US2836760A - Electron multiplier - Google Patents

Description

May 27, 1958. E. WINTER-Y r ELECTRON MULTIPLIER Filed Feb. 28, 1956 35 l l l l l igl l l l l l "F 38 F|G.1

Input FIG-Z FIG.3

FIGLS a1 FIGA INYENTOR ATTORNEY ELECTRON MULTIPLIER Erno Winter, Budapest, Hungary, assignor to Egyesult Izzolampa es Villamossagi Reszvenytarsasag, Budapest, Hungary, :1 Hungarian enterprise Application February 28, 1956, Serial No. 568,326 Claims priority, application Hungary March 8, 1955 4 Claims. Cl. 315-42 This invention relates. to improvements in electron multipliers and particularly to electron multipliers adapted to be operated as voltage amplifiers.

In conventional multistage electron multipliers the original input electron current usually originated from a photoelectric current source and the amplification prob-.

lem has been overcome by application of plinciples of secondary-emission. For this purpose inside a sealed envelope there are provided a photoelectric cathode, a.

series of electrodes, so-called target electrodes, all surfacetreated with secondary emitting materials and operated at successively higher positive potentials and, a collector electrode As thesecondary emission coefficient is usually greater than unity, amplification oflthe originalbeam current is achieved by multiplication of the electrons,

the gain of the multiplier being 6", where 6 is the number offelectrons emitted (from the first target electrode,

for each electron leaving the cathode and n the number'of the target electrodes on which secondary emission is employed, With as many as 15-20 stages ,gains of several million are possible in a singletube. Work has been done on magneticand lateron electric-field focusing ar- U i States Patent a, 2,836,760 Patented May 27, 1958 ice , target electrodes located between said cathode and said l electrode collecting the electron-leaving the last target 7 rangements and the result of the latter development are target electrodes, 'calleddynodes, which has-been so shaped and arranged obliquely to one another as to give a focusing efliect for electrons proceeding from one target electrode to the nextr Although. secondary emission multipliers have also been applied to amplification of thermionic currents, these a devices ..were not adapted to voltage-amplification In factin electron discharge tubes the amplitude of the D.-C. component of the anode-current surpasses by orders ofimagnitude the amplitude of the A.-C.' component of theranode' current. By amplifying very low voltages for iustanceof the order of magnitude of one microvolt, the D.-C. component, the so-called carrier component of the' anode-current, would cause an'anodercurrent onthe' collectorof: 100-300 amperes. An anode-current of this magnitude would not only be unbearable, but would cause an unavoidable noise in the output circuit completely smothering the amplified signal. i 1 I l The principal object of the present-invention, therefore,

islto providean improvedelectron-multiplier, especially suited for voltage-amplification.

"Another object of the invention is to providean electron-multiplier adapted to be operated with a current of the order ofmagnitude of one microampere striking the firsfltarget electrode of the electron multiplier with a simultaneous amplifying 'voltage of the same order of magnitud'e, thus causing an anode current on the collectorot about the same order of magnitude asthe anode-current ofthe'terminal tube of a multistageamplifier, whereby theamplitude of the "Dl-C, carrier component of said? anode-current and theaniplitude of the AFC. component of said anode-current are of the same order of magnitude."

Thus a single electron multiplier according to the invencollector, a set of electrodes for focusing the electronbeam emitted by said cathode and located between said cathode and at least one further electrode located between the .last focusing electrode and the first secondary emitting target electrode, the geometrical dimensions of the cross-section of said electron-beam, its electron den- 'sity, i, e. the number of electrons contained in the unit volume of the electron current flow and the distance between the last focusing electrode and said further electrodes being chosenso as to set up a virtual cathode upon at least one of said further electrodes. Hereby the electron current flow striking the first secondary emitting ,target electrode is of the order of magnitude of about one microampere.

The electron current flow leaving the cathode is by means of said focusing electrodes concentrated on a slit in one of said further electrodes located between the last focusing electrode and the first secondary emitting target electrode. This further electrode may be a grid, for instance the control-grid of the electronmultiplier and is preferably provided with one: single slit. The virtual' cathode is thus set up on said grid or at least in its close proximity, acting practically as apotential minimum set up by the space charge upon said further electrode so that this electrode may be regarded as the virtual source of the electrons flowing to'the first target electrode; As a result' of this 'virtual cathode the slope of the characteristic in "the plane of the electrode actinguas a virtual cathode will ensure an advantageous actual performance even for l-ma. plate current. With this small electron D.-C. cur-. rent the noise-level of the tube is low and the multiplier is adapted to amplify such small currents up to 0.1 ampere, r that is. up to the magnitude of the plate current of the terminal tube of a high performance multistage amplifier.

One embodimentjofmy invention is shown in the accompanying drawing in'which- .Fig. 1 is a sectional view of an electron amplifier embodying my present invention. Q 1

Figs. 24 are three different modifications of a grid electrode, adapted tobe appliedin' an electron amplifier according'to my present invention, shown on an enlarged scale.

