US3444414A - Photomultiplier - Google Patents

Description

May 13, 1969 BETOULE ET AL 3,444,414

PHOTOMULT IPLIER Filed Jan. 4, 1967 INVEN'IORJ MICHEL BETOULE REN PETIT JEAN-CLAUDE REBUFFIE AGENT nited States Int. Cl. lj 39/12 U.S. Cl. 31396 4 Claims ABSIRACT OF THE DISCLOSRE A high-eurent fast-response multiplier employing an anode having a small aperture therein pos1t1oned between the last two dynodes and in front of the last dynode.

The present invention relates to a photomultiplier the exit stage of which consists of three electrodes for the emission, the focussing, the multiplication and the collection et a flux of electrons, namely the next to the last dynode, the last dynode and the anode, the anode bemg positioned between the next to the last dynode and the last dynode and just in front of the latter dynode. The invention particularly concerns a high current photomultiplier With a fast response.

In a photomultiplier a photocathode liberates a flux of electrons upon capture of a flux of photons. The flux of electrons is multiplied by successive secondary emission of a plurality of electrodes, called dynodes, each dynode thereby focussing the flux of electrons onto the next dynode. In the exit stage of the photomultiplifier the electrons originating from the next to the last dynode are focussed onto the last dynode, which like the preceding dynodes gives rise to secondary emission. The electrons originating from the last dynode are focussed onto and collected by an anode, from which the output signal can be taken. There are known photomultipliers wherein the anode is a grid positioned between the next to the last dynode and the last dynode and just in front of the latter, so as to obtain a strong electric field and a maximum active surface.

These multipliers have some disadvantages, which are particularly noticeable in the case of high currents With fast variations. As a matter of fact, a part of the electrons originating from the next to the last dynode are directly captured by the grid without being subjected to multiplication by the last dynode. On the other hand, a part of the electrons originating from the last dynode and being attracted toward the grid, which has a higher potential than this dynode, pass through the grid and this introduces a perturbation of the output signal.

It is an object of the invention to provide a photomultiplier, which being free from said disadvantages has improved characteristics concerning the fastness of the response and its degree of linearity.

In accordance With the invention, in a photomultiplier, the exit stage of which consists of three electrodes for the emission, the focussing, the multiplication and the collection of a flux of electrons, namely the next to the last dynode, the last dynode and the anode, the anode being positioned between the next to the last dynode and the last dynode and just in front of the latter dynode, the anode has a single opening of a small cross-section With respect to the next to the last dynode.

Electron tubes for the generation or the amplification of oscillations of very high frequencies are known, in which anodes are used having a single opening in order to reduce the time of transit of the electrons. These tubes atcnt O show a slight resemblance to the exit stage of the photomultiplier according to the invention. Like this exit stage they include an electrode for the production of a flux of electrons, an electrode for the multiplication of this flux by means of secondary emission and an anode for the collection of electrons. However, they essentially diier from the exit stage of the photomultiplier in that they include at least a fourth electrode for the focussing of the electrons, without which they would not serve their purpose. Besides they have no preceding electrode structure. As a matter of fact these tubes belong to a diflerent art and the problems arising in photomultipliers are by no means comparable With the ones in said tubes. The differences originate mainly from the fact that photomultipliers are measuring apparatus, in which there must be conserved a linearity in the a prior unknown pulse heights. Moreover, the electron beams in the photomultipliers have large cross-sections, s0 their focussing problems are of a very particular nature.

Nevertheless it has been observed that the flux of electrons originating from the next to the last dynode was focussed in a cross-section of a noticeable smallness by a structure of the three electrodes of the exit stage being convenient in every respect. This permitted the use of an anode having a single opening of small dimensions With respect to the surfaces facing it.

In this structure, provided with an anode having a single opening of small dimensions, almost the total flux of electrons originating from the next to the last dynode passes through the opening in the anode. On the other hand the smallness of the dimensions of this opening prevents almost completely the passage of electrons through the anode in the opposite direction.

