Showing papers in "Review of Scientific Instruments in 1994"

Neural networks and their applications

Abstract: Neural networks provide a range of powerful new techniques for solving problems in pattern recognition, data analysis, and control. They have several notable features including high processing speeds and the ability to learn the solution to a problem from a set of examples. The majority of practical applications of neural networks currently make use of two basic network models. We describe these models in detail and explain the various techniques used to train them. Next we discuss a number of key issues which must be addressed when applying neural networks to practical problems, and highlight several potential pitfalls. Finally, we survey the various classes of problem which may be addressed using neural networks, and we illustrate them with a variety of successful applications drawn from a range of fields. It is intended that this review should be accessible to readers with no previous knowledge of neural networks, and yet also provide new insights for those already making practical use of these techniques.

Vacuum Arc Ion Sources

Abstract: The vacuum arc is a rich source of highly ionized metal plasma that can be used to make a high current metal ion source. Vacuum arc ion sources have been developed for a range of applications including ion implantation for materials surface modification, particle accelerator injection for fundamental nuclear physics research, and other fundamental and applied purposes. The beam parameters can be attractive, and the source has provided a valuable addition to the spectrum of ion sources available to the experimenter. Beams have been produced from over 50 of the solid metals of the periodic table, with mean ion energy up to several hundred keV and with beam current up to several amperes. Typically the source is repetitively pulsed with pulse length of order a millisecond and duty cycle of order 1%, and operation of a dc embodiment has been demonstrated. Here the source fundamentals and operation are reviewed, the source and beam characteristics summarized, and some applications examined.

Thermal conductivity, thermal diffusivity, and specific heat of thin samples from transient measurements with hot disk sensors

Abstract: Transient measurements of thermal conductivity, thermal diffusivity, and specific heat capacity have been performed with hot disk sensors in thin samples of metallic materials. With this new variation of the hot disk method the sample size can be reduced to a volume less than ten cubic centimeters for copper at room temperature. It is also shown that the specific heat capacity can be conveniently measured in transient recordings of slightly longer duration. On comparing with standard values the accuracy turns out to be better than 1% while the precision (standard deviation of the mean from six measurements) on the average is about 0.5% for all values recorded.

Lateral, normal, and longitudinal spring constants of atomic force microscopy cantilevers

Abstract: For a V‐shaped atomic force microscopy cantilever beam, the spring constants in the three principal directions are given in terms of the beam geometry and material properties. For the lateral stiffness, a closed‐formed expression is presented. Also, the normal and the longitudinal stiffness are obtained from a few simple equations. The results are compared with a finite element study and found to be very accurate. All spring constants depend strongly on the cantilever thickness, which is difficult to measure. In addition, the lateral and longitudinal stiffness are sensitive to the location and the height of the attached pyramid.

The Gaseous Electronics Conference radio‐frequency reference cell: A defined parallel‐plate radio‐frequency system for experimental and theoretical studies of plasma‐processing discharges

Abstract: A “reference cell” for generating radio-frequency (rf) glow discharges in gases at a frequency of 13.56 MHz is described. The reference cell provides an experimental platform for comparing plasma measurements carried out in a common reactor geometry by different experimental groups, thereby enhancing the transfer of knowledge and insight gained in rf discharge studies. The results of performing ostensibly identical measurements on six of these cells in five different laboratories are analyzed and discussed. Measurements were made of plasma voltage and current characteristics for discharges in pure argon at specified values of applied voltages, gas pressures, and gas flow rates. Data are presented on relevant electrical quantities derived from Fourier analysis of the voltage and current wave forms. Amplitudes, phase shifts, self-bias voltages, and power dissipation were measured. Each of the cells was characterized in terms of its measured internal reactive components. Comparing results from different cells provides an indication of the degree of precision needed to define the electrical configuration and operating parameters in order to achieve identical performance at various laboratories. The results show, for example, that the external circuit, including the reactive components of the rf power source, can significantly influence the discharge. Results obtained in reference cells with identical rf power sources demonstrate that considerable progress has been made in developing a phenomenological understanding of the conditions needed to obtain reproducible discharge conditions in independent reference cells.

