摘要:

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.

ISSN:0034-6748    

DOI:10.1063/1.1144830

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A 492 GHz quasioptical superconductor‐insulator‐superconductor (SIS) heterodyne receiver for submillimeter astronomy has been constructed with a double‐sideband noise temperature of 170±25 K, where the system response is an average over a 600 MHz i.f. bandwidth centered at 1.5 GHz. The excellent sensitivity is achieved through use of a planar twin‐slot antenna feeding a pair of Nb–Al2O3–Nb junctions linked by an inductance. A main beam off/15 has been measured. The performance of this receiver is comparable to that of the best 492 GHz waveguide receivers currently in operation.

ISSN:0034-6748    

DOI:10.1063/1.1144831

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

An inward precipitator collects particles initially dispersed in a gas throughout either a cylindrical or spherical chamber onto a small central planchet. The instrument is effective for particle diameters greater than about 1 &mgr;m. One use is the collection of interplanetary dust particles which are stopped in a noble gas (xenon) by drag and ablation after perforating the wall of a thin‐walled spacecraft‐mounted chamber. First, the particles are positively charged for several seconds by the corona production of positive xenon ions from inward facing needles placed on the chamber wall. Then an electric field causes the particles to migrate toward the center of the instrument and onto the planchet. The collection time (of the order of hours for a 1 m radius spherical chamber) is greatly reduced by the use of optimally located screens which reapportion the electric field. Some of the electric field lines terminate on the wires of the screens so a fraction of the total number of particles in the chamber is lost. The operation of the instrument is demonstrated by experiments which show the migration of carbon soot particles with radius of approximately 1 &mgr;m in a 5‐cm‐diam cylindrical chamber with a single field enhancing screen toward a 3.2 mm central collection rod.

ISSN:0034-6748    

DOI:10.1063/1.1144832

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The design of a novel electrode structure for electric‐field ionization of Rydberg atoms in a fast atomic beam is described. It allows the resolution of the ion signal from Rydberg atoms with adjacent effective principal quantum numbersn* in the range of 14≤n*≤40 and minimizes the volume where field ionization takes place. These features drastically reduce the number of background events arising from collisions of fast atoms with residual gas molecules and thus are essential for trace analysis applications of the technique of resonance ionization spectroscopy in collinear geometry.

ISSN:0034-6748    

DOI:10.1063/1.1144833

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Soft x‐ray photons, generated by the interaction of intense 610 nm subpicosecond laser pulses with rare gases, are used to carry out tunable angle‐resolved photoemission on a number of systems. When the intensity at the focus of the input radiation exceeds ∼1014W/cm2significant quantities of harmonic photons at odd multiples of the input 610 nm (2.04 eV) light are produced. To date, harmonics from the 5th (10.2 eV) to the 25th (51 eV) have been generated with fluxes sufficient for generating photoemission spectra. The experimental apparatus for harmonic generation and several experiments demonstrating the applications of this light are described.

ISSN:0034-6748    

DOI:10.1063/1.1144834

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The odd‐hydrogen radicals OH and HO2are 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 1014in 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 electronic state (A˜ 2&Sgr;+(v’=1) ←X˜2&Pgr;3/2(v‘=0)) near 282 nm. The laser light is produced by a high‐repetition rate pulsed dye‐laser powered with all solid‐state pump lasers. HO2is 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 (≳30) achieved in short integration times (< 20 sec).

ISSN:0034-6748    

DOI:10.1063/1.1144835

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The instrumentation, the luminescence microprobe, and synchronously scanned luminescence spectroscopy technique described here can be used to classify microliter quantities of oil such as those in fluid inclusions in cements from petroleum reservoirs. It is primarily constructed to obtain synchronously scanned luminescence spectra from microscopic sized samples to characterize the organic classes of compounds that predominate. At present no other technique can so readily analyze a single oil‐bearing fluid inclusion. The data collected from the technique are pertinent to evaluating systems and providing quantitative data for solving problems in oil migration and maturation determinations, oil‐to‐oil and oil‐to‐source correlations, oil degradation, and episodes and chemistry of cementation.

ISSN:0034-6748    

DOI:10.1063/1.1144836

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A phase measurement distance meter has been devised using a frequency‐stabilized intermode optical beat generated from a two‐mode He‐Ne laser (14‐cm‐long cavity) and a conventional phase meter. The beat frequency (1096 MHz) is stabilized to within 1 kHz fluctuation by a feedback control system. This frequency was converted to an electrical signal and reduced to 6 MHz by a rf‐heterodyne technique in order to match the best frequency response range of the phase meter. Experimental results show that the total distance error of the optical distance meter due to the frequency fluctuation of the laser and the performance of the phase meter is 1 and 19 &mgr;m, respectively, for a mirror target located 1 m away from the laser source. Further improvement in the resolution is possible if a better phase meter is used.

ISSN:0034-6748    

DOI:10.1063/1.1144837

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A displacement sensor utilizing a light mode attenuation principle in a hollow metal waveguide is investigated. This sensor can be used to control and measure displacements up to 20 cm. The sensor characteristics are studied both experimentally and theoretically. The sensor design allows a variation of its specified characteristics in a broad range making it suitable for every practical application. A very simple and cheap design has been obtained.

ISSN:0034-6748    

DOI:10.1063/1.1144838

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

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) functionsSeffbetween −0.15 and −0.25, the exact value being determined by the inelastic energy loss window selected. The corresponding scattering efficienciesI/I0are in the range ∼6–2×10−3resulting in optimized efficiencies &eegr; (≡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 ∼104mm2sr eV. It is fully UHV compatible and is suitable for application in a wide range of spin‐dependent studies.

ISSN:0034-6748    

DOI:10.1063/1.1144839

出版商:AIP    

年代:1994

数据来源: AIP