Volume 31, Issue 3, March 1960
Index of content:
31(1960); http://dx.doi.org/10.1063/1.1716948View Description Hide Description
A pulsed neutron source has been developed whose principal element is a sealed‐off vacuum tube in which a discharge between titanium tritide surfaces produces tritons, which then are accelerated to a deuterium‐loaded target. The construction of a tube producing approximately 107neutrons in microsecond pulses and having a life of several thousand pulses is described.
31(1960); http://dx.doi.org/10.1063/1.1716949View Description Hide Description
A study of the factors influencing the design of pulsed neutrongenerators is made. By using the reaction between tritium and deuterium, peak rates of 1013/sec should be obtainable from quite small sources.
A method of keeping the pressure of deuterium gas substantially constant at the working pressure was evolved. This enabled a ``sealed‐off'' and hence portable source to be made.
The development of a fast neutrongenerator or neutron tube based on these considerations is described. This was capable of producing neutrons at a rate of 5×1012/sec at a voltage of 100 kv. The tube was suitable for operation at pulse lengths of 20 μsec, and the neutron content of each pulse was 108. No difficulty was experienced in operating at 1 pulse/sec, and higher pulse repetition frequencies may be possible.
31(1960); http://dx.doi.org/10.1063/1.1716950View Description Hide Description
Second‐order perturbation theory has been used to calculate the electron orbits in a ``flat'' beta‐ray spectrometer with an azimuth‐independent magnetic field having a symmetry plane. Abandoning first‐order z focusing, it is possible to obtain a much higher resolution at a given transmission compared with the usual π√2 instrument. The resolution depends, in the lowest power, only on fourth‐order terms of the radial and axial emission angles. The resolution does not depend on the first power of the source height. The focusing principle is applicable for a set of angles θ, and the dispersion increases strongly with increasing angle. Several examples are discussed.
31(1960); http://dx.doi.org/10.1063/1.1716951View Description Hide Description
The paper describes the basic circuitry required for a high precision direct reading resistance thermometer with dials calibrated in degrees C and any standard platinum resistance thermometer as sensing element. A circuit has been developed which, through linear variations of the resistance elements, gives a quadratic law having the form R=R 0(1+AT+BT 2), where B is negative, as is required for a platinumthermometer. At the same time it allows separate adjustments for slope A and curvature B or B/A of the resistance vs temperature relation of the particular thermometer being used and also for the ice point resistance R 0. The auxiliary circuitry is given for comparing the computing circuit with the thermometer and for maintaining a constant recorder sensitivity of all temperatures; stability requirements for work to 0.001°C over a 700°C range are discussed.
31(1960); http://dx.doi.org/10.1063/1.1716952View Description Hide Description
This paper presents the details of circuitry and construction for a direct reading resistance thermometer which has been built utilizing the design criteria formulated in Part I. By using any standard 25.5Ω platinum resistance thermometer as a sensing element, the bridge gives subdivision of the temperature scale to 0.001°C between −50° and +700°C with an accuracy exclusive of that of the thermometer of about ±0.001°C. Methods are described that allow for variations of thermometer constants, for autocalibration of the bridge circuit, and for maintaining constant sensitivity by variation of the thermometer current. The bridge as described has performed satisfactorily for three years and has proved to be a useful and convenient laboratory tool.
31(1960); http://dx.doi.org/10.1063/1.1716953View Description Hide Description
A new and relatively simple type of mass spectrometer ion detector is described. The positive ion is accelerated onto an aluminum surface releasing secondary electrons, and these in turn are accelerated onto an organic scintillator, viewed by a sealed‐off photomultiplier. Counting methods are used to measure the ion beams. The detector has a low noise level, 4×10−20 amp, and the mass discrimination is small for ions in the high and low mass range.
Admission of air to the vacuum system does not affect the gain of the detector, since no activated surfaces are situated within the vacuum.
In the event of a fault occurring in the photomultiplier a new one can be substituted in a few minutes without letting air into the vacuum system.
31(1960); http://dx.doi.org/10.1063/1.1716954View Description Hide Description
An experimental high vacuum furnace with a platinum‐rhodium resistance heating element is described; it is operable also in oxygen or other gases up to pressures of at least 10−3 mm Hg. The furnace is assembled from readily obtainable equipment, and knife‐edge vacuum seals are used throughout. The uniform hot zone is approximately in. in diameter and 2 in. long. The power required for 1400°C, the maximum safe operating temperature, is about 1 kw; at this temperature the pressure is ∼10−8 mm Hg. This furnace is suitable for electrical conduction studies as well as for heat treatment, etc.
31(1960); http://dx.doi.org/10.1063/1.1716955View Description Hide Description
The problem of determining the distribution of path lengths of an isotropic flux of particles passing through a cylinder is solved analytically. The distribution function is shown graphically for cylinders of various height‐to‐diameter ratios and the mean path length is calculated in each case. The distribution function for some special cases of shielded cylinders also is discussed.
31(1960); http://dx.doi.org/10.1063/1.1716956View Description Hide Description
An electromechanical shutter is described which will open and close a 20‐μ spectrograph slit in 5 μsec after a delay of 20 μsec. The shutter incorporates a movable slit which is propelled across the stationary spectrograph slit by the thermal expansion of a Joule‐heated hairpin‐shaped Nichrome wire.
