US3634713A

US3634713A - Electron multiplier having means for altering the equipotentials of the emissive surface to direct electrons towards the anode

Info

Publication number: US3634713A
Application number: US855889A
Authority: US
Inventors: Theodore Foote
Original assignee: Bendix Corp
Current assignee: Bendix Corp
Priority date: 1969-09-08
Filing date: 1969-09-08
Publication date: 1972-01-11
Anticipated expiration: 1989-01-11
Also published as: DE2031115A1; FR2060549A5; GB1322257A

Abstract

A plurality of electron conducting paths are arranged in a pattern along the sides of the emissive surface of an electron multiplier to cause emitted electrons to travel towards the anode along a path defined by the sides of the emissive surface.

Description

United States Patent Theodore Foote Holley, N.Y.

Sept. 8, 1969 Jan. 1 l, 1972 The Bendix Corporation Inventor Appl. No, Filed Patented Assignee ELECTRON MULTIPLIER HAVING MEANS FOR ALTERING THE EQUIPOTENTIIALS OF THE EMISSIVE SURFACE TO DIRECT ELECTRONS [56] References Cited UNITED STATES PATENTS 2,668,184 2/1954 Taylor et a1 313/94 X 2,705,764 4/1955 Nicoll 313/68 2,932,768 4/1960 Wiley 328/243 X 3,321,660 5/1967 Ramberg 313/103 Primary Examiner Robert Segal AttorneysRaym0nd J. Eifler and Flame, Arens, Hartz, Hix

and Smith ABSTRACT: A plurality of electron conducting paths are arranged in a pattern along the sides of the emissive surface of an electron multiplier to cause emitted electrons to travel towards the anode along a path defined by the sides of the emissive surface.

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POWER SUPPLY /////////x///*-F AV LD 22 FIGURE 2 FIGURE 3 FIGURE 4- 70- K2] 7 FIGURE 5 ATTORNEY ELECTRON MULTIPLIER HAVING MEANS FOR ALTERING THE EQUIPOTENTIALS OF THE EMISSIVE SURFACE TO DIRECT ELECTRONS TOWARDS THE ANODE BACKGROUND This invention relates to electron multipliers and more specifically to a magnetic electron multiplier of the type having a source of primary electrons, an electron emissive surface having a secondary emission ratio greater than 1, and an anode or collector for detecting the electrons generated by the source of primary electrons and the secondary electron emissive surface.

Although the electron multiplier is useful for general amplification purposes and for photomultiplication, its usefulness for particle detection, such as in mass spectrometry, has created the need for a high resolution electron multiplier. To obtain high resolution it is essential thatelectrons, from the electron emissive surface, travel towards the anode within the path defined by the sides or edges of the emissive surface. Escape and/or reentry of the electrons from the defined path causes unstable gain, noise and adversely affects the transit time for electrons traveling to the anode.

An early attempt to control'the flow of electrons was an invention directed at subjecting the electrons to a gating action as they traveled along the electron emissive surfaces towards the anode. (U.S. Pat. No. 2,932,768 filed 1955). But, although this invention was useful for gating electrons traveling to the anode, it did not prevent electrons from escaping the path defined by the sides of the emissive surface. Further it' also required applying different potentials to different points along the emissive surface. A later invention (U.S. Pat. No. 2,983,845 filed 1959) was directed to stabilizing the gain by preventing feedback of escaped electrons to the source of primary electrons. This was accomplished by a barrier or baffle that was located perpendicular to and surrounding the reverse side of the emissive surface. However, such a baffle is structurally undesireable as its location and size is limited to the space available around the emissive surface and, although the baffle does stop escaped electrons from returning to the source of primary electrons, it does nothing to prevent electrons from escaping the path defined by the outer edges of the emissive surface.

SUMMARY OF THE INVENTION By the present invention substantially all the electrons traveling from an electron emissive surface towards an anode are confined to a path defined by the edges of the emissive surface. This is accomplished by arranging a plurality of electron conducting paths on the emissive surface so that when a potential is applied to the surface the potential gradient along the surface is shaped in a manner that causes at least a portion of the equipotentials of the potential gradient to assume a curved shape. The electron conducting paths may be comprised of a family of lines or curvilinear paths extending from the sides of the emissive surface. The configuration of each line is characterized by the fact that all or substantially all of the line is not in a direction perpendicular to any side of the emissive surface and that a substantial portion of each line is further from the anode than the point where the line meets the side (edge) of the emissive surface. The lines themselves are characterized by the fact that the material which comprises the lines is a much better conductor than the emissive surface and in most instances has a resistance less than 1/ l ,000 of the resistance of the emissive surface.

