US2903595A

US2903595A - Electron multiplier

Info

Publication number: US2903595A
Application number: US477493A
Authority: US
Inventors: George A Morton
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1954-12-24
Filing date: 1954-12-24
Publication date: 1959-09-08
Anticipated expiration: 1976-09-08

Description

- l 5% 5%2/7 `zz'mfara/v L 4 l 7% aufm/r l Sept. 8, 1959 G. A. MoRToN 2,903,595

ELECTRON MULTIPLIER Filed Dec. 24, 1954 2 Sheets-Sheet 1 I 200M '"l 6/ zooov. 200V. 40am faz se" f IMM V. 300M 500V IN V EN TOR. Gien/g' e AMO/'fon i I e Wlw orweq G. A. MoR'roN 2,903,595

ELEcTRoN MULTIPLIER 2 Sheets-Sheet 2 seppe, 1959 Filed Dec. 24, 1954 A N n QQNA Qi Y NQQW SQQN NQQQN ELECTRON MULTIPLIER George A. Morton, Princeton, NJ., assignor to Radio Corporation of America, a corporation of Delaware Application December 24, 1954, Serial No. 477,493

12 Claims. (Cl. Z50-207) The present invention relates to improvements in electron multiplier tubes.

An electron multiplier is a device utilizing secondary electron emission to amplify or multiply the electron current from a primary electron source, such 'as a photocathode or a thermionic cathode. The usual electron multiplier comprises a series or cha-in of secondary emitting elements, called dynodes, interposed between a primary electron source and an output collector or anode. The electrodes are constructed and arranged to` form an electron optical system for directing primary electrons from the primary source onto the first dynode, releasing therefrom several secondary electrons for each primary electron. These secondaries are directed by the electron optical system onto the next dynode where each produces more secondaries. This process is repeated at each succeeding dynode or stage of the multiplier, thus producf ing a greatly multiplied electronic current from the final dynode to the collector. The number of dynodes or stages may be from one to twenty or more depending on the amount of amplification needed. Each succeeding dynode in the chain is maintained at a potential substantially higher, e.g. 100 volts, than the preceding dynode, to accelerate the secondaries from element to element, and the dynodes are preferably shaped to direct and focus the electrons emitted thereby to the next dynode.

Electron multipliers are particularly useful for amplifying electron currents produced by weak signals, such as light, nuclear radiations or radio waves. When used for detecting and/ Or counting rapidly recurrent signals such as nuclear particles, it is necessary that the multiplier have sufficient speed and a resolving time low enough to distinguish between successive signals or particles.

The speed of a multiplier can be increased by reducing the overall transit time of primary and secondary electrons between the primary source and the collector. The resolving time of a multiplier is limited by the transit time spread of electrons through the multiplier chain, that is, the difference between the transit times of the fastest and slowest electrons. This transit time spread is primarily due to differences in the trajectories taken by various electrons through the multiplier and differences in the initial velocities of secondary electrons. The time differences for different trajectories can be reduced to some extent by careful design of the dynode shapes.

The overall transit time and the time spread due to different initial velocities and to different path lengths can be decreased by using higher accelerating voltages between successive dynodes. However, if the voltage per stage is made Very high, the overall voltage is dangerously high; the device is unstable; cold discharge occurs; and, worst of all, the secondary emission ratio of each dynode is reduced so much that the gain per stage is inadequate.

Therefore, the principal object of the present invention is to provide a high speed electron multiplier having considerably lower resolving time than previously known multipliers. Other objects and advantages will be apparent from the following description of the invention.

nited States arent O fuice in accordance with the present invention, the advantage of the high voltage is achieved, without the disadvantages mentioned above, by providing a high voltage accelerating electrode between successive dynodes, while retaining the ordinary Voltage steps to 200 volts per stage) between successive dynodes. This electrode is preferably part of the electron optical system for directing electrons from each dynode to the next dynode.

The invention will be understood from the following detailed description, and by reference to the accompanying drawing in which:

Fig. 1 is a side view of an electron multiplier embodying one form of the invention;

Fig. 2 is a longitudinal sectional View of an electron multiplier tube of the photomultiplier type incorporating the invention; and

Fig. 3 is a cut-away View similar to Fig. 2 showing a portion of a beam-deflection amplifier tube with an electron multiplier output section incorporating the invention.

