscopy (CARS) system is directly related to the half width at half maximum of the instrument slit function. Therefore, an accurate knowledge of the instrument slit function is necessary to determine temperature with CARS. However, in turbulent systems, the input slits of the spectrometer may be removed in order to guarantee signal throughput and establish the necessary dynamic range. In this case, the physical input slits of the spectrometer are replaced with apparent slits created by focussing the input beams near the entrance plane of the spectrometer. The slit function will then depend on the physical relationship among all of the optical components, the probe volume, and the dispersive performance of the spectrometer and detector, as well as the optical path through density and temperature gradients which may not be invariant in a turbulent system. The presence of high temperatures and turbulence levels can effect the size of the CARS signal origin and the optical path, and as a result, the slit function is not invariant. Ignoring these changes can result in large root mean square temperatures (decreased precision) as well as mean temperature errors. The variability of the slit width can be accounted for on a shot-to-shot basis by using a two parameter (HWHM of the slit function and temperature) fitting routine. For temperatures greater than 1200 K there is convergence on a best curve implying both a temperature and a slit width. This method can be used alone or in concert with various weighting schemes to improve the precision. There are two major advantages gained by allowing the slit function to vary in a CARS system: (a) it allows an increase in the precision; (b) it allows temperatures to be calculated without the assumption that the slit function does not change with temperature or turbulence or position within the flame. In fact, it allows the temperature to be determined with almost no previous knowledge of the slit function except the general shape. These two advantages combine to significantly simplify the study of turbulent combusting systems with CARS and to improve the precision, both point-to-point and shot-to-shot, of CARS.
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  • 57
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    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A multichannel single photon counting detection system for steady state, time-resolved luminescence spectra, time-gated imaging, or time-gated Raman is described. This system, which has approximately 80 ps time resolution, is based on a microchannel plate photomultiplier with a position-sensitive anode (Mepsicron). This detector can operate at very low light levels with excitation wavelengths from 200 nm to the near infrared. For time-resolved luminescence applications, the system simultaneously collects 1024 decays of up to 1024 spectral channels. The measured time dispersion for spectral bandwidths greater than 100 nm are shown to be negligible.
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  • 58
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    Review of Scientific Instruments 62 (1991), S. 2105-2108 
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    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: An acousto-optic (AO) element made of As2Se3 glass was used for spectroscopic gas sensing. With the AO element, desired wavelengths in the infrared can be monitored by simply switching the oscillation frequency of the ultrasonic transducer. The concentration of the CO2 gas was measured by using the absorption band at 4.3 μm wavelength. To avoid measurement error due to the fluctuation in the sensing system, a probe light of 4.3 μm and a reference light of 4.7 μm were measured alternatively by switching the acoustic frequency between 50 and 54 MHz. It has been demonstrated in a preliminary experiment that the AO element is a promising device for constructing a fast, compact, and inexpensive measurement system for monitoring pollutants.
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  • 59
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    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Commissioning and performance of a water-cooled quasi doubly bent crystal monochromator for Compton scattering experiments are described. The monochromator consists of an array of twenty singly bent crystals. Each crystal is half immersed in liquid Ga-ln alloy contained in a water-cooled holder. This is necessary because the monochromator must be able to withstand heat load of 4 kW from synchrotron radiation from an ellipsoid multipole wiggler at the 6.5 GeV accumulation ring. The alignment procedure for twenty singly bent crystals is described in detail. Its performance is as follows; the focused beam size of 60 keV x rays is about 3 mm (vertical) and 8 mm (horizontal). The flux is 6×1012 photons/s at stored currents of 30 mA and the energy resolution is 88 eV for 60 keV x rays. Heating of various adjusting mechanisms by Compton scattered x rays from both the absorber used to suppress low energy x rays and the monochromator crystals is found to be a very serious problem for designing a high energy x-ray beamline.
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  • 60
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    Review of Scientific Instruments 62 (1991), S. 1743-1745 
    ISSN: 1089-7623
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    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A novel experimental apparatus designed to study, using in situ Raman spectroscopy, chemical and structural modifications induced by ions impinging on frozen gases and solids at low temperatures (10–300 K) is described.
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  • 61
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    Review of Scientific Instruments 62 (1991), S. 1746-1748 
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    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: This article describes an inverse photoelectron spectrometer based on the novel design of a magnetically focused low energy electron gun. The article discusses results of the ray tracing procedure conducted for this gun and some of the advantages of the magnetic focusing.
