Flexible Channel Multiplier

Abstract

A flexible channel secondary electron multiplier tube for use in detector of a mass spectrometer, a detector for the photons and charged particles in the cosmic space and an image intensifier, which is made up of electron-conductive polymers having a secondary electron emitting effect and which, therefore, can easily be produced by extrusion molding and which do not require the inner surface thereof to be coated with a secondary electron emitting substance. The secondary electron emission coefficient .delta. of the tube is 2.0 with respect to primary electron of 250 eV, and the gain thereof is higher than that of conventional glass tube channel secondary electron multipliers. The tube can be used by bending it at an optional curvature so as to eliminate the undesirable influence of ion feedback, and can be produced at a lower cost than a conventional one.

Parent Case Text

This is a continuation of application Ser. No. 887,052, filed Dec. 22, 1969, now abandoned.

Description

The present invention relates to a flexible channel secondary electron multiplier tube which is adapted for use in the detector of a mass spectrometer, the detector of a device for measuring low energy ions or electrons from cosmic space which is mounted in an artificial satellite or space observation rocket, an image intensifier or the like.

A secondary electron multiplier tube, when supplied with photons, ions or electrons as an input, readily emits secondary electron and amplifies them in a stable condition, and hence is applicable to such an apparatus which measures or indicates a faint input signal by amplifying the same. The flexible channel secondary electron multiplier tube has the property of conducting electrons and is produced by extrusion-molding a polymer composition, having a secondary electron emission coefficient .delta. greater than 1, into a tubular shape. By suitably selecting the electric field to be applied, a gain of 10.sup.6 or greater can easily be obtained.

The polymer compound of the type described itself has a secondary emitting effect and has substantially the same processability, moldability and flexibility as those of the commonly used plastics. Therefore, the flexible channel secondary electron multiplier tube can be produced very easily by simply molding the polymer composition into a tubular shape, it being unnecessary to coat the inner wall of the tube with an electron emitting substance. As compared with a glass-made channel multiplier, the flexible channel secondary electron multiplier tube of a polymer composition has the advantages that it can be processed more easily and is more resistive to shock and that it can be used by fixing at a curvature at which the multiplying efficiency is highest. Furthermore, it is not inferior to the glass-made multiplier tube in multiplication gain.

Conventional channel multipliers are classified into parallel plate multipliers and capillary multipliers. The parallel plate multiplier is also called a continuous dynode multiplier and comprises, as shown in FIG. 1 mentioned later, quartz plates of a size, for example, of 25.4 mm in length, 9.5 mm in width and 1 mm in thickness, arranged in parallel relation at an interval of 0.3 mm and each being coated with a film of a resistance of the order of 10.sup.9 .OMEGA. which is formed by evaporation-depositing alumina and molybdenum.

In this figure, numeral 1 designates a thus formed dynode, 2 a collector, 3 an ammeter for output current and 4 an input current path. The secondary electron emission coefficient .delta. is normally 2.45 with respect to a primary electron of 150 eV. The gain changes with the voltage impressed, which is shown as a continuous dynode characteristic in FIG. 9 also mentioned later. When a direct-current voltage of 3,000 V is impressed on the dynodes, the gain is about 5 .times. 10.sup.5. This multiplier has the drawback that, while the secondary electron emission coefficient .delta. is large, the gain is small and further the operation is unstable by reason of the positive feedback process caused by using parallel plates.

The glass-made capillary multiplier has been developed for the purpose of directly surveying the exosphere by mounting it on a rocket or artificial satellite, by taking advantage of its high gain, small size, light weight and simplicity. However, the most serious drawbacks of this system are that it is extremely difficult to coat on the inner surface of the fine pipe a resistance film on the order of 10.sup.9 .OMEGA. which has a stable secondary electron emitting effect, and that it is easily subject to breakage even under a slight shock. The construction of the capillary multiplier is shown in FIG. 2 mentioned later, in which numeral 5 designates a glass tube, 6 resistance coating, 7 a secondary electron track, and 8 injected ion, and the gain characteristic thereof is shown as a capillary multiplier characteristic in FIG. 9. The secondary electron emission coefficient .delta. of the capillary multiplier is 2.50 with respect to a primary electron of 150 eV. The gain of the multiplier when impressed with an acceleration voltage of 3,000 V is 5.0 .times. 10.sup.7.

