Cathode Diameter Research Articles

The accuracy and spatial resolution of the measurement of tissue oxygen removal rate

An analysis is presented of factors determining the accuracy and spatial resolution of a method for the estimation of oxygen removal rate in tissues using a mutli-cathode electrode placed on the tissue surface. The effect of oxygen consumption by the cathodes is shown to be small for 25-μm diameter cathodes, but covering the tissue surface with a thin oxygen-permeable plastic film can introduce a systematic error into the estimate of removal rate, the size of which depends upon the oxygen diffusion coefficient and solubility in the plastic compared with those in the tissue. Spatial resolution is shown to be adequate for distinguishing changes in removal rate over distances of a few hundred microns, and the averaging of removal rate in the direction normal to the tissue surface is weighted heavily towards the superficial tissue layer.

Excitation of the 7s2P1/2, 3/2 levels of Xe+ due to charge exchange in the negative glow of a glow discharge in a neon-xenon mixture

It is shown that in the negative glow of a glow discharge in mixtures of neon with xenon (xenon contents of 5.5·10−3 to 2% of the total pressure P of the mixture) with P=15 torr and discharge currents i=40 mA (cathode diameter 30 mm), the 7s2P1/2, 3/2 levels of the Xe+ ion are excited by the charge exchange reaction Xe m + Ne+ → Xe+* + Ne.

Reliability of Cutaneous Oxygen Measurement by Skin Sensors with Large-Size Cathodes

The oxygen partial pressure may be measured cutaneously by directly heated PO2 sensors attached to the surface of the skin. The use of a large-size cathods (diameter 4 mm) allows one to obtain an average PO2 value over a sufficiently large skin area. The permeability of the membrane for oxygen has to be kept low to prevent a disturbance of the oxygen profile in the cutaneous tissue as a result of the oxygen consumption of the sensor. When using of 6 micrometer Mylar membrane, the current can be limited by the diffusion of oxygen through the membrane and is a measure of the arterialized cutaneous PO2. One effect of using membrane of low permeability is that the response time of the sensor is long (tau 90% = ca 45 sec). The reliability of correctly reproducing cPO2 variations by sensors having different response times may be evaluated by determining the transfer function of the membrane which allows the calculation of the frequency characteristics for various membranes. The cutaneous tissue layer also acts as a filter for rapid PO2 fluctuations. The characteristic of this filter is essentially similar to that of a "slow" membrane such as Myler 6 micrometer. The information gained by using a "rapid" membrane such as Telfon 13 micrometer is negligible. In 490 comparative measurements in newborns performed with sensors heated to 44 degree C, the correlation between arterial PO2 and cutaneously measured PO2 is significant (correlation coefficient = 0.934) and the regression line does not deviate significantly from the identity line.

Membrane-covered oxygen electrodes: I. Electrode dimensions and electrode sensitivity

Based on measurements with Clark-type, membrane-covered oxygen electrodes of various cathode diameters, ranging from 0.8 mm to 0.02 mm, a theoretical model is proposed. This model allows for the estimation of the sensitivity of micro Clark electrodes, considering radial diffusion of oxygen under the membrane of the electrode.

Axial density and temperature variations in a magnetically confined arc

Axial variations of electron density ne and temperature Te have been found in magnetically confined arcs assumed by other workers to be axially uniform. Results of measurements using plasma radiation, swiveling water-cooled electrostatic probes, and Thomson scattering of dye laser radiation are presented. The axial variations depend only weakly upon magnetic field strength, arc current, distance between the electrodes, and cathode diameter, but depend strongly upon gas pressure p. Te does not always vary monotonically with distance, but ne usually decreases away from the cathode. For all measurements reported here, 4×1014 <ne<2.2×1015 cm−3 and 0.6 <Te<2.6 eV.

Characteristics of a multicathode polarographic oxygen electode.

Recent work on Clark-type pO2 electrodes has shown that significant performance improvements can be achieved through the use of a multicathode arrangement. To provide a sound basis for the design of such electrodes, we present an analysis and discussion of their properties. The electrode is initially modelled by asuming the cathodes consist of noninteracting membrane-covered discs. Expressions are derived for the pO2 distribution and polarographic current. The influence of cathode diameter, membrane thickness and permeability are examined, particularly in relation to flow velocity dependence. An extension of the analysis to the situation where interaction between the cathodes becomes significant is also presented. The analysis and results provide a proper basis for the optimal design of multicathode p O2 electrodes.