Preferably the anode is substantially equidistant to the emissive surface of the last dynode in order that the electrons originating from the last dynode substantially have the same time of flight.

Preferably the emissive surface of the last dynode presents a convexity oriented to the opening in the anode. For instance it may have the form of a dihedral angle. This convexity provides an additional reduction of the number of electrons originating from the last dynode that pass through the opening in the anode.

The invention will be described with reference to the accompanying drawing.

The drawing shows a cross-section of the tube of a photomultiplier. This tube has the form of a cylinder of revolution its axis being perpendicular to the plane of the figure. The electrodes in the tube have the form of cylindrical surfaces with generatrices parallel to this axis. The photocathode 10 consists of an opaque silver foil covered with caesium antimony alloy.

The photocathode =10 emits a flux of electrons upon capture of a flux of photons indicated in a general manner by means of the arro=ws 1. The electrode 2 has a positive potential of some hundreds of volts With respect to the cathode 10 in order to deflect the electrons emitted by the cathode 10 in the direction of the first dynode 11, which has a positive potential with respect to the cathode 10. The impact of these primary electrons on the dynode 11 gives rise to the emission of s0-called secondary electrons, the number of which is usually three to five times the number of the primary electrons. These secondary electrons are in their turn attracted by the dynode 12 and give rise to secondary emission from the dynode 12. The process is repeated at the dynodes 13, 14 and &15. The dynodes consist of fols of a silvermagnesium alloy. In the space between the dynodes are shown the metal posts 21, 22 and 23. These serve t0 improve the trajectories of the electrons by means of an electrostatic action, which permits a better repartition of the action On the dynodes. Moreover, they reduce the time of transit of the electrons and the influence of the space charge, which limits the flux of electrons. The posts 21 and 22 have the potentials of the dynodes 14 and 15. The post 23 has a potential intermediate between the potentials of the dynodes 15 and 16. The last dynode 16 presents an emissive surface which has the form of a dihedral angle. The anode consists of two parts: a post 17a parallel to the generatrices of the dynodes and opposite to one face of said dihedral angle anda part With a fiat surface 17b parallel and just opposite to the other face of said dihedrai angle. The space between the two parts of the anode (which have the same potential of the order of 3 kV. with respect to the cathode) forms an opening 18 the length of which is parallel to the generatrices of the dynodes. A secondary electron emitted by the dynode :15 passes through the opening 18, impacts on the dynode 16 and liberates several electrons, which are collected by the anode 17. The potentials of the difierent elements are adjusted so as to obtain a focussing in the plane of the opening 18. Conductors 34 and 35 support getters. The references 40 to 46 are bars, which support the structure within a glass cylinder. Moreover, these bars serve as conductors.

It is possible to conceive several modifications of the electron multiplier, wvhich remain within the scope of fe invention, particularly concerning the number of dynodes.

What is claimed is:

:1. A photomultiplier comprising a source of primary electrons, a plurality of dynodes in succession and an anode for collecting electrons liberated by the last of the dynodes, said anode being positioned between the last dynode and the next preceding dynode, said anode having a single aperture therein the crosssection of which is small relative to the next preceding dynode, said last dynode having a convex emissive surface orientcd toward said anode aperture.

2. A photomultipler according to claim 1, character ized in that all points on the anode surface are substantially equidistant from the emissive surface of the last dynode.

3. A photomultper according to claim 2, characterized in that the emissive surface of the last dynode has the form of a dihedra] angle.

4. A photomultipfier according to claim 3, character ized in that the anode comprises a post parallel and just opposite to one face of said dihedral angle, and a part having a fiat face parallel and just opposite to the other face of said dihedral angle.

References Cited UNITED STATES PATENTS 2,198,233 4/1940 Snyder 313 2,231,693 2/1941 Snyder 313105 2,285,126 6/1942 Rajchman et a]. 313105 3,260,878 7/1966 L6g0ux 313104 IAMES W. LAWRENCE, Primary Examiner.

DAVID OREILLY, Assistant Examiner.

U.S. Cl. X.R.

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