A femtojoule calorimeter using micromechanical sensors

Abstract: We describe a highly sensitive new type of calorimeter based on the deflection of a ‘‘bimetallic’’ micromechanical sensor as a function of temperature. The temperature changes can be due to ambient changes, giving a temperature sensor or, more importantly, due to the heat absorbed by a coating on the sensor, giving a heat sensor. As an example we show the results of using the sensor as a photothermal spectrometer. The small dimensions and low thermal mass of the sensor make it highly sensitive and we demonstrate a sensitivity of roughly 100 pW. By applying a simple model of the system the ultimate sensitivity is expected to be of the order of 10 pW. The thermal response time of the cantilever can also be determined, giving an estimate of the minimum detectable energy of the sensor. This we find to be 150 fJ and again from our model, expect a minimum value of the order of 20 fJ.

Resonance response of scanning force microscopy cantilevers

Abstract: A variational method is used to calculate the deflection and the fundamental and harmonic resonance frequencies of commercial V‐shaped and rectangular atomic force microscopy cantilevers. The effective mass of V‐shaped cantilevers is roughly half that calculated for the equivalent rectangular cantilevers. Damping by environmental gases, including air, nitrogen, argon, and helium, affects the frequency of maximum response and to a much greater degree the quality factor Q. Helium has the lowest viscosity, resulting in the highest Q, and thus provides the best sensitivity in noncontact force microscopy. Damping in liquids is dominated by an increase in effective mass of the cantilever due to an added mass of the liquid being dragged with that cantilever.

Thin film microcalorimeter for heat capacity measurements from 1.5 to 800 K

Abstract: A new microcalorimeter for measuring heat capacity of thin films in the range 1.5–800 K is described. Semiconductor processing techniques are used to create a device with an amorphous silicon nitride membrane as the sample substrate, a Pt thin film resistor for temperatures greater than 40 K, and either a thin film amorphous Nb–Si or a novel boron‐doped polycrystalline silicon thermometer for lower temperatures. The addenda of the device, including substrate, is 4×10−6 J/K at room temperature and 2×10−9 J/K at 4.3 K, approximately two orders of magnitude less than any existing calorimeter used for measuring thin films. The device is capable of measuring the heat capacity of thin film samples as small as a few micrograms.

Measurement of small forces using an optical trap

Abstract: A simple and sensitive method for detecting small forces applied to an optically trapped microsphere utilizes the transmitted beam of the trapping laser to monitor microsphere deflections within the potential well of the trap. The rms dynamic displacement detection sensitivity measured in the frequency range from 1 Hz to 10 kHz is ∼1 nm in the radial direction and ∼10 nm along the z (optic) axis. Radiation trapping force constants were calibrated against viscous drag on microspheres in the range between the Rayleigh (r<0.2λ) and Mie (r≫λ) size regimes. For a 1‐μm‐diam polystyrene sphere trapped with a 60 mW beam the rms spontaneous thermal motion limits the force sensitivity to better than 10−12 N and lateral spatial resolution to ∼10 nm in a frequency range from 1 Hz to the viscous rolloff frequency ∼1 kHz. The measured maximum trapping efficiencies are compared with the theoretical predictions of the ray‐optics approximation.

Practical nitric oxide measurement employing a nitric oxide‐selective electrode

Abstract: An NO‐selective electrode was developed as an easily applicable tool for a real‐time nitric oxide (NO) measurement. The working electrode (0.2 mm diam) was made from Pt/Ir alloy coated with a three‐layered membrane. The counterelectrode was made from a carbon fiber. When a stable NO donor, S‐nitroso‐N‐acetyl‐dl‐penicillamine, was applied, the electrode current increased in a dose‐dependent fashion. The current and calculated NO concentration showed a linear relationship in the range from 0.2 nM (S/N=1) to 1 μM of NO. The response of the electrode was 1.14±0.09 s. The effects of temperature, pH, and chemicals other than NO on the electrode current were also evaluated. Electrodes which were placed in the luminal side of rat aortic rings exhibited 30 pA of current due to NO generation induced by the addition of 10−6 M of acetylcholine. The current was eliminated in the presence of 50 μM NG‐monomethyl‐L‐arginine, an inhibitor of NO synthase. Thus, this NO‐selective electrode is applicable to real‐time NO assay in biological systems.