31(1960); http://dx.doi.org/10.1063/1.1716957View Description Hide Description
The performance of a windowless resistance strip magnetic multiplier for detection of extreme ultraviolet is described. The detector is characterized by a high spectral sensitivity for wavelengths below 1400 A and a low sensitivity to longer wavelength radiation. It is reproducible in spectral response and gain after exposure to air or after cleaning. A current gain of 108 can be realized with the multiplier for an over‐all voltage of approximately 2000 v. The photomultiplier can be operated as a photoelectron counter with a well‐defined counting plateau for pressures below 10−4 mm Hg. The background counting rate of the detector, at room temperature and typical operation in the plateau region, is less than 0.1 count/sec.
31(1960); http://dx.doi.org/10.1063/1.1716958View Description Hide Description
A test set is described which permits the display of the V‐I (volt‐ampere) characteristic of a tunnel diode on an oscilloscope. The circuit instabilities which usually exist when a negative resistance device is tested on a V‐I curve tracer are suppressed in the unit described. The principles of design, construction, and operation are presented.
31(1960); http://dx.doi.org/10.1063/1.1716959View Description Hide Description
A semiautomatic instrument for ionization measurements in emulsion is described briefly. The instrument was used in the investigation of the various ionization parameters. Deviations from a purely exponential gap‐length distribution were observed, a result found earlier by Cortini et al. This observation limits the usefulness of the mean gap length as a parameter. A new parameter based on the blob‐length distribution, which is useful over the entire ionization range, is proposed. The relation of this parameter to grain density is discussed. It is shown that the behavior of the latter (determined from the blob‐length distribution) as a function of restricted ionization loss seems to be in agreement with theoretical expectations, with the exception of the very dense region, where appropriate corrections are proposed.
31(1960); http://dx.doi.org/10.1063/1.1716960View Description Hide Description
A high power, low inductance vacuum spark gap combination (crowbar and main switch) is described which is capable of dc operation over a wide voltage range. The electrical properties are discussed in regard to shorting and multiple switch operation. The principal difficulty of vacuum spark gaps, the coating of the inner surface of the insulator with evaporated and sputteredelectrode material, is absent in this design after conditioning. A mechanism to account for this, based on the establishment of a large number of nucleation centers on the insulating walls, is shown to be consistent with observation.
31(1960); http://dx.doi.org/10.1063/1.1716962View Description Hide Description
A heating element design is described, applicable to graphite or sheet metal, which has an electrical resistance comparable to a helix and a structural rigidity comparable to a cylinder. Incomplete cuts perpendicular to the axis create a number of electrically long paths in parallel, each with relatively short distances between regions of mechanical support.
31(1960); http://dx.doi.org/10.1063/1.1716963View Description Hide Description
A new system for control of the x‐ray energy from a 25‐Mev betatron is described. Modifications in the method of energy control and orbit expansion lead to improved operation. Calibration of the energy scale is based on reaction thresholds for D(γ,n), Bi209(γ,n), Cu63(γ,n), and the threshold for excitation of the 15.116‐Mev state in C12. The resulting energy scale is linear with respect to electron momentum to better than ±20 kev.
31(1960); http://dx.doi.org/10.1063/1.1716964View Description Hide Description
A device is described which is capable of continuously measuring the thickness of moving conducting materials. The details of two independent methods, one utilizing the amplitude and one the phase of reflected microwave energy, are explained and compared. Experimental results verify that increments of the order of 1/40 mm are clearly detectable.
31(1960); http://dx.doi.org/10.1063/1.1716965View Description Hide Description
There are two fundamental problems in ebulliometry. The first is to find a detector with sufficient sensitivity to measure the very small boiling point elevations obtained with dilute solutions of high polymers. This problem is solved easily with thermistor bridges and modern high gain amplifiers. The second problem is that of random thermal fluctuations or background noise in the ebulliometer itself. When this study began, it was considered that there were three probable sources of noise, viz., (1) superheated liquid from the Cottrell pump, (2) pressure fluctuations and differences within the ebulliometer, and (3) the presence of foam which usually appears when a polymer solution is maintained at the boil. However, the tests carried out with the ebulliometers described here seem to indicate that the difficulties may be inherently associated with the polymer solutions themselves.
31(1960); http://dx.doi.org/10.1063/1.1716966View Description Hide Description
A versatile high current regulator using transistors for the series regulating element and capable of accuracies of a few parts per million is described. A solid state dc power supply capable of delivering 50 amp at 50 v also is described. The performance of the system as a magnet current regulator is discussed briefly.
31(1960); http://dx.doi.org/10.1063/1.1716967View Description Hide Description
Piezoresistivity, the change in resistivity with applied stress, offers a tool for analyzing the conduction mechanism in semiconductors and also offers potential transducer applications. A discussion of this effect in semiconductors and an analysis of the fourth‐rank piezoresistivetensor for two crystal symmetries are presented. The magnitude of the effect in semicondictors is tabulated, including two materials, TiO2 and PbTe, which the authors have determined to be of considerable interest. The design of devices based on piezoresistivity is discussed and illustrated by working models.