US2664515A - Magnetic electron multiplier - Google Patents

Description

2 sheets-sheet 1 L. G. SMITH MAGNETIC ELECTRON MULTIPLIER D ec. 29, 1953 Filed June 22, 1951 Dec. 29, 1953 L. G. SMITH MAGNETIC ELECTRON MULTIPLIER 2 Sheets-Sheet 2 Filed June 22, 1951 l N VE N TO R L'lmmfv @i SMITH BY ,4M/awww ATTCRNEY Patented Dec, 29?. i953' riso srarss Lincoln G. Smith, Center Morches, N. Y., assigner to the United States of America as represented hy the Commission United States Atomic Energy Application June 22, 1951, Serial No. 232,893

6 Claims. l

The present invention relates to an electron multiplier capable of operating in a magnetic eld. In mass spectrometers and other similar' analytical devices wherein a beam of ions is projected into a magnetic analyzer which resolves the beam into its characteristic mass components, there is a need for apparatus capable ofdetecting ion pulses of extremely small magnitudes. For example, certain types of mass spectrometers such as the time-of-iiight mass spectrometer disclosed in application S. N. 83,258 (S. A. Goudsmit) require associated apparatus capable of detecting with accuracy short ion pulses of very weak intensity. Moreover, in this type of instrument it is also desired to have the time between the formation of a pulse in the ion source of the spectrometer and its arrival at the detector represent an integral number of periods of rotation oi an ion in a homogeneous magnetic eld. It is accordingly necessary to have the sensitive surface of the detector located in the magnetic field. Conventional elecron multipliers, such as are used in electron multiplier phototubes, will have their operation seriously impaired if they are located in the magnetic field of the spectrometer. By contrast, the apparatus of the present invention makes use of the magnetic eld to focus secondary electrons from each ernissive electrode on to the next.

It is accordingly an object of the present invention to provide a new and improved method and apparatus for detecting moving charged particles.

Another object of the invention is to provide a new and improved ion detector which can be used in a magnetic field.

A third object of the invention is to provide a new and improved magnetic electron multiplier with a high gain and a short response time.

A further object of the present invention is to provide a new and improved magnetic electron .multiplier that utilizes the focusing properties `of a uniform electrostatic field perpendicular to a magnetic eld.

Other objects and advantages will be in part obvious and in part pointed out hereinafter.

More particularly, a preferred embodiment of the apparatus of the present invention comprises the combination including a homogeneous magnetic field, an electrically conductive plate located in said magnetic field and parallel to the lines of iiux of said magnetic eld, a plurality of secondary electron emission dynodes located in said magnetic neld and disposed in echelon parallel .relation with respect to said conductive plate,

means for establishing a uniform electrostatic eld between said plate and said dynodes, said electrostatic field being perpendicular to said magnetic neld, means for causing the moving charged particles to be detected to impinge on the dynode, most remote from said plate whereby secondary electrons are produced which are attracted and amplied by the remaining dynodes in successive steps and means for collecting the ampliiied number of secondary electrons produced by the last dynode.

The many objects and advantages of the present invention may best be appreciated by reierence to the accompanying drawings, the figures of which illustrate apparatus incorporating a preferred embodiment of the present invention and capable of carrying out the method oi the invention.

Figure l is a schematic View showing the eX- terior of the magnetic electron multiplier in relation to the conventional instruments connected thereto.

Figure 2 is a perspective View of the apparatus rotated through degrees from the position shown in Figure l and with two side plates removed to show the internal constructional details.

Figure 3 is a perspective View oi a second embodiment of the apparatus of the present invention with a portion of the side plates broken away.

Figure 4 is a transverse sectional view taken along the line li-l of Figure 3.

Figure 5 is a transverse sectional view of three successive dynodes of Figure 2.

Referring to Figure 1 the magnetic electron multiplier it is shown positioned in a homogeneous magnetic iield the direction ci which is represented by arrows Ell. The moving charged particles to be detected enter the multiplier iii through Window H containing grid wires l2. Operating potentials for multiplier It are pro- Vvided by power supply 3e connected to the multiplier by conductors i3 and it.

The amplined output of the multiplier is connected on conductor I6 to a conventional amplifier it. Ampliiier t is used only if it is desired to further amplify the electron pulse received from multiplier l. The output of amplifier it can be connected to an oscilloscope 5o by conductor il or to any other conventional indicating instrument. Also shown on one end wall ci multiplier it are pressure screws I 8 and l 9 used for mounting purposes as will be described hereinhelow.