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  • 62
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    Review of Scientific Instruments 62 (1991), S. 1735-1742 
    ISSN: 1089-7623
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    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A novel signal processing approach, which results in a significant improvement in the measurement resolution for a fluorescent decay-time-based fiber optic temperature sensor, is proposed. The technique discussed is based on the phase sensitive detection (lock-in) of the fluorescent signal. As with the other phase-sensitive detection techniques, it has the significant advantage of a high noise-suppression ability, but it differs from existing phase sensitive detection methods in several aspects which are discussed to yield this improvement. When the system is used to monitor the fluorescence lifetime of ruby, in the range from 40 °C to 100 °C, the measurement resolution achievable due to the signal processing scheme is ±0.04 °C, and from 100 °C to 200 °C, it is ±0.2 °C. It should be noted that other measurement errors must be reduced to achieve a high accuracy of measurement comparable with this. Finally, the proposed technique can be used not only for the monitoring of fluorescence lifetime, but also for the highly accurate monitoring of the time-constants of other appropriate first order systems, such as a capacitive sensor, operating under high noise conditions.
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  • 63
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    Review of Scientific Instruments 62 (1991), S. 1727-1734 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We report the creation of two-dimensional fluorescence lifetime images, based on a sinusoidally modulated image intensifier that is operated as a radio-frequency phase-sensitive camera, synchronized to a mode-locked and cavity dumped picosecond dye laser. By combining the image intensifier with a CCD camera and applying digital image processing, lifetime-selective signal suppression can be realized even for fluorophores with comparable lifetimes. This phase-sensitive technique can be used to create fluorescence lifetime images, that is, images in which the contrast is based upon the fluorescence lifetimes rather than upon local probe concentration and/or intensity. Because the lifetimes of many dyes are sensitive to the chemical environments surrounding the fluorophore, fluorescence lifetime imaging (FLIM) can reveal the local chemical composition and properties of the molecular environment that surrounds the fluorophore. As an example we created images of rhodamine 6G (4 ns) and rhodamine B (1.5 ns) solutions in which the difference in lifetimes results in 100% contrast, whereas the total fluorescence intensity is similar. In order to estimate the time resolution obtainable with our phase-sensitive imaging setup, we also performed measurements using distance-selective suppression of optical signals backscattered from laser-illuminated targets. In these studies, we have created distance-selective images with a resolution of 3.75 cm, which corresponds to a lifetime resolution of 0.25 ns for fast-decaying fluorophores. Since we observed nearly 100% contrast for this 0.25 ns difference, still smaller distances and/or lifetime differences could be observed, which seems to imply some advantages of phase fluorometric lifetime imaging. Using these same intensifiers in the pulse-gating mode one expects a typical 5 ns gating time, which result in a distance resolution of only 75 cm. The distance-selective imaging principle has the potential for manifold applications related to robotics, construction, and even to spacecraft maneuvering. Fluorescence lifetime imaging, in combination with fluorescence microscopy, can have numerous applications to biochemistry, biophysics, and cell physiology.
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  • 64
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    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 1749-1753 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The relative response of Kodak SB film has been measured with a pulsed-laser Al plasma x-ray source in the energy region delimited by the absorption edges of Br (1599 eV) and of Ag (3351 eV) in the photographic emulsion. The data of optical density versus exposure were obtained by recording spectra at different exposures which were controlled by placing stepped Be filters. A significant variation has been observed with earlier results obtained with continuous x-ray sources at high optical densities. The data may be fit with a simple expression of exposure L determined with an arbitrary constant L0 as a function of optical density D with only two parameters, Dsat and k: L = L0/(Dsat/D − 1)1/k. It is found that Dsat=3.45 and k=0.92 over the entire energy range.
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  • 65
    Electronic Resource
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    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 1754-1766 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Direct detection of thin-film and interfacial electron-spin resonance (ESR) pushes conventional techniques to the noise limit for those techniques and requires use of large-area samples. Several microstrip resonator structures are compared here to conventional techniques and experimental results reported. Aluminum microstrip resonators are shown to have from 3 to 10 times the sensitivity to two-dimensional systems as waveguide cavities, while needing only about 1/20th the sample area. A microstrip resonator metal-oxide-superconductor field-effect-transistor structure for study of inversion layer conduction electron-spin resonance is reported in which the resonator functions as the gate, and ohmic contact is made to the inversion layer. A "microstrip induction spectrometer'' is described in which a bimodal microstrip resonator is used along with a cryogenically cooled amplifier for study of lines with very long relaxation times. The ultimate sensitivity to Curie law spins of this spectrometer is 1×108 spins in an area of 1 cm2 at 4.2 K for a 1-G linewidth at the onset of saturation. Superconducting microstrip resonators for ESR are reported and their use discussed.
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  • 66
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A novel digital integrator circuit has been engineered for constant-current mode STM feedback control. In contrast to analog integrators that rely on charging or discharging of a capacitor in an amplifier feedback loop, the integrator uses a 16-bit digital counter that is rapidly incremented or decremented depending on whether the tunneling current is higher or lower than a reference value. The counter drives a digital-to-analog converter (DAC) whose output is amplified and used to control the STM Z-axis scanner. A computer's parallel input port records the DAC output word at each sampling point, directly giving the digital value of the tip position. The integrator is used in conjunction with a two-axis 16-bit scan generator. The Z-axis feedback reference circuit is used to dynamically control the STM's lateral scanning rate, and scan rates of 300 μm s−1 are demonstrated. The digital integrator and scan generator facilitate fast switching between different modes of STM operation, allowing glitch- and drift-free locking of tip position, interchange of axes, scanning or switching the sign of the tip bias, and flexible tailoring of the feedback response.
