FR2466851A1 - SECONDARY EMISSION ELECTRODE, IN PARTICULAR FOR A PHOTOMULTIPLIER - Google Patents

Abstract

The invention relates to a secondary emission electrode, comprising a metal support 1 and a layer 3 with a high secondary emission coefficient, made of an alkaline antimony compound 4, remarkable in that it also comprises an intermediate layer 2, forming office barrier layer, in a material selected from the group formed by rhodium, ruthenium and palladium. These various layers are preferably deposited electrolytically. Secondary emission electrodes find their place in particular in photomultiplier tubes. Application: scintillation measurement; spectrophotometry. (CF DRAWING IN BOPI)

Description

ELECTRODE WITH SECONDARY EMISSION, IN PARTICULAR FOR PHOTOMULTIPLIER.

  The invention relates to a secondary emission electrode,

  very generally taking at least one metal support and one layer to

  high secondary emission factor. The invention-finds its application

  in the manufacture of photomultiplier tubes, for use in

  various situations, in particular for scintillation

  research or nuclear medicine ...), or for spectrophotometry

lysis of image ...).

  A photomultiplier is a tube that groups together

  hen, a photocell and a current amplifier,

  the secondary emission of electrodes (dynodes) brought to increasing electrical potentials. The gain of the amplifier is directly related

  number of dynodes and their secondary emission factor.

  Initially, the dynodes were made among other things in

  copper and beryllium binding (2%) and had an emission factor

  secondary of the order of 4, for electrons accelerated under 100 volts.

  In order to produce electrodes with better coeffi-

  In the case of secondary emission, it is known from the prior art to deposit on any metal substrate, an antimony layer, by various deposition methods, such as evaporation, cathodic sputtering, or

  electrolysis, then to activate this material by evaporation under vacuum, of

  alkaline elements, for example cesium, potassium or rubidium, from a Joule-effect generator and forming, at least in the superficial region of the antimony layer, various alkaline compounds (for example Sb Cs3 , SbK3 ...) or even bi-alkaline (Sb Cs2 K,

  SbK2 Cs ...), these layers then having an emission coefficient se-

  of the order of 5.5 (in the case of Sb Cs3), in the same way

ditions.

  However, while the metal substrate usually used

  For historical reasons, it is copper-beryllium, and since the layer with a high secondary emission coefficient is an alkaline antimony compound, a reaction occurs between the copper and the antimony, which strongly degrades the properties. emissives of said layer, as a result of

  the formation of an alloy between copper and antimony.

  In order to avoid this harmful reaction, a first improvement

  It has consisted in subjecting said metal support to a heat treatment.

  under a vacuum (for example, heating to 600 ° C. for half an hour)> in order to make the beryllium migrate to the surface of the support, then to admit a partial pressure reduced by oxygen, so as to form a thin

  protective layer of Be0. This protective layer of BeO presents the

  to form a barrier between antimony and copper, but

  also very resistive, which prevents the deposition of the

  electrolyte superior antimony.

  The object of the invention is to overcome this disadvantage by

  troduction of an intermediate layer forming a barrier between copper and antimony and sufficiently conductive to allow the deposit of

  top layer electrolytically.

  According to the present invention, the electrode is characterized in that it comprises an intermediate layer of a metal chosen from the group formed by rhodium, ruthenium and palladium, this metal being deposited directly on the metal support, preferably by

electrolysis.

  According to a preferred embodiment of the invention, this intermediate layer is a rhodium layer between the support

  metallic copper-beryllium, and a layer of antimony.

  The following description, with reference to the accompanying drawings,

  given in a non-limitative way, will help to better understand how

vention can be realized.

  FIG. 1 represents the succession of layers composing the

secondary emitting electrode.

  FIG. 2 represents a particular electrode geometry (dynode) and FIG. 3 represents a diagram of a photomultiplier tube,

including such dynodes.

  A metal support, for example copper-beryllium,

  1, is covered with a first intermediate layer 2, a metal selected from the group, formed by rhodium, ruthenium and palladium, having a thickness of substantially between 0.1 μm and 1 μm. pm,

  and a second antimony layer 3 activated at least in its region

  4 by the formation of alkaline antimony compounds, presented

  both a high secondary emission coefficient, for example an antimony and cesium compound Sb Cs3, or potassium Sb K3 or rubidium Sb Rb3; this region 4 having a thickness of the order of a few

hundreds of Angstr m.

  The support 1 is necessarily metallic because it must be

  brought to a certain voltage, so as to accelerate the electrons pro-

  coming from a lower dynode. Moreover, he must not possess any

  magnetic properties, it must be able to be evacuated (which excludes aluminum) and finally it must be formable from a sheet of metal and not be expensive. These criteria make it possible to retain several

  metals or alloys, preferably copper-beryllium, but also

nickel ...