According to the present invention, a capillary second electron multiplier can be obtained, simply by molding a polymer composition into a tubular shape which has a secondary electron emitting effect, and the multiplier thus obtained can be used by curling and fixing it at a curvature at which the gain is highest, by taking advantage of the flexibility of the material.

The aforesaid moldable and flexible secondary electron emission polymer composition can be produced by any one of the following three processes:

1. A charge transfer complex consisting of metals in Group I of the Periodic Table or onium compounds as electron donor and tetracyanoparaquinodimethane or tetracyanoethylene as electron acceptor, has excellent secondary electron emitting effect and is capable of conducting electrons. An organic charge transfer complex is fully compatible and can homogeneously be blended with thermoplastic or thermosetting resins. A moldable and flexible secondary electron emitting polymer composition having a large secondary electron emission coefficient .delta. can be produced by blending such an organic charge transfer complex having a high secondary electron emitting effect with a matrix polymer.

2. The polymeric charge transfer complex consisting of poly-2-vinylpyridine as donor and tetracyanoparaquinodimethane as acceptor, has a secondary electron emission coefficient .delta. of 2.0 or greater and is electron-conductive, the volume resistivity being 2.0 .times. 10.sup.10 .OMEGA. cm. This polymeric charge transfer complex is moldable but poor in flexibility. Therefore, by copolymerizing vinylpyridine with styrene, ethylacrylate, etc., a flexible polymeric charge transfer complex can be synthetized.

A secondary electron emitting polymer composition can be produced by using this polymeric charge transfer complex along or by blending it with other polymers for the purpose of further improving the moldability and flexibility.

3. Polyethylene, polystyrene, polyvinyl chloride and epoxy resin which are insulating polymers, have a secondary electron emitting effect and are moldable and flexible. Therefore, these compounds can easily be molded into a multiplier tube. However, the multiplier tube thus produced does not show a stable multiplying effect because it is charged in a positive polarity due to the lack of electrons which will result upon emitting secondary electrons. Therefore, a secondary electron emitting polymer composition can be produced by adding carbon black or a fine metal powder to the insulating polymers for the purpose of imparting conductivity thereto.

The secondary electron multipliers made of the secondary electron emitting polymer compositions produced by the processes described above are believed to be much superior to the glass-made capillary secondary electron multipliers in the following six points:

1. The structural material itself has the property of emitting secondary electrons.

2. They can be produced in large quantities and with a uniform quality.

3. The production cost is low.

4. Owing to their flexibility, they can be used at an optional curvature and hence stable operation can be obtained.

5. They have great mechanical strengths and are highly resistant to shock.

6. The available percentage is high. Gain characteristics for channel multiplier is high.

The present invention will appear more clearly from the following detailed description when taken in conjunction with the accompanying drawings, showing the examples of the invention, in which:

FIGS. 1 and 2 are diagrams showing the theoretical structures of conventional multipliers, the former being a view of a continuous dynode multiplier and the latter being a view of a capillary multiplier;

FIG. 3 is a set of views showing the structure of an embodiment of the flexible channel multiplier of the present invention, in which FIG. 3a shows an input cone for ions or electrons and FIG. 3b shows a side elevation;

FIGS. 4 and 5 are sets of views showing other embodiments of the flexible channel multiplier of the invention respectively, in which FIGS. 4a and 5a show input cones for ions or electrons and FIGS. 4b and 5b shows the side elevations of the respective multipliers;

FIG. 6 is a diagram exemplifying the use of a flexible channel multiplier, similar to those shown in FIG. 4, which is formed in a spiral shape;

FIG. 7 is a diagram showing an experimental arrangement in counting mode;

FIG. 8 is an output wave-form diagram of the multiplier of FIG. 3 in the single electron injection;

FIG. 9 is a diagram showing in comparison the relationships between the acceleration voltage and the gain, of the present flexible channel multiplier and the conventional one;

FIG. 10 is a diagram showing the relationship between the primary electron energy and the secondary electron emission coefficient .delta., of the flexible channel multiplier of Example 5; and

FIG. 11 is a diagram showing the relationship between the acceleration voltage and the gain, of the multiplier of Example 5.

