Volume 24, Issue 8, August 1953
Index of content:
24(1953); http://dx.doi.org/10.1063/1.1770793View Description Hide Description
Here described is a high stability UHF source which is used as a microwavefrequency standard.Frequency measurement requires the use of a calibrated communication receiver to measure the difference between a known harmonic of the UHF source and the unknown microwave frequency. Frequencies as high as 27 kmc have been measured and the frequency stability is approximately 1 part per million.
24(1953); http://dx.doi.org/10.1063/1.1770794View Description Hide Description
A quantitative study of distortion in nuclear emulsions, based on a method of measuring and calculating distortion to known accuracy, has been made. The method has been applied to a study of variations in fixing washing and drying procedures. Distortion reduction up to one‐half was found to occur with the use of a concentrated sodium sulfate hypo solution. The reduction of vertical shrinkage and the use of thicker emulsions also appear to reduce distortion. The method of distortion measurement is accurate to better than 10 percent using sea‐level cosmic‐ray background tracks as a distortion indicator.
24(1953); http://dx.doi.org/10.1063/1.1770795View Description Hide Description
Equations are obtained for the focusing by a system of two magnets with nonuniform fields such that n>0 in the first magnet and n<0 in the second. For each magnet a lens equation is set up, and then the system of the two magnets is itself treated as a thick lens. It is shown that for very large n, there is in general one object distance for which double focusing takes place. The theory is also applied to deflecting systems with moderate values of n, and it is shown that the entrance and exit angles can be reduced to values of the order of 20° even for small deflections (20°) of the charged particle.
24(1953); http://dx.doi.org/10.1063/1.1770796View Description Hide Description
Described is a simple device for measuring the total mass and total charge of a powder cloud. The instrument consists of a sensitive electrometer circuit and a metallic collection chamber. The powder is collected on a filter paper sealed between two aluminum tubes which, if their length is three times greater than their diameter, provides sufficient shielding of the charge collected on the filter paper to justify assumption of an ``Ice Pail.'' The shielding error is less than 0.5 percent. A null‐reading circuit is used with a Cenco electrometer. The necessary counter voltage for the null‐reading circuit is supplied by a potentiometer circuit, and a vacuum‐tube voltmeter is used to measure the voltage. Because it is desired to obtain explicitly the mass and charge of a given cloud of particles rather than the ratio of charge to mass, enough mass must be collected for weighing and, in addition, the total charge measured. This is to be accomplished without disturbing appreciably the normal flow rate of powder from the aerosol generator.
24(1953); http://dx.doi.org/10.1063/1.1770797View Description Hide Description
The theory of the fixed frequency cyclotron is developed and illustrated at each step with numerical results for an example. The conditions for ion transmission with respect to horizontal motion are derived, and it is shown that they are satisfied by all ions in most cyclotrons. A more correct and more complete focusing formula than that found in the literature is derived and used to integrate the vertical motion. It is characterized by a region in which the net force is defocusing causing the ions to diverge exponentially from the median plane.
The theory is applied to find the maximum energy attainable as a function of dee voltage and magnetic field fall‐off. The problem of orbital precession is treated quite generally, and equations for calculating the center of curvature as a function of time are derived. Some considerations in the magnetic field design are presented. A proof is given that ions experience strong radial grouping. The effect of ion source ``feelers'' is discussed.
24(1953); http://dx.doi.org/10.1063/1.1770798View Description Hide Description
Experiments on cylindrical ionization chambers operating at high‐current densities showed that more voltage was required to produce saturation when the central electrode was negative than when it was positive with respect to the outer cylinder. An analysis is made which indicates this phenomenon might be explained by the positive ion space charge. Under the condition of uniform ionization density within the chamber, a relation is developed for the ratio of voltages required to overcome the space charge for the two collection polarities. Also, approximate analyses are made for the cases of the ionizing particles originating at the surface of either electrode. There is only rough agreement between the calculations and experimental data.
24(1953); http://dx.doi.org/10.1063/1.1770799View Description Hide Description
A pulsed cold cathodeion source, giving peak proton currents in excess of one milliampere and average currents up to 50 microamperes, is described. The source is based on the Penning discharge using axial extraction of the ions. Some discussion of the discharge mechanism and the role of the cathode material is included. Installation and operation of this source in the Berkeley 4‐mv Van de Graaff and its performance in that machine is discussed.
24(1953); http://dx.doi.org/10.1063/1.1770800View Description Hide Description
A pulsed x‐ray method for measuring the decay time of luminescence of organic scintillators is described. The design of the x‐ray tube and the arrangement of the accompanying electrical apparatus is outlined. Particular care was taken to minimize the time dispersion effect of the apparatus and to increase the pulse size of the scintillation in order that statistical fluctuations would be minimized. The luminescence decay curves for anthracene, stilbene, and diphenylacetylene scintillators are shown.
24(1953); http://dx.doi.org/10.1063/1.1770802View Description Hide Description
A modified Rossi circuit has been extended to a form in which it allows the recording of as many as six coincidence and four anticoincidence pulses with a resolving time of 3×10−9 second. All input pulses are negative and of any amplitude > 3 volts. A complete schematic is given of the circuit which has operated for six months without possibility of adjustment.
24(1953); http://dx.doi.org/10.1063/1.1770803View Description Hide Description
A set of accurate scale models has been developed for use in studies of the structures of amino acids,peptides, and proteins. Models representing atoms or groups of atoms built from hard wood to the scale 1 in.=1A are connected by a clamping device which maintains desired molecular configurations. These accurate models have been used as substitutes for calculation in investigations of the probable configuration of the polypeptide chain in proteins. Analogous models constructed of rubber‐like plastic to the scale 1 in.=2A and connected by snap fasteners are designed for qualitative studies of protein structure.
