US2701319A - Electrometer amplifier tube - Google Patents

Description

Feb. 1, 1955 J. H. COLEMAN ELECTROMETER AMPLIFIER TUBE 2 Sheets-Sheet 1 Filed Nov. 20, 1950 l/ll/I/II INVENTOR.

I/IIIIIl/I BY K (DR-Ma... imwfi Feb. 1, 1955 J. H. COLEMAN 2,701,319

ELECTROMETER AMPLIFIER TUBE Filed Nov. 20, 1950 2 Sheets-Sheet 2 INIVENTOR.

United States Patent its! ELECTROMETER ANIPLIFIER TUBE John H. Coleman, Palm Beach, Fla., assignor to Radiation Research Corporation, West Palm Beach, Fla., a corporation of Florida Application November 20, 1950, Serial No. 1%,553

13 Claims. (Cl. 313-69) This invention relates generally to electrometer amplifier tubes, and more particularly to a unique and new combination of tube elements with a deflection type control providing high sensitivity and low control electrode currents.

In practical operations, ordinary amplifier tubes fail to approach the requirements of an electrometer tube, defined by Victoreen Electrometer tubes for measurement of small currents, Proceedings of the Institute of Radio Engineers, volume 37, page 432, 1949, as a tube in which the control element has a leakage current of less than amperes.

In conventional vacuum cathode tubes, the limitations on the control grid resistance have been found, as reported by G. F. Metcalf and B. T. Thompson, A low grid current vacuum tube, Physical Review, volume 36, page 1489, 1930, to be current from the following factors: (1) positive ions formed by collisions of the electron beam with gas in the tube; (2) ions emitted by the filament; (3) photo-electrons emitted by the control grid under'action of light from the filament; (4) photo-electrons emitted by the control grid under action of soft X-rays produced by the normal anode current.

After careful processing, as reported by Victoreen, a standard tetrode can be made to operate with low control grid current by adjusting the plate voltage to a low value to eliminate ionization (factor 1) and soft X-rays (factor 4); and by operating the first grid at a slightly positive potential to repel positive ions from the cathode (factor 2); and by operating the cathode at a low temperature to reduce photo-electric emission from the control grid (factor 3). The voltage gain for this type of operation is less than unity;however, the current amplification is sufficient to operate an additional standard amplifier, which is necessary and required, however, to measure and record low current.

Among the prior approaches to the control of electron streams are the beam deflection type amplifiers, in which instead of changing the current density of the electron stream as does the grid of an ordinary vacuum amplifier, the control electrode of the tube electrically deflects the electron stream between an array of collecting plates; the electron ray tube, which is similar to the deflection tube in that the control elements change the direction of the electron paths in the beam, but differs from the deflection tube in that a fluorescent screen is used to indicate the change in beam direction instead of a system of collector plates; the post deflection-acceleration cathode ray tube, which is similar to the electron ray tube except for the beam which is sharply focused to a spot instead of being a Wide angle stream; and, tubes employing positive ion trapping in a field-free region for space charge neutralization to given a sharply focused, high current beam. One of these tubes was used as an amplifier by varying the amount of trapping by electrically removing some ions from the field-free region by the application of the signal as a sweeping out potential to vary the beam density accordingly.

In all of the deflection tubes, electron ray tubes, and cathode ray tubes described in the preceding paragraph, the control electrode current is excessive due to the positive ions formed in the long beam path. The long path increases deflection sensitivity in direct proportion to length; however, the path for ionization is also increased proportionately. The spacecharge neutralized amplifier 0 was fundamentally unsuited for low current work because .1

2,701,319 Patented Feb. 1, 1955 the control electrode had to collect current for the operation of the tube.

The present invention, on the other hand, by a unique and new combination of the beam deflecting principle and positive ion trapping, provides a high deflection sensitivity by a long drift space for the electrons after deflection, and provides a low control electrode current by retaining in the drift space all positive ions formed therein. Also, a special combination of radiation shielding for the control elements with positive ion trapping is provided to further sensitivity.

, Accordingly, an object of this invention is to provide an improved method of measuring small currents, and an improved electrometer amplifier tube to measure small currents.

Another object of this invention is to provide means for measuring small currents comprising a source of electrons in an evacuated medium, an electrical system for buiding up a potential in accordance with the small current, varying the direction of the electron trajectories in accordance with the potential on the said electrical system, projecting the electron stream through a second and substantially field-free region of said medium whereby the space charge within the beam is neutralized by positive ions, and colleeting the electron stream in such a manner as to indicate visibly or electrically the magnitude of the change in direction of the electron trajectories and consequently the magnitude of the impressed small current.