Confining Process of Negative Glow in Concentric Cylinder Type Hollow Cathode Laser Tube and Design Principle

The confining process which attracts and maintains a negative glow within a hollow cathode of cylindrical geometry and the design principle of a concentric cylinder type laser tube have been clarified with reference to model tubes. The confining process can be interpreted in close relation to the transition process from Townsend discharge to normal glow discharge. The two design principles to manufacture an efficient laser tube are: (1) make the product (pl) smaller than 2.5 Torr·m, and (2) make δ equal to D, where p is the gas pressure, l is the difference between the inner radius of the anode and the outer radius of the cathode, δ is the hole diameter of the cathode wall and D is the inside diameter of the hollow cathode. An example of laser tube thus designed and tested is also shown.

Experimenteller Nachweis des Pendel-Effektes in einer zylindrischen Niederdruck-Hohlkathoden-Entladung in Argon / Experimental Evidence of the Existence of the Pendel Effect1 in a Low Pressure Hollow Cathode Discharge in Argon

Abstract The energy of electrons crossing the glow boundary of a cylindrical hollow cathode discharge has been directly controlled in a small, test region by the use of a wall probe installed in the side of the cathode cylinder. Varying the potential of this probe disclosed that some electrons ejected from the probe surface and hence from all cathode walls penetrate into the dark space opposite to the surface of their origin. These electrons can reach the opposite wall if the pressure and the cathode diameter are too low, in which case they will be removed. This removal causes the hollow cathode discharge to extinguish at higher pressures than is the case for linear discharges. Electrons that cross the cathode cavity entirely can pass through a small hole in the cathode wall into a high vacuum chamber and be analysed energetically. The same experiment also discloses the energy distribution of electrons leaving the cathode surface. A comparison with the theory recently published 3 shows that pendelelectrons can be responsible for the currrent amplification observed in the hollow cathode discharge.

Miniature Photomultiplier for Scintillation and Space Use

A rugged miniature photomultiplier has been developed which is only 8 mm in diameter. The electron multiplying element is a single channel multiplier. Photoelectrons from the 5 mm diameter cathode are electrostatically focused into the channel multiplier without need for a funnel or cathode-to-channel bias. This photomultiplier is a two-terminal device and uses no external resistors. The high gain characteristics (~108 at 3,000V), and the single-electron pulse height distribution (FWHM ⩽0.6), make the tube ideal for pulse-counting applications. The small diameter of this tube suggests applications in close-packed arrays and mosaics for scintillation imaging. This tube can be used as a scintillation spectum analyzer with good resolution even at high gains where there are greater than 109 anode electrons per scintillation event. The structure can also be employed in a windowless configuration as a particle or x-ray detector. In this modification apertures as large as 5 × 7 mm2 are possible. One use is as the detector in an Auger apparatus, where the high-gain output pulses allow precision digital data handling techniques and long drift-free integration time.

Arc discharges in supersonic flows: H Huegel et al,Rep DLR-Mitt-67-17, Oct 1967, 25–37 ( in German) ( Sci Tech Aerospace Reps, 6 (5), 722, N68-13926).

The chapter presents a short review on hollow cathode, penning, and electron-beam excitation sources. A type of discharge that has been found particularly suitable as a vacuum ultraviolet (VUV) radiation source is the windowless high-current hollow cathode source. Hollow cathode discharges are a special type of glow discharge. The characteristic feature of these sources is its dominant negative glow. Because the pressure is chosen so that the mean free path of the electrons is larger than the diameter of the hollow cathode these electrons, the so-called “beam electrons,” oscillate between the two opposing cathode falls until their kinetic energy is transferred to the gas atoms by collision. Hollow cathode discharges are typically non-LTE plasmas. Penning discharges were originally investigated for use as ionization gauges for low-pressure measurements. The characteristic feature of a Penning discharge is the application of a strong magnetic field parallel to the electric field of the cathode fall. When the high-energy beam electrons in the hollow cathode or Penning discharge are replaced by electrons produced from an electron gun no erosion of the electrode takes place and a highly stable source of VUV line radiation can be created. With known excitation cross sections the absolute flux emitted by such a source can be determined from the operating parameters and the geometry of the source.

