DE1295614B - Storage screen for an image pick-up tube - Google Patents

Description

The invention relates to a storage screen for an image pickup tube consisting of one for photoelectrons permeable carrier electrode and a storage element located thereon, which is a Has porous layer of insulating material, which when bombarded with primary electrons in their cavities Secondary electrons generated. Such a storage screen is from the German patent 1,138,482 known.

can charge to a potential value that is greater than the potential of the first zero crossing for reflective Secondary emission of the storage material is. When reading the result on the storage screen This is because the charge pattern causes the slow electron beam to produce a reflective secondary emission of the storage screen. The secondary electrons generated by the slow electrons will be of which are at a relatively high potential-

One application of such a storage screen is attracted to the grid electrode, and the open surface äl i dth Ptt 1240 549 area of the storage screen is becoming increasingly positive

in the earlier own German patent 1240 549 described. The storage screen is built into an image pick-up tube and serves to collect it of the electron image generated by the photocathode. The stored electron image is stored in Read the form of a video signal by directing a slow electron beam onto the storage screen will. Due to the irregular movement of the slow electron beam, it is often necessary to to attach a grid electrode in close proximity to the surface of the storage screen to collect the electrons to bring in a substantially perpendicular to the screen surface path. To this For example, a voltage in the range from 200 to 400 volts can be applied to the grid electrode.

In operation, the conductive support electrode of the storage screen is at a potential which is approximately +15 volts with respect to the normally grounded cathode of the electron beam source. That of the photo surface of the storage screen becomes more and more positive because it is depleted of electrons. This process continues until the potential of the free surface is substantially equal to the potential of the grid electrode. However, a breakthrough in the storage layer usually occurs even beforehand. According to the teaching of the earlier patent, this breakthrough can be prevented by using a second grid between the grid electrode used to align the reading beam and the storage screen. A positive voltage is applied to this auxiliary grid which is below the first zero crossing of the secondary emission curve of the storage layer. This prevents the surface of the storage panel from assuming a potential that would lead to breakdown. _:

Because of the relatively low zero potential for most materials that can be used, this must be

cathode outgoing electron image of the primary electrode 30 potential of the auxiliary grid at a low value

tronen is accelerated to an energy (about 10 keV), which is sufficient to penetrate the carrier electrode and to enter the porous layer, thereby generating the energy of the primary electrons numerous slow secondary electrons is destroyed. Most of the slow secondary electrons are under the influence of the polarizing electric field through the cavities of the porous Layer removed and reach the carrier electrode. This creates a flow of secondary electrons, the the free surface of the porous layer makes more and more positive until the potential of this layer im "is essentially the same as that of the carrier electrode. This results in a charge pattern on the free one Surface resulting from the secondary electron conduction. Local charging is a function the number and energy of the primary electrons, the strength of the polarizing electric field and the capacity of the storage layer.

The storage screen has the further property that secondary electrons from its free surface emerge, which are accelerated by the external electric field of the grid electrode. This from the Secondary electrons emerging from the storage screen with regard to the carrier electrode of the storage screen being held. Under these circumstances, the electric field strength is between the support electrode and the auxiliary grid extremely small, so that almost no external secondary emission to the free Surface of the porous layer occurs and the storage screen almost exclusively through the inner Secondary emission is charged.

The situation is similar in the case of the storage screen according to US Pat. No. 3,128,406. Here it is a picture tube for direct playback, in which the image to be recorded is directed to an input screen and that with regard to Brightness and / or contrast enhanced image can be viewed on an output screen. In this tube there are several storage screens between the input screen and the output screen, to thereby multiply the electrons emitted by the input screen. An auxiliary grid

so can be applied to the emitted secondary electrons to accelerate and also to limit the potential to which the free surface of the porous layer of the storage screen can rise. To get the electrons from a storage screen on

increase the potential of the free surface above that of the next one behind it those of the carrier electrode on an equilibrium the successive storage screens with additional value operated between the potential of the grid electrode and increasing positive voltages that of the carrier electrode. This becoming equal. Without using an auxiliary grid weight potential creates an electric field at which the storage screen is easily destroyed, because constant-

