US3099764A

US3099764A - Photomultiplier tube

Info

Publication number: US3099764A
Application number: US27066A
Authority: US
Inventors: Arthur F Mcdonie; Robert M Matheson
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1960-05-05
Filing date: 1960-05-05
Publication date: 1963-07-30
Anticipated expiration: 1980-07-30
Also published as: GB975909A; DE1220050B; NL264400A

Description

y 1963 A. F. MCDONIE ETAL 3,099,754

PHOTOMULTIPLIER TUBE Filed May 5, 1960 INVENTORJ' Arthur F. MDonie Robert M. Matheson United States Patent 3,099,764 I PHOTOMULTIPLIER TUBE Arthur F. McDonie and Robert M. Matheson, Lancaster,

Pa., assignors to Radio Corporation of America, a corporation of Delaware Filed May 5, 1960-, Ser. No. 27,066 1 Claim. (Cl. 313-95) This invention relates to photoemissive devices and has special reference to improvements in the electron multiplier section of a high vacuum type photomultiplier tube.

Photomultiplier tubes, having a large area photoemissive cathode surface that is exposed to a light source, have been used as scintillation counters. In the operation of such a tube, when radiations, such as those from radioactive matreials, are caused to fall upon a phosphor and activate the phosphor to luminescence, the light from the phosphor causes photoemission of electrons from the photocathode of the tube. The photoelectrons are then directed, by an electrostatic electron-optical field, to an electron multiplier structure within the tube. The electron multiplier structure usually comprises a staggered array of electrodes each adapted to produce secondary electron emission when subjected to electron bombardment. Amplification of the current represented by the photoelectrons from the cathode is thus achieved by secondary electron emission phenomena.

The photomultiplier tubes known in the prior art have been limited in the magnitude of the output current that can be obtained by the magnitude of the potentials which could be applied to the various stages of electron multiplication. The reason for the limitation on the magnitude of potentials that could be applied to the electron multiplying electrodes is that certain dark currents occur within the tube, and the magnitude of these dark currents generally increases as the magnitude of the potential that is applied to the electron multiplier electrodes, or dynodes, is increased. The specific causes of these regenerative dark currents are not completely understood. One of the known origins of these dark currents is that light is generated by electrons strikinga dynode surface 'at a high velocity. The light then couples back to the photocathode which produces a spurious photoemission. Other reasons for these dark currents include ion feedback from the dynode structure to the photocathode, meta stable atom or molecule feed-back from the dynode to the photocathode, and X-ray feed-back from the dynode structure to the cathode. The problem of ion feed-back occurs generally in the area between the photocathode and the first dynode since the potentials are normally arranged so that the ions are collected by a dynode structure if the ion is originated in any other area of the device. Also, ions generated in the anode region of the multiplier sometimes strike the glass bulb with sufficient energy to cause fluorescence or luminescence of the glass bulb. The light so generated may feed back to the photocathode to cause spurious photoemission. The meta stable atom or molecule, is a particle having a zero electrical charge, can cause photoemission from the photocathode if it originates in any area of the tube since it is not attracted to and absorbed by a dynode structure. Also, radiations such as X-rays can cause spurious photoemission from the photo cathode when they originate in any area of the tube since these radiations are not attracted to particular dynodes.

In the prior art many attempts have been made to decrease the regenerative dark current that has been caused by one or combinations of the above mentioned phenomena, so that higher voltages may be applied to the dynodes and thus, higher output signals obtained. These attempts have been unsuccessful in that the magnitude of the dynode potentials can not be substantially increased by using any of these known devices.

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It is therefore an object of this invention to provide an improved photomultiplier tube.

It is a further object of this invention to provide a new and novel dynode sub-assembly for use in a photomultiplier tube.

These and other objects are accomplished in accordance with this invention by providing an imperforate electrically conductive shield closely spaced around the electron multiplier structure.

The invention is described in greater detail in connection with the accompanying single sheet of drawing wherein:

The single FIGURE is a sectional view of a photomultiplier tube in accordance with this invention.

The tube shown in FIG. 1 comprises an evacuated envelope 10 closed at one end thereof with a wall section or face plate 12 which extends substantially transversely of the tube axis. On the inner surface of the transparent face plate 12, there is provided a photoemiss-ive photocathode 14 in the form of an electrically continuous photoemissive surface. The photoemissive surface may ,be formed of any of the known materials such as, for example, manganeseantimony-oxygen-cesium or a multialkali photoemissive film.

The tube is provided with a cup-shaped dynode shield electrode 16 which is spaced from the photosurface 1'4 and which terminates in a base member having an aperture 18 therein. Spaced inside the dynode shield 16 is a focus electrode 20 which is a hollow tubular member that is spaced below the top of the dynode shield 16 and which includes an aperture 22 that is spaced around an inner ring 24. The ring 24 is connected to the dynode shield 16. The inner ring 24 serves to protect the electrode elements in the tube from contamination by the material which is evaporated from the dynode side of the accelerating electrode or dynode shield 16, during a step in the manufacture of the photocathode 14.

