US2942133A - Electron multipliers - Google Patents

Description

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ATTORN 5 June 21, 1960 J. D. McGEE x-:LEc'rRoN MULTIPLIERS Filed June 3. 1954 FIG. 1

States atent w 2,942,133 ELECTRON MULTIPLIERS James Dwyer McGee, Ealing, London, England, assignor i to Electrical & Musical Industries Limited, Hayes, England, a British company y Filed June '3, 1954, Ser. No. 434,220 Claims priority, application Great Britain June 5, 1953 4 claims. (cl. 313-104) Thisinventionrelates to electron multipliers.

Y vElectron multipliers for multiplying a beam of electrous by causing the beam to impinge on one or a plurality of secondary electron emitting electrodes are wellknown. These multipliers can be operated to obtain a gain of between and 1010 by employing a suitable number of multiplication stages and whilst in the past various proposals have been made for multiplying an electron image, such attempts have not been satisfactory owing to the difficulty of obtaining a coherent image at the output of the multiplier. The necessity for maintaining the multiplied electron beam in yfocus throughout the various stages gives rise to such difliculties in maintaining a composite image at the output of the dev-ice that no satisfactory practical results in obtaining imagefmultiplication have yet been obtained.

",The object Aof the present invention is to provide an improved electron discharge device with a view to overcoming these diiculties.

' jA further object of the inventionis to provide an electro'n discharge device having means for generating an electron image, means for projecting said image on tov :1 -plurality of s'econdary'electron emitting electrodes such that electrons representing different image points can pass directly from-one electrode to the next in separate streams' defined by passages in said electrodes so that the definition of the image is preserved throughout the multiplication process.

v To obtain adequate definition in the final image derivedffrom the secondary electron emitting electrodes, thefcross-sectional area of the passages must be very small in relation to the cross-sectional areal of the image on entry intov the first secondary electron emitting electrode, since it Vwill be appreciated that the number' of image points in the final image is limited to the number ofpassagesvin each secondaryV electron emitting electrode.A

Therefore in order that this ratio may be decreased-when it is not practicable to decrease the cross-sectional 'area of the passages is is desirable for the cross-sectional.

a'r'e'a" ofthe electron image to be large and this can be achieved by producing an original electron image of large dimensions, for examle by deriving it from a photo-.electrically sensitive cathode of large dimensions and employinganI optical system for focussing a light image of correspondingly large dimensions on the photo-cathode. This proposal has however, the disadvantage that the mass ofthe optical lenses required tends to be excessive and therefore according to a preferred feature of the present invention means are provided for magnifying the electron image before said image is projected on the first secondary electron emitting electrode, the secondary electron emitting electrodes being of larger dimensions than the dimensions of the original electron image. Said means for magnifying' the electron image may be such as to produce.; alinear magnification by a factor of about 10 or more.

In order that the said invention may be clearly understood and readily carried into effect, the same will now be more fully described with reference to the accompanying drawings, in which:

Figure l illustrates diagrammatically and in sectional view one example of an electron `discharge device in accordance with the present invention, y

A2,942,133 Patented June 21,

Figure 2 is a detailed sectional view of part of Figure 1, the section being taken on the line lI-II of Figure 3, and v Figure 3 is a sectional Yview on the line III-lll of Figure 2.

Referring to the drawing, the electron discharge device comprises an evacuated glass envelope 1 having a hat end window 2, on the inner surface of which is deposited a photo-electrically sensitive cathode 3. The device is associated Iwith a lens system represented by the reference 4 for focussing light images on the cathode 3. The envelope has a minimum diameter at theend wall 2 and its diameter increases towards the other end of the envelope and has a maximum in a region 4 in which is mounted a series of secondaryelectron emitting electrodes, or so-called-dynodes. In this example of the invention, the dynodes are twelve in number and are denoted by 'the references 51, 52 .y 513. The construction of these dynodes will be described in more detail later. The dynodes are generally parallel to the photocathode 3 and the effective area of the photo-cathode may be, for example, about 2 inches square, whereas the effective area of each of the dynodes may be 10 inches square.l An accelerating electrode 6 which may be in the form of .a conductive coating on the inner surface of the envelope I1, is provided for accelerating electrons released from the photo-cathode 2 towards the first dynode 51, and an electro-magnet represented by the reference 7 is provided for focussing the electrons from the cathode 3 on to the first dynode in the form of a coherent electron image. the electrode 6 and the electro-magnet 7 is, moreover,

