Electron tube

Information

  • Patent Grant
  • 6198221
  • Patent Number
    6,198,221
  • Date Filed
    Wednesday, December 9, 1998
    25 years ago
  • Date Issued
    Tuesday, March 6, 2001
    23 years ago
Abstract
The present invention relates to an electron tube comprising, at least, a cathode electrode, a face plate having a photocathode, and an electron entrance surface provided at a position where the electron emitted from the photocathode reaches. The object of the present invention is to provide an electron tube which can reduce its size and has a structure for improving the workability in its assembling process. In particular, the electron tube according to the present invention has a bonding ring, provided between the face plate and the cathode electrode, for bonding the face plate and the cathode electrode together. The bonding ring is made of a metal material selected from the group consisting of In, Au, Pb, alloys containing In, and alloys containing Pb.
Description




BACKGROUND OF THE INVENTION




1. Field of the Invention




The present invention relates to an electron tube utilized as a photodetector for quantitatively measuring weak light. In particular, the present invention relates to an electron tube equipped with a sensing device having an electron entrance surface such as a semiconductor device for multiplying and outputting electrons emitted from a photocathode.




2. Related Background Art




There have conventionally been known electron tubes in which electrons emitted from a photocathode are accelerated and converged by an electron lens and then are made incident on a semiconductor device to yield a high gain. Such electron tubes are disclosed, for example, in U.S. Pat. No. 5,120,949, U.S. Pat. No. 5,374,826, and S. Base et al., “Test Results of the First Proximity Focused Hybrid Photodiode Detector Prototypes,”


Nuclear Instruments and Methods in Physics Research,


A330 (1993), 93-99. In particular, the above-mentioned Base reference discloses an electron tube such as that shown in FIG.


1


. This electron tube has an electrical insulating bulb


102


which secures electrical insulation between an anode


100


and a cathode electrode


101


. The inner diameter of the cathode electrode


101


is made greater than that of the bulb


102


, whereby a photocathode


103


has a large area, allowing a semiconductor device


104


to have an increased effective area (e.g., 100 mm


2


). Accordingly, it can be seen that the electron tube shown in

FIG. 1

has a large size. The cathode electrode


101


employed in this electron tube is constituted by two pieces of cylindrical metal members


101




a


and


101




b


having inner diameters different from each other disposed concentrically with a gap therebetween.




SUMMARY OF THE INVENTION




Having studied the above-mentioned prior art, the inventors have found the following problems to be overcome. The cathode electrode


101


of the electron tube shown in

FIG. 1

can be configured into various sizes and forms as two pieces of cylindrical metal members


101




a


and


101




b


are combined together. Though it is suitable for a large electron tube since a gap must be formed between these metal members


101




a


and


101




b,


such a gap is hard to secure in a small electron tube (with a diameter of about 10 mm, for example). Also, in order to assemble such a cathode electrode


101


, each of two planar sheets must be pressed and then sealed by welding or the like into a cylindrical form, thereby yielding a low efficiency in the assembling operation.




It is thus an object of the present invention to provide an electron tube which can reduce its size and has a structure for improving the workability in its assembling process.




In order to achieve this object, the electron tube according to the present invention comprises, at least, a face plate on which a photocathode for emitting a photoelectron in response to incident light is disposed; an electron entrance surface (corresponding to the electron entrance surface of an avalanche photodiode or the like) on which the photoelectron emitted from the photocathode is incident, the electron entrance surface being arranged so as to face the photocathode; a cathode electrode positioned between the face plate and the electron entrance surface; and a bonding member, provided between the face plate and the cathode electrode, for bonding the face plate and the cathode electrode together.




The electron tube according to the present invention further comprises a body. The body can be selected from at least one of a pipe type having a first opening and a second opening opposing the first opening, and an envelope type having one opening and a bottom portion. The electron tube having the pipe type body preferably has a stem, and wherein the cathode electrode is arranged on the first opening side of the pipe type body and the stem is arranged on the second opening side of the pipe type body. The stem functions to define a distance between the photocathode and the electron entrance surface facing the photocathode. On the other hand, in the envelope type body, the cathode electrode is arranged on the opening of the envelope type body, and the bottom portion functions as the above-mentioned stem.




In particular, the bonding member is made of a metal material selected from the group consisting of In, Au, Pb, alloys containing In, and alloys containing Pb. In the electron tube according to the present invention, in order to allow its size to decrease, after the step for forming the photocathode (heating to about 300° C.), the cathode electrode and the body are bonded together in an atmosphere at a temperature much lower than that in the photocathode-forming step. Accordingly, as the material for the bonding member, materials which can sufficiently deform at a pressure of about 100 kg in the atmosphere at room temperature are preferable, whereas metals such as aluminum are unfavorable.




The cathode electrode has a through-hole for transmitting therethrough the photoelectron from the photocathode toward the electron entrance surface. In the case of selecting the pipe type body, the electron tube comprises a welded electrode arranged at the second opening side of said body and positioned between the body and the stem. This welded electrode also has a through-hole for transmitting therethrough the photoelectron transmitted through the through-hole of the cathode electrode toward the electron entrance surface.




The electron tube according to the present invention may further comprise an anode having a through-hole for transmitting therethrough the photoelectron transmitted through the cathode electrode (first embodiment). This anode is supported by the welded electrode such that at least part of the anode is positioned between the cathode electrode and the electron entrance surface, thereby constituting an electron lens together with the cathode. In the first embodiment, it is preferred that the through-hole of the anode has an area equal to or smaller than the electron entrance surface. It is due to the fact that, when a photoelectron from the photocathode reaches the surroundings of the electron entrance surface, the device is deteriorated or charged. Alternatively, a part of the welded electrode may be configured to function as the anode (second embodiment). Also in the second embodiment, it is preferred that the through-hole of the anode has an area equal to or smaller than the electron entrance surface.




In addition, the welded electrode comprises a portion to be resistance-welded to the stem. The stem has a mounter section, projecting toward the photocathode, for holding a semiconductor device.