The flexible channel secondary electron multipliers according to the present invention are produced by molding flexible secondary electron emitting polymer compositions having a large secondary electron emission coefficient into the shapes described in the following examples at a suitable temperature and under a suitable pressure, by extrusion-molding, injection-molding or casting methods. These flexible channel secondary electron multipliers are respectively so designed as to incrase the gain, to elimiate the undesirable effect of ion feedback and to enhance the collection efficiency. The secondary electron emitting polymer composition used in Examples 1--4, illustrated hereinafter, has the following composition:

Lithium-tetracyano-paraquinodimethane-- 35 g.

Polyvinyl chloride (PVC) (Average

polymerization degree 1,200)--30 g.

Stabilizer (equivalent mixture of

tribasic lead sulfate and Cd-Ba type

stabilizer)--5 g.

Low molecular weight polyurethane

(Ultramole PU of Bayer)--30 g.

After weighing precisely, the component materials were thoroughly blended for 30 minutes in a ribbon blender, while maintaining them at a temperature of 60.degree. C. and successively thereafter the blend was kneaded for 10 minutes on a mixing roll heated at a temperature of 165.degree. - 170.degree. C. and taken out in the shape of a sheet. The sheet material thus obtained was pelletized by a pelletizer and used as a sample in the respective Examples.

Example 1

The flexible secondary electron emitting polymer composition was continuously molded into a tube by extrusion molding under the conditions of a temperature of 180.degree. C. and a pressure of 200 kg/cm.sup.2, and the tube was cut in a length of 100 mm. The pipe thus obtained had an inner diameter of 1 mm, an outer diameter of 2 mm and a total length of 100 mm as shown in FIG. 3. The electric resistance of the pipe across the opposite ends thereof was 6.0 .times. 10.sup.11 .OMEGA. at room temperature and the secondary electron emission coefficient .delta. thereof was 2.0 with respect to a primary electron having an energy of 300 eV. These measurements were taken by fixing the pipe at a radius of curvature of 30 mm upon bending.

Example 2

The flexible secondary electron emitting polymer composition was molded by extrusion-molding under the conditions of a temperature of 180.degree. C. and a pressure of 200 kg/cm.sup.2 into a pipe of the shape as shown in FIG. 4, which consists of a conical input cone of 10 mm in diameter and 15 mm in depth and a tube of 1 mm in inner diameter and 2 mm in outer diameter connected with each other integrally.

The electric resistance of the pipe across the opposite ends thereof at room temperature was 6.5 .times. 10.sup.11 .OMEGA. and the secondary electron emission coefficient thereof was 2.0 with respect to a primary electron having an energy of 300 eV. These measurements were taken by fixing the pipe at a radius of curvature of 30 mm upon bending.

Example 3

The flexible secondary electron emitting polymer composition was molded by extrusion molding under the conditions of a temperature of 180.degree. C. and a pressure of 200 kg/cm.sup.2, into a pipe of the shape as shown in FIG. 5, which consists of a pyramidal input cone of 10 .times. 4 mm in bottom surface area, 15 mm in height and 0.5 mm in thickness and a tube of 2 mm in outer diameter and 1 mm in inner diameter, connected with each other integrally, and has a total length of 115 mm. The electric resistance of the pipe across the opposite ends thereof at room temperature was 6.5 .times. 10.sup.11 .OMEGA. and the secondary electron emission coefficient was 2.0 with respect to a primary electron having an energy of 300 eV. These measurements were taken by fixing the pipe at a radius of curvature of 30 mm upon bending.

Example 4

A multiplier which is the same as that of Example 2 except that the length of the tube was 150 mm, was molded and fixed upon bending it three turns at a radius of curvature of 5 mm into a spiral shape as shown in FIG. 6, for use as a secondary electron multiplier, in which the upper arrow mark indicates an injection part for electrons and ions and the lower arrow marks indicate connection with a synchroscope.