porous layer, the one for initial polarization has opposite polarity and is usually large enough to breakthrough the porous To evoke layer. Even if the intensity or duration of the primary electron exposure is limited is to stop this process before the breakdown voltage of the porous layer is exceeded the storage screen is not safe from destruction if its free surface is dig a secondary emission takes place and the free surface of the porous layer consequently the Seeks to accept the potential of the neighboring electrode. The increase in potential is fundamentally due to this exclusively from the external secondary emission. In order to limit the increase in potential, a Auxiliary grid required. It must be kept at such a tension that between the auxiliary grid and the carrier electrode of the associated storage

cherschirms no longer have an electric field strength than about 150 volts / cm can occur.

The use of an auxiliary grid in a television camera tube or a direct image intensifier tube but is disadvantageous for various reasons. Not only do additional parts and their brackets need to be in the tube, but the required DC voltages must also be applied and kept at the right value. In addition, the image quality is increased by the auxiliary grid greatly deteriorated.

The object of the invention is accordingly to create a storage screen of the type mentioned at the beginning, which no longer requires an auxiliary grid in the image pickup tube in order to break through the storage element to prevent.

The solution to this problem is that the porous storage layer on their the side facing away from the carrier electrode is provided with a barrier layer, the density of which is greater than that of the storage layer and that of the escape of electrons from the free surface of the storage element prevented.

The denser cover layer increases the probability of the secondary electrons escaping decreased from the surface of the memory element.

In one embodiment of the invention, the porous storage layer and the barrier layer are as separate Layers formed from the same insulating material, while in another embodiment of the invention the density of the memory element steadily from a more porous area on the side of the carrier electrode to a denser area changes on the free surface of the storage element. In a third embodiment, there is Barrier layer made from a thin, discontinuous coating of electrically conductive material.

There has already been proposed a storage screen for image pick-up tubes which is based on a glass plate with a transparent, electrically conductive coating a porous layer of a photoconductive material and then a non-porous or semi-porous layer of photoconductive Material is located. By using such a cover layer made of photoconductive material the sensitivity of the storage screen is to be increased, especially if it is separate is prefabricated for subsequent installation in an image pickup tube. So this is a storing photocathode on which an image is immediately drawn around the photoconductive layer to make more or less conductive depending on their exposure. According to the invention, however, it is not a photoconductive memory element, but a memory element made of insulating material, in which by Secondary electrons are released from the bombardment of electrons released elsewhere. Also is The purpose of the invention is not an increase in sensitivity as in the older proposal, but rather the prevention of the escape of electrons from the free surface of the storage element.

The invention is explained below with reference to the drawing. Herein shows

F i g. 1 the schematic representation of an image pickup tube with the storage screen according to the invention,

Fig. 2 is a schematic representation of a direct image intensifier tube with the storage screen according to the invention and

F i g. 3 to 5 greatly enlarged sectional views of storage screens according to the invention, which are shown in FIGS Orders of Fig. 1 and 2 can be applied.

Fig. 1 shows the application of the invention to a television camera tube 40. It has a piston 42 made of insulating material with an end plate 44

ίο at one end. The face plate 44 is transparent to the desired type of light, so that the image of a Scene 62 can be designed onto a photocathode 45 located on the inside of the face plate 44. There is an electron gun at the other end of the tube 50, which emits a slow, pin-shaped electron beam onto the storage screen 24 can judge. The electron gun consists, for. B. from the cathode 52, the control grid 54 and the accelerating electrode 55. A field electrode 56 is coated on the inner surface of the piston 42 formed. Deflection coils 58 are used to deflect the scanning electron beam. Furthermore is a Concentration coil 60 is provided, which not only contains the electron beam emanating from the electron beam generator 50, but also the photoelectrons emitted by the photocathode 45 onto the storage screen 24 sets up. Between the storage screen 24 and the photocathode 45 there are several acceleration and concentration electrodes 46 and

48. There is also a thin wire mesh at a short distance from the free surface of the storage screen 24 64, which is held at a potential of 200 to 400 volts against the grounded cathode 52 will. The wire mesh serves to transfer the electrons emanating from the cathode 52 to the storage screen 24 to give a vertical direction of impact.