In the electron tube according to the present invention, light incident on the face plate from the outside is converted into electrons by the photocathode. While the orbit of the electrons is converged by an electron lens effect formed by the cathode electrode and anode cooperating together, the electrons reach the electron entrance surface of the semiconductor device or the like. Here, the cathode electrode has a cylindrical form and can be made easily by any of various integral-molding methods such as press molding, injection molding, or cutting. Also, a small cathode electrode can easily be materialized when required, allowing the electrode to further decrease its size. Since each of the cathode electrode, body, and welded electrode is formed like a ring, they can easily be mounted on each other concentrically. Accordingly, in order to form a vacuum case, the operation for assembling the case is facilitated. As the electron tube is made smaller, the present invention can satisfy a strong demand in the fields of high energy and medical instruments for using 1,000 to 10,000 pieces of electron tubes arranged in a limited space. Also, when a ring-shaped member made of indium is disposed between the cathode electrode and face plate in the case, and the face plate (provided with a photocathode beforehand) and the cathode electrode are pressed against each other while a high pressure of about 100 kg is applied thereto in a vacuumed transfer apparatus (within a vacuum chamber) and a vacuum region can easily be formed within the electron tube. Accordingly, it is unnecessary for the case to be provided with an exhaust tube, and a large number of electron tubes can be produced within the transfer apparatus.




In this case, it is preferable that the cathode electrode, the body, and the cylindrical main part of the welded electrode have substantially the same cross-sectional form. In such a configuration, the outer face of the case can be made free of irregularities, thereby yielding a simple form without roughness. Accordingly, a number of electron tubes can be arranged densely. Also, the electron tube can become easy to handle, while yielding a structure which is tolerant to a pressure as high as 150 kg.




Also, it is preferable that the inner peripheral wall face of the cathode electrode be positioned on the inside of the inner peripheral wall face of the body. In other words, the inner diameter of the cathode electrode is preferably smaller than that of each of the first and second openings in the body. In this configuration, stray electrons generated at unintentional places on the photocathode side can be prevented from impinging on the body. Accordingly, the body is kept from being charged upon impingement of the stray electrons thereon and thereby influencing the electron orbit.




The welded electrode is preferably connected to the stem by resistance welding. In this case, as the stem is resistance-welded to the welded electrode of the case, the second opening of the case can easily be closed with the stem.




One end of the cylindrical main part of the welded electrode is provided with a first flange section (first edge section) projecting outward, whereas the other end of the cylindrical main part is provided with a second flange section (second edge section) projecting inward from the inner wall of the body, and the outer periphery of the stem is provided with a cutout edge section which is secured to the first flange section of the welded electrode. In this configuration, the stem can be attached to the welded electrode by a simple assembling operation in which the first flange section of the welded electrode is resistance-welded to the cutout edge section of the stem. Further, the attachment of the case (including the cathode electrode, body, and welded electrode) to the stem can be improved. Also, since the second flange section of the welded electrode projects into the electron tube, the second flange section itself can function as an anode (second embodiment). Alternatively, an anode having a given form may simply be secured to the second flange section by welding or the like (first embodiment).




By contrast, in the case where a distance between the photocathode and the electron entrance surface is small, the above-mentioned envelope type body (third embodiment) can be selected because a consideration for the impinging of the stray electrons is not necessary.




Further, in the electron tube according to the present invention (fourth and fifth embodiments), the body comprises at least two insulating members, each of which has a through hole extending from the photocathode toward the electron entrance surface, and at least one conductive member provided between, of the insulating members, those adjacent to each other. The conductive member has a through-hole extending from the first opening toward the second opening. The body of the electron tube is constituted by the insulating and conductive members alternately mounted on each other. Obtained in this configuration is a case in which the cathode electrode is attached to one end (end portion where the first opening is positioned) of the body, whereas the welded electrode is attached to the other end (end portion where the second opening is positioned) of the body.




In addition, in order to control the photoelectron emitted from the photocathode and prevent the inner wall of the insulating members from being charged, it is preferable that the through-hole of the conductive member has a smaller area than the through-hole of each insulating member.




Namely, the inner peripheral wall face of the cathode electrode is positioned on the inside of the inner peripheral wall face of the insulating members of the body, whereas the conductive member (intermediate electrode) projects inward from the inner peripheral wall face of the insulating members of the body. In this configuration, stray electrons generated at unintentional places on the photocathode side can be prevented from impinging on the insulating members of the body. Accordingly, the insulating members are kept from being charged upon impingement of the stray electrons thereon and thereby influencing the electron orbit.




Also, it is preferable that voltages supplied to the cathode electrode and the photocathode be the same, voltages supplied to the anode (or a part of the welded electrode) and the welded electrode be the same, and a predetermined voltage not lower than that supplied to the cathode electrode but not higher than that supplied to the anode be supplied to the intermediate electrode. In this configuration, dielectric breakdown does not occur even when a strong negative voltage is applied to the photocathode.




The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.




Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.











BRIEF DESCRIPTION OF THE DRAWINGS





FIG. 1

is a view showing a cross-sectional configuration of a conventional electron tube;





FIG. 2

is a perspective view showing a partial cross-sectional configuration of an electron tube according to the present invention (first embodiment);





FIG. 3

is a view showing a cross-sectional configuration of the electron tube according to the first embodiment of the present invention taken along line I—I in

FIG. 2

;





FIG. 4

is a view showing a cross-sectional configuration of a semiconductor device (APD) in the electron tube according to the first embodiment shown in

FIG. 3

;





FIG. 5

is a view for explaining an assembling process of the electron tube according to the present invention (first embodiment);





FIG. 6

is a cross-sectional view showing the configuration of the electron tube according to a second embodiment of the present invention, at a cross section corresponding to that taken along line I—I shown in

FIG. 2

;





FIG. 7

is a view showing a cross-sectional configuration of a semiconductor device (PD) in the electron tube according to the second embodiment shown in

FIG. 6

;





FIG. 8

is a plan view showing a modified example of an anode in the electron tube according to the second embodiment shown in

FIG. 6

;





FIG. 9

is a cross-sectional view showing the configuration of the anode taken along line II—II in

FIG. 8

;





FIG. 10A

is a top view showing the electron tube according to a third embodiment of the present invention; whereas

FIG. 10B

is a cross-sectional view showing the configuration of the electron tube according to the third embodiment taken along line III—III shown in

FIG. 10A

;





FIG. 11

is a cross-sectional view showing the configuration of the electron tube according to a fourth embodiment of the present invention, at a cross section corresponding to that taken along line I—I shown in

FIG. 2

;





FIG. 12

is a view for explaining an assembling process of the electron tube according to the present invention (fourth embodiment); and





FIG. 13

is a cross-sectional view showing the configuration of the electron tube according to a fifth embodiment of the present invention, at a cross section corresponding to that taken along line I—I shown in FIG.