Accordingly, it is an object of this invention to confine substantially all the electrons traveling towards one end of an emissive surface in a magnetic electron multiplier to the path defined by the sides of the emissive surface.

lt is another object of this invention to establish a potential gradient along an emissive surface wherein the equipotentials of the gradient along a portion of the surface are shaped to cause electrons emitted from said surface to range within the path defined by the edges of the emissive surface.

It is still another object of this invention to arrange on an electron emissive surface, a plurality of electron-conducting paths having a resistance less than the emissive surface, so that when a potential is applied to said emissive surface the equipotentials along said surface are not straight lines.

It is a further object of this invention to improve the performance of electron multipliers.

It is still a further object of this invention to improve the means for ion detection in a mass spectrometer.

The above and other objects and features of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings and claims which form a part of this specification.

BRIEF DESCRIPTION OF DRAWING FIG. 1 is a partial diagrammatic side view of a magnetic electron multiplier illustrating the cycloidal path of electrons traveling between the plates of the multiplier of the anode.

FIG. 2 is a plan view of one of the plates of the electron multiplier showing one arrangement of conducting lines located on the surface of that plate.

FIG. 3 is an arrangement showing conducting lines that travel from one edge of an electron-emitting surface to another.

FIG. 4 is an arrangement of curvilinear lines located on an emissive surface.

FIG. 5 indicates the shape of the equipotentials when a potential is applied to the emissive surface shown in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, FIG. I shows a portion of a magnetic electron multiplier (e.g. U.S. Pat. No. 2,841,729 or 2,983,845) having a source of primary electrons 10, a pair of

parallel plates

20, 40, and a collector or anode 30 for detect ing secondary electrons. The source of primary electrons 10 may be a cathode which emits electrons when impinged by ions and is positioned to introduce electrons onto one of the

parallel plates

20,40. During operation of the multiplier the cathode is maintained at a higher negative potential (e.g., 3,200 volts) than the

plates

20, 40.

The

plates

20, 40 are spaced from each other at a distance of approximately one-fourth inch, and a pair of pole pieces (not shown) are disposed adjacent the

plates

20, 40 to provide a magnetic field in the region between the plates that is substantially parallel to their inner surfaces. The

plates

20, 40 are provided with a conductive strip or coating 21, 41 on their inner surface. The

coatings

21, 41 are comprised of a secondary electron emissive material such as a tin oxide or a carbon compound which has a relatively high resistance and a secondary electron emission ratio greater than 1. However, since during operation the combination of the electric and magnetic field forces electrons to follow a cycloidal path and impinge on only the emissive surface 21 of plate 20 (called the dynode), the emissive coating 41 on plate 40 (called the field strip) may be made from a material which has a relatively high resistance but which does not have the secondary emissive qualities required for coating 21. The

plates

20, 40 themselves are made of insulating or nonconducting material such as glass or bakelite.

Terminals

24, 25 are provided on the opposite ends of the plate 20 to receive a potential for the conductive strip 21. Similarly, terminals 42, 45 are provided on the op posite ends of plate 40. Different combinations of voltages may be applied to the

terminals

23, 25, 43, 45 by power supply 5 to achieve different results, e.g., perpendicular or slanting electric fields between the

plates

20, 40 but essential to the proper operation of the electron multiplier is a potential applied to

terminals

23, 25. Preferably, a direct negative potential less than that applied to the cathode, such as -3,000 volts, is applied across

terminals

23, 25 with terminal 25 being at ground potential. This establishes a negative potential gradient along the emissive surface 21 which decreases as it approaches the anode 30.

The anode plate 30 is preferably disposed in a substantially perpendicular relationship to the

plate

20, 40 to receive electrons produced by secondary emission from the surface 23. To facilitate collection of electrons the anode 30 may be connected to a source of positive potential (e.g., +1 ,500 volts) to attract the electrons emitted from the surface 21.

In a magnetic electron multiplier of the type shown in FIG. 1, electrons emitted by the cathode are subjected to the combined action of the magnetic and electric field between the plates and 40. This causes the electrons to travel in a cycloidal path 50 and to strike the emissive surface 21 which emits a proportionately increased number of electrons. The electrons emitted from the surface 21 travel in a cycloidal path 52 and impinge upon another part of the surface 21 to again emit a proportionately increased number of electrons for cycloidal travel to another part of the emissive surface 21. In his way successively emitted electrons travel across the surface 21 in successive cycloidal paths to multiply the number of electrons initially emitted by the cathode 10. Finally, the electrons emitted from the end of the surface 21 impinge upon the anode 30 for detection.