Fig. 1 shows an electron multiplier comprising a primary cathode 1, a chain of

dynodes

3, 5, 7 and 9, and a collector 11, arranged in a conventional zig-Zag array. In operation, the various electrodes would be maintained at suitable D.C. potentials beginning with the cathode at zero volts and increasing by 100 volts, for example, at each dynode, as indicated on the drawing. The collector potential of 500 Volts is applied through an output resistor 13.

ln accordance with the invention, a high voltage apertured accelerating electrode 15 is positioned in the paths of secondaries between successive dynodes. The accelerating electrode 15 may comprise grids or otherwise apertured portions 17 connected together at their ends to form a single Zig-zag member. Each grid 17 is located substantially midway between the two associated dynodes and preferably extends substantially perpendicular to the average path of secondaries therebetween. lf desired, the electrode 15 may extend across the path of primaries from the cathode 1, and also across the path of secondaries between the last dynode 9 and the collector 11, as shown in Fig. 1. An envelope 19 provides an evacuated enclosure for the electrodes.

ln operation, the accelerating electrode 15 is main` tained at a D.C. potential of several thousand volts with respect to the cathode, for example, at 5000 volts. This potential is much higher than that of any of the dynodes (or the collector). It, therefore, greatly increases the electric field strength at each dynode surface. As each secondary is emitted it is rapidly accelerated away from the emitting dynode. After traversing the accelerating electrode aperture, the electron is slowed down by the retarding field between the accelerating electrode and the next dynode and impinges upon the latter at substantially the same velocity it would have had in the absence of the accelerating electrode, determined by the potential difference between the two dynodes. Thus, there is no reduction in the secondary emission ratio, and consequent loss of gain, caused by the use of the high voltage accelerating electrode. However, the transit time of each electron across the space between two successive dynodes is greatly reduced. In fact, the reduction in transit time is nearly as great as though the voltage between the dynodes were equal to that between a dynode and the accelerating electrode. In the region between the cathode l and the first dynode 3 the average velocity is approximately half that produced by 500() volts. The ratio of this average velocity to that which would be produced by the 100 volt step without the accelerating electrode 15 is approximately equal to 35. Thus, the transit time across the first stage of the multiplier is reduced by a factor of about 7 by use of the accelerator electrode. The corresponding reduction factor between the last dynode 9 and the collector 11 is approximately 6.3. Since a large reduction in transit time is produced in each stage, the overall transit time is correspondingly reduced, and hence, the speed of the multiplier is correspondingly increased.

The reduction in transit time produced by the high voltage accelerating electrode is accompanied by a corresponding reduction in the transit time spread, and resolving time, of the multiplier. This is because the differences in initial velocities constitute a much smaller fraction `of the velocities themselves, and also because the differences in path lengths are greatly reduced by the high accelerating elds produced by the accelerating electrode.

Fig. 2 shows a photomultiplier tube comprising a glass envelope 21 containing a photocathode 23, three

annular focusing electrodes

25, 27 and Z9, an electron multiplier chain 31 embodying the invention and a collector 33. The photocathode 23 may be in the form of a photoconductive coating on the inner surface of the end wall of the envelope 21, as shown. Usually, the coating would be electrically connected to the first focusing electrode 25 for operation at Zero potential. Preferably, the photocathode 23 is concave in shape, as shown, in order to assist in focusing the primary electrons emitted thereby through a restricted aperture in the third focusing electrode 29 onto the inclined first dynode of the multiplier.

The multiplier chain 31 comprises two spaced parallel rows of

curved dynodes

37, 38, 39, 40, 45, 46, 47, 4S and 49, which constitute dynodes Nos. 1, 2, 3, 4, n, 11+ 1, 11-1-2, n|3 and n-j-4, respectively. The dynodes in each row are staggered with respect to those in the other row and each dynode except the last one is inclined to face the next dynode, as shown, to assist in focusing secondaries from each dynode onto the next dynode. The last dynode 49 is preferably of cylindrical form surrounding the collector 33 except for a relatively narrow entrance aperture 51, which may be covered by a grid 53. The collector 33 is substantially flat and disposed in a plane parallel to the path of secondaries entering the last dynode 49, to minimize collection of secondaries other than those emitted by the last dynode. Thervarious electrodes may be mounted within the envelope 21 by any suitable conventional means (not shown). The dynodes 37 to 49 and collector 33, shown in transverse section in Fig. 2 are sheet-like members which may be elongated in the direction perpendicular to Fig. 1 to permit a relatively wide angle of divergence of the secondaries.