2


.











DESCRIPTION OF THE PREFERRED EMBODIMENTS




In the following, embodiments of the electron tube according to the present invention will be explained with reference to

FIGS. 2

to


9


,


10


A,


10


B and


11


to


13


.





FIG. 2

is a perspective view showing a partial cross section of an electron tube


1


according to a first embodiment of the present invention.

FIG. 3

is a cross-sectional view showing the configuration of the electron tube


1


according to the first embodiment taken along line I—I in FIG.


2


. As shown in

FIGS. 2 and 3

, the electron tube


1


has a cylindrical case


10


. The case


10


is constituted by a ring-shaped cathode electrode


11


, which is made of a highly conductive covar metal by any of various integral-molding methods such as press molding, injection molding, or cutting; a ring-shaped body


12


made of an electrical insulating material (e.g., ceramics); and a ring-shaped welded electrode


13


made of a covar metal. These members


11


,


12


, and


13


are mounted on each other with their center axes AX coinciding with each other. While the body


12


is disposed between the cathode electrode


11


and the welded electrode


13


, one end of the body


12


(on the side of a first opening


14


) is butted against a flat end face


11




a


of the cathode electrode


11


and then is secured thereto by brazing or the like. The other end of the body


12


(on the side of a second opening


15


) is butted against a flat end face


13




a


of the welded electrode


13


and then is secured thereto by brazing or the like. Accordingly, the case


10


includes the cathode electrode


11


, the body


12


, and the welded electrode


13


, which are easily united together by brazing.




Further, the cathode electrode


11


, the body


12


, and a cylindrical main part


13


A of the welded electrode


13


have substantially the same cross-sectional form (e.g., circular form having a diameter of 14 mm here). Accordingly, the outer face of the case


10


can be made free of irregularities, yielding a simple form without roughness. As a result, obtained is an electron tube which is easy to handle, and a number of such electron tubes can be arranged densely even in a narrow space. Also, thus obtained electron tube has a structure which is tolerant to a high pressure. Here, the ring-like cathode electrode


11


, the body


12


, and the welded electrode


13


may have a polygonal cross-sectional form.




An inner peripheral wall face


11




b


of the cathode electrode


11


is positioned on the inside of an inner wall face


12




a


of the body (insulating member)


12


, whereby the inner diameter of the cathode electrode


11


is made smaller than that of the insulating member


12


. In other words, the through-hole of the cathode electrode


11


has a smaller area than that of each of the first and second openings in the body


12


. Accordingly, stray electrons generated at unintentional places on the side of a photocathode


22


, which will be explained later, can be prevented from impinging on the body


12


. Consequently, the body


12


is kept from being charged upon impingement of the stray electrons thereon and thereby influencing the electron orbit. Here, each of the through-holes


11




b


and


12




a


has a circular cross section. The inner diameters of the cathode electrode


11


and body


12


are respectively 10 mm and 11 mm, for example. The through-holes


11




b


and


12




a


may have either identical or different cross-sectional forms and may be either circular or polygonal. Here, the length of the cathode electrode


11


is preferably 3.5 mm, whereas the length of the body


12


is preferably 6.5 mm.




Firmly attached to the cathode electrode


11


in the case


10


is a face plate


21


made of glass which transmits light therethrough. The face plate


21


has a photocathode


22


on the inner face and is disposed at one end of the case


10


(on the side of the first opening


14


in the body


12


). After the photocathode


22


is made, the face plate


21


is integrated with the cathode electrode


11


by way of a bonding member (bonding ring)


23


made of a metal material selected from the group consisting of In, Au, Pb, alloys containing In, and alloys containing Pb. Disposed at the peripheral portion of the photocathode


22


is an electrode


25


made of a thin film of chromium for electrically connecting together the photocathode


22


and the bonding member


23


(referred to as “indium ring” hereinafter) containing indium. The inner diameter of the electrode


25


, i.e., 8 mm, defines the effective diameter of the photocathode


22


. The indium ring


23


is formed so as to project from the inner side face of a hollow cylindrical auxiliary member


24


(conductive material). When the indium ring


23


and the face plate


21


are successively disposed on the cathode electrode


11


, and then the cathode electrode


11


and the face plate


21


are pressed against each other at a high pressure of about 100 kg, the indium ring


23


deforms and functions as an adhesive, whereby the face plate


21


is integrated with the case


10


. The auxiliary member


24


functions not only to prevent the indium ring


23


deformed upon a predetermine pressure applied thereto from projecting to the outside but also as an electrode for applying a predetermined voltage to the photocathode


22


.




As the material for the adhesive member


23


, since the cathode electrode


11


and the face plate


21


having the photocathode


22


are bonded together after the manufacturing process for the photocathode


22


, materials which can sufficiently deform at a pressure of about 100 kg in the atmosphere at room temperature are preferable, whereas hard metals such as aluminum are unfavorable.




Firmly attached to the welded electrode


13


in the case


10


is a disk-shaped stem


31


made of a conductive material (e.g., covar metal). The stem


31


is disposed at the other end of the case


10


(on the side of the second opening


15


in the body


12


). Here, one end of the cylindrical main part


13


A of the welded electrode


13


is provided with a circular first flange section


13


B projecting outward so as to be utilized for joining with the stem


31


, whereas the other end of the cylindrical main part


13


A of the welded electrode


13


is provided with a circular second flange section


13


C projecting inward so as to be utilized for joining with the body


12


. Formed at the outer periphery of the stem


31


is a cutout edge section


31




a


for attaching to the first flange section


13


B. Accordingly, the welded electrode


13


and the stem


31


can easily be joined together by a simple assembling operation in which the first flange section


13


B of the welded electrode


13


is resistance-welded to the cutout edge section


31




a


of the stem


31


. Also, in this configuration, the attachment of the case


10


to the stem


31


is quite improved. A lead pin


32


insulated by a glass member


34


is secured to the stem


31


. The electron tube


1


is integrally formed by the case


10


, the face plate


21


, and the stem


31


, such that its inside is kept in a vacuum state.