Another object of this invention is to provide in combination with the means aforesaid, means to shield the control electrode from internally generated radiation, such as photo-electrons from the cathode and X-rays from the bombardment of the collecting anode by the electron stream; and/ or means for electro-statically repelling positive ion emission from the source of electrons away from the deflection control region; and/or means for electrostatically shielding the deflection control region from positive ions formed by collision of the electron stream with neutral gas particles in the control region.

A still further object of this invention is to provide a single tube detection and measuring system for small currents.

The foregoing and other objects and advantages of this invention will be more apparent from the following description, in conjunction with the drawings, forming a part thereof, wherein:

Fig. 1 is a transverse cross-sectional view of the tube showing the basic elements of this invention with collector plates as an output system;

Fig. 2 is a transverse cross-sectional view of the tube showing the basic elements of this invention with three control electrodes and a fluorescent screenas a visual output system;

Fig. 3 is a cross-sectional view of a longitudinal section of the tube showing the basic elements of the invention with end plates providing axial deflection, electrical lead lines shown in elevation;

Fig. 4 is a view similar to Fig. 3 with a fluorescent screen as a visual output system;

Fig. 5 is a transverse cross-sectional view of the basic elements of the invention employing an outer electrode with a plurality of control electrodes and collector plates;

Fig. 6 is a cross-sectional view of a longitudinal section of the tube showing a frustro-conical shaped fluorescent screen as a visual output system, electrical lead lines shown in elevation;

Fig. 7 is a cross-sectional view similar to Fig. 1 with a plurality of control electrodes and two collector plate systems;

Fig. 8 is a cross-sectional view of a longitudinal section of the tube showing a modification of the invention emloying an outer cathode with axial deflecting plates and a two system collecting anode, electrical lead lines shown in elevation;

Fig. 9 is a cross-sectional View of some elements of the invention showing the location of a space charge grid to repel positive ions from the cathode;

Fig. 10 is a view similar to Fig. 5 modified to show shields between collector plates and control electrodes and between control electrodes and the cathode;

Fig. 11 is a cross-sectional view of part of the elements of this invention showing the use of a screen grid to provide electro-static shielding of the control elements from the accelerating potentials on the trapping grid;

Fig. 12 is a cross-sectional view taken along the transverse line 12+12 of Fig. 11.

Referring to the drawings, wherein like members are given the same reference numeral, a cathode .1, usually and preferably cylindrical in form, has spaced therearound or equi-distant therefrom a plurality of control electrodes 2, specifically 2 2 22 2 2 2 2 The cathode 1 provides a source of electrons which are attracted by a positive potential on the accelerating and trapping grid mesh 3 and .on to the collecting member 5, specifically '5 "55 5 5 5 5 and 10, which may be collector plates in one embodiment of the invention and a fluorescent screenin-another embodimentof the invention. The grid mesh 3 is usually in the form of a cylinder and positioned intermediatethe control electrodes 2 and the collecting members 5 with which the grid mesh 3 forms a field-free region 18 for positive ion trapping. In the case of the fluorescent screen 7 the collector 5 is electrically connected to the trapping grid 3. In the case in which collecting plates 5 are used, the individual plates can be operated at the same potential as the trapping grid 3; however, separate leads are brought out to indicate the collected current. The elements are enclosed in a glass or other transparent vacuum envelope 6, having the usual and necessary electrical lead lines sealed therethrough and therein. These are the basic elements of the invention, and the-additions, modifications, and changes will be described with respect to the particular drawing illustrating them. It is understood, however, that the invention is not limited to the additions and modifications described, as they are shown in illustration and not in limitation.

