EP1846939B1 - Photomultiplier tube with least transit time variations - Google Patents

Abstract

A single-channel photomultiplier tube having a sealed envelope, of which one wall includes an internal face having a concavity with a central axis, turned toward the inside of the tube, having a plane of symmetry and containing a photocathode, inlet optics including electrodes, an electron multiplier including a plurality of dynodes, an anode, and a mechanism connecting the dynodes, the photocathode, electrodes of the optics, and the anode, at their respective operating voltages. The electron multiplier is composed of parts physically distinct from one another, and having between them a symmetry of revolution with respect to the central axis of the concavity.

Description

TECHNICAL AREA

The present invention relates to a single electron multiplier tube.

STATE OF THE PRIOR ART

A photomultiplier tube generally comprises, inside a gas-tight vacuum envelope, a light-sensitive electrode, called a photocathode, an electronic focusing optic, an electron multiplier for multiplying the electrons emitted by the photocathode and a photocathode. anode that collects the multiplied electrons.

The patent application FR 1,288,477 corresponding to the US patent having the deposit number 27066 , assigned to Radio Corporation of America, describes in connection with the single figure of this patent, a single-channel photomultiplier tube, comprising a sealed envelope 10. The sealed envelope 10 comprises a transparent window 12 to photons. The window 12 has an outer face and an inner face. The inner face has a concavity having a central axis. The concavity is turned towards the inside of the tube. It has a plane of symmetry containing the central axis.

• une optique de focalisation comportant plusieurs électrodes focalise les électrons provenant de la photocathode sur une première dynode 31 d'un multiplicateurs d'électrons à structure linéaire focalisée située en aval de l'optique dans le sens de parcours des électrons. Le multiplicateur comporte une pluralité de dynodes 31-40 dont une première dynode 31, des dynodes intermédiaires, une avant dernière dynode et une dernière dynode. Le tube comporte également une anode 42. Des moyens 18 de raccordement traversent l'enveloppe étanche 10 et comportent des contacts 18 de raccordement extérieurs à l'enveloppe 10, eux même raccordés à des liaisons électriques internes de raccordement, et permettent de raccorder respectivement les dynodes, la photocathode 14, des électrodes 16, 20, 22, 24 formant ensemble l'optique de focalisation, et l'anode 42, à leur tension respective de fonctionnement,

A photocathode 14 is disposed on the inner face of the wall forming the transparency window to receive light photons having passed through the transparency window,

• a focusing optics comprising a plurality of electrodes focuses the electrons from the photocathode on a first dynode 31 of a focused linear structure electron multipliers located downstream of the optics in the direction of travel of the electrons. The multiplier includes a plurality of dynodes 31-40 including a first dynode 31, intermediate dynodes, a penultimate dynode and a last dynode. The tube also comprises an anode 42. Connection means 18 pass through the sealed envelope 10 and have external connection contacts 18 to the envelope 10, themselves connected to internal connection electrical connections, and enable the respective connections to be connected. dynodes, the photocathode 14, electrodes 16, 20, 22, 24 together forming the focusing optics, and the anode 42, at their respective operating voltage,

The single-channel tube described in this application is used in applications where the homogeneity of transit time between the moment when an electron is emitted by the photocathode and a moment when a bundle of electrons resulting from the multiplication of this electron by the multiplier is an important factor. A perfect tube would have transit times equal to each other regardless of the place of emission on the photocathode and the initial energy of the emitted electron. In the single-channel tubes described above, the dispersion of the transit times between photocathode and The first dynode of the multiplier is reduced by the fact that the photocathode is mounted on a hemispherical surface. Because of this form the distance between the different points of the photocathode and a center is equal. This geometry contributes to reducing the dispersion of the transit times as a function of the emission site of an electron on the photocathode.

STATEMENT OF THE INVENTION

The subject of the invention is a single-channel photomultiplier tube having an improved temporal resolution compared to single-channel tubes known from the prior art. This object is attained by the fact that an electron multiplier is provided in the tube composed of several multiplying parts physically distinct from each other, and presenting between them a symmetry of revolution with respect to the central axis concavity. Each multiplier part is actually an autonomous multiplier.

