US3436590A - Electron multiplier - Google Patents

Description

April 1969 G. WOLFGANG ETAL. 3,436,590

ELECTRON MULT IPLIER Sheet of 3 Filed larch 2. 1966 2 N o0 E C A R E wa SE HI'P'I' IN VEN TOPS G N W 1 MOT F RA vi G H E DT N LR R AE 0 w wDRM A V! B April 1969 L. a. WOLFGANG ETAL 3,436,590

ELECTRON MULTIPLIER Filed larch 2, 1966 Sheet 2 of a udzu'RE awoLFeANe DONALD ROARLO ROBERT H. CLAYTON BY #MM! 4444 ATTORNEYS April 1, 1969 1.. G. WOLFGANG ETAL. 3,

ELECTRON MULTIPLIER Filed March 2, 1966 Sheet of 3 wvmrons LOZURE GWOLFGANG DONALD ammo AT TOR/VEYS United States Patent m 3,436,590 ELECTRON MULTIPLIER Lozure G. Wolfgang, Donald R. Carlo, and Robert H.

Clayton, Fort Wayne, Ind., assignors to International Telephone and Telegraph Corporation, Nutley, N.J., a

corporation of Maryland Filed Mar. 2,1966, Ser. No. 531,232 Int. Cl. H01j 31/48 US. Cl. 315-11 11 Claims ABSTRACT OF THE DISCLOSURE An electron multiplier includes two facing surfaces having secondary emissive material thereon. Electrons enter a central aperture in one surface and are accelerated by potential applied to a plurality of dynodes extending radially outward from the center between the two surfaces. A peripheral electrode collects the electrons.

The present invention relates to electron multipliers, and more particularly to electron multipliers of disc-like shape.

One of the primary objectives of conventional electron multiplier tubes is to provide linear amplification over a wide range of input signal amplitudes. One of the major limitations in this regard i the formation of a space charge inside the tube, which repels electrons away from the anode, thereby decreasing amplification at high levels. Because of the nature of secondary emission, space charge in the vicinity of the anode is usually the limiting factor.

Additionally, such prior art multipliers in a great many instances are complicated structurally, relatively large, and not as rugged as desired for certain uses.

It is therefore an object of this invention to provide an electron multiplier of disc-like geometry which is rugged, compact, has high-current handling capacity and is simple to construct.

It is yet another object of this invention to provide an electron multiplier capable of minimizing space charge limitations a well as provide larger current-carrying capabilities for a given internal volume.

It is still another object of this invention to provide an electron multiplier wherein succeeding dynodes provide larger areas and volumes for handling and multiplying the larger space currents to which they are subjected.

In the accomplishment of this invention there is provided an electron multiplier of disc-like geometry wherein the electron paths extend radially from a centrally located primary electron impact point. Such an electron multiplier comprises supporting means having spacedapart facing surfaces, secondary emissive material on said surfaces, means for applying an electric potential between a centrally located portion of said surfaces and radially outwardly disposed peripheral portions thereof, said electric potential having a distribution which increase in magnitude radially outward from said central portion, thus providing an electric 'field upon said 'surfaces and within the region between surfaces, means including said field and the spacing between said surfaces for causing primary electrons entering into the region between the central portion of said surface to eject secondary electrons from the central portion of one of said surfaces and for developing radially outwardly directed secondary electron trajectories having multiple radially spaced impact points on said surfaces, and means adjacent to said peripheral portions for collecting electron which follow said trajectories.

The above-mentioned and other features and objects of 3,436,590 Patented Apr. 1, 1969 this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

. 1 is a top plan view of one embodiment of this 1nvent1on;

FIG. 2 is a cross-sectio taken substantially along section line 2--2 of FIG. 1;

FIG. 3 is a top plan view, partially broken away for clarity, of another embodiment of this invention;

FIG. 4 is a partial cross-sectional view taken substantially along section line 4-4 of FIG. 3;

FIG. 5 is a fragmentary sectional view of another embodiment of this invention; and

FIG. 6 is a fragmentary sectional view of still another embodiment of this invention.

