US2429772A - Phototube multiplier - Google Patents

Description

A F. ROMAN PHOTOTUBE MULTIPLIER feet; 28, 1947.

Filed March 4, 1946.

- INVENTOR ALFRED F. ROMAN ATTORNEY Patented Oct. 28, 1947 PHOTOTUBE MULTIPLIER Alfred F. Roman, Fort Wayne, Ind., assignor to Farnsworth Television & Radio Corporation, a

corporation of Delaware Application March 4, 1M6, Serial No. 651,909

6 Claims.

This invention relates to electron multipliers and particularly to electron multipliers suitable for use in conjunction with phototubes.

Phototube multipliers have been used frequently in apparatus where it is desired to develop signals in response to received light impulses. A photosensitive element which is capable, in

response to light incident thereon, of producingelectrical signal efiects generally requires in conjunction therewith additional apparatus for magnifying the signal effects sufficiently for the desired use which is to be made thereof. A commonly employed device for magnifying these relatively weak electrical effects generated by a photosensitive device is an electron multiplier. Such a device is particularly useful for the reason that the electrical efiects produced by light falling upon a photosensitive device are electronic in character and, therefore, need not be converted into other forms of energy for amplification purposes.

There has been an increasing need for phototube multipliers which are capable of gathering light through a relatively wide angle. Consequently, it is desired wherever possible to mount the photosensitive element in an evacuated envelope as close to one wall of the envelope as possible. However, in view of the fact that the photoelectrical emission from the cathode or photosensitive element occurs on the surface which is located adjacent the tube envelope, it is difficult and at times impossible to employ conventional electron multiplier structures in conjunction with such apparatus, for the reason that these multiplier structures must be located with respect to the cathode in order to receive the photoelectrons in such a manner that they obstruct light from the cathode. The sensitivity of the device therefore is reduced.

There have been proposed for use in relatively wide angle phototube multipliers, various multiplier structural arrangements which will not materially obstruct light from the cathode. However, most of such prior art arrangements have required considerable space for mounting. Accordingly, where space limitations of the equipment in which phototube multiplier apparatus is to be incorporated are encountered, it is essential that the phototube multiplier be contained in an envelope of minimum size. As a consequence of such requirements, wide angle phototube multipliers heretofore have been limited as to the number of multiplier stages which could be employed. Hence, the magnitude of the signal eiiects derived from the multiplier have not been in all cases as great as desired.

It therefore is an object of the present invention to provide a, phototube multiplier which has a relatively wide light collecting angle and a re,- latively high amplification ratio, and at the same time over-all dimensions which are relatively small.

In accordance with a preferred embodiment of the invention, there is provided a photosensitive cathode located adjacent to one end of an evacuated envelope. There also is provided at least one box-type dynode located adjacent to the cathode but not in the path of light which itis desired to project onto the cathode. The boxtype dynode or the last one of a series of such dynodes, if such is employed, is located adjacent to a first or entrance opening of an annular dynode. In addition, there is provided at least one perforate dynode located adjacent to a second or exit opening of the annular dynode. Finally, there is provided adjacent to the other end of the envelope, an electron collecting electrode.

In a particular embodiment of a phototube multiplier in accordance with the instant inven tion, there may be provided a plurality of series of box-type dynodes. The first one of each series of box-type dynodes is located in position to receive photoelectrons from the cathode and the last one of each series of box-type dynodes is located around the periphery of the entrance opening of the annular dynode. Also, this form of the invention includes a plurality of perforate dynodes arranged in a series extending from the exit opening of the annular dynode to the vicinity of the collecting electrode.

For a better understanding of the invention together with other and further objects thereof,

reference is had to the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the accompanying drawing:

Fig. 1 is a side view in section of a phototube multiplier, embodying the invention in what at present is considered to be a preferred form;

Fig. 2 is a top view of the apparatus showing the relationship between the cathode and the first stage dynodes of two series of box-type dynodes; and

Fig. 3 is a sectional view taken on the line 33 of Fig. 1 showing the relationship between the last of the two series of box-type dynodes with the annular dynode.

