DE1054608B - Photocell with a photocathode of a very small surface - Google Patents

Description

Secondary multiplier photocells are generally used with large photocathodes equipped to capture as large a fraction as possible of that transmitted by a diaphragm or lens Absorbing luminous flux and converting it into a stream of electrons. For a special purpose However, a photocell with a secondary electron multiplier was also specified, which is a very small one Has cathode and therefore only has a correspondingly small dark current. Such cells offer the possibility of amplifying very low luminous fluxes with a relatively low level of noise. The well-known However, the cell has some drawbacks which the present invention aims to overcome.

In the known tube, the transparent photocathode is on a transparent pivotable support arranged, which is folded away from the multiplier part during the production of the photo layer, so - That a relatively large-area cathode is produced with the good homogeneity guaranteed by this can be. To operate the arrangement, the photocathode is against a very narrow entrance aperture Folded in front of the multiplier so that most of the photo layer is covered by the diaphragm and at the same time is in contact with the aperture. Such a tube has a low dark current, but it can because of the large distance between the photocathode required by the flap mechanism and light entry window only relatively narrow bundles of rays, as z. B. for astronomical measurements occur, record. It is also not possible to get the photocathode closer to a light source than corresponds to the distance between the window and the entrance panel. With a diffuse heater therefore a large part of the amount of light emitted is lost. There is another disadvantage nor that due to the position of the photocathode to the entrance of the secondary electron multiplier the secondary electron current is essentially restricted to the center of the dynodes, which results in larger photocurrents result in a distortion of the output signal due to space charge saturation can.

The above-mentioned disadvantages are avoided in the construction of a photocell with a miniature cathode carried out according to the invention. According to the invention, a photocell with a photocathode has a very small surface area, which is created by the fact that most of a large-area photocathode, apart from a small, geometrically precisely defined area inside the tube, is covered in such a way that only electrons from the exposed area are covered The collecting electrode, the photocathode on the light entry window of the photocell, and the photocell with a photocathode with a very small surface area

Applicant: Television G. m. B. H., Darmstadt, Am Alten Bahnhof 6

Dr. Werner Tretner, Darmstadt,
has been named as the inventor

The same is covered by a vapor deposition layer.

The evaporation source can advantageously be used for re-evaporation of the metal covering the photocathode can be used for the base metal of the photocathode. The vaporized layer will be in such thickness applied that the unshielded part of the photocathode shows practically no photo effect or becomes impermeable.

An, embodiment of the invention is explained in more detail with reference to the drawings, of which
Fig. 1 shows a photocell in longitudinal section and

Fig. 2 shows the photocathode in plan. In Fig. 1, 1 is a tubular piston in which a secondary amplifier 2 with an inlet aperture 3, an evaporation furnace 4 and a photocathode 5 is mounted. Also located in the tube is a disc 6, which is suspended in a hinge-like manner from a wire and which is used to produce the desired micro-cathode. According to the method according to the invention, a layer of antimony is first deposited on the photocathode surface by the evaporation furnace 4, a ring provided with antimony beads and heated with high frequency. Then cesium is introduced into the tube and the photocathode is formed to the highest sensitivity using the usual measuring devices and tools.

The disk 6 is then brought out of its angled back position 6 'in front of the photocathode 5 by knocking. By heating the evaporation furnace 4 again, antimony is again deposited on the photocathode until the cathode surface, apart from the covered part, has become practically opaque. After melting, the cover 6 is folded back into the starting position, so that the photocell with the smallest cathode is ready for use. the

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Claims (3)

Phatocathode then has the appearance of FIG. 2. There is a recessed area of z. B. 3 mm diameter, which is still photosensitive. The photocell produced in this way has a very small dark current, which is particularly suitable for measuring the radioactive radiation of very small samples. For example, with a photocell moved in a grid over a surface, an area contaminated with radioactive traces can be scanned and the location of the traces determined. Patent claims: 1. Photocell with a photocathode very small surface, which results from the fact that the Largest part of a large-area photocathode except for a small, geometrically precisely defined Surface inside the tube is covered so that only electrons · from the exposed surface to the Collecting electrode, thereby indicates that the photocathode is located on the light entry window of the photocell and that the cover is made by a vapor deposition layer. 2. A method for producing a photocell according to claim 1, characterized in that after the completion of the large-area cathode a screen covering the intended area for the actual cathode folded in front of the photocathode ίο and then the remaining part of the photocathode with a little photo-sensitive material, for . B. the base metal of the photo layer, is evaporated, whereupon the screen is removed again. 3. The method according to claim 2, characterized in that the post-evaporated layer is practically is opaque. Considered publications:
German Patent No. 926 804;
U.S. Patent No. 2,555,423. 1 sheet of drawings & 809 7297415 3.5 »