Secondary Emission Ratio Research Articles

A two electron gun technique for the measurement of secondary emission characteristics of a variety of materials

A new technique and apparatus are described whereby the secondary emission ratio of a variety of materials can be measured. The experimental setup employs two electron guns in a demountable vacuum system and associated electronics. The system, which operates at a pressure of about 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> torr, has the advantage of taking data under steady-state conditions rather than employing pulsing techniques. Conductors, semiconductors, and dielectrics can be tested in the demountable. The principal features of this technique are 1) use of a flood electron gun to place the target surface at some fixed voltage, and 2) the use of an amplitude modulated probing electron beam and a narrow-band amplifier centered at the probing beam modulating frequency, to measure the collected probing current at the target. Secondary emission measurements on semiconductors are described for primary energies from zero through first crossover to approximately 50 eV above maximum. Modification of the system to enable measurements up to 10 kilovolts is also explained. Advantages of this apparatus as an important laboratory tool for the measurement of the secondary emission characteristic of materials for use in camera tubes, image converters, storage tubes, electron multipliers, and other devices, are emphasized.

Die zählratenabhängigkeit des verstärkungsfaktors und des auflösungsvermögens bei photomultipliern vom typ RCA 6810 A

The variations in gain and resolution of RCA 6810 A photomultiplier tubes have been investigated with reference to changes in the illumination of the photocathode. The tubes were mounted to a NaI(Tl) crystal, irradiated with γ-rays of different energies and intensities. Changes in the counting rate result in reversible fast and slow variations in gain. An increase of the irradiation level causes a rapid positive initial change with a continuing slow rise followed by a gradual decline to a stable level. Magnitude and exact time dependence vary with multiplier load and past history. Moreover, the amplitude is different for every tube. A reduction of counting rate results in a sudden negative initial shift with a continuing slow decrease. The long-term instabilities are discussed in detail. They may be attributed to reversible changes in the secondary emission ratio of the dynode surfaces. The resolution was found to deteriorate with increasing irradiation intensity.

Electron Multipliers Utilizing Continuous Strip Surfaces

Thin films of semiconducting materials with secondary electron emission ratios greater than one have been used for some years in the Bendix crossed electric and magnetic field electron multiplier for ion detection. In these devices the electric field to accelerate the electrons is established by the potential gradient created by external current flowing through the resistive strip. A low background counting rate, insensitivity to radiation in the visible and near ultraviolet, and stability on exposure to atmospheric gases are properties which permit this device to have interesting photon detection applications in the extreme ultraviolet in dynamic vacuum systems and outer space environment. A new form of electron multiplier utilizing continuous thin film surfaces, but not requiring a magnetic field, has been developed. Small diameter tubes (a few mils I.D.) coated on the inside surface give gains up to 106. This "channel" multiplier has been operated singly as a miniature electron or photon detector, and in arrays in image intensifiers. The principle of operation and some applications of these channel amplifiers will be discussed.

Analysis of Noise in the Image Orthicon

The problem of how to design a camera tube to operate at light flux levels lower than those presently used without reduction of the signal-to-noise ratio is examined. It is shown that any increase in photocathode sensitivity or mesh transmission allows a corresponding reduction in the light required for equal performance. On the other hand, increases in the secondary-emission ratio of the target or the first dynode, even if indefinitely large, can only produce a limited reduction in the light required for equal performance.

A high-power microwave duplexer using secondary electron resonance

A high-power S-band duplexer has been developed, in which power activated switching is accomplished through the mechanism of secondary electron resonance (multipactor) rather than through gas discharge. Advantages of the multipactor discharge, which occurs in high vacuum (10-9mm Hg), include low noise, very long life, and short recovery times of the order of an RF cycle. The multipactor discharge occurs in a plane-parallel, high-voltage gap within an RF resonator, if the frequency, field strength, and gap spacing are related as follows: 1) Electrons, accelerated by the electric field, traverse the gap in one half RF period. 2) The impact energy of the electrons is such that the secondary emission ratio is larger than one.