The electric resistance of the multiplier across the opposite ends thereof was 9.0 .times. 10.sup.11 .OMEGA. at room temperature and the secondary electron emission coefficient thereof with respect to a primary electron having an energy of 300 eV was 2.0.

The construction of a measuring circuit, using the flexible channel secondary electron multiplier of Example 1, is shown in FIG. 7. The multiplier tube used had a total length L of 100 mm, an inner diameter r.sub.i of 1 mm, an outer diameter r.sub.o of 2 mm and a radius of curvature r.sub.c of 30 mm. In addition the arrow mark 9 indicates the flexible channel multiplier, 10 incident electrons and 11 connection with a synchroscope. The resistance value of the tube across the opposite ends thereof was 1 .times. 10.sup.9 .OMEGA.. The tube was connected to a direct-current source Va of a voltage of 3,000 V through a load resistance (R.sub.L) of 50 .OMEGA.. Character C.sub.c designates a coupling condenser, the electrostatic capacity of which is 0.01 .mu.F. While maintaining the multiplier tube in a vacuum atmosphere of 1 .times. 10.sup.-.sup.6.sub. Torr, a faint electron stream of the order of 10.sup.-.sup.18 A having an energy of 300 eV was fed as an input and the output was taken out from a cable which was connected to the output end of the tube through the coupling condenser C.sub.c. A voltage change corresponding to the output current was read by a synchroscope through a 50.OMEGA. load resistance R.sub.L '. As will be clear from FIG. 8, in which one scale represents 40 mV on the axis of ordinates and one scale 10 n sec. on the axis of abscissa, it was found that a pulse output of a width of about 30 n sec. can be obtained for one electron injected, in case of the acceleration voltage being 3,000 V. By calculating the area surrounded by the output waveform, the gain of the multiplier tube can be obtained. The curve of Example 1 in the diagram of FIG. 9 shows the relationship between the thus obtained gain and the acceleration voltage. In this case, a suitable value of the acceleration voltage is in the proximity of 3,000 V because the expansion of pulse height distribution is relatively small. The gain in this case was 6.0 .times. 10.sup.8. The gains of Examples 2, 3 and 4 were 7.0 .times. 10.sup.8, 6.5 .times. 10.sup.8 and 8.0 .times. 10.sup.8 respectively.

In Examples 2, 3 and 4, it was ascertained that the collecting effect was enhanced due to the fact that the input ends of the respective multiplier tubes are shaped in a trumpet-like configuration. In Example 4, it is evident that the gain is improved.

Other polymer compositions which are also suitable for use in the production of the present secondary electron multiplier will be described hereinafter:

Example 5

A mixture consisting of

Sodium-tetracyano-paraquinodimethane-- 35 g.

Polyvinyl chloride (PVC) (Average

polymerization degree 1,200)--45 g.

Low molecular weight polyurethane

(Ultramole PU of Bayer)--55 g.

Stabilizer (equivalent mixture of

tribasic lead sulfate and Cd-Ba

type stabilizer)--10 g.

was sufficiently agitated in a ribbon blender for about 30 minutes at a temperature of 60.degree.C. Successively thereafter, the mixture was kneaded on a hot roll heated at 165.degree. - 170.degree. C. and taken out in the form of a sheet having a thickness of about 1 mm. The sheet was pelletized by a pelletizer. Using the resultant pellets, a tube was molded continuously at a temperature of 180.degree. C. under a pressure of 60 kg/cm.sup.2 by an extruder of 1 mm in inner diameter and 2 mm in outer diameter, and a multiplier having an inner diameter of 1 mm, an outer diameter of 2 mm and a total length of 100 mm was obtained from the tube. The electric resistance of the multiplier across the opposite ends thereof was 6.5 .times. 10.sup.11 .OMEGA. at room temperature and the secondary electron emission coefficient .delta. thereof was 2.0.

Example 6

A mixture consisting of

Potassium-tetracyano-paraquinodimethane-- 35 g.

Polyvinyl chloride (PVC) (Average

polymerization degree 1,200)--30 g.

Stabilizer (equivalent mixture of

tribasic lead sulfate and Cd-Ba

type stabilizer)--5 g.