The hemispherical photocathode is thus divided virtually into as many portions of cathodes as there are parts of multipliers. When the photocathode has a shape of revolution about an axis, the photocathode portions are angular sectors whose apex coincides with the axis of revolution. Each photocathode sector corresponds to a dedicated multiplier. Due to the symmetry of revolution the sectors are equal to each other. Thus, according to the invention, there is a zone in which the electrons emitted by each of the photocathode sectors are focused in common by a common optical focus, as many first dynodes as sectors. Each first dynode is a dynode of an autonomous multiplier multiplying the electrons from the photocathode sector corresponding to this dynode. Like the set of dynodes, these first dynodes of each of the multipliers are symmetrical with respect to the axis of the tube.

Since the electrons coming from a single photocathode sector have trajectories which have angles of divergence between them less than the angles of divergence presented to each other by the trajectories of the electrons coming from the entire cathode, and therefore differences in length. For shorter journeys, the transit time differences of electrons from the photocathode to the first dynode of each multiplier are smaller.

On the other hand, the trajectories of the electrons between the first dynode D1 and the second dynode D2 of each multiplier also have differences in their path lengths between them which are smaller than the differences in path lengths that one would have with a single large first. dynode returning the electrons to a single large second dynode. As a result, the differences in electron travel time between the first and second dynodes of each multiplier are also reduced. The same is true, albeit to a lesser extent, of the travel times between consecutive stages of each of the multipliers.

This produces a single-channel tube having a transit time dispersion smaller than that of the tubes of the prior art.

• une enveloppe étanche, ayant une paroi formant une fenêtre de transparence à des photons et comportant une face externe et une face interne présentant une concavité ayant un axe central, tournée vers l'intérieur du tube, et ayant un plan de symétrie contenant l'axe central,
• une photocathode disposée sur la face interne de la paroi formant la fenêtre de transparence de façon à recevoir des photons lumineux ayant traversé la fenêtre de transparence,
• une optique de focalisation comportant une ou plusieurs électrodes,
• un multiplicateur d'électrons à structure linéaire focalisé situé en aval de l'optique dans le sens de parcours des électrons, comportant une pluralité de dynodes dont une première dynode, des dynodes intermédiaires, une avant dernière dynode et une dernière dynode,
• une anode,
• des moyens de raccordement traversant l'enveloppe étanche et comportant des contacts de raccordement extérieurs à l'enveloppe, eux même raccordés à des liaisons électriques internes de raccordement, pour raccorder respectivement les dynodes, la photocathode, des électrodes formant

l'optique de focalisation, et l'anode, à leur tension respective de fonctionnement,
caractérisé en ce que

• le multiplicateur d'électrons est composé de parties physiquement distinctes l'une de l'autre, chaque partie formant un multiplicateur autonome, les multiplicateurs autonomes présentant entre eux une symétrie de révolution par rapport à l'axe central de la concavité.

In summary, the invention relates to a single-channel photomultiplier tube with fewer differences in transit time comprising

• a sealed envelope, having a wall forming a photon transparency window and having an outer face and an inner face having a concavity having a central axis, facing the inside of the tube, and having a plane of symmetry containing the axis central,
• a photocathode disposed on the inner face of the wall forming the transparency window so as to receive light photons having passed through the transparency window,
• focusing optics comprising one or more electrodes,
• an electron multiplier with a focused linear structure located downstream of the optical in the direction of travel of the electrons, comprising a plurality of dynodes including a first dynode, intermediate dynodes, a penultimate dynode and a last dynode,
• an anode,
• connecting means passing through the sealed envelope and having connection contacts external to the envelope, themselves connected to internal electrical connection connections, for respectively connecting the dynodes, the photocathode, electrodes forming

the focusing optics, and the anode, at their respective operating voltage,
characterized in that

• the electron multiplier is composed of physically distinct parts of each other, each part forming an autonomous multiplier, the autonomous multipliers having between them a symmetry of revolution with respect to the central axis of the concavity.

In one embodiment the sealed envelope comprises a cylindrical insulating sleeve centered on the central axis of the concavity carrying the photocathode, the transparent window wall being connected to one end of said sleeve, and the focusing optics comprises an electrode. accelerator and focusing device, a focusing correction electrode formed by a conductive thin film in the form of a cylindrical surface portion deposited on the inner wall of the sleeve having an end close to the photocathode in a zone situated between the photocathode and the accelerating electrode, favoring the initial acceleration of the photo-electrons of the peripheral zone by increasing the electric field in their vicinity.

