US3622827A - Matrix assembly for aligning electron multiplier components - Google Patents

Abstract

An apparatus for precisely locating the components at the input and output of an electron multiplier which includes mounting plates having precisely located holes for securing a plurality of arms in a predetermined arrangement. Mounted on the arms of the compoents which are fixed in their proper position when the apparatus is assembled.

Description

United States Patent [72] lnventor Willaim H. Shriner Blanchester, Ohio [21 Appl. No. 867,997 [22] Filed Oct. 21, 1969 [45] Patented Nov. 23, 1971 [73] Assignee The Bendix Corporation [54] MATRIX ASSEMBLY FOR ALIGNING ELECTRON MULTIPLIER COMPONENTS 1 Claim, 4 Drawing Figs.

[52] U.S.Cl 313/103, 250/41.9 R, 313/243 [51] Int. Cl ..H0lj 43/16, H01j 39/34, BOld 59/44 [50] Field of Search 313/103- [56] References Cited UNITED STATES PATENTS 2,615,135 10/1952 Glenn,.lr. 250/41.9

Primary Examiner-Herman Karl Saalbach Assistant ExaminerMarvin Nussbaum Attorneys-Raymond .l. Eifler and Plante, Arens, Hartz, Smith and Thompson ABSTRACT: An apparatus for precisely locating the components at the input and output of an electron multiplier which includes mounting plates having precisely located holes for securing a plurality of arms in a predetermined arrangement. Mounted on the arms of the compoents which are fixed in their proper position when the apparatus is assembled.

PATENTEUuuv 23 I97! SHEET 1 [IF 2 N WEDGE WILLIAM H. SHRINER INVENTOR.

BY ATTORNEY PAIENTEDuuv 23 I971 SHEET 2 OF 2 FlGURE 3 FlGURE 4 WILLIAM H. SHRINER INVENTOR.

ATTORNEY MATRIX ASSEMBLY F OR ALIGNING ELECTRON MULTIPLIER COMPONENTS BACKGROUND This invention relates to an improved gating apparatus for an electron multiplier of a mass spectrometer.

The mass spectrometer is an instrument that permits rapid analysis of molecular species by measurement of the masses of the different ions after ionization of the molecules. In operation, a small amount of gas to be analyzed is admitted through a sample inlet into an ionization chamber where the gas is ionized by electrons emitted from a filament. The ions are then directed or accelerated by an electric field from the ionization chamber and into a region where the ions are separated according to their mass to charge ratio (rule). The ions then impinge upon the cathode of an electron multiplier to achieve a gain of or greater at the output. The output signal is then synchronized on an oscilloscope or gated to an analog for strip chart recording to indicate the mass spectrum of the gas under analysis. A multiplier having more than one gate gives the spectrometer a built-in capability to monitor multiple mass peaks of the spectrum simultaneously by the addition of analog scanners. Each scanner is capable of scanning the mass range from 0 to 750 atomic mass units, or any portion thereof. A Bendix Time of Flight Mass Spectrometer, Model 3012, is one mass spectrometer which allows up to six analog scanner plug-in units to be used simultaneously with the oscilloscope output.

The ability to distinguish between mass peaks on the spectrum is called resolution and, the better the resolution, the better the ability to identify the constituents of the sample under analysis. Because of improvements to other portions of the mass spectrometer to improve resolution, the precise arrangement of the input and output components of the electron multiplier has become important. These components, e.g. the cathode at the input and the gating plates at the output when not precisely arranged, adversely affect the resolution between mass spectrum lines. For example, when the output gates are not precisely arranged, portions of the output signals are misdirected and/or lost, resulting in peak broadening of the mass spectrum (i.e., the mass spectrum line does not return to the base reference line between mass peaks). Further, the arrangement of the cathode at the multiplier input must be precise to maintain the proper separation of incoming signals. The efiects and advantages of precisely arranging the cathode is disclosed in copending application entitled Mass Spectrometer Having Means Compensating for Electron Transit Time Across the Cathode of the Electron Multiplier" by D. C. Damoth and W. H. Shriner, filed Sept. l5, I969, Ser. No. 858,058.

