CONGRESSES AND CONFERENCES

CONFERENCE ON CATHODE PHENOMENA IN VACUUM AND RAREFIED GASES

I. From July 4 to 8 of this year, sessions of the Conference on Cathode Phenomena in Vacuum and Rarefied Gases, convened by the Institute of Physics of the Academy of Sciences of the Ukrainian SSR, were held in Kiev. Although the subject matter of this conference was expanded, as compared with the subject matter of the Conference on Secondary Emission and the Photoelectric Effect¹ (held in 1939 in Leningrad), by the inclusion of a number of reports on cathode phenomena in a gas discharge, in the main this conference was the successor to the aforementioned conference, as is evident from what follows.

The conference program included the following sections:

1. Photoelectron, secondary-electron, and thermionic emission of various cathodes (reports on experimental studies—12 papers).

2. Antimony–cesium cathodes (physical properties, technology, operational characteristics—8 papers).

3. Discussion on the nature of the secondary emission of complex cathodes (4 papers and a number of presentations).

4. Electronic instruments and their applications (technology and properties of photoelements with the external photoelectric effect and of electron multipliers, measurement of the parameters of photoelements, use of photoelements in sound cinema and phototelegraphy, certain properties of iconoscopes—8 papers).

5. Cathodes in a gas discharge (operating regimes of cathodes in gas-discharge devices, phenomena in mercury rectifiers—9 papers).

Thus, in all, 41 papers were scheduled in the program, of which 32 papers concerned cathode phenomena in a gas discharge—more than one and a half times the number of communications at the Conference of 1939. This shows that research work in this field continues to develop rapidly. Another distinctive feature of this conference was that its program included, alongside fundamental studies, practical questions connected with the use of electrovacuum devices (section 4). This circumstance is highly characteristic for a whole series of reasons. It reflects, first, the striving to establish closer contact between research laboratories and organizations directly interested in the practical application of new electrovacuum devices. Secondly, it shows that at the present time some developments (antimony–cesium photoelements) have already passed beyond the laboratory stage and have found important applications in technology. It should be added to this that, as was quite clearly revealed in the discussion on photoelements for sound cinema, antimony–cesium photoelements have every basis for being used much more widely than hitherto.

Another indicator of the progress represented at this conference was the fact that among the papers there were three presented by laboratories of the Moscow Electric-Lamp Plant—the largest producer of photoelements in the USSR.

Thus, evaluating the conference as a whole as an indicator of the development of the work since the last conference, one may note the quantitative growth of research, the establishment of closer ties among research organizations, industry, and direct consumers, a broader coverage of research organizations and, it should be especially emphasized, that one of the lines of work has already yielded quite concrete and valuable results.

II. Among the reports of the first of the sections indicated above, it is first of all necessary to note two communications made by S. A. Vekshinskii: “On equilibrium states of the oxide-silver-cesium photocathode” and “The microstructure of antimony-cesium photocathodes.” They are distinguished by a new and original experimental technique, which, despite its fundamental simplicity, made it possible with complete certainty to detect the finest inhomogeneities in the emission properties of the surface of photocathodes and to establish the connection of these inhomogeneities with the crystalline structure of the layers. The essence of this method consists in examining the test surface with a light spot of very small dimensions (about \(0.05 \times 0.5\ \mathrm{mm}^2\)), obtained with the aid of a microscope objective, and in the continuous automatic recording of galvanometer readings (photocurrent) on a photographic plate. Automatic and continuous recording is achieved by combining the motion of the light source with the microscope objective and the photographic plate, mechanically linked to one another. The small size of the spot and the automatic recording ensured a very detailed and rapid examination of the surface, impossible by other methods. In application to Cs—O—Ag cathodes, the nature of the distribution of sensitivity over the cathode surface as a function of the degree of silver rarefaction was established; the dependence of the sensitivity of various oxide-coated surface regions on the time of their treatment with cesium vapors was established. Data of this kind, in agreement with the results of earlier works,^2 lead to the conclusion that the maximum sensitivity of the cathode corresponds to a definite ratio between the amounts of oxygen and cesium. Examination by the same method of the surface of antimony-cesium cathodes also showed the presence of significant inhomogeneities of sensitivity. This led to an investigation of the structure of the original antimony layers (obtained by evaporation in vacuum), which showed that these inhomogeneities must be attributed to crystallization of the layer, beginning immediately after its formation. Further, many interesting and practically important data were reported on the process of crystallization and on the formation of compounds of antimony with various metallic substrates (Cu, Ni). One of the few metals that does not form compounds with antimony turns out to be tungsten.

