US2338885A - Electron multiplier - Google Patents

Description

Jan. 11, 1944. :F. .1. s. VAN DEN BOSCH 5 ELECTRON MUL'T IBLI ER "Filed June 16, 1941 Patented Jan. 11, 1944 ELECTRON MULTIPLIER Francois Joseph Gerard van den Bosch, East Groydon, England, assignor to Vacuum-Science Products Limited, London, England, a British company Application June 16, 1941, Serial No. 398,341 In Great Britain February 23, 1940 3 Claims.

This invention relates to multiplier apparatus and is concerned with such apparatus of the kind comprising an electron multiplier having a series of electrodes comprising a plurality of secondary cathodes and a collector electrode, and means for applying to this series of electrodes progres sively increasing positive potentials. The aforesaid series of electrodes are enclosed in an enve lope together with a primary cathode, which may be thermionic or photo-electric, and usually a control grid and an accelerator electrode for the primary emission. Electrons emitted by the primary cathode are caused to impinge on the first of the series of secondary cathodes and liberate other electrons which impinge on the next secondary cathode, and so on, in succession to the collector.

Usually, the series of electrodes comprising the secondary cathodes and collector are uniformly separated one electrode from another and the potentials applied to these electrodes are such that the potential difference between each secondary cathode and the next following electrode is of uniform value, for instance 300 volts.

In general it is of advantage to have adequate separation of the secondary cathodes so as to permit requisite acceleration of the electrons from one secondary cathode to the next following electrode. It is found, however, that there is in some cases a falling oiT in the response of the electron multiplication as regards the later multiplier stages, and an object of the invention is to improve the operation of the multiplier in this respect.

This invention comprises an electron multiplier apparatus of the kind specified wherein the potentials applied to at least some of the said series of electrodes and/or the spacings of these electrodes are so proportioned that the rate of increase of the field strength along the path of electrons is progressively increased.

The invention further comprises an electron multiplier apparatus of the kind specified wherein the electric field between alternate electrodes of the series has a zone located in front of the intermediate electrode corresponding to the potential applied to the intermediate electrode.

The spacing of adjacent electrodes of the series decreases towards the collector electrode. Furthermore, the electron multiplier may be constructed with the spacing of adjacent electrodes of the series decreasing towards the collector electrode and operated with the potential difference of adjacent electrodes of the series increasing towards the collector electrode.

It has been found that a falling off in the response of an electron multiplier when operating with high frequencies can be attributed to the electron transit time through the multiplier. The transit time between successive secondary cathodes is greatest for the later stages of the multiplier, due to the density of electrons passing between cathodes at the end of the series of electrodes being greater than that of electrons passing between cathodes at the beginning of the series. The increasing rate of the rise of applied potentials and/or the relatively close spacing of the secondary cathodes, particularly at the output end of the series, considerably improves the response of the multiplier without adversely affecting the multiplication per stage. Also, secondary electrons emitted by a secondary cathode are influenced by the strong field obtained by the increased potential applied to the following electrode and/ or by the relatively close proximity of the following electrode so that these second ary electrons are more completely accelerated towards this following electrode. The multiplication obtained for the later stages of the electron multiplier is thus materially improved. The transit time between secondary cathodes at the beginning of the series is not of great importance and wide spacing of these secondary cathodes has the advantage that it permits adequate accelerations of the electrons to be obtained.

The invention also comprises an electron multiplier apparatus as aforesaid wherein at least some of the secondary cathodes consist of grid, perforated plate or like apertured members and wherein the number of apertures in the secondary cathodes increases for successive secondary cathodes towards the collector electrode.

Specific embodiments of the invention are illustrated by way of examples in the accompanying drawing, in which:

Figures 1, 2 and 3 are diagrams showing different forms of multiplier apparatus according to the invention.

In Figure 1 there is shown an electron multiplier ii! comprising a thermionic primary cathode ii together with a control grid i2 and an accelerator 13 for the primary electrons. In addition, there is a plurality of secondary cathodes id each in the form of a grid, perforated plate or like apertured member, a final secondary cathode it in the form of an imperforate plate and a grid-like collector it. Positive potentials with respect to the cathode H are applied to the accelerator, the secondary cathodes and the collector from appropriate tappings on a potential divider ll, across which an electric supply indicated at it! is connected. A signal input device [9 is connected to the control grid 52 and the cathode i l and an output device it is connected the collector electrode Hi. The spacing between successive electrodes of the series consisting of the secondary cathodes and collector electrode progressively decreases. Thus, the spacing of the first two secondary cathodes id is greater than that between the second and third secondary cathodes it. Similarly, the spacing between the second and third secondary cathodes it is greater than that between the third and fourth secondary cathodes l4, and so on, to the collector I5. Uniformly increasing potentials are applied to the series of electrodes comprising the secondary cathodes and collector electrode from the potential divider ll, for example a potential diiference of 300 volts.

