RU2661887C2 - Photoelectric multiplier - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: photoelectron multiplier (PEM) with a massive (reflecting) photocathode and a photoelectron focusing system is proposed. Inside vacuum cylinder 1 massive photocathode 3 with metal base 4 is installed facing input light flux. Photocathode covers only a part of the area of input window 2. Light flux passes through the free part of the input window to cathode chamber 7. Mirror reflector 6 a), which can be part of the photoelectron focusing system, redirects the light flux to emission surface 5 of massive photocathode 3. Focusing system 6 collects and focuses flux of photoelectrons emitted from the photocathode to the input of multiplication system 8. Geometry of specular reflector 6 a), dimensions of massive photocathode 3 and their relative position in cathode chamber 7 determine photomultiplier angle of view. Thermoelement 11 can be installed in the PEM to cool photocathode 3.

EFFECT: photoelectron multiplier (PEM) with a massive (reflecting) photocathode and a photoelectron focusing system is proposed.

1 cl, 2 dwg

Description

The invention relates to photomultiplier tubes (PMTs) and is a highly sensitive high-speed optical radiation receiver that converts the energy of this radiation into an electrical signal.

BACKGROUND

Two fundamentally different designs of vacuum photoelectronic multipliers (PMTs) are known: 1) PMTs with a translucent photocathode and 2) PMTs with a massive photocathode (reflective photocathode). As analogues of the invention taken: R374 (with a translucent photocathode) and R636-10 (with a massive photocathode) from the Hamamatsu Photonics catalog "PMT (photomultiplier tubes) CATALOG". The scope of such PMTs is a wide class of scientific and medical devices, control and navigation systems, astronomy and astrophysics, High Energy Physics HEP, especially in those cases where the highest light sensitivity and speed are required. Accordingly, a wide range of products manufactured by the above company and many other companies.

The appearance and circuit diagram of the internal analog device are shown in FIG. 1a) - PMT with a translucent photocathode (schematic longitudinal section), and FIG. 1b) - PMT with a massive photocathode (schematic cross section). Both analogues have a vacuum cylinder 1, usually in the form of a body of revolution, in particular a cylinder, with an entrance window 2 for receiving the light flux. For PMTs with a translucent photocathode FIG. 1a) the input window is located on the upper end of the container, and for a PMT with a massive photocathode, FIG. 1b) on the side wall. It can be seen that for PMTs with a translucent photocathode, the internal elements are located along the direction of the input light flux and, in general, symmetry is fulfilled, while for PMTs with a massive photocathode, the location of the internal elements does not coincide with the direction of the input light flux and there is no symmetry in the relative position of the photocathode and other elements. The angle of view is the direction from which the photomultiplier is able to receive the light flux, for both types of photomultiplier it is close to the hemisphere (2π), and in some special applications (High Energy Physics) a wider angle of view is also desirable (RF patent 2503082).

The principle of the device and operation common to both types of PMTs (Fig. 1a) and b) is as follows: the energy of the light flux entering through the input window 2 and incident on the emission layer 5 of the photocathode 3 causes the emission of photoelectrons into the vacuum volume of the cylinder 1. Voltages applied from external circuits to the photocathode 3 and other electrodes create electrostatic fields, and under the influence of these fields the photoelectrons are sent to the input of the secondary electron multiplication system 8. A multiplied stream of electrons arrives at the anode (collector) 9 and is output through the output (s) 10 in the form of an electrical signal to the outside. Thus, the PMT works as a photocell using an external photoelectric effect and, at the same time, as a multiplying amplifier, operating on the principle of secondary electron emission.

The significant difference between the PMTs of FIG. 1a) in that the translucent photocathode 3 receives the light flux on the one hand, and emits photoelectrons on the other hand along the way with the direction of the light flux. This allows you to organize the effective focusing of photoelectrons using electrodes 6 (electrostatic lenses) located in the PMT bulb immediately behind the photocathode 3. This part of the internal volume and PMT device is called the “cathode chamber” 7 and is usually clearly visible from the outside. The difference between the PMTs Fig. 1b) in that its photocathode (“massive photocathode”), having an opaque metal base 4, emits photoelectrons towards the input light flux. Therefore, all known PMTs with a massive photocathode have no photoelectron focusing system as such.

The disadvantages of a photomultiplier with a semitransparent photocathode are the disadvantages of the photocathode itself, which are further discussed in terms of comparing two types of photocathodes. PMT developers are forced to put up with these shortcomings of the translucent photocathode for the sake of efficient focusing of photoelectrons. The bottom line is that the cathode chamber 7, with the photocathode 3 and focusing electrodes 6, is a projection system of the electronic image of the photocathode, similar to that used in electron-optical converters. This property of the focusing system makes it possible to use not only dynodic secondary electron multiplication systems, but also channel ones (PerkinElmer, Channel Photomultiplier), as well as multichannel and microchannel plates (SU 1295953, РМТ Hamamatsu R3809U-50) and create PMTs with various properties for various applications.