In Fig. 1, the cathode 1 is cylindrical in form and positioned along the longitudinal axis of the tube 6 and emits electrons, which are accelerated by the positive potential on the grid mesh 3 of any suitable conductor material permeable to electrons and attracted to the collecting plates 5 and 5', being semi-cylindrical. The grid mesh 3 is a cylinder, preferably of tungsten or molybdenum, and connected on each end by electrical conductors to enclose the region between the grid mesh 3 and the collecting plates 5 and 5 The distribution of the current of the electron stream 17, defined in the drawing between the lines there shown, to the collecting plates 5 and 5 is determined by the relative deflecting potentials on the control electrodes 2 and 2'. An increase in negative potential on 2 for example, increases the current to the opposite collecting plate 5 be attached to the collecting plates 5 and 5 with any type of load in conventional beam deflection plate circuits. When the accelerating, trapping grid 3 and collecting plates 5 are operated at the same potential, the potential of the inter-electrode space '18 remains negative with respect to these electrode potentials due to the etfect of the negative charges of the electron stream 17. The mutual repulsion of the negative charges ordinarily re- .sults in a limitation in the current density of the electron stream 17; however, positive ions formed by collision of the electrons with the neutral gas are attracted into the electron stream 17 until the space charge depression of potential is neutralized, resulting in two favorable effects. First, the positive ions formed in the inter-electrode space or trapping space 18 do not pass through the trapping grid 3 to the control electrodes 2; and, secondly, deflection sensitvity of the .control electrodes 2. is increased by the removal of current density limitations in the trapping or drift space 18 of the beam or electron stream '17 before impinging upon the collecting plates 5.

In Fig. 2, two modifications of Fig. l are shown. First a fluorescent screen 7 is employed to indicate visually the variation in direction of the electron stream 17 in accordance with the potential on the control electrodes 2 by fluorescence. The collector S can be evaporated on a phosphor, according to the teaching of D. W. Epstein and L. Pensak, RCA Review, volume 7, page 5, March 1946, which phosphor can then be deposited directly upon the envelope 6. Secondly, a third control electrode has been added. The spacing between the control electrodes 2 2 and 2 can be variedto give varying degrees of sensitivity. For example, in the drawing the electron stream 17 pass ing between the closest spaced electrodes, 2 and 2 is the most sensitive in deflection to the control voltage.

There are several additions and modifications of the Conducting leads can invention as illustrated in Fig. 3. The electron stream 17 emitted by the cylindrical cathode 1 is deflected axially between the cylindrical collector plates 5 and 5 by the potential on the control electrodes, the disc shaped one 2 and annular ring 2 An increase in negative potential on the control electrode 2 results in an increase in the current to 5 The positive ion trapping is enhanced by providing trapping grid plates 12, in the form of annular rings attached to the respective ends of the grid mesh 3 and extending at least to the wall of the cylindrical collector plates 5 to which they are electrically connected, thus completing the electrical enclosure of the field-free space. Some details of the tube construction are shown in the drawing, such as the filament leads 8 of the cathode 1 being brought down to the tube base. Also, the leads to the control elements 2 can be brought out the top of the envelope 6 to provide a long surface leakage path over the glass envelope 6.

In Fig. 4 the axial deflection system illustrated in Fig. 3 is combined with the visual indication illustrated in Fig. 2. In this modification, the collector and phosphor combination isdeposited upon a glass cylinder, here designated the collector 5; however the collector phosphor screen is provided as previously described.

An inverted arrangement of the elements is illustrated in Fig. 5, to provide a large area cathode 1; this outer cathode 1 is illustrated as a cylinder, however, it could be a series of cylindrical cathodes ,or any other convenient form. The cathode 1 may conveniently have the active coating only on the surface between the control rods 2. In the embodiment illustrated, the control rods 2 and collector plates 5 are increased in number to four each; any number of these elements in any desired shape can be used the same as in the case of the tube with the inner cathode.

A change in visual output screen in the basic embodiment is illustrated in Fig. 6, in which the geometrical design of the phosphor collector screen is frusto-conical. In this design of the collector 5, it does not have to be transparent when the trapping plate 12 is constructed in the form of a grid mesh to permit observation of fluorescence on the side of the collector system 5.

In Fig. 7, there is illustrated a modification in the collecting plate system. The cylindrical cathode 1 is surrounded by a plurality of control electrodes 2, which are again shielded by the trapping grid mesh cylinder 3 from the positive ions formed by the electron stream 17 before collection by the plates 5 and 10. The series of collecting anodes 5, equal in number to the control electrodes, are placed equidistant from each other and on an equal radius. The collecting cylinder 10 is on a greater radius than the collecting anodes 5, and employed therewith to obtain desired field strength at the edges of the collecting anodes 5. Thus, the electron stream 17 can be deflected by the control electrodes 2 between the collecting plates 5 and cylinder 10 with higher sensitivity than the system illustrated in the previous figures, when the relative potentials of adjacent electrodes in the collecting system vary in magnitude with the corresponding current and load of the particular electrode. The collecting anodes decrease in potential due to the drop across the load in the output circuit; however, one electrode can have a reduction in potential of at least 10% below other potentials before space charge neutralization is lost by sweeping out an appreciable number of positive ions even at low pressures. If, one the other hand, either collector system were reduced in potential by a sufficient amount to sweep out positive ions, only the deflection sensitivity would be reduced by space charge limitations. The control electrode current would not be increased in any case due to the positive ion potential barrier formed by the trapping grid mesh 3 with respect to any reduction in the potential of the collecting anode systems.

