Volume 23, Issue 8, August 1952
Index of content:
23(1952); http://dx.doi.org/10.1063/1.1746317View Description Hide Description
The construction of a torsion pendulum with an improved method of alignment and minimized friction losses is described. An optical recordingsystem is employed, using a moving film camera with film velocity variable from 1 inch per hour to 60 inches per second to record amplitude decay of freely vibrating specimens. Simplicity of operation permits semiskilled personnel to obtain 6 to 10 records per hour of high damping Mn–Cu specimens. Sample records demonstrate how variations from 0.002 percent to over 33 percent in damping capacity may be measured at torsional stresses ranging from several hundred to about 20,000 psi.
23(1952); http://dx.doi.org/10.1063/1.1746319View Description Hide Description
A linear accelerator comprising 5 cylindrical cavities operating in the TM 010 mode on the divided power of a 10‐cm magnetron (nominally 1 megawatt) has produced electron beams up to 1.4 Mev. The first cavity is a phase buncher; the remaining ones, accelerators. Power is fed to the cavities through series tee connections to a master wave guide which is loaded with dielectric rods for adjusting the relative phases. The fields are of the order of 100 kilovolts per cm.
The maximum output has been 0.25 microampere average, 5 percent of the injected current. The accelerated electrons consist of a high energy bunch containing 50 percent of the total output current, with the remainder of the electrons spread over the lower energies. The energy of the bunched component can be varied widely by changing the magnetron voltage and by operating with fewer cavities.
This accelerator has proven a reliable research tool in a study of secondary electron emission of solids as a function of primary electron voltage over the range of 18 kilovolts to 1.4 Mev.
23(1952); http://dx.doi.org/10.1063/1.1746322View Description Hide Description
A precision megohm ratio unit consisting of 100 resistance units wound of manganin wire is described. Stability of the order of a few parts per million is achieved through careful construction. Variations due to temperature changes are reduced by operating the resistor in a circulating oil bath maintained at a temperature of 33°C, where the temperature coefficient of the resistor is essentially zero. The resistor is capable of continuous operation at a potential of 5 kv and intermittent operation up to 25 kv.
A general method is also reviewed for setting up accurate resistance ratios by comparing two groups of resistors in appropriate series and parallel arrangements. Employing this method and using the units of the described megohm ratio unit, high ratios, consistently accurate to one part per million, have been set up.
23(1952); http://dx.doi.org/10.1063/1.1746324View Description Hide Description
The combination of fast and slow phosphors with a single photomultiplier has coincidence and anticoincidence properties. The fast phosphor gives a sharp spike on the leading edge of the pulse due to the slow phosphor if a charged particle traverses both phosphors. This makes possible the construction of highly directional phosphors,phosphors responding only to neutral radiations, and phosphors free of wall effects. The transparency of sodium iodide (thallium) and stilbene to each other's radiations is demonstrated; these phosphors form a possible, though not ideal, pair.
23(1952); http://dx.doi.org/10.1063/1.1746326View Description Hide Description
The performance of the 6BN6 gated‐beam tube as a fast coincidence circuit for short positive pulses is investigated experimentally. Design considerations are discussed and curves of performance are shown. Resolving times better than 3×10−10 second have been observed with pulses from a mercury‐switch pulser. The circuit is simple and reliable in operation. It has been applied to coincidence measurement of pulses from photomultipliers.
23(1952); http://dx.doi.org/10.1063/1.1746329View Description Hide Description
23(1952); http://dx.doi.org/10.1063/1.1746331View Description Hide Description
A low temperature cell is described that permits spectralabsorption and emission measurements, as well as high energy irradiation of materials held at liquid helium temperatures. The liquid helium capacity of one liter is sufficient for up to 24 hours of continuous operation. Any substance whether it be solid, liquid, or gas at room temperatures can be studied. The all‐metal cell is entirely portable and can be employed with a number of instruments without modification. A sylphon bellows arrangement permits vertical displacement within the cell of the samples under study.
23(1952); http://dx.doi.org/10.1063/1.1746333View Description Hide Description
The use of radioactive Bi210 as a tracer in thin Bi films prepared by evaporation in a vacuum has resulted in a simple and direct method for the measurement of film thickness in the range 1–300A. The radioactivity of a standard, prepared by evaporating a weighable amount of active Bi on a piece of glass, is compared to that of a specimen of unknown thickness. For films 200A or less in thickness, the ratio of counting rates of the ``unknown'' and standard yields the mass of Bi on the unknown which, together with the film area (measured with a planimeter) and the density of Bi, yields the average Bi film thickness. Two levels of activity occurring in the standards were assigned after an electron diffraction study to a difference in structure of the Bi films. The method is accurate for extremely thin films (probable error is less than 4 percent if counting statistics are carried to a probable error of 3 percent), simple procedures and calculations are used, calibration is continuous, and the specimen is not altered in any way.
23(1952); http://dx.doi.org/10.1063/1.1746336View Description Hide Description
A method of timing particles, falling freely in a fluid medium, is described. The method utilizes the light reflected from a particle as it passes the fields of view of two photomultiplier cells. The method has been used in a study of shape and density effects on settling rates of particulate matter in air and in water.
23(1952); http://dx.doi.org/10.1063/1.1746338View Description Hide Description
An instrument is described for measuring the dynamic rheological properties of viscoelasticliquids and solids. The design of this instrument is based on that of Goldberg and Sandvik, but differs from theirs in that it is not limited to use at a point of resonance. Examples are given of results obtained by its use on a Newtonian liquid, a viscoelasticliquid, and a viscoelastic solid. A derivation of the equation governing the instrument when used for solids is given in an appendix.
23(1952); http://dx.doi.org/10.1063/1.1746340View Description Hide Description
A type of magnetic potential divider for shaping the magnetic field near a pole edge is discussed. This method of terminating a pole has been applied to a β‐ray spectroscope magnet recently constructed at Carnegie Institute of Technology. The maximum usable radius has been increased by about fifteen percent over the value without it. The method is applicable to many low field magnets.
- LABORATORY AND SHOP NOTES