The storage screen 24 is fastened in the piston 42 with a support ring 66 which, for. B. from a nickel-iron-cobalt alloy (Trade name Kovar) can exist. The structure according to the invention of such a storage screen 24 is shown in FIG. 3 shown. It consists of an insulating support layer 27, for example made of aluminum oxide, on which a electrically conductive electrode layer 26, for example made of aluminum, is located. On the senior Electrode 26 is a cohesive porous layer 28 made of an insulating secondary emitting Material, e.g. B. Potassium Chloride. Instead, for example, barium fluoride, lithium fluoride, Magnesium nitride, magnesium oxide, cesium iodide and sodium chloride can be used. the Layer 28 is applied in a known manner, for. B. cast down as smoke. A second layer 30 is located is on the layer 28 and has the property that its density is greater than that of the layer 28.

The layer 30 typically consists of the same material as the layer 28, but can also other insulating materials can be used.

In the operation of the camera tube according to FIG a potential of about 15 volts is applied across the cathode 52 to the electrode 26 of the storage screen 24. The slow electrons emitted from the electron gun 50 are from the field electrode 56 and the concentration coil 60 are focused on the surface of the layer 30 of the storage screen 24. As a result, the surface of layer 30 essentially depletes the potential of the cathode 52, i.e. earth potential. Furthermore, a picture of the

Scene 62 designed on the photocathode 45, and the one where they are multiplied again. The one from the last Photoelectrons are emitted by the concentration storage screen 24 from electrons coil 60 in its distribution corresponding to the image of the potential source 35 with a voltage of about focused on the screen 24, whereby they by the 5 kV on the surface of the luminous layer 23 gerich acceleration electrodes 46 and 48 so strongly that a corresponding visual representation is accelerated that it gives the insulating base layer.

27 and the electrode 26 penetrate. To manufacture the storage screen 24 according to FIG

the primary electrons penetrate the layer 28 F i g. 1 to 3 can be done as follows and create in the cavities of this porous den. An aluminum plate is oxidized first, and Layer large amounts of slow secondary electronics then the metallic aluminum is etched away, like this nen. This changes the potential of the free that the insulating support layer 27 with a thickness Surface of the screen 24, mainly of about 1000 Angstroms remains. Then it will be as a result of the dissipation of the secondary electrons by aluminum on the support layer 27 in a thickness of layer 28 to electrode 26 and secondly evaporated about 1000 angstroms to the electrode as a result of a noticeable but far less significant 15 26 form. Now the base layer 27 with the Tending emission of secondary electrons from the electrode 26 brought into a recipient, wherein Layer 30, which is then absorbed by electrode 64, an inert gas (e.g. argon or nitrogen) men will be. This results in a recorded pressure of about 1 Torr. One menen image corresponding charge distribution on the specific amount (z. B. 25 mg) of a corresponding free surface of layer30, which is known in ao material (e.g. potassium chloride), is made in an ent-way can be scanned. In Fig. 1 is e.g. B. distance of about 7.5 cm evaporated to be on the one. Vidicon type reading device provided. Electrode 27 knock down. The evaporation

Fig. 2 shows as a further example a direct image is made at a temperature that only intensifier tube 10 according to the invention. It has a little above the melting point of potassium chloride a flask 12, the z. B. consists of glass. The 35 lies. The potassium chloride is completely evaporated, vacuum flask 12 has a cylindrical portion 13, whereupon it is found that the density of the layer 28 has a window 14 on one end face and about 1 to 10% of its density in the compact state window 16 on the other Face. On the inside is. In order to achieve the desired secondary emission from the surface of the window 14, there is an electrically conductive layer 18 inside the cavities of the layer 28, which layer 18 is to generate for the radiation to be absorbed, the surface density of the layer 28 should be permeable and, for example, consists of tin oxide about 25 to 200mg / cm ² , and their density. A photocathode 20, e.g. B. cesium anthnonide, should be between 10 and 30 microns, det is on the layer 18. The inner surface of the window. Next, according to the invention, a second sters 16 is with a transparent electrode 22 layer 30 of a suitable material, such as potassium chloride - (e.g. made of tin oxide), on which a 35 rid, vapor-deposited, namely in an inert atmospheric luminous layer 23 (e.g. made of zinc cadmium suffide), at a pressure of about 0.1 Torr or finds. The photocathode 20 becomes less of one scene 15. This ensures that the layer 30 is exposed and provides a corresponding electron density than the layer 28. To the same picture. The luminous layer 23 emits light at electrical potential on the surface of the layer 30 upon electron bombardment. 40 to bring a favorable value, without the own

