EP0230694B1 - Multiplying element with high collection efficiency, multiplying device equipped with such an element and application to a photomultiplier - Google Patents

Description

The present invention relates to an electron multiplier element with secondary emission composed, on the one hand, of a first metal plate pierced with at least one hole, called multiplier hole, having an inlet opening and an outlet opening, and whose wall is endowed with emissive power, and, on the other hand, a second metal plate, parallel to the first, pierced with at least one hole, called auxiliary hole, disposed opposite the outlet opening of the hole multiplier, the second plate, electrically isolated from the first, being brought to an electrical potential greater than the electrical potential of said first plate.

The invention also relates to an electron multiplier device comprising N multiplier elements in accordance with the preamble and an application of said electron multiplier device to a photomultiplier tube.

The invention finds a particularly advantageous application in the field of photomultiplier tubes.

A multiplier element of the type described in the preamble is known from French patent application No. 2,549,288 (see also EP-A-0 131 339). This patent application shows a multiplier element whose multiplier holes are either symmetrical, that is to say that the inlet and outlet openings are coaxial, or dissymmetrical, that is to say that the inlet and outlet openings outlet are offset from each other, the outlet opening however remains opposite the inlet opening. This structure of electron multiplier element has the disadvantage of offering a collection efficiency limited by the fact that many incident electrons can pass through the multiplier element without undergoing multiplication on the wall of the multiplier holes, passing directly to through the inlet and outlet openings. On the other hand, this loss of collection efficiency is reproduced on each stage of a multiplier device comprising N multiplier elements of the known type, and therefore results in a loss of gain, a lack of linearity and an extension of the time of response, for example when said multiplier device is incorporated into a photomultiplier tube.

The object of the invention is to remedy these drawbacks by proposing an electron multiplier element having an increased collection efficiency.

Indeed, according to the present invention, an electron multiplier element with secondary emission composed, on the one hand, of a first metal plate pierced with at least one hole, called multiplier hole, having an inlet opening and a outlet opening, and the wall of which is emissive, and, on the other hand, a second metal plate, parallel to the first, pierced with at least one hole, called an auxiliary hole, disposed opposite the opening outlet of the multiplier hole, the second plate, electrically isolated from the first, being brought to an electrical potential greater than the electrical potential of said first plate, is particularly remarkable in that the straight projection of the outlet opening of the multiplier hole on a plane parallel to the first metal plate is at least partially located outside the corresponding projection of the inlet opening.

Thus, most of the incident electrons arriving on the electron multiplier element according to the invention, with the exception of those, few in number, occurring under too great an incidence, meet the wall of the multiplier hole and therefore undergo a multiplication. Tests carried out by the Applicant on multiplier holes with totally offset openings have shown that the collection efficiency of such a multiplier element is very significantly increased, and this although one might think that, given the relatively small dimensions large of the multiplier hole, multiplied electrons could return to the wall of said hole and would be lost. This experimental fact gives credence to the idea of a possibility of rebound without loss of electrons on the wall of the multiplier hole. r.

In a very general embodiment of the multiplier element according to the invention, said first metal plate is pierced with a plurality of multiplier holes arranged in a regular plane network. This regular planar network can be square or hexagonal, just as the inlet and outlet openings are circular, square or hexagonal.

The multiplier element according to the invention can be advantageously used to make an electron multiplier device comprising N multiplier elements according to the invention, remarkable in that the second metal plate of the ith multiplier element is brought to an electrical potential identical to the potential electric of the first metal plate of (i + I) th multiplier element.

This ensures a better collection of electrons between a multiplier element and the next, when these are relatively far apart. In addition to improved collection efficiency, the electron multiplier device according to the invention also offers the possibility of image formation. Two types of geometry can be envisaged, one in which said N multiplying elements are in parallel configuration with respect to each other. In another more advantageous geometry, said N multiplier elements are in head-to-tail configuration with respect to each other, which allows, with each multiplication, to fold the electron beam back on itself.

The multiplier device according to the invention advantageously applies to a photomultiplier tube comprising a photocathode and n adjacent anodes. In this application, it is provided, according to the invention, that said multiplier device is placed near the photocathode and is divided into n secondary multiplier devices by electron-tight partitions, located substantially opposite the separation zones of two contiguous anodes so as to produce n secondary photomultiplier tubes in the same photomultiplier tube.

Finally, a method of producing a first metal plate of an electron multiplier element according to the invention is mainly remarkable in that the two faces of the same metal plate are simultaneously attacked, each using sets of masks, the successive windows of which increase in size and are offset with respect to each other, the windows of the last set of masks reproducing respectively the shape of the entry opening and the exit opening of the multiplier hole.