Separate leads through the envelope 21 are provided for each electrode for applying suitable D.C. potentials thereto, as indicated on the drawing. For example, the first dynode 37 may be maintained at 100 Volts relative to the cathode 23 and each succeeding dynode may be 100 volts higher than the preceding dynode, in which case the nth dynode would have a potential V equal to 10011 volts. Preferably, the collector potential would be about 1000 volts higher than the last dynode, or Vl1500 volts applied through an output resistor 55.

In accordance with the invention, a series of high voltage accelerating and focusing electrodes 57 to 60 and 65 to 69 is mounted by suitable means (not shown) between the two rows of dynodes 37 to 49. Each of these electrodes, except the first and last ones, is located opposite the edges of a pair of adjacent dynodes in one row, so that each pair of adjacent accelerating electrodes lie on opposite sides of one of the secondary electron paths between successive dynodes and forms an accelerating and focusing aperture for that path. The first accelerating electrode 57 is located opposite the leading edge of the second dynode 38 to form an accelerating aperture with accelerating electrode 58. Similarly, the last accelerating electrode 69 is located so as to form an accelerating aperture with accelerating electrode 68. The accelerating electrodes are preferably in the form of relatively narrow strips having a length perpendicular to the plane of Fig. 2 at least equal to the length of the dynodes 37 to 49.

Means are provided for maintaining the accelerating electrodes 57 to 69 at D.C. potentials which are high as compared to the dynodes between which they are interposed. For example, electrodes 58 to 69 may be electrically connected together Within the envelope, by conductors schematically shown at 71 in Fig. 2, and provided with a common lead 73. Electrode 57 may be provided with a separate lead to permit the application of a high potential different from that applied to the other electrodes.

In operation, electrodes 58 to 69 are maintained at a D.C. potential of several thousand volts, e.g. 5000, to reduce the transit time and transit time spread of secondary electrons through the multiplier in a manner similar to that described above for the embodiment shown in Fig. l. Due to the use of apertures, instead of the grids used in Fig. l, the secondary electrons will be focused as well as accelerated by the accelerating electrodes. Preferably, electrode 57 is maintained at substantially lower potential than the next accelerating electrode 58, e.g. at 2000 volts, to deflect the secondaries from the first dynode in the direction toward the other dynodes. By adjustment of the potential of electrode 57, the secondaries in succeeding paths can be centered within the accelerating apertures.

In operation, an input signal in the form of a flash of light or a nuclear radiation, for example, liberates primary electrons from the photocathode 23 which are focused onto the first dynode 37. The total electron current is multiplied by secondary emission at each stage of the high speed electron multiplier of the invention to produce a greatly amplified output signal having a time duration which is not substantially greater than the input signal.

Fig. 3 shows the invention incorporated in a beamdeflection amplifier tube having an electron multiplier output. The tube comprises a glass envelope 81 containing a thermionic cathode 83, accelerating and focusing

electrodes

85, 87 and 39,

deecting plates

91 and 93, a knife-edge electrode 95, accelerating electrode 97, electron multiplier chain 99 and a collector electrode (not shown). The multiplier chain 99 comprises dynodes 101 to 105 similar to the dynodes 37 to 49 in Fig. 3. Conventional D.C. potentials may be applied to the various electrodes as indicated in Fig. 3.

In accordance with the invention, high voltage accelerating electrodes 107 to 111 are mounted between the dynodes 101 to 105, and high accelerating potentials are applied to electrodes 107 and 111, in the same manner as explained above in connection with Fig. 2.

In operation of the tube of Fig. 3, a signal pulse, for example, is applied to the deflecting

plates

91 and 93 to deflect the beam of primary electrons from cathode 83 relative to the knife-edge electrode 95 and thereby valy the primary electron current reaching the rst dynode 101. This current variation is amplified by the high speed multiplier 99 to produce a greatly amplified output pulse which is not substantially greater in width on a time scale than the input pulse.

What is claimed is:

1. An electron multiplier tube comprising an envelope containing a multiplier chain of dynode electrode elements, a source of primary electrons at one end of said chain in position to supply primary electrons to the first element thereof, a collector at the other end of said chain in position to receive secondary electrons from the last element thereof, and unipotential electron-permeable accelerating electrode means comprisingV a series of accelerating electrodes intenposed between successive dynode elements; said accelerating electrodes being insulated from said source, said dynode elements and said S collector, and being electrically connected together within said envelope, and being provided with an external lead for applying a high direct-current potential thereto.

2'. An electron multiplier tube according to claim l, wherein said source of primary electrons comprises a photocathode and means for directing primary electrons from said photocathode onto said first dynode element.