Referring now more particularly to FIGS. 1 and 2, a pair of discs 1 and 2 of insulating material such as glass, ceramic, natural lava or the like are disposed in parallel spaced relation as shown. The disc 1 is provided with a centrally located aperture 3 fitted with a metallic sleeve 4 having a flange 5 overlying the upper surface 6 of the disc 1. On the facing surfaces 8 and 9 of the disc 1 and 2 are deposited or coated resistive film dynodes 10 and 11, respectively. These films 10' and 11 are extended around the peripheries '12 and '13 of the discs 1 and 2 and are conductively connected to terminal rings 14 and 15, respectively, suitably adhered to the outer surfaces of the two discs 1 and 2 as shown.

Encircling the peripheral portions of the film 10 and 11 and secured thereto are two rings 16 and 17, respectively, of insulating material of glass, ceramic or the like. Coaxially secured to these rings 16 and 17 is a sleeve-shaped, conductive collector or anode 18. All of the parts thus far described are secured together in hermetically sealed relation uch that the space 19 between the two films 10 and 11 may be evacuated upon closure of flange -5 by a hermetic window. As FIG. 2 shows more clearly, this space 19 is of disc-like shape. A pin-like supply terminal '20 is sealed within a suitable aperture in the center of the disc 2 as shown in conductive contact with the geometric center of the film 11. In thi position the pin 20 is axially aligned with the center of the aperture 3 and the sleeve 4.

A power supply, such as batteries indicated generally by the numerals 21 and 22, is connected between the collect-or 18 as shown and the terminal 20 on the one hand and the conductive sleeve 4 on the other. The batteries 21 and 22 are, in one embodiment of this invention, identical such that the same potential is applied betwee the sleeve 4 and the collector 18 as between the terminal 20 and the collector.

The structure of FIGS. 1 and 2 described thus far may be enclosed within an envelope or housing (not shown) which may be evacuated. Within this housing is mounted a source of electrons such as an electron gun indicated by the umeral 23 which is capable of emitting a pencil-like electron beam 24 and of directing the same through the center of the sleeve 4 against the center 25 of the film 11 as shown. The gun 23 may be of conventional construction.

The films 10 and 11 are secondary emissive at a ratio greater than unity. Typical materials for such films may be aluminum oxide doped with an impurity such as molybdenum or chromium. The requirements for such films is that they be semi-conductive (resistive) yet be emissive of secondary electrons upon particle impact. Being resistive, it will be obvious that a potential gradient will be established between the center and the outer peripheral portions of the structure, this creating a radial electric field in the space 19. Electrons or other particles or radiations entering the aperture of the sleeve 4 and striking the center 25 of the dynode film 11 produces secondary electrons which are drawn radially outwardly toward the collector 18. The dashed lines 26 and 27 depict representative radial electron trajectories having impact points on both dynode films and 11. The potentials applied, the spacing between the films 10 and 11, the resistivity and emission ratio of the film dynodes 10 and 11 determine the electron multiplication and ultimate current collected by the anode 18. Typical dimensions for a practical working embodiment of this invention lie in the vicinity of an outside diameter for the multiplier of from one to three inches, voltages for the supplies 21 and 22 of about 1000 volts, spacing between the films 10 and 11 of about 0.050 inch, and size of the aperture in the sleeve 4 of from 0.0005 to 0.20 inch. Obviously, other dimensions and materials may be used without departing from the spirit and scope of this invention.

More effective control and development of the electric field established in the space 19 is achieved by using a series of radially spaced, concentric, conductive rings for applying potentials to radially successive portions of the film dynodes 10 and 11. These rings are indicated by the numerals 28 through 32 which are recessed into companion grooves provided in the surface 8 as shown. The rings are radially spaced and are concentric about the center 25. They are also in conductive contact with radially spaced portions of the film dynode 10 as shown. Another series of concentric rings 33 through 37 are secured in the face 9 of the disc 2 as shown, these being preferably opposite the respective rings 28 through 32. The film 11 is in conductive contact with these rings.

The voltage supplies 21 and 22 are incrementally tapped and connected to the respective rings as shown such that the rings outwardly from the center will have successively higher, dynode potentials applied thereto. Preferably, opposed rings such as rings 32 and 37 will have the same potentials applied thereto. As will now be apparent, the radial potential distribution may be accurately and precisely controlled by the potentials applied to the rings. The operation of this structure as well as the multiplication of electrons is the same as described hereinbefore.