FIG. 2 shows the emissive surface 21 with the preferred arrangement of conducting lines 22 to accomplish the purposes of this invention. The conducting lines 22 may be on the surface of the plate 20 but preferably the plate 20 is first coated with an electron emissive material 21 such as tin oxide and then a plurality of electron conducting paths 22, having a resistance less than the emissive surface 21 is located (e.g., by sputtering) on the emissive surface 21. Preferably, the resistance of the conducting paths 22 are in the order of 1/l,OOO to l/ l ,0O0,000 or less than the resistance of the surface 21 and are comprised of a conductive material such as chromium, copper, gold platinum or stainless steel deposited as a thin film on the emissive surface 21. The thin film of conducting material 22 may be applied by any conventional means such as spraying, sputtering, painting or vaporizing. The direction of the conducting paths shown is such that each line, when viewed from the anode, makes an acute angle A with the sides (C,D) of surface 21, having sides C and D. Therefore, each line has a direction which intersects side C, D, of the emissive surface and the plane in which the anode is located at an acute angle. Shown also on the surface are the terminals 23, to which a negative potential is applied to establish a gradient (not shown) along the length of the surface 21. The gradient is a series of negative equipotentials (imaginary parallel lines) decreasing in magnitude from one to another in the direction of the anode (when a higher negative potential is applied to terminal 23 than terminal 25). Power supply 5 supplies the potentials to properly bias the anode and the emissive surface 21 between

terminals

23, 25. The preferred relationship of the conducting paths 22 to the anode and cathode is such that the conducting paths extend on the emissive surface 21, away from the sides (C, D) of the plate 20 and travel in a direction away from the anode 30 and towards the cathode 10. Also, the conducting paths 22 are arranged in parallel relationship along the sides of the surface and although it is not necessary for each line 22 to extend completely across the surface 21, it is preferable to have the conducting paths extend to one edge (C or D) ofthe surface 21.

FIG. 3, is an alternate pattern of conducting paths 22 arranged in a parallel relationship. Each conducting path 22 extends from one edge (C) to the emissive surface 21 to the other edge (D) and may be considered as being comprised of at least two straight lines that intersect at an angle on the surface. The surface shown is rectangular in shape to conform to the shape of the plate 20. However, regardless of the shape of the emissive surface, the conducting paths are arranged to cause equipotentials to shape themselves such that the apex 27 of each equipotential is in a direction ofa greater negative potential. This arrangement causes an electron moving over the surface to seek a path towards the longitudinal axis of the emissive surface.

FIG. 4 shows a series of conducting paths 22 which are curvilinear in shape. Each path is arranged so that the concave or hollow portion 29 of each peak of an equipotential faces a direction of a lower potential (decreasing negative or increasing positive) to cause an electron moving over the surface 21 towards an edge (C or D) to be attracted away from an edge (C or D) and towards the anode 30.

FIG. 5 shows the shape of the equipotentials when a potential is applied to the multiplying surface shown in FIG. 2. Preferably a direct negative potential is applied to

terminals

23, 25 with terminal 23 being at a higher negative potential than terminal 25. This establishes a potential gradient along the longitudinal axis 60 of the emissive surface 21. The equipotentials 61, which would normally be a series of straight lines each perpendicular to both sides. C, D, of the surface 21, are distorted by the conducting paths so that a portion 63 of the equipotential 61 is curved. The fact that the equipotentials are not straight lines creates a second potential gradient on the emissive surface 21 along a line perpendicular to the axis 60 of the first potential gradient. The second potential gradient along line 70 is characterized by the fact that the negative potential along line 70 decreases in magnitude when approaching the longitudinal axis 60 of the first gradient.

OPERATION Considering the plate shown in FIG. 2 an actual embodiment of the invention utilized in a magnetic electron multiplier of the type shown in FIG. 1 had the following features: The conducting paths 22, of a width of about 0.030 inch, were arranged in a parallel relationship wherein one end of each line 22 terminated away from the edges (CD) of the emissive surface 21 and the other end extended to one edge (C or D) of the surface 21. Each line 22 extended approximately one-sixth of the distance across the surface at an angle of 45 (angle A), and because the plate and emissive surface were rectangular in shape, each of the lines 22 had a direction that was not perpendicular to any side of the emissive surface 21 (rectangle C, D, E, F). When a negative potential of 3,000 volts was applied to terminal 23 and terminal 25 was grounded, a potential gradient was established along the surface 21 between

