Index of content:
Volume 25, Issue 7, July 1954
25(1954); http://dx.doi.org/10.1063/1.1771146View Description Hide Description
A narrow, well‐collimated beam of soft x‐rays has been used for the purpose of measuring distortion in cloud chambers. The path of the x‐ray beam in the cloud chamber is made visible by the ionization of the short‐range photoelectrons. The main advantage of this method is that it can be used in the presence of a magnetic field. A limitation of the method is the finite width of the track. In argon it was not difficult to get a track 0.06 cm wide, allowing measurement to 0.004 cm. The appearance of the beam in both a diffusion and an expansion chamber is described. In addition to its use in measuring distortion, its applications to measuring rates of droplet growth and length of cloud chamber sensitive times are discussed.
25(1954); http://dx.doi.org/10.1063/1.1771147View Description Hide Description
A simple and inexpensive radiationdetection device has been constructed sensitive to gamma fluxes in the range from a few to a few hundred roentgen per hour. Thus one can distinguish between dangerous and relatively safe radiation levels such as one might expect from radiological warfare, low level atomic blasts, or from ``rigged'' hydrogen bomb explosions.
A typical device consists of two gamma‐activated crystals in a light‐tight container which can be held close to one eye to exclude external light. A radium‐activated phosphorescent material is included for light intensity comparison purposes. The more sensitive crystal can be discerned in a gamma radiation of a few R/hr, and its light output matches that of the fluorescent material at levels of 20 to 30 R/hr. The light from the less sensitive crystal matches that of the phosphorescent material at about 100 R/hr.
25(1954); http://dx.doi.org/10.1063/1.1771148View Description Hide Description
A automatic system, employing a metal bellows, was designed and constructed for the purpose of studying the catalytic combination of hydrogen and oxygen on a submerged catalyst in a stirred solution. The reaction was carried out under conditions of constant temperature, pressure, and volume. This system can be used to study the reaction for unlimited lengths of time; it is completely automatic, and can be used to study reaction rates quantitatively. It can be modified for the study of other reactions.
25(1954); http://dx.doi.org/10.1063/1.1771149View Description Hide Description
A description is given of an instrument to determine the temperatures of rapidly varying flames. This instrument uses a photoelectric modification of the line‐reversal method, wherein the light beam from a temperature‐variable incandescent source is split into two monochromatic beams by means of mirrors and filters to impinge upon 931A multiplier phototubes. The phototube signals are sent to a mu‐bridge null detector, whose output is fed to an oscilloscope for photography of results. The instrument can detect 25°K temperature differences between about 1540° and 3100°K and three millisecond time differences. Typical time‐temperature results are given, along with the analytical procedures used.
25(1954); http://dx.doi.org/10.1063/1.1771150View Description Hide Description
A double‐focusing mass spectrograph for the analysis of low concentrations of impurities in solids has been developed. The instrument is of the Mattauch type and is designed for either photographic or electrical ion detection. The construction and performance of the instrument are discussed. Bulk concentrations below 0.1 part per million, and surface contaminants of less than 0.1 monolayer, can be detected in short exposures using a photographic plate.
25(1954); http://dx.doi.org/10.1063/1.1771151View Description Hide Description
This paper describes a method for presenting on a fluorescent screen the time varying velocity and density modulation of an electron beam, modulated at a frequency of 3000 Mc, as it emerges from the cavities of a klystron, the wave guide in a traveling wave tube, or similar structures.
In order to deflect an electron beam at this frequency, each pair of the usual deflection plates is replaced by a two‐wire transmission line, resonant at the beam modulating frequency. Under proper phase conditions a circular trace is obtained, where one complete revolution of the beam corresponds to exactly one complete cycle of events on the beam at the point of its deflection. Since electrons having different velocities will be deflected differently, a complete velocity and density picture of the beam at a given point of interest is obtained.
This is demonstrated by observing the bunching action in the drift region of a velocity modulated electron beam.