A barrier-grid tube for binary digital storage

An improved electrostatically focused and deflected barrier-grid tube for high speed random access binary digital storage is reported. The tube has a high resolution electron gun incorporating a dual focus lens and dual interdeflection plate shields. The barrier-grid Consists of a four-layer stack of tungsten wire grids (wound at 450 turns per inch). A gold coating on the grid wires provides a uniform secondary emission ratio. The useful storage area of the target is 1\frac{7}{8} \times 1\frac{7}{8} . The tube is used in a system to store 16,384 bits with a performance error rate of less than 1 in 1010operations. The cycle time is 2.5 microseconds per bit with a single read or write operation requiring 0.7 microsecond. Under these conditions the read-around number exceeds 300 for an average output amplitude change of 10 percent.

Image Intensification by Transmission Secondary Electron Emission

The successful operation of a multistage image intensifier using a series of aluminum dynodes coated with thin layers of BaF/sub 2/ and reinforced with very fine wide-mesh grid ia reported. The electron gain in the best of these tubes is 2400, corresponding to sn average secondary emission ratio of 7 per dynode. The photon gsin with 15kw acceleration from the last dynode to a P11 output phosphor is about 10,000. At low light levels, resolution is limited by photoelectron noise. This tube, when fully developed, will be well suited to the amplification of extremely low light level images such as those encountered in astronomy, nuclear particle track photography, spectroscopy, and x-ray fluoroscopy. (D.L.C.)

Preparation and Properties of Thin Film MgO Secondary Emitters

Some simple methods of preparing MgO thin films on Mg–Ag alloy with high secondary emission ratios are discussed. The oxidation procedure and a discussion of the underlying phenomena are presented. Films with a secondary yield of 12 at 600 v may be produced by oxidation in either oxygen or carbon dioxide. Results of a study of some of the factors influencing the life of a secondary emitter are given. These include the effects on the secondary emitting characteristics of evaporation products from an oxide-coated cathode and the deterioration of the thin film under electron bombardment.

A method of investigating the secondary emissive properties of insulators

Methods are described for measuring the secondary emission ratio (δ) of an insulator together with the energy distribution of the emitted electrons. The secondary emission ratio is obtained by means of a type of electron beam voltmeter which is used to measure the potential of the bombarded surface thus allowing the rate of change of charge to be determined. The results show good agreement with those obtained by another method and also check with d.c. measurements on metal surfaces. The energy distribution of the secondary electrons from an insulator surface is measured using the electron beam voltmeter and also by a method employing cathode potential stabilization. As the electron beam voltmeter method for measuring δ depends on the ratio of two time measurements it should be capable of greater accuracy than methods which involve the measurement of output pulses from an amplifier.

The Effect of Organic Vapor on the Secondary Emission of Phosphors

Experiments are described to show that, when a phosphor surface contaminated with organic vapor is bombarded with electrons, the secondary emission ratio is rapidly reduced. If the bombarding voltage is sufficiently high, this will result in a progressive decrease in potential of the phosphor surface and a corresponding decrease in apparent fluroescent efficiency. The relation between this effect and true ``phosphor burn'' is discussed as well as its possible connection with the phenomenon of ``cross burn'' encountered in the cathode-ray tube industry.

The use of radioactive tracer techniques in the study of cathode problems

Reasonably long-lived isotopes of barium strontium and calcium are available from the Oak Ridge Laboratories of the Atomic Energy Commission. If these are in­ corporated in oxide cathode coatings, a convenient means is available for studying a number of problems which arise in the operation of electron tubes utilizing oxide coated cathodes. Studies have been made of the evolution of material from oxide coated cathodes both during activation and life. It is found that the principle evaporant from equi-molar oxide coatings based on a pure nickel is barium oxide. The second most important evaporant is metallic strontium. It is believed that approximate equal amounts of barium and strontium metal are evolved but the barium metal evolved is masked by the evaporation of barium oxide. It is believed that under cert in circumstances the strontium itself may provide an index for the· reducing potentials of the base nickel. These techniques have also been applied in a study of the effects of cathode evaporation on the secondary emission ratio of a target exposed to an oxide coated cathode.