Low molecular weight polyurethane

(Ultramole PU of Bayer)--30 g.

was sufficiently agitated in a ribbon blender for about 30 minutes at a temperature of 60.degree. C. Successively thereafter, the mixture was kneaded for about 10 minutes on a hot roll at a temperature of 165.degree. - 170.degree. C. and taken out in the form of a sheet having a thickness of about 1 mm. The sheet was pelletized by a pelletizer and using the resultant pellets, a tube was molded continuously at a temperature of 180.degree. C. under a pressure of 60 kg/cm.sup.2 by an extruder having an inner diameter of 1 mm and an outer diameter of 2 mm. A multiplier tube having an inner diameter of 1 mm, an outer diameter of 2 mm and a total length of 10 cm was obtained from the tube. The electric resistance of the tube across the opposite ends thereof was 5.0 .times. 10.sup.11 .OMEGA. at room temperature and the secondary electron emission coefficient .delta. thereof was 2.2.

Example 7

A styrene-ethylacrylate-2-vinylpyridine copolymer (at a ratio of 40-40-20) was dissolved in toluene and quaternarized by the addition of hydriodic acid in an amount 1.5 times the mol number of the 2-vinylpryridine. The hydriodic acid-quaternarized styrene-ethylacrylate-2-vinylpyridine copolymer was dissolved in acetonitrile and reacted with lithium-tetracyano-paraquinodimethane, to form a charge transfer complex. This complex is high flexible and moldable, and is electron-conductive, the volume resistivity being 5.0 .times. 10.sup.9 .OMEGA. cm. The complex was molded under the conditions of a temperature of 130.degree. C. and a pressure of 30 kg/cm.sup.2, using an extruder with a die having an inner diameter of 1 mm. and an outer diameter of 2 mm. The resultant molding was cut into a length of 10 cm and an electrode was formed at each end with silver paint, to obtain a multiplier tube. The electric resistance of the tube across the opposite ends thereof was 3.0 .times. 10.sup.11 .OMEGA. and the secondary electron emission coefficient .delta. was 2.1.

Example 8

100 g. of poly-2-vinylpyridine was dissolved in 200 ml. of ethanol and quaternarized with 127 g. of 67 percent hydriodic acid. 100 g. of the poly-2-vinylpyridine hydriodate was dissolved in 200 ml. of an equivalent mixed solvent of water and ethanol. On the other hand, 145 g. of lithium-tetracyano-paraquinodimethane was dissolved in 500 ml. of water. Both solutions were mixed with each other and the mixture was thoroughly stirred, whereby a dark bluish purple precipitate was formed. This precipitate is the poly-2-vinylpyridine-tetracyano-paraquinodimethane charge transfer complex. This complex is highly conductive and highly moldable but not sufficiently flexible. The complex was blended with polyvinyl chloride which had been plasticized with low molecular weight polyurethane. The blending ratio was as follows:

Poly-2-vinylpyridine-tetracyano-paraquinodimethane--50 g.

Polyvinyl chloride-- 30 g.

Stabilizer (tribasic lead sulfate,

Cd-Ba type stabilizer)--5 g.

Low molecular weight polyurethane

(Ultramole PU of Bayer)--25 g.

The blend was kneaded for about 10 minutes on a hot roll at a temperature of 160.degree. - 170.degree. C. and formed into the shape of a sheet. After pelletizing the sheet, the resultant pellets were molded into a tube by an extruder. The molding was effected at a temperature of 160.degree. C. under a pressure of 30 kg/cm.sup.2. The electric resistance of the thus produced multiplier tube across the opposite ends thereof was 1.0 .times. 10.sup.11 .OMEGA. and the secondary electron emission coefficient .delta. was 2.3.

Example 9

An electron-conductive polymer composition consisting of 90 g. of an intermediate density polyethylene and 10 g. of carbon black blended therewith, was used. Namely, the polyethylene was used to make use of its own secondary electron emitting effect, and the electron-conductive carbon black was added in a small quantity as electron carrier, for quickly neutralizing the hole after the emission of electrons. The composition had excellent moldability and flexibility as it consisted essentially of polyethylene. The material was molded by extrusion-molding into a pipe having an inner diameter of 1 mm, an outer diameter of 2 mm and a total length of 100 mm, at a temperature of 110.degree. C. under a pressure of 60 kg/cm.sup.2.