Between the windows 14 and 16 there are shafts of the storage screen 24, which are unfavorable to impair, the number of which depends on the density of the layer 30 should have the desired enhancement of brightness or 5 to 25 mg / cm ² . Their thickness should depend roughly between contrasts. In certain cases a 0.5 and 5.0 micron range will suffice. The density of the layer 30 single storage electrode. The structure of the memory 45 is about 10 to 50% of the density in the compact screen24 is identical to that of Fig. 1 state,
and 3. As mentioned, the in the porous sponge

Between the photocathode 20 and the first layer 28 penetrating primary electrons an An-storage screen 24 is a voltage source number of slow free electrons. Part of this 31 that provides an accelerating voltage. Voltage sources 33 and 35 that speak secondary electrons can emerge from the storage screen 24 and thereby leave a positive impression on the first and second storage screen and charge on the surface of layer 30. In the course of time between the last storage screen 24 and that of time increases the surface charge Q is constantly on, luminescent screen 23 is provided. To concentrate the layers 28 and 30 as a very high speci-electron z. B. have a 55 fish resistance surrounding the tube. As a result, permanent magnet 34 develops. A potential difference F develops across layers 28

30 corresponding to the flatness F = f, where C

Voltage of about 4 kV in the direction of the first accelerates the total capacitance of layers 28 and 30 against storage screen 24. They penetrate the G ₀ above the conductive electrode 26. In the case of an insulating carrier layer 27 and the electrode 26, the potential difference V increases, the strength of the increases and releases numerous Se secondary emission currents in the porous layer 28 because the leakage secondary electrons are released. As pointed out in the above-mentioned US Pat. No. 3,128,406 formed in layer 28, the electron is increasing. Potentialdiffe exceeds the porosity of the layer 28 is an important factor for 65 rence V emen-specific value, then a discharge occurs achieve a high amplification. The electrons emerging from the first through the layers 28 and 30 because the electron storage screen 24 is free are electrons and / or conduction electrons are accelerated by the impact to the second storage screen 24, the primary electrons of high energy (5 to 10 kV)

Are available. This conduction effect limits the positive charge on the surface of the storage screen 24. A balance between the charge of the surface by secondary emission and the discharge by conduction effect results when the conduction current due to secondary emission equals the emission current due to secondary electrons, i.e. when the inner secondary emission is equal to the outer one Secondary emission will. This state corresponds to a voltage drop across layers 28 and 30, which can be referred to as the equilibrium voltage Vgl.

The equilibrium voltage V _{gi of} typical storage screens is now normally higher than the breakdown voltage of a porous layer. As a result, a breakthrough with corresponding destruction of the storage element already occurs before the equilibrium voltage defined above is reached. The equilibrium voltage can, however, be reduced if the external secondary emission current is reduced and / or the internal secondary emission current is increased. According to the invention, this can easily be achieved by making the surface layer 30 more dense than the actual secondary emission layer 28. In a denser area, the probability of the secondary electrons escaping from the surface is lower and, at the same time, the solid-state conduction in the layer is increased. This reduces the dependence of the equilibrium potential on the external electrical field, so that an auxiliary grid to limit the external electrical field is unnecessary.

It is evidently not necessary to use a memory element with two special layers as shown in FIG. 3 to use. Instead, a memory element 74 according to FIG. 4 can be provided, which has a single secondary emission layer 72, in which the area which adjoins the surface 78 of an electrode layer 76 has a density of approximately 1 to 10% of the density in the compact state, while those areas which the free surface 80 are adjacent have a density of about 30% of the density in the compact state. This structure not only reduces the amount of secondary electrons emerging from the free surface, but also increases the conduction current through the layer 72 resulting from the secondary emission, whereby the equilibrium voltage Vgi is further reduced.

