Band Amplifier Research Articles

Electron Ionization Cross Sections using Chopped Beams

The paper describes the development of a crossed beam method suitable for measuring ionization cross sections of unstable gases such as atomic hydrogen. Results obtained in He and H2 are compared with those of Tate and Smith. The method makes use of a narrow band amplifier associated with a beam chopper and phase sensitive detector, to eliminate the current due to ionization of the background gas. The electric field in the collision region is such that limitation of the beam crossover volume can make the uncertainty in electron energy much smaller than is otherwise possible, without resorting to magnetic collimation or pulsed collection. An analysis of the sources of error due to the collection of spurious currents at signal frequency is given.

Photoelectric observations of the electron solar corona.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Photoelectric observations of the electron solar corona. Wlerick, Gerard ; Axtell, James ; Lee, Robert Abstract The High Altitude Observatory staff has recently installed in Boulder a new instrument designed for the detection without eclipse of the electron solar corona. This component of the corona, often called the K-corona, is strongly polarized, the direction of the polarization being practically radial. It was successfully detected without eclipse in 1930 by B. Lyot at Pic du Midi with a coronagraph and a visual polarimeter able to detect a percentage of polarization as low as 0.1 per cent. Lyot's observations went as far as 0.35 R0 from the limb. The new instrument, called the K-coronameter, has been designed to permit observations at a greater distance from the edge of the sun, of the order of 0.5 to i.o R0. It is a coronagraph supplemented by a sensitive photoelectric polarimeter. The analyzer of polarization consists essentially of an electrically birefringent crystal excited by an a.c. voltage, a polarizer and a Lallemand photomultiplier. The a.c. signal from the multiplier is fed to a narrow band amplifier with synchronous detection. The band width is of the order of a few cycles per second. To reduce as much as possible the instrumental polarization, the coronal observations are always made on the axis. Thus, when scanning around the limb, the axis of the instrument describes a cone. A complete scan, at one given distance from the limb, takes 4 minutes. During the scan, a half-wave plate rotating at the speed of I round/ 8 min. keeps the plane of polarization of the light entering the analyzer in a fixed direction. With a relatively clear sky, Brightness B5 i~-~ B0, it has been possible in Boulder to detect the main features of the corona streamers up to 0.5 R0 and in one case up to o.66 R0. There is generally a good correlation between the brightness of the green coronal emission line measured with a conventional coronograph. It is planned to move the instrument to Climax this summer, at elevation I I 000 feet where better sky conditions prevail, and to make routine observations on clear days during the International Geophysical Year. The principal development work on this instrument was supported by the U. S. National Committee for the I.G.Y. under a grant from the National Science Foundation; a portion was supported by Radio Corporation of America. High Altitude Observatory, Boulder, Colo. Publication: The Astronomical Journal Pub Date: 1957 DOI: 10.1086/107631 Bibcode: 1957AJ.....62R..39W full text sources ADS |

Demonstrations of Electromagnetic Waves in Helices

Demonstrations of standing microwaves in helices are described for helix circumferences ranging from 0.1 to 3 wavelengths. Special attention is given to dimensions of helices used in (1) traveling wave tubes as broad band amplifiers, (2) axial mode antennas for producing a circularly polarized axial beam, and (3) radial mode antennas for broadcasting a horizontally polarized radial beam. The demonstrations are made with 2450 megacycle/sec educational equipment.

Measured Amplitude Distribution of Noise Voltages

In this paper a brief theory is given about the amplitude and probability density distribution of a narrow band shot noise. It is found that the probability density distribution follows the normal law and the amplitude distribution is the integral of the normal function. However, if the noise voltage is applied across a nonlinear impedance such as a type 1N34 crystal diode, both the amplitude and probability density distributions are changed. The change depends upon the current-voltage characteristic of the nonlinear impedance. An experimental method is described which can be used to measure the amplitude and the probability density distribution. The noise source was a photomultiplier tube followed by a narrow band amplifier centered at 16 kc. The measured results agree with that obtained by calculations.

Measurement of Time Varying Optical Absorption

Time sampling techniques have been applied to the study of optical absorption transients. The method is described in terms of its use for the study of the decay of the concentration of metastable atoms following a pulsed discharge. Light from a capillary source is passed through the discharge tube. The transmitted intensity of one of the spectral lines which is absorbed by the metastable atoms is measured using an interference filter and a gated photomultiplier. The photomultiplier gate occurs at twice the discharge frequency so that alternate pulses of the photomultiplier output are reduced by absorption. The component of the photomultiplier output at the discharge frequency is proportional to the absorption and can be measured with a narrow band amplifier and synchronous detector. Since the time resolution of the system is determined by the width of the photomultiplier gate, the response of the synchronous detector can be made slow enough to average the absorption signal over many decay periods. Fluctuations in the number of electrons leaving the cathode of the photomultiplier limit the useful sensitivity of the present system to approximately two parts in 104. This represents a hundredfold improvement in sensitivity over that available with previous techniques. The results of some studies of helium metastables are presented in order to illustrate the detail which can be obtained with this technique.

