摘要:

A new method of obtaining three‐dimensional nuclear magnetic resonance images is introduced. This imaging method, Fourier reconstruction zeugmatography, reconstructs true three‐dimensional images directly from transient NMR signals by a Fourier transform technique. A theoretical analysis and experimental results are presented. The method is simple in operation and provides a safe, efficient, and versatile alternative to other methods for medical diagnostic imaging.

ISSN:0021-8979    

DOI:10.1063/1.329728

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The application of photoacoustic spectroscopy to the spectroscopy of layered samples with thicknesses of tens of microns is demonstrated both theoretically and experimentally. Theoretical expressions for the signal damping and phase delay are developed in terms of the thermal diffusion length, which is determined by the chopper frequency and the thermal properties of the sample. The theory is supported by measurements of both amplitude and phase on a color photographic film containing three light absorbing layers. The application of the method is further illustrated by a simple study of fading leaves.

ISSN:0021-8979    

DOI:10.1063/1.329729

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

In the present work two types of crossed field separators are described. The first separator has parallel rectangular magnetic poles and the second has tilted poles. The spectra of equal‐energy electrons are compared for each of the separators. The same is done for protons. Indications of astigmatism elimination are observed at the mass focus using the tilted poles configuration for both the electrons and protons.

ISSN:0021-8979    

DOI:10.1063/1.329730

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The various aspects of injecting ions into a three‐dimensional radio‐frequency quadrupole trap and their trapping for considerable periods of time are investigated by a numerical method. It is shown that for suitably chosen experimental parameters sufficiently long trapping times can be achieved to make this method attractive for on‐line high‐resolution spectroscopy of short‐lived isotopes and of highly charged stable ions. The narrow range of the phase of the driving voltage during which trapping of injected ions occurs is determined. Injection through an end cap electrode is more favorable than ring electrode injection. It is also shown that in the absence of collisions injection through the gaps between the electrodes of the quadrupole ion trap is not feasible. The process of moderating high‐energy ions by a thin foil before trapping and the various mechanisms of energy reduction after trapping are discussed.

ISSN:0021-8979    

DOI:10.1063/1.329731

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The effects of geometrical parameters (aspect ratio, aperture size, gaps) and electrical parameters (voltage distribution, beam energy) on single‐beamlet profiles are investigated theoretically and experimentally on a two‐stage ion accelerator over both a broad range of beam conditions and a wide range of angles (0 to ±11.5°). In general, the intensity distribution is non‐Gaussian. Under optimum conditions, divergence angles half‐width at half maximum less than 0.5° could be achieved even with large holes (0.9 mm). Shaping the plasma grid reduced the non‐Gaussian wings and improved the beam transmission efficiency.

ISSN:0021-8979    

DOI:10.1063/1.329732

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The Wilson theory has been modified for the calculation of the continuous slowing‐down approximation (CSDA) range of 50‐keV–100‐MeV electrons in the absorbers of atomic numbers from 1 to 92 and in some detector materials, minerals, organic compounds, and gaseous compounds. A correction factor which depends on energy as well as on atomic number is evaluated. Values of the CSDA range obtained by the present approach have shown an agreement with reported values within a discrepancy of 6–7%. An attempt has also been made to correlate the CSDA range with the available experimentally measured projected range. Calculated projected range and experimentally measured projected range are found to be in good agreement.

ISSN:0021-8979    

DOI:10.1063/1.329733

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The spherical aberration of electrostatic two‐foil lenses which are round and symmetrical about the center of the lens, are investigated. Functional relationships betweenA, a measure of the aberration, andBn, the coefficients of the Fourier series representing the axial potential distribution, are illustrated graphically, and a local minimum is indicated. This minimum can be used as a starting point for a steepest descent minimization procedure. The two‐dimensional hyperfaces ofA=A(−1,B3,B5,B7) resemble intersecting valleys, of which the faces are plane near the local minimum. The plane faces and their rectilinear intersections allow analytical methods to increase the efficiency of the search for a minimum. It is also shown that if the axial potential distribution is perturbed by functions of high spatial frequency but small amplitude (1 part in 107), large (for example, nine fold) increases in the spherical aberration can occur.

ISSN:0021-8979    

DOI:10.1063/1.329734

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

A four‐stage linear electron accelerator is described which uses pulsed radial transmission lines as the basic accelerating units. An annular electron beam produced by a foilless diode is guided through the accelerator by a strong axial magnetic field. Synchronous firing of the injector and the acccelerating modules is accomplished with self‐breaking oil switches. The device has accelerated beam currents of 25 kA to kinetic energies of 9 MV, with 90% current transport efficiency. The average accelerating gradient is 3 MV/m.

ISSN:0021-8979    

DOI:10.1063/1.329735

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

We consider the transmission and reflection of electromagnetic waves by an obstacle inside a waveguide. The method employed is an adaptation of the null‐field approach (T‐matrix method). The geometry is more or less arbitrary, but only for a waveguide with constant cross section can we obtain the scattered field in a not too formal way. For a circular cross section we derive comparatively explicit expressions for the transmission and reflection coefficients of the obstacle, and we give numerical results for spherical and spheroidal obstacles in rotationally symmetric geometries. As an important part of the theory, we have derived some apparently new transformations between the cylindrical and spherical vector waves and also expansions of the free‐space Green’s dyadic in cylindrical vector waves.

ISSN:0021-8979    

DOI:10.1063/1.329736

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The mechanism of the appearance of a knee on the voltage‐current curve of a plane Langmuir probe with an incident ion beam is investigated experimentally. The two‐dimensional shape of the sheath formed in front of the probe is measured by an emissive probe. The knee is found to be due to the enhanced expansion of an electron sheath by beam reflection, which increases the effective area of the probe. A potential profile similar to a double layer is formed in front of the probe. A weak axial magnetic field, which reduces a radial electron flow and which makes inverted V‐shaped two‐dimensional potential contours, quenches the knee.

ISSN:0021-8979    

DOI:10.1063/1.329737

出版商:AIP    

年代:1981

数据来源: AIP