GB1558662A - Electron multiplying arrangements - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO ELECTRON MULTIPLYING ARRANGEMENTS (71) E.M.I. LIMITED, Blyth Road, Hayes, Middlesex, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present inventon relates to electron multiplying arrangements and it relates especially to focusing electrodes used in such arrangements.

In United Kingdom Patent Specification No. 1470162 there is described a multichannel electron multiplier. The electron multiplying stages for the multiplier comprise a plurality of parallel plates slit to form parallel elements which are twisted out of the plane of the plates so as to be perpendicular to the plate. This arrangement of the multiplying stages, or dynodes, provides the multiplier with particular advantages over multipliers with continuous dynodes. Among these advantages are improvements in the linearity, transit time and efficiency of the multiplier.

In an example of an electron multiplier described in the aforementioned Patent Specification a focusing electrode is employed to focus the electrons, associated with particular signal input channels, into separate channels of the electron multiplier. An arrangement comprising such a focusing electrode is shown in Figure 1. A photocathode 1 receives light onto one surface and emits electrons from its other surface in the known fashion. These electrons are directed toward electron multiplying stages (dynodes) 2 via the action of the focusing electrode 3a.In general the focusing electrode will be at a potential of between 5 to 15V and the dynodes 2 at 1 ooV. In certain circumstances, when comparing light outputs from separate channels for example, it is necessary to ensure that electrons emitted from a particular part of the photocathode should remain in the channel corresponding to that part of the photocathode.

In general the focusing electrode 3a ensures this but, in practice, a small fraction of the electrons strike the electrode; many of these electrons produce secondary electrons and due to the divergent effect of the electrostatic field immediately in front of the focus electrode 3a,these secondary electrons may be directed into neighbouring channels. These misdirected secondary electrons create an error signal referred to as cross-talk. Tracks of electrons are shown as dotted lines 4, 5 and 6. Electron 4 is correctly directed but the secondary electrons emitted by electrons 5 and 6 are misdirected and so give rise to cross-talk.

It is an object of the present invention to provide a focussing electrode in an electron multiplier which substantially reduces the incidence of cross-talk.

According to the present invention there is provided an electron multiplier comprising an electron source, separate electron multiplying channels, each channel comprising a plurality of electron multiplying stages arranged successively, and each stage within a channel being constructionally separate from a neighbouring stage within that channel and a plurality of focussing means each disposed between the electron source and an adjacent pair of first electron multiplying stages at a potential intermediate that of the source and the said pair, and each focussing means comprising an electrode having a conductive surface facing the electron source and conductive side elements projecting towards the source thereby forming a channel having an open facing the source.

In order that the invention may be clearly understood and readily carried into effect an embodiment will now be described, by way of example only, with re ference to the drawings accompanying the Provincial Specification in which: Figure 1 has already been referred to, and Figure 2 is a part of an electron multiplier comprising focus electrodes accordto an aspect of the invention.

Referring now to figure 2 references 1, 2 and 4 are as for figure 1, a focusing means is now inserted in place of focus electrodes 3a. In this example the focusing means 3b is a focus electrode in the form of a U-section channel where the depth of the channel is approximately equal to its width.

The sides of the electrodes 3b are able to improve the cross-talk error in two distinct ways. Firstly they intercept many of the secondary electrons which would have otherwise entered incorrect channels of the photomultiplier and secondly they modify the electrostatic field between the cathode 1 and the first dynode 2 immediately in front of the focus electrode in such a manner that a convergent lens is formed. The potentials formed by the electrode 3b are shown by the continuous lines 8. This modification of the field means that most of those secondary electrons which do escape from the focus electrode are returned to the focus electrode or are directed into their appropriate channel, lelectron 7 being an example of the former kind.

The improvement in cross-talk can be further implemented by coating the focus electrode with materials for reducing secondary emission, such materials are well-known in the art and include for example soot and porous aluminium.

In a typical example of a channel photomultiplier in which the invention has been employed reductions in cross-talk from about 5% to 2% have been found.

WHAT WE CLAIM IS: 1. An electron multiplier comprising an electron source, separate electron multiplying channels, each channel comprising a plurality of electron multiplying stages arranged successively, and each stage within a channel being constructionally separate from a neighbouring stage within that channel, and a plurality of focussing means each disposed between the electron source and an adjacent pair of first electron multiplying stages at a potential intermediate that of the source and the said pair, and each focussing means comprising an electrode having a conductive surface facing the electron source and conductive side elements projecting towards the source thereby forming a channel having an opening facing the source.

2. An electron multiplier according to Claim 1 in which said channel is of substantially equal width and depth.

3. An electron multiplier according to any of the preceding claims in which the focussing electrode is coated to reduce secondary emission.

4. An electron multiplier including a focussing means substantially as herein described with reference to Figure 2 of the drawings accompanying the Provisional Specification.

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**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. ference to the drawings accompanying the Provincial Specification in which: Figure 1 has already been referred to, and Figure 2 is a part of an electron multiplier comprising focus electrodes accordto an aspect of the invention. Referring now to figure 2 references 1, 2 and 4 are as for figure 1, a focusing means is now inserted in place of focus electrodes 3a. In this example the focusing means 3b is a focus electrode in the form of a U-section channel where the depth of the channel is approximately equal to its width. The sides of the electrodes 3b are able to improve the cross-talk error in two distinct ways. Firstly they intercept many of the secondary electrons which would have otherwise entered incorrect channels of the photomultiplier and secondly they modify the electrostatic field between the cathode 1 and the first dynode 2 immediately in front of the focus electrode in such a manner that a convergent lens is formed. The potentials formed by the electrode 3b are shown by the continuous lines 8. This modification of the field means that most of those secondary electrons which do escape from the focus electrode are returned to the focus electrode or are directed into their appropriate channel, lelectron 7 being an example of the former kind. The improvement in cross-talk can be further implemented by coating the focus electrode with materials for reducing secondary emission, such materials are well-known in the art and include for example soot and porous aluminium. In a typical example of a channel photomultiplier in which the invention has been employed reductions in cross-talk from about 5% to 2% have been found. WHAT WE CLAIM IS:

1. An electron multiplier comprising an electron source, separate electron multiplying channels, each channel comprising a plurality of electron multiplying stages arranged successively, and each stage within a channel being constructionally separate from a neighbouring stage within that channel, and a plurality of focussing means each disposed between the electron source and an adjacent pair of first electron multiplying stages at a potential intermediate that of the source and the said pair, and each focussing means comprising an electrode having a conductive surface facing the electron source and conductive side elements projecting towards the source thereby forming a channel having an opening facing the source.

2. An electron multiplier according to Claim 1 in which said channel is of substantially equal width and depth.

3. An electron multiplier according to any of the preceding claims in which the focussing electrode is coated to reduce secondary emission.

4. An electron multiplier including a focussing means substantially as herein described with reference to Figure 2 of the drawings accompanying the Provisional Specification.

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