摘要:

Scientific advancement is often spurred by the development of new instruments for investigation. Over the last several decades, many new instruments have been produced to further our understanding of the physiology of the human brain. We present a partial overview of some of these instruments, paying particular attention to those which record the electrical activity of the human brain. We preface the review with a brief primer on neuroanatomy and physiology, followed by a discussion of the latest types of apparatus used to investigate various properties of the central nervous system. A special focus is on microelectrode investigations that employ both intracellular and extracellular methods of recording the electrical activity of single neurons; another is on the modern electroencephalographic, electrocorticographic, and magnetoencephalographic methods used to study the spontaneous and evoked field potentials of the brain. Some examples of clinical applications are included, where appropriate. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149245

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Frequency stabilization of a rf excitedCO2laser on the peak of the Doppler broadened gain curve using the optogalvanic effect generated from the laser itself is achieved. The optogalvanic signal is directly coupled from a rf discharge chamber via a capacitor into a detector and a lock-in stabilizer. The frequency stability is estimated to be better then3×10−8.©1998 American Institute of Physics. 

ISSN:0034-6748    

DOI:10.1063/1.1149246

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We present the design of a highly efficient, pulsed ammonia laser which is optically pumped by a commercial TEACO2laser and is line tunable in the spectral range of 780–930 cm−1. The power, spectral, and temporal characteristics of the laser have been investigated for different configurations of the optical cavity. Using a stable resonator, the laser operates with an efficiency of 24&percent; (940 mJ) in a multiline configuration and 16.5&percent; on a single line. Changing the resonator to an unstable configuration produces a diffraction limited output on a single line with an efficiency of 13&percent;. These output characteristics make this laser suitable for experiments in the field of infrared multiphoton dissociation. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149247

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Recently there has been considerable interest in the Thomson scattering of intense laser from relativistic electron beams, and a new kind of x-ray source named laser synchrotron source (LSS) has been discussed based on this mechanism which is expected to have extremely good performances. In order to verify this theory, an experiment is going to be carried out by our group using a frequency-doubled yttrium–aluminum–garnet laser and a 25 MeV electron beam generated by the accelerator in the Applied Department of the Institute of High-Energy Physics. In this article some theoretical results have been calculated for this experiment, including the scattered x-ray’s wavelength and intensity distribution. The results at 180° and 90° have been given and the 180° result should be very close to the experimental geometry of 178.5°. In Sec. III, figures of the spatial distribution of x-ray are drawn. The future of LSS is prospected finally in Sec. IV. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149248

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Ultrashort-pulse, laser-produced plasmas have become very interesting laboratory sources to study spectroscopically due to their very high densities and temperatures, and the high laser-induced electromagnetic fields present. Typically, these plasmas are of very small volume and very low emissivity. Thus, studying these near point source plasmas requires advanced experimental techniques. We present a new spectrometer design called the focusing spectrometer with spatial resolution (FSSR-2D) based on a spherically bent crystal which provides simultaneous high spectral(&lgr;/&Dgr;&lgr;≈104)and spatial resolution (≈10 &mgr;m) as well as high luminosity (high collection efficiency). We described in detail the FSSR-2D case in which a small, near point source plasma is investigated. An estimate for the spectral and spatial resolution for the spectrometer is outlined based on geometric considerations. Using the FSSR-2D instrument, experimental data measured from both a 100 fs and a nanosecond pulse laser-produced plasma are presented. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149249

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A compact charge coupled device (CCD) camera system has been designed and characterized for use in the six inch manipulator (SIM) at the Nova laser facility. The camera system has been designed to directly replace the 35 mm film packages currently used in SIM-based x-ray imaging diagnostics. The unit’s electronic package has been constructed for small size and high thermal conductivity which reduces the overall camera size and improves its performance when operated within the vacuum environment of the Nova target chamber. Measurements of the x-ray imager’s contrast transfer function (CTF) were made under a variety of operating conditions on a static x-ray Manson source using both the CCD and Kodak T-Max 3200 film as recording media. The CTF data were converted to an equivalent modulation transfer function (MTF). The MTF plots show that the microchannel plate has a uniform response within our measurement accuracy along its strips. In a direction normal to the strip, however, the MTF is reduced due to the slant angle of the pores in the MCP. The measurements show that the CCD camera has a lower MTF response than T-MAX film for all spatial frequencies and configurations measured. However, data obtained from the film exhibited reciprocity failure and border effects that are not observed in the CCD data. Measurements indicate that the signal-to-noise ratio for the CCD data is four to six times larger than that obtained with film and higher photon flux levels were recorded. The CCD-based diagnostic offers immediate access to the data, improved dynamic range, and reduced turnaround time, while eliminating the need for film development, digitization, equipment, and personnel. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149250

