Index of content:
Volume 27, Issue 12, December 1956
27(1956); http://dx.doi.org/10.1063/1.1715471View Description Hide Description
An iron‐free intermediate‐image beta‐ray spectrometer has been constructed having a momentum spread (full width at half‐maximum) for a point source of conversion electrons continuously variable between limits of 0.5% and 4% with corresponding transmissions of 1% and 8% of 4π, respectively. The line half‐width contribution due to source size is 0.3% per mm of diameter. At a power dissipation of 100 kw the spectrometer focuses electrons of 9‐Mev energy allowing positron‐electron pair lines corresponding to transitions of up to 19 Mev to be measured using a double‐crystal statistical‐separation detector together with a fast coincidence circuit. In tests with the 6‐Mev nuclear transition in O16 using a Van de Graaff beam spot 1 mm in diameter the momentum half‐width of the pair line can be varied from 0.56% to 2.5% with corresponding absolute transmissions of 0.07 to 6.3 pair counts per 105source transitions, respectively. The energy of this transition has been determined as 6.065±0.009 Mev. Electron‐gamma and electron‐electron coincidence measurements on radioactive sources may be made with the spectrometer in conjunction with crystal detectors placed near the source.
27(1956); http://dx.doi.org/10.1063/1.1715437View Description Hide Description
The design of a magnet for producing a uniform field is described. The magnet model used in the calculations is that of cylindrical cores surrounded by rims. The positions of the central pole faces and rims can be adjusted independently. A calculation, based on the assumption that the pole pieces are uniformly magnetized, shows that, for a certain positioning of the cores and rims, the z 2 and z 4 correction terms to Hz can be simultaneously canceled. Practical limitations to uniformity that result from a slight asymmetry in the geometry and from the effect of nonuniformity in magnetization because of domain structure in the iron also are discussed.
27(1956); http://dx.doi.org/10.1063/1.1715438View Description Hide Description
Small variations in the fields of magnets, caused by structures or domains in the pole faces, were investigated by moving a small coil in a circular path. It was found that for a given pole face the variations in the field were all about the same size, and decreased exponentially from the pole face. None of the materials tested as pole faces produced strikingly better fields than another.
27(1956); http://dx.doi.org/10.1063/1.1715439View Description Hide Description
Ferromagnetic wires, that have been suitably conditioned, generate electrical potentials when subjected to changes in elastic strain. Conditioning the wires involves alignment of the circumferential component of the magnetic field, due to domains, in a preferred direction. This is accomplished by the magnetic field associated with a current passed through the wire. A disturbance which changes this established circumferential field causes a potential to appear between the ends of the wire.
27(1956); http://dx.doi.org/10.1063/1.1715440View Description Hide Description
A high‐temperature x‐ray diffraction unit for powder specimens is described, which can be attached to standard room‐temperature cameras. The evacuable heater‐specimen space is surrounded by a beryllium cylinder with cellophane windows and the heater, either a small, gap‐divided spiral, holding the specimen capillary, or a Pt wire with coated‐on specimen, can be slipped in it and rocked during exposures. Temperatures up to 1000°C can be obtained with power consumption of ca 10 w. Specimen temperatures are determined from line‐shifts in the pattern of added internal standard or the pattern from the platinumheater wire.
27(1956); http://dx.doi.org/10.1063/1.1715441View Description Hide Description
This article describes the construction of a conductive glass resistor intended for operation in a sealed high vacuum system. The resistor is small, having a body size of 0.195 in. long and ⅛ in. diameter, and resistance values from 0.25 to 15 megohms can be achieved. A power dissipation of 25 milliwatts is normal and in vacuum, the operating temperature is approximately 65°C. After suitable initial outgassing, no appreciable amount of gas is evolved into the vacuum system as a result of operating the resistors.
27(1956); http://dx.doi.org/10.1063/1.1715442View Description Hide Description
Two temperature control thyratron relays for use with contact thermoregulators are described, posessing long life, high reliability and low control power requirements. In addition, simple phase shift circuits have been incorporated allowing continuous control of system thermal hysteresis.
27(1956); http://dx.doi.org/10.1063/1.1715443View Description Hide Description
The theory of the method of measurement of liquid level by measurement of the resistance of a heated wire is developed, and it is applied to the design of a liquid‐nitrogen‐level indicator having a power dissipation of 3.4 mw/in. of length and a deviation from linearity in the calibration of less than 0.1 in. over its working length of 2.0 in. The apparatus should be applicable to the measurement of level of low viscosity liquids and its sensitivity is limited only by the temperature coefficient of resistance of the heated wire.
27(1956); http://dx.doi.org/10.1063/1.1715444View Description Hide Description
A simple reliable process is described for recording the amplitudes of individual pulses on magnetic tape. This enables the pulse analysis to be performed after the experiment, and under optimum conditions. Low counting rates may be recorded with the tape moving slowly, and then speeded up on playback, so that, for example, a twenty‐four hour run may occupy only seven minutes analyzer time. The recording process is strictly linear over a range of signal/rms noise of up to 300/1. A maximum rate of 4000 pps may be recorded. The resolution varies with pulse amplitude, but, over the upper two‐thirds of the linear range the relative spread assumes a very nearly constant value of about 3%.
A further development is described in which gated time coincident pulses are recorded simultaneously on twin tracks of the same tape. In data gathering power this system is better than an array of thirty conventional thirty channel pulse analyzers. It may also be applied to the simultaneous recording for separate analysis of the outputs of up to about ten independent detectors.