In the preferred embodiment, the tube comprises two multipliers, the concavity is hemispherical and the focusing optics and the two multipliers comprise a plane of symmetry which is a plane of symmetry of the concavity. This solution makes it possible to put two multipliers in parallel with a common axis on the plane of symmetry.

In this embodiment the angular sectors are 180 °.

In a variant of the preferred embodiment, the first dynodes of each multiplier have a portion that is closest to the tangent photocathode at the same point in said plane of symmetry and each have a concavity, the respective concavities of each of the first dynodes. not being turned towards each other. This solution makes it possible to put two multipliers in parallel with a common point on the plane of symmetry.

BRIEF DESCRIPTION OF THE DRAWINGS

• La figure 1 représente une coupe longitudinale d'un tube photomultiplicateur selon l'invention effectuée selon un plan de symétrie du tube. Des trajectoires d'électrons dans ce plan de symétrie, entre une première moitié d'une photocathode et la première dynode d'un premier multiplicateur d'électrons sont également représentées.

The invention will now be described with the aid of the accompanying drawings in which:

• The figure 1 represents a longitudinal section of a photomultiplier tube according to the invention carried out along a plane of symmetry of the tube. Electron trajectories in this plane of symmetry between a first half of a photocathode and the first dynode of a first electron multiplier are also shown.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

The figure 1 represents a longitudinal section of a photomultiplier tube 1 with two multipliers according to the invention.

The photomultiplier tube 1 comprises a sealed envelope 4, formed by a set of walls assembled together. In the example shown, a first wall 3 has a cylindrical sleeve shape, of axis AA '. The cylindrical sleeve is made of preferably in an insulating material, for example glass. The sleeve is completed at one end by a wall 5 forming a photon transparency window. It is completed at the other end by a bottom wall 8. Pins 12 connecting the different electrodes located inside the sealed envelope 4 pass sealingly, and in a manner known per se through the bottom wall 8. When the tube is in operation, these pins 12 are respectively coupled to voltage sources, applying operating voltages to the different electrodes of the tube.

The wall 5 forming the window of transparency of the tube has a flat outer face 6 and an inner face 7 having a concavity turned towards the inside of the tube. This concavity is in the example shown a spherical cap, whose center is located on the axis AA 'of the tube. It therefore presents a plan of symmetry materialized on the figure 1 by the axis AA '. The figure 1 is an axial section along a plane containing this axis of symmetry. A photocathode 2 is disposed on the inner face 7 of the wall 5 forming the window 5 of transparency, so as to receive light photons having passed through the transparency window 5. In itself known manner the photocathode 2 is constituted by a layer of a light emitting material, for example a layer of multi-alkaline material or silver-oxygen-cesium, or cesium-antimony. It may also be another light emitting material. The material is chosen according to its spectral characteristics of photo emission and wavelengths of the photons to which the photomultiplier tube will be applied. Fictitiously, the photocathode 2 comprises two parts 21, 22 symmetrical to one another with respect to a plane of symmetry, whose intersection with the plane of the figure is materialized on the figure 1 by the axis of symmetry AA 'of the spherical cap.

From the photocathode 2 to the bottom wall 8, the tube comprises, in order, a focussing optics 9 comprising an accelerating and focusing electrode 13. The focusing optics 9 may also comprise, as in the example shown, a focusing correction electrode 15. In the example shown, this focusing correction electrode 15 is formed by a conductive thin film in the form of a cylindrical surface portion deposited on the inner face of the sleeve 3. The focusing correction electrode 15 has in the axial direction a close end of the photocathode 2 in an area between the photocathode 2 and a portion which is the most upstream of the accelerator and focusing electrode 13. In what is described here, the upstream and downstream are in the direction of travel of the electron flow from the start, so upstream of the photocathode and directed downstream so the anode. The focusing optics 9 is thus common to the two autonomous multipliers 24, 26 of the tube 1.