A metallic wall coating 26, which may, for example, be a relatively thin film or layer of aluminum, is formed on the inner surface of the tube envelope 10, in the area above the bottom of the dynode shield 16 and by any well known aluminizing techniques. The metallic wall coating 26 extends from the photocathode axially down the tube wall and thus forms a cylindrical focusing electrode coaxial with the dynode shield 16. Electrical contact (not shown) is made by means of a lead which connects the photocathode through the wall coating 26 to one of the lead in pins 28 that is sealed through the base of the tube envelope. The coating 26 not only functions as a part of the electron optical field which directs electrons from the photocathode 14 into the photomultiplier structure, but also functions so as to prevent the collection of charges on the envelope wall adjacent to the photocathode 14.

The photomultiplier structure 30 is of the straight throu-g type and includes a plurality of elongated electron multiplier electrodes or dynodes 31 through 40. The dynodes 31 through 40 may be made of any conventional material having a high secondary electron emission, such as, for example, .a copper beryllium alloy. The dynodes 31 through 40 are disposed in a staggered array on opposite sides of a plane so that photoelectrons from the photocathode 14 impinge upon the first dynode 31 and initiate secondary electron emission therefrom having a ratio that is greater than unity. This secondary electron emission is accelerated and directed by fixed electrostatic fields along curved paths to successive dynodes 31 through 40. The accelerating fields are formed by biasing each successive dynode at a predetermined potential which is positive, for example, 300 volts positive, with respect to the dynode previous to it. Positioned adjacent to the last dynode 40 is a mesh type anode 42 from which output signals are obtained.

In accordance with this invention the dynodes 31 through 40, as Well as the anode 42, are enclosed by a substantially imperforate dynode cage shield 44 which extends from the bottom of the [dynode shield 16 around all of the dynodes. The spacing between the'dynode cage shield and dynode structure is as close as practical considering the voltage applied. A spacing of 0.050 is normally used. Tubes having spacings Within the approximate range of 0.030 to 0.50 inch have been used. The dynode cage shield 44 is closely spaced adjacent to each of the dynodes so that an electrical potential is provided in an area that is closely adjacent to the dynodes and the potential is that of the annular shield 16'. The dynode cage shield 44 may be made of any electrically conductive material such as Nichrome. The dynodes 31 through 40, the anode 42, land the dynode cage shield 44, are supported between electrically insulating dynode spacer sheets 46. One of the dynode spacer sheets 46 has been removed for simplicity of illustration. nection is made to the various dynodes by the support wires passing through the dynode spacer sheets to connect to a particular dynode. These wires extend to the lead-in pins 28 passing through the base of the envelope '10.

During operation of the device 10, potentials are applied to the various structures such as, for example, those shown in the following chart:

It has been found that, due to the presence of the dynode cage shield 44, it has been possible to operate ten multiplier stage tubes with up to 4000 volts D.C. applied between the cathode and anode and to increase the gain of the ten stage photomultiplier as much as ten times that which was possible using the prior art tube structures. In other words, the gain of the tube may be increased as much as ten times that previously known by increasing the dynode potentials by about two times that possible in the prior art structures before regenerative Electrode con- 4 feed-back to the photocathode occurs in an amount equal to that round in the prior art.

As was previously stated, the reason as to why this invention operates in the manner described is not clearly understood. Other devices, such as a mesh screen in the same position as the shield 44, a conductive coating on the inner walls of the envelope 10, an opaque coating on the outer Wallsof the envelope 10 or an imperforate shield adjacent to the dynode 31 have been tried and have failed to produce any substantial change in the magnitude of the gain obtainable without increasing the feed-back action. It is believed that the presence of the dynode cage shield 44 tends to improve the transit time spread, to collect positive ions before they bombard the photocathode, absorb ions which may tend to cause fluorescence on the bulb walls, absorb light generated by electrons striking the dynode, and collect meta stable particles so that these particles, as well as certain generated radiations, do not tend to be returned to the photocathode to produce the spurious emission.

What is claimed is:

A photomultiplier tube comprising an elongated evacuated envelope, a photoemissive cathode on the inner surface of one end of said envelope, a tubular cup shaped shield spaced from said cathode and having a central aperture in the base thereof, a tubular dish shaped focus ring positioned within said shield and having a central aperture, said central apertures being in alignment, a plurality of elongated electron multipliers being positioned to receive electrons fromsaid cathode through said central apertures, the vertical projection of the aperture in the base of said tubular shield enclosing only the first of said electron multipliers, a pair of electrically insulating support members positioned adjacent to the ends of said electron multipliers, an imperforate multiplier cage shield extending around all of said electron multipliers and between and supported by said support members, said cage shield being spaced from each of said electron multipliers a distance less than the length of the shortest of said multipliers, and said cage shield being connected to said tubular shield.

References Cited in the file of this patent UNITED STATES PATENTS