In the apparatus shown in Figure 2 the secondary cathodes it and it are equally spaced and the potentials applied to these secondary cathodes from the potential divider H instead of being of uniformly increasing value the rate of increase of these potentials also increases. the potential difference of the first two secondary cathodes H is less than the potential difference across the second and third secondary cathodes and this latter potential difference is again less than the potential difference across the third and fourth secondary cathodes, and so on. For example potentials applied to the secondary cathodes and collector may progressively increase by 390 volts plus an additional per cent of the total voltage value thus obtained, although good results have been obtained by employing a potential difference between successive elec trodes which rises by 50 volts for each stage.

In the arrangement shown in Figure 3 the secondary cathodes and collector are difierently spaced in the manner shown in Figure 1 and have an increasing potential difference between successive secondary cathodes according to the arrangement shown in Figure 2. By arranging alternate electrodes of the series of secondary electrodes and collector in any of the ways described with reference to the drawing there is obtained a potential Zone of the same value as the potential applied to the intermediate secondary cathode which lies to the front of this secondary cathode. In this way secondary electrons emittted by this intermediate electrode are subjected to a strong field of the following electrode penetrating the apertures of the electrode from which the secondary electrons are emitted and serving to accelerate these electrons in greaternumber towards the following electrode than is the case with the normal arrangement of equally spaced secondary cathodes operated with a uniform potential gradient.

The invention employs apertured secondary cathodes It in which the total number of apertures increases for the successive cathodes in the direction towards the final collector. In a construction in which the secondary cathodes consist of wire grids or meshes the grid or mesh may be finer for the cathodes at the end of the series than for the cathodes at the beginning of the series; for instance, the grid or mesh of the cathodes may be progressively finer throughout the series. In the case of secondary cathodes formed of perforated plates the number of perforations may be increased, for instance, progressively for the several secondary cathodes. In the case where the secondary cathodes consist of perforated plates with apertures tapering in cross-section, as described in the specification of my United States Patent No, 2,254,128, these apertures may be formed of less depth for secondary cathodes at the end of the series than for secondary cathodes at the beginning of the series, so as to enable close spacing of the secondary cathodes at the end of the series. Auxiliary electrodes may be provided for each of the secondary cathodes and in front thereof, as described in the specification of my Unitedstates Patent No. 2,285,848.

I claim:

1. An electron multiplier apparatus comprising an electron discharge tube having a primary cathode and a series of electrodes consisting of a plurality of secondary cathodes and a collector in said tube, at least some of said secondary cathodes having multiple apertures for the passage of electrons and the number of apertures in the secondary cathodes increasing for successive secondary cathodes towards the collector, and means for applying to at least some of said series of electrodes positive potentials with respect to the primary cathode of progressively increasing value so that the potential differences between succeeding pairs of electrodes increase, the spacing of said series of electrodes decreasing towards the collector and the rate of increase of the field strength along the path of electrons between successive electrodes being progressively increased.

2. An electron multiplier apparatus comprising an electron discharge tube having a primary cathode and a series of electrodes consisting of a plurality of secondary cathodes and a collector in said tube, at least some of said secondary cathodes having multiple apertures for the passage of electrons and the number of apertures in the secondary cathodes increasing for successive secondary cathodes towards the collector and the spacing of at least some of said series of electrodes decreasing towards the collector, and means for applying to said series of electrodes substantially uniformly increasing positive potentials with respect to the primary cathode.

3. An electron multiplier apparatus comprising an electron discharge tube having a primary cathode and a substantially uniformly spaced series of electrodes consisting of a plurality of secondary cathodes and a collector in sai tube, at least some of said secondary cathodes having multiple apertures for the passage of electrons and the number of apertures in the secondary cathodes increasing for successive secondary cathodes towards the collector, and means for applying to said series of electrodes, positive potentials which are of progressively increasing value towards the collector and which provide potential differences which increase between successive pairs of electrodes towards the collector, the spacing of at least some of said series of electrodes decreasing towards the collector.

FRANCOIS JOSEPH GERARD VAN DEN BOSCH.

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