FR2653269A1 - MULTICHANNEL PHOTOMULTIPLIER TUBE WITH HIGH RESOLUTION BETWEEN SIGNALS. - Google Patents

Abstract

Photomultiplier tube with N parallel channels, comprising an input window (10) and an electron multiplier (20) of the "hole-plate" type, partitioned into N elementary multipliers (20a), and the first stage of which comprises a input electrode (30) and a first emitter electrode (40). According to the invention, the walls (41) of the holes (42) of said first emitting electrode (40) are covered with a photoemissive material (43). Application to the detection of elementary particles in nuclear physics.

Description

The present invention relates to a photomulti-tube

  plicator with N parallel tracks, comprising a window

  trée and an electron multiplier of the "hole plate" type, partitioned into N elementary multipliers, and of which

  the first stage includes an input electrode and a pre-

first emitting electrode.

  The invention finds a particularly advantageous application in the field of nuclear physics and, more particularly, in the detection and precise localization of

elementary particles.

  There is known from European Patent No. 131,339 a photomultiplier tube conforming to the preamble, the first emitting electrode of which is a first emitting half-dynode, and the upper stages of which, beyond the first stage, are

  made up of dynodes made up of two half-dynodes present

  as much, as the first emitting half-dynode, like conductive plates pierced with holes; the first half-dynode

  is an extractor half-dynode, while the second half

  dynode, provided with secondary emission, is the half-dynode

  transmitter. The two half-dynodes of the same dynode are pre-

  views to be brought to substantially the same electrical potential

  that and, in operation, one of the roles played by the half-dyno-

  of extractor is to attract towards the emitting half-dynode, by passing them through its own holes, the secondary electrons produced on the walls of the holes of the

  previous transmitting half-dynode, itself raised to a poten-

  lower electric tiel. Generally, the half dynode ex-

  tractor and the preceding emitting half-dynode with which it is associated are placed a short distance from each other and have the same structure in the sense that their respective holes correspond. In the particular case of the first stage, the extractor half-dynode is not a plate with holes like the other half-dynodes of the multiplier, but is an input electrode constituted by a single or double grid of high transparency opposite photoelectrons, emitted by the photocathode deposited on the input window, and that said input electrode is responsible for attracting, for secondary multiplication, the first emitting half-dynode. Furthermore, each extracting half-dynode, as well

  that the input electrode also serves to shield the electric

  only the emitting half-dynode of the same pair so that the electric field in the space between these two half-dynodes is weak, except for the field created by the following extracting half-dynode associated with said emitting half-dynode. In fact, in the absence of shielding, the secondary electrons produced by the emitting half-dynode would be subjected as soon as they leave to an excessive electric field which would immediately cause them to fall back to the same place where they have

been created.

  The photomultiplier tube known from the state of the art is used in nuclear physics for the precise detection of elementary particles. To this end and with the aim of increasing the spatial resolution of the observation, this tube is partitioned so as to produce N (N = 4,9,16, ...) elementary photomultiplier tubes in the envelope of a single tube. This is why the electron multiplier itself is divided by electron-tight partitions into N elementary multipliers which lead to N contiguous anodes located near and opposite the output of the

  respective elementary multipliers.

  In order to define, on the photocathode deposited on the

  tube entry window, N corresponding elementary photocathodes

  corresponding to the N multiplication pathways and avoid any di-

  photie between said elementary photocathodes, the multipli-

  The hole plate electron generator is placed in the immediate vicinity of the photocathode. In this way, any photon reaching a given elementary photocathode produces photoelectrons which will all be collected by the multiplier.

corresponding elementary.

  However, despite the precautions taken, it turns out

  re experimentally that the partitioned photomultiplier tube

  known nevertheless presents a certain diaphotia between channels.