The collecting anode arrangement employed as illustrated in Fig. 7 may be inverted with respect to the other elements as illustrated in Fig. 8. The outer cathode 1, in the form of a cylinder and provided with heating coils 9, emits electrons controlled by axial deflecting plates 2, accelerating and trapping grid 3 and trapping grid plates 12 electrically connecting the grid 3 with the collecting anodes 5 and collecting plate 10.

In Fig. 9 there is illustrated the use of the conventional space charge grid 11 which can be operated at a positive potential to repel positive ions back into the cathode 1. This figure also illustrates a linear type of beam structure. The cathode 1 andother elements are shown as rectangular; however, any other geometrical design, such as spherical and cylindrical, can be used as a single unit instead of the multiple unit shown in previous figures.

The basic arrangement of elements as illustrated in Fig. is modified inFig. to show additional elements shielding the control electrodes 2 and improving sensitivity. The space charge grid 11, in the form of a mesh cylinder similar to and of similar material as the trapping grid' 3, is positioned intermediate the control electrodes 2 and cathode 1 and provided with solid strips 15, of lead, tungsten or molybdenum foil opaque to photons, to shield the control electrodes 2 from the photons emitted from the filament. Although the structure illustrated is similar to that of Fig. 5, any other type of structure may be used and any form of light shields located between the cathode 1 and control electrodes 2 as long as the path of the electron stream 17 is not restricted between the cathode 1 and collecting anodes 5. Another modification and improvement on the basic elements of this invention is also shown in this drawing, namely the shields 13, similar to the shields 15, positioned on the trapping grid mesh 3 to absorb X-ray radiation from the collecting anodes 5 under bombardment by the electron stream 17. These shields can also be located on other electrodes and can have any desired shape as long as the electron stream 17 is not restricted. These shields 13 and 15 may each or both be used with the basic elements of the invention.

In Fig. 11 another feature that may be added to the basic elements of the invention to determine the output characteristics is illustrated. A screen grid mesh 16, similar to the grid meshes 3 and 11, is positioned intermediate the trapping grid mesh 3 and and the control electrodes 2 and can be operated at such a potential as to increase deflection sensitivity, to restrict ionization by the electron stream 17 in the control electrode region, and to focus the positive ions that are formed in the high velocity region between the trapping grid 3 and screen grid 16 away from the control electrodes 2. Without the screen grid 16 some positive ions are formed in the region between the cathode 1 and the trapping grid 3 or between the trapping grid 3 and the space charge grid 11, by the collision of the electron stream 17 with neutral gas in this region, despite the close spacing of these electrodes. Normally, some of these positive ions will be collected by the negative control electrodes 2 despite other forces tending to carry them into the cathode 1. Thus, the screen grid 16 can be operated below the ionization potential of the gaseous medium to restrict any ionization in the control region between the cathode 1 and the screen grid 16. A further advantage of the screen grid 16 is that its geometry can be adjusted to cause the positive ions formed between the trapping grid 3 and screen grid 16 to be injected into the control region between the cathode 1 and the screen grid 16 with the control electrodes 2 a less favorable target. The cylindrical structure, better illustrated in Fig. 12, is an example of a geometrical design which tends to focus the positive ions away from the control electrodes 2 into the cathode 1 where they are collected as current.

This invention eliminates the necessity for using two tubes in electrometer work to obtain any voltage amplification of small currents because the plate voltage is not restricted.

The tube as illustrated and described herein can be used with any standard amplifier circuit. When it is used in the measurement of small currents, this tube eliminates the necessity of an additional amplifier tube to obtain voltage gain, but is electrically connected in the circuit the same as an amplifier tube would be connected.

The mutual conductance and amplification factor of the tube of this invention is determined by the relative dimensions and geometry of the tube elements, and can be determined by conventional calculations. Extending the drift or field-free space increases the mutual conductance.

A practical application of the invention is in the detection of small currents encountered in nuclear work. For example, a visual type tube could be used with an ionization chamber as a simple radioactive detector; and meters can be used in the output circuit for operation directly as a sensitive electrometer.

