DE1214808B - Electron optics for an electron tube with a large photocathode - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY GERMAN mjtVm PATENT OFFICE

EDITORIAL

Int. α .:

HOIj

German class: 21g-29/20

Number:
File number:
Registration date:
Display day:

February 9, 1960
April 21, 1966

The invention relates to electron optics for an electron tube with a large area Photocathode, in particular photomultiplier tube, consisting of at least two coaxially arranged electrodes and which focuses the electrons on a collecting electrode.

Tubes with electron optics of this type are known which contain at least two electrodes. For example, triode optics can consist of the photocathode, one im essentially cylindrical and concentrically attached to the photocathode focusing electrode and a suction electrode approximately in the shape of a truncated cone, the structure and the Potentials of this optics are chosen such that the surface of a controllable part of the Photocathode emitted photoelectrons under the action of an almost uniform field of sucked away from this part and concentrated on the input of a multiplier.

Furthermore, photomultiplier tubes are known with a large-area photocathode, with a electron multiplier system comprising several dynodes, in which the first dynode is compared to one to the photocathode has small dimensions, and with electron optics consisting of at least two electrically independent, coaxially arranged electrodes, each with an electron passage opening with the electrode closest to the photocathode serving as the accelerating electrode.

In practice it has been found that when using such a photomultiplier with a photocathode a usable diameter of 40 mm work satisfactorily, this is no longer the case if this diameter is chosen to be larger, e.g. 110mm, since the fields are reversed at the same potentials Decrease in proportion to magnification. If you want to keep a high value for the suction field, so one must increase the potentials proportionally, which in a much too high numerical way Value for the potential of the first dynode results. The impinging electrons then have too great a speed.

In the case of electron optics of the type mentioned at the outset, this disadvantage is avoided if, according to the invention, a delay electrode which is at a lower potential than the electrode is arranged between the electrode which is at high potential and the collecting electrode. The deviation in the transit time along the axially and on the outside running paths within electron optics for an electron tube is also included
large area photocathode

Applicant:

NV Philips' Gloeilampenfabrieken,
Eindhoven (Netherlands)

Representative:

Dr. rer. nat. P. Roßbach, patent attorney,

Hamburg 1, Mönckebergstr. 7th

Named as inventor:

Georges Pietri,

Bellevue, Seine-et-Oise (France)

Claimed priority:

France of February 11, 1959 (786 339)

admissible limits, and there are only slight fluctuations in the running times (the are related to the distribution of the initial speeds).

In this way it is possible to bring the collecting electrode to a potential which is the same or almost the same is equal to that of the delay electrode; behind this electrode are the photoelectrons then in a field that is practically zero. Thus, for the potential of the collecting electrode, z. B. the first dynode of a photomultiplier system, a permissible value can be selected, although to release the photoelectrons from the photocathode the mentioned electrode with very high positive potential is available.

Preferably, the delay electrode is in the form of a screen which is at high potential surrounding electrode and which has an axial opening that has a much smaller diameter than the starting diameter of the mentioned electrode, and the deceleration electrode is arranged in such a way that the mentioned opening is practically in

so near the focussing point of the photoelectrons between the electrode which is at high potential and the collecting electrode.

609 559 / 32a

It should be noted that an arrangement for eliminating the image blurring in cathode ray tubes with post-acceleration grids is known, the one between a grid lying on positive potential and that on a much higher positive one A further collecting electrode that is at potential is more negative than the first grid Lattice potential is arranged in order to dissipate the secondary electrons occurring at the grids. There is no reduction in the speed of impingement on the impingement electrode.

The device described is explained in more detail with reference to the drawing, in which an exemplary embodiment of electron optics is shown schematically is.

These electron optics are based on an accelerating electrode F ₃ with a very high potential (e.g. 1500V), whereby the suction field on the surface of the photocathode C is considerably increased and is perpendicular to it, so that at least the equipotential lines closest to this surface run practically parallel to this surface.