The arrangement of the cathode and gating plates with precision is a problem because tolerances such as, 10.00l inch and angles less than I", are difficult to obtain. Also, it is different to obtain precise uniformity of arrangement from one multiplier to another. Therefore, the components to be combined with each multiplier are individually adapted to each other when each spectrometer is assembled. This results in a time-consuming, expensive and inefiicient method of assembly. Further, the present mounting methods and apparatus are not precise enough to obtain the improved resolution between mass peaks required by the advancing biological sciences.

SUMMARY OF THE INVENTION To improve the performance of a mass spectrometer, the resolution between mass peaks of the spectrum at the spectrometer output is improved by precisely arranging (spacing) the gating components of the electron multiplier so that electrons when gated to a particular collecting anode strike that anode. The invention is characterized by an assembly which includes mounting plates having rigidly fixed and precisely located arms or supports extending from the mounting plates so that gating components, such as the gating wall, gating electrode, anode plate, and collecting plate secured to the supports, are positioned in their proper places. The advantage of the assembly is that the precise alignment and spacing of components is easily accomplished in one operation by precisely locating the holes for the arms by jig boring the mounting plates. Jig boring also facilitates duplication of the precise location of the holes from plate to plate. The invention is especially useful in improving the performance of the gating apparatus shown in US. Pat. No. 3,049,638 and for precisely orienting the cathode as required in the aforementioned copending application.

Accordingly, it is an object of this invention to provide an apparatus which permits precisely orienting the gating components associated with an electron multiplier.

It is another object of this invention to provide an apparatus which permits precisely orienting the cathode of an electron multiplier.

It is still another object of this invention to improve the performance of a gating apparatus for charged particles.

It is a further object of this invention to improve the performance of a mass spectrometer.

It is still a further object of this invention to improve the resolution between mass peaks over the entire mass spectrum range.

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 the specification.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagrammatic view of a time of flight mass spectrometer utilizing the invention.

FIG. 2 is a more detailed partial plan view of the gating apparatus shown in FIG. 1.

FIG. 3 is a top view of the matrix assembly which precisely locates the gating components of a gating apparatus with respect to the dynode of an electron multiplier.

FIG. 4 is a side view of the matrix assembly showing the precisely located jig boreholes which hold the anns that support the gating components.

DESCRIPTION OF PREFERRED EMBODIMENT Referring now to the drawings, FIG. I shows a mass spectrometer of the type described in US. Pat. Nos. 2,765,408 and 2,685,035. Molecular species entering the ionizing region I from the sample inlet 3 are ionized by electrons from filament 5. The ions 10 are then accelerated into the drift tube 7 by accelerating grids 9. Because of the length of the drift tube 7 and the different velocities of the ions, the ions are separated according to their mass to charge ratio (m/e) striking the cathode 20 at different times. The different times of flight (T) for ions of different masses (m) to travel from the ionizing region to the cathode may be calculated mathematically from the equation T=k(m/e)" where k is a constant depending on physical dimensions. For example, the time of flight of a singly charged nitrogen ion m=28 atomic mass units in a Bendix Time of Flight Mass Spectrometer is 5 microseconds and under usual conditions the time width of the nitrogen pulse at the spectrometers output is about 0.015 microsecond (about 0.060 microsecond for atomic species of 400 AMU). A magnetic electron multiplier 30 (e.g. U.S. Pat. Nos. 3,049,638 and 2,841,741) is used to detect and amplify the ion bunches. The ions pass through aperture 2 of grid 4 and strike the cathode 20 to produce secondary electrons 40. The cathode 20 may be disposed at an acute angle with a plane that is perpendicular to the longitudinal axis 8 of the drift tube 7 to decrease the time interval for all the electrons, propogated from the cathode by ions of the same mass, to strike the dynode. The electrons emitted from the cathode then follow a cycloidal path 41 under the influence of the mutually perpendicular electric and magnetic fields in the multiplier to strike the dynode strip 31 multiplying in number to achieve a gain of approximately I0 K The resulting output signal is then synchronized on an oscilloscope, or gated (gates 35) to an analog (not shown) for strip chart recording. Plates 70 and arms 80 form a matrix that precisely locates, with respect to each other, the gates 35, (and gating components), the field strip 32, dynode 31 and rail 50. The matrix may also be used at the input to the electron multiplier 30 to precisely angle the cathode 20 where the electron multiplier is of the type disclosed in the aforementioned copending application. At the input, the arms 80 are placed in the holes 72 having predetermined locations so that when the arms 80 are placed in the holes 72, the cathode is held in a position'relative to the location of the holes 72.