A High‐Speed Air‐Driven Shutter for Controlling Exposures to a Convergent Beam of High‐Intensity Thermal Radiation25(1954); http://dx.doi.org/10.1063/1.1771152View Description Hide Description
A high‐speed two‐bladed shutter was built to deliver square‐wave pulses of energy from a converging light beam with an average intensity of about 5 cal/cm2/sec, and a diameter of 3 in., in the plane of the shutter. The blades open or close in less than 0.01 sec. They move in the same direction, so that all parts of the target are exposed for very nearly the same length of time. The shutter makes use of a new type of air cylinder which is suitable for driving high‐speed mechanisms, and has a dynamic shock absorbing system to stop the moving parts in a short distance with little rebound. It is electrically controlled and fully automatic. Exposure times may be varied from 0.07 to 5 sec by setting an electronic interval timer. Longer exposures are controlled manually. To date it has been used more than 15 000 times and has required only two overhauls.
25(1954); http://dx.doi.org/10.1063/1.1771153View Description Hide Description
Modifications of an electron impact ion source for a 60° mass spectrometer are described. The changes consist of (1) an almost completely enclosed ionization region which greatly reduces the contribution of ions from thermal decomposition products on the filament and reduces the contribution of background peaks and (2) a built‐in aligned and stronger magnetic field which eliminates sourcemagnet adjustment, collimates the electron beam better and permits the use of a higher repeller voltage thereby increasing the collection efficiency for ions with initial kinetic energy. The performance of the source is described.
25(1954); http://dx.doi.org/10.1063/1.1771154View Description Hide Description
An energy‐analyzing detector of the pair‐spectrometer type has been used to observe the angular distribution of bremsstrahlung produced by 17‐Mev monokinetic electrons in molybdenumbetatron targets. Distributions were measured for gamma‐ray energies of 7, 11, and 15 Mev, for each of three targets whose thicknesses were approximately 2, 5, and 21 mils. Reproducible variations of spectrum with angle were found, the variations becoming more pronounced with increasing target thickness. The experimental distributions are compared here with the theoretical distributions of Schiff and Lawson, the former being significantly narrower. Axial relative spectra are also presented.
25(1954); http://dx.doi.org/10.1063/1.1771155View Description Hide Description
A null instrument for the measurement of intense beams of radiant energy (from about 5 to 300 cal cm−2 sec−1) is described. The instrument utilizes two identical hollow blackbody receivers whose walls are provided with coils carrying a stream of cooling water. The inflowing water divides equally between each chamber. The temperature difference between the two outflows is determined by a differential thermopile. One of the chambers is exposed to the radiant energy beam, while the other chamber is heated electrically until a null is obtained on a galvanometer connected to the differential thermopile. It is shown that measurements of the electrical power dissipated in the chambers under null conditions and a measurement of the aperture areas of the chambers is sufficient to give the radiant flux density.
25(1954); http://dx.doi.org/10.1063/1.1771156View Description Hide Description
A liquid scintillation counter suitable for measuring H3 and C14 activities is described. A photomultiplier tube with a high sensitivity and signal‐to‐noise ratio is used, and is cooled to −10°C to further reduce the tube noise background. The phototube is mounted in a chassis containing its preamplifier, and the pulses are then led to a high‐stability amplifier scaler. The radioactive materials are dissolved in aliquots of a xylene solution containing p‐terphenyl (1.9 g/liter) and diphenylhexatriene (0.020 g/liter). Substances which quench the scintillations or color the scintillator solution cannot be counted by this method. A minimum of 4.3×10−10 curie of H3, or 1.8×10−11 curie of C14 may be measured by the counter.
25(1954); http://dx.doi.org/10.1063/1.1771157View Description Hide Description
A machine is described which solves the equations of motion for a charged particle in a two‐dimensional magnetic field. Its accuracy is not limited by the steepness or the asymmetry of the field. Guided by remote control, the mechanism moves continuously over a contour map just as a particle would move through the field itself. The operator matches position, or local field value, with a parameter of the steering control, or curvature of the trajectory. He is able to interpolate continuously between lines of constant field value, thus speeding the process of solution and increasing the accuracy. In five minutes a 40‐inch orbit can be drawn over which the field values vary through 23 000 gauss. Five minutes are required for introducing initial conditions, including momentum. For regions in which the field is more than half maximum, the orbits of the mechanical particle and the electromagnetic particle coincide in their radii of curvature to within 0.5 percent. Accuracy is less for low field values. The performance characteristics have been determined from a working instrument which is fully described. An alternative computer design to the one already in use is also discussed.