  The explanation of this parasitic phenomenon is found in the re-

  statistical partition of the initial velocities of the electrons se-

  condaries produced by the transmitting half-dynodes. It appears

  rait that the majority of the secondary electrons have, of course, an initial speed sufficiently low so that, under the effect of the extractor field, they cross the half emitting dynode by the exit opening even of the hole on the wall of which they took birth. These secondary electrons do not give rise

  with diaphotia. However, we note that a certain number of

  secondary electrons acquire sufficient initial velocity

  high which can be equal to that of the electrons inci-

  teeth, these are the backscattered and elastic electrons that can travel distances equivalent to several

  holes. In the upper stages of the multiplier, these elect

  sections are stopped by the bulkheads, but in the

  first stage, devoid of partitioning, retro electrons

  elastically diffused, produced by the first emitting half-dynode are likely to pass from one channel to another

  and thus cause the observed crosstalk.

  Also, the technical problem to be solved by the object

  of the present invention is to produce a photomulti-tube

  plifier conforming to the preamble which would not present any

  diaphoty at the level of the first floor, while maintaining the information

  dispensable proximity focusing of the tube known to the state

of technique.

  The solution to the technical problem posed consists in the present invention, in that the walls of the holes of the first emitting electrode are covered with a material

  photoemissive. Thus, the invention amounts to carrying out the trans-

  duction of incident photons into photoelectrons not on a photocathode conventionally deposited on the entry window of the photomultiplier tube, but on the holes of the first emitting electrode itself, the function of which

  instead of multiplying incident electrons, is then to pro-

  to deduce photoelectrons whose particularity is to have all a low initial speed which does not allow them to cross the first emitting electrode by another hole

  than the one where they were created. The characteristic technical effect

  tick resulting from the invention is therefore to avoid the phenomenon of "elastic rebound" observed in the known tube previously described, and, therefore, not to cause crosstalk

  on the first stage of the electron multiplier, consistent

  ment to the objective assigned with regard to the technical problem pos-

  se. In the photomultiplier tube according to the invention,

  the input electrode associated with the first emitted electrode

  this no longer plays the role of photoelectron extraction, as in the known tube, since these are produced downstream and no longer upstream of said input electrode. On the other hand, it still retains its role of shielding the first emitting electrode, which has become photocathode in accordance with

  provisions of the present invention.

  In order to obtain maximum sensitivity, the input electrode must offer the greatest optical transparency. For this purpose, said input electrode may be in the form of a conductive grid similar to

  that already used in the state of the art, but for the-

  which the required transparency was exercised in an equivalent way

  te with respect to incident photoelectrons. The op-

  the tube entry stage can then be further improved

  improved when, according to the invention, the input electrode is a conductive grid made up of wires arranged opposite the holes of the first emitting electrode and reflecting the incident light towards the walls of said holes. Thus, the light which would be lost by rear reflection on the wires of the grid is recovered by the mirror effect in the direction of the photoemissive holes of the first emitting electrode. Another means of obtaining satisfactory optical transparency consists, according to the invention, in that the input electrode

  is made of a thin layer of conductive material

  placed on the entrance window. This particular embodiment

  linking has the advantage of being easy and inexpensive to implement

reuse.

  Finally, the transduction efficiency of the photoca-

  method of the photomultiplier tube according to the invention can be

  further increased if the walls of the holes of the first elect

  emitting trode comprise a layer of a reflective material on which said photoemissive material is deposited. Of this

  way, the path of incident light through the material

  the photoemissive beam is elongated by reflection, which tends to increase the probability of transformation of photons into photoelectrons.

  The following description next to the drawings

  attached, given by way of nonlimiting examples, will make it clear what the invention consists of and how it can

be realized.

  Figure 1 is a sectional view of a photo tube

multiplier according to the invention.

  Figure 2 shows a section of a variant of

  reading of the input stage of the photomultiplier tube of the

figure 1.