Magnetic manipulation instrumentation for medical physics research

Abstract: The noncontact magnetic manipulation of probe masses within the body is an area of research that has received substantial attention from the medical physics community, especially during the past three decades. The therapeutic and diagnostic possibilities arising from such technology include site‐specific drug delivery within the central nervous system, advancement of techniques for navigation and selective catheterization of vessels within the cardiovascular and cerebrovascular systems, and the nonsurgical exploration of the alimentary and respiratory tracts. In this review, we examine the physical principles underlying in vivo magnetic manipulation systems, and catalog the various types of instrumentation used for such purposes to date. Thereafter, we evaluate the different methods of image‐based localization used to identify the position of the probe within the body. Finally, we appraise an emerging technology known as nonlinear magnetic stereotaxis, a technique that permits minimally invasive access to difficult‐to‐approach parts of the brain. We close the review with a few comments on the directions for future work within this field.

Atomic force microscope with magnetic force modulation

Abstract: We have constructed a scanned stylus atomic force microscope (AFM) with direct force modulation and integrated microfluorescence optics. The instrument was designed to image the surface of massive samples under various ambient conditions. In force modulation microscopy the imaging force is modulated during the scanning process via an external magnetic field that acts directly on the magnetic AFM tip. Polymeric Langmuir–Blodgett films on silicon oxide were imaged to evaluate the application range of the instrument. We demonstrate that direct force modulation microscopy permits the quantitative recording of the local complex compliance both as a function of the location and as a function of the frequency. In a novel imaging mode referred to as sample resonance mode, the contrast of the image can be selectively enhanced based on local elasticity differences.

Aero‐acoustic levitation: A method for containerless liquid‐phase processing at high temperatures

Abstract: A method for containerless liquid‐phase processing was developed which has practical application in process and property research on virtually any material which is involatile at the melting point. It combines aerodynamic and acoustic forces to support and position the levitated material. The design provides forced convection control of the thermal boundary in the gas surrounding beam‐heated specimens, which stabilizes the acoustic forces and allows acoustic positioning necessary to stabilize the aerodynamic levitation forces on molten materials. Beam heating and melting at very high temperatures was achieved. Experiments were conducted on specimens with diameters in the range 0.25–0.4 cm, of density up to 9 g/cm3, at temperatures up to 2700 K, and in oxygen, air, or argon atmospheres. Unique liquid‐phase processing results included deep undercooling of aluminum oxide, glass formation at exceptionally small cooling rates, complete melting and undercooling of YBa2Cu3Ox superconductor materials, direct form...

A flat flame burner as calibration source for combustion research: Temperatures and species concentrations of premixed H2/air flames

Abstract: A commercially available flat flame burner with a sintered porous bronze disk was used to stabilize premixed H2/air flames at atmospheric pressure. Temperatures for various stoichiometries, flow rates, and heights above the burner disk have been measured by coherent anti‐Stokes Raman scattering with an accuracy of ≊2.5%. The corresponding exhaust gas compositions have been derived from equilibrium calculations. This burner together with the data presented in this article can be used for the verification or calibration of a variety of measuring techniques in combustion research.

Evaluation of tortuosity in acoustic porous materials saturated by air

Abstract: Tortuosity is an important parameter for the prediction of the acoustical properties of porous sound absorbing materials. The evaluation of tortuosity by resistivity measurements is now used in several laboratories, although this method presents several drawbacks. In particular, the complete saturation by a conducting fluid of a porous foam having a high flow resistivity is difficult to obtain without partially damaging the structure of the cells. A simple technique based on ultrasonic wavespeed measurements in a material saturated by air is described. This method has been used previously only for water or superfluid helium saturated materials.

New fast response photofragment fluorescence hygrometer for use on the NASA ER‐2 and the Perseus remotely piloted aircraft

Abstract: We have developed an in situ instrument to measure water vapor on the NASA ER‐2 as a prototype for use on the Perseus remotely piloted aircraft. It utilizes photofragment fluorescence throughout the stratosphere and the upper to middle troposphere (mixing ratios from 2 to 300 ppmv) with simultaneous absorption measurements in the middle troposphere (water vapor concentrations ≳5×1014 mol/cc). The instrument flew successfully on the NASA ER‐2 aircraft during the 1993 CEPEX and SPADE campaigns. The 2σ measurement precision for a 10 s integration time, limited by variation in the background from scattered solar radiation, is ±6% and the data were tightly correlated with other long‐lived stratospheric tracers throughout the SPADE mission. Its accuracy is estimated to be ±10%, based on laboratory calibrations using a range of water vapor concentrations independently determined by both standard gas addition techniques and by absorption. This accuracy is confirmed by in‐flight absorption measurements in the trop...