Further, as shown in

FIG. 4

, a semiconductor device


40


operating as an APD (avalanche photodiode) is secured onto a mounting surface


310


of the stem


31


by way of a conductive adhesive


50


. The semiconductor device


40


comprises, as a substrate material, a silicon substrate


41


containing a high concentration of an n-type dopant. Formed at the center portion of the substrate


41


is a disk-shaped p-type carrier-multiplying layer


42


. Formed at the outer periphery of the carrier-multiplying layer


42


is a guard ring layer


43


having the same thickness as that of the carrier-multiplying layer


42


and containing a high concentration of a n-type dopant. Formed on the surface of the carrier-multiplying layer


42


is a breakdown-voltage control layer


44


containing a high concentration of a p-type dopant. The surface of the breakdown-voltage control layer


44


is formed as an electron entrance surface


44




a.


An oxide film


45


and a nitride film


46


are formed so as to link the peripheral portion of the breakdown-voltage control layer


44


and the guard ring layer


43


together. Disposed on the outermost surface of the semiconductor device


40


are an electrode


47


formed by circularly deposited aluminum for supplying an anode potential to the breakdown-voltage control layer


44


and a peripheral electrode


48


for connecting with the guard ring layer


43


. The peripheral electrode


48


is spaced from the electrode


47


with a predetermined distance therebetween. Preferably, the electron entrance surface


44




a


is positioned within the opening of the entrance surface electrode


47


and has a diameter of about 3 mm.




The n-type silicon substrate


41


of the semiconductor device


40


is secured to the stem


31


by way of the conductive adhesive


50


. As the conductive adhesive


50


is utilized, the stem


31


and the n-type substrate


41


are electrically connected to each other. By way of a wire


33


, the electrode


47


is connected to the lead pin


32


insulated from the stem


31


.




As shown in

FIGS. 2

to


4


, in the electron tube


1


according to the present invention, a planar anode


60


is disposed between the semiconductor device


40


and the photocathode


22


. The outer peripheral end portion of the anode


60


is secured to the second flange section


13


C of the welded electrode


13


. Also, the anode


60


is positioned in the body


12


on the side of the second opening


15


and is formed by a pressed thin stainless sheet having a thickness of 0.3 mm. Preferably, the distance between the anode


60


and the semiconductor device


40


is 1 mm.




An opening section


61


is formed at the center of the anode


60


, i.e., at the region opposing the electron entrance surface


44




a


of the semiconductor device


40


. Further, integrally formed with the anode


60


is a cylindrical collimator section (collimator electrode)


62


projecting toward the photocathode


22


so as to surround the opening section


61


. The collimator section


62


is disposed so as to project toward the photocathode


22


and surround the opening section


61


. Preferably, the collimator section


62


has an inner diameter of 2.5 mm and a height of 1.5 mm. Here, the anode


60


may be formed on an extension of the second flange section


13


C of the welded electrode


13


beforehand such that the welded electrode


13


can also serve as the anode


60


.




In the following, an assembling process for the electron tube


1


(first embodiment) will be explained with reference to FIG.


5


. First, the semiconductor device


40


is die-bonded to the stem


31


. Subsequently, the electrode


47


and the lead pin


32


are connected to each other by the wire


33


. On the other hand, the anode


60


is secured to the welded electrode


13


in the case


10


by resistance welding, and the welded electrode


13


and the stem


31


are secured to each other by resistance welding. Then, the face plate


21


, the indium ring


23


, and the case


10


, in which the stem


31


and the cathode electrode


11


are integrated together, are separately introduced into a vacuum apparatus (vacuum chamber) which is known as a transfer apparatus. Then, after being baked in a vacuum chamber for about 10 hours at 300° C., one side of the face plate


21


is provided with the photocathode


22


. In order to form the photocathode


22


, after vapor deposition of antimony, vapors of potassium, sodium, and cesium are successively introduced. Alternatively, it may be formed when cesium vapor and oxygen are alternately introduced onto a GaAs crystal which has been integrated with the face plate


21


beforehand.




The case


10


and the face plate


21


already provided with the photocathode


22


are joined together by way of the indium ring


23


. As a pressure of about 100 kg is applied to this assembly (to the face plate


21


and the stem


31


in the directions indicated by arrows A and B in FIG.


11


), the indium ring


23


, which is the softest member therein, is crushed. Here, the gap between the face plate


21


and the cathode electrode


11


, in which the indium ring


23


is positioned, is sealed with the auxiliary ring


24


. As a result, the indium ring


23


functions as an adhesive. Accordingly, as the inside of the apparatus is kept in a vacuum state, a vacuum is produced in the electron tube


1


. Finally, the vacuum in the transfer apparatus is caused to leak out, thereby accomplishing a series of steps. Typically, in the making of the electron tube


1


in the transfer apparatus, materials for about 50 pieces of electron tubes are set at once to make the photocathode


22


. Accordingly, in such a method, a large amount of electron tubes


1


can be made homogeneously at a low cost.




As shown in

FIGS. 2 and 3

, in the electron tube


1


, a voltage of −8 kV is applied to the photocathode


22


and the cathode electrode


11


, whereas the anode


60


is supplied with 0 V (grounded). Here, the cathode electrode


11


and the anode


60


cooperate to form an electron lens. The photoelectrons emitted from the photocathode


22


having an effective diameter of 8 mm are reduced, in terms of their extent, to a diameter of 2 mm, which is smaller than the inner diameter of the collimator section


62


, and then are guided onto the electron entrance surface


44




a


of the semiconductor device


40


. On the other hand, in order to apply a reverse bias to the pn junction in the semiconductor device


40


, a voltage of −150 V is applied to the breakdown-voltage control layer (anode)


44


of the semiconductor device


40


, whereas the silicon substrate (cathode)


41


is supplied with 0 V (grounded). Accordingly, an avalanche-multiplying gain of about 50 times is obtained in the APD.