Having thus described the invention, what is claimed as new and desired to secureby grant of United States Letters Patent is:

1. In a vacuum tube the combination comprising a cathode, control electrodes, a series of collector plates concentrically spacedfrom the cathode and exposed directly to the latter in a field free region, a trapping grid mesh intermediate said collector plates and control electrodes and having the same potential as said collector plates, and shields between said cathode and collector plates to absorb X-rays of bombardment in the cathode stream.

2. In a vacuum tube the combination comprising a cathode, control electrodes, a series of collector plates concentrically spaced from the cathode and exposed directly to the latter in a field free region, a trapping grid mesh intermediate said collector plates and control electrodes and having the same potential as said collector plates, a positive ion repelling grid positioned intermediate said cathode and control electrodes, and X-ray absorbing slliields positioned intermediate said cathode and collector p ates.

3. In a vacuum tube the combination comprising a cathode, control electrodes, a series of collector plates concentrically spaced from the cathode and exposed directly to the latter in a field free region, a trapping grid mesh intermediate said collector plates and said control electrodes and having at least the potential of said collector plates, photon absorbing shields positioned intermediate said cathode and said control electrodes, and X-ray absorbing shields positioned intermediate said cathode and collector plates.

4. In a vacuum tube the combination comprising a cathode, control electrodes, a series of collector plates concentrically spaced from the cathode and exposed directly to the latter in a field free region, a positive ion repelling grid mesh intermediate said control electrodes and said collector plates, X-ray absorbing shields intermediate said cathode and said collector plates, and a screen grid mesh intermediate said control electrodes and positive ion repelling grid mesh.

5. In a vacuum tube the combination comprising a cathode, control electrodes, a single collector system, uniformly and concentrically spaced from the cathode and exposed directly to the latter in a field free region, a positive ion repelling grid intermediate said collector and said control electrodes, photon absorbing shields positioned intermediate said cathode and control electrodes, X-ray absorbing shields positioned intermediate said cathode and collector system, and a screen grid mesh intermediate said positive ion repelling grid.

6. In a vacuum tube the combination comprising a cathode, deflecting electrodes, a series of collecting members, a trapping grid intermediate said collecting members and deflecting electrodes, trapping plates intermediate said trapping grid and collecting members and electrically connected to said trapping grid, and said trapping grid and collecting members having the same potential.

7. An electronic tube comprising an envelope enclosing a cathode, a segmental anode spaced therefrom, deflecting electrodes mounted intermediate said cathode and anode, and a positive ion trapping electrode intermediate said deflecting electrodes and said anode.

8. An electronic tube as claimed in claim 7 and including a positive ion repelling grid positioned intermediate said cathode and said deflecting electrodes to repel positive ions emitted by said cathode.

9. An electronic tube as claimed in claim 7 and including photon absorbing shields between said cathode and said deflecting electrodes to absorb photons from the cathode.

10. An electronic tube as claimed in claim 7 and including a screen grid intermediate said positive ion trapping electrode and said deflecting electrodes to change the electrical field at the deflecting electrodes to prevent positive ions between said cathode and said positive ion trapping electrode returning to said deflecting electrodes.

11. An electronic tube as claimed in claim 7 and including a positive ion repelling grid positioned intermediate said cathode and said deflecting electrodes, and photon absorbing shields positioned intermediate said cathode and said deflecting electrodes.

12. An electronic tube as claimed in claim 7, and including a positive ion repelling grid positioned intermediate said cathode and said deflecting electrodes, and

a estee gici in ermesl ate sa d p s t ve i n trapping e Re er n 3W i 1. 1? fi e 9 t is -,p en trode and e ecting electro es. 7

13- ;An electronic tube as plaimed in ,elaim 7 and in- UNITED STATES PATENTS plugling photon absql bing shields positioned intermediate 2,175,700 Roberts Oct. 10, 1939 saidpathode and -s aid defie etipgeleetrodes, and a spreen 5 2,197,041 Gray Apr. 16, 1940 grid intermediate said positive ion itrapping electrode and 2,243,408 Anderson et al. May 27, 1941 said deflecting ele etr odes. 2,273,800 Schliemann Jensen Feb. 17, 1-942 7 2,278,630 Winter Apr. 7', 19.42

2,311,672 Le Van Feb. 23, 1 943 0 12,470,732 Visscher May 17 1949

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