SU1054847A1 - Photo-cell - Google Patents

Abstract

1. A PHOTOCELL with an asymmetrical stripline containing a photocathode and an anode located in a vacuum envelope with an entrance window, characterized in that, in order to improve the temporal resolution and also to simplify the connection of the photocell to the load, the asymmetrical stripline is formed by two parallel plates inserted between photocathode and anode and having areas that are transparent to the electron and light fluxes, with one of the plates facing the photocathode grounded, and the other has a galvanic connection with the anode, the maximum capacity of which relative to the grounded plate of the strip line should not exceed that determined by the formula 1-t С fnR. 2Л f, В вr where R 6 is the wave resistance of the strip line; fgr upper bound frequency; S tp is the ratio of the voltage at the load at the upper cut-off frequency to the voltage at the load within the uniform part of the frequency response. 2. A photocell according to claim 1, characterized in that the anode is made in the form of a body with a cavity open on the side of the cathode. SP jiib 00 4: -k |

Description

The invention relates to electronic equipment, in particular, to photo cells with an external photoelectric effect, and can be widely used in the design of fast-acting photocells for the study of fast processes, such as high-current high-speed photo cells with an external photoelectric effect, are widely used in the study of fast processes up to 10 c, and coaxial or strip photocells are used for this purpose.

A coaxial photocell is known (a photocell with a biplanar electrode system) containing a photocathode located at the end of the internal mud of a coaxial line or a dis, connected to it, and a mesh, anode located parallel to the photocathode at a small distance from it and connected to the outer sheath of the coaxial line Cl J.

The improvement in the temporal resolution of photocells of both coaxial and stripline meets the limitations associated with the effect of transit time and reactance of structural elements. In the case of a coaxial photocell, even with a cathode diameter of 3 mm and an anodcathode distance of 0.4 mm, the capacitance value of the anode cathode is 0.15 pF and its shunting effect does not allow obtaining a transient response of the beam D1e, the transit time at a voltage of 2000 V is 3 10 with. The possibility of further increase. The time resolution in this design is limited, since it is necessary to increase the voltage or decrease the anode-cathode distance in order to reduce the transit time. As experience with instruments shows, it is not possible to obtain a working voltage with a gap of 0.4 mm greater than 2000 V. If, however, the anode-cathode distance is reduced, then due to an increase in capacitance, an even smaller photocathode area must be used, which leads to a decrease in linear currents and prevents high sensitivity photocathodes. At the same time, the use of photocells in the paths of registration of light pulses with a duration of tens of picoseconds requires, along with a sufficient time resolution, also significant output currents. Such a combination of the required characteristics causes a major difficulty in the design.

Also known is a photocell with an asymmetrical strip line, which is located in the vacuum envelope with an entrance window of the asymmetrical strip line with a massive photocathode on

one of the line plates and the mesh anode forming the other plate. Strip line plates are connected to a coaxial connector by means of a smooth, matched transition. But even in such a design, it is difficult to obtain a temporal resolution better than 3.10 s, since an acceptable transit time is provided at an anode-cathode distance of 1 mm only at 10,000 V spacing, and such a working voltage cannot be obtained in instruments (length the cathode should not exceed 9 mm). If, however, the transit time is reduced by decreasing the distance of the anode cathode, then the transverse size of the anode of the stripline must be reduced, since the characteristic impedance must be kept constant. At an anode-cathode distance of 0.5 mm, the anode width is l / 3 mm at 2g 50 ohms, and the transit time at a research institute of 2000 V is 45 PS. At an anode-cathode distance of 0.3 mm, the width of the anode is l / 1.8 mm, and the time of flight of a voltage of 1500 V is 25 in C2.

Such a design is difficult to implement, since it is necessary to provide a 0.3 mm gap between the electrodes over a period of 20-50 mm. Meanwhile, the time resolution of the considered construction is no better than 39 ps.

The aim of the invention is to increase the temporal resolution and simplify the scheme of shading the photocell to the load.

To achieve this goal, in a photocell with an asymmetric 1LOK line containing a photocathode and anode, an asymmetrical strip line is formed by two parallel plates inserted between the photocathode and anode and having portions that are transparent to the electron and light streams, one of the plates facing the photocathode, which are transparent to the electron and light fluxes, one of the plates facing the photocathode, which are transparent to the electron and light fluxes, one of the plates facing the photocathode and the cathodes are transparent to the electron and light fluxes. is grounded, and the other has galvanic connection with the anode, the maximum value of which relative to the grounded plate of the strip line should not exceed that determined by the formula

1st

WITH

I

vg

where Re is the wave resistance of the strip line;

fgr upper bound frequency; m is the ratio of the voltage at the load at the upper limit frequency to the voltage at the load within the uniform part of the frequency response.

B. The anode proposed by the photocell can be made in the form of a body, with a cavity open with. cathode sides

FIG. 1-3 shows variants of the construction of a photocell made according to the invention.