When light is incident on the electron tube


1


, a photoelectron is emitted from the photocathode


22


into the vacuum. Thus emitted photoelectron is accelerated and converged by the electron lens, so as to be made incident on the electron entrance surface


44




a


of the APD


40


with an energy of 8 keV. As this photoelectron generates one piece of electron-hole pair each time it loses 3.6 eV of energy within the APD


40


, it is multiplied by about 2,000 in this initial multiplying step and then by 50 in the subsequent avalanche multiplication, thereby yielding a gain of about 1×10


5


in total.




The initial multiplication factor of the electron tube


1


is about 2,000, which is higher than that of the typical photomultiplier by about three digits, thereby enabling detection with a very high S/N. In practice, when about four electrons on average are emitted from the photocathode


22


in response to very weak pulse light incident thereon, the number of input photoelectrons (number of incident photons) that has been indistinguishable by the conventional PMT becomes discernible. Such a characteristic obtained by the above-mentioned electron tube


1


is quite effective for quantitatively observing fluorescence emitted from a trace biomaterial. Also, it is quite important for the electron tube itself to stably operate over a long period of time.




In the electron tube


1


according to the first embodiment, a voltage of −150 V is applied from a power supply to the electron entrance surface


44




a


of the semiconductor device


40


by way of the lead pin


32


, the wire


33


, and the entrance surface electrode


47


. On the other hand, a voltage of 0 V is applied to the anode


60


by way of the welded electrode


13


. Namely, the anode


60


has a positive potential with respect to the electron entrance surface


44


of the semiconductor device


40


. Consequently, the positive ion generated at the electron entrance surface


44




a


is subjected to a reverse bias, whereby thus generated positive ion cannot return to the photocathode


22


or the case


10


through the opening section


61


of the anode


60


.




Namely, since the anode


60


is kept at a positive potential with respect to the electron entrance surface


44




a,


i.e., at a reverse potential with respect to the positive ion generated at the electron entrance surface


44




a,


such a positive ion generated at the electron entrance surface


44




a


cannot return to the photocathode


22


or the insulating portion in the body


12


of the case beyond the anode


60


. Accordingly, the photocathode


22


of the electron tube


1


is not influenced by such ion feedback and therefore does not deteriorate upon long-time operations. Further, since the positive ion does not return to the insulating portion of the case


10


either, the latter is prevented from being charged. Thus, the positive ion neither influences the orbit of electrons, which are emitted from the photocathode


22


so as to reach the semiconductor device


40


, nor emits secondary electrons from the case


10


to generate pseudo signals. Accordingly, the electron tube realizes a quite stable operation over a long period of time.




Here, supposing that the ions generated at the electron entrance surface


44




a


of the semiconductor device


40


return to the photocathode


22


, since the positive ion returns to the photocathode


22


with an energy as high as about 8 keV due to the potential difference between the photocathode


22


and the electron entrance surface


44




a


, the material constituting the photocathode


22


is sputtered with the positive ion. Accordingly, under the circumstances where the ions generated at the electron entrance surface


44




a


return to the photocathode


22


, the photocathode sensitivity may remarkably deteriorate even in a short-time operation.




In the following, with reference to

FIGS. 6 and 7

, the configuration of an electron tube


100


according to a second embodiment of the present invention will be explained. Hereinafter, while its differences from the first embodiment will be explained, the constituent parts in the drawings identical or equivalent to those of the electron tube


1


according to the first embodiment will be referred to with the marks identical thereto without their overlapping explanations repeated. Also, the assembling process for the electron tube


100


according to the second embodiment is similar to that in the first embodiment shown in FIG.


5


.




As shown in

FIG. 6

, the electron tube


100


differs from the electron tube


1


in that the length of the cathode electrode


11


is 2 mm, the length of the body


12


is 8 mm, the diameter of an opening section


71


of an anode


70


is 7 mm, and a PD (photodiode) is employed as a semiconductor device


80


. In this embodiment, the operation of the electron lens is changed as the length of the cathode electrode


11


is altered, whereby the extent of the electrons emitted from the photocathode


22


having an effective diameter of 8 mm is converged to a diameter of about 5 mm and made incident on the semiconductor device


80


. Further, the anode


70


(part of the welded electrode


13


) is formed on an extension of the second flange section


13


C of the welded electrode


13


beforehand such that the welded electrode


13


can also serve as the anode


70


.




Thus configured electron tube


100


is supposed to be usable in a strong magnetic field exceeding 1 T (tesla) as well. In such a strong magnetic field, the advancing direction of electrons is determined by the direction of the magnetic field alone, and the electric field can be used only for accelerating the electrons. Namely, in such a strong magnetic field, no electron lens formed by the electric field can operate. Accordingly, the substantial effective diameter of the photocathode


22


is restricted by the size of an electron entrance surface


84




a


of the semiconductor device


80


. Thus, in order to keep the effective diameter of the photocathode


22


as large as possible, the semiconductor device


80


having the large electron entrance surface


84




a


is required.




As shown in

FIG. 7

, the semiconductor device


80


, i.e., PD, comprises a diffusion wafer as its substrate


82


, in which phosphorus, i.e., an n-type impurity, is deeply dispersed with a high concentration into a high-resistance n-type wafer from the rear side thereof. A high concentration of phosphorus is ion-implanted into the peripheral portion of the surface of the substrate


82


, whose rear side has become an n-type high-concentration contact layer


81


, so as to form an n-type channel stop layer


83


. A high concentration of boron is diffused into the surface of the substrate


82


at the center portion so as to form a disk-shaped p-type entrance surface layer (breakdown-voltage control layer)


84


. Formed at the peripheral portion of the entrance surface layer


84


are an oxide film


85


and a nitride film


86


which cover the surface of the channel stop layer


83


. Further, disposed in contact with the entrance surface layer


84


is an entrance surface electrode


87


made of an aluminum film for supplying a voltage to the entrance surface layer


84


. At a position distanced from the entrance surface electrode


87


is an antistatic electrode


88


made of an aluminum film in contact with the channel stop layer


83


. The electron entrance surface


84




a


of the PD


80


is substantially defined by the inner diameter of the entrance surface electrode


87


. Preferably, the diameter of the electron entrance surface


84




a


is 7.2 mm.