A semitransparent photocathode deposited in the form of a thin layer of a semiconductor on the inner surface of the PMT input window exhibits during operation properties that, in terms of the claimed invention, can be considered as disadvantages. The loss of part of the light energy associated with the through passage of a significant part of the light flux through the emission layer is not the most dramatic loss. More significantly, the light begins to excite electrons at the boundary with the solid - the glass of the input window - and in order to get out into the vacuum, the photoelectrons have to overcome the entire thickness of the emission layer. Losses along the way are inevitable. Another drawback of the translucent photocathode, also from its semiconductor nature, is its low conductivity. Replacing the losses of electrons emitted from various parts of the photocathode comes from external power circuits connected to the thin emission layer of the translucent cathode along the periphery of its area, and the resistance along the thin emission layer is very high. The emerging defects in the potential topography of the emission layer are aligned slowly, which reduces the speed and degrades the noise characteristics of the PMT.

The massive photocathode is free from the above disadvantages and has several advantages. Electron excitation by light incident on the emission layer of the photocathode begins at the interface with the vacuum. The part of the light that has passed through the emission layer is reflected from the metal base, once again passes through the emission layer. Both of these factors significantly increase the efficiency of photoelectron emission compared to a translucent photocathode. The low conductivity of the emission layer does not affect the operation of the massive photocathode in any way, since the loss of emitted photoelectrons is compensated from the metal base not across the thin emission layer, but across.

Details of the device and the operation of the PMT in general and photocathodes, in particular, are considered in many educational, scientific, methodological and information sources. In patents (for example, patent US 8212457 B2) relating to improving the composition and properties of the emission layers of photocathodes, the advantage of a massive photocathode is certainly noted, and in all respects, both energy and technological parameters.

The remarks related to the influence of the conductivity of the emission layer on the operation of two types of photocathodes can be summarized as follows: a translucent photocathode is critical for light loads on the photocathode, and a massive photocathode is not critical and can withstand any integral and pulsed light load.

To date, no constructive solution has been found for using a massive photocathode in conjunction with a focusing system. However, in view of the crucial role played by the photoelectron focusing system in the operation of the PMT, both in the analogs discussed above and in this invention, a PMT with a translucent photocathode was taken as a prototype of the invention.

It is necessary to additionally note the features of the prototype, which in terms of this invention are considered as disadvantages. It was mentioned above that the angle of view of both types of PMTs is close to the hemisphere (2π), and sometimes a wider one is also desirable. This property is usefully used in many applications, especially in the field of High Energy Physics, when the direction from which the light pulse “flies” to the PMT is unpredictable. However, many other applications of PMTs in scientific and medical devices, in control and navigation systems require aperture matching with an object - a radiation source, which in this case is performed using additional optical-mechanical means. This leads to loss of light energy and to the complexity of the equipment. In terms of this invention, the lack of the prototype the ability to set the optical orientation of the PMT due to the structural parameters of the elements is considered as a disadvantage.

The disadvantage of the prototype is also the difficulty of cooling the photocathode in order to reduce PMT noise. A known photocathode cooling system (Patent EP 2615621 A1), as well as a series of modules C10372b 3-73 from Hamamatsu Photonics. These systems are units external to the PMT. The disadvantage of these and other similar systems is cumbersomeness and complexity. Another system is known, (USSR Patent 1091253), with a very low power consumption for cooling, but requiring a significant complication of production technology. In all cases, cooling the semitransparent photocathode, reducing the PMT noise, entails a deterioration in its frequency characteristics due to a decrease in the conductivity of the emission layer.

SUMMARY OF THE INVENTION

The aim of the invention is to create a photomultiplier, which combines the use of a massive photocathode, with its advantages, and a photoelectron focusing system, which will make it possible to produce a photomultiplier with a set of parameters that are absent from analogues, including with certain angles of view set during production, as well as with effective cooling the emission layer of the photocathode through a metal substrate at extremely low energy costs for this cooling.

This goal is achieved by the fact that in accordance with the invention, a massive photocathode in the form of a flat part separate from the glass from the bulb is used in a photoelectronic multiplier, mainly similar to the prototype in geometry and functions. This photocathode according to the invention is installed in close proximity or close to the inner surface of the input window so that its base is facing towards the input light flux. The area of the photocathode is selected smaller or substantially smaller than the area of the input window. The light flux can pass through the free part of the input window into the cathode chamber, where the photoelectron focusing system is located. A mirror reflector is installed on the side of the cathode chamber opposite the input window, designed to change the direction of the input light flux and send it to the emission layer of the photocathode. This reflector can be an independent functional and constructive element or be part of an electrode system for focusing photoelectrons. The ratio of the sizes of the photocathode and the reflector, their shape and relative position in the cathode chamber determine the angle of view of the PMT. The photocathode can be made mosaic, in the form of several separate emission sections on a common metal base to give the PMT sensitivity in angular direction, realized using a multi-channel multiplication system.