If desired, the rings 28 through 37 may have other than circular shapes in order to achieve non-axially symmetrical fields. Also, the shape or position of the rings in the face 8 may be different than that of the rings in the face 9 to yield an axial as well as a radial field.

The embodiment of the invention illustrated in FIGS. 3 and 4 is quite similar to that of FIGS. 1 and 2. Two glass or the like discs 38 and 39 are peripherally sealed together in such a manner as to define a disc-like space 40 therebetween. The interior faces 41 and 42 of the discs 38 and 39, respectively, are not flat and parallel as in the case of the embodiment of FIGS. 1 and 2, but instead are machined or otherwise formed into a coaxial series of annular grooves 43 and 44, respectively. The grooves 43 are formed in the face 41 while the grooves 44 are formed in the face 42. Each groove 43, 44 as shown has a cross-section or profile similar to that of a dynode in a conventional Rajchman multiplier structure; however, as will appear from the following discussion, other shapes may be used without departing from the spirit and scope of this invention. All of these grooves are of substantially the same size and shape, the grooves 44, however, being radially staggered with respect to the grooves 43. As shown in FIG. 4, the crests 45 of the grooves 43 are axially aligned near the valleys 46 of the grooves 44. The disc 38 is provided with an aperture 3 which is equipped with a metallic sleeve 4 as previously described in connection with the embodiment of FIGS. 1 and 2.

Deposited or coated upon the surface 41 is a thin resis- 4 tive film 47 of finely divided metal or carbon. The same kind of resistive film 48 is applied on the face 42.

A suitable secondary emissive material having an emission ratio greater than unity is deposited or otherwise applied to the valley portions of the films 47 and 48 in annular segments, one such segment being indicated by the numeral 49 and another such segment by the numeral 50. These segments are radially separated by the crests 45 of the various grooves so that the only electrical connection therebetween is that provided by the resistive films 47, 48. The material of these annular segments may be of any conventional type, silver-magnesium being typical.

In the outermost groove 51 in the disc 38 is deposited a highly conductive metallic film 52 of silver or the like which serves as the collector electrode. A pin '53 penetrating the envelope formed by the two discs 38 and 39 is conductively connected to the collector 52 for applying a supply voltage thereto. Another terminal 54 penetrates the disc 38 and makes contact with the resistive film 47 in the next to the outer groove 55, while still another terminal 56 in the disc 39 makes contact with the film 48 in the outer groove 57. The central portion 58 of the surface 42 is dish-shaped as shown and is provided with a film 59 of secondary emissive material. A terminal 60 is connected between the center of this film 59, the center of the resistive film 48 and the negative terminal of the supply battery 61. The positive terminal of this battery is connected to terminal 56 previously described. Another unidirectional voltage supply 62 is connected between the sleeve 4 and the terminal 54 with the polarity connections as shown, and still a further supply 63 is connected between the two terminals 53 and 56. The operation of this embodiment is essentially the same as that of FIGS. 1 and 2, a primary beam of electrons 24a impacting the center of the film 59, dislodging secondaries therefrom which are drawn radially outward through the space 40 by reason of the electric field established therein through the resistance of the two films 47 and 48. The potentials applied are such that secondaries liberated by the film 59 will impact the dynode segment 49 first, secondaries from this dynode then being directed to the next radially outward dynode 50 and dislodging secondaries therefrom. This process is repeated in radially outward successive steps according to the dashed line trajectory 64 until the current is finally collected by the anode 52. While in the drawings the electron trajectory is indicated as being in a single path lying in a single plane, it will be obvious that the trajectory will be volumetric radially throughout the space 40.

While the operation of the multiplier has been explained in connection with an externally generated beam 24a of electrons, it is possible to construct and operate various embodiments of this invention as photomultipliers. An example of this is given in connection with FIG. 4. In this connection, a transparent glass plate indicated by the dashed line 65 is sealed over the opening in the sleeve 4 and the film 59 is made of a photoelectric material instead of material that is only secondary emissive. Light penetrating the cover 65 and striking the photoemissive film 59 will cause liberation of electrons from the latter which will be multiplied, as already explained. Also, a photoemissive material may be applied to the underside of the cover 65 such that electrons emitted thereby may be used to impact the secondary emissive film 59 causing dislodgement of secondaries therefrom. Obviously, the space 40 must be evacuated as already explained in connection with FIG. 2.