Having reference now to the drawing, the phototube multipliera-pparatus is shown as contained Within an evacuated vessel having an envelope ll. Generally, the entire envelope will 3 be entirely of transparent material, such as glass. However, in any case, the upper end wall 12 of the envelope will be transparent. There is mounted within the envelope adjacent to the end wall l2, a photoelectric cathode l3. As shown herein, thiscathodeis generally cup-shaped having an annular lip or flange Hi extending substantially entirely around the periphery of the cathode. A screen mesh I5 is supported by and stretched across the upper edge of the flange M. Preferably, this screen is made of relatively ifine wire which is loosely eno gh woven to obstruct a minimum of light which .is ;to (be projected through the end wall I2 of the tube envelope onto the upper sensitized surface of the cathode I3. There are formed in the flange M a plurality of openings such as l6 and H. For the purpose of illustrating the underlying principles of the present invention, the illustrative embodiment thereof includes two such openings. However. it is considered tobe Within .the scope of the invention to providesimilar openings in the 'flange lii greater or lesser in number :accordingto the particular requirements of the apparatus.

There .is mounted adjacent the flange opening I6 a box-type dynode 18. The configuration of this dynode is such that electrons emitted from the sensitized surface of the cathode 13 under the influence of light are drawn easily into the dynode l;8. Furthermore, thisdynode is shaped 'toprov'ide eXit'for'the secondary electrons emitted thereby 'toward the' underside of the cathode. A series .of box-typedynodes such as l-9,:2'I and 22 may "be arranged somewhat conventionally as shown, to receive electrons "from the dynode i8 and to develop ,multiplied :nurribers of electrons for additional multiplication by other dynodes of the apparatus to be described. Similarly, there is mounted adjacent the other flange opening H, a first stage box-type dynode 2,3 with which 'is associated another series or box-type dynodes such as 24, 25 and 26. The two series of 'boxtype dynodes are substantially identical in form and number and, as shown .and described, are symmetrically arranged relative to the cathode 13.

Additionally, there is provided annular dynode .211 having an entrance opening disposed gen- .erallycn the nppersidethereotfand an exit opening disposed generally on the underside thereof. The entrance opening of the dynode "21 may be covered 'by a screen mesh '28 and is located generally-adiacent to the box-type dynodes 22 and '25 7 so .as to receive thesecondary electrons emitted thereby.

Under the annular dynode 21 "there ,is mounted in serial arrangement a plurality of perforate dynodes such as the screens 29 and 31. The "first screen-type dynode 29 of the series is "located adjacent to the .exit opening :of the annular .dynode 21 so .as to receive secondary electrons emitted by the .annular dynode. 'If desired, the

screen-type dynodes may-be of increasing diameter so as to have agreater surface area and .thereby 'be capable of handling "the increasing number of electrons developed as the number of stages is increased.

connection made thereto through the tube envelope H to a power source (not shown). The first stage dynodes l8 and 23 of the two series of box-type dynodes may be electrically connected together either inside of or outside of the tube andfurther connected to the power source at a more positive potentialithanthe cathode. In like manner, the box-type dynodes [9-24, 2I--25 .and 22-26 are connected to the power supply at increasingly positive potentials according to conyentional practice. Similarly, the annular dynode 21 and the series of screen-type dynodes, such as 2,9 and 3!, are connected to still more positive points of the power supply. The collector electrode '33 maybe connected through a suitable load impedance :device to the most positive point of the power supply. Inasmuch as the connections of all of the electrodes of the device are conventional and well known to those skilled in the art, they have'not been-shown herein since they form-no part of the instant invention.