such as to produce a magniiication of the electron image focussed on-the first dynode 5, by a factor equal to the ratio of the effective dimension of the dynode 5 to that of the photo-cathode 3. The magniiication factor in the present example is of the order of 5. As will hereinafter appear, the electron image focussed on the first dynode Stundergoes successive stages ofmultiplication in the series of dynodes 51 512 and the electronsfrom the last dynode 512 are projected directly on to aliluorescent screen denoted by the reference 8, the fluorescent screen being deposited on a conductive but substantially transparent layer provided on, or constituting, a transparent support forthe fluorescent screen. Leads from the various electrodes project to the exterior of the envelope 1 and for operation of the device the photo-cathode 3 may, lfor example, be grounded and a potential of S00 volts applied to the accelerating electrode 6. Moreover, successively higher potentials in the range om 500 to 1,500 volts are applied to the successive dynodes '51 512, means forl applying such potentials being represented in the drawing as a potentiometer l0 connected across a suitable potential source. Moreover, a potential in the range from 5,000 to 10,000 volts is applied'to the conductivev support for the fluorescent screen 8.

In Figures 2 and 3 the construction of two of the dynodes is illustrated and' it will be assumed that .these- -dynodes are the dynodes 51 and 52. Each dynode is of the so-called Venetian blind type 'and comprises an arraythe suppression of secondary electrons-liberated from theV respective dynode by penetration of the relatively negative electrical field from the previous dynode orv elet.:-v

The electron optical system formed by trode. The mesh 12 is carried on vertical extensions 13 which project forwardly from the inclined portions of the slats 11. This has the effect of allowing better penetration of the electrical tield from each dynode (other than the rst) into the preceding dynode so as to facilitate the extraction of secondary electrons liberated at the front edge of the preceding dynode. Each dynode comprises a further array of parallel ribs also in the form of slats perpendicular to the array of slats 11. A few of these ribs are represented bythe reference 14 and they serve as stiffening members to provide accurate spacing of the slats 11 and in conjunction with the slats they partition each dynode into a two dimensional array of box-like passages which Vprevent secondary electrons spreading along the direction of the slats '11. The ribs 14 and the slats 11 produce focussing elds which steer the secondary electrons representing different image points directly from one dynode to the next in separate streams, in `such a way that the electrons representing each image point .when they emerge from the final dynode 512 are discrete and not associated with electrons which have been derived from other image points. To facilitate the isolation of the streams the dynodes are mounted as close to one another as possible and the individual passages formed by the intersecting slats and ribs inthe successive dynodes are aligned as accurately as possible as represented for the case of the dynodes 51 and 52 in Figures Zand 3 so that the exits from the passages in one dynode register with the entrances to the corresponding passages in the next dynode. The ribs 14 do not extend throughout the Whole depth of the slats 11, to prevent excessive screening of the extracting electrical field.

Furthermore, the cross-section of the passages formed Y by the slats and ribs are arranged to be very small so as to provide for a very large number of image points and thereby ensure high denition. For example, with dynodes having the overall dimensions above indicated there may be one hundred slats and one hundred ribs, providing 104 elemental electron multipliers in each dynode and thus 104 picture points, corresponding to an image having one hundred lines. The cross-section of each elemental electron multiplier is then 0.1 x 0.1. The uorescent screen 8 is mounted close to the last dynode as shown and produces a bright display of the original optical image focussed on the photo-cathode 3 with a definition corresponding to the number of elemental electron multipliers. The above dimensions are given by way of example and it will be understood that the number of slats and ribs may be greater, and the dynodes may be of larger overall size.