The storage screen 74 may e.g. B. be produced as follows: an insulating carrier layer 77 of aluminum oxide and electrode 76 of aluminum are formed as above. Then the Secondary emission 72 evaporated from potassium chloride. The material is in the process of evaporation at a distance of about 7.5 cm from the electrode 76. The evaporation occurs again at a temperature which is only slightly above the melting point of the potassium chloride, in one Protective gas atmosphere, e.g. B. argon. Initially, the evaporation occurs at a pressure of about 2 to 3 torr carried out. During the evaporation process, the pressure is continuously reduced by pumping out, so that at the end of the evaporation process the residual pressure is about 0.1 torr or less. Consequently the initially deposited parts of the layer 72 are very porous, while the later vapor deposited Parts are getting denser.

Otherwise it is not necessary to use the outer layer of the storage element made of the same material as the actual secondary emission layer form. For example, it is in the case of a camera tube according to FIG. 1 may be desirable, another Material with low secondary emission gain and higher zero crossing potential than for to choose the material directly on the electrode. An example of this is shown in FIG. 5 shown. The storage screen 84 again has an insulating support layer 87 and one deposited thereon Electrode 90. A porous layer 88 is deposited thereon in the same manner as before. Then comes a conductive layer 86, e.g. B. aluminum, the density of which is about 100% of the density in compact state. It is evaporated in a high vacuum and results in a thin one, not coherent coating only a few molecules deep (i.e. 10 to 20 angstroms) on the particles of layer 88. The conductive material does not only cover the outer surface the layer 88, but also seeks to penetrate into the cavities of the same and to line their boundaries, thereby improving conduction through the layer. Such a storage screen 84 has possibly not only the property that the equilibrium potential is lower than the breakdown potential is, but the equilibrium potential can also be lower than the potential of the first zero crossing for reflective secondary emission.

As mentioned, the invention has the advantage that the equilibrium potential of the storage screen is below the breakdown potential without an external auxiliary grid having to be used. So results There is a considerable simplification in the manufacture and operation of image storage tubes.

Claims (9)

Patent claims: 1. Storage screen for image pick-up tubes, consisting of a photoelectron-permeable Carrier electrode and a storage element located thereon, which has a porous layer made of insulating material, which when bombarded with primary electrons in their cavities Secondary electrons generated, characterized in that the porous storage layer (28) is provided with a barrier layer (30) on its side facing away from the carrier electrode (26) is, the density of which is greater than that of the storage layer (28) and the exit of Prevents electrons from the free surface of the storage element. 2. Storage screen according to claim 1, characterized in that the density of the barrier layer (30) is between 10 and 50 ° / e that of the material in compact form. 3. Storage screen according to claim 1 or 2, characterized in that the surface density of the material in the barrier layer (30) is about 5 to 25 mg / cm ² and the thickness of the barrier layer is about 0.5 to 5.0 microns. 4. Storage screen according to claim 3, characterized in that the surface density of the material in the storage ^{layer (28) is about 25 to 200 mg / cm 2} and the thickness of the storage layer is about 10 to 30 microns. 5. Storage screen according to claim 1, characterized 909 521/335 characterized in that the density of the porous material (72) continuously increases in the barrier layer in the direction of the free surface (80). 6. Storage screen according to claim 5, characterized in that the density of the entire Storage element from the surface (78) adjoining the electrode (76) to the free surface (80) increases continuously. 7. Storage screen according to claim 6 or 7, characterized in that the density of the porous Material (72) in the storage area less than 10% and on the one adjacent to the electrode (76) Surface is down to 1%, so that 10 against in the direction of the free surface (80) beyond 10% of the density in compact form grows. 8. Storage screen according to claim 1, characterized in that the barrier layer consists of a incoherent layer (86) made of electrically conductive material, the density of which is roughly equal to the density of the material in compact form. 9. Storage screen according to claim 8, characterized in that the conductive layer is about 10 up to 20 Angstroms deep into the porous area (88) penetrates. 1 sheet of drawings