Index of content:
Volume 26, Issue 12, December 1955
26(1955); http://dx.doi.org/10.1063/1.1715197View Description Hide Description
The theory of conditioned probabilities is applied to the problem of momentum, charge, and mass determination from Coulomb scattered tracks in a magnetic field. An optimum procedure is derived, which makes use of both random and systematic track curvature.
It is shown that application of this procedure to a highly relativistic nuclear emulsion track of 1‐cm length in a field of 300 000 gauss will yield the momentum information of a 9.2‐cm track in zero field. If the scattering information were neglected, the effective length of the track would be 8.2 cm. At 100 000 gauss, the improvement in effective track length due to the inclusion of scattering information is shown to be by a factor of 2.3.
26(1955); http://dx.doi.org/10.1063/1.1715198View Description Hide Description
Over the last few years, a method has been developed for obtaining ionization probability curves with essentially monoenergetic electrons. A retarding potential is applied to the electron beam to yield an energy distribution with a sharp low‐energy limit. By varying the retarding potential slightly, a new low‐energy limit of the distribution can be selected. The difference in the ionization produced in the two cases is ionization by those electrons with a small energy spread selected from the original distribution. By pulsing the electrons and ions, it is possible to eliminate the adverse effect of the ion‐drawout field on the electron energy. With this retarding potential difference (RPD) method, a detailed analysis of ionization probability curves is possible. A full description of this method is given in this paper with a discussion of its advantages and limitations. The mass spectrometer used in this series of studies is described, particular attention being given to a description of the ion source. The various electrodes of the electron beam slit system are described in terms of their influence on the electron energy, and on the shapes of ionization probability curves.
26(1955); http://dx.doi.org/10.1063/1.1715199View Description Hide Description
Two flip coils are rotated through 180° to give a precision measurement of the ratio of separate magnetic fields. The output voltages are connected in opposition and balanced in a resistance potentiometer using a sensitive galvanometer as a null indicator. The ratio of the fields is a linear function of the potentiometer setting. In fields of several thousand gauss and using a common shaft for the coils, readings were reproducible to within 0.01%. With separate selsyn motor drives the precision was 0.02%.
26(1955); http://dx.doi.org/10.1063/1.1715200View Description Hide Description
The construction and design principles of a focusing atomic beam apparatus are reported. This apparatus, which is of the radio‐frequency magnetic resonance type and which has been used to measure the spins and hyperfine splittings of four radioactive nuclides as reported elsewhere, utilizes the six‐pole focusing magnet of Friedburg and Paul and of Korsunskii and Fogel but in a way which avoids velocity aberrations. From an analysis of the optimization of intensities in the conventional and the focusing types of atomic beam apparatus it is concluded that the latter has an advantage of a factor of roughly twenty‐five.
26(1955); http://dx.doi.org/10.1063/1.1715201View Description Hide Description
An electronic system is described for modulating the beam intensity of a cathode‐ray oscilloscope in such a manner as to reduce variations in trace brightness due to differences in writing speed. A voltage derived by a push‐pull differentiating amplifier from the vertical deflection signal is added to the instrument's intensity‐control voltage. By use of this system, improved photographs of wave forms can be obtained.
26(1955); http://dx.doi.org/10.1063/1.1715202View Description Hide Description
This paper describes in some detail a radiometer for measurement of ultraviolet radiation from sun and sky. As designed, the instrument is selectively responsive to a narrow band of wavelengths and is capable of detecting and measuring low‐energy levels. The radiometer consists of a photoelectric sensing element, an ultraviolet transmitting window, and a dry‐cell battery mounted in a small housing. Calibration of the instrument shows satisfactory agreement with Lambert's cosine law and a linear relationship between energy flux and output of the phototube. For recording the signal from the radiometer the output is fed to a direct currentamplifier which in turn drives a recorder. A typical recording for a cloudy day is presented.