Downstream of the focusing optics 9, the tube 1 comprises an electron multiplier 11 formed by a set of two multiplying parts 24, 26 physically separate from each other and symmetrical to each other with respect to the plane of symmetry of the tube. These multiplying parts constitute autonomous multipliers 24, 26. Each of the multipliers 24, 26 comprises dynodes in a Rajchman focusing linear structure. By physically distinct, we mean that the dynodes composing each of the multipliers are physically distinct from the dynodes composing the other multiplier. This does not exclude that dynodes of the same rank of the two multipliers 24, 26 are connected to the same voltage source, and therefore there is a common connection part. This common connection part may be outside or inside the envelope 4. Similarly, this does not exclude that two dynodes of the same rank in each of the multipliers 24, 26 have a point or a contact zone with each other.

Each multiplier 24, 26 of electrons comprises a plurality of dynodes including a first dynode 31, 32, respectively, a second dynode 23, 25, respectively intermediate dynodes 33, 34 respectively, a second to last dynode 35, 36 respectively and a last dynode 37, 38 respectively located downstream of the optics 9 in the direction of travel of the electrons.

Downstream of the last dynode 37, 38, in the direction of travel of the electrons, the tube comprises an anode 16 formed by two conductors 17, 18 respectively, electrically connected to each other to form a single anode of the multiplier 11 .

Thus, a first multiplication channel of the tube 1 is materialized by the first half 21 of the photocathode 2, the common optic 9, the first multiplier 24, and the portion 17 of the anode 16. The second multiplication channel of the tube 1 is materialized by the second half 22 of the photocathode 2, the common optic 9, the second multiplier 26, and the portion 18 of the anode 16.

In the example shown on the figure 1 , the dynodes, 32, 34, 36, 38 and 31, 33, 35, 37 of the same rank of the two multipliers 24, 26 with the exception of a gain tuning dynode 30, 39 in each multiplier are connected to a same connection pin respectively. the dynodes 30, 39 for setting respectively each of the two multipliers 24, 26 have a connection allowing independent voltage adjustment for each of them.

In the example shown figure 1 , the first dynodes 31, 32 of each multiplier 24, 26 respectively are symmetrical to each other with respect to the plane of symmetry of the concavity of the transparency window 5. Each of these first dynodes 31, 32 has a part 27, 28 respectively which is closest to the photocathode 2. The portions 27, 28 of each of the first dynodes 31, 32 are respectively tangent at one and the same point to each other and to said plane of symmetry. The first dynodes 31, 32 have a concavity whose respective centers of curvature are symmetrical to each other with respect to the plane of symmetry. The centers of curvature of each of the first dynodes 31, 32 respectively are located on the same side of the plane of symmetry as the corresponding dynode. We can see on the figure 1 that each of the first dynodes is constituted by a set of four planar portions, the overall curvature resulting from the fact that two consecutive planar portions form a dihedron. In the section plane shown, it is considered that a center of curvature of a dihedron is the center of the circle tangent to each of the two faces of the plane portions forming the dihedron.

• De façon en elle -même connue, lorsqu'un électron est émis par la photocathode 2, cet électron est accéléré et dirigé par l'optique 9 vers l'une ou l'autre des premières dynodes 31, 32. Des trajectoires temporisées d'électrons émis par la partie 21 de la photocathode 2 sont représentées sur la figure 1. Les électrons provenant de la partie 21 sont majoritairement dirigés vers la première dynode 31 appartenant au premier multiplicateur 24. Les électrons sont multipliés par la première dynode 31 du premier multiplicateur 24. Les électrons provenant de la première dynode 31 sont projetés sur la seconde dynode 23 du premier multiplicateur 24. Les électrons sont ensuite multipliés de dynode en dynode et le flux multiplié atteint la partie 17 de l'anode unique 16.

The operation is as follows:

• In itself known way, when an electron is emitted by the photocathode 2, this electron is accelerated and directed by the optics 9 towards one or the other of the first dynodes 31, 32. Time trajectories of electrons emitted by the part 21 of the photocathode 2 are represented on the figure 1 . The electrons coming from the part 21 are mainly directed towards the first dynode 31 belonging to the first multiplier 24. The electrons are multiplied by the first dynode 31 of the first multiplier 24. The electrons coming from the first dynode 31 are projected onto the second dynode 23 of the first multiplier 24. The electrons are then multiplied from dynode to dynode and the multiplied flux reaches the portion 17 of the single anode 16.