Another communication concerning photoemission was S. S. Prilezhaev’s report, entitled “Secondary emission and the photoeffect from potassium as a function of the coverage of potassium by hydrogen.” Study of the change in the work function of potassium upon adsorption of atomic hydrogen (at low temperature), of the course of the spectral curve, of the magnitude of the coefficient of secondary emission, and of those changes which take place as a result of heating, compels one to consider that the high quantum yield of KН cathodes and the change in \(\sigma\) should be attributed to potassium particles embedded in KН.

The question of secondary emission in this section was represented by four communications.

In the report of P. V. Timofeev and R. M. Aronovich, “Barium and magnesium emitters of secondary electrons,” the properties of these emitters and the method of their manufacture were described.^3 D. I. Arkad’ev and T. Ya. S’ora (Odessa State University) reported on a study of the secondary emission of metallic calcium.

M. M. Budynskii reported on his new studies of the secondary emission of KCl, TlCl, AgCl, and CuJ, concerned primarily with the question of the stability of the secondary emission of dielectric layers. Budynskii believes that stable secondary emission of KCl is possible under the condition that, between the density of the primary beam and the concentration of the alkali metal atomically distributed in the layer, there exists a definite ratio.

In other words, this means that (at a given temperature) there must exist a definite relation between the density of the beam and the electron conductivity of the layer. This is consistent with the statement made earlier^4 that only an electronic semiconductor can be a stable emitter of secondary electrons.

Some of Budynskii’s propositions concerning the ideas about the structure of layers and their changes as a result of secondary emission, as well as the experimental technique used by Arkad’eva and Sbor, were criticized.

In the report by M. E. Gurtovoi (Kiev State University), “Secondary Electron Emission upon the Impact of Ions and Atoms on the Surface of Doped Tungsten,” the results of work with potassium and metastable mercury ions, as well as with potassium and cesium ions, were presented. The conclusions reached by the author are essentially that kinetic ejection has an impact-ionization character, and also that the ejection of electrons occurs not only from the upper layer, which directly experiences the impact.

The remaining four reports in this section were devoted to thermionic emission and thermal ionization on heated cathodes. T. P. Kozlyakovskaya (VEI) reported on an experimental study of alkaline-earth oxides; N. G. Sushkin (MEI), on a study of emission from an oxide cathode by means of an electron microscope.

In the report by L. N. Dobriova (State University in Alma-Ata) on the heat of evaporation of electrons from doped tungsten, an interesting voltage-stabilization circuit was described, permitting the constancy of the potential difference to be maintained with an accuracy of up to \(10^{-7}\ \mathrm{V}\). The main conclusion from the experimental results obtained is that, in the region of the anomalous Schottky effect, the influence of the field on the emission of electrons consists in a change in the work function, and that changes in the heat of evaporation of electrons are sufficient to explain the anomalous Schottky effect without assuming a dependence of Richardson’s constant \(A\) on the field.

N. I. Ionov (Leningrad State University), in the report “Formation of Negative Ions of Halogens on the Surface of Heated Tungsten,” reported on a method he had developed for detecting and measuring small currents of negative ions in the presence of large electron currents. With the aid of this method, the formation of negative halogen ions was detected during the interaction of molecules of alkali-halide salts with the surface of heated tungsten, and the values of the electron affinity of fluorine, chlorine, bromine, and iodine atoms were determined (4.11; 3.75; 3.64; and 3.31 eV, respectively).

III. In the section devoted to antimony–cesium cathodes, fundamental and theoretical questions, questions of technological character, as well as the properties essential for the use of these cathodes in photoelectric devices, and the secondary-emission properties of antimony–cesium layers, were addressed.

S. Yu. Luk’yanov, in the report “On the Mechanism of Operation of the Antimony–Cesium Cathode,” proposed an energy scheme for such a cathode and discussed the applicability of Fowler’s method to semiconducting cathodes in general. In his opinion this method is more or less applicable to photoemission in the region close to the long-wavelength limit.

The communication by P. G. Borzyak (Physical Institute of the Academy of Sciences of the Ukrainian SSR), “Some Properties of Cathodes of the Antimony–Cesium Type,” contained an account of studies of the internal photoeffect in Sb—Cs layers.

These experiments, more direct than the earlier ones^5, showed the presence of an internal photoeffect in this semiconductor and also provided new evidence of the electronic nature of conductivity in them. Further, P. G. Borzyak proposed a conduction mechanism for Sb—Cs cathodes, and also gave a description of the properties of cesium compounds with other metals of Group IV of the periodic system, which, as a rule, turn out to be considerably less effective electron emitters than Sb—Cs layers.

