Volume 65, Issue 3, March 1994
Index of content:
65(1994); http://dx.doi.org/10.1063/1.1145242View Description Hide Description
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.
65(1994); http://dx.doi.org/10.1063/1.1145118View Description Hide Description
A novel discharge circuit that reduces significantly the amount of energy conducted by the switch in a TEA CO2 laser is reported. We demonstrate this circuit on a working multijoule TEA CO2 laser in which the switch is shown to conduct typically only 5% of the total input pulse energy. The laser has worked reliably in gas mixtures that place stringent demands on the discharge and circuit. It has produced an output of 7 J at an efficiency of 9.6% using a CO2:N2:He (1:1:4) gas mixture at atmospheric pressure and further using a helium‐free gas mixture (CO2:N2:H2 at 400 mbar) it has produced a maximum efficiency of 14.6% for an output of 7.8 J. The great simplicity and high efficiency of the new discharge circuit allow it to be incorporated in the existing laser designs with minimal modifications.
65(1994); http://dx.doi.org/10.1063/1.1145119View Description Hide Description
Fabry–Perot interferometers are routinely used in the Tore Supra tokamak in order to measure the time evolution of the electron temperature of the confined plasmas.Calibration of such interferometers requires the detection of very low dc levels (0.1 nV) with signal‐to‐noise ratios less than 10−5, which is generally not compatible with standard detection methods. A new correlation method is proposed to achieve this absolute calibration. It is based on a proper noise autocorrelation technique combined with an optimized filtering involving Fourier analysis. The advantages of the method are detailed and experimentally compared to standard averaging techniques, such as coherent addition and synchronous detection. The method can be used in a more general context every time very small amplitude signals are to be measured.
65(1994); http://dx.doi.org/10.1063/1.1145120View Description Hide Description
A simple step‐by‐step procedure, including several novel techniques discussed in the Appendices, is given for minimizing ac phase noise in typical interferometric systems such as two‐beam interferometers,holographic setups, four‐wave mixers, etc. Special attention is given to index of refraction fluctuations, direct mechanical coupling, and acoustic coupling, whose importance in determining ac phase noise in interferometric systems has not been adequately treated. The minimization procedure must be carried out while continuously monitoring the phase noise which can be done very simply by using a photodiodemeasurement of the interferometer output. Supplementary measurements using a microphone and accelerometer will also be helpful in identifying the sources of phase noise. Emphasis is placed on new techniques or new modifications of older techniques which will not usually be familiar to most workers in optics. Thus, the necessity of eliminating the effects of index of refraction fluctuations which degrade the performance of all interferometers is pointed out as the first priority. A substantial decrease of the effects of all vibrating, rotating, or flowing masses (e.g., cooling lines) in direct contact with the optical table will also have to be carefully carried out regardless of the type of interferometric system employed.
It is recommended that this be followed by a simple, inexpensive change to a novel type of interferometer discussed in Appendix A which is inherently less sensitive to mechanical vibration. Such a change will lead to a reduction of both low‐frequency and high‐frequency ac phase noise by more than an order of magnitude and can be carried out for all interferometers with the exception of multiple pass optical systems and high‐resolution FFT spectrometers. It is pointed out that most homemade air bladder vibration isolators are used incorrectly and do not provide sufficient reduction in the contribution of floor vibrations to phase noise. Several simple trampoline‐type air bladder vibration isolator systems are described which are comparable in performance to commercial systems. With the exception of very nonrigid or undamped optical tables, the dominant source of ac phase noise at this point will usually be due to acoustic coupling to the optical components and mounts themselves. This means not only that the optical components and mounts must be rigid but that the mechanical coupling between the table and the mounts, as well as the coupling between the mounts and components themselves, be as rigid as possible.
An additional damping of optical mounts beyond that generally found in commercial mountings will also have to be carried out to obtain a further reduction of phase noise. A simple damping technique employing an additional mass and an intermediate damping layer is described which will significantly improve the performance of both homemade and commercial optical mounts. Similar damping techniques which are especially suitable for homemade optical tables and breadboards are also considered.
65(1994); http://dx.doi.org/10.1063/1.1145121View Description Hide Description
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 (±0.035 RI units) with a resolution of 10−6 RI units, which is sufficient for determining the specific refractive index increment of most polymer solutions.