25(1954); http://dx.doi.org/10.1063/1.1771158View Description Hide Description
An interferometer for measuring gas density as a function of time during a transient flow is described. This is an absolute instrument capable of measuring density changes over an approximate range from 10−2 to 10−5 g/cm3. Since the instrument requires only small optical components, it is relatively simple and inexpensive to construct.
25(1954); http://dx.doi.org/10.1063/1.1771159View Description Hide Description
The construction of a specimen holder and furnace for use as an auxilary piece of equipment with a North American Phillips Company high angle goniometer is described. The apparatus can be used for obtaining x‐ray diffraction patterns at temperatures up to 1600°C, and at pressures in the range of 2×10−6 to 3×10−5 mm of mercury.
25(1954); http://dx.doi.org/10.1063/1.1771160View Description Hide Description
A fast neutrondetector has been prepared by suspending ZnS phosphorpowder in ``Bio‐Plastic''. The detector can be easily prepared using only simple laboratory equipment. In the presence of the gamma rays from a Ra‐Be source, the neutrons from the source can be counted using this detector with about one percent efficiency.
25(1954); http://dx.doi.org/10.1063/1.1771161View Description Hide Description
A cylindrical chamber, 8.5 in. in length and 8.5 in. in diameter, containing two sets of parallel multiple‐wire grids has been built and operated as a proportional counter for fast neutrondetection. The collecting wires are 0.001 in. in diameter. Filled with pure CH4 at about atmospheric pressure and operated at 3400 v, it serves to detectneutrons up to 10 Mev, with an average efficiency of 0.17 percent. The energy distribution of neutrons from 0.3 to 3 Mev can be estimated. The multiple‐wire counter is completely insensitive to gamma radiation under these operating conditions. This instrument has been used to make a survey of fast neutrons around the U. C. L. A. cyclotron. At higher pressures the instrument can presumably be used to measure neutron spectra of higher energy.
25(1954); http://dx.doi.org/10.1063/1.1771162View Description Hide Description
The electron beam of the National Bureau of Standards betatron has been removed by the use of a pulsed magnetic extractor. The electrons were spiraled outward into the extractor that had created a sharp step magnetic field canceling the betatron guiding field for 19° of azimuth. This step magnetic field was produced by a current pulse flowing in an array of parallel wires. The removed beam was well focused. Initial testing up to 24 Mev showed that about 60 percent of the accelerated electrons were extracted in the beam.
25(1954); http://dx.doi.org/10.1063/1.1771163View Description Hide Description
A new type of goniometer has been designed and constructed for neutron diffraction analyses, and particularly for single‐crystal studies. The neutrondetecting system employs a scintillation crystal and photomultiplier, or a miniature high‐pressure BF3 counter; and the detector and diffracting crystal are located within a cylindrical shielded region, in such a manner that the moving parts are entirely free of heavy shielding elements. When a miniaturized BF3 counter is used, no shielding except B4C in plastic immediately surrounding the counter is required. Rotations are accomplished by servo‐controlled motors, with selsyn indicators coupled to an external servo system. This servo control is designed to permit future introduction of automatic crystal and detector setting. The system is small and inexpensive, and has the particular advantage of readily permitting three‐dimensional diffraction measurements.
25(1954); http://dx.doi.org/10.1063/1.1771164View Description Hide Description
A photoelectronic apparatus has been developed which makes possible the continuous simultaneous measurement of the depth of penetration, the speed, and the deceleration of a nondeforming small‐caliber projectile dring armor penetration. The basic operating principle of the apparatus is to have the flight path of the projectile pass perpendicularly through a thin parallel light beam of uniform intensity which activates a vacuum‐type phototube. The output of the phototube is proportional to position or the depth of penetration under appropriate geometry. A voltage signal derived from the phototube circuit is preamplified and differentiated to give voltage signals proportional to speed and deceleration. Permanent records of the signals are made by photography of cathode‐ray tube traces and analyzed. The apparatus is capable of measuring real transient accelerations or decelerations as great as 108 ft/sec2 (3×109 cm/sec2) with average error of 2 percent. Possible applications of the system to mechanical problems as an accelerometer, speed, and displacement measuring device are indicated.