Referring now to Figure 2 it can be seen that the multiplier lo is in a homogeneous magnetic iield which is in a direction represented by arrows 20. nt the left hand portion of Figure 2 is window l I in which are mounted the grid1 wires l2. Directly opposite window l! is an L-shaped secondary electron emissive dynode 2l which is mounted on two insulated rods and spaced from end wall Ed of multiplier l@ by two electrically insulated spacers 22 and 23. Spacer 22 is shown broken away so that one of the insulated mounting rods 25 may be visible. The insulated mounting rods run the entire length of the multiplier and are terminated in pressure screws i3 and i9 mounted in the end wall' of the multiplier. Pressure screws is and i9 are used to force the rods securely into recesses in the opposite end wall Eel. Also mounted on the rods are a plurality of L-shaped secondary electron einissive dynodes which are separated by*y insulated spacers oi a construction similar to spacers and 23. The dynodes are electrodes made or or coated with a material capable or high secondary emission. When moving charged particles imple on the electrode a number or" electrons are emitted. When a high secondary ernissve material is used more electrons are emitted by the electrode than arrive thereon resulting electron amplication. Mounted along one side of multiplier lo is an electrically conductive plate 2i. Each of the dynodes has one surface parallel to the plate il. The distance between each of these surfaces and plate 2i decreases in successive steps progressing iroin left to right in the multipli r so that the nrst dynode 2l is most remote from plate 2l and the last dynode 28 is closest to the plane of the plate 2i. Accordingly, these dynode surfaces are disposed in echelon parallel relationship with respect to plate 2l. That is, each of the dynode surfaces is successively closer to plate 2'." and the surfaces are parallel to one another. Mounted next to the last dynode 23 is a collector plate 29 of the same general shape as the secondary electron emissive dynodes. At the right hand portion of the multiplier, plate 2l is depressed so that the parallel. surface of collector 2t lies in the plane of the plate l. Attached to collector' plate 2Q by means or a clip 3i is a conductor t2 which is connected to output conductor l@ by ineans of a conventional connector mounted in the end wall 2t oi multiplier lil. Attached to the iirst dynode 2i by :neans of a clip Sil is a conductor 3G that is connected to power supply conductor i3 through a conventional connector 3l mounted in the end wall 2liv of multiplier it). Also connected to dynode 2! by means or" a clip 3S is one end of the resistor 39. rihe other end or" resistor 3S is connected to the dynode immediately adjacent dynode 2l by means of clip. All. in the saine manner, resistors are connected between adiacent dynodes mounted in multiplier i@ up to and including resistor @l2- connected to the last dynode 2%. Dynode .28 isconnected to the remaining power supply lead Hl by means of conductor and connector [if In operation the multiplier it. is placed in the magnetic held so that plate 22H-is disposed parallel to the i'lux lines of the magnetic iield represented oy arrows 2d. The ions to be detected, which travel in aplane perpendicular to the lines of ilus: of the magneticeld, arrive from the direction indicated by arrows 3. A negative operating potential is applied on. lead i3 to the rst dynode 2i by means of connector 3l, conductor sa clip 3ft. This negative potential is reduced in successive steps by the resistors connected between adjacent dynodes so that dynode 2! is at the most negative potential and dynode 2S is at the least negative potential. The electrically conductive plate 2i is connected to ground potential. This connection is not shown but may be easily carried out by a suitable connection to the mounting screws holding the multiplier assembly together. Collector plate 29 is. connected to ground through an external resistance, for example, the input resistor of amplier d. rEhe connection of the operating potentials as described hereinabove results in a uniform electric field between plate 2l and the surfaces of the dynodes parallel to plate 'El oecause the potential of each dynode is proper'- tional toits distance from plate That is, as each dynode is spaced closer to plate 2l its potential is made proportionately less negative due to the voltage dropping resistors. riherefore, when the ions to be detected arrive at window il they are immediately attracted to dynode 2i which is maintained at the most negative potential in multiplier lil. Due to the secondary electron emissive properties of dynode iii the impingernent of the ions thereon gives rise to secondary electrons. These electrons would nirnially be attracted to plate El' which is maintained at the moet positive potential in multiplier il. However, due to the action of the magnetic iield the path oi the electrons, represented by dotted line d3, curves in the direction shown by the arrow le so that the electrons iin 'ne on dynode adjacent dynode il. orbit represented by dotted line it is trochoidal with a cusp at each dynode. it is well known that the orbit of electrons in a crossed magnetic and electrostatic field is a trochoidal orbit. Therefore by properly adjusting the spacing potentials or adiacent dynodes the secondary electrons are made to follow the path represented by dotted line i3 the cusps of the orbit to occur at each adjacent dynode. More secondary electrons are emitted from the next dynode than thereon due to its r potential and -sive properties. These emitted electrons continue to be ainpliiied in the saine ina-nner 'throughout the reina ng dyncdes with successive amplification taking place du to last that each succeeding dynode is positive than the one previous to. it. rEhis amplification continues until the amplified secondary electrons reach the last dynode 23. They are thereupon attracted to the collector plate rise to an output pulse on conductor which is transmitted to output. conductor IL-,

