Index of content:
Volume 27, Issue 10, October 1956
27(1956); http://dx.doi.org/10.1063/1.1715380View Description Hide Description
The more important properties of end‐window Geiger counters, side‐window proportional counters, and NaI(Tl) scintillation counters are reviewed and their performance in the 5 to 50 kvp x‐ray analysis region is compared. Proportional and scintillation counters have resolving times of the order of 0.25 μsec and virtually eliminate the nonlinearity limitation of the Geiger counter. They produce pulse amplitudes proportional to the energy of the x‐ray quanta, and thus simple pulse height discrimination techniques can be used to obtain greatly improved peak‐to‐background ratios with little loss of intensity of the characteristic x‐rays. They are as reliable and nearly as easy to use as Geiger counters. The electronic discrimination method is described and explained with some practical applications. For a given experimental arrangement, the measured intensity is determined by the quantum counting efficiency of the counter tube, and the peak‐to‐background ratio is determined by the spectral response of the detector‐discriminator combination.
27(1956); http://dx.doi.org/10.1063/1.1715381View Description Hide Description
A pulsed, cold‐cathode ion source has been developed to produce multiply charged ions for use in a linear accelerator. Milliampere currents of such ions as He4 2+, C12 2+, N14 3+, O16 3+, Ne20 3+, and smaller currents of S32 5+ and A40 6+ have been produced in focused beams.
Mass spectra of the various ionization states produced by the arc are presented. Constructional details of the source and the associated electronic equipment are given, and operating characteristics of the source and ion extraction are discussed.
27(1956); http://dx.doi.org/10.1063/1.1715382View Description Hide Description
Transpiration of gas in the inlet tube between the sample reservoir and the ion source is eliminated if the thermal gradient occurs in the low‐pressure region between the leak and the ion source. Since transpiration was found to be a total pressure effect, sensitivity response is not affected if gas mixtures and calibrating gases are run at the same pressure, even though transpiration is present.
27(1956); http://dx.doi.org/10.1063/1.1715383View Description Hide Description
The construction and operation of a compact automatic Toepler pump of the mercury piston type which has many advantages over previous designs are described. The mercury motion is controlled by a switching relay activated by electrodes so placed that at the ultimate of its stroke the air leak to the lower mercury reservoir is closed and an oil vacuum pump is started to evacuate it, and at the bottom of the stroke the vacuum pump is stopped and the air leak opened to allow the mercury to rise in the piston chamber. Pumping action is effected by two glass valves with conically ground seats in series in the gas inlet and outlet tubes.
27(1956); http://dx.doi.org/10.1063/1.1715384View Description Hide Description
A xenon‐filled electronic flash tube with a cylindrical lighted volume of 1.2‐mm diam and 6‐mm length has been developed for use as a small source in optical systems, such as the schlieren and interferometer types. The lamp has been operated at 6000 flashes per second from a hydrogen‐thyratron driver circuit to record the density in air flow in an interferometer. Flash duration varies from 0.3 to 3 μsec, depending upon the circuit conditions. A peak brightness of 10 million candlepower per sq cm has been achieved.
27(1956); http://dx.doi.org/10.1063/1.1715385View Description Hide Description
It is shown that the heat flow in a single homogeneous medium of thermal conductivity σ(T), between boundaries at temperatures T 2 and T 1 is the same as that for the same geometry in a medium of unit thermal conductivity and boundary ``thermal potentials'' Σ(T 2) and Σ(T 1), where . Some applications are given, as well as a short table of Σ(T) for common cryogenic temperatures and materials.
27(1956); http://dx.doi.org/10.1063/1.1715386View Description Hide Description
A direct method of measuring the injection ratio of rectifying contacts is described. With this method, the injection ratio is determined by comparing the area of a current injection pulse with the area of the resulting hole storage pulse as observed on an oscilloscope screen.
27(1956); http://dx.doi.org/10.1063/1.1715387View Description Hide Description
To assess the effectiveness of plastic phosphors in the reduction of Compton effect in scintillation spectrometers, a spectrometer with a small NaI(Tl) crystal surrounded by a large plastic phosphor has been constructed. The output from the plastic phosphor is put in anticoincidence with the output from the NaI(Tl) crystal. A reduction of Compton contribution in the scintillation spectrum to about 35% of original value is obtained. Comparison of results with, and without, the shield enables certain spectral features to be identified. Possible applications are discussed. Transistor anticoincidence circuits are used.
27(1956); http://dx.doi.org/10.1063/1.1715388View Description Hide Description
This paper describes the utilization of a simple universal vacuum joint, as the support for a magnetic shield type of deflector in the form of a cylindrical soft iron pipe. Variable insertion of this pipe and change of direction in azimuth permit the extraction, with up to 70% efficiency, of all the microtron orbits, making available, in this case, electrons between 1 and 4 Mev energy, in steps of 0.5 Mev. Radiation intensity, or electron current, from any particular orbit, up to the limit determined by the peak for that orbit, may be set at any desired level by small changes in the insertion of the deflector.
27(1956); http://dx.doi.org/10.1063/1.1715389View Description Hide Description
An analog method is described for solving some of the mechanical problems associated with the recording of ballistocardiograms. An L‐C‐R circuit is used as the analog of the body coupled to a fixed table. It is concluded that no technical obstacles exist to the widespread use of ballistocardiography, through equipment similar to that described here. Clinical problems in the interpretation are not discussed.