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  • 67
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 1767-1771 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Probes having two mechanically fixed, electrically independent tips were proposed ("twin probes''). A control system and mechanism for STM with a twin probe has been designed and fabricated. Two types of twin probes are fabricated. The first had Pt-Ir tips with 1.5 mm distance between the tips. The other had tungsten tips with 0.1 μm distance. Since the lengths of the two tips are not equal, the system controls not only the distance between the probe and the sample surface but also the angle between the probes and the sample to allow detection of tunneling currents with both tips of the probe at the same time. The sum of the tunneling currents is used to control the height of the probe and the difference between currents is used to control the angle of the sample. Using this system with the Pt-Ir probe, two independent images of graphite atoms from one sample were simultaneously obtained.
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  • 68
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Deflection of carbon dioxide and helium-neon laser beams has been used to measure plasma and neutral density gradients during the operating mode and after the shorting time of a long-pulse field-emission electron beam diode. Plasma density gradients of (1014–1015) cm−4 were observed throughout the diode during the final microsecond of the 2–3 μs electron beam pulse. The neutral density gradient was less than 1×1018 cm−4 during the electron beam pulse. Upon diode shorting, neutral density gradients increased to (1018–1019) cm−4 over ∼1 μs, and decayed over many microseconds. Plasma density gradients of ∼1015 cm−4 were also observed after shorting. These experiments demonstrate the value of carbon-dioxide laser and helium-neon laser deflection for diagnosing plasma and neutral particles in long-pulse electron beam diodes.
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  • 69
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 1783-1786 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A flow cryostat allowing independent cooling of eight pipe guns in a multishot deuterium pellet injector is described. The pipe guns are placed symmetrically around the flow cryostat and with a liquid helium consumption of 4–5 l/h the cooling is sufficient for simultaneous formation of eight pellets at 8–9 K with a diameter of 2.1 mm containing up to 8×1020 atoms/pellet. The thermal interaction between the eight pipe guns is sufficiently low to allow successive firing of the guns with time intervals of up to 1 s.
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  • 70
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    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 1787-1795 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The components of a typical rotamak device are described and the constructional details of an inexpensive version are given. The operating range of this particular version is described and, as an example, a detailed set of measurements made for one set of initial conditions are presented. Typically, steady toroidal currents of ∼100 A are generated and maintained for the full 40 ms of operation by a ∼1-MHz rotating magnetic field of a few Gauss in amplitude. Plasmas having electron temperatures lying between 5–10 eV and number densities in the 1017-m−3 range are routinely and very reproducibly produced.
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  • 71
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: This article describes a simple apparatus to perform ac inductive measurements on superconductors. The ac susceptibility, the penetrated and trapped magnetic flux and the magnetization of a sample placed in an ac magnetic field are measured by processing the signal induced in a pick-up coil that surrounds the sample. The complex harmonic analysis of the signal is also performed. The setup consists of an amplifier, a three-channel recorder, and a standard desktop computer. The main advantages of the apparatus are its simple design and low cost. Its main drawback is the noise affecting the signals. Tests were performed on a high Tc superconductor.
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  • 72
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The microstrip resonator technique is a convenient way to sensitively measure the temperature dependence of the magnetic penetration depth, λ(T), in superconducting thin films. Because the method relies on measuring the resonant frequency of a microwave transmission line resonator, one can very precisely measure small changes in λ(T). This technique can resolve changes in λ on the order of several angstroms, allowing a direct measurement of the low-temperature behavior of λ(T), which is a measure of the low-lying pair breaking excitations of the superconductor. Absolute penetration depth values can also be obtained from a self-consistent fit to the data to an assumed temperature dependence. Measurements of the penetration depth of Nb and NbCN film give results that are consistent with the predictions of BCS theory in which 2Δ/kTc is treated as an adjustable parameter, while YBa2Cu3O7−δ films give results that are not completely understood at this time. We also compare this technique with other methods of measuring the penetration depth of superconducting thin films, and discuss the systematic errors present in the measurement.
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  • 73
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A variable-temperature (1.4–320 K) superconducting quantum interference device magnetometer, operating in magnetic fields up to 8 T, has been built. The sensitivity is 2×10−7 emu in low fields and 2×10−6 in high fields. The apparatus allows magnetization and anisotropy measurements to be performed and is entirely automated. It is of simple construction and has proved over a number of years to be very robust.
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  • 74
    ISSN: 1089-7623
    Source: AIP Digital Archive