US2245895A - Electron multiplier - Google Patents

Description

Filed Feb. 4, 1939 II J INVENTOR' Fe/Tz BELOW BY M 2 TTORNEYS Patented June 17, 1941 A lTE A dr orrrce L 1523453952 i v j ELECTRON MULTIPLIER v FritzBelow, Klein-Machn'ow, near Berlin, Germany, assignor to Femseh Aktiengesellschaft, Berlin-Zehlendorf, Germany 2 Claims.

This invention relates to electron multipliers using a plurality of electron-permeable secondary-emitting electrodes and in which modulation of the multiplied electron stream is to take place.

It is known to amplify an electron stream generated by an electron source by causing the electron stream to impact an electron-permeable electrode having a surface of a secondary-emission factor greater than unity, and causing this process to be repeated at a number of successive electron-permeable secondary-emitting electrodes and finally collecting these electrons. It is also known to apply a control voltage to one or several of the secondary-emitting electrodes in order to modulate the electron stream. In general when a stream of electrons approaches an electron-permeable electrode from one side, it is clear that a certain portion of the arriving electrons will impact the electrode and liberate secondaries, while another portion of the arriving stream will pass through the openings in the electron-permeable electrode without impacting and without producing secondary electrons. Upon arrival of a successive stage the electron stream will thus consist of a portion of secondary electrons and a portion of primary electrons, wherein the primary electrons will have a higher velocity than the secondary electrons because the velocity is proportional to the potential difference between the electrode from which the electrons were emitted and the electrode which they are about to impact. It may be seen, therefore, that in a multiplier having a plurality of electron-permeable multiplying stages the velocity distribution of the electron beam will increase from stage to stage. Consequently, if the electron stream is to be modulated, a considerable amount of control voltage is necessary to out off the electron stream because of this wide range of velocity distribution.

It is the object of my invention to overcome this disadvantage, Broadly considered, my invention provides for reducing the velocity distribution of the electron stream to that normal for secondary emission at a single secondary-emitting electrode before modulating the electron stream, thereby greatly reducing the control voltage necessary to cut off the elec ron stream.

The invention shall now be described in detail in connection with the drawing, which shows an embodiment of the invention.

In the drawing, the vacuum receptacle houses an electron source 2 which is shown to be a filamentary cathode, but which, however, may

be any suitable electron source. The grid-like electron-permeable electrodes 3 are sensitized to a secondary-emission ratio greater than unity. Next in succession are a solid secondary-emitting electrode 6, a control grid, which may or may not be sensitized to secondary emission at a ratio greater than unity, two more grid-like secondaryemitting electrodes 5 and 5a, and a solid plate I for electron collection. The filament-current source is indicated at I3. The cathode 2, secondary-emitting electrodes 3 and 6, control grid 4, secondary-emitting electrodes 5 and 5a and collector are held at increasingly positive potentials derived from voltage divider H across voltage source IZ. An output resistor 8 is in the output lead of collector l. Amplified signals can be taken across resistor 8 through coupling condenser 9. Control signals are applied to control grid 4 through condenser I0.

In operation, electrons are emitted from source 2 and are directed toward the first of the secondary-emitting electrodes 3, As stated above, a portion of the arriving electrons will impact this electrode and liberate secondaries, while another portion will pass through the openings. Upon arrival at the second of the multiplying electrodes 3, primary electrons from source 2 will have a greater velocity than secondaries emitted at the first multiplier stage. Again a portion of the arriving electrons will impact the electrode and liberate secondaries, while another portion will pass through the openings of the electrode without impacting the same and without liberating secondaries, This increases the velocity distribution and it may be seen that this process is repeated at each multiplying stage. The electron stream emanating from the last of the multiplying electrodes 3 is guided upon solid secondary-emitting electrode 6. All electrons in the stream will impact this electrode and thereby produce a stream of secondary electrons in which the velocity distribution is that normally obtained and may lie, say, between zero and 5 volts. This multiplied stream is then guided toward control electrode 4 by virtue of its increased positive potential. It may be seen that a voltage swing of the order of about 5 volts is suflicient to fully modulate the electron stream emitted from electrode 6. The modulated electron stream can then be subjected to further multiplication as is shown, by way of example, at electron-permeable electrodes 5 and 5a., and finally collected at collector 1. It may, however, be understood that no multiplication need take place after control grid 4 is passed by the electrons, or that any other type of electron multiplication can be used.

While I have shown my invention in connection with a specific type of electron multiplier, I

am well aware that it is applicable to all cases in which it is desired to modulate an electron stream by means of a control voltage.

What I claim is:

1. An electron multiplier including an electron source, a plurality of successive grid-like secondary-emitting electrodes in alignment therewith, a solid imperforate secondary-emissive member positioned at an oblique angle to said electrodes, a second electrode system comprising a control grid, a plurality of secondary-emitting electrodes

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