US2418574A - Electron multiplier - Google Patents

Description

April 8, 1947. M. CAWElN ELECTRON MULTIPLI'ER Fil ed Dec. 20, 1943 INVENTOR MADISON CAWEIN ATTORNEY Fatente Apr. 8. i947 STATES PATENT ELECTRON MULTIPLIER Madison Cawcin, Fort Wayne, Ind., assignor to Farnsworth Televisionand Radio Corporation, a corporation oi. Delaware Application December 20, 1943, Serial No. 515,004

1 Claim.

tively small numbers of electrons. An electron multiplier of this character consists of a plurality of secondary electron emissive electrodes arranged in such a manner that the electrons travel from one electrode to another. Upon the impact of the electrons with one of the secondary emissive electrodes a multiplied number of electrons are produced and are drawn toward another electrode. After enough of such impacts the number of electrons is so materially increased from a small original number that they may be withdrawn from the multiplier andcaused to traverse an impedance element for the development therein of a signal voltage. A collector electrode is usually provided adjacent the last multiplier secondary emissive electrode to intercept the elec-- trons and to conduct them by suitable connections to an external impedance device. The collection of the electrons is accomplished by impressing upon the collector electrode a potential which is more highly positive than that impressed upon the last secondary emissive electrode.

As is well known in the art, however, the electron current flow from the collector electrode through the external impedance device produces a potential variation which is dependent upon the magnitude of the electron current flow. The effective potential of the collector electrodethen is subject to considerable variation which causes a. corresponding distortion of thesignal voltages developed in the external impedance device. This type of distortion is particularly noticeable when multiplier do not correspond thereto.

It is an object of the present invention, therefore, to provide an electron multiplier structure of such a, character that signal distortions caused by potential variations of the output electrode are eliminated.

In accordance with the present invention, there is provided a collector electrode disposed in proximity to the final secondary electron emissive electrode and normally "operated at a positive potential with respect to the final secondary emissive electrode. Also, there is disposed between the collector electrode and the final secondary 5 emissive electrode an auxiliary electrode which is maintained at a substantially constant positive potential. The electron current flow to the collector electrode then is determined primarily by the unvarying potential impressed upon the intervening auxiliary electrode.

For a better understanding of the invention,

together with other and further objects thereof,

reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the accompanying drawing;

Fig. 1 is a diagrammatic representation of a dissector tube embodiment of the invention;

Fig. 2 is a schematic wiring diagram showing the electrical characteristics of the pertinent components of a-dissector tube drawn to an enlarged scale; and

Fig. 3 illustrates a modified form of electron multiplier embodying the invention.

Having reference now to Fig. l of the drawing, there is illustrated a sectional view of a dissector tube provided with an evacuated envelope ll. Mounted within and adjacent to one end of the envelope is a photoelectric cathode l2. Also,

within the envelope and adjacent to the end onposite thev cathode there is mounted a main-- stage electron multiplier housed within a metallic shielding anode l3. The anode'is provided with a recessed portion M at the end of which is formed a primary seaming aperture I5 which is disposed substantially centrally with respect to the cathode l2 and faces the cathode. An

accelerating anode is provided in the form of an 0 interior wall coating is between the cathode l2 and the anode l3.

A focusing coil i1 is disposed on the outside of the tube envelope H in a manner to surround substantial1y completely the space within the tube between the cathode and the anode. A battery It or other suitable source of electrical energy is connected, to the focusing coil for the energization thereof to establish the desired focusing field within the tube envelope whereby an electron image lsformed substantially in the plane of the anode It. The tube also is provided with horizontal and vertical scanning coils l9 and 2!, respectively. Each of these coils is energized by respective sources 22 and 23 of appropriate saw-toothed wave form voltages.

In conjunction with the dissector tube there also is provided an optical-system represented by a lens 24, whereby an optical image of a sub- Ject 25 is focused into the plane of the photoelectric cathode l2.

Referring now 'to Fig. 2 of the drawing, a more detailed description of the electrical connections to the photoelectric cathode l2 and the electron multiplier willbe given. The multiplier which is housed within the anode shield l3 comprises a plurality of box-lik electrodes such as 26, 21, 28 and 29. In one form of a so-called eleven-stage multiplier, the device includes eleven of such box-like electrodes, the eleventh stage also being in the form of a box type electrode such as 3D. A collecting electrode of suitable form such as aplate 3i is located adjacent the eleventh stage electrode 30. Also a grid-like auxiliary electrode 32 in accordance with this invention is located between the collector electrode 34 and the eleventh stage electrode 30. The first stage electrode 26 is provided with a secondary scanning aperture 33 in alignment with the primary scanning aperture 15 formed in the anode shield I3. The secondary aperture ordinarily is smaller than the primary aperture, being of substantially the same size as the desired elemental scanning area.