Resolution in surface plasmon microscopy

Abstract: In this article we demonstrate how to obtain the ultimate lateral resolution in surface plasmon microscopy (SPM) (diffraction limited by the objective). Surface plasmon decay lengths are determined theoretically and experimentally, for wavelengths ranging from 531 to 676 nm, and are in good agreement. Using these values we can determine for each particular situation which wavelength should be used to obtain an optimal lateral resolution, i.e., where the plasmon decay length does not limit the resolution anymore. However, there is a trade‐off between thickness resolution and lateral resolution in SPM. Because of the non‐optimal thickness resolution, we use several techniques to enhance the image acquisition and processing. Without these techniques the use of short wavelengths results in images where the contrast has vanished almost completely. In an example given, a 2.5 nm SiO2 layer on a gold layer is imaged with a lateral resolution of 2 μm, and local reflectance curves are measured to determine the laye...

A broadband method for the measurement of the surface impedance of thin films at microwave frequencies

Abstract: We present a new technique to measure the complex surface impedance of the mixed state of superconducting thin films over the broad frequency range 45 MHz–20 GHz. The surface impedance is extracted from measurements of the complex reflection coefficient made on the film using a vector network analyzer. The technique takes advantage of a special geometry in which the self‐fields from currents flowing in the film are everywhere parallel to the film surface, making it an ideal configuration in which to study vortex dynamics in superconductors. The broadband nature of the measurement system allows us to explore a region of magnetic field–temperature–frequency parameter space of superconductors previously inaccessible with other measurement techniques. The power of the technique is illustrated by measurements on thin films of the high temperature superconductor YBa2Cu3O7−δ.

One‐dimensional scanning of moisture in porous materials with NMR

Abstract: A versatile and modular nuclear magnetic resonance (NMR) instrument is described that is particularly suited for the study of moisture transport in porous media such as various building materials in which moisture can give rise to several kinds of damages. Quantitative measurements of one‐dimensional moisture profiles and their time evolution can be performed on cylindrical samples having a diameter up to 20 mm with a spatial resolution better than 1 mm. Water absorption and drying experiments on various building materials demonstrate that the instrument can also be used for materials containing relatively large amounts of magnetic impurities, which until now were hardly accessible by NMR techniques.

Incorporation of a differential refractometer into a laser light-scattering spectrometer

Abstract: A new differential refractometer, which mainly consists of a laser light source, a position‐sensitive detector, and a temperature‐controlled refractometer cuvette has recently been developed In comparison with a conventional differential refractometer, it has a different optical design so that the effect of laser beam drift can be greatly reduced In our design, a very small pinhole is illuminated by the laser light and the illuminated pinhole is imaged to the detector by a lens located in the middle between the detector and the pinhole in a 2f‐2f configuration The cuvette is placed just before the lens The pinhole, the cuvette, the lens, and the detector are mounted on a small optical rail The refractometer can be easily incorporated into any laser light‐scattering spectrometer, in which the laser, the thermostat, and the computer are shared This not only reduces the total cost (at least ten times cheaper than a commercial differential refractometer), but also enables us to measure the specific refractive index increment and the scattered light intensity under the identical experimental conditions, such as wavelength and temperature This novel refractometer has a wide linear detection range (±0035 RI units) with a resolution of 10−6 RI units, which is sufficient for determining the specific refractive index increment of most polymer solutions

Design and implementation of a low temperature near‐field scanning optical microscope

Abstract: The design and implementation of a low temperature (T≥1.5 K), near‐field scanning optical microscope are described herein. This microscope, which is based on the recently developed tapered fiber probe, is optimized for luminescence imaging and spectroscopy of mesoscopic semiconductor systems.