In operation, the light which is incident upon the sensitized surface of the cathode [3 throughout a relatively wide anglejby reason of the relatively unobstructed character oif the cathode, effects the emission of photoelectrons from substantially all portions of 'the cathode surface. "In the present case, wherein two series of boxtype dynodes areemployed, the'electron emission from one-half of the cathode will be drawn into the dynode l8, while the emission irom the other half of thecathode will be drawn into thedynode 23. It iscontemplatedthat,where there is used a-ca'thcde surface of greaterextent than that disclosed herein, more than two series of box type dynodes-will be arranged at suitable points around the periphery of the cathode to utilize the entire electron emission from the cathode. The two electronic signal effectsthus produced in the re spective series of box-type dynodes-areseparately multiplied in-the succeeding stages of this portion ofthe multiplier. The output of all of the series of box-type dynodes are "drawn into the annular dynode21 for the production of more secondary electrons. The secondary electrons emitted by the annular dynode are successively multiplied by each of the screen-type dynodes such as 29 and 3'-! and the resultant electron stream is 'collected'by the electrode -33 for the development of an electrical signal effect in the load impedance device connected thereto in a well-"known manner (not shown). 7

In the present phototube multiplier structure embodying the invention, it is necessary to use for the first multiplier stage, a dynodeof such a character that it will be capable of receiving photoelectrons emitted by the cathode and a'tthe same time, be of such a character and/or configuration to obstruct substantially no light from the "cathode. The box typ'e dynodes such as 18 and 23 are well suited to sucha use. This type of electrode may be formed somewhat in the manner shown to receive the' photoelectrons from the cathode and to subsequently provide exit for the secondary electrons toward the underside and away from the cathode for further multiplication. It-is preferable that electrodes for use as the first stage dynodes in a device of the character described have solid secondary electron emissive surfaces in order to effect the development of a maximum number of secondary electrons. The photoelectrons emitted 'by the cathode are relatively few in number. It, therefore, is important to'make the maximum-possible'use of these electrons. Hence, dynode structures such as $8 and such stages.

23 are used to accomplish this result. Furthermore, in the interest of economizing materials and minimizing the cost of fabrication of devices of this nature, it is entirely adequate to provide a few relatively small box-type dynodes spaced around the periphery of the cathode at suitable points rather than to entirely surround the cathode with an annular dynode of the same general character as the dynode 21. For substantially the same reasons, the first 'few following multiplier stages may utilize conventional box-type dynodes.

However, while the box-type dynodes operate to efiect electron multiplication at relatively high efiiciency, they require so much space for mounting that to use them entirely throughout the device would make the over-all dimensions of the tube more than can be tolerated for certain uses of a device of this character. Consequently, after having effected a substantial electron multiplication by means of a relatively few box-type dynodes, some sacrifice in efiiciency may be made in order to meet the requirements of space limitations. Such a result may be obtained by the use of a number of relatively closely spaced screen-type dynodes such as 29 and 3|. This type of dynode does not have the high electron multiplication efficiency of the box-type dynodes by reason of the perforate character thereof which permits some electrons to pass through the meshes thereof without effecting the emission of secondary electrons. However, the lower efficiency of the screen-type dynodes may be compensated for by the use of a greater number of Inasmuch as such dynodes may be mounted more compactly in a tube envelope, it is possible to use more stages without exceeding the over-all dimensions'of the tube.

In order to make the maximum use of the secondary electrons emitted from the last of several series of box-type dynodes such as 22 and 26, in accordance with the present invention, there is provided the annular-type dynode 21. The configuration of this dynode is such that the entrance opening thereof is disposed to receive substantially all of the secondary electrons emitted by the box- type dynodes 22 and 26. In addition, the annular dynode is provided with an exit opening of such a character to direct substantially all of the secondary electrons emitted by the annular dynode towardthe first screen-type dynode 29 of a series of such dynodes arranged substantially as shown and described.