The basic multiplier configuration of FIGS. 1 through 4 may undergo certain structural modifications for the purpose of varying the operation thereof. For example, by a slight modification, the multipliers may be operated as trackers, the forms of the invention as shown in FIGS. 5 and 6 being examples. 'In FIG. 5, the structure thereof is identical to that of FIG. 4 with the exception that the central portion 58a of the surface 42 is elevated into the shape of a cone. The film 59a thereon has the same shape. A primary electron beam 24b ofi center as shown impacting one side of the film 59a will initiate a secondary electron trajectory which is directed angularly outwardly therefrom instead of being evenly radially distributed throughout the space 40. By making the collector 52 (FIG. 4) in circumferentially separated segments, the angular position of the beam 24b can be determined.

A similar result can be achieved by the further embodiment shown in FIG. 6 wherein a pin electrode 66 is coaxially positioned in the disc 39 in axial alignment with the opening in the sleeve 4. This pin 66 is held in position by means of a glass or the like sleeve 67 hermetically sealed thereto which is in turn hermetically sealed to a metal sleeve 68 conductively connected to film 58. The secondary emissive film 590 has a clearance surrounding the pin 66 as shown. A battery 69 is connected between the pin 66 and the sleeve 68 which in turn is connected to the resistive film 48 on the surface 58.

In operation, a beam 240 which is slightly eccentric will be deflected more in the direction of the eccentricity by the pin 66' as shown by the dashed line trajectory. The potential supplied to the pin 66 is made sufiiciently negative with respect to the beam 240 to produce the deflection. Using a segmented collector as described in con nection with FIG. 5, the angular position of the beam 240 can be easily determined.

Advantages residing in the disc-like geometry of the multipliers disclosed in the preceding are in part as follows. The structure is physically more compact than other multipliers, particularly in axial extent. The larger elemental volume in the outer peripheral portions of the multiplier permits greater current-handling capability. As radius increases, the volume of the multiplying space for a given increment of radius increases. In this connection, current growth is approximately exponential while volume increase is linear with radius.

In the form of the invention in FIGS. 1 and 2 where the ring electrodes 28 through 37 are not used, the resistive dynode films 10 and 11 are relied upon to provide the necessary field for accelerating the electrons radially outwardly. This eliminates the need for applying incremental voltages at various points along the radius of the multiplier. The dynodes may simply be evaporated or deposited onto the preformed substrates, leading to a simpler and more rugged structure.

If the films 10 and 11 are of uniform thickness and radial resistivity, a logarithmic potential increase with radius is produced such that the electric field decreases with increased radius. This results in some reduction in allowable current density adjacent to the collector which offsets to some extent the advantages gained by the increase in multiplier volume in this same region. A linear potential increase with radius may be achieved by varying the thickness or resistivity of the films 10 and 11 such that the electric field remains constant with radius or increases with radius to reduce the depressive effect of space charge in the peripheral region.

Certain advantages in the arrangement of FIGS. 3 and 4 over the embodiment of FIGS. 1 and 2 include the simplicity of fabrication and resulting greater ruggedness in the finished structure. The collector may be made as an integral part of the structure such that the two supporting discs 38 and 39 may be sealed together at the outer edges thereof. The resistivity requirement of the dynode material is not constricted to provide also for proper distribution of potential; thus providing greater freedom in materials and design, and the number of impacts and integrated time of flight is held relatively constant, thereby improving linearity and minimizing time distortion.

The preceding embodiments can be made optically transparent so as to transmit light. In this respect, not only would the supporting discs 1 and 2 (FIG. 2, for ex- 6 ample) be transparent, but also the electrodes and the resistive and dynode materials.

Additionally, instead of using, in the arrangement of FIG. 2, a single film which serves both resistive and secondary-emissive functions, two superposed layers may be used, the substrate being of resistive material and the outer one being of secondary emissive material. Thus, the functions of potential distribution and secondary emission yield would be separated In the event an output signal voltage is desired, a load resistor is connected in series with the collector lead.

While the invention has been disclosed primarily in conjunctionwith electrons, it will be understood that electron multiplication can be initiated by alpha, beta and gamma radiation as well as ions.