Instead of projecting the final electron image from the dynode 512 on a uorescent screen, it may be projected on a target electrode of the charge storage type such as` u sed, for example, in conventional television pick-up tubes. Picture signals representative of the original light image can then be derived from this target by scanning it with a beam of electrons, preferably of low velocity. Alternatively, where a uorescent screen such as 8 is provided, picture signals may be derived from it by utilising a conventional television pick-up tube to pick-up the light image directly from the uorescent screen. The device may also nd application in colour television to give successive pictures in red, blue and green which are directed by shutters sequentially into three pick-up tubes from which picture signals are derived by conventionalV scanning.

The discharge device in accordance with the invention can be utilised not only as part of or in conjunction with a television pick-up tube, but it can also be used for direct visual observation, for example in astronomy or as an aid to vision at night or in conditions of poor visibility. The device can also be used in photography, for example for making time exposures of images such as faint spectra'. Furthermore, if the photo-cathode 3 is formed inclose juxtaposition to an X-ray sensitive uorescent screen, the

device could be employed for X-ray image intensification. Alternatively, the device in the form shown could be. used for direct observation of an X-ray fluorescent screen as now used. On the other "hand, .the device can be used as an ultra-violet light microscope byV makingv the end -wall 2 in the form of a quartz window.

What I claim is: j

l. An electron discharge device comprising means for generating an electron image, a plurality of secondary electron emitting electrodes, means for projecting said image along a path onto `the first of said' electrodes, each said electrode comprising a first array of parallel slats, said array being parallel to said image, and saidslats being inclined to said path of said image, and a second array of parallel ribs, transverse to said irst array with the ribs intersecting the slats, so that the slats and ribs of said rst and second arrays dene a two dimensional array of passages oblique to said path, in .which saidielectrodes are arranged adjacent one another and are disposed so that the exit of each passage in each electrode is in alignment with the entrance of a single passage in the next electrode, so that electrons representing different image points can pass directly from one electrode to the next in separate streams defined by said passages, and a target arranged for exposure to the individual electron streams from the last secondary electron emitting electrode.

2. An electron discharge device comprising means for generating an electronimage, a plurality of secondary electron emitting electrodes, `means for projecting said image along a path onto the first of said electrodes, each said electrode comprising a first array of parallel slats, said array being parallel to said image, and said slats being inclined to said path of said image, and a second array of parallel ribs transverse to said first array with the ribs intersecting the slats, so that the slats and ribs of said rst and second arrays dene a two dimensional array of passages oblique to said path, and a mesh extended in `front of each electrode to prevent the suppression of secondary electrons liberated from the respective electrode, in which said electrodes are arranged adjacent one another and are disposed so that the exit of` each passage in each electrode is in alignment with the entrance of a single passage in the next electrode, so that electrons representing different image points can Vpass directly from one electrode to the next in separate streams defined by said passages, and a target arranged for exposure to the individual electron streams from the last secondary electron emitting electrode. A

3. An electron discharge device according to claim l in which said means for projecting said electron image along a path onto the tirst of said secondary electron emitting electrodes comprises means for magnifying said electron image.

4. An electron discharge device according to claim.2 in which said means for projecting said electron image along a path onto the first of said secondary electron emitting electrodes comprises means for magnifying said electron image.

References Cited in the tile of this patent UNITED STATES PATENTS 2,237,679 Lubszynski et al. Apr. 8, 1941 2,254,617 McGee Sept. 2, 1941 2,443,547 -Weimer June 15, 1948 2,612,610 Marshall et al Sept. 30, 1952 2,645,734 Rajchman July 14, 1953 2,727,182 Francken Dec. 13, 1955 v2,821,637 Roberts et al Ian. 28, 1958 2,836,755 Sommer May 27,1958

FOREIGN PATENTS 883,415 France Mar. 22,'1943

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