While there has been described what, at present, is considered'the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and, therefore, it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A phototube multiplier comprising an evacuated envelope, a cathode having a photosensitive surface located adjacent one end of said envelope, an electron multiplier including a boxtype dynode located in said envelope adjacent to said cathode for emitting secondary electrons generated under the control of photoelectrons received from said cathode, an annular dynode located adjacent to said electron multiplier and having a first opening for receiving secondary electrons from said electron multiplier, a perforate dynode located adjacent a second opening of said annular dynode for receiving secondary electrons emitted by said annular dynode, and a collector electrode located adjacent to the other end of said envelope for receiving secondary electrons generated under the control of the electrons received by said perforate electrode.

2. A phototube multiplier comprising, an evacuated envelope, a cathode located in one end of said envelope and having a photosensitive surface facing outwardly through one wall of said envelope, a plurality of electron multipliers having dynodes of a first type, the first dynode of each of said multipliers being mounted at spaced points around and adjacent to the periphery of said cathode, an annular dynode having entrance and exit openings and being mounted with said entrance opening adjacent to all of the last dynodes of said electron multipliers, electron multiplying means including a plurality of perfora'te dynodes mounted with the first one of said'perforate dynodes adjacent to the exit opening of said annular dynode, and an electron collector electrode mounted in-the other end of said envelope adjacent to the last one of said perforate dynodes.

3. A phototube multiplier comprising, an evacuated envelope, a cup-shaped cathode located in one end of said envelope and having a photosensitive surface facing outwardly through one end wall of said envelope, a plurality of series arrangements of box-type dynodes, the first dynodes of said series being mounted adjacent to the periphery of said cathode, an annular dynode having entrance and exit openings and being mounted with said entrance opening adjacent to the last dynodes ,of said series of box-type dynodes, electron multiplying means including a perforate dynode mounted adjacent to the exit opening of said annular dynode, and an electron collector electrode mounted in the other end of said envelope adjacent to said electron multiplying means.

4. A phototube multiplier comprising, an evacuated envelope, a cup-shaped cathode located in one end of said envelope and having a photosensitive surface facing outwardly through one end wall of said envelope, said cathode having an annular lip formed thereon provided with a plurality of spaced openings, a plurality of series arrangements of box-type dynodes, the first dynode of each of said series being mounted adjacent to respective openings of said cathode lip, an annular dynode having entrance and exit openings and being mounted with said entrance opening adjacent to the last dynodes of said series of box-type dynodes, a first screen-type dynode mounted adjacent to the exit opening of said annular dynode, a second screen-type dynode mounted for communication with said firstscreen-type dynode, and an electron collector electrode mounted in the other end of said envelope adjacent to said second screen-type dynode.

5. A phototube multiplier comprising, an evacuated envelope, a cup-shaped cathode located in one end of said envelope and having a photosensitive surface facing outwardly through one end wall of said envelope, said cathode having an annular lip formed thereon provided with two spaced openings, a first pair of box-type dynodes mounted adjacent to respective openings of said cathode lip, a second pair of box-type dynodes mounted for communication with respective ones of said first pair of box-type dynodes, an annular dynode having entrance and exit openings and being mounted with said entrance opening adjacent to said second pair of box-type dynode's, a series arrangement of a plurality of screen-type dynodes mounted with the first one of said series adjacent to the exit opening of said annular dynode, and an electron collector anode mounted in the other end of said envelope adjacent to the last one in said series of screen-type dynodes.

6, A phototube multiplier comprising, an evacuated envelope, a cup-shaped cathode located in one end of said envelope and having a photosensitive surface facing outwardly through one end wall of said envelope, said cathode having an annular lip formed thereon provided with two oppositely disposed openings, two series arrangements of a plurality of box-type dynodes, the first dynode of each of said series being mounted adjacent to respective openings of said cathode lip. an annular dynode having entrance and exit openings and being mounted with said entrance opening adjacent to the last dynodes of said series of box-type dynodes, a series arrange- ALFRED F. ROMAN.

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