The secondary emission from copper and silver films obtained with primary electron energies below 10 eV

An apparatus for the investigation of the secondary electron emission from freshly evaporated copper and silver films obtained with primary electron energies between thermal energies and 10 eV is described. The measured secondary emission ratios show a definite dependence upon primary electron energy which is different from that determined by previous workers. Furthermore, the values of the secondary emission ratio are lower than those hitherto recorded, both silver and copper possessing a secondary emission ratio less than 0·1 in the primary energy range 1 to 5 eV. The difference between the results presented in this paper and those obtained by earlier workers is attributed to the improved vacuum conditions which were used in the present work, thereby preventing the troublesome effects of contamination of the surfaces under investigation by the residual gases in the experimental tube. The energy distribution of the secondary electrons was measured and showed that at very low energies (less than 5 eV) these electrons are in fact elastically reflected primary electrons.

The Mechanism of Field Dependent Secondary Emission

In recent experimental investigations, it was found that secondary emission ratios as high as 10,000 to 1 could be attained utilizing field dependent secondary emission from magnesium oxide. Early tests showed the mechanism causing the high gains to be fundamentally different from the more standard secondary emission phenomenon.The hypothesis was made that the mechanism of field dependent secondary emission was a process similar to that of the "Townsend avalanche" occurring in gas discharges. As the surface of the dielectric film was bombarded with primary electrons, the high resistivity of the material in combination with the secondary emission current caused the surface to charge to the potential of the collector grid, producing a high field within the dielectric. Electrons released within the material could then gain enough energy to liberate additional electrons, and an avalanche type process resulted.Experiments were conducted to test this hypothesis and each proved to be consistent with the above theory. The main content of these experiments can be summarized in the following statements:(1) The high yields appeared to be independent of the base material. This indicated that the surface or volume effects were most important, implying that a Fowler type field emission from the base metal was not a significant factor.(2) In studying the secondary current as a function of field, the gas discharge equations were found to be correct. These equations predicted a straight line plot of the $\mathrm{ln}\mathrm{ln}\ensuremath{\delta} \mathrm{vs} \frac{1}{E}$, and in addition, gave a close estimate of the magnitude of the secondary emission ratio.(3) By means of retarding potential measurements, the energies and mean free paths of the emitted secondary electrons were determined. These data were in good agreement with the results in item (2).(4) The rise time for surface charging was determined by using square wave variations of bombarding currents, and was found to be consistent with the original hypothesis.

Field-Dependent Secondary Emission

Experiments have been conducted with magnesium oxide surfaces in which high dc fields were applied to the surface, while at the same time secondary emission measurements were being made. Very high secondary emission ratios were obtained reproducibly (100 to 1 and greater). Under static conditions it was found that the ratios would increase exponentially with increasing fields. In addition, for constant fields, the ratio of liberated electrons to bombarding electrons was constant, over a wide range of bombardment energy. Measurements were made of secondary emission with a square wave field applied to the surface and time lag effects were noted. Decay time in the order of 30 microseconds were measured. The rise time was found to be dependent upon field and bombarding currents. The mechanism of field dependent secondary emission was shown to be related to an avalanche effect, triggered by the bombarding electrons.

Secondary Emission of Nickel-Barium Mixtures and Rhenium When Bombarded by Electrons with Energies from 50 to 8000 Electron-Volts

The experimental tube contains a contamination-free gun which directs an electron beam at normal incidence onto a small thin sheet target which may be heated by conduction. The target is located at the center of a conducting sphere to permit a measure of the secondary current, and also of the energy distribution of the secondary electrons by means of the retarding potential method. The maximum value of δ, the secondary emission ratio, for a cast alloy of nickel containing 1.5 percent barium is 2.8 and occurs at 800 to 900 volts after a critical amount of previous heat treatment of target. The value of δmax increases with heat treatment of target, subsequent to baking of tube, to the above value and then decreases with further heating. For the critical condition the value of δ above 4000 volts is sensitive to the amount of previous electron bombardment of the target. A δmax of 1.3 for rhenium after thorough out-gassing was obtained at 900 volts. The most probable energy of low speed secondary electrons from rhenium is approximately 5 ev.

Measurement of secondary electron emission from dielectric surfaces.