The improvement oi the performance oi the embodiment hereinabove described is pri due to the unit rin electrostatic held between the active surfaces oi the dynodes and electrically conductive plate 2l. structional feature which accomplisl sult is the progressively smaller distance tween each dyncde and plate 2l. the slight overlapping oi each emissive surface preve `ts any: abrupt break in the uniformity cleotric iield. ln this manner the required potential diierence oi several hundred volts between acent dynodes isv established without resort to a non-uniform electric held. This useful feature also requires only one-half the number of insulated plates normally used in magnetic electronv multipliers. due to the fact that only one accelerating electrode, namely conductive plate i, is used. Another advantage in the embodiment shown in Figure 2 is that there is no component of the electrostatic iield which tends to draw ions from the region near the output stage toward the input stage thereby causing a breakdown or noise due to ion feedback.

Referring to Figure 3 a second embodiment employing the features of the present invention is shown. The magnetic eld in which this multiplier is located is in a direction represented by arrows 2t. The ions to be detected enter the multiplierthrough window 6|. Located opposite window EBI is the secondary electron emissive surface of dynode 62. The dynodes of the multiplier shown in this figure are co-planar and mounted at an acute angle with respect to the electrically conductive plate 63, with the first dynode d2 being farthest frornplate 63 and the last dynode lid being closest to plate 63. The dynodes all have their longitudinal edges parallel toplate 53. Following the last dynode S-i is collector plate t6. The dynodes and the collector plate t5 are all mounted on two insulated rods 6l and t8 each of which have one end secured in the insulated end plate 69 and the other end secured in end plate 1I. The dynodes are secured to the rods 61 and 68 by a suitable adhesive material, lil, which is shown only on the rst two dynodes in order to make clear the construction of the dynodes. The ends of the dynosles are bent around rods 5'! and 63 to permit convenient electrical connection thereto and resistors such as resistor 'i2 are connected between adjacent dynodes in the manner shown. In order to supply operating potential to the first dynode t2, power supply conductor 'I3 is connected to the interior of the multiplier through a conventional connector 14. Connection is made from the last dynode 64 to the return power supply conductor 76 by means of clip Ti, conductor lt and connector 19. The amplified output of the multiplier is taken from collector plate 66 by means of clip 8l, conductor 82, connector 83 and external conductor 85. Collector plate 65 is connected to ground in a similar manner to collector plate 29 of Figure 2. Mounted on the insulated end plate 69 is` a second electrically conductive plate 84 which is parallel to plate S3 in the operating area of the multiplier, Plate 8d is supported at its other end in a curved slot in insulated end plate 1|. Electrical connection to plate 84 is made by means of clip 86, conductor Si and connector 83 leadingr to the external conductor 89. The operation of this embodiment will be described hereinbelow with respect to Figure 4.