The electric resistance of the pipe across the opposite ends thereof was 1.0 .times. 10.sup.11 .OMEGA. and the secondary electron emission coefficient .delta. was 1.9.

Example 10

30 g. of polyamine (Thomaid of Fuji Polymer Chemical Company) was added to 100 g. of an epoxy resin (Epikote of Shell International Chemicals Corporation), to which was added by stirring 10 g. of a conductive silver powder for painting with stirring. The mixture was poured into a tubular metal mold and cured therein for 1 hour at a temperature of about 60.degree. C.

A multiplier tube having an inner diameter of 1.0 mm, an outer diameter of 2.0 mm and a total length of 100 mm was obtained. The resistance across the terminal ends of the tube was 1.0 .times. 10.sup.11 .OMEGA. and the secondary electron emission coefficient .delta. was 1.7.

The secondary electron emission coefficients .delta. of the secondary electron emitting polymer compositions produced by the processes of the respective Examples were measured in a superhigh vacuum from a sample film of each composition having a thickness of 20 .mu. and a diameter of 20 mm. The gains of the respective multiplier tubes were measured by incorporating the tube in a measuring circuit as shown in FIG. 7.

Referring to FIG. 10, there is shown the relationship between the secondary electron emission coefficient .delta. and the primary electron energy, which was obtained on a film of the secondary electron emitting polymer composition produced by the process of Example 5. Namely, the circuit of FIG. 7 was formed using the secondary electron multiplier tube molded of the composition and a faint electron stream of the order of 10.sup.-.sup.18 A having an energy of 300 eV was fed to the multiplier tube as the input. To the output side was connected a load resistance of 820 .OMEGA. and a voltage change corresponding to the output current was observed on a synchroscope. The relationship between the voltage impressed and the gain is shown in FIG. 11. A noise-free, stable emitting effect is obtained in the proximity of 3,000 V. The gain at this time is 3.0 .times. 10.sup.8.

The secondary electron emission coefficients .delta. of the multiplier tubes of Examples 5-10 with respect to a primary electron having an energy of 300 eV and the gains of the same at an acceleration voltage of 3,000 V were measured, the results of which are summarized in Table 1 below: ##SPC1##

As has been described hereinabove, the flexible channel multiplier of the present invention is superior to the prior art multipliers in gain and is less inferior in characteristics. Further, the flexible channel multiplier of the invention is far superior in the uniformity of surface condition to the glass multiplier with a coating on the inner surface thereof, owing to the fact that the material of the tube itself is a uniform polymer composition having a secondary electron emitting effect. Another advantage of the present multiplier tube is that because of its flexibility, it is highly resistive to shock, has excellent mechanical strengths and can be used at a suitable curvature for the purpose of eliminating the instability caused by ion feedback. The material can easily be synthetized and simply molded by applying the techniques which are commonly used for processing plastics. The flexible channel secondary electron multiplier is adapted to mass production and is of high industrial value. The flexible channel secondary electron multiplier can be used as a detector element of such measuring instruments which are mounted in an artificial satellite, space observation rocket or the like for the measurement of ions, electrons an photons, or as an ion detector of a mass spectrometer. It is also expected to find practical use in an image intensifier in the future which utilizes the secondary electron emitting effect.

Claims

1. A flexible channel secondary electron multiplier comprising at least one flexible pipe fixed in an arcuate shape as a unitary piece, said at least one pipe having a conically or pyramidally-shaped injection port, said pipe and injection port being molded of a composition comprising at least one thermosetting resin selected from the group consisting of polyvinyl chloride and polyethylene having a large secondary electron emission coefficient and at least one selected from the group consisting of an electron-conductive charge transfer complex and carbon black, said composition thus possessing a volume resistivity no greater than 3.0 .times.10.sup.10 ohm-cm and wherein said electron-conductive charge transfer complex is a salt selected from the group consisting of potassium-tetracyano-paraquinodimethane, sodium-tetracyano-paraquinodimethane, lithium-tetracyano-paraquinodimethane and polyvinylpyridine-tetracyano-paraquinodimethane.