摘要:

A new NMR imaging technique using a quadratic nonlinear field gradient is described. The center position of the nonlinear field gradient where the field intensity is minimum is moved spatially at a fixed amount in each imaging pulse sequence to produce nonlinear phase encoding as a function of position. A NMR image is reconstructed from the collected data by the computer data processing. The principle of this technique and many experimental results using a low field imaging system operating at 0.0192 T are presented. Theoretical and experimental comparison of theS/Nof the image in this technique with that of the conventional Fourier imaging technique are made and the results show that both techniques have almost the sameS/N. Moreover, the theoretical and experimental results show that this technique has a feasibility of a spatially localized imaging without using a conventional localization technique such as the use of the selective excitation in the presence of a linear field gradient.

ISSN:0034-6748    

DOI:10.1063/1.1143401

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

This article describes a system implemented on an IBM compatible personal computer (PC) for acquisition and processing of NMR data and for the spectrometer control. The homemade hardware includes a 12‐bit analog‐digital conversion board and a simplified pulse sequence controller. The software, written in assembly language and in Turbo Pascal, handles these two boards and, thanks to a versatile pulse generator, allows easily to run most of the modern NMR experiments in simple or in quadrature detection with spectral widths up to 25 000 Hz. Owing to its simplicity, its low cost, and its versatility, this system is specially suited for upgrading old NMR spectrometers or can be considered as the basis of a routine spectrometer.

ISSN:0034-6748    

DOI:10.1063/1.1143402

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The design of an electron nuclear double resonance (ENDOR) resonator suitable for single crystal studies is reported. An outline is given of the mechanical and electronic construction of the resonator. This paper will focus on the construction suitable for studies at 4.2 K, but the design can be easily adapted to a flow cryostat for variable temperature measurements. The rf‐coil efficiency has been measured and is high and fairly constant over the frequency range from 0.3 to 20 MHz. Measurements on a test compound, show that the baseline stability is good over this frequency range. Even for low abundant nuclei as13C, the signal to noise ratio is very good.

ISSN:0034-6748    

DOI:10.1063/1.1143403

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The development of a computer‐controlled electrochemical impedance spectrometer, based on a Fourier transform algorithm is described. Together with a fast potentiostat the system can be used in the frequency range 1 mHz to 100 kHz. The perturbation signal is a superposition of sine waves with properly chosen frequencies. The overall measurement time is limited only by the lowest frequency in the spectrum and by the data transfer to the computer, thus, time‐resolved impedance spectra measurements can be performed. The principle of operation and technical details are presented and discussed.

ISSN:0034-6748    

DOI:10.1063/1.1143404

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We present a force microscope which uses a feedback‐controlled differential fiber interferometer for the measurement of the deflection of the force‐sensing cantilever. Due to the differential principle, and the feedback control, the influence of thermal and mechanical drifts or fluctuations is minimized. Topographic images in both modes, the attractive ac mode and the repulsive dc mode, have been taken to demonstrate a first performance of the instrument.

ISSN:0034-6748    

DOI:10.1063/1.1143405

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A simple method has been developed for measuring the propagation of liquid surface waves over a frequency range from 500 Hz to 40 kHz. A new type of electromagnetic wire transducer was successfully employed to generate liquid surface waves at such high frequencies. Surface waves were detected by an optical probe with spatial resolution as high as 30 &mgr;m, which determined the high‐frequency limit of the measurement.

ISSN:0034-6748    

DOI:10.1063/1.1143406

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Recent developments in time‐resolved photoacoustic calorimetry (PAC) are discussed. An equation is derived relating the amplitude of the maximum photoacoustic signal to parameters of a photoacoustic cell, to physical properties of the solvents, and to the energy deposited as heat. The equation is validated by a series of scaling experiments: it correctly correlates the dependence of the PAC signal on cell thickness, on energy deposition, and on solvent properties. The dominant sources of background signal have been determined and the background signal reduced substantially relative to previous work. It is now possible to measure energies and lifetimes of transients in solutions with optical densities as low as ∼5×10−4. Data are presented for the energy of 2‐cyclopentenone triplet, a transient for which interpretable PAC measurements were previously not possible. Its triplet energy is 73.1±1.1 kcal/mole. With reduced background and a faster digital storage oscilloscope, a quite short transient lifetime, 7.53±0.2 ns, and a triplet energy of 51.0±2.1 kcal/mol have been measured for 1,1‐bis‐(p‐bromophenyl)ethene. The lifetime agrees very well with that determined by kinetic absorption spectroscopy and the triplet energy agrees with expectation based on the unsubstituted 1,1‐diphenylethene.  

ISSN:0034-6748    

DOI:10.1063/1.1143407

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A specific heat calorimeter for the temperature range 0.07–1.3 K with a novel sample support of four commercial pencil leads is described. It is shown that this support substantially reduces the vibrational heat input to the sample assembly. The thermal shield of the calorimeter is vacuum tight to allow measurements with gas adsorbing samples. We estimate and derive expressions for the temperature dependence of the thermal conductances of the AuCu thermal link and of the pencil leads support.

ISSN:0034-6748    

DOI:10.1063/1.1143842

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

An analysis is presented that establishes how correction‐free radiance measurements or pyrometry can be performed on targets in furnaces where there may be significant additional radiance from heated furnace walls or from reflecting surfaces close to the target. These configurations are common in microgravity and containerless processing applications where a specular target is radiantly heated in a hot‐wall furnace, or inductively heated from the fields arising from nearby closely spaced electromagnetic coils. Proper design requires that the pyrometer does not view any images of the background features in the specular target so that stray radiation will not be reflected into the viewing optics of the pyrometer. To analyze the problem, conventional paraxial optics analysis is shown to be significantly in error. Instead a method of nonparaxial ray tracing, based on the derivatives of the ray paths, shows how the images must be located. This leads to analytical results that can be used to design such systems. The findings are applied to the furnace geometries that are presently being considered for flight in low‐earth orbit. However, the same results may be used in other applications where any radiance measurement is being made on a specular target, as is the case for any molten material in a furnace.

ISSN:0034-6748    

DOI:10.1063/1.1143408

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Ultrahigh frequency rf‐SQUID magnetometer employing a cryogenic preamplifier and a commercially available thin film rf‐SQUID sensor have been designed, manufactured, and tested. The system has been operated at 316 MHz, which is the maximum pump frequency accepted by the thin film sensor. The cryogenic preamplifier has been built around a microwave high‐electron‐mobility transistor which was made unconditionally stable at ultrahigh frequency band by means of an appropriate circuit design, namely, a cascode configuration. The total flux noise of the magnetometer has been found to be 8.4 × 10−6&Fgr;0/&sqrt;Hz at 4.2 K and 5.8× 10−6&Fgr;0/&sqrt;Hz at 1.4 K. The temperature dependence of the fractional step rise &agr; of the current‐voltage rf characteristic has been measured to be proportional to the 2/3 power of the SQUID sensor temperature. The noise performance of the system at 4.2 K has been limited by the intrinsic SQUID noise only, in good agreement with theoretical predictions.

ISSN:0034-6748    

DOI:10.1063/1.1143409

出版商:AIP    

年代:1992

数据来源: AIP