A millimeter/submillimeter spectrometer for !high resolution studies of transient molecules

Abstract: A design is presented for a millimeter/submillimeter direct absorption spectrometer for studies of the pure rotational spectra of metal‐bearing free radicals. The spectrometer operates in the frequency range of 65–550 GHz with an instrumental resolution of 200–1000 kHz and an absorption sensitivity of a few ppm. The instrument utilizes phase‐locked Gunn oscillators as the tunable, coherent source of radiation from 65–140 GHz. Higher source frequencies are obtained with Schottky diode multipliers. The gas cell and optics path are designed utilizing Gaussian beam optics to achieve maximum interaction between molecules and the mm‐wave radiation in the reaction region. Scalar feedhorns and a series of PTFE lenses are used to propagate the source signal. The gas cell is a cylindrical tube 0.5 m in length with a detachable Broida‐type oven. The detector for the spectrometer is a helium‐cooled InSb hot electron bolometer. Phase‐sensitive detection is achieved by FM modulation of the Gunn oscillators and use of a lock‐in amplifier. Spectra are recorded by electrical tuning of the Gunn oscillator, which is done under computer control. The millimeter and sub‐mm rotational spectra of several free radicals have been observed for the first time using this instrument, including CaOH, MgOH, CaH, MgF, and BaOH.

Stabilization and precise calibration of a continuous‐wave difference frequency spectrometer by use of a simple transfer cavity

Abstract: A novel, simple, and inexpensive calibration scheme for a continuous‐wave difference frequency spectrometer is presented, based on the stabilization of an open transfer cavity by locking onto the output of a polarization stabilized HeNe laser High frequency, acoustic fluctuations of the transfer cavity length are compensated with a piezoelectric transducer mounted mirror, while long term drift in cavity length is controlled by thermal feedback A single mode Ar+ laser, used with a single mode ring dye laser in the difference frequency generation of 2–4 μm light, is then locked onto a suitable fringe of this stable cavity, achieving a very small long term drift and furthermore reducing the free running Ar+ linewidth to about 1 MHz The dye laser scan provides tunability in the difference frequency mixing process, and is calibrated by marker fringes with the same stable cavity Due to the absolute stability of the marker cavity, precise frequency determination of near infrared molecular transitions is achi

High-efficiency retarding-potential Mott polarization analyzer

Abstract: A compact retarding‐potential Mott polarimeter is described that employs a thorium target. When operating at an electron accelerating voltage of 25 kV, the instrument provides effective asymmetry (Sherman) functions Seff between −0.15 and −0.25, the exact value being determined by the inelastic energy loss window selected. The corresponding scattering efficiencies I/I0 are in the range ∼6–2×10−3 resulting in optimized efficiencies η (≡S2effI/I0) of ∼1.6×10−4. These efficiencies are much larger than have been obtained previously with similar analyzers. The instrument is simple to construct, is stable in operation, and has a large electron‐optical acceptance, estimated to be ∼104 mm2 sr eV. It is fully UHV compatible and is suitable for application in a wide range of spin‐dependent studies.

Friction effects in the deflection of atomic force microscope cantilevers

Abstract: The conventional deflection‐mode atomic force microscope operates by optically monitoring the slope near the end of a microcantilever in contact with the sample surface. This signal is usually interpreted as a measure of height change. Lateral forces from friction, surface geometry, or inclination of the cantilever to the surface also affect the slope due to cantilever buckling. We calculate the deflection of a hollow triangular model cantilever subject to both lateral and normal forces. The measured response of the servo circuit to an inclined, loaded cantilever is then determined. This shows (1) errors are always present in height measurements of structures on inhomogeneous surfaces; (2) the sensitivity to buckling can be reduced by repositioning the laser; (3) friction measurements can be accurately made by scanning in two directions and applying the proper calibration.

Aircraft-borne, laser-induced fluorescence instrument for the in situ detection of hydroxyl and hydroperoxyl radicals

Abstract: The odd-hydrogen radicals OH and HO2 are central to most of the gas-phase chemical transformations that occur in the atmosphere. Of particular interest is the role that these species play in controlling the concentration of stratospheric ozone. This paper describes an instrument that measures both of these species at volume mixing ratios below one part in 10(exp 14) in the upper troposphere and lower stratosphere. The hydroxyl radical (OH) is measured by laser induced fluorescence at 309 nm. Tunable UV light is used to pump OH to the first electric state near 282 nm. the laser light is produced by a high-repetition rate pulsed dye-laser powered with all solid-state pump lasers. HO2 is measured as OH after gas-phase titration with nitric oxide. Measurements aboard a NASA ER-2 aircraft demonstrate the capability of this instrument to perform reliably with very high signal-to-noise ratios (greater than 30) achieved in short integration times (less than 20 sec).