• Les figures la a et 1 b montrent, en coupe et en vue de dessus, un premier mode de réalisation d'un élément multiplicateur d'électrons selon l'invention.
• Les figures 2a et 2b montrent, en coupe et en vue de dessus, un deuxième mode de réalisation d'un élément multiplicateur d'électrons selon l'invention.
• Les figures 3a et 3b montrent, en coupe et en vue de dessus, un troisième mode de réalisation d'un élément multiplicateur d'électrons selon l'invention.
• La figure 4 est une vue en coupe d'un premier dispositif multiplicateur d'électrons selon l'invention.
• La figure 5 est une vue en coupe d'un deuxième dispositif multiplicateur d'électrons selon l'invention.
• La figure 6 montre en coupe un tube photomultiplicateur comportant un dispositif multiplicateur analogue à celui de la figure 5.
• La figure 7 illustre par une vue en coupe un procédé de réalisation d'une première plaque d'un élément multiplicateur conforme à l'invention.

The description which follows with reference to the appended drawings, given by way of nonlimiting examples, will make it clear what the invention consists of and how it can be implemented.

• Figures la and 1b show, in section and in top view, a first embodiment of an electron multiplier element according to the invention.
• Figures 2a and 2b show, in section and in top view, a second embodiment of an electron multiplier element according to the invention.
• Figures 3a and 3b show, in section and in top view, a third embodiment of an electron multiplier element according to the invention.
• Figure 4 is a sectional view of a first electron multiplier device according to the invention.
• Figure 5 is a sectional view of a second electron multiplier device according to the invention.
• FIG. 6 shows in section a photomultiplier tube comprising a multiplier device similar to that of FIG. 5.
• FIG. 7 illustrates by a sectional view a process for producing a first plate of a multiplier element according to the invention.

Figures 1, 2 and 3 show in section (Figures 1a, 2a, 3a) and in top view (Figures 1b, 2b, 3b), an electron multiplier element 10 with secondary emission, composed of a part, of a first metal plate 11 pierced with holes 12, called multiplier holes, having an opening 13 of entry and an opening 14 of exit. The inner wall 15 of the multiplier holes 12 is endowed with emissive power. For this, the first metal plate 11 is made of a material capable of secondary emission such as the copper-beryllium alloy, after heating, migration of the beryllium and oxidation. It can also be made of an inexpensive material, such as mild steel, covered with a secondary emission material: layer of oxidized copper-beryllium alloy or layer of manganese oxide.

On the other hand, the multiplier element 10 is composed of a second metal plate "16, parallel to the first, pierced with holes 17, called auxiliary holes, arranged opposite the openings 14 for the exit of the multiplier holes 12. This second metal plate 16 is electrically isolated from the first plate 11, the electrical insulation of the two plates 11 and 16 being able to be carried out, for example, using small glass beads 30, 100 to 200 μm in diameter, sealed at the periphery of said plates The second metal plate 16 is brought to an electrical potential V1 greater than the electrical potential VO of said first plate 11, the second plate 16 thus playing the role of accelerating electrode.

As can be seen in all of Figures 1 a, 1 b, 2a, 2b and 3a, 3b, the right projection 18 of the opening 14 out of the hole MULTIPLICA _- tor 12 on a plane P parallel to the first metal plate 11 is known at least partially, here entirely, located outside the corresponding projection 19 of the inlet opening 13. This configuration offers the incident electrons 50 whose angle of incidence is not too large a maximum surface for capture by the multiplier wall 15. In other words, most of the electrons entering the multiplier hole 12 through the inlet opening 13 will not be able to exit directly from the outlet opening 14, but will give rise to secondary emission thereby contributing to a significant increase in the collection efficiency of the multiplier 10, as the Applicant has observed experimentally. This suggests that electrons which strike the multiplier wall 15 relatively far from the outlet opening 14 and which do not exit directly after multiplication can bounce without loss on the wall before exiting through said outlet opening.

As can be seen in Figures 1b, 2b and 3b, said first metal plate 11 is pierced with a plurality of multiplier holes 12 arranged in a regular plane network. According to FIG. 1 b, said regular planar network is a hexagonal network and said inlet and outlet openings (13, 14) are circular. FIGS. 2b and 3b give two configurations which make it possible to increase the useful multiplication surface of the first plate 11. According to FIG. 2b, the regular plane network of multiplier holes (12) is a hexagonal network, said inlet openings 13 are hexagonal and the circular outlet openings 14 while with reference to FIG. 3b, the regular plane array of multiplier holes 12 is a square array, the inlet openings 13 are square and the outlet openings 14 circular.