  The first intermediate layer 2 plays the role of a barrier layer 1 between the metal support on the one hand and the layer on the other

  emissive. Metals, capable of fulfilling this function, are typically

  only noble metals that are not oxidizable and hardly react with other elements. Among these, the Applicant has selected those of the platinum family (with the exception of platinum which reacts with copper), which are electrolyzable: the group retained is then constituted by rhodium, ruthenium and palladium. But in order to realize the best mode of realization, it is better to use

  rhodium, because palladium reacts weakly with copper and ruthenium

  nium is more difficult to implement, in the form of an electrical bath.

  trolysable, with optimal temperature around 70 or 800C, resulting in

significant evaporation of water.

  The deposition of these layers of rhodium and antimony can be done by any means (evaporation under vacuum, sputtering, etc.) but preferably, it is carried out electrolytically, by means of a suitable electrolyte solution. Indeed, this route is easily industrialized and therefore less expensive for the deposition of a layer of a material. However, electrolysis is only possible if the object has a relatively low surface resistance (which is not the case in

  presence of a layer of BeO on the surface).

  This electrolytic deposition is thus carried out directly on the unoxidized copper-beryllium metal support, by the usual route, namely immersion in a suitable electrolyte solution, brought to a suitable temperature generally specified by the producing firm, and connected at the negative terminal of a current generator,

  whose other terminal is connected for example to a platinum electrode.

  Thus, for the deposition of a layer of rhodium on a support

  of copper-beryllium, an aqueous sulphate solution should be used.

  rhodium fate, in sulfuric medium (ph-1), with a quantity of rhodium ion close to 5 g / liter. Such a solution is marketed

  by ENGELHARD Industries, under the reference "Rhodium Electro-

  glossy lytic S 100 ", and by the Continental Society PARKER, under the reference" T30 rhodium-rich bath. "The deposition of a layer of 0.25 pim, requires the establishment of a current of a density close to 1 A / dm 2

  for about two minutes at a temperature between 30 and 40 C.

  Electrolytes baths for the deposition of ruthenium and palladium are

  also available from the same suppliers.

  Similarly, for the deposition of the antimony top layer, it is advantageous to use an aqueous solution of antimony chloride.

  (Sb C13), in a hydrochloric medium, with a quantity of antimony ions

  sine of 20 g / liter. A layer of a thickness of the order of a few hundred Angstroms is then deposited by establishing a current with a density of about 2 A / dm 2 for a period of 4 to 8 seconds (on average).

at room temperature.

  The antimony layer is then exposed to vapors of

  alkali - usually cesium and / or potassium, - produced in an enclosure subjected to a fairly high vacuum, by means of a generator at

  dichromate by Joule effect. There is a chemical reaction between the

  alkaline element and antimony, which increases with temperature. The neck-

  antimony and the enclosure are heated to a sensible temperature

  between 130 and 150 ° C., so as to accelerate this chemical reaction and to prevent the condensation of cesium on cold regions of the enclosure. It then occurs, at least in a superficial region 4,

  a few hundred Angstroms thick, the formation of com

  alkaline antimony, with a high emission factor

  ondary. FIG. 2 is a perspective view of a particular dynode, whose geometry is adapted to the position and the adjustment of the potentials, such that the electrons emitted by a lower dynode are best captured. This geometry is not limiting in itself of the invention and is given here only as an example, while there is

  multiple forms of marketed dynodes.

  FIG. 3 represents a diagram of a type of photomultiplication.

  with a semi-transparent end photocathode. The incident light

  hit a photocathode 10, which emits electrons that are focussed

24 6685 1

  in the central orifice of an electrode 11, which then fall on a first dynode 12, having a high secondary emission coefficient 6. For an incident electron, there are then 6 emitted electrons, which are received, thanks to a suitable geometry and a growing potential, on a second dynode, the phenomenon continuing right through.

  Thus, if n is the total number of dynodes, the gain of the

  Thus, it is obvious that, to improve the gain G, the number of dynodes can be increased but also the secondary emission coefficient must be improved, in accordance with the spirit of the present invention.

as claimed below.

Claims (5)

CLAIMS:   1. Secondary emission electrode, comprising at least one   metal port and a layer with a high secondary emission coefficient, characterized in that it also comprises an intermediate layer of a metal selected from the group consisting of rhodium, ruthenium and palladium.   Secondary emission electrode according to claim 1,   characterized in that the selected metal is rhodium.   3. Secondary emission electrode according to one of the claims   1 or 2, characterized in that the metal support is in one copper-beryllium alloy (2%).   4. Secondary emission electrode according to one of the   cations 1 to 3, characterized in that the high coefficient layer   secondary emission is an alkaline antimony compound.   5. Secondary emission electrode according to claim 4, characterized in that the alkali metal is chosen from the group formed by cesium and potassium.   Photomultiplier comprising at least one electrode   secondary emission according to one of claims 1 to 5.