27(1956); http://dx.doi.org/10.1063/1.1715445View Description Hide Description
A recording microdensitometer is described that has a resolving power of 800 lines/mm when the scanning aperture is 1 μ wide and 200 μ long. The response is linear in density to 3.0 for the same aperture. By narrowing the aperture to 0.1 μ, the resolving power can be increased to 1800 lines/mm. The record is made either on chart paper with rectangular coordinates, on punched cards, or on both.
27(1956); http://dx.doi.org/10.1063/1.1715446View Description Hide Description
A cryostat is described which will maintain optical cells at any temperature from −190°C to 0°C while they are being scanned in a recording spectrophotometer. Liquid air is used as the refrigerant and the temperature is adjusted by means of an electrical resistanceheater. Fogging of the optical system is avoided by circulation of the dry refrigerant vapors and by window heaters.
27(1956); http://dx.doi.org/10.1063/1.1715447View Description Hide Description
A simple creep apparatus for approximating a constant stress in tensile specimens of large cross sections, where large applied loads are required, is described. The apparatus consists of a lever with a large built‐in mechanical advantage, together with means for removal of incremental weights from the applied load.
27(1956); http://dx.doi.org/10.1063/1.1715448View Description Hide Description
A frequency‐sensitive switching circuit is described which automatically selects between two ranges of an otherwise conventional frequency meter by switching the leak resistor of the integrating circuit. The basis of the unit is a diode pump integrator operated as a nonlinear device to obtain maximum sensitivity. The sensitivity is such that the ``on‐off'' backlash of the switching relay corresponds to less than 1% of the switching frequency.
27(1956); http://dx.doi.org/10.1063/1.1715449View Description Hide Description
A simple slide rule has been developed which will permit a rapid and accurate determination of radiographic sample transmission, of absorption index, μm, and of the photographic film density corresponding to the direct x‐ray beam by means of a single setting of the sliding scale.
27(1956); http://dx.doi.org/10.1063/1.1715450View Description Hide Description
An apparatus for measuringwork functions by the Kelvin method is described in which the customary vibrating motion is replaced by rotary motion. A set of three coplanar samples is mounted on a turntable which rotates below a set of three stationary samples. This arrangement permits simultaneous observation of three electrode pairs in all permutations and provides ready access to the sample surfaces for purposes of treatment, illumination, inspection, etc., without the necessity for repositioning. The rigid structure also permits the application of electrostatic fields between electrodes, which is difficult in a vibrating system. Measurements can be made in high vacuum by driving the turntable inductively with coils located outside the glass envelope. The instrument has a sensitivity of ±1 millivolt.
27(1956); http://dx.doi.org/10.1063/1.1715451View Description Hide Description
A fast, high‐efficiency detector of heavy particles that results in negligible changes in the velocity and direction of the detected particles has been built. The detector consists of an electron accelerator and a scintillator that responds to delta particles emitted by the passage of each particle through a thin foil. For fission fragments, time resolutions of 3×10−9 sec, with efficiencies greater than 95%, have been obtained. The fragment velocity loss in traversing the thinnest foils used was less than 1%.
27(1956); http://dx.doi.org/10.1063/1.1715452View Description Hide Description
In a sectorial cyclotron the ``force functions'' and the frequencies of the linearized normal and axial betatron oscillations are given as power series in the inverse‐square of the number of sectors. The coefficients in these series are expressed either in terms of parameters characterizing the magnetic field or in terms of those specifying the family of equilibrium orbits. The first‐order terms agree with those in existing approximate expressions. Among other features of the higher order terms, they show that it is possible to design magnetic fields with momentum compactions varying over wide ranges and frequencies of betatron oscillations remaining constant. Together with a scheme proposed earlier of transferring ions continually between circular accelerators, this opens the possibility of accelerating ions continually in several stages of constant‐frequency sectorial cyclotrons to multi‐Bev energies. This method of analysis can also be applied to the study of nonlinear betatron oscillations.
27(1956); http://dx.doi.org/10.1063/1.1715454View Description Hide Description
Semiconductorcarrier lifetimes as short as 10−8 sec may be obtained from the decay of bombardment conductivity following pulsed excitation by high energy (700 kev) electrons from a Van de Graaff accelerator. Bombardment damage of the sample is minimized by the use of short pulses (2×10−8 or 3×10−4 sec) and a low repetition rate (15 or 30 pulses per sec). Modification of a standard accelerator for these measurements is discussed in some detail and some typical results are presented.
27(1956); http://dx.doi.org/10.1063/1.1715455View Description Hide Description
A theoretical study is made of the surface temperature of a front heated slab in the presence of small holes drilled to within a fraction of a centimeter of the heated surface. In addition, equations and calculations are presented which yield an estimate of the error incurred when the surface temperature is measured by a transducer located at the end of such a hole.
In the absence of a practical exact solution to the problem, the method of attack chosen here leads to an upper bound on the additional surface temperature or hot spot caused by a cavity. The upper bound is justified because it is a close upper bound and, for cavity dimensions in the range of interest, the resulting hot spot is quite small. It is shown, for instance, that a ½‐mm hole drilled to within 1 mm of the exposed surface will cause a time variant surface hot spot that is no more than 1.5% of the surface temperature. If the hole depth is reduced to ½ mm, then the hot spot is less than 5.5% of the surface temperature.
An additional finding is that the magnitude of the hot spot is nearly independent of the temporal shape of the surface heat pulse for a large variance in the latter.