Resonant side-coupled cavity electron accelerators: E A Knapp et al,Rep LA-DC-8834; MP-3-4, Sept 1967, 18 pages ( Sci Tech Aerospace Reps, 6 (5), 643, N68-14324).

The chapter presents a short review on hollow cathode, penning, and electron-beam excitation sources. A type of discharge that has been found particularly suitable as a vacuum ultraviolet (VUV) radiation source is the windowless high-current hollow cathode source. Hollow cathode discharges are a special type of glow discharge. The characteristic feature of these sources is its dominant negative glow. Because the pressure is chosen so that the mean free path of the electrons is larger than the diameter of the hollow cathode these electrons, the so-called “beam electrons,” oscillate between the two opposing cathode falls until their kinetic energy is transferred to the gas atoms by collision. Hollow cathode discharges are typically non-LTE plasmas. Penning discharges were originally investigated for use as ionization gauges for low-pressure measurements. The characteristic feature of a Penning discharge is the application of a strong magnetic field parallel to the electric field of the cathode fall. When the high-energy beam electrons in the hollow cathode or Penning discharge are replaced by electrons produced from an electron gun no erosion of the electrode takes place and a highly stable source of VUV line radiation can be created. With known excitation cross sections the absolute flux emitted by such a source can be determined from the operating parameters and the geometry of the source.

Equivalent capacitance derivation of space-charge-limited current from a filamentary cathode

Dow has outlined a method of calculating the space-charge-limited current and obtained a formula that is valid for cathode diameter to anode-to-anode spacing ratio of less than 40 percent. A new formula has been derived that will extend the limit and give excellent result for as high a cathode diameter to anode-to-anode spacing ratio as 70 percent.

810. Experimental comparison of conventional-hollow and coaxial-hollow cathode low-pressure arcs: E L Boyer and J F Holt, Rep AFAPL-TR-67-63, AD-659124, June 1967, 21 pages (Sci Tech Aerospace Reps, 6 (1), 120, N68-10945).

Abstract : A one-meter coaxial-hollow cathode argon arc was investigated experimentally and the results obtained were compared with already-documented data on conventional-hollow cathode low-pressure argon arcs. The outer diameter and inner diameter of the hollow cathode, used for comparison purposes, were 1.91 and 0.953 centimeters, respectively, while the outer diameter of the coaxial cathode was 1.91 centimeters and its inner diameter was 1.27 centimeters. Radial profiles of the electron temperature and density in the arc plasma were measured by a Langmuir probe; current profiles were measured by an electrical profile scanner device. A plasma sweeper provided data on electron density. The data obtained by all methods for the low-pressure arc of the coaxial-hollow cathode were consistent in repeated tests and consistent with the data available on the low-pressure arc of the conventional-hollow cathode. The maximum electron temperature was 4.8 electron volts; the electron density average for the arc column was 3 x 10 to the 12th power per cu cm. With the coaxial arrangement for a cathode of any fixed dimensions, the coaxial type can be run at higher power than a conventional single-element hollow cathode of the same dimensions. (Author)

Molecular plating, V. The influence of some experimental factors on the deposition yield

The effects of cathode diameter, electrode distance, initial current and temperature on the deposition yield using the molecular plating method were studied. 90Sr/ 90Y was used as a model nuclide. The best conditions to obtain a uniform layer with 100% yield are reported.

A Radar Computer for the Closest Point of Approach

The paper describes an experimental analogue computer used with a civil marine radar set for determining the closest point of approach of selected ship's echoes and the time to closest approach. The equipment was installed on shore and measurements made of shipping passing to and from Portsmouth and Southampton harbours. The projected track of a ship echo was displayed on a 5-in. diameter cathode ray tube and the closest point of approach read as the distance of this track from the centre of the tube. The results show a high probability that under these conditions errors in C.P.A. will not exceed ±0·5 n.m. for targets between 10 and 6 n.m. range, and ±0·3 n.m. for targets nearer than 6 n.m.