Application of a Statistical Correlation Function for Determining Daughter Half‐Lives in the Range 10−4 to 10−1 Second27(1956); http://dx.doi.org/10.1063/1.1715390View Description Hide Description
Discussed herein is the application of a statistical correlation method [Goldanski and Podgoretski, Doklady Akad. Nauk. S.S.S.R. 100, 237 (1955)] to the determination of the lifetime of an activity in equilibrium with a long‐lived parent activity. It is shown that this method has a definite advantage over delayed‐coincidence methods when the lifetime to be measured is of the order of 10−4 sec or longer. An electronic apparatus designed to perform the operations required by the statistical correlation method is described. This equipment has been used for a new determination, 0.293±0.020×10−3 sec, for the half‐life of Y88m .
27(1956); http://dx.doi.org/10.1063/1.1715391View Description Hide Description
An absolute anemometer suitable for measurements in gases under a wide variety of conditions is described. The instrument, based on an idea first used by Kovasznay, utilizes the temperature fluctuations in the wake of a sinusoidally heated fine wire as tracer. A second hot‐wire detects the fluctuations, and the phase change between two successive positions of this wire can be used to determine the free stream velocity. The technique is shown to have good spatial resolution and an accuracy better than 2% in the velocity range 40–400 cm/sec. Sources of error and various experimental conditions are discussed in detail.
27(1956); http://dx.doi.org/10.1063/1.1715392View Description Hide Description
The fringing field of a magnetic sector analyzer produces a curved image of a line object. This is because rays off the median plane traverse a slightly different fringing field from that traversed by those on the median plane. In this paper, the shape of the image is calculated for a general paraxial ray traveling in a plane inclined to the median plane, under the assumption that the fringing field is two‐dimensional. It is found that the image is not only curved, but is broadened by an amount depending on the exact nature of the fringing field. In the special case in which all paraxial rays travel in planes parallel to the median plane, and in which the analyzer is symmetrical, a straight‐line object is imaged in a line having a radius of curvature of R/2, where R is the sector radius. In this special case, the shape of the image is independent of the exact nature of the fringing field.
27(1956); http://dx.doi.org/10.1063/1.1715393View Description Hide Description
A dual objective optical micrometer for strain measurements of up to 8% is described. In this instrument the images of two reference marks on a specimen are brought to a focus in one field on the reticle scale of a filar micrometer eyepiece. The relative displacement of the images is measured by the eyepiece and the readings are reduced to strain by the use of a calibrated magnification factor. Since only the separation distance of the images is of interest, the exact position of the images in the field is not critical. No external point of reference on the instrument mount is necessary, hence alignment problems are minimized and the instrument may be moved between readings. The described calibration procedure shows that the least count of the instrument with a 2.64‐in. gauge length is 42×10−6 in./in. and measurements can be made with an accuracy of ±13×10−6 in./in.
27(1956); http://dx.doi.org/10.1063/1.1715394View Description Hide Description
A method has been found for solving an algebraic equation for a real or complex root on an analog computer (electronic differential analyzer). The computer connections are arranged so that the root locations may be shifted along the real axis of the complex plane by adjusting a ganged potentiometer. A maintained transient is indication that a root has been shifted to the imaginary axis, allowing the real and imaginary parts of the root to be read. The accuracy is reduced when a second nearby root is present; however, poor tracking in the ganged potentiometer has about the same effect as an improper setting of a coefficient of the equation.
27(1956); http://dx.doi.org/10.1063/1.1715395View Description Hide Description
Two organic scintillating solutions, containing the same total number of electrons, but differing by more than a fourfold in the total number of hydrogen atoms, are used to detectneutrons in the presence of a gamma‐ray background. By suitable adjustments of their luminescence their pulse‐height spectra can be made to coincide in the presence of gamma radiation; hence by subtracting the spectrum of one solution from that of the other it is possible to obtain the pulse‐height spectrum due to the excess of proton recoils in the latter. Although refinements in the interpretation of its response must wait for basic information on the luminescent efficiency of both protons and electrons of various energy, this detector is simple to build, stable in time, isotropic in response, and suitable to evaluate radiation hazards in the presence of mixed radiations. Typical responses to gamma radiation, D‐D, and D‐T neutrons are presented.
27(1956); http://dx.doi.org/10.1063/1.1715396View Description Hide Description
A high‐temperature x‐ray powder diffraction camera has been designed and constructed. The design specifications were as follows: (1) to permit operation at temperatures up to at least 2000°C, in a high vacuum, (2) to be able to work in controlled atmosphere at somewhat lower temperatures, and (3) to permit successive exposures of the specimen held at temperature. The completed apparatus appears to meet these requirements.
27(1956); http://dx.doi.org/10.1063/1.1715397View Description Hide Description
A simple method for calibration of calorimeters used in solutioncalorimetry has been devised. The heat of vaporization of water is taken as a chemical standard for this process and shown to be unaffected by the speed and pressure employed in pumping it out of the evaporator described.
27(1956); http://dx.doi.org/10.1063/1.1715398View Description Hide Description
A device has been developed for the alpha counting of solid materials of extremely low activity. It utilizes two identical detectors of novel design inside a common gas envelope and a programming mechanism to permit the automatic alternation of samples between the counters. The detectors are rectangular, screen wall, proportional counters which operate at an average background of 3.8 counts per hour. Thin, flat samples of 141 cm2 area may be accommodated.