The photocell (Fig. 1) contains a photocathode 1, a strip line, the wide plate 2 of which is grounded, and the narrow plate 5 is galvanically connected to the anode 4, made in the form of a cylinder, the end of which, facing the entrance window, is closed and provided with a light-transparent section flow and electrically conductive coaxial vacuum tight pins 5 and 6, yoke 7 for fastening the anode -. A photocell structure with a developed photocathode surface (Fig. 2) into which a focusing acceleration system is inserted, formed with a common center of curvature and representing parts of spherical surfaces by a photocathode 1 and a grid electrode 8, and to simplify the design, this electrode is connected to a grounded plate 2 strip lines. The anode can be, for example, a cylinder. The anode 4 is made (Fig. C) in the form of a cylindrical glass, closed from the end facing the entrance window. Ioloskovaya line can be enabled on - pass (Fig. 1 and 2) or open at one end (Fig.3)

The device works as follows.

The electrons emitted from the photocathode 1 are accelerated by a voltage difference between the photocathode 1 and the stripline 2 (Fig. 1), and for the structures of Figs. 2 and 3, they are also focused into a converging electron beam. The electrons fly into the space between the electrodes of the strip line at a speed determined by the voltage difference between the photocathode 1 and the strip line. When electrons pass through this space, induced current arises, which is transmitted through connectors 5 to the load. There is no need to apply voltage between the plates of a strip line, since electrons are accelerated by a voltage from one to several thousand h volts, therefore, a coaxial coupling capacitor is not needed. Passed through the strip line, the electron flow enters the internal cavity of the anode 4 of a particular structure and is deposited on its surface. The absence of a coaxial capacitor in the signal circuit greatly simplifies the use of a photocell and improves its temporal resolution in the subnanosecond region, since the design and manufacture of coaxial capacitors for this region presents a serious and still not sufficiently solved problem. Such a construction allows to achieve

higher temporal resolution than poloskov (and coaxial), primarily due to the fact that, unlike a strip photocell, electrons fly over a strip

line with speed V - Y (I

I m

about i

-nineteen

where e is the electron charge 1, b; t is the electron mass of 9.1, kg; UQ accelerating voltage. B, and the flight time is thus

where d is the distance

between the strip line plates. For famous strip photoelean sZ

2m -t

ment t

-5- 7G i.e. 2 times more. In addition, the time of flight.

The proposed design can be made even smaller by using higher voltages than in a strip photocell, with the same distance between the plates of the strip line. So, for example, if in a strip cell with a 5 v3ope anode – cathode 1 mm turns out to be a permissible voltage (JQ, then at a distance of 2-3 mm between the photocathode and the accelerating electrode in the proposed design we obtain a much higher operating voltage, which is additionally reduces the time of flight of electrons between the plates of the strip line. The best temporal resolution allows to obtain the structure shown in Fig. 1. In a real photocell structure with a gap between the electrodes 2 and 3 of the strip line 0.8 mm and the distance between The photocathode 1 and the grounded electrode 2 of the strip line 2 lyw have a working voltage of 8000 V and, accordingly, a flight time of 15 ps, and the anode capacitance relative to the grounded plate does not exceed 0.03 pF, which provides the upper

5 cutoff frequency 35 Hz, i.e. the corresponding time resolution component is 10 ps. At the same time, the scatter in the electron transit time from the photocathode to the stripline, caused by the scatter of the initial photoelectron velocities, does not exceed 2-. Consequently, the temporal resolution of the proposed design is much better than that of a glossy photocell with the same gap in the strip line (ps). The diameter of the photocathode with the specified dimensions was chosen to be 3 mm, since the width of the narrow electrode of the stripline with a gap of 0.8 mm and ohms is 4 mm. As for the diameter of the holes in the ends of the anodes of various configurations (Figs. 1 and 2), they must be such that the walls of the anode do not obscure the photocathode from the wall.

It is quite enough to choose this diameter of at least 1.1 times the diameter of the photocathode. An excessive increase in this diameter over 1.21, 3 of the diameter of the photocathode worsens the interception of electrons and therefore leads to the need to increase the length of the anode, which for the structures of FIG. 2 must be 4-5 rae larger than the focal length of the accelerating focusing system. For the design.

shown in FIG. 3, the diameter of the anode is chosen from the conditions of effective interception of electrons and preventing shading of a considerable photocathode AREA, which is achieved by choosing its size in the range 0.1-0.4 of the photocathode diameter, while the anode obscures 1-15% of the photocathode area ...

The ejection allows for an improvement in the temporal resolution of the instrument.

Claims (2)

1. A photocell with an asymmetric strip line, containing a photocathode and anode located in a vacuum shell with an input window, characterized in that, in order to increase the time resolution, as well as to simplify the connection of the photocell to the load, an asymmetric strip line about ^C '2L f _vg wave strip line resistance; upper cutoff frequency; the ratio of the voltage at the load at the upper cutoff frequency to the voltage at the load within a uniform portion of the frequency response. 2. The photocell according to claim 1, wherein the anode is made in the form of a body with a cavity open from the side of the cathode. where ^ br m X & cl Q0 * 4