A technique is developed for the rapid and precise measurement of the secondary electron emission properties of certain dielectric surfaces. The dielectric surface is bombarded by primary electrons having energies which produce a secondary emission ratio greater than unity. This establishes the surface potential at a known value. With the known potential difference between the dielectric surface and primary electron source, the energies of the primary electrons are automatically determined for the first instant of surface bombardment. The net current to the dielectric surface at any moment is measured by using a cathode-ray oscillograph to observe the transient current to a metal plate which backs the dielectric. The method has been used to study metallic surfaces and has yielded results which agree very closely with those obtained using the conventional d.c. method of measurement.

A new secondary cathode

One of the main difficulties in the use of secondary-emission multiplier stages in thermionic valves is the instability of the secondary cathode, which is usually characterized by a gradually decreasing secondary-emission ratio during its useful life. In this paper, experiments are described showing that this instability is due to the bombardment of the secondary cathode by the primary electron beam, and it is shown that this bombardment causes disintegration of the surface layer. A new type of secondary cathode, formed from a mixture of magnesium oxide and barium carbonate, has been found to have a stable life of over 1 000 hours, when operating at 400° C and with a primary bombarding current density of 20 mA/cm2. The use of this secondary cathode has made possible the development of a single-stage thermionic electron-multiplier valve, with a mutual conductance of 20 mA/volt.

Secondary Emission of Beryllium

The secondary electron emission of beryllium films prepared by evaporation in a high vacuum has been studied. As the beryllium is deposited the secondary emission ratio (defined as the ratio of the number of electrons leaving the surface, to the number of primary electrons impinging on it) rises from that of the support metal to a value in the neighborhood of 2.5. The position of the maximum with respect to energy of primary electrons remains near that of the support metal. As the thickness of the layer is increased the ratio falls to less than unity and the voltage of the primary electrons necessary to reach the maximum drops to about 200 volts. Heating the thick beryllium layer to a bright red for a few minutes raises the emission to about 1.6 with the maximum at a primary energy of about 630 volts. Oxidation of the surface increases the emission to about 4.1. Explanations of these effects are discussed and arguments are presented favoring the acceptance of the value of 1.6 as the maximum secondary emission ratio to be expected for pure bulk beryllium.

Shot Effect of Secondary Electrons from Nickel and Beryllium

The secondary emission ratios and shot effects from nickel and beryllium were measured over a range of primary energies up to 1600 volts. The results are consistent with the following conclusions. At high primary energies the true secondary electrons, as distinguished from the reflected primaries, are emitted in groups whose distribution in size about the average is closely Bernoullian. There are many primaries which are buried in the body of the metal and produce no secondaries; their number has been determined. Disregarding reflected and buried primaries, the secondary emission ratio is nearly proportional to the square root of the primary energy. At low primary energies the effects of reflection, both internal and external, can be followed in detail.

Television Pickup Tubes with Cathode-Ray Beam Scanning

Television pickup tubes which use cathode-ray beam scanning, although only one class of television pickup devices, may be made in a variety of ways, a number of which are described in this paper. In these tubes, the function of the electron beam is to release secondary electrons from the target, the number escaping being modulated by electrostatic fields, magnetic fields, orientation of electrodes or changes in the secondary emission ratio of the target. The Iconoscope is a well-known example of modulation by electrostatic fields produced by photoemission from the target. A conducting photocathode when used as a target, however, acted as if its secondary emission ratio were decreased by light. A copper plate oxidized and treated with caesium transmitted a picture with some time lag. Photoconductive materials exposed to light and scanned by an electron beam were made to develop potential variations over their surface and thereby transmit a television picture. Aluminum oxide and zirconium oxide, treated with caesium, were used in this manner. Selenium, used as a photoconductive material, also transmitted a picture. Germanium used as a target sensitive to heat radiation was able to transmit a picture, probably as a result of some thermoelectric effect. The most sensitive tubes tested were those in which an electron picture was focused upon a scanned, secondary electron emissive target. The scanning and picture projection operations may be separated by using a two-sided target. Coupling between the two sides was obtained by conducting plugs through the target.