Detector time jitter in He 3 and BF 3 proportional counters

The delayed-coincidence technique has been used to measure the electronic time jitter for a number of neutron-detecting proportional counters. A selection of BF 3 counters with filling pressures of 1–2 atm and cathode diameters of one and two inches were investigated. Several 1″ diameter He 3 counters with fillings of helium and various polyatomic gas additives in pressures up to 12 atm were also studied. Procedures for the use of these results in the design of neutron time-of-flight detectors are discussed.

Noise reduction schemes in transverse modulation tubes

The paper is concerned with a general analysis of noise in d.c. pumped amplifiers based on transverse waves. It is shown that any amplifier involving either the cyclotron waves or the synchornous waves has the same minimum noise temperature determined by the physical conditions at the cathode. The noise temperature can be reduced to an arbitrarily small value by increasing the magnetic field at the cathode or by reducing the cathode diameter. It is also shown that the minimum noise temperature of the transverse waves is reduced by the smoothing effect of the potential minimum near the cathode. The minimum noise temperature can be realized by inserting a suitable noise transformer between the cathode and the amplifier. In general, structures with electric or magnetic fields having rotational symmetry such as used in normal electron guns can perform the required transformation. Some specific examples of noise transformers are discussed.

Minimum noise temperature of DC-pumped transverse-wave electron beam amplifiers

The paper is concerned with a general analysis of noise in dc-pumped amplifiers based on transverse waves. It is shown that any amplifier involving either the cyclotron waves or the synchronous waves has the same minimum noise temperature determined by the conditions at the cathode. The noise temperature can be reduced to an arbitrarily small value by increasing the magnetic field at the cathode or by reducing the cathode diameter. It is also shown that the minimum noise temperature is reduced by the smoothing effect of the potential minimum near the cathode. The minimum noise temperature can be realized by inserting a suitable noise transformer between the cathode and the amplifier. In general, structures with electric or magnetic fields having rotational symmetry such as used in normal electron guns can perform the required transformation. Some specific examples of noise transformers are discussed.

Fatigue effects on the cathode of self-quenching GM counters

The work functions of Cu and Ni cathode tubes and the plateau length and slope of Geiger-Mueller tubes after prolonged operation were studied. The tubes used were of identical geometry with a cathode diameter of 20 mm, a sensitive anode length of 80 mm, and a gas volume of about 25 cc. All tubes contained a gas mixture of the same composition and pressure throughout the measurements. In the course of fatigue operation the work function of the Ni cathode increases rapidly at first and then gradually the rate of increase becomes slower. The plateau length also shows a sharp increase, but the plateau slope shows a strong decrease. Similar results were obtained for the Cu cathode tubes. (J.S.R.)

Recent Developments in the Scintillation Counter Field

Rarely does a device having the conceptual simplicity of the scintillation counter gain the wide recognition accorded this detector. It has become an essential in most phases of nuclear research, and its usefulness extends beyond this into the fields of radiochemistry, biology, tracer technology, radiation and contamination protection, and prospecting for oil and fission fuel. This importance rests upon its extremely high sensitivity, its superior time resolution capabilities, and its ability to measure the energy of the incident nuclear particles. Furthermore, the instrument is rugged, reliable, and flexible. The scintillation counter consists of a clear phosphor scintillation, a multiplier phototube, and the required presentation circuitry. Incident nuclear radiation excites light from the phosphor. This light releases photoelectrons in the multiplier which, after secondary emission amplification by the cascaded dynodes, appear as a current pulse at the anode. The presentation circuitry analyzes and displays or records the results. The instrument's high sensitivity depends upon the availability of high-absorbing, efficient phosphors and upon photomultipliers which can detect very small light flashes. These requirements have been responsible for the development of scintillators of a wide range of sizes and characteristics, and of more sensitive multipliers ranging in cathode diameter from 3/4 inch to 16 inches. The very high speed of the scintillation counter is a consequence of the extremely short fluorescent lifetimes of certain organic phosphors and the very small time dispersion in multiplier phototubes. With presently available phosphors and multipliers, durations can be measured in the one-tenth millimicrosecond range.