摘要:

This review concerns the interpretation of the signal provided by SQUID magnetometers designed for measurements of magnetization and magnetic susceptibility. ‘‘Image effect’’ refers to the influence of superconducting elements in the vicinity of the sample and detection coil. Several exact and approximate methods are presented for calculation of how the device sensitivityG(r) varies with positionrof a unit dipole sample in the presence of a superconducting shielding tube. In particular, the relative decrease ofG(r) is approximately given by the third power of the ratio of the detection coil and shield diameters, and the effect of the shield’s finite length is found to be negligible in practical situations. A survey of complex detection coils (Helmholtz, saddle‐type, gradiometer, and second derivative coil) includes calculation of the optimum spacing of the Helmholtz pair in the presence of a superconducting cylinder. The image effects due to the properties of the SQUID circuit and the field‐dependent effects in high‐field SQUID magnetometers are also discussed. Change in the instrument calibration due to sample size, shape, and location is considered for arbitrary samples as well as for specific cases of small, medium, and very long samples. A spherical harmonic expansion ofG(r) makes it possible to derive simple formulas describing sample geometry effects for medium‐sized samples with regular geometries (thin rod, cylinder, and rectangular parallelepiped). The results are compared to published experimental data.

ISSN:0034-6748    

DOI:10.1063/1.1144306

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A one‐turn coil of 20 cm diameter and 30 cm length produces a field up to 1.7 kG with a rise time of 30 ns. The rate of rise of field, nearly 6 T/&mgr;s, is faster than for any other coil of this size. Powering the coil is a transformer‐charged pulse‐forming line machine operating at up to 28 kA, 280 kV without deleterious arcing. The field is uniform to 5% over the coil length, focusing an electron beam passing along its axis.

ISSN:0034-6748    

DOI:10.1063/1.1144307

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Feedback between optical elements can be a major source of noise when trying to attain high sensitivity in infrared absorption experiments. We find that a conventional White‐cell optical arrangement introduces e´taloning fringes that modulate the peak‐to‐peak amplitude of our signals by 1 part in 16 666, a fractional change of 6×10−5. Although relatively small, this ‘‘noise’’ is systematic and adds coherently with averaging, obscuring interesting absorption features. An easily constructed multipass optical system suited for performing high‐resolution infrared spectroscopy in molecular beams is described. The design is based on a variation of the White cell and has been optimized for use with lead salt diode lasers. One of the key components in the improved design is the addition of an oscillating mirror for spoiling optical feedback generated by laser scatter and/or poor mode coupling of the laser to the multipass optics.

ISSN:0034-6748    

DOI:10.1063/1.1144308

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Impulsive stimulated Brillouin scattering is applied to a new high‐resolution phonon spectroscopy, where phonons created by two crossing pump pulses are observed using diffraction of a cw probe light in both time and frequency domains. In time domain, real‐time behavior can be detected sensitively using a digital oscilloscope, while in frequency domain, measurements with high resolution are attainable by means of a spherical Fabry–Perot interferometer. Several examples of the measurements on liquid samples are demonstrated for clarifying various aspects of the phonon generation by the present method: the mechanism of the generation under pulsed light irradiation, the propagation of the generated phonon, the resonance excitation, and the interference with the other nonlinear optical processes. The relation with the ordinary light scattering is also discussed.

ISSN:0034-6748    

DOI:10.1063/1.1144309

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

An apparatus and data analysis methodology is described which allows determination of response time to oxygen concentration changes of luminescent oxygen sensor coatings. Utilizing a solenoid valve, a sample chamber creates a pressure jump from 0.1 to 700 Torr in 600 &mgr;s that is followed by 15 ms of ringing. An optical detection system measures the response of porphyrin‐based luminescent oxygen sensors to the pressure jump. The pressure in the chamber is measured simultaneously and independently with a piezoresistive pressure transducer. Data analysis techniques using nonlinear least‐squares and numerical convolution of the luminescent response to the pressure rise allow determination of response times of the oxygen sensor. The response to pressure jumps of several luminescent oxygen sensitive coatings designed for video luminescent barometry are measured with this computer‐controlled instrument. Several coatings were studied with response times of ∼2.5 s, ∼400 ms, 11 ms, 1.5 ms, and <25 &mgr;s. Studies of the system suggest that we can determine response times down to about 10 &mgr;s.