Apparatus for Nuclear Magnetic Resonance

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.

Nuclear Gyromagnetic Ratios. III

The super-regenerative oscillator techniques described in earlier reports have been modified by introduction of a tuned narrow band amplifier with chart recorder and by improvements in magnetic-field regulation. Magnetic resonance frequencies of various nuclei have been compared with the resonance frequencies of ${\mathrm{H}}^{1}$, ${\mathrm{D}}^{2}$, ${\mathrm{Li}}^{7}$, and ${\mathrm{Sc}}^{45}$, which have been used as "standards." Ratios of the observed resonance frequencies to the proton resonance frequency in the same magnetic field are given for the following nuclear species: ${\mathrm{Be}}^{9}$, ${\mathrm{B}}^{11}$, ${\mathrm{F}}^{19}$, ${\mathrm{Na}}^{23}$, ${\mathrm{Al}}^{27}$, ${\mathrm{P}}^{31}$, ${\mathrm{V}}^{51}$, ${\mathrm{Mn}}^{55}$, ${\mathrm{Cu}}^{63}$, ${\mathrm{Cu}}^{65}$, ${\mathrm{Br}}^{79}$, ${\mathrm{Br}}^{81}$, ${\mathrm{Rb}}^{87}$, ${\mathrm{Nb}}^{93}$, ${\mathrm{I}}^{127}$, ${\mathrm{Cs}}^{133}$, ${\mathrm{La}}^{139}$, ${\mathrm{Tl}}^{203}$, and ${\mathrm{Tl}}^{205}$. The nuclear magnetic moments of these species have been determined in terms of the proton moment from the known spins and the observed frequency ratios after making diamagnetic corrections; values for the nuclear magnetic moments are listed.

Transient Nutations in Nuclear Magnetic Resonance

Transient nutations of the resultant nuclear magnetic moment vector are set up by applying radiofrequency power in the form of pulses in the neighborhood of resonance ($\ensuremath{\omega}=\ensuremath{\gamma}{H}_{0}$). The nutations have an initial amplitude depending on the state of magnetization at the start of a pulse and on the proximity to resonance, and are damped by spin-spin and spin-lattice interaction. The thermal relaxation time can be directly found by observing the dependence of initial amplitude on the time between pulses. The spin-spin time constant ${T}_{2}$ can be found from the rate of decay even in the presence of normally disturbing inhomogeneity in magnetic field. Sensitivity is in many cases comparable to that obtained in the modulation method with narrow band amplifiers. The fast response due to the relatively wide band widths used can be applied to a rapid search for unknown resonances. The effects observed are in qualitative accord with predictions based on the Bloch theory.

Nuclear Gyromagnetic Ratios II

By means of super-regenerative oscillator techniques, magnetic resonance absorption peaks have been observed for ${\mathrm{Be}}^{9}$, ${\mathrm{P}}^{31}$, ${\mathrm{Cl}}^{35}$, ${\mathrm{Rb}}^{85}$, ${\mathrm{Cs}}^{133}$, and ${\mathrm{La}}^{139}$. By simultaneous observation of each of these resonances and of the proton absorption peak in the same magnetic field, the ratio of the resonance frequency $\mathrm{nu}$ of each of these nuclei to the proton frequency ${\ensuremath{\nu}}_{p}$ in the same applied field has been determined. The experimental results are as follows: From these experimental results, the gyromagnetic ratio for each nucleus can be determined in terms of the gyromagnetic ratio of the proton provided suitable corrections for the diamagnetism of the electrons are applied.

Short Time Delays in Geiger Counters

There are at least two types of short time delays in the firing of Geiger counters. The first is due to the transit time of the secondary electrons as they migrate toward the central wire. This time can be measured when beta particles are sent parallel to the axis of the counter, creating secondaries at known distances from the axis. In a typical counter, 2.2 cm in diameter, the maximum secondary electron transit time is measured at about 11×10−8 second. The second delay occurs between the time the electron arrives near the center wire, and the time a pulse is detected on the output of a wide band amplifier connected to the center wire. Increasing the voltage on the counter from the threshold to an over-voltage of 250 volts, decreases this delay by 10 to 20×10−8 second. It is suggested that this delay is due to the time required to form the initial part of the ion sheath.

On the Distortionless Reception of a Modulated Wave and Its Relation to Selectivity

The importance of overtones in the faithful reproduction of speech and music. Overtones are transmitted by the extreme side bands of a modulated wave. Distortionless reception of the full side bands necessary for faithful reproduction. Crowding of the air channels brings adjacent waves into such close juxta-position that selectivity demands a sharp cut-off at the limits of the band. Selectivity by the usual resonance methods trims the side bands. Attempts at compromise by employing damped resonant circuits do not achieve distortionless reception and sacrifice selectivity. Description of an amplifier giving uniform amplification over the entire band width with a sharp cut-off. Description of a band selector having an approximately rectangular frequency characteristic. Frequency control and volume control. Various applications of the band amplifier and the band selector to broadcast reception.