Volume 21, Issue 4, April 1950
Index of content:
21(1950); http://dx.doi.org/10.1063/1.1745562View Description Hide Description
This paper describes a simple and accurate method for the measurement of the phase difference between two sinusoidal voltages and an instrument for making the measurement. It is shown that measurement accuracy sufficient for most ordinary purposes can be obtained through the use of very simple circuitry, and that measurement errors can be made arbitrarily small by resorting to more complex circuitry. The instrument is compact and convenient in use.
21(1950); http://dx.doi.org/10.1063/1.1745563View Description Hide Description
21(1950); http://dx.doi.org/10.1063/1.1745564View Description Hide Description
A method for cooling x‐ray tubes was devised, based on the principle of relative motion between the target and focal spot. The electron beam is deflected by a rotating magnetic field so that the focal spot moves in a circle upon the target face. Simultaneously, the tube itself is gyrated (without rotation about its axis) so that any point on the target describes an equal circle. The two rotations are equal in frequency but phased 180 degrees from each other. Thus the focal spot remains stationary in laboratory space. The advantages are (1) it is adaptable to most x‐ray tubes with target face perpendicular to the tube axis, and target diameter twice that of the longest focal spot dimension, without any modification of tube design; (2) it combines the benefits of a rotating target, water‐cooled target, and line focus without the need for rotating joints or vacuum connections. Tubes, operating continuously for hours at a time at a gyrating frequency of 6 r.p.s. have been cooled successfully by this method without observable shifting of the focal spot in laboratory space.
21(1950); http://dx.doi.org/10.1063/1.1745565View Description Hide Description
The preparation and use of calibrated RaDEF equilibrium sources for absolute beta‐counting is described. The sources are essentially weightless and are mounted on a thin backing to reduce self‐absorption and back‐scattering correction problems. Calibration was done by determination of the alpha‐disintegration rate of RaF in a parallel plate alpha‐ionization chamber. No secondary standards are required and the sources can be recalibrated at any time.
As an example of the application of such standards, details of the calibration of a P32 solution distributed by the Bureau of Standards for intercalibration in February 1949 are given. The final result is 88.8 mrd/ml at 8 A.M. C.D.T. March 1, 1949. The precision of the measurement is 0.3 percent. The accuracy is estimated to be 1–2 percent.
21(1950); http://dx.doi.org/10.1063/1.1745566View Description Hide Description
A multiple gamma‐ray counter is described, composed of 24 counter tubes connected in parallel. The tubes have Bi‐plated brass screen cathodes and are placed inside the same envelope. Relative and absolute values for the efficiency of the counter were determined experimentally in the range 0.28 to 2.62 Mev. Compared to a single brass counter of similar dimensions, the multiple counter shows for gamma‐rays of, say, 0.7 Mev energy a tenfold increase in efficiency. Its starting voltage is 5–600 volts lower.
The ``effective resolving time'' of the multiple counter appears to be 3 to 4 μseconds.
The counting rate curve seems to follow the inverse‐square law down to the shortest counter‐source distances. The solid angle subtended by the multiple counter for sources, placed in its middle, is nearly 4π.
21(1950); http://dx.doi.org/10.1063/1.1745567View Description Hide Description
The high density, transparency, and chemical inertness of calcium and cadmium tungstates, together with their reasonable luminescent efficiencies when stimulated with high energyradiation, strongly suggest their use as phosphors in scintillation counters for gamma‐ray detection. Some of the properties of these crystals are reported and compared with anthracene. It is shown that with proper design, the total anode current produced by a given source is several times greater with the tungstates than with anthracene, although the latter has a more favorable pulse size distribution. Cadmium tungstate has a zero temperature coefficient of luminescence up to 40°C and shows no afterglow after exposure to intense radiation.
21(1950); http://dx.doi.org/10.1063/1.1745568View Description Hide Description
The low frequency range of an audio oscillator can be extended to d.c. by means of a heterodyning circuit using power line frequencies as a reference. The voltage wave forms are essentially sinusoidal and can be used for certain novel tests. A theoretical discussion of the beating phenomenon is included with particular applications and observed results.
21(1950); http://dx.doi.org/10.1063/1.1745569View Description Hide Description
The sensitivity of the RCA 1P21 photo‐multiplier to gamma‐radiation has led to the investigation of the characteristics of this tube as a gamma‐ray detector. The range, linearity, directional dependence, temperature dependence, applied voltage regulation requirements, and radiation energy dependence were determined for several tubes. Preliminary results concerning the ability of the tube to maintain a calibration are given. A basic circuit for such a detector is given.
21(1950); http://dx.doi.org/10.1063/1.1745570View Description Hide Description
An apparatus is described by means of which the size and the charge of large numbers of microscopic particles can be simultaneously determined. The method is based on Hopper and Laby's work on the determination of the electronic charge, i.e., horizontal deflection of particles settling under gravity recorded photographically. The procedures involved are given in some detail, and the nature and limitations of the general results are discussed. Simplicity of operation and design are emphasized to render the instrument a workable tool in industrial research.
21(1950); http://dx.doi.org/10.1063/1.1745571View Description Hide Description
The spectral sensitivities of Kodak No‐Screen and Kodak Industrial Type K X‐Ray Films were determined in the region between 0.2 and 2.5A using the filtered Kfluorescence of several irradiated elements as approximately homogeneous radiation. Exposures in roentgens were obtained with an air ionization chamber designed especially for the purpose. Curves are given showing the number of roentgens incident on the film which will give a developed photographic density of 1.0 above fog for the various wave‐lengths. The K limits of silver and bromine are prominent. The maximum sensitivity in terms of roentgens occurs at about 0.4A, but the marked decrease found by other observers at shorter wave‐lengths is probably obscured by inaccuracies. From the energy value of the roentgen, the sensitivity of the films is expressed as the incident ergs per square centimeter required to produce a density of 1.0 above fog. Computations from these results show that as the wave‐length increases the films utilize the absorbed energy more efficiently. On the other hand, high energy quanta are more efficient than low energy quanta but not in proportion to their energy. In addition to the more or less gradual change with wave‐length of efficiency of utilization of absorbed energy, there is evidence for sudden jumps at the K limits of Ag and Br. Qualitative explanations are given for the observed changes in efficiency.