Figures 4 and 5 show in section two electron multiplier devices comprising N (here N = 3) multiplier elements of the type described above in Figures 1, 2 and 3. The electrical potentials applied to each of the first 11 and second 16 plates of each multiplier element are such that the second metal plate 16 of the ith multiplier element is brought to an electrical potential Vµ identical to the electric potential VO _{(i + 1)} of the (i + 1) th element N8um therefore we have the equalities:

The multiplier device of FIG. 4 is such that the multiplier elements 10 are in parallel configuration with respect to each other. This configuration, if it maintains a one-to-one correspondence between the electrons leaving the (i + 2) th multiplier element and the electrons entering the th multiplier element, however, leads to a spatial offset between the incoming electrons and the electrons leaving the multiplier device. The device shown in FIG. 5 makes it possible to avoid this offset, in the sense that the multiplier elements 10 are in head-to-tail configuration consecutively with respect to each other.

The electron multiplier device according to the invention finds a particularly advantageous application in the field of photomultiplier tubes, in particular so-called proximity focusing tubes. FIG. 6 shows, in a sectional view, an example of such an application to a photomultiplier tube comprising a photocathode 20 and n (here n = 2) adjacent anodes 21. According to FIG. 6, said multiplier device 22 is placed near the photocathode 20 and is divided into n secondary multiplier devices 23 by electron-tight partitions 24, situated substantially opposite the separation zones of two contiguous anodes 21 so as to produce n secondary photomultiplier tubes in the same photomultiplier tube. Tubes of the type shown in FIG. 6 can advantageously be used in nuclear physics for the precise localization of the elementary particles detected. The watertight partitions 24 are produced in a clasical manner by masking and photoengraving of a metal plate.

FIG. 7 illustrates a process for producing a first metal plate 11 of an electron multiplier element of the kind described above. According to this method, the two faces of the same metal plate 11 are simultaneously attacked, by photoengraving, using sets of masks, 31/41, 32/42 and 33/43, the successive windows of which increase by size and are offset with respect to each other, the windows of the last set of masks 33/43 reproducing respectively the shape of the opening 13 of entry and the opening 14 of exit of the multiplier hole 12. The Applicant has produced , by this method, a metal plate with multiplier holes whose thickness was 0.15 mm, for respective dimensions d1, d2 of openings of 0.6 mm and 0.3 mm.

Claims (9)

1. An electron multiplier element (10) for secondary emission, consisting of a first metal plate (11) which has at least one hole (12), referred to as multiplier hole, having one input aperture (13) and one output aperture (14) and whose wall (15) has emissive power, and of a second metal plate (16) in parallel with the first plate (11), which has at least one hole (17), referred to as auxiliary hole, and is disposed opposite the output aperture (14) of the multiplier hole (12), the second plate (16), which is electrically insulated from the first (11), being brought at an electric potential (V1) which is higher than the electric potential (VO) of the first plate, characterized in that the straight projection (18) of the output aperture (14) of the multiplier hole (12) onto a plane which is parallel to the first metal plate (11) is at least partly located outside the corresponding projection (19) of the input aperture (13).

2. A multiplier element as claimed in Claim 1, characterized in that said first metal plate (11) has a plurality of multiplier holes (12) arranged in accordance with a regular plane network.

3. A multiplier element as claimed in Claim 2, characterized in that regular plane network of multiplier holes (12) is a square-shaped network, and in that said input aperture (13) is square-shaped and said output aperture (14) is circular.

4. A multiplier element as claimed in Claim 2, characterized in that said regular plane network of multiplier holes (12) is a hexagonal network and in that said input and output apertures (13, 14) are circular.

5. A multiplier element as claimed in Claim 2, characterized in that said regular plane network of multiplier holes (12) is a hexagonal network and in that said input aperture (13) is hexagonal and said output aperture (14) is circular.

6. An electron multiplier device comprising N multiplier elements as claimed in any one of Claims 1 to 5, characterized in that the second metal plate (16) of the (i)th multiplier element is brought at an electric potential which is identical to the electrical potential of the first metal plate (11) of the (i+1)st multiplier element.

7. An electron multiplier device as claimed in Claim 6, characterized in that said N multiplier elements (10) are arranged in a parallel configuration with respect to one another.

8. An electron multiplier device as claimed in Claim 6, characterized in that said N multiplier elements (10) are consecutively arranged in a head-to-tail configuration.

9. A photomultiplier tube comprising a photocathode (20), n adjacent anodes (21) and an electron multiplier device as claimed in any one of Claims 6 to 8, characterized in that said multiplier device (22) is placed in the proximity of the photocathode (20) and is divided into n secondary multiplier devices (23) by partitions (24) which are impervious to electrons and which are located substantially opposite separation zones of two contiguous anodes (21) in such a manner that n secondary photomultiplier tubes are obtained in one and the same photomultiplier tube.

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