摘要:

We describe a digital technique for measuring the spectral density of resistance fluctuations with simultaneous background subtraction. The technique implements either of two equivalent ac bridge techniques that rely on extremely good orthogonality between the two channels of a dual‐phase lock‐in amplifier. We use a digital signal processor (DSP) chip and a personal computer in place of both the lock‐in amplifier and spectrum analyzer. The digital system can measure sample 1/fnoise that is 100 times smaller than the experimental background noise; this performance equals or surpasses that of any analog system. The system is low cost, very flexible, and can function as a stand‐alone digital lock‐in amplifier or low‐frequency spectrum analyzer.

ISSN:0034-6748    

DOI:10.1063/1.1143732

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A field portable, multiwavelength, digital image processing system for the analysis of thin layer chromatography plates is described. The imaging module consists of a two‐dimensional charge coupled device detector, a xenon strobe light source, a stepper driven filter wheel for wavelength selection, and hardware to digitize images. The instrument is controlled by a laptop computer resulting in a compact device the size of a 6‐in. thick briefcase. As a representative application, software has been developed for the automated analysis of sulfa residues in pork products by thin layer chromatography (USDA SOS‐test). Response to analyte concentration is linear over two and one half orders of magnitude with a correlation coefficient of 0.9998. Detection limit is 80 pg for sulfamethazine spotted on a plate.

ISSN:0034-6748    

DOI:10.1063/1.1143733

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

X‐ray diffraction patterns were obtained using a tapered lead aperture. A detailed analysis of the effect of the tapered aperture is given. Experimental studies include results on W (423 GPa), on Pt (440 GPa, obtained using type IIadiamonds), and Mo (560 GPa). These results extend earlier studies in which we achieved for the first time static pressures greater than the pressure at the center of the earth.

ISSN:0034-6748    

DOI:10.1063/1.1143734

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A vertically dispersing high‐energy spherical grating monochromator (HESGM) has been installed on the synchrotron radiation source (SRS) at the Daresbury Laboratory. The instrument has no entrance slit and provides a high intensity source of x rays of energy 250–1400 eV focused into a small spot suitable for surface spectroscopy. Photoabsorption features of 240 meV have been resolved, but the resolution of the instrument is dependent on the SRS stored beam current. The degree of linear polarization has been measured at the carbonKedge to be 0.80.  

ISSN:0034-6748    

DOI:10.1063/1.1143735

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A method of evaluation of gas amplification in proportional counters, independently of gas gain measurements, is presented. The approach, particularly useful for high regions of the reduced electric field, (S=E/P), involves the fit of an analytic expression to all theSregions of the Townsend first ionization coefficient, (&agr;/P), experimental data. This approach is also useful for the estimation of gas gain for established gas mixtures but of nonstandard proportions such as Ar/Xe/CO2. The constants obtained in the process were used to evaluate the fundamental gas gain equation. An appropriate formula was then found to fit the resulting data. A component of the method is the generation of (&agr;/P) data for gas mixtures. Comparison of this work with various gas amplification models in the literature was effected. Physical phenomena which may be responsible for differences between calculated and experimental gas gain are discussed.  

ISSN:0034-6748    

DOI:10.1063/1.1143846

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The development of a noninvasive PC‐based computer automated radioactive particle tracking (CARPT) facility for investigation of phase recirculation and turbulence in multiphase systems such as fluidized beds and bubble columns is presented. In this facility, the motion of a single radioactive particle, which is dynamically similar to the recirculating phase, is monitored by an array of scintillation detectors which surround the test section. An on‐line computer is used to map the flow field of the recirculating phase. The data acquisition is achieved by commercially available nuclear instrumentation via a modular, high‐speed GPIB‐CAMAC system through assembly language software. Using CARPT, solids’ motion in gas‐fluidized beds and liquid motion in bubble columns have been investigated. The capabilities and versatility of the CARPT facility is described by illustrating some typical results for mean recirculation in gas‐fluidized beds with and without internals and in a gas‐liquid bubble column. The results include the mean circulation profiles and turbulence parameters such as the Reynolds normal and shear stresses and the turbulent eddy dispersion coefficients. Potential applications of CARPT technique to other recirculating systems are also discussed.

ISSN:0034-6748    

DOI:10.1063/1.1143736

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

In this paper the limitations and perspectives of a simple remanent field technique for the determination of the critical current density (jc) in high‐temperature superconductor (HTSC) thin films and in HTSC thin‐film multilayers at 77 K were examined. As the comparison with resistivejcmeasurements on microbridges shows,jcvalues down to 104A/cm2can easily be measured with high accuracy. The method is nondestructive, fast, and inexpensive. It is especially suitable forjcmeasurements of double‐sided coated substrates, where the determination of the individualjcvalues of both thin‐film layers is possible.    

ISSN:0034-6748    

DOI:10.1063/1.1143737

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We describe a large (78 cm) diameter liquid‐helium‐cooled blackbody absolute reference cold load for the calibration of microwave radiometers. The load provides an absolute calibration near the liquid‐helium (LHe) boiling point, with total uncertainty in the radiometric temperature of less than 30 mK over the 2.5–23‐cm wavelength (12–1.3 GHz) operating range. Emission from those parts of the cold load not immersed in LHe is ≤25 mK and the reflection coefficient is ≤3.5×10−4. This cold load has been used at several wavelengths at the South Pole, Antarctica and at the White Mountain Research Station, California to calibrate spectral measurements of the cosmic microwave background radiation. For the instruments operated at 20‐, 12‐, 7.9‐, and 4.0‐cm wavelength at the South Pole, the total corrections to the LHe boiling‐point temperature (∼3.8 K) were 48±23, 18±10, 10±18, and 15±16 mK, respectively. In operation, the average LHe loss rate was ≤4.4l/h, allowing day‐long periods of operation without a LHe fill. The boiloff rate is not strongly dependent on the radiative load at the aperture, yielding very stable operation and radiometric performance. Design considerations, radiometric and thermal performance, and operational aspects are discussed. A comparison with other LHe‐cooled reference loads including the predecessor of this cold load is given.

ISSN:0034-6748    

DOI:10.1063/1.1143738

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The recently developed dynamic plane source (DPS) technique for simultaneous determination of the thermal properties of fast thermally conducting materials with thermal conductivities between 200 and 2 W/mK has now been extended for studying relatively slow conducting materials with thermal conductivities equal or below 2 W/mK. The method is self‐checking since the thermal conductivity, thermal diffusivity specific heat, and effusivity of the material are obtained independently from each other. The theory of the technique and the experimental arrangement are given in detail. The data evaluation procedure is simple and makes it possible to reveal the distortions due to the nonideal experimental conditions. The extension to the DPS technique has been implemented at room temperature to study the samples of cordierite‐based ceramic Cecorite 130P (thermal conductivity equal to 1.48 W/mK), rubber (0.403 W/mK), and polycarbonate (0.245 W/mK). The accuracy of the method is within ±5%.

ISSN:0034-6748    

DOI:10.1063/1.1143739

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

In this paper we describe the use of so‐called open‐photoacoustic cell (OPC) for thermal characterization of transparent liquid samples. The samples used to demonstrate the usefulness of the OPC configuration are water, glycerol, and diffusion pump oil.

ISSN:0034-6748    

DOI:10.1063/1.1143740

出版商:AIP    

年代:1992

数据来源: AIP