As a result, the photoelectrons are greatly accelerated and could hit the collecting electrode A ₄ , the first dynode of a photomultiplier, at too great a speed, so that the latter would not be effective under optimal conditions. Therefore, the electrode F _{2 is} attached with a lower potential (500 V) than that of the preceding electrode, which has a disc-like or umbrella-like edge F _{2 b bent behind the electrode F 3} , so that the electrode acts as a delay electrode and the photoelectrons decelerates, which pass through the _{aperture consisting of the part F 2 &} with a smaller opening diameter than that of the electrode F ₃ . The potential of the first dynode A ₄ can therefore be equal to or almost equal to that of the electrodes F ₂ .

The front part F _{2 a of} the electrode F ₂ protrudes forward past the electrode F ₃ and is slightly wider, so that when the photocathode C, as usual, is provided with a cylindrical focusing _{electrode F 1} , which opposite the photocathode, z. B. is brought to + 200V, the part F _{20 of} the electrode F ₂ fulfills the function of an accelerating and focusing electrode, through which the shape of the equipotential lines can be influenced. These lines, which each correspond to a specific potential, which is shown in the drawing as a function of the voltages applied to the electrodes, are shown in dashed lines. As can be seen, the optics according to the invention are equivalent to two lenses; the first is converging and the second is diverging. One also sees an inner screen F _{3 a} , _{which forms a whole with F 3} , which serves to give the field the appropriate course.

Following the first dynode A ₄ , the three successive dynodes A ₅ , A _e , A ₁ are shown, which can be followed by any desired number of similar stages.

Each electrode F ₁ , F ₂ , F ₃ , A ₄ , A ₅ , A ₆ , A ₇ is (here schematically) with a connection terminal P indicated by a corresponding number for applying the required voltage (P ₁ : 200 V, P ₂ : 500 V, P ₃ : 1500 V, P ₄ : 500 V). ^;

The parts F ₂₀ and F _{2 b} are connected in the drawing by a dashed line to indicate that they can optionally also be separated and possibly brought to different potentials. In this case, the part F ₂₀ would have to be connected to a special clamp (not shown).

It is not absolutely necessary to intercept the photoelectrons with a multiplier system; this system could optionally be replaced by a normal anode.

Claims (8)

Patent claims: 1. Electron optics for an electron tube with a large photocathode, in particular a photomultiplier tube, which consists of at least two coaxially arranged electrodes and which focuses the electrons on a collecting electrode, characterized in that between the electrode (F ₃ ) and the collecting electrode (A ₄ ) a delay electrode (F ₂ ) which is at a lower potential than the electrode (F _{3) is arranged.} 2. Electron optics according to claim 1, characterized in that the delay electrode has the shape of a screen with an axial opening with a diameter which is smaller than the diameter of the electrode (F ₃ ) lying at high potential. 3. Electron optics according to claim 2, characterized in that the opening of the delay electrode in the vicinity of the focusing point of the electrons between the high potential electrode (F ₃ ) and the collecting electrode (^ t ₄ ) is arranged, the collecting electrode of said opening opposite. 4. Electron optics according to claim 1, characterized in that the potential of the collecting electrode is equal to that of the delay electrode. 5. Electron optics according to one or more of claims 1 to 4, characterized in that that the collecting electrode is the first dynode of an electron multiplier system. .6. Electron optics according to one or more of Claims 2 to 5, characterized in that the delay electrode consists of a disk arranged perpendicular to the optics axis which has a sufficiently small opening to ensure that the field strength in the space between this disk and the collecting electrode is almost the same Is zero, and that the disc is provided with a cylindrical part (F ₂₀ ) which is arranged coaxially to the electrode (F ₃ ) which is at high potential and has a larger diameter than it. 7. Electron optics according to claim 6, characterized in that the cylindrical part (F ₂ a) of the delay electrode continues in the direction of the photocathode (C) with a part _{passing by the electrode (F 3} ). 8. Electron optics according to one or more of claims 1 to 7, characterized in that the delay electrode consists of two electrically isolated parts (F ₂₀ , F ₂₆ ) arranged one behind the other in the axial direction and that these separate parts are connected to different potentials.