Piezoresistive cantilevers utilized for scanning tunneling and scanning force microscope in ultrahigh vacuum

Abstract: Piezoresistive cantilevers have been utilized in a novel ultrahigh vacuum scanning probe microscope which allows in situscanning tunneling microscopy(STM), contact atomic force microscopy(AFM), and noncontact atomic force microscopy. The instrument uses interchangeable tungsten tips (for STM imaging) and piezoresistive cantilevers (for AFM or STM imaging) and is capable of atomic resolution in both STM and AFM modes of operation. In situ tip exchange under vacuum conditions is performed quickly and reliably using a high precision rotary/linear feedthrough and a tip/cantilever storage system.Piezoresistive force‐sensing cantilevers provide a new detection scheme for AFM, using an all‐electronic detector that requires no alignment or optical detectionsystem. The microscope features a high‐resolution, dual‐axis, inertial‐drive translation stage with an open access sample mount designed to optimize vibration isolation.

First use of the UV Super‐ACO free‐electron laser: Fluorescence decays and rotational dynamics of the NADH coenzyme

Abstract: Significant improvements in the performances of the Super‐ACO storage ring free‐electron laser (FEL) at 800 MeV have been obtained recently: enhancement of the output power in the ultraviolet, laser duration of 10 h for the same injection of positrons, long‐term stability with a micropulse of 60 ps FWHM. A first series of experiments using this FEL has then been successfully performed. Taking advantage of the time structure, the polarization and the high power of the source at 350 nm, the polarized fluorescence decays of the reduced nicotinamide adenine dinucleotide coenzyme (NADH) were studied in aqueous solution, using the single‐photon counting (SPC) technique. The experimental setup is described, including the Super‐ACO FEL characteristics and diagnostics. The FEL working point has been first optimized by monitoring the SPC apparatus function. A complete fluorescence experiment required about 30 min of data acquisition, during which the best integrated instrumental response had a FWHM of 110 ps. Measu...

Scanning tunneling microscope calibration and reconstruction of real image: Drift and slope elimination

Abstract: Drift, slope of sample, and indeterminate sensitivity of piezoceramics are considered as the origin of the linear scanning tunneling microscope (STM) image distortions. A special algorithm of STM scanning and reconstruction of image of unknown atomic structures is suggested and tested. This algorithm allows us to measure three components of the drift velocity and two angles, characterizing the average slope of scanning surface. On the one hand, using this algorithm, one can perform the STM calibration by a known surface structure (for example, the highly oriented pyrolytic graphite surface) even in the presence of a drift. This enables us to determine all the three piezoceramics constants of STM piezoscanner and the deviation of the real scanner axes X and Y from orthogonality. On the other hand, using such a calibrated STM and the described algorithm, it is possible to obtain the real STM image and make measurements for unknown surfaces with atomic resolution without the distortions mentioned above. As i...

Hyper‐Rayleigh scattering in solution with tunable femtosecond continuous‐wave laser source

Abstract: Hyper‐Rayleigh scattering has been developed as a generally applicable technique for the determination of the first hyperpolarizability β of a wide class of molecules [Clays and Persoons, Rev. Sci. Instrum. 63, 3285 (1992)]. In this article, we show that it is possible to replace the low repetition rate, nanosecond pulse, Q‐switched laser source and fast gated electronics by a self‐mode‐locked, femtosecond pulse, broadly tunable continuous‐wave laser and phase‐sensitive detection. This results in a faster and much simpler measurement procedure. The accuracy of the femtosecond hyper‐Rayleigh scattering set‐up was checked by measuring the first hyperpolarizability β of the octopolar molecule crystal violet in methanol [(450±100)×10−30 esu at 800 nm]. The sensitivity of the instrument allows concentration‐dependence studies, down to the pure solvent.