Here, in the electron tube


100


, a voltage of −8 kV is applied to the photocathode


22


and the cathode electrode


11


, whereas 0 V is applied to the anode


70


. At this time, the cathode electrode


11


and the anode


70


cooperate to form an electron lens. The photoelectrons emitted from the photocathode


22


having an effective diameter of 8 mm are reduced, in terms of their extent, to a diameter of 5 mm, which is smaller than the inner diameter of the opening section


71


of the anode


70


, and then are guided onto the electron entrance surface


84




a


of the semiconductor device


80


, i.e., PD. On the other hand, in order to apply a reverse bias to the pn junction of the PD


80


, a voltage of −50 V is applied to its anode side, whereas 0 V is applied to its cathode side.




When light is incident on thus configured electron tube


100


, a photoelectron is emitted from the photocathode


22


into the vacuum (within the electron tube


100


). Through the electron lens formed by the cathode electrode


11


and anode


70


, thus emitted photoelectron is accelerated with its orbit converged. After passing through the opening section


71


of the anode


70


, the photoelectron is made incident on the PD


80


with an energy of 8 keV. As this photoelectron generates one piece of electron-hole pair each time it loses 3.6 eV of energy within the PD


80


, it is multiplied by about 2,000, which becomes the gain of the electron tube


100


.




The electron tube


100


mentioned above, in which the face plate


21


has a large light-receiving surface, can stably operate in a strong magnetic field and can be employed in high-energy experiments using an accelerator. In an example of such experiments, 10,000 pieces of electron tubes are disposed within an experimental apparatus generating a strong magnetic field of 4 T (tesla) so as to capture light emitted by a scintillator. When a number of electron tubes are arranged in a limited space for the experiment, it is important for the electron tubes to have a small size and uniform characteristics. Since the electron tube


100


adopts a vacuum seal technique employing the indium ring


23


, it can be made with a small size. Also, since a large number of the electron tubes


100


can be made at once in a transfer apparatus, homogenous electron tubes having uniform characteristics in terms of sensitivity of the photocathode


22


and the like can be realized.




Further, in the electron tube


100


, since there is no shielding member blocking the photoelectron emitted from the photocathode


22


, a large effective diameter can be obtained even in a strong magnetic field. In general, in a strong magnetic field of about 4 T, no electron lens made by an electric field can operate, whereby the photoelectron emitted from the photocathode


22


cannot be converged into a small area by means of an electric field. Accordingly, in the electron tube


100


which is tolerant to such a use, the photocathode


22


having an effective diameter of 8 mm and the semiconductor device


80


having the electron entrance surface


84




a


with an effective diameter of 7.2 mm which is substantially equivalent to the former are disposed, whereas only the anode


70


(part of the welded electrode


13


) having the opening section with a diameter of 7 mm is disposed therebetween. When the electron tube


100


is operated in a strong magnetic field of 4 T having the same direction as the incident light (coinciding with AX shown in FIG.


2


), the photoelectron emitted from the center region of the photocathode


22


(portion with a diameter of 7 mm) is made incident on the semiconductor device


80


without being blocked. Accordingly, in the electron tube


100


, an effective diameter of 7 mm can be obtained in a strong magnetic field. It is needless to mention that a typical photomultiplier (PMT) cannot be used in such a strong magnetic field.




The present invention should not be restricted to the foregoing embodiments. For example, in the electron tube


100


according to the second embodiment, as shown in

FIGS. 8 and 9

, a grid-shaped mesh electrode


72


can be disposed at the opening section


71


of the anode


70


(part of the welded electrode


13


). In order to form the mesh electrode


72


, the anode


70


made of stainless is partially etched. In this case, the line width and pitch of the mesh electrode


72


are 50 microns and 1.5 mm, respectively. Electrons are transmitted through the mesh electrode


72


at a rate corresponding to the open area ratio (93%≐(1.5-0.05)


2


/(1.5)


2


×100) of the mesh electrode


72


.




The mesh electrode


72


is disposed at the opening section


71


of the anode


70


since the opening section


71


of the anode


70


is increased in view of the electron entrance surface


84




a


of the semiconductor device


80


. Namely, it is due to the fact that, when the opening section


71


of the anode


70


is made large, the valley of minus potential on the side of the photocathode


22


penetrates through the anode


70


from the opening section


71


, thereby lowering the effect for suppressing the feedback of the positive ion generated at the electron entrance surface


84




a


of the semiconductor device


80


. When the mesh electrode


72


is additionally provided, the minus potential from the photocathode


22


can be prevented from intruding into the electron entrance surface


84


, whereby the effect for suppressing the ion feedback can be maintained. Here, the maximum diameter of the opening section


71


of the anode


70


is preferably equal to or smaller than the electron entrance surface


84




a


of the PD


80


.




As explained in the foregoing, in accordance with the present invention (first and second embodiments), the case is configured to comprise a ring-shaped cathode electrode integrally made of a conductive material, which is disposed on the photocathode side so as to form, together with an anode, an electron lens for irradiating a semiconductor device with an electron emitted from the photocathode, and is connected to a face plate by way of a bonding member made of indium or the like; a ring-shaped welded electrode, positioned on the stem side, having an outer end secured to the stem; and a ring-shaped body made of an electrical insulating material, positioned between the cathode electrode and the welded electrode, having one end secured to an end face of the cathode electrode and the other end secured to an end face of the welded electrode; while they are mounted on each other with their center axes coinciding with each other. With this configuration, an electron tube can be made smaller such that a number of electron tubes can be arranged densely within a limited narrow space, and an electron tube with a very high workability in its assembling process can be obtained.