N. D. Morgulis and B. I. Dyatlovitskaya (“Emission Properties of Antimony–Cesium Cathodes”) considered the secondary-emission and photoelectric properties, and the phenomena associated with them, in the case of antimony–cesium cathodes located both on a conducting substrate and directly on glass.

N. S. Khlebnikov reported on new measurements of the volt-ampere and light characteristics of antimony–cesium photocells (with cathodes on substrates and without substrates), extended also to the region of large luminous fluxes (up to 100 lm), and also on a study of the fatigue of these photocells over a very long period (up to 5000 hours). As it turns out, in photocells with cathodes on a substrate saturation is always observed, and the light characteristic remains linear up to luminous fluxes of 100 lm. In the regime of luminous fluxes of the order of 0.02–0.04 lm (close to the sound-film regime), even under continuous operation, antimony–cesium photocells reduce their sensitivity by less than 50% over 5000 hours, in many cases surpassing all other photocells in constancy. This is especially important in view of their high integral sensitivity.

G. S. Ryabinin (MPEI) reported on the secondary-emission properties of antimony–cesium layers obtained by various methods. As it turns out, the greatest value of $\sigma_{\max}$ (up to 13.6) is given by antimony layers treated with cesium vapors without heating the antimony layer.

We have already spoken above about the work of S. A. Vekshinskii, very important for understanding the technology of antimony–cesium cathodes. Another report concerning questions of technology was made by V. S. Parkhomenko, who reported on a new method of uniformly applying an antimony layer to flat and cylindrical surfaces and on certain properties of antimony layers (this method was developed for the purpose of creating multipliers with resistance distributed over the layer, first described by P. T. Farnsworth).

IV. The discussions on the nature of secondary emission were preceded by four reports: P. S. Tartakovskii, summarizing existing views on the phenomenon of secondary emission, N. D. Morgulis (“Certain Factors Affecting the Secondary Emission of Complex Cathodes”), I. M. Dikman (“Theory of Secondary Emission of Complex Emitters”), and A. E. Kadyshevich (“Secondary Emission from Dielectrics”).

The most essential new data were contained in the last of the reports listed. Kadyshevich extended his method of considering secondary emission (applied at first to metals) to the case of a dielectric and, refining the calculation (allowing for secondary electrons emitted after several elastic collisions), showed that in accord with experiment, the values of $\sigma_{\max}$ in the case of dielectrics may exceed by an order of magnitude the $\sigma_{\max}$ for metals, and that the $V_p$ corresponding to the maximum of the curve $\sigma = (V_p)$ must be greater for dielectrics than for metals.

To everyone’s surprise, the discussion itself, unlike all the preceding cases, passed to the highest degree placidly. This, however, was for quite substantial reasons. The point is that at the conference two points of view were presented, the principal difference between which consists in the fact that whereas the first (Khlebnikov, Morgulis, Kadyshevich) regards the phenomenon as a volume effect, the second (Timofeev) believes that it is determined by phenomena on the surface of the emitter (in the case of complex surfaces). But since the results reported by Kadyshevich in his paper testified with great persuasiveness in favor of the first point of view, no one (including the author himself) showed much desire to defend the second. In general, insofar as it was possible to establish by personal questioning, the general opinion comes down to the fact that phenomena on the surface (ionization of surface atoms) may be supposed merely to be superimposed on the basic phenomenon (which has a volume character) as a certain higher-order correction of small magnitude.

V. The first report in the section on electronic devices and their applications was the report by L. A. Kubetskii, who reported on the high-speed method he had developed for measuring the weakest luminous fluxes with the aid of elec-

...tron multipliers, which he called an “integral-balance system.” The essence of the method, hidden behind this somewhat complicated name, consists in charging a certain capacitance from the light flux under investigation and then from a known light flux over a definite interval of time, and in comparing the accumulated charges. This method thus has an immediate predecessor in the “charging method” when working with an electrometer.

Representatives of the laboratory of the Moscow Electric-Lamp Plant delivered three communications. L. G. Leiteizen reported on work on electron multipliers being carried out in this laboratory; T. N. Rabotnova reported on the major work done in the laboratory in standardizing methods and conditions for determining the parameters of photocells of various types. The third communication concerned a comparative assessment of gas-filled photocells operating at high (240 V) and low (90 V) anode voltages. The extensive statistical material collected in the laboratory shows that filling a photocell with gas for operation in the “low-voltage” regime gives real advantages by virtually all criteria.

G. S. Wildgrube reported on the design and properties of an electrostatic multiplier of the Farnsworth system, with a surma cathode and emitter, developed by him jointly with V. S. Parkhomenko.