FIG. 2 shows a gating assembly 35, of the type shown in U.S. Pat. No. 3,049,638, which receives electrons from the electron multiplier 30. Mounting plate 70, which forms part of the matrix assembly that precisely arranges the gating components, is shown in phantom lines. Only a portion of the field strip 32 and dynode 31 of the multiplier 30 are shown. The field strip 32 has an intermediate electrode 34 which receives a potential from a power source 61 and has an electrode 36 at one end which is grounded. The dynode 31 has an intermediate electrode 42 and an electrode 44 at one end of the dynode which receive potentials from a power supply 33.

The gating assembly 35 includes the rail 50 which is aligned with dynode 31 and connected to electrode 44. The magnetic field of the electron multiplier 30 extends through the rail 50 so that the cycloidal path of the electrons continues, but is changed a predetermined amount by an electric field between the rail 50 and gates, so that electrons do not strike the rail 50. Precisely spaced from the rail 50 and apart from each other are gating walls 52 which define the gating area. The gating walls 52 are connected to ground and electrode 36 of the field strip 32. Between the gating walls 52 are a plurality of gating electrodes 54 each of which is precisely spaced from rail 50 and each of which has a transverse rod 56 secured (preferably welded) to one end thereof. Connected to each of the gating electrodes 54 is a pulser 58 which maintains the electrodes 54 at zero potential until it is desired to divert the path of the electron cycloid 41 to a collecting anode 62 by applying a negative potential (preferably a negative voltage greater than that applied to the rail 50, such as -70 volts) to the appropriate gating electrode 54. Precisely spaced from each electrode 54 is an anode plate 60 which is maintained at a constant voltage (preferably a negative voltage greater than that applied to the collecting anode, such as l volts) by power supply 61. Adjacent to each anode plate 60 in the collecting anode 62 which is connected to an output device, (not shown) such as an oscilloscope, recorder or other indicating and/or actuating devices. The anode plates 60 are preferably spaced from collecting anodes 62 a precise distance, which is less than the cycloid height to provide sufficient space for the electrons traveling along cycloidal path 41 (diverted by a negative pulse to the proper electrode 54) to strike the collecting anode 62. The gating electrodes 54 are precisely spaced from the anode plates 60. collecting anodes 62 and ground plates 64 to establish the proper electric field therebetween. A grounded plate 64 is precisely spaced from the anode plate 60 and collecting anode 62 to prevent electrons directed into one output to cross over into another output. The power supplies 33, 61 and 58, supply the voltages to the gating components to establish at predetermined time intervals the electric fields necessary to direct electrons to the proper output. Although 3 outputs are shown, more or less may be used as desired.

FIG. 3 is a plan view of a portion of the matrix assembly which precisely locates the gating components. Mounting plates 70 have a plurality of holes 72, at predetermined locations. Mounted in the holes 72 are arms 80. The arms are preferably wires having a diameter less than 0.060 inches and in their preferred positions extend perpendicular from the surface of the plate 70 and in parallel relationship to each other. Secured to arms 80 (preferably welded) are the field strip 32, gating wall 52, collecting anode 62, anode plate 60, and ground plate 64. Not shown is the dynode 31 which is also secured to an arm 80. The holes 72 are precisely oriented so that when the arms, holding the gating components, are placed in the holes 72 and secured in place to the mounting plates 70, the gating components are arranged in their proper relationship with respect to field strip, dynode and each other.