Conversion of the Finnigan‐MAT TSQ‐70 thermospray ionization interface to an electrospray ionization interface65(1994); http://dx.doi.org/10.1063/1.1145122View Description Hide Description
The conversion of a thermospray ionization (TSI) feature of a Finnigan‐MAT TSQ‐70 mass spectrometer to electrospray ionization (ESI) and its performance are described. The existing source, pumping capacity, flanges, and temperature controller of the TSQ‐70 TS feature were used with a few modifications. Conversion of the commercial TS option to a simple and economically viable ES option has made the analysis of large biomolecules possible without expensive upgrades. To preserve the simplicity of the conversion, desolvation is effected by a heated‐capillary tube (HCT). The HCT and its housing are inserted inside the TSQ‐70 TS flange like a solid probe. Mass spectrometric results of low and high molecular weight biomolecules, the mass accuracy, sensitivity, and charge states of the observed ions are comparable to published results by other laboratories. Adequate spectral quality was obtained at short scan times, a required characteristic for interfacing ESI with separation methods such as capillary zone electrophoresis.
65(1994); http://dx.doi.org/10.1063/1.1145123View Description Hide Description
A pinhole small‐angle x‐ray scattering (SAXS) instrument was constructed at the SUNY X3A2 beamline, National Synchrotron Light Source, Brookhaven National Laboratory. The three pinholes were mounted in a thick‐walled stainless steel pipe and prealigned by using a portable laser source and a charge‐coupled device (CCD) area detector. After the prealignment, incorporation of the collimator to the synchrotron x‐ray source required only maximization of the incident x‐ray intensity passing through the pinholes, which could be done easily by using a scintillation counter after proper attenuation. The entire synchrotron SAXS instrument setup took only a few hours even without stepping motor control for the pinhole collimator unit. By combining this collimator with a CCD‐based x‐ray area detector which could be assembled by using commercially available components, the SAXS instrument showed good performance for structural scales up to an order of 100 nm. The CCD‐based x‐ray area detector used a computer‐ (or manually) controlled intensified unit with a variable gain setting in order to accommodate the changing x‐ray flux and to protect the detector from over exposure, a necessary feature for operation of an area detector at a synchrotron light source.
65(1994); http://dx.doi.org/10.1063/1.1145124View Description Hide Description
A complete micropositioning unit based on high precision, manually controlled X‐Y‐Z translators, related metrology system, and sighting microscope is described. It has been specifically developed for the alignment of collimating pinholes (5–10 μm diam) on cryogenic x‐ray detectors, 10–50 μm in size, deposited both on transparent and opaque substrates. The main characteristics of this flexible and convenient system are the capability to handle a complete test fixture ready for further measurements at cryogenictemperature, coupled with the possibility to verify the precision attained. Such microalignment equipment will find application in optical/UV/x‐ray photon counting experiments, whenever a highly collimated illumination is required or in any test involving precision positioning of small experimental units onto microdevices or detectors.
65(1994); http://dx.doi.org/10.1063/1.1145125View Description Hide Description
New cassettes for 201 mm×252 mm (8‘×10‘) and 201 mm×400 mm (8‘×15.75‘) storage phosphor plates have been developed at the Synchrotron Resource of the Howard Hughes Medical Institute. The purpose for this work was mainly twofold. Firstly, to diminish the number of manual operations when putting the storage phosphor plate into the cassette or when extracting it from the cassette. Secondly, to render such a cassette much lighter than the former metal cassette previously in use. These two goals were achieved by making new cassettes that are operated as one piece instead of two or three independent parts as with the former systems. The cassettes have been extensively tested and found to be very useful.