Suitable electron accelerating potentials are.

impressed upon the multiplier electrodes and the photoelectric cathode by appropriate connections to taps on a voltage divider which, as illustrated, comprises the series connection of resistors 34 to 45, inclusive. A source of relatively high potential, such as a battery 46, is connected to the terminals of the voltage divider. The negative terminal of the battery 46 is connected to the photoelectric cathode l2. The junction point between resistors 34 and 35 is connected to the shielding anode l3 and also, by means of an internal connection such as represented by a conductor 41, to the first stage multiplier electrode 26. Thus, the anode shield and the first stage electrode are operated at the same positive potential with respect to the cathode I2.

Similarly the electrodes comprising multiplier stages 2 to ll inclusive, the latter of which comprises the electrode 30, are connected respectively to other-taps on the voltage divider in a manner to impress increasingly higher positive potentials on succeeding multiplier electrodes. The eleventh stage electrode 30 is connected to the junction point between resistors 44 and 45. The other terminal of the resistor 45 is connected to the grounded positive terminal of the battery 46 and also to one terminal of an output resistor 48.

The other terminal of th output resistor is connected to the collecting electrode 3| of the multiplier and also to one of a pair of output circuit terminals 49 of which the other terminal is connected to ground. The screen. grid electrode 32 is connected to the grounded junction point between resistors 45 and 48.

Referring now to the operation of the illustrated embodiment of the invention, consider first the electrode voltage relationships in the case where the multiplier is completely inoperative by reason of no electrons being admitted thereto. Since there is no electron current flow through the output resistor 48 the potential impressed upon the collector electrode 3! is sub stantially the potential of the junction point between resistors 45 and 48. This same potential also is impressed upon the screen grid 32. At

the same time there is impressed upon the eleventh stage electrode 30 the potential derived from the junction point between resistors 44 and 45. This potential is less positive than the potentials impressed upon electrodes 3| and 32 by the amount of the voltage drop in the resistor 45 produced by the flow or the circulating current derived from the battery 46.

Assume now that a small number of electrons are admitted to the multiplier through apertures l5 and 33. After being multiplied in numbers by successive multiplier stages in a well known manner, th secondary electrons emitted by the eleventh multiplier stage electrode 30 are drawn toward the collector electrode 31 through the meshes of the grid-like electrode 32. By omitting from consideration for the moment the eflfect the screen grid 32, the electrons emitted by the electrode 3! will be collected by the collector electrode3l which is at a higher positive potential. The collected electrons are caused to traverse the output resistor 48 to produce a current flow therein whereby a signal voltage corresponding in magnitude to the number of collected electrons is developed.

The number of electrons which are caused to traverse the output-resistor 48 is a function of the number of electrons emitted by the electrode 30 and the voltage difference between this electrode and the collector electrode 3|. Assume for the purpose of this consideration that the voltage derived from the junction points between resistors 44 and 45 is solely dependent upon the circulating current through this resistor from the battery 46 and is in on way afiected by the relatively small electron currents traversing this resistor. In such a case the potential of the electrode 30 is maintained at a substantially constant value with respect to ground.

However, it is evident that the efiective potential impressed upon the collector electrode 3| is dependent upon the magnitude of the electron current flow through the output resistor 48. Con sequently, it is seen that the collector electrode potential varies as a function of the output circuit electron current flow.

Variations of the collector electrode potential at relatively small electron concentrations do not materially affect the response characteristic of the multiplier. In other words, the output voltage variations in the resistor 48 correspond to variations in the strength of the primary electron concentrations at the multiplier input. In this case the multiplier is said to have a linear response characteristic.

