摘要:

A practical method which may be used to measure small four‐terminal resistances at liquid helium temperatures with a precision of 0.1 ppm is described. Sensitivity sufficient to do this with a measuring current of 30 mA in a 500‐&mgr;&OHgr; resistor is achieved by the use of a SQUID null‐detector in a circuit with a high precision MacMartin–Kusters type direct current comparator. A major problem is the production of suitable comparison resistors to use as standards at low temperatures. The choice of material and construction is discussed for a pair of resistors with which it was possible to obtain noise voltages in liquid helium at 4.2 K that were near the Johnson noise in value, and to obtain resistance ratio measurements that were independent of current from 30 to 80 mA within 0.1 ppm. The method may be extended to resistances from 1 m&OHgr; to 10 &mgr;&OHgr; in value and even lower if thermoelectric voltages can be suitably reduced.

ISSN:0034-6748    

DOI:10.1063/1.1134755

出版商:AIP    

年代:1976

数据来源: AIP

摘要:

It is shown that, under suitable conditions, the power absorbed by a thin semiconductor slice in an oscillating magnetic field is accurately proportional to the material conductivity. The magnitude of this power absorption can be used to determine the conductivity by coupling the semiconductor to an amplitude‐stabilized marginal oscillator and noting the power needed to maintain the demanded level of oscillation. Instruments of this type have been constructed exhibiting ∼1% linearity over a 100:1 range of sample conductivity, a resolution of ∼1 in 104and a limiting sensitivity of ∼10−5mho/&laplac;. The method may be applied to the measurement of essentially any conducting material from semiconductors to metals.

ISSN:0034-6748    

DOI:10.1063/1.1134756

出版商:AIP    

年代:1976

数据来源: AIP

摘要:

A new instrument is described which permits the travel time of ultrasonic waves to be measured accurately and conveniently by the pulse‐echo‐overlap method (PEO). The accuracy and versatility of the PEO method are reviewed, and optimal methods of preparing and interrogating samples are presented. Extensive data on a large blank of fused silica are presented. Longitudinal and shear wave velocities were calculated from the wave travel times through measured thicknesses, and the elastic moduli were subsequently calculated from the velocities and the measured density. Error analysis shows that the measurement of travel time contributed less than 20 ppm to the total error of about 400 ppm in the calculated moduli.

ISSN:0034-6748    

DOI:10.1063/1.1134757

出版商:AIP    

年代:1976

数据来源: AIP

摘要:

Apparatus has been developed for producing a beam of fast hydrogen atoms having energies between 10 and 3000 eV. The procedure involves generation of a beam of hydrogen ions (H−), appropriate ion acceleration and trajectory definition, and ion neutralization by a photodetachment process. The magnitude of the resulting neutral atom flux can be determined to within an uncertainty of less than ±3% by a technique described. An atom beam intensity in excess of 1011atoms/sec at 1000 eV is readily obtainable, with the intensities at other energies scaling approximately inversely with the primary ion velocity. The advantages of the method over other neutral beam formation procedures are discussed, and techniques allowing considerable enhancement of the beam intensity over that presently achieved are suggested.

ISSN:0034-6748    

DOI:10.1063/1.1134758

出版商:AIP    

年代:1976

数据来源: AIP

摘要:

A new electrostatic optical system designed for high angular and energy resolution in ion–molecule‐scattering experiments is described. This device is based upon a theoretical lens model (’’zero gap electrostatic double‐aperture lens’’) proposed by Read. A first pair of these lenses accelerates a low‐energy ion beam, with a relatively wide angular spread, up to a chosen collision energy and makes the beam parallel in the scattering region, without any system of slits. A second pair of lenses, symmetrical to the first, decelerates the ions scattered at a given angle and focuses them on the entrance slit of a 127° electrostatic analyzer. This analyzer and the final lens are scanned in order to obtain energy loss spectra. A simple theoretical study of the system which allows the angular spread of the ion beam in the collision region to be predicted as a function of the initial energy and the chosen scattering energy (between about 100 and 3000 eV) is given, the respective positions of the lenses being adjustable parameters. These positions are chosen according to the theoretical results in order to achieve the best angular resolution. The first experimental data obtained are in very good agreement with the theoretical predictions. An example of application to elastic He+–He collisions is presented.

ISSN:0034-6748    

DOI:10.1063/1.1134759

出版商:AIP    

年代:1976

数据来源: AIP

摘要:

A predetection technique for cancelling flutter noise in FM multichannel magnetic analog tape recorders is described. Cancellation of all correlated flutter noise components is done automatically in the phase‐frequency domain so the technique is insensitive to amplitude variations and does not depend on any gain adjustments in the system.

ISSN:0034-6748    

DOI:10.1063/1.1134746

出版商:AIP    

年代:1976

数据来源: AIP

摘要:

The design of a sample positioning mechanism for use in a multiexperiment ultrahigh vacuum apparatus is described. It permits positioning of a solid sample for surface studies at four points on a 7.6‐cm circle and the independent rotation of the sample about an axis through it at any one of these positions.

ISSN:0034-6748    

DOI:10.1063/1.1134747

出版商:AIP    

年代:1976

数据来源: AIP

摘要:

A highly sensitive, fast rise‐time detector has been developed that is capable of neutron time‐of‐flight (TOF) measurements in the presence of an intense bremsstrahlung background such as that from a relativistic electron beam (REB) accelerator. This detector, used in conjunction with thermal neutron counters, forms an effective neutron detection system. Such a system was used to measure neutrons from the HYDRA REB accelerator. By using a fast‐acting and reproducible gating circuit, the inherent sensitivity of the detector has been retained while suppressing all deleterious effects caused by the intense bremsstrahlung burst. At no time during the measurements were any spurious afterpulses observed in the TOF detectors. The timing resolution obtained was sufficient to distinguish between neutrons produced by thermal and fast deuterium ions.

ISSN:0034-6748    

DOI:10.1063/1.1134748

出版商:AIP    

年代:1976

数据来源: AIP

摘要:

A total‐reflecting mirror of 120‐cm length was designed and built to focus synchrotron radiation emanating from the electron–positron storage ring at the Stanford Linear Accelerator Center (SPEAR). The reflecting surface is of unpolished float glass. The bending and tilt mechanism allows very fine control of the curvature and selectability of the critical angle for wavelengths ranging from 0.5 to 3.0 A˚. Elliptical curvature is used to minimize aberrations. The mirror is placed asymmetrically onto the ellipse so as to achieve a tenfold demagnification of the source. The bending mechanism reduces nonelastic deformation (flow) and minimizes strains and stresses in the glass despite its length. Special design features assure stability of the focused image. The mirror reduces the intensity of shorter wavelength harmonics by a factor of approximately 100.

ISSN:0034-6748    

DOI:10.1063/1.1134749

出版商:AIP    

年代:1976

数据来源: AIP

摘要:

A convenient piezoelectric driven Kelvin probe for the measurement of work function changes is described. The probe has a simple construction, is small, bakeable, and can be mounted on any 35‐mm‐i.d. UHV flange. As the piezoelectric device is inside the vacuum chamber, only one electrical feedthrough is needed for operating the probe. The distance between reference electrode and sample can be varied within ±0.5 mm simply by applying a dc voltage to the piezoelectric device in addition to the ac oscillator‐driving voltage, not requiring any mechanical driving and feedthrough provisions. The probe was tested studying the adsorption of O2on Pt(111). The sensitivity for contact potential measurements is better than 0.1 mV at a response time of 10 sec.

ISSN:0034-6748    

DOI:10.1063/1.1134750

出版商:AIP    

年代:1976

数据来源: AIP