Inert gas purgebox for Fourier transform ion cyclotron resonance mass spectrometry of air‐sensitive solids65(1994); http://dx.doi.org/10.1063/1.1145126View Description Hide Description
A sealed rigid ‘‘purgebox’’ makes it possible to load air‐ and/or moisture‐sensitive solids into the solids probe inlet of a Fourier transform ion cyclotron resonance (FT/ICR) mass spectrometer. A pelletized sample is transferred (in a sealed canister) from a commercial drybox to a Lucite(R) purgebox. After the box is purged with inert gas, an attached glove manipulator is used to transfer the sample from the canister to the solids probe of the mass spectrometer. Once sealed inside the inlet, the sample is pre‐evacuated and then passed into the high vacuum region of the instrument at ∼10−7 Torr. The purgebox is transparent, portable, and readily assembled/disassembled. Laser desorption FT/ICR mass spectra of the air‐ and moisture‐sensitive solids, NbCl5. NbCl2(C5H5)2, and Zr(CH3)2(C5H5)2 are obtained without significant oxidation. The residual water vapor concentration inside the purgebox was measured as 100±20 ppm after a 90‐min purge with dry nitrogen gas. High‐resolution laser desorption/ionization mass spectrometry of air‐sensitive solids becomes feasible with the present purgebox interface. With minor modification of the purgebox geometry, the present method could be adapted to any mass spectrometer equipped with a solid sample inlet.
65(1994); http://dx.doi.org/10.1063/1.1145127View Description Hide Description
RELAX is a resonance ionization, time‐of‐flight mass spectrometer to which a cryogenic sample concentrator has been added. This has resulted in an increase in sensitivity by a factor greater than 100. The sample concentrator consists of a localized cold spot in the ion source, onto which the sample condenses, and a heating laser to release the condensed sample into the ionization region. The lifetime against detection of a sample atom is close to 20 min, which corresponds to a count rate of 1 cps from a sample of 1000 atoms, while the mass resolution is 300 (10% peak height). Sensitivity depends on the return time of sample atoms to the cold spot (10 s) and the fraction of these atoms subsequently ionized (∼1%). The minimum sample size which can be measured is limited only by blank, which is currently 2×10−15 cc STP total xenon and isotopically atmospheric (this can be attributed to the large aliquots of xenon admitted to the instrument during development, and so may be expected to decrease with time). The precision of abundancemeasurements has been improved by the incorporation of pulse height discrimination and pulse counting detection for the less abundant isotopes. The design, construction, and operation of the spectrometer in its new configuration are described with particular attention to abundance extraction. The effects of the sample concentrator on ionization efficiency and discrimination are discussed in detail, as are interferences from nonresonantly ionized hydrocarbons and the means of accounting for them.
65(1994); http://dx.doi.org/10.1063/1.1145128View Description Hide Description
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 semiconductorsystems.
65(1994); http://dx.doi.org/10.1063/1.1145129View Description Hide Description
The design and characterization of a scanning electron microscope based microindentor is presented. Dynamic, high magnification imaging of the indentor–specimen contact zone is possible, permitting observation of indent events. Applied load as a function of indentor tip displacement is continuously monitored during indentation. The maximum applied load capability of 20 N is measured to a resolution of 1 mN with a piezoelectric transducer mounted on the indentor shaft. Displacement is measured with a specially developed capacitance gauge that is again mounted on the indentor shaft near the indentor tip and records tip displacement with respect to the specimen surface to a resolution of 10 nm over a 100 μm range. The instrument is vacuum compatible, capable of remote operation, has a short measurement loop, and a potentially high bandwidth response. Results from a fiber push‐down test on a SiC fiber reinforced glass ceramic are reported to illustrate the capability of the instrument in performing measurements across the nanoindentation and microindentation ranges.
65(1994); http://dx.doi.org/10.1063/1.1145130View Description Hide Description
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.
Difference between the forces measured by an optical lever deflection and by an optical interferometer in an atomic force microscope65(1994); http://dx.doi.org/10.1063/1.1145131View Description Hide Description
Using a simple model, we investigated the difference between the forces measured by an atomic force microscope(AFM) with an optical lever deflection method and that with an optical interferometer method. Then, using a mica with an atomically flat surface as a test sample, we confirmed experimentally the above difference, which says that the optical lever deflection method detects not only the surface corrugation but also the frictional force, while the optical interferometer method detects only the surface corrugation. Based on the above results, we proposed a method to measure the three‐dimensional force vector by using both the optical lever AFM/LFM and the optical interferometerAFM simultaneously. Furthermore, we mention that the measurement of three‐dimensional spatial distribution of the force vector will implement the computed tomography technique into AFM measurements, which yields a three‐dimensional spatial distribution of the force origin.