Referring to Figure 4 the lines of ux of the magnetic eld are now directed perpendicular to the plane of Figure 4. The ions to be detected arrive in a direction indicated by arrows El. Dynode t2 is connected to a source of negative potential with the return lead of the source connected to dynode iid. Since resistors are connected between adjacent dynodes each dynode is successively less negative with respect to the one before, progressing from said iirst dynode S2 to the last dynode 5ft. Plate Sli is connected to a source of potential more negative than dynode E2. Plate 63 is connected to ground potential. The negative potential of each dynode and of plate 8d is proportional to its distance from plate t3. Therefore when the ions arrive at the window '5l they are attracted to dynode 62, the most negative dynode. Secondary electrons arise upon the impingementv of the ions. These electrons are attracted towards grounded platel S3 but are deflected towards the next dynode by the magnetic eld so that they follow the path represented by dotted line 92 and arrow 93. Since there are spaces between adjacent dynodes the uniformity of the electrostatic field between the dynodes and the plate 53 would be disturbed were it not for the inclusion of plate Sei which provides a uniform electrostatic neld between plates 63 and Si throughout the operating region. Therefore with a homogeneous magnetic eld and a perpendiculai'uniform electrostatic eld the secondary electrons follow trochoidal orbits with cusps at each succeeding dynode as represented by the dotted line 92 of Figure 4. The secondary electrons are successively attracted to each succeeding dynode, with more electrons being emitted from the dynodes than impinge thereon, until the last dynode iid is reached. Here the arnplied number of electrons are attracted to the collector plate et giving rise to an output pulse on the conductor connected thereto. if necessary this pulse can be further amplified by conventional amplifying equipment located outside the magnetic field and visually indicated on an oscilloscope.

The embodiment illustrated in Figures 3 and 4 may generally be used in more intense magnetic fields than the embodiment of Figure 2. This results from the fact that the distance between successive cusps of the trochoidal orbit is proportional to the value of the electrostatic iield and inversely proportional to the square of the value of the magnetic neld. This may be represented by the equation, halt/H2 where b is the distance between cusps, E is the value of the electrostatic field and H is the value of the magnetic field. It is apparent that the minimum distance between cusps is the width of a dynode. Therefore, if it is desired to double the intensity of the magnetic field for a given width of dynode, it would be necessary to quadruple the value of the electrostatic field in order to maintain the distance, b, constant. As the intensity of the magnetic field is increased to higher levels, the

. electric eld would have to be increased to such a value that an electrical discharge would occur between the dynodes. The construction of the embodiment of Figures 3 and 4 overcomes this when the spaces between dynodes are made equal to the width of a dynode. ln this event, the secondary electrons can be made to` undergo two trochoidal cycles between successive dynodes as illustrated by the dotted line fifi of Figure 4. In this manner, the distance, c, between cusps can be kept the same in higher magnetic iields without the danger of a discharge occurring between the dynodes. It is evident that the distance between successive dynodes can be made equal to any integral number of dynode widths and the saine result obtained by causing the secondary electrons to go through an equal integral number of trochoidal cycles between dynodes.

The following materials and dimensions were found satisfactory in the construction of the em`- bodiment of Figure 2: l5 dynode plates whose active surfaces` are parallel and slightly overlapped are used; the dynodes are l high, made of commercial beryllium copper 9.007" thick, supported on two 1/8 O. D. insulated tubes made of a suitable material, as for example the material commercially known as steatite and spaced by sections of 1/4 O. D. insulated tubing such as steatite; the grid window is made wide by 1/2 high; the distance between corresponding points on adjacent active dynode surfaces is rst said conductive plate whereby secondary electrons are produced which are attracted and amplified by the remaining dynodes in successive steps and means for collecting the number of electrons produced by the last -dynode 5. A magnetic electron multiplier for the detection of moving charged particles in a homogeneous magnetic eld which comprises an accelerating electrode located in said magnetic eld and parallel to the lines of flux of said field, a plurality of L-shaped beryllium copper dynodes, each of said dynodes having one surface located in said magnetic ield, said surfaces being disposed in echelon parallel relation with respect to said accelerating electrode, means for establishing a uniform electrostatic field between said dynode surfaces and said electrode, said electrostatic iield being perpendicular to said magnetic field, said accelerating electrode containing an aperture opposite the dynode most remote from said electrode whereby said moving charged particles enter said electrostatic eld through said aperture and impnge on said dynode surface most remote from said electrode thereby producing secondary electrons which are attracted and ampliiied by the remaining dynodes in successive steps and means for collecting the electrons produced by the last dynode.

6, The apparatus of claim 5 wherein the means for establishing a uniform electrostatic field comprises means for producing a direct current po tential difference between each of said dynodes and said accelerating electrode, the value or" said potential dierence being proportional to the distance between said dyncdes and said accelerating electrode.

LINCOLN G. SMITH.

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