terminals

23, 25. Because of the paths 22 on the surface 21, the equipotentials 61 which are normally parallel, were distorted into the shape shown in FIG. 5. The conducting paths, therefore, established a second potential gradient along a line 70 perpendicular to the longitudinal axis 60 of the first potential gradient. It is the function of this second potential gradient to accelerate electrons away from the edges of the emissive surface 21 and towards the axis 60 of the first potential gradient whose function is to accelerate electrons to the anode. Under the above conditions, when a primary electron is emitted form the cathode 10 it follows a cycloidal path 50 in a direction towards the anode 30 and impinges the secondary electron emissive surface 21 releasing a plurality of electrons. The multiplication process is repeated along the entire length of the surface until the electrons are collected at the anode 30. During the process, some of the secondary electrons emitted from the surface 21 have a velocity vector with a direction which could result in the electrons leaving the path defined by the edges (CD) of the surface 21. However, as these electrons travel towards an edge (C or D) of the surface 21, the second potential gradient created by the conducting paths 22 subjects the electrons to an electric field that changes the direction of the velocity vector of the electrons away from the edge (C or D) of the surface 21. This confines substantially all the electrons leaving the emissive surface 21 to the path defined by the sides (C, D) of the surface.

While a preferred embodiment of the invention has been disclosed, it will be apparent to those skilled in the art that changes may be made to the invention as set forth in the appended claims, and, in some cases, certain features of the invention may be used to advantage without corresponding use of other features. For example, the shape of the emissive surface 21 is rectangular to conform to the shape of the plate 20, however, the shape of the emissive surface and the arrangement of the conducting paths may assume any configuration that would prevent the escape of electrons from the surface. Accordingly, it is intended that the illustrative and descriptive materials employed herein be used to illustrate the principles of the invention and not to limit the scope thereof.

l claim:

1. A magnetic electron multiplier comprising:

a secondary electron emissive means for producing secondary electrons upon impact of primary electrons, said emissive means including a substrate having a first and second end, with a longitudinal axis therebetween, a resistive secondary electron emissive material disposed on one surface thereof, and a plurality of electron conducting paths, disposed on aid emissive surface in spaced relationship to each other, each of said paths extending in a direction generally transverse of said longitudinal axis and having at least a portion of said path extending in a direction towards the first end of said substrate;

means for producing primary electrons disposed at said first end of said emissive means;

means for establishing an electric field having a component substantially perpendicular to said emissive means;

means for applying an electric potential between said first and second ends of the substrate of said emissive means;

means for establishing a magnetic field having a component substantially perpendicular to said electric field so the secondary electrons emitted from said emissive surface follow a cylindrical path; and

anode means disposed at said second end of said substrate for collecting secondary electrons leaving the second end of said substrate.

2. The combination as recited in claim 1 wherein said conducting paths have a direction which intersects the anode plane at an acute angle.

3. The combination in claim 1 wherein said conducting paths are curvilinear and arranged in a spaced relationship.

4. The combination as recited in claim 1 wherein the resistance of the conducting path is less than 1/ l ,000 of the resistance of the emissive surface.

5. The combination as recited in claim 1 wherein said conducting paths are lines each having at least one end terminating away from the edge of said emissive surface.

6. The combination as recited in claim 1 wherein each of said conducting paths comprises at least two straight lines that intersect at an angle.

7. The combination as recited in claim 1 wherein said conducting paths are straight lines arranged in a parallel relationship, said lines having a length less than the distance across said emissive surface.

8. The combination as recited in claim 1 wherein said emissive surface is rectangular in shape and wherein said conducting paths are straight lines arranged in a parallel relationship to each other along at least one side of said rectangle, each of said lines having a direction which is not perpendicular to any side of said rectangle.

9. The combination as recited in claim 1 wherein said conducting paths are curvilinear and arranged in a spaced relationship to each other.

10. The combination as recited in claim 4 wherein said conducting paths are lines each having at least one end terminating away from the edge of said emissive surface.

11. The combination as recited in claim 4 wherein the material comprising the conducting path is selected from the group consisting of chromium, copper, gold, platinum or stainless steel.

Claims

1. A magnetic electron multiplier comprising: a secondary electron emissive means for producing secondary electrons upon impact of primary electrons, said emissive means including a substrate having a first and second end, with a longitudinal axis therebetween, a resistive secondary electron emissive material disposed on one surface thereof, and a plurality of electron conducting paths, disposed on aid emissive surface in spaced relationship to each other, each of said paths extending in a direction generally transverse of said longitudinal axis and having at least a portion of said path extending in a direction towards the first end of said substrate; means for producing primary electrons disposed at said first end of said emissive means; means for establishing an electric field having a component substantially perpendicular to said emissive means; means for applying an electric potential between said first and second ends of the substrate of said emissive means; means for establishing a magnetic field having a component substantially perpendicular to said electric field so the secondary electrons emitted from said emissive surface follow a cylindrical path; and anode means disposed at said second end of said substrate for collecting secondary electrons leaving the second end of said substrate.

4. The combination as recited in claim 1 wherein the resistance of the conducting path is less than 1/1,000 of the resistance of the emissive surface.