摘要:

Current sources in the human body can be localized by measuring the biomagnetic fields with multichannel SQUID systems. Important system aspects are the noise level, the ambient field suppression, the dynamic range, the reliability, the number of channels, and the arrangement of gradiometers. From the user’s point of view the most important quality factor is the accuracy with which a current dipole can be localized. A test procedure is proposed to determine the localization power of the system. A 31‐channel‐SQUID system is presented together with the results of the test. The crucial parts of the system determining the accuracy are pointed out.    

ISSN:0034-6748    

DOI:10.1063/1.1144359

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

An average laser output power as high as 5 kW has been produced from a transversely excited atmospheric CO2laser pumped with a new solid state exciter. The exciter providing a 100 J transfer energy consisted of a high voltage pulse generator and a two stage magnetic pulse compressor. A stack of new high power semiconductor switches, called insulated gate bipolar transistors, was employed for the main switch of the pulse generator. A sophisticated discharge head with the corona preionization behind a perforated anode was designed to realize fast and uniform gas flow in cooperation with acoustic dampers. The maximum average laser power of 4.6 kW was attained at a repetition rate of 550 pps with an overall efficiency as high as 10.5%. The maximum repetition rate achieved was 1.1 kpps, at which the resulting average laser power was 3.4 kW.

ISSN:0034-6748    

DOI:10.1063/1.1144360

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A novel gas target was developed for use in laser‐atom harmonic‐generation experiments. It operates on the principles of free molecular flow rather than those of fluid flow as does a gas jet. This allows much lower gas pressures to be achieved. The device is a small, cylindrical, double‐ended hole through which the focused laser beam passes. Gas enters from the middle of the cylindrical wall and quickly disperses as it emerges out the hole openings. As with the gas jet, the purpose of the gas target is to provide a very narrow and localized distribution of gas for laser interaction. Some of the advantages of the target relative to the jet are that it is easier to systematically align to the laser beam and it can operate at much lower pressures. Results of a Monte Carlo simulation of the gas density and flow rate within the target and experimental measurements of the density profile made by imaging the recombination emission are presented herein. It is found that the gas density decreases rapidly once outside the target hole openings along the symmetry axis. The calculations are in good agreement with experimental measurements. The gas target continues to function without serious deviation at pressures up to ten times above the molecular flow range.

ISSN:0034-6748    

DOI:10.1063/1.1144361

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A degenerate‐four‐wave‐mixing apparatus is described, with novel data acquisition, and sample handling techniques. Particular attention is paid to methods of noise reduction and elimination. Four‐wave mixing is performed with Nd:YAG laser light, at both 532 and 1064 nm, and with a pulse length of 20 ns.

ISSN:0034-6748    

DOI:10.1063/1.1144476

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The position and angle measurements discussed in the paper are carried out by application of a displacement‐angle interferometer which employs a single laser and is capable of resolutions better than 1 pm and 1 nrad. Angle and displacement values are simultaneously obtained by measuring the phase shifts between four points of the interference pattern with the use of a position‐sensitive detector. Though the instrument resembles an optical lever, it is largely insensitive to beam movements, since angle values are obtained by phase rather than position measurements. The interferometer design and performance are discussed, emphasis being given to applications in x‐ray/optical interferometry.  

ISSN:0034-6748    

DOI:10.1063/1.1144362

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

An optical interferometric approach is described for the measurement of axial eye length. The technique uses the short coherence length characteristics of the light source to locate, interferometrically, the position of the front surface of the eye and the retina. Results are presented from studies on an idealized model eye and the precision of the measurement is discussed. The advantages of the technique and a comparison with previously used short coherence measurements are also reviewed.

ISSN:0034-6748    

DOI:10.1063/1.1144363

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The construction of a practical UV micro‐Raman spectrograph capable of selective excitation of bacterial cells and other microscopic samples has been described. A reflective objective is used to focus cw laser light on a sample and at the same time collect the scattered light at 180°. With the aid of a quartz lens the image produced is focused on the slits of a spectrograph equipped with a single 2400 grooves/mm grating optimized for 250 nm. Spectra were detected by means of a blue‐intensified diode array detector. Resonance Raman spectra ofBacillussubtilisandFlavobacteriumcapsulatumexcited by the 257.2 nm output of a cw laser were recorded in the 900–1800 cm−1region. Bacterial cells were immobilized on a quartz plate by means of polylysine and were counted visually. Cooling was required to retard sample degradation. Sample sizes ranged from 1 to 50 cells with excitation times varying from 15 to 180 s. Excellent spectra have been obtained from 20 cells in 15 s using a spectrograph having only 3% throughput.

ISSN:0034-6748    

DOI:10.1063/1.1144313

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

We describe a design for a paraboloidal electrostatic reflector (PER) adapted for molecular‐beam time‐of‐flight (TOF) photoelectron spectrometers. The PER offers a nearly two orders of magnitude improvement in detection efficiency over standard line‐of‐sight TOF detection. The energy resolution &Dgr;E/Eis nominally about 0.02, but can be improved to about 0.005 (to a current limit of &Dgr;E=10 meV) at some expense in sensitivity. The PER makes possible sensitive measurements for inherently weak ionization experiments. We have used our spectrometer in applications involving low‐power picosecond pulses, low‐density molecular clusters, and ionization through dissociative states.

ISSN:0034-6748    

DOI:10.1063/1.1144314

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A detection scheme for zero‐kinetic‐energy photoelectron spectroscopy (ZEKE spectroscopy) adapted to the time structure of multibunch vacuum ultraviolet synchrotron radiation from BESSY is described. A resolution of 50 cm−1(6 meV) is achieved at high stored ring currents providing high photon flux and resolution for one‐photon ZEKE spectroscopy and electron‐ion coincidence measurements.

ISSN:0034-6748    

DOI:10.1063/1.1144315

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A cryogenic photoacoustic cell has been developed for intracavity use in a tunable ring laser. Its design ensures uniform cooling of the gas sample and minimum loss to the laser cavity. Also implemented is a flexible microphone mounting system that reduces thermal stress during temperature changes.

ISSN:0034-6748    

DOI:10.1063/1.1144474

出版商:AIP    

年代:1993

数据来源: AIP