On the other hand, in the case where the distance between the photocathode and the electron entrance surface of the semiconductor device, an envelope type body can be selected because a consideration for the impinging of the stray electrons.

FIG. 10A

is a top view showing the electron tube according to a third embodiment of the present invention, and

FIG. 10B

is a cross-sectional view showing the electron tube according to the third embodiment taken along line III—III in FIG.


10


A.




The electron tube


200


according to the third embodiment comprises a face plate


21


with a photocathode


22


, an envelope type body


10


, and a photodiode


80


having an electron entrance surface


84


. The face plate


21


is shaped like a square with four sides of 37 mm. A photocathode electrode


25


has a through-hole for defining an effective area


210


(28 mm×28 mm) and is provided so as to surround the effective area


210


on an inner surface of the face plate


21


while electrically connecting to the photocathode


22


.




The electron tube


200


further comprises a body


10


having one opening


14


and a bottom portion


10




a


. A cathode electrode


11


, which has a through-hole for transmitting photoelectrons emitted from the photocathode


22


, is provided such that its edge portion


11




a


is fixed on the opening edge of the body


10


while the face plate


21


is supported by the cathode electrode


11


through a bonding member


23


.




On an inner surface of the bottom portion


10




a


of the body


10


, a photodiode


80


having the electron entrance surface


84


is mounted, and outer lead pins


32


pass through the bottom portion


10




a.






Further, in the third embodiment, the bonding member


23


is made of a material selected from the group consisting of In, Au, Pb, alloys containing In, and alloys containing Pb. Also, the electron tube


200


according to the third embodiment is fabricated in a same manner as the first and second embodiments (see FIG.


5


).




In the following, with reference to

FIGS. 11

to


13


, the configuration and assembling process of electron tubes according to the present invention (fourth and fifth embodiments) will be explained. Here, the configuration and assembling process of an electron tube


300


according to the fourth embodiment shown in

FIG. 11

are identical to those of the electron tube


1


according to the first embodiment except for the structure and assembling step of the body


12


. Also, the configuration and assembling process of an electron tube


400


according to the fifth embodiment are identical to those of the electron tube


100


according to the second embodiment except for the structure and assembling step of the body


12


.





FIG. 11

is a cross-sectional view showing the configuration of the electron tube


300


according to the fourth embodiment of the present invention. As depicted, the electron tube


300


has the cylindrical case


10


. The case


10


is constituted by the ring-shaped cathode electrode


11


, which is made of a highly conductive covar metal by any of various integral-molding methods such as press molding, injection molding, or cutting; the ring-shaped body


12


made of an electrical insulating material (e.g., ceramics); and the ring-shaped welded electrode


13


made of a covar metal. The body


12


further comprises a first bulb (insulating member)


12


A, a second bulb (insulating member)


12


B, and a ring-shaped intermediate electrode


90


made of a covar metal held and secured between the insulating members


12


A and


12


B. The members


11


,


12


(including the members


12


A,


12


B, and


90


), and


13


are mounted on each other with their center axes coinciding with each other. While the body


12


including the intermediate electrode


90


is disposed between the cathode electrode


11


and the welded electrode


13


, one end of the body


12


(on the side of the first opening


14


) is butted against the flat end face


11




a


of the cathode electrode


11


and then is secured thereto by brazing or the like. The other end of the body


12


(on the side of the second opening


15


) is butted against the flat end face


13




a


of the welded electrode


13


and then is secured thereto by brazing or the like. In order to form the body


12


, the outer peripheral end portion of the intermediate electrode


90


is held between the first bulb


12


A and the second bulb


12


B, and their joint portions are brazed. Thus, the case


10


can easily be united by brazing.




As with the above-mentioned first and second embodiments, the ring-shaped cathode electrode


11


, the body


12


(including the bulbs


12


A and


12


B and the intermediate electrode


90


), and the welded electrode


13


may have a polygonal cross-sectional form.




The inner peripheral wall face


11




b


of the cathode electrode


11


and the inner wall face


12




a


of the first and second bulbs


12


A and


12


B each have a circular cross-sectional form. The inner diameters of the cathode electrode


11


and body


12


are respectively 10 mm and 11 mm, for example. Here, the through-holes


11




b


and


12




a


may have either identical or different cross-sectional forms and may be either circular or polygonal. Preferably, in the fourth embodiment, the lengths of the cathode electrode


11


, first bulb


12


A, and second bulb


12


B are 3.5 mm, 3.5 mm, and 3 mm, respectively.




Here, the intermediate electrode


90


projects inward from the inner peripheral wall face


12




a


of the first and second bulbs


12


A and


12


B, while the inner diameter of an opening section


90




a


of the intermediate electrode


90


is minimized (preferably 7 mm) within a range which does not interfere with the electron orbit. Accordingly, the insulating members


12


A and


12


B are prevented from being charged with stray electrons. Also, even when the insulating members


12


A and


12


B are charged for some reason, the potential in a space near the electron orbit is made constant by means of the intermediate electrode


90


, whereby the charge of the insulating members


12


A and


12


B can be prevented from affecting the electron orbit. Preferably, the thickness of the intermediate electrode


90


is 0.5 mm.




Firmly attached to the cathode electrode


11


in the case


10


is the face plate


21


made of glass which transmits light therethrough. The face plate


21


has the photocathode


22


on the inner face and is disposed at one end of the body


12


on the side of the first opening


14


. After the photocathode


22


is made, the face plate


21


is integrated with the cathode electrode


11


by way of the bonding member (bonding ring)


23


made of a metal material selected from the group consisting of In, Au, Pb, alloys containing In, and alloys containing Pb. Disposed at the peripheral portion of the photocathode


22


is the electrode


25


made of a thin film of chromium for electrically connecting together the photocathode


22


and the bonding member


23


(referred to as “indium ring” hereinafter) containing indium. The inner diameter of the electrode


25


, i.e., 8 mm, defines the effective diameter of the photocathode


22


. The indium ring


23


is formed so as to project from the inner side face of the hollow cylindrical auxiliary member


24


. When the indium ring


23


and the face plate


21


are successively disposed on the cathode electrode


11


, and then the cathode electrode


11


and the face plate


21


are pressed against each other at a high pressure of about 100 kg, the indium ring


23


deforms and functions as an adhesive, whereby the face plate


21


is integrated with the case


10


.