However, at relatively high electron concentrations, the variation of the collector electrode potential is of sufllcient magnitude to cause the voltages developed in the output resistor 48 .to no longer correspond in magnitude with the primary electron concentrations at the multiplier input;

- the electron current flow from the electrode 30 to the collector electrode 3| is determined almost entirely by the potential impressed upon the screen grid 32 with respect to the eleventh stage electrode 30. It is evident that, neglecting the effect of the relatively small electron current flow through voltage divider resistor 45, this potential remains substantially uniform regardless of the magnitude of the electron concentration in the multiplier. It is true, of course, that by reason of the variable electron current flow in the out put circuit including the resistor 48 the effective potential of the collector electrode 3| will vary, but since this potential no longer has any substantial efiect upon the electron current flow, the output voltages are more nearly representative of the primary electron concentrations at the multiplier input.

Fig. 3 shows another form of electron multiplier in which the present-invention may be embodied. In such a form the arrangement of the final multiplier stages is somewhat different than in the form illustrated in Fig. 2. Therefore, the

multiplier is shown fragmentarily to include only the portion thereof which differs from that previously described.

In this case the eleventh stage secondary emissive electrode is in the form of a plate 30' and is connected to the junction point between the voltage divider resistors 44 and 45. The collector electrode 3!! is in this case a grid-like structure and is connected to the ungrounded terminal of the output resistor 48. The auxiliary grid-like electrode 32 is interposed between the eleventh stage electrode and the collector electrode as in the previous case and is connected to the grounded junction point between resistors 45 and 48. It is seen, therefore, that the relationship of the auxiliary electrode with respect to the eleventh multiplier stage and the collector electrodes and the connections of these electrodes to the respective sources of potential is the same as in the previous instance.

By reason of the interposition of the collector electrode 31' and the auxiliary grid-like electrode 92 between two of the secondary emissive multiplier electrodes the mode of operation of this form of device is slightly different from that described. The secondary electrons emitted by the tenth multplier stage electrode 29 are drawn I toward the eleventh stage electrode 30' through the meshes of the grid-like electrodes 3! and 32.

sive surface of the secondary emissive surface 30' with sufilcient force to cause the release of a:

multiplied number of secondary electrons. These positive potential impressed upon'the auxiliary electrode 32 and therefore, are accelerated sufil- 6 eleventh stage electrode 30' is produced by the potential of the auxiliary electrode 32 :and since this potential, in accordance with the present invention, is-constant irrespective of the magnitude of the electron concentrations, the number of electrons which are collected by the electrode and subsequently are caused to traverse the output resistor 48 is not affected by the'voltage variations of the collector electrode.

It is contemplated to be within the skill of those versed in the art to incorporate other types of auxiliary electrodes, according to the present invention, between the final multiplier electrode and the collector electrode. For example, in Fig. 2 a control grid may be introduced between the screen grid 32 and the electrode 30 to control the electron flow to the collector 3| in accordance with voltages impressed thereon. In this manner blanking and/or synchronizing signaling impulses may be injected into the video signals where such a device is used in combination with a television dissector tube as indicated herein.

While there have been described what, at present, are considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that variou changes and modifications may be made therein without departing from the invention, and therefore, it is aimed in the appended claim to cover all such. changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

In a multistage electron multiplier, a plurality of secondary electron emissive electrodes, a collector electrode disposed intermediate of the last two of said secondary emissive electrodes, means It is probable that some of these electrons will be collected by the electrode 3|. but the larger; portion of them will strike the secondary emis-i ciently to pass through the meshes of. this elec-f trode for collection by the collector electrode 3i;

- It will be noted that, as in the previous instances,

including an impedance device coupled to said collector for impressing upon said collector electrode a. positive potential with respect to said last two secondary emissive electrodes, ascreen grid electrode disposed intermediate of said collector electrode and the last of said secondary emissive electrodes, and means including a source of direct current energy coupled to said screen grid electrode for impressing upon said screen grid electrode a fixed positive potential with respect to said last two secondary emissive electrodes.

MADISON CAWEIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 1 Number Name Date 2,198,233 Snyder, Jr. Apr. 23, 1940 2,256,523 Lubszynski et a1. Sept. 23, 1941 2,307,035 Gabor Jan. 5, 1943 2,271,716 Salzberg Feb. 3, 1942 2,163,966 Snyder June 2'7, 1939 2,205,071 Skellett June 18, 1940 Certificate of Correction Patent No. 2,418,574. April 8, 1947.

MADISON OAWEIN It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 6, line 38, in the claim, after the word collector, first occurrence, insert electrode;

and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 6th day of September, A. D. 1949.

THOMAS F. MURPHY,

Assistant Commissioner of Patents.

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