An unexpected and very lively discussion arose in connection with the report by K. A. Lamagin (Leningrad Institute of Motion-Picture Engineers), “On the Question of the Use of Photocells in Sound Cinema.” This report showed that the sensitivity of existing vacuum antimony-cesium photocells is more than sufficient for their use in sound cinema in place of the gas-filled oxygen-silver-cesium cells currently employed there, and that, consequently, the objections raised against antimony-cesium photocells on account of their allegedly insufficient sensitivity do not withstand any criticism. Representatives of the Scientific-Research Institute of Film Engineering (A. A. Khrushchev, Prof. P. V. Timofeev) presented the opposite point of view.

A very interesting report, accompanied by a large amount of excellently selected illustrative material, was delivered by A. M. Gurevich (NIIS NK Svyazi) on the topic “The Use of Antimony-Cesium Photocells in Phototelegraphy.” The report vividly outlined the great and already successfully realized practical possibilities opened up for phototelegraphy by the creation of highly sensitive vacuum antimony-cesium photocells and by their mastery by our industry (Moscow Electric-Lamp Plant). These include: the transmission (in two-color reproduction) of color images, including maps, colored texts, and authentic documents; a great increase in the reliability of phototelegraph communication and a significant reduction in operating costs. Equally great possibilities are opened up (thanks to their constancy and especially the distribution of the sensitivity of these photocells) for objective photoelectric pyrometry.

An interesting and profound analysis of the physical factors determining the course of the sensitivity-distribution curve of iconoscope mosaics was given by I. G. Kesaev (NIIS NK Svyazi) in the report “Light and Spectral Characteristics of Iconoscopes.”

VI. The following reports were devoted to cathode phenomena in a gas discharge:

1. A. A. Shemaev (VEI), “Cathodes of Gas-Discharge Devices.”
2. E. A. Yakuninsky (LIKI), “Operating Conditions of Cold Cathodes in Gas-Discharge Devices.”
3. N. D. Morgulis and Ya. L. Lyubarsky (Kiev State University), “Sputtering Threshold of Oxide Cathodes.”
4. A. V. Vorob’ev (LII), “Cathodes of Gas-Discharge Devices.”
5. E. Yu. Kleiner (MEI), “On the Temperature Regime of Oxide Cathodes in a Low-Pressure Gas Discharge.”

1. M. E. Gurtovoi and T. I. Kovalenko, “The Use of Thermal Ionization of Cesium on Surfaces in Electron Tubes.”

2. I. A. Sokolov (Rostov Institute of Railway Transport Engineers), “The Phenomenon of Discharge at a Large Cathode Surface.”

3. N. N. Petukhov (Elektrosila Plant), “The Occurrence of a Cathode Spot on the Anode of a Mercury Rectifier and Its Investigation by Means of a Pulse Generator.”

4. M. D. Gabovich (Institute of Physics, Academy of Sciences of the Ukrainian SSR), “Reverse Ignition and Secondary Emission on a Graphite Anode in a Mercury-Vapor Discharge.”

5. A. E. Askinazi (LII), “An Investigation of the Probability of Reverse Ignitions.”

VII. In summing up the results of the conference, it is necessary to note the following most important outcomes.

1. The reported recent theoretical works (especially Kadyshevich’s work) provide a quantitative basis for one of the points of view on the phenomenon of secondary emission, which means the establishment of the unity of the mechanism of the phenomenon in all cases, regardless of the nature of the emitting material, i.e., a new step in theoretical generalization.

2. A number of reports were a brilliant demonstration of the overwhelming advantages of antimony–cesium (vacuum) photoelements over those currently in use. This circumstance will serve as an incentive for their further and still broader use, in particular in one of the most widespread applications—in sound cinema.

Thus, while reflecting the growth of scientific work in this field by all indicators, this conference also testifies to the successful practical realization of new developments which only a year ago had not yet gone beyond the limits of laboratories and were merely the subject of discussion in principle.

N. S. Khlebnikov. Moscow.

LITERATURE

1. Uspekhi fizich. nauk, 22, 105, 1939.

2. N. S. Zaitsev and N. S. Khlebnikov, Zhurnal tekhnich. fiziki, 8, 1023, 1938.

3. P. V. Timofeev and R. M. Aranovich, Zhurnal tekhnich. fiziki, 10, 32, 1940.

4. N. S. Khlebnikov, Uspekhi fizich. nauk, 21, 333, 1939.

5. N. S. Khlebnikov and N. S. Zaitsev, Zhurnal tekhnich. fiziki, 9, 44, 1939.

6. A. E. Kadyshevich, J. Phys. Acad. Sc. USSR, 2, 115, 1940.