65(1994); http://dx.doi.org/10.1063/1.1145132View Description Hide Description
Charge exchangeproperties of H atoms passing through thin carbon foils at incident energies from 0.5 to 120 keV/u are discussed in the context of charge transfer models. A model is presented in which the charge state equilibrium in the solid is explained by the overlap of the atomic and the solid‐state electron wave functions in k space. Outside the solid, near the surface,charge exchange occurs by tunneling of electrons between the carbonsurface and the exiting projectile.
65(1994); http://dx.doi.org/10.1063/1.1145133View Description Hide Description
A mechanical beam chopper with small dimensions is presented. The shape of the generated neutral beam pulses is calculated and experimental results are discussed. The microchopper is based on the operation principle of neutron choppers. It is operated in vacuum and generates molecular beam pulses at a repetition rate of 10 kHz with pulse lengths easily adjustable between 5 and <1 μs. The chopper can be used for time‐of‐flight applications and inherently acts as a high velocity pass filter.
65(1994); http://dx.doi.org/10.1063/1.1145134View Description Hide Description
Removal of the plasma contained in a gadolinium atomic beam produced by electron beam heating was investigated. A positive or negative electric potential was applied to the plasma removal electrodes which were a pair of parallel electrodes put along the atomic beam. When a positive potential was applied to the plasma removal electrodes, the plasma could not be removed at high evaporation rates. On the other hand, the plasma could be removed by applying a high negative potential to both removal electrodes, even at high evaporation rates. The potentials applied to the electrodes required to remove the plasma were estimated using the model that a plasma at ground potential flows with the atomic beam; ions are extracted from the plasma by negatively biased removal electrodes. The estimated potentials required to remove the plasma agreed well with experimental values.
65(1994); http://dx.doi.org/10.1063/1.1145135View Description Hide Description
In laboratory experiments related to space plasma physics it is often desirable to produce plasmas with characteristics as close as possible to various naturally occurring plasma regimes. In the near‐earth region space plasma densities typically vary from 103–107 cm−3 and temperatures range from a few tenths of an eV to the order of 1 eV. The plasma parameters of electron density, electron temperature, and ion species are primary variables which are often not easy to reproduce in a chamber environment which is dependent upon conventional gas discharge or arc sources for plasma production. A simple microwave discharge device was developed which is easily tunable and capable of producing the moderate range of electron densities without an external magnetic field. The Asmussen‐type microwaveplasma source described here covers and exceeds the parameter ranges required, is relatively easy to construct, and is inexpensive. The device makes use of an air dielectric coaxial coupler to couple magnetron output to a resonantcavity. Estimates of effective electric fields and source densities and temperatures suggest that similar devices can easily be constructed and fashioned to produce these parameters, depending upon requirements, over a wide range of values. The use of widely available commercial magnetrons manufactured for microwave ovens allows a certain ease in the construction of these devices in that available cavityQ’s can range to lower levels and therefore resonant lengths can be adjusted more easily. The design is discussed relative to desired experimental parameter ranges and some discussion is given of expected source current densities, electric fields, and temperature ranges.
65(1994); http://dx.doi.org/10.1063/1.1145136View Description Hide Description
30‐cm‐class long plasmas were generated using 2.45‐GHz microwave slot antennas on a rectangular waveguide with a T‐shaped ridge for the development of high performance ion sources or for plasma reactors for large‐area material processing. The microwave coupling efficiencies above 85% were achieved for Ar, N2, and O2 over large flow rate ranges. From the electric fields on the inner wall E side of the waveguide, standing waves with the maximum electric field strength of about 30 kV/m were expected to be excited in the waveguide, depending on the location of the T‐shaped ridge. The plasma density for Ar was in the order of 1017 m−3 and for N2 and O2 1016 m−3 in a discharge chamber in front of the slot antennas. The electron temperature for Ar ranged from 3 to 4 eV and for N2 and O2 from 3 to 8 eV. The spatial profiles of the ion saturation current for Ar were almost flat in the discharge chamber although the profiles for N2 and O2, with large flow rates or near the antennas, were slightly rough.