21(1950); http://dx.doi.org/10.1063/1.1745572View Description Hide Description
A pulse counter of decade type, which does not require any components with narrow tolerances, has been designed. A direct reading system for the total count has been used, which eliminates all calculations. Several instruments of the type described here have been put to daily use for more than half a year without any failure.
21(1950); http://dx.doi.org/10.1063/1.1745573View Description Hide Description
A null‐method magnetometer is described for the measurement of magnetization of small single crystals of polycrystalline specimens of ferromagnetic materials. It may also be used for measurements on para‐ and diamagnetic materials. The specimen is placed in an inhomogeneous magnetic field inside a ``null‐coil'' of known geometry, and the current through the coil is adjusted so as to reduce the net magnetic moment of coil and specimen to zero. This coil is rigidly attached to the lower end of a sensitive pendulum and the null‐position of the pendulum is observed with the aid of a projection‐microscope. This method eliminates the necessity of measuring the force on the specimen and the gradient of the magnetic field. The conditions are discussed under which the absolute error may be reduced to arbitrarily small values. The secondary field arising from the current in the null‐coil is, in general, negligible; however, a simple method is described for its elimination.
21(1950); http://dx.doi.org/10.1063/1.1745574View Description Hide Description
Observation of the wave form of current during a Geiger discharge indicates the presence of a component due to electron collection ammounting to as much as 50 percent of the total energy involved in the discharge. It appears, therefore, that the initial ion sheath can have a mean radius of the order of 10 to 15 times the radius of the center wire. Apparatus is described, wave forms are shown and the method used to evaluate electron component and mean radius of ion sheath is indicated. Results for two tubes with different quenching vapors are tabulated.
21(1950); http://dx.doi.org/10.1063/1.1745575View Description Hide Description
Suitably shaped electrodes attached to a thin sheet of isotropic conducting material, such as one of the newly developed conducting plastics, can make available a specified distribution of potential at a contact sliding along an edge of the sheet. Electrode contours are evaluated for the assumption of a logarithmic potential distribution along the straight‐line edge of a voltage‐divider strip cut out of the conducting sheet. The ratio of the total length for contact travel to the maximum width of the strip controls the attenuation rate. For a specified attenuation rate (db/in.), the three non‐straight edges of the divider strip can be scribed or punched by repeated use of a single curved template or die. Alternative electrode configurations for attachment to rectangular strips of material offer simplification of construction at some sacrifice of linearity of the logarithmic scale.
21(1950); http://dx.doi.org/10.1063/1.1745577View Description Hide Description
The 6AS6 pentode has been used in a very stable counting or control ring whose input is essentially independent of amplitude and wave shape. Values of supply voltages and circuit parameters are not critical. Control voltages corresponding to one or more stable states of the ring may be obtained very easily.
21(1950); http://dx.doi.org/10.1063/1.1745578View Description Hide Description
When studying transformations at sub‐zero temperatures it is often convenient to be able to carry out treatments such as cold‐working and annealing and to transfer the specimen, at temperature, to the x‐ray diffraction camera. Several cameras have been designed, primarily for the x‐ray spectrometer, in which this is easily done. For constant temperature work a flask, having a nozzle which allows cold liquid to flow over the specimen, is used. The holder is such that the specimen can be put in, in correct alignment, while in a cold bath. Two types of cameras giving variable temperatures are described. One, using cold gas blown over the specimen for cooling, permits putting the specimen in place under liquid nitrogen and keeping it at this temperature while it is aligned in the spectrometer. Rapid changes and accurate control in the range up to nearly room temperature are possible. A more versatile camera with a working range from 77°K to over 375°K uses liquid nitrogen as a coolant. Temperature control is by balancing the heat conduction along a tube with a heater near the specimen. This camera allows inserting the specimen at any chosen temperature in the working range but does not allow as rapid temperature changes in the cold gas camera.
21(1950); http://dx.doi.org/10.1063/1.1745579View Description Hide Description
A pulse type 1000‐v variable high voltage supply is described. This supply utilizes a 6BG6 tube to interrupt the current through a standard inductance.Rectification is by means of a CK1013 cold‐cathode rectifier which eliminates a well‐insulated filament transformer. The output is shunt regulated by means of a 2C53 tube. An advantage of these supplies is the small size 5×5×6 in. and the economy in cost. Standard parts are used throughout. Ripple at 1000 v is 80 mv. Regulation curves are given.
21(1950); http://dx.doi.org/10.1063/1.1745580View Description Hide Description
The fundamental thermistor equations are presented as functions only of electrical quantities and constants, and independent of temperature measurements. The universal curves allow one to predict the static characteristics of thermistors as circuit elements. The two most significant characteristics are the power coefficient of resistance, and the negative incremental resistance. These can be computed, or read from the universal curves, if the resistance and four parametric constants are known. These five values can be determined from six convenient measurements: the resistance and change of resistance with current at very low current, at the maximum voltage point, and at maximum current.
21(1950); http://dx.doi.org/10.1063/1.1745581View Description Hide Description
The thermistor sinusoidal and transient response characteristics are presented, as a function of the dissipation constant and incremental resistance. Equations are given for time constant, critical frequency, equivalent inductance, and Q. The construction of a low frequency oscillator is discussed, as well as possible non‐linear filter applications.