The averages of the travel times of the different electrons between the photocathode 2 and the first dynode 31 of the first multiplier 24 appear opposite the starting points of the electrons on the photocathode 2. These averages of courses vary between 6.24 and 6.40 nanoseconds. The initial differences in travel time are therefore very small. These differences in travel time will be further reduced during the multiplication. The improvement in the homogeneity of the travel times is due to the fact that there is a smaller distance of travel between the electrons from a sector such as 21 or 22 of the photocathode and the first dynode of each multiplier. It is the same between first and second dynode of each multiplier.

The tube having a symmetry, all that has been said about the first multiplication channel applies mutatis mutandis to the second way of multiplication. The electrons emitted by the second part 22 of the photocathode are directed mainly towards the first dynode 32 of the second multiplier 26. The signal is collected on the part 18 of the single anode 16.

Despite the precautions taken to have as much symmetry as possible between the two channels, the manufacturing tolerances make the two paths are not as symmetrical to each other as would be desirable. Therefore, it is advantageous to provide in each of the multipliers 24, 26 a gain adjustment dynode, 30, 39 respectively. Gain tuning dynodes are dynodes which unlike other dynodes of the same rank of each multiplier are not connected to voltage sources of the same value. These dynodes 30, 39 thus each have a connection pin 12 of its own and can be connected to a source of voltage that is specific to each gain adjustment dynode. The dynodes 30, 39 make it possible to balance the overall gain of each of the multipliers 24, 26 and an equalization of the transit times between the multiplication channels.

Claims (6)

1. Single-channel photomultiplier tube (1) with lower transit time variations, comprising:

   - a sealed envelope (4), having a wall (5) forming a photon-transparent window comprising an external face (6) and an internal face (7) which has an internal concavity with a central axis (AA'), turned toward the inside of the tube, and having a plane of symmetry containing the central axis (AA'),

   - a photocathode (2) arranged on the internal face (7) of the wall (5) forming the transparency window (5) so as to receive light photons having passed through the transparency window (5),

   - focusing optics (9) comprising one or more electrodes,

   - an electron multiplier (11) with a focused linear structure located downstream of the optics (9) in the direction of travel of the electrons, comprising a plurality of dynodes (23, 25, 30-39) including a first dynode (31), intermediate dynodes (33), a penultimate dynode (35) and a final dynode (37),

   - an anode (16),

   - connection means (12) passing through the sealed envelope and comprising contacts (12) for external connection to the envelope (4), themselves connected to internal electrical connections, for respectively connecting the photocathode (2), the dynodes (23, 25, 30-39), electrodes (13, 15) forming together the focusing optics (9), and the anode (16), at their respective operating voltages,

   characterised in that

   - the electron multiplier (11) is composed of parts (24, 26) physically distinct from one another, with each part forming an autonomous multiplier (24, 26), and the autonomous multipliers (24, 26) having between them a symmetry of revolution with respect to the central axis of concavity.
2. Photomultiplier tube (1) according to claim 1, characterised in that one (30, 39) of the dynodes (23, 25, 30-39) of each multiplier part (24, 26) is a gain setting dynode (30, 39), with each of the gain setting dynodes (30, 39) having its own connection means (12).
3. Photomultiplier tube (1) according to one of claims 1 or 2, characterised in that the sealed envelope (4) comprises a cylindrical insulating sleeve (3) centred on the central axis of the concavity holding the photocathode (2), with the wall (5) forming a transparency window being connected to an end of said sleeve (3),
   and in which the focusing optics (9) comprise an accelerating and focusing electrode (13), a corrective focusing electrode (15) formed by a conductive thin film in the form of a cylindrical surface part deposited on the internal wall of the sleeve (3) having an end close to the photocathode (2) in an area located between the photocathode and the accelerating and focusing electrode (13).
4. Photomultiplier tube (1) according to one of claims 1 to 3, characterised in that the internal concavity of the transparency window (5) is hemispheric and the focusing optics (9) and the two multiplier parts (24, 26) comprise a plane of symmetry that is a plane of symmetry of said internal concavity.
5. Photomultiplier tube (1) according to claim 4, characterised in that the first dynodes (31, 32) of each multiplier part (24, 26) have a part that is closest to the photocathode (2), which is tangential in the same point to said plane of symmetry and each having a concavity, wherein the respective concavities of each of the first dynodes (31, 32) are not turned toward one another.
6. Photomultiplier tube (1) according to one of claims 1 to 5, characterised in that the external face (6) of the transparency window (5) is planar.

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