Additionally, in a special design, the photocathode cooling system can be included in the composition of the internal PMT components. The system consists of a thermocouple additionally included in the design of the PMT, a metal base of the photocathode in contact with the “cold” side of the thermocouple, and a glass of the PMT cylinder in contact with the “hot” side of the thermocouple.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 - explanations of the essential features of analogues related to this invention.

FIG. 2 is a general diagram of a PMT with a massive photocathode, a focusing system, and a reflector in the cathode chamber. This diagram shows for simplicity that the reflector is made as part of the focusing electrode. This particular case does not limit the possibility of a separate design of the reflector as an independent part.

MODES FOR CARRYING OUT THE INVENTION

The embodiments of the invention described below are intended only to explain the essential differences of the invention, but not to limit the invention to these specific examples.

The feasibility of the invention is based on the opportunities developed in the field of design and production of PMTs. The inclusion of a substantially new element - a reflector - presents no technical difficulties. The inclusion of the cooling function of the photocathode is based on promising cooling designs for the chip industry (RMT LLC) and on the specifics of the heat balance of the cooled element in vacuum.

The invention is illustrated by the diagram of FIG. 2. According to the invention, the massive photocathode 3 is mounted with an opaque base 4 facing the input light flux and blocks part of the light flux incident on the input window 2 from the outside. These light flux losses in the case of the photocathode are relatively large, as in FIG. 2), are approximately 1/4, which is less than the losses associated with the through transmission of light by a translucent photocathode. These light energy losses in the PMTs of the present invention are compensated by the higher quantum efficiency of the massive photocathode. Reflector 6 a) in this diagram is shown as part of the electrode focusing system 6 a) and 6 b). This design is feasible, since metal electrodes, or part of their surface, can be made mirror. The reflector can be made as an independent element, separate from the electrodes of the photoelectron focusing system. Reflector 6 a) redirects the luminous flux that has passed into the cathode chamber 7 towards the photocathode 3, and depending on the specific application, various optical focusing options are possible.

The symmetric arrangement of the components in the cathode chamber of the PMT according to this invention and the small area of the photocathode 3 in comparison with the full size of the input window 2 creates the advantageous conditions for focusing the photoelectrons to the input of the secondary electron multiplication system 8. The photoelectron focusing system can be configured both for the sharp focusing of photoelectrons at the input of the dynode multiplication system, and for the projection of the electronic image of the photocathode onto the input surface of the multi-channel multiplication system, equipped in this case with a multi-anode collector and the corresponding number of outputs. (Similar technical solutions for secondary electron multiplication systems are known, are used in PMTs (patent SU 12959534, manufactured by Hamamatsu Photonics R3809U-50) and are not distinguishing features of this invention.)

The use of the massive photocathode according to this invention is possible in conjunction with the use of a translucent photocathode deposited on the inner surface of the input window.

Additionally, a photocathode cooling system may be included in the PMT of the present invention to reduce its thermal noise. In a particular embodiment, the cooling system includes a cooling element 11, the cold side of which has thermal contact with the base 4 of the photocathode 3, and the hot side has thermal contact with the walls of the cylinder. The heat influx to the photocathode in contact with the cold side of the element 11 is negligible, since this inflow is possible only due to radiant energy and through the cathode supply wire, and the sufficient thickness of this wire is very small. Even with a low efficiency of a cooling element, for example, a Peltier element (such as those developed by RMT LLC for cooling microchips and photodiodes), the glass of a PMT cylinder will be able to remove heat from the “hot” side of the element. In this case, the formation of condensate on the surface of the input window is excluded due to its heating from element 11, and for the PMT operation, heating the glass of the input window by units of degrees with respect to the ambient temperature does not matter. The formation of condensate on the emission surface of the cooled photocathode is excluded, since it is entirely in vacuum.

Claims (2)

1. A photomultiplier tube having a vacuum bulb with an input window for receiving the light flux and a photocathode located inside the bulb, a photoelectron focusing system, the electrodes of which together with the photocathode occupy a part called the “cathode chamber” in the bulb volume, and a secondary electron multiplication system, characterized in that the photocathode is a part separate from the glass of the bulb, is a massive photocathode, and the photocathode area is set smaller than the input window area, and this photocathode is towards the light flux at a distance from the inner surface of the input window or close to it, and on the side of the cathode chamber opposite to the input window, a mirror reflector is installed to reflect the input light flux onto the emission layer of the photocathode, and this mirror reflector can be made as part of the electrode system focusing photoelectrons. 2. The photomultiplier according to claim 1, characterized in that the cathode chamber is additionally equipped with a thermocouple, the cold side of which has mechanical and thermal contact with the base of the photocathode, and the hot side of the thermocouple has mechanical and thermal contact with the glass of the balloon.

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