While there have been described above the principles of this invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of the invention.

What is claimed is:

1. An electron multiplier comprising supporting means having two spaced-apart facing surfaces defining an electron path, secondary emissive material on each of said surfaces, means for applying an electric potential between a centrally located portion of said surfaces and radially outward disposed peripheral portions thereof, said potential having a distribution which increases in potential in a direction radially outward from said central portion, an electron receiving aperture in said central portion of one of said surfaces, means including said potential and field generated thereby and the spacing between said surfaces for causing primary electrons received by said aperture in the region between the central portion of said surfaces to eject secondary electrons from the central portion of one of said surfaces and for developing radial secondary electron trajectories having multiple radially spaced impact points on each of said surfaces, and means adjacent to said peripheral portions for collecting electrons which follow said trajectories.

2. The multiplier of claim 1 wherein said surfaces define the opposite side of a radially extending space, said aperture being in the center thereof, said potential-applying means including electrode means carried by said supporting means, said electrode means extending along said sides, said potential causing an electric field which accelerates electrons within said space to velocities which eject secondary electrons upon impact with said secondary emissive material.

3. The multiplier of claim 1 wherein said secondary emissive material is in film form on said surfaces and is semi-conductive, said potential-applying means including a source of unidirectional potential having the negative terminal thereof connected to the central portion of said film and the positive terminal thereof connected to a radially outer portion thereof.

4. The multiplier of claim 1 wherein said potentialapplying means includes a plurality of electrode elements which encircle said centrally located portion and are radially spaced from each other along said surfaces, said elements being electrically insulated from each other, said elements being carried by said supporting means, and means for applying potentials to said elements of progressively increasing value radially outwardly from said central portion.

5. The multiplier of claim 1 wherein said facing surfaces include two disc-like members of insulating material which are parallel and spaced apart, a plurality of concentric radially spaced annular electrode elements mounted on the opposed surfaces of said members, said secondary emissive material being in film-like form on said surfaces, a conductive collector ring coaxially surrounding the space between said members, said ring and said members being hermetically sealed together into an assembly which may be evacuated, and terminal means connected to all of said elements and said ring for applying potentials thereto of progressively increasing value radially outward from the center of said members.

6. The multiplier of claim 1 wherein said facing surfaces are generally parallel, said surfaces having a plurality of radially adjacent coaxial annular grooves having crests and valleys, respectively, the crests of the grooves in one surface being axially opposite the valleys of the grooves in the other surface, said secondary emissive material being in coaxial annular segments lying in the valley portions, respectively, of the grooves, said segments being radially separated at the crests between grooves, and terminal means for applying potentials to said segments of values which progressively increase in a radially outward direction.

7. The multiplier of claim 6 including a collector electrode encircling the outermost segment, said grooves having Rajchmann profiles.

8. The multiplier of claim 6 wherein the portion of said supporting means providing said facing surfaces is of insulating material, the central portion of the surface opposite said aperture having a shape which is one of the group of concave and convex shapes, said central surface portion having secondary emissive material thereon, and means for applying a potential to the last-mentioned secondary emissive material which is negative with respect to the potentials applied to said segments.

9. The multiplier of claim 8 having a deflecting electrode mounted in the side of said supporting means opposite said aperture, said deflecting electrode being of pinlike shape coaxially aligned with said aperture, and means for applying a potential to said deflecting electrode which is negative with respect to the potential applied to said last-mentioned secondary emissive material.

10. The multiplier of claim 6 wherein said supporting means is of insulating material and including a film of resistive material on said surfaces with said segments being superposed on said resistive material.

11. The multiplier of claim 6 wherein said supporting means is of insulating material and including a film of resistive material on said surfaces with said segments being superposed on said resistive material, the central portion of the surface opposite said aperture having a shape which is one of the group of concave and convex shapes, said central surface portion having secondary emissive material thereon, and means for applying a potential to the last-mentioned secondary emissive material which is negative with respect to the potentials applied to said segments, a collector electrode encircling the outermost segment, said grooves having Rajchmann profiles.

References Cited UNITED STATES PATENTS 5/1942 Gebauer et a1 3l3103 5/1967 Ramberg 313103

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