Fig. '5' shows the relationship betweenplate current and control voltage, the current being plotted in microamperes, thevoltage of 'thecontrol -grid in microvolts. U and U areparametersi; i v

The device comprises an evacuated sealed envelope 2 of glass orany other suitable material in whichthere is provided ,a thermionic or field emission cathode, 3, secondaryemitting staggered target electrodes 9, 10, 11, 12, 13, 14 and 15 and a collector electrode 16. The target electrodes 9-5-15 are preferably. so shaped as to provide curved electric fields thus increasing the elfect for electrons proceeding fromone target to the next.

Suitable accelerating potentials are impressed upon the target electrodes,9-'-15 and the cathode 3 by'appropriate connectionsto taps 'on a voltage divider which, as 'illustrated, comprises the series connection of resistors'33 to 37 inclusive; A source ofrrelatively high potential,

as }a"battery'38 is connected to the terminals of the voltage divider. The negativeterminal of the battery 38 is connected to the cathode 3. Between the cathode 3 and a control grid 8 there are provided four focusing electrodes 4, 5, 6 and 7. The control grid 8 has but one tiny slit 17 and the electron beam leaving the cathode 3 is concentrated by means of said focusing electrodes 4-7 just on the slit 17. The narrow beam passing through the slit 17 strikes the first secondary emitting target electrode 9 and the secondary electrons released on the surface of this electrode are attracted by the next following target electrode and, so on thus producing an amplification due to bombardment of the secondary emitting surface in cascade, which are operated at successively higher positive potentials. The secondary elec trons released on the surface of the last target electrode of the series are collected by the collector electrode 16.

The first focusing electrode 4 is connected to a tap movable along a standard resistor 41in series connection to a voltage-source 42, in order to adjust the potential of said first focusing electrode 4. Suitable potentials are impressed upon the focusing electrodes 5, 6 and 7 by appropriate connections to a voltage divider-43, to the terminals of which there is connected a battery 44. In order to control the voltage of the focusing electrode 6 the respective tap is movable along the resistor 43.

The potential minimum set up by the space charge and referred to as a virtual cathode, because it may be regarded as the virtual source of the electron flowing to the plate, i. e. in the tube according to the-invention to the first secondary emitting target electrode, forms at a critical value l of the electron current leaving: the cathode.

where the value of K is a constant if the voltage is'substituted in volts I results in milliamperes. F is the cross-section of the electron beam and D the distance between the accelerator-grid and the plate, i. e. in the tube according to the invention the distance between the last focusing electrode 7 and the control grid 8 located between said last focusing electrode and the first target electrode. Vgzeff is the eflective voltage of the last focusing electrode 7 and Vaeff the effective voltage of the grid 8 on the hole of which the virtual cathode is setup. (The effective voltage of an electrode is the value of the voltages of all other electrodes located behind said one, reduced to the position of "that one elec trode.)

Let V volts, V =100 volts,D=1 cm.' and F=1 cmF, then:

If, according to the invention, an electron-beamis produced with a quadrilateral cross-section measuring 0.1 0.01 cm., or 0.002 0.5 cm., the value of F drops from 1 cm. to 0.001 cm. and D left unchanged the critical electron current I drops from 11.3 ma. to 11,3 microamperes. With these conditions assumed the voltage amplification aimed at will then be realized. Various other values may also be varied. Thus, for instance, with an accelerating voltage, i. e. potential of the last focusing electrode 7 of only 45 volts effective and a control voltage on the grid '8 of only 9416 volts effective and the cross-section of the electron beam measuring 0.001 cm. the virtual cathode will be set up with an electron current of 1 microampere. Owingto the fact of D being an quadratic factor its variation highly effects the value of I Since the distance between the virtualca'thode and control grid 8 is extremely small, practicallyequal to zero and the input attenuation is of a small value, the multiplier tube according to the invention is well adapted to amplify signals in the ultra high frequency range.

The focusing electrode 4 is preferably operated at a negative potential, the focusing electrodes 5, 6 and 7 at positive potentials. The positive potential of the focusing electrode 7 is preferably higher than the positive potential of the focusing electrode 5; the positive potential of the focusing electrode 6 may be varied by' means of the potentiometer 43 and tap 45, but is 'preferably always lower as the potential corresponding to the position of the electrode 6.

To simplify the drawings, only four multiplying stages are shown in Fig. 1. It' is to be understood, however, that as many stages as desired may be incorporated into the device.

The efliciency of concentration or focusing of the electron beam leaving the cathode 3 may be increased by means of deflection-plates or pairs of such plates setting up electrostatic deflecting fields, these plates being applied in addition to the focusing electrodes 4, 5, 6 and 7 and arranged between the electrodes 7 and 8. g

In Figs. 2-4 there are shown three modified forms of the control grid 8 of the multiplier tube according to the invention.