26(1955); http://dx.doi.org/10.1063/1.1715203View Description Hide Description
A rotary device was developed to produce uniform drops of liquids in the diameter range of 50 to 700 microns. A horizontally rotating blade detaches drops in a steady stream of regular trajectory, from a stabilized liquid mass fed under constant head through a stationary capillary. Its operation is described, and performance characteristics with oil and aqueous test solutions are given. Drop size is calculated from the mass of drops emitted in a given interval at a known generation frequency. Variation in size of individual drops,electrostatic effects, and some uses for the machine are noted.
26(1955); http://dx.doi.org/10.1063/1.1715204View Description Hide Description
Characteristics are described for the collection of cyclotron beams in a conducting medium separated from the accelerator by an insulating window. The measuredcurrent can be interpreted in terms of an absolute beam current if charge displacement within the insulator is taken into account. Collection of the beam in this manner is particularly advantageous in a region external to the cyclotron magnetic field where stray currents due to air ionization and secondary electron emission are pronounced. A comparison is made between particle energies determined by the charge‐input method and by calorimetry. A simplified arrangement for particle range determination is described.
26(1955); http://dx.doi.org/10.1063/1.1715206View Description Hide Description
A timing system is described which provides sharp, one‐millisecond, electric pulses at one‐second intervals. These pulses are used to actuate a trigger circuit which is used to control one or more relays. The accuracy of the time scale can easily be checked against radio station WWV. Auxiliary apparatus can be controlled for time intervals of any even number of seconds. The accuracy of this interval can be determined to better than one millisecond. This system has proved to be particularly useful in calorimetry.
26(1955); http://dx.doi.org/10.1063/1.1715207View Description Hide Description
The limitations of alpha counters using air in the proportional amplification region were investigated with a view toward a better design that would provide improved performance and reliability. A new detector is described which incorporates a shielded guard‐ring insulator assembly that provides freedom from insulator failure at high humidities. It is believed that spurious pulses also arise from the effect of humidity on the conductivity of foreign particles adhering to the anode wires. Reducing the relative humidity (rh) by the use of a small heater which maintains the detector 20°F above room temperature prevents this type of failure and provides reliable operation at 98% rh. The associated amplifier and discriminator were designed to detect the slow components of the ionization pulses associated with the transit time of negative oxygen ions. The resulting efficiency is 39% of the total disintegrations, with a plateau of over 150 volts having a slope of less than 4% per 100 volts.
26(1955); http://dx.doi.org/10.1063/1.1715208View Description Hide Description
Apparatus and procedures for conversion of sample carbon to carbon dioxide, quantitative hydrogenation of carbon dioxide to methane, and radiocarbon assay of the methane are described. Two sizes of counters are used. At the operating pressure of two atmospheres, the larger size contains 0.85 g of carbon and the smaller 0.22 g. If the sample and background are both counted for 24 hours in a 0.85‐g counter, a sample 34 000 years old (26 000 years old in the 0.22‐g counter) gives a C14 count equal to twice its standard statistical error. If desired, this method can readily be adapted to larger samples with a consequent increase in the maximum measurable age.
26(1955); http://dx.doi.org/10.1063/1.1715209View Description Hide Description
A divergent beam of monoenergetic charged particles can be refocused by a homogeneous magnetic field whose sharply defined boundaries are straight lines or circles. The transverse aberration of order α3 at the image point is evaluated and a general third‐order focusing condition is given. Design equations for third‐order instruments are obtained and the results presented graphically. Many third‐order focusing designs follow, both symmetrical and asymmetrical.
26(1955); http://dx.doi.org/10.1063/1.1715210View Description Hide Description
A dc amplifier that uses the condenser‐charging principle of electron counting has been constructed and used for the measurement of stars as faint as magnitude 20.8 with the 82‐inch and 100‐inch telescope. The device is simple (15 tubes), will measure stars of any brightness permitted by the photomultiplier, and has a simple automatic recording feature using a Brown recording meter.