3. An electron multiplier tube according to claim 1, wherein said source of primary electrons comprises a thermionic cathode, means for varying the density of primary electrons from said cathode, and means for directing said primary electrons onto said first dynode element.

4. An electron multiplier tube according to claim 1, wherein said accelerating electrodes are grids extending across the path of secondary electrons between successive dynode elements and connected together at their ends.

5. An electron multiplier tube according to claim l, wherein said accelerating electrodes are spaced apart along said chain, one located on each side of the path of secondary electrons from each dynode element to the next dynode element.

6. An electron multiplier tube according to claim 5, wherein each of said accelerating electrodes forms a common part of the boundaries of the accelerating apertures for two adjacent secondary electron paths.

7. An electron multiplier tube according to claim 5, wherein said accelerating electrodes constitute part of the electron optical system for directing electrons from each dynode to the next dynode.

8. An electron multiplier tube comprising an envelope containing a multiplier chain made up of two parallel rows of curved dynode elements, the elements of each row facing and staggered with respect to the elements of the other row, a source of primary electrons at one end of said chain in position to supply primary electrons to the rst element thereof, a collector at the other end of said chain in position to receive secondary electrons from the last element thereof, Iseparate leads for said cathode, said dynode elements and said collector for applying increasing direct-current potentials thereto, and means for reducing the total transit time and the transit time spread of electrons through said multiplier chain comprising a high-voltage unipotential electron-permeable accelerating electrode means interposed between said rows of dynode elements and insulated from said source, said dynode elements and said collector, said accelerating electrode means providing an accelerating aperture between each dynode element and the next dynode element in the other row, each accelerating aperture being located substantially midway between said elements, and a separate lead for said accelerating electrode means for applying a high direct-current potential thereto.

9. An electron multiplier tube according to claim 8, wherein said accelerating electrode means has a zig-zag shape in cross-section.

10. An electron multiplier tube according to claim 8, wherein the planes of said apertures are substantially perpendicular to the paths of electrons therethrough.

11. An electron multiplier comprising an envelope containing a multiplier chain made up of two parallel rows of curved dynode elements, the elements of each row facing and staggered with respect to the elements ofthe other row, a source of primary electrons at one end of said chain in position to supply primary electrons to the rst element thereof, a collector at the other end of said chain in position to receive secondary electrons from the last element thereof, separate leads for said source, said dynode elements and said collector for applying increasing direct-current potentials thereto, and means for reducing the total transit time and the transit time spread of electrons through said multiplier chain comprising a highvoltage unipotential electron-permeable accelerating electrode means interposed between said rows of dynode elements and insulated from said source, lsaid dynode elements and said collector, said accelerating electrode means providing an accelerating aperture between each dynode element and the next dynode element in the other row, each accelerating aperture being located substantially midway between said elements, a separate lead for said accelerating electrode means for applying a high direct-current potential thereto and `a source of directcurrent voltage connected to the leads of said cathode, dynode elements and collector for maintaining said electrodes at potentials increasing from zero at the cathode to several hundred volts at the collector, said source being also connected to the lead for said accelerating electrode for maintaining the latter at a potential of several thousand volts higher than said collector.

l2. An electron multiplier comprising an envelope containing a multiplier chain of dynode electrode elements, a source of primary electrons at one end of said chain in position to supply primary electrons to the rst element thereof, a collector at the other end of said chain in position to receive secondary electrons from the last element thereof, unipotential electron-permeable accelerating electrode means comprising a series of accelerating electrodes interposed between successive dynode elements; said accelerating electrodes being insulated from said source, said dynode elements and said collector, and being electrically connected together within said envelope, and being provided with an external lead for applying a high direct-current potential thereto; and a source of directcurrent voltage connected to said electron source, said dynode elements and said collector for maintaining the same at potentials increasing from Zero at the electron source to several hundred volts at the collector, said voltage source being also connected to said lead for said accelerating electrodes for maintaining the same at a potential several thousand volts more positive than said collector.

References Cited in the le of this patent UNITED STATES PATENTS 1,903,569 Jarvis et al Apr. 11, 1933 2,147,756 Ruska Feb. 2l, 1939 2,157,529 Drewell et al. May 9', 1939 2,160,798 Teal May 30, 1939 2,200,722 Pierce et al May 14, 1940 2,207,355 Shockley July 9, 1940 2,285,126 Rajchman et al June 2, 1942 2,434,895 Arditi Jan. 27, 1948 2,675,431 Miller Apr. 13, 1954 2,702,865 Herzog Feb. 22, 1955