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A double-resonance spectrometer, suitable for the investigation of low lying intermolecular vibrational modes and of tunneling transitions of van der Waals complexes, is described. The instrument utilizes a pulsed molecular beam Fourier transform microwave spectrometer (frequency range 4–26 GHz) for signal detection, and a high frequency, phase stabilized, backward wave oscillator (258–375 GHz), as well as a millimeter wave synthesizer (78–118 GHz), as pump radiation sources. A brief description of the Fourier transform microwave spectrometer and of the phase stabilization system of the backward wave oscillator is given. The experimental arrangement is such that the molecular ensemble travels through the submillimeter radiation field prior to the pulsed excitation microwave experiment, in which the resulting population changes are detected. The sensitivity and the resolution capabilities of the new technique are illustrated with double resonance spectra of the van der Waals complexes Ar–CO andCO–N2.©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149251

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A normally “on” linear-focused 14-stage end-window photomultiplier tube (PMT) (Electron Tubes Limited 9893Q/100B), designed for fast photon counting, has been gated through control of the voltage applied to the first dynode. The gating circuit reduces the gain of the PMT during a laser pulse, in order to discriminate against the detection of scattered light, and then increases the gain promptly to observe extremely low levels of laser-induced fluorescence (LIF). An extinction factor for the laser scattered photons of>105was observed, and has enabled count rates for photons due to LIF as low as 1 Hz to be measured for a laser pulse-repetition frequency of 7 kHz. The rise of the PMT gain is monitored directly by observation of the fluorescence using time-resolved photon counting, and the PMT turn-on time is 30 ns. No significant distortion of the temporal profile of the fluorescence was observed during PMT turn-on. The system, which can also be operated in ungated mode, is rugged and reliable, and has been employed outdoors during atmospheric field measurements of the hydroxyl radical using LIF. The gating circuit is suitable for the rejection of laser scattered light in applications with extremely low levels of fluorescence on the submicrosecond timescale. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149252

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

When laser Doppler anemometry (LDA) is used to measure sound fields in water it is important to take into account the refractive index variations in the water due to the sound wave. These have the effect of creating a phase difference between the two laser beams in the LDA setup so that when they intersect they create moving fringes. In some situations this acousto-optic effect can dominate over the movement of the particles due to the sound wave, thereby influencing the Doppler signal. This article determines in which situations the acousto-optic effect can be ignored and in which situations it has a dominant effect. Theoretical expressions are derived for the magnitude of the acousto-optic effect on the LDA signal in terms of the distance of the laser beam propagation and acoustic wave number. The results show that varying the value of the wave number, the distance the laser beams have traveled in the direction perpendicular to the optical axis, or the angle of the sound wave, has an effect on the amplitude,yamp,of the fringe movement. For low wave number values, an angle of 0, ±&pgr; from the optic axis (ydirection) corresponds to the situation in which the laser beams have been affected to the same degree by refractive index changes in the water and thus the path difference is zero and the value ofyampis a minimum. A maximum value ofyampis obtained for an angle of ±&pgr;/2 from theydirection for low wave number values. It is shown that the ratio of the acousto-optic effect to the amplitude of the particle movement due to the sound wave varies as the square of both the acoustic wave number and the distance of the laser beam propagation for low wave number values. An equation for the apparent motion of the particles in the fringes is determined and is used to show that the acousto-optic effect dominates as the value of the wave number and the distance of propagation of the beams increases. Also, for very low wave numbers and short distances, the acousto-optic effect is negligible. This is because at low wave numbers, corresponding to low sound frequencies, the refractive index gradients, and hence the phase changes along the lengths of the laser beams, are small. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149253

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

This article deals with the most recent performance of the superconducting electron cyclotron resonance ion source (SERSE) working at 14 GHz with high magnetic fields after the required conditioning and optimization of several operating parameters. SERSE has now achieved an outstanding level of performance in delivering highly charged ion beams in argon and oxygen gases: the results obtained while operating in a stainless steel chamber and with an aluminum liner are shown and discussed. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149254

出版商:AIP    

年代:1998

数据来源: AIP