26(1955); http://dx.doi.org/10.1063/1.1715211View Description Hide Description
Weak hyperfinesatellites have been observed with very large signal‐to‐noise ratios when microwave power at frequencies corresponding to these satellites is introduced into a ``maser'' which is oscillating at the frequency of the main inversion line of NH3. The satellite transitions are accompanied by a reduction of the amplitude of oscillation at the main‐line frequency. A composite oscilloscope trace showing the structure of the weakest of the four quadrupolesatellites associated with the 3–3 inversion line of N14H3 is presented.
26(1955); http://dx.doi.org/10.1063/1.1715212View Description Hide Description
A new type of ion gun is described which greatly improves the resolution of a nonmagnetic time‐of‐flight mass spectrometer. The focusing action of this gun is discussed and analyzed mathematically. The validity of the analysis and the practicability of the gun are demonstrated by the spectra obtained. The spectrometer is capable of measuring the relative abundance of adjacent masses well beyond 100 amu.
26(1955); http://dx.doi.org/10.1063/1.1715213View Description Hide Description
Scattering of the electron beam by the residual gas in the vacuum chamber is calculated for the 1 Bev strong focusing Cornell Mesotron. It is shown that the elasticscattering by the nucleus and the inelastic scattering by the atomic electrons are the only important contributions to the loss due to the presence of gas. These two effects are calculated in detail. Radiation and space charge effects are not discussed. Because of the small scattering angles involved the details of the screening by the atomic electrons must be taken into account accurately. Multiple scattering is shown to be of little importance because of the damping of the beam oscillations as the energy increases. The calculations are carried out for two different injection conditions and little difference is found between them as far as the loss due to the scattering itself is concerned. The numerical results for the Cornell Mesotron show that about half of the beam gets lost at a vacuum of 5×10−6 mm Hg. The scattering losses will depend on the field lattice of the synchrotron.
26(1955); http://dx.doi.org/10.1063/1.1715214View Description Hide Description
A fast neutron coincidence spectrometer utilizing stilbene scintillators is described. The principle of the spectrometer is based upon the pulse‐height analysis of the recoil protons resulting from a 45 degree scattering of the incident neutron beam. This is achieved by requiring the scatteredneutron to be recorded in a secondary ring of ten stilbene scintillators. With delays corresponding to a fixed time of flight, a coincidence between the primary and secondary crystals gates the primary pulse. At 4 Mev the efficiency is of the order of 10−4 per incident neutron, and the pulse‐height distribution has a width at half‐maximum of 17%. The corresponding energy width is 10%. The time‐of‐flight characteristic is used to reject gamma rays. Limitations caused by accidentals and pulse‐height resolution are discussed.
26(1955); http://dx.doi.org/10.1063/1.1715215View Description Hide Description
A description is given of a three‐terminal cell for dielectricmeasurements on polymericmaterials over a wide temperature range. Use is made of a lightly spring‐loaded, floating electrode to maintain good electrical contact with the specimen when it expands or contracts thermally. A differential‐capacitor, built into the movable electrode, is used with a simple detector circuit to indicate the position of the electrode with respect to its micrometer‐driven carriage. Simultaneous measurements can be made of cell capacitance and electrodeseparation. The latter can be determined with a precision of better than 10−4 inch. Provision is made to confine the specimen when it softens at temperatures above its glass‐transition temperature. The cell has been operated successfully at frequencies between 20 cps and 1 Mc and at temperatures between 20 and 200°C.
26(1955); http://dx.doi.org/10.1063/1.1715216View Description Hide Description
A device is described which converts a train of electrical impulses into a succession of hyperbolic wave forms of amplitude proportional to the interval between successive impulses. These wave forms when applied to an oscilloscope furnish a display capable of showing rapid changes in the frequency of an electrical signal. Unlike conventional counting‐rate meters response to sudden changes in frequency is immediate so that averaging over several cycles is not required. The instrument as constructed operates in the lower audio‐frequency range and is being used in the study of the electrical discharge of single fibers of the optic nerve.