ISSN:0034-6748    

DOI:10.1063/1.1144310

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

An analytical solution of the recursion formula for convolution of a sum‐of‐exponentials fluorescence impulse (&dgr;) response with an apparatus response (‘‘lamp’’) function approximated locally by a general polynomial of thenth degree is derived. The five lowest approximations are tested by comparison with properly simulated fluorescence evolution‐and‐decay data. These were obtained byanalyticalconvolution of selected &dgr; responses with ananalyticallydefinedapparatus function chosen closely to simulate typically observable flash‐lamp apparatus functions, taking account of the integration within channels inherent in the experimental collection of both ‘‘lamp’’ and fluorescence response curves. A precise comparison of the recovery of various test theoretical mono‐ and sum‐of‐exponentials impulse‐response parameters for the different approximations was attained by performing all calculations for both simulation and nonlinear least‐squares optimization analysis in double precision. The results highlight the advantages of using local higher‐order polynomial approximations under various circumstances. The improved time resolution in particular suggests more expedient regimes of data collection than heretofore possible.

ISSN:0034-6748    

DOI:10.1063/1.1144311

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A novel method for reconstructing the apparatus response function required for accurate detailed analysis of nanosecond time‐resolved fluorescence decay data is proposed and tested with properly constructed simulated data sets for a variety of pertinent cases, using the high‐accuracy convolution algorithm previously developed [J. Vecˇerˇ, A. A. Kowalczyk, and R. E. Dale, Rev. Sci. Instrum.64, xxx (1993)]. The veracity of recovery of test mono‐ and multiexponential decay responses by this method, involving the use of two appropriately chosen monoexponential reference decay responses, is shown to be essentially identical to that attainable with the ‘‘true’’ apparatus response function. The method is also demonstrated to exhibit significant advantages in a variety of situations over currently the most widely employed method of overcoming extant problems in thedirectdetermination of an appropriate apparatus function:indirectanalysis against a monoexponential reference decay response (&dgr;‐function convolution method,F‐Fdeconvolution).  

ISSN:0034-6748    

DOI:10.1063/1.1144312

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The combination of a variable wavelength interference filter and a position‐sensitive photon‐counting detector provide spectra of low‐intensity luminescence. This method has been demonstrated on two luminescent materials, europium (II)‐doped barium fluorobromide [BaFBr/Eu(II)] and uranyl nitrate [UO2(NO3)2⋅6H2O]. Pixel‐by‐pixel division of a luminescence image taken through the filter by an image without the filter corrects for variations in image intensity. The intensity along the long axis of the filter after correction for detector sensitivity constitutes a luminescence emission spectrum. BaFBr/Eu(II) gave delayed emission with a maximum intensity at 395 nm, in reasonable agreement with the measured Eu 5d→4ffluorescence maximum that occurs at 389 nm. The same technique applied to UO2(NO3)2⋅6H2O gave peaks at 511, 539, 567, and ∼600 nm. These did not correspond to experimental fluorescence maxima due to differing self‐absorption effects.

ISSN:0034-6748    

DOI:10.1063/1.1144263

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A time‐resolved spectral photometer, i.e., an instrument for simultaneously measuring intensity, time, and wavelength (I‐t‐&lgr;) is described. The ‘‘spectro‐streak’’ apparatus comprises a grating objective coupled to a commercial streak camera with 2‐ps resolution, a channel plate amplifier, a CCD camera, and a computer for data processing. The necessary calibration and correction procedures are discussed in detail; these are sweep nonlinearity correction, shading correction, geometric distortion correction, wavelength calibration, spectral intensity correction, and dispersion correction. The performance of the instrument is demonstrated with an example of current interest, the measurement of the solvatochromic dynamic Stokes shift of fluorescence from a charge‐transfer system in solution.

ISSN:0034-6748    

DOI:10.1063/1.1144264

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A phase‐sensitive flow cytometer has been developed that combines flow cytometry and fluorescence lifetime spectroscopy measurement principles to provide unique capabilities for making phase‐resolved measurements on fluorochrome‐labeled cells and particles. Stained cells are analyzed as they intersect a high‐frequency intensity‐modulated (sinusoid) laser excitation beam. Fluorescence is measured orthogonally using only a single‐channel optical detector. The detector output signals, which are phase shifted from a reference signal and amplitude demodulated, are processed by phase‐sensitive detection electronics to resolve signals from heterogeneous fluorescence emissions and quantify single‐component decay times. Results show signal phase shift and amplitude demodulation on fluorospheres; a detection limit threshold of 300–500 fluorescein molecules equivalence for excitation frequencies 1–30 MHz; a measurement precision (coefficient of variation) of 1.8% on alignment fluorospheres and 3.6% on cells stained for DNA content; the resolution of fluorescence signals from cells stained in combination with two fluorochromes, based on differences in their decay times; and the measurement of fluorescence lifetimes directly by the two‐phase ratio method.

ISSN:0034-6748    

DOI:10.1063/1.1144265

出版商:AIP    

年代:1993

数据来源: AIP