  Figure 1 shows, in section, a photomul-

  multiplier with N parallel paths, N up to 64 for example. This tube has an entry window 10, made of glass or

  in quartz, and an electron multiplier 20 of the "flat" type

  with holes ", partitioned into N elementary multipliers

  at. The first stage of the electron multiplier 20 comprises

  takes an input electrode 30 and a first emitting electrode 40. Then, there are in the upper stages a series of dynodes made up of two half-dynodes in the form of plates with holes, one of which, such as D'2 or D'3, is the extracting half-dynode, and the other, like D2 or D3, is the emitting half-dynode which, endowed with secondary emission, has the role of multiplying the electrons incident on the walls of

  its holes. The two half-dynodes of the same dynode are carried

  tees at the same electrical potential, while each dynode is

  brought to an electrical potential higher than that of the dyno-

  from previous. This is why the transmitting half-dynodes 40,

  D2, D3, ..., are separated from the following extractor half-dynodes

  vantes D'2, D'3, ..., by insulating spacers 60, such as small resin beads for example. As shown in FIG. 1, the N elementary multipliers 20a lead to N adjacent anodes 60a and are separated from each other by electron-tight partitions 21a, produced by

masking and photoengraving.

  As can be seen in FIG. 1, the walls 41 of the holes 42 of said first emitting electrode 40 are covered with a photoemissive material 43. By way of example, said photoemissive material can be an alkaline antimonide

  composed of antimony and one or more alkalines among the po-

  tassium, sodium and cesium. The first electrode emits-

  trice 40 then plays the role of photocathode which transforms

  incident photons 70 into photoelectrons 71 whose speed initiates

  is not sufficient to pass from one hole to another, and which therefore cannot cause crosstalk

  between the tracks. In the embodiment shown in fig.

  re 1, the input electrode 30 is made of a thin layer

  of a conductive material deposited on the entry window 30.

  This input electrode ensures good optical transparency with respect to incident photons 70 and, on the other hand, being brought to an electrical potential equal to or slightly lower than that of the first emitting electrode 40, it ensures the shielding of said first emitting electrode so as to avoid the return of the photoelectrons 71 on the walls 41 of the

holes 42.

  Figure 2 shows another embodiment of

  the invention in which the walls 41 of the holes 42 of the first

  emitting electrode 40 comprise a layer 44 of a material

  reflective material on which said phosphor material is deposited

  toémissif 43. The reflective layer 44 can be, for example, constituted by aluminum. This advantageous arrangement of the invention has the effect of increasing the path of the incident photons 70 inside the photoemissive material 43 in order to improve the photon transduction efficiency.

photoelectron incidents 71.

  Still in FIG. 2, it can be seen that, in the case where the input electrode 30 is a conductive grid made up of wires 31 arranged opposite the holes 42 of the first emitting electrode 40, said wires 31 have a shape such that 'They reflect the incident light 70 on the walls 41 of said holes. The optical transparency of the grid is then increased, as well as the efficiency of the tube.

  photomultiplier according to the invention.

Claims (4)

  1. Photomultiplier tube with N parallel channels,   carrying an input window (10) and a multiplier (20)   of "hole-plate" type electrons, partitioned into N multi-   elementary pliers (20a), and the first stage of which   takes an input electrode (30) and a first emitting electrode (40), characterized in that the walls (41) of the holes (42) of said first emitting electrode (40) are   covered with a photoemissive material (43).   2. Photomultiplier tube according to claim 1, characterized in that the walls (41) of the holes (42) of the first emitting electrode (40) comprise a layer (44)   of a reflective material on which said material is deposited photoemissive line (43).   3. Photomultiplier tube according to one of the claims   1 or 2, characterized in that the input electrode (30)   is made of a thin layer of conductive material   placed on the entry window (10).   4. Photomultiplier tube according to one of the claims   1 or 2, characterized in that the input electrode (30) is a conductive grid made up of wires (31) placed opposite the holes (42) of the first emitting electrode (40) and reflecting the incident light ( 70) on the walls (41) said holes.