FIG. 4 is a side view of the matrix assembly showing the location of the holes 72 in mounting plate 70. By locating the arms in the precisely arranged holes 72, the gating components held by the amis are also precisely arranged. The gating components shown are the gating wall 52, collecting anode 62, anode plate 60, ground plate 64 and rail 50.

The preferred method of making and locating the holes 72 is by jig boring. Jig boring the holes 72in each plate 70 precisely locates the holes in each plate without repeated time-consuming measurement. Further, accurate duplication from plate to plate which facilitates alignment of the arms 80 is achieved.

Although one mounting plate 70 could be used, two mounting plates are preferred, with supports extending between them. In this manner, a more precisely arranged matrix assembly is formed. I

OPERATION In describing how the matrix assembly improves the performance of the gating apparatus 35, reference is made to FIG. 2 and the cycloidal path 41 of the electrons as they leave the dynode 31. The cycloid is formed through the cooperation of the electrical and magnetic field existing between the field strip 32 and the dynode 31. At each contact point on the cycloid with the dynode 31 secondary electrons are emitted resulting in a multiplying action. The cycloid moves in a direction toward electrode 44. The magnitude of the cycloid may be varied by varying the voltage applied to the electrode 34 of the field strip 32. The cycloidal path of the electrons is lifted from the dynode strip 31 by the electric field established by electrodes 42, 44, 36 and 34. By the time the electrons reach the rail 50, they are traveling in a cycloidal path 41 that has been elevated a predetermined amount by the application of a proper electric field. To maintain a uniform electric field so that the electron cycloid travels parallel to rail 50, the gating electrode must be exactly spaced from the rail. This is accomplished by arms 80 (FIG. 4) which hold the electrodes and rails. The electrons continue to travel in a cycloidal path parallel to the rail 50 until one of the gating electrodes is pulsed in an appropriate manner. For example, when gating electrode 54 of output gate number 3 receives a negative voltage greater than the voltage applied to the rail 50 from pulser 58, the cycloid is diverted towards gating anode 62. An electric field between the gating anode 62 and anode plate 60 then directs the electrons towards the collecting anode 62. To prevent electrons from bypassing the collecting anode 62 on the side adjacent to the gating electrode 54, a grounded plate 64 is precisely located by arms 80 (FIG. 3) between the collecting anode of gate 3 and the anode plate 60 of gate 2. So that no electrons are misdirected or miss their intended targets, it is essential that all the gating electrodes 54, anode plates 60, collecting anodes 62, and ground plates 64 be precisely spaced from each other. The matrix assembly makes the precise spacing possible.

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. Accordingly, it is intended that the illustrative and descriptive materials herein be used to illustrate the principles of the invention and not to limit the scope thereof.

Having described the invention, what is claimed is:

1. In combination with a magnetic electron multiplier of the type wherein electrons traveling in cycloidal paths in the space between a field strip and a dynode are accelerated from the dynode to a gating apparatus having a gating components a plurality of wires extending between said plates in fixed positions. said wires in generally parallel and spaced relationship with respect to each other, each of said wires having attached thereto only one component of said gating components, so that said gating wall, collecting anode, anode plate and ground plate are precisely spaced and oriented with respect to each other and said field strip and said dynode.

Claims (1)

1. In combination with a magnetic electron multiplier of the type wherein electrons traveling in cycloidal paths in the space between a field strip and a dynode are accelerated from the dynode to a gating apparatus having gating components comprising a gating wall, a collecting anode, an anode plate, a ground plate and means for mounting said gating components in predetermined relationship and wherein the electrons are directed, by potentials applied to said gaiting components, to impinge said collecting anode, the improvement wherein said mounting means comprises: a pair of plates disposed in spaced relationship with respect to each other at one end of said dynode, said plate adjacent to and generally perpendicular to said dynode; and a plurality of wires extending between said plates in fixed positions, said wires in generally parallel and spaced relationship with respect to each other, each of said wires having attached thereto only one component of said gating components, so that said gating wall, collecting anode, anode plate and ground plate are precisely spaced and oriented with respect to each other and said field strip and said dynode.

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