The Adaptation of the Norelco High‐Angle Spectrometer to the Measurement of the X‐Ray Diffraction of Fibers24(1953); http://dx.doi.org/10.1063/1.1770804View Description Hide Description
The use of the North American Philips spectrometer for the radial and azimuthal scanning of the transmitted x‐ray diffraction of rayon tire yarns involved a change in the slit system from convergent to divergent, design of a rotating sample holder, a reproducible sample preparation and positioning, and calibration and control checks which guarantee that the x‐ray intensity remain constant to ±2 percent as automatically recorded by the rate meter. Typical data are presented and compared with data obtained photographically.
24(1953); http://dx.doi.org/10.1063/1.1770805View Description Hide Description
A circuit is described for amplitude stabilization of a regenerative detector for use in nuclear magnetic resonance experiments. A remotely located two‐stage dc amplifier converts variations in oscillator grid leak voltage into compensating changes in plate supply potential. The stabilizer circuit adds little stray capacity and does not appreciably affect the oscillator design. The level of oscillation is adjustable from zero to approximately 0.3 volt peak rf signal at the detector grid. Once set, it remains essentially constant over the tuning range (4.8 to 6.3 Mc/sec). A noise figure of 2 or better is obtained. Circuit parameters for the regenerative detector and the stabilizer are given.
24(1953); http://dx.doi.org/10.1063/1.1770806View Description Hide Description
Tests were made on a 27‐megacycle, inductively coupled, radio‐frequency ion source standardized in design for use at the Los Alamos Scientific Laboratory. Total deuteron beam current and probe current characteristics were found, in tests made before installation, as a function of gas pressure of the discharge, rf power input, probe voltage, and extraction system geometry to determine the best values of these variables to use. These characteristics showed a maximum total beam current of 600 μa to be obtained with roughly 200 watts of rf power input. Tests made after installation in an accelerator gave 92 percent monatomic deuteron beams of up to 80 percent the strength found under similar conditions in the tests made before installation. A platinum‐coated envelope gave a triatomic beam of about one‐eighth the strength of the monatomic beam obtained with an uncoated envelope. The source has been used at 150‐psi external pressure in a Van de Graaff tank.
24(1953); http://dx.doi.org/10.1063/1.1770807View Description Hide Description
A regulator has been built to hold the accelerating potential of a Cockcroft‐Walton accelerator to within 0.015 percent of any preset potential in the range of 20–250 kilovolts. The regulator is a degenerative type where the error signal is amplified by a galvanometer‐phototube dcamplifiersystem which controls a variable impedance transformer in series with the high‐voltage power transformer primary. The regulator is very simple to put into operation. Its action is independent of the potential desired; therefore the range of operation could easily be extended.
24(1953); http://dx.doi.org/10.1063/1.1770809View Description Hide Description
Geiger‐Müller tubes have been constructed which use visibly transparent nonmetallic electrically conducting films as cathodes. These tubes have the following advantages: (1) a long plateau; (2) no photosensitivity; (3) an almost indefinite operating life; (4) immunity from damage arising from heavy discharges; (5) straightforward filling procedure devoid of any ``passivising'' or saturating techniques; (6) good response to ionizing radiation throughout the length of the counter; (7) freedom from use of critical materials such as copper or stainless steel No. 446 which require special cleaning and polishing procedures.
24(1953); http://dx.doi.org/10.1063/1.1770810View Description Hide Description
Apparatus and procedures are described for the high‐resolution measurement of resonance shifts and for the observation of broad line shapes. Included are a discussion of magnet design and a summary of empirical results obtained in the construction and field homogenization of two large permanent magnets. Narrow, complex resonance lines, with components separated by as little as a milligauss, are resolved by a combination of homogeneous applied magnetic field, small samples, and slow‐sweep field modulation. Broad absorption line shapes are plotted at fixed frequency by a system incorporating a regenerative oscillator, a narrow band amplifier, a recording potentiometer, and an electronic control for varying the applied magnetic field linearly in time. A simple cryostat provides stable temperatures from 85° to 500°K. In the case of samples with short spin‐lattice relaxation times, such as the metals, improved signal‐to‐noise ratios are obtained by adjusting the oscillator to super‐regenerative operation and observing the frequency modulation associated with the dispersive component of the resonance.
24(1953); http://dx.doi.org/10.1063/1.1770811View Description Hide Description
24(1953); http://dx.doi.org/10.1063/1.1770812View Description Hide Description
A fast neutron coincidence spectrometer of high efficiency has been constructed applicable to the approximate energy range of 1 Mev to 20 Mev. Design considerations and the working model are discussed.
To illustrate the operation of such an instrument, the line spectra of the gamma‐rays from Co60 and of 3.3‐ Mev D‐D Neutrons are presented. Discrimination between gamma‐rays and neutrons is obtained by utilizing the difference in the time of flight of the two particles. In conclusion possible improvements are suggested.
24(1953); http://dx.doi.org/10.1063/1.1770813View Description Hide Description
A differential manometer which photographically records pressure as a function of time is described. A fluorescent lamp in conjunction with a simple louver system provides a sharp image of the interface between indicator liquid and gas. The instrument can be used to study reactions whose half‐lives range from a few seconds to several hours.