The control grid shown in Fig. 2 consists of 'two' plane metal sheets 18 and 19 connected to each other by mean's' of strips20 and 21 in such a way that there remains a tiny slit 22 surrounded by said members 18-21. The hole 22 has been drawn on an enlarged scale. Its dimensions, i. e. its cross-section is preferably of the order of magnitude of the beam concentrated by the focusing electrodes 4-7' on said slit 22.

The control grid according to Fig. 3 consists of two plane sheets 23 and 24, connected to each other by means of strips 25 and 26 in such a way that thereremains a narrow gap 27 of an also drawn on an enlarged scale.

gap 27.

This gap may for instance have a height of'l mm. and

plates 28 and 29 and the linking strips 30 and 31, said gap having somewhat greater Width. In this gap "there is provided a set of very thin wires 32 preferably made of tungsten and each having a diameter of about a few microns. The wires 32 may be arranged longitudinally in the gap as shown in Fig. 4, or crosswise or longitudinally and crosswise; they are provided in the plane of the electrode and span the gap.

The grid 8 to be applied in the tube according to Fig; 1 may also be made of one single piece of sheet metal and be provided with a slit or gap in any known manner.

The material, the various electrodes of the tube are made of, may be any of the commonly used metals or alloys, for instance molybdenum or tungsten. The wires 32'of the grid shown in Fig. 4 may preferably be coated with a thin layer of gold.

The secondary emitting target electrodes 9-15 may be made of an alloy of silver and magnesium or-silver and beryllium or brass and may be coated with alkaline metals. Such electrodes are described in Patent No. 868,086 and the Hungarian 133,334. Other materials may also be used.

Fig. 5 shows the relationship between the plate current and the voltage of the control grid. The lower curve shows the slope for a voltage of -2 volts of the' Patent No.

1 first focusing electrode 4, the higher curve for a voltage of 1 .volt of the same focusing electrode. to theories prevailing up to now the virtual cathode, ire."

cross-section of the electron oblong form. This gap is Its dimensionsai'ef preferably equal to the dimensions of the electron beam" concentrated by the focusing electrodes 4-7 on saidthe French According 1 the potential minimum of the space charge is set up with a certain discontinuity. The diagram in Fig. 5 shows no such phenomenon. Discontinuity would result with a perfectly dense cathode, without any hole or gap in it. I have found that the virtual cathode is first set up on the two edges of the gap of the electrode 8, for instance on the longitudinal edges of the gap of the grids shown in Figs. 3 and 4 and then continually expands towards the middle of said gap. The setting up of the virtual cathode is accelerated by means of the thin wires 32 provided in said gap.

The gap 17 in the grid 8 may, however, be of any suitable form, such as a guad, a circular hole, or the like.

Although only one embodiment of the electron multiplier according to the invention has been described, it will be understood by those skilled in the art that this specific form may be modified and the electron-multiplying may be performed according to other known principles without departing from the spirit of my invention and I desire to avail myself of such modifications as come within the scope of the appended-claims.

I claim as my invention:

1. A grid-controlled electron multiplier tube for voltage amplification, comprising a thermionic or field emission cathode, a set of secondary emitting target electrodes, a collector electrode, a control electrode having an aperture and disposed between said cathode and said target electrodes, means coacting with said cathode for establishing a space charge having predetermined properties in the region of said aperture, said means comprising a set of focusing electrodes disposed between said cathode and said control electrode for concentratingthe electrons emitted by said cathode on said aperture in said control electrode, and the physical and electrical parameters of said means and their interrelation being determined by the formula:

KF 2( V en "err) where 1 is the electron beam current to said control electrode expressed in milliamperes and is of the order of magnitude of one microampere; K is a constant, the numerical value of which is 2.33 l0 F is the crosssectional area in cm. of the electron beam leaving the focusing electrode closest to said control electrode, D denotes the distance between the last-mentioned focusing electrode and said first target electrode in cm., Vggefi is the effective voltage of the last focusing electrode and Vaeff is the effective voltage of the control electrode, both in volts.

2. An electron multiplier according to claim 1, in which four focusing electrodes are present.

3. An electron multiplier circuit comprising an electron multiplier according to claim 2, and means for applying suitable voltages to the electrodes of said electron multiplier, in which a negative voltage is applied to the first focusing electrode, positive voltages to each of the other focusing electrodes, to the target electrodes and t0 the collector electrode, and a control voltage is applied to the control grid.

4. An electron multiplier circuit according to claim 3, in which the positive voltage applied to the fourth focusing electrode is higher than that applied to the second focusing electrode, the positive voltage applied to the third focusing electrode being adjustable and lower than the voltage of said fourth focusing electrode.

References Cited in the file of this patent UNITED STATES PATENTS 2,157,585 Zworykin May 9, 1939 2,205,071 Skellett June 18, 1940 2,228,121 Krenzien Jan. 7, 1941 2,473,031 Larson June 14, 1949 FOREIGN PATENTS 1,050,892 France Sept. 9, 1953 OTHER REFERENCES Terman: Radio Engineers Handbook, McGraW-Hill,

0 New York, 1943, p. 317.

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