Firmly attached to the welded electrode


13


in the case


10


is the disk-shaped stem


31


made of a conductive material (e.g., covar metal). The stem


31


is disposed at the other end of the case


10


(on the side of the second opening


15


in the body


12


). As with the above-mentioned first and second embodiments, the welded electrode


13


comprises the cylindrical main part


13


A; the circular first flange section


13


B, positioned at one end of the cylindrical main part


13


A, projecting outward so as to be utilized for joining with the stem


31


; and the circular second flange section


13


C, positioned at the other end of the cylindrical main part


13


A (on the body side), projecting inward so as to be utilized for joining with the body


12


. Formed at the outer periphery of the stem


31


is the circular cutout edge section


31




a


to be secured to the first flange section


13


B.




Further, disposed on the mounting surface


310


of the stem


31


in the electron tube


300


according to the fourth embodiment is the semiconductor device


40


having the same configuration as that of the APD (avalanche photodiode) in the first embodiment (see FIG.


4


). Preferably, the diameter of the electron entrance surface


44




a


on the inside of the entrance surface electrode


47


is 3 mm.




As shown in

FIGS. 4 and 11

, the planar anode


60


is disposed between the semiconductor device


40


and the intermediate electrode


90


, and the outer peripheral end portion of the anode


60


is secured to the second flange section


13


C of the welded electrode


13


. This configuration is similar to that in the electron tube


1


in the above-mentioned first embodiment. The anode


60


is formed by a pressed thin stainless sheet having a thickness of 0.3 mm. Preferably, the distance between the anode


60


and the semiconductor device


40


is 1 mm.




In the following, the assembling process for the electron tube


300


according to the fourth embodiment will be explained with reference to FIG.


12


. This assembling process is the same as the assembling process (

FIG. 5

) for the electron tubes


1


and


100


according to the first and second embodiments explained earlier, except for the assembling step of the body


12


.




In the transfer apparatus (vacuum chamber), the case


10


(including the cathode electrode


11


, first and second bulbs


12


A and


12


B, intermediate electrode


90


, and welded electrode


13


) and the face plate


21


are joined together by way of the indium ring


23


, and a pressure of about 100 kg is applied to thus formed assembly (to the face plate


21


and the stem


31


in the directions indicated by arrows A and B in FIG.


12


), whereby the indium ring


23


, which is the softest member therein, is crushed. As a result, the indium ring


23


functions as an adhesive. Accordingly, as the inside of the apparatus is kept in a vacuum state, a vacuum is produced in the electron tube


300


. Finally, the vacuum in the transfer apparatus is caused to leak out, thereby accomplishing a series of steps.




As shown in

FIG. 11

, in the electron tube


300


, a voltage of −12 kV is applied to the photocathode


22


and the cathode electrode


11


, the anode


60


is supplied with 0 V (grounded), and their in-between voltage of −6 kV is applied to the intermediate electrode


90


. Here, the cathode electrode


11


, the anode


60


, and the intermediate electrode


90


cooperate to form an electron lens. Accordingly, the photoelectrons emitted from the photocathode


22


having an effective diameter of 8 mm are reduced, in terms of their extent, to a diameter of 2 mm, which is smaller than the inner diameter of the collimator section


62


, and then are guided onto the electron entrance surface


44




a


of the semiconductor device


40


. On the other hand, as in the case of the above-mentioned first embodiment, in order to apply a reverse bias to the pn junction in the semiconductor device


40


, a voltage of −150 V is applied to the breakdown-voltage control layer (anode)


44


of the semiconductor device


40


, whereas the silicon substrate (cathode)


41


is supplied with 0 V (grounded). Accordingly, an avalanche-multiplying gain of about 50 times is obtained in the APD. Here, a method of applying a predetermined voltage, which is not lower than the voltage applied to the photocathode


22


but not greater than the voltage applied to the anode


60


, to the intermediate electrode


90


can be realized with a Cockcroft-Walton power supply. Alternatively, the applied voltage may be divided by means of a resistance.




When light is incident on the electron tube


300


according to the fourth embodiment, a photoelectron is emitted from the photocathode


22


into the vacuum (within the electron tube


300


). Through the electron lens, thus emitted photoelectron is accelerated with its orbit being converged, so as to be made incident on the electron entrance surface


44




a


of the APD


40


with an energy of 12 keV. As this photoelectron generates one piece of electron-hole pair each time it loses 3.6 eV of energy within the APD


40


, it is multiplied by about 3,000 in this initial multiplying step and then by 50 in the subsequent avalanche multiplication, thereby yielding a gain of about 2×10


5


in total.




In addition to the effects similar to those of the above-mentioned first embodiment, the electron tube


300


according to the fourth embodiment yields the specific effects as will be explained hereinafter. In typical electron tubes, bulbs made of an insulating material may be charged under the influence of stray electrons, ions, or X-rays. The charge of the inner peripheral wall face may trigger dielectric breakdown.




In the electron tube


300


according to the fourth embodiment, by contrast, the body


12


is divided into two pieces of the first and second bulbs


12


A and


12


B made of ceramics, whereas the intermediate electrode


90


is inserted between the first and second bulbs


12


A and


12


B. Since a predetermined voltage between the voltages respectively applied to the photocathode


22


and the anode


60


is applied to the intermediate electrode


90


, no dielectric breakdown occurs even when a strong negative voltage is applied to the photocathode


22


. Also, since the intermediate electrode


90


is inserted between the first and second bulbs


12


A and


12


B made of ceramics, the insulating parts (first and second bulbs


12


A and


12


B) are hard to be charged with stray electrons, ions, X-rays, or the like. Further, since the intermediate electrode


90


is set to a middle potential, dielectric breakdown will not occur in the first and second bulbs


12


A and


12


B even if these insulating parts are charged. Accordingly, in the electron tube


300


, a high gain can be obtained even when a strong negative voltage is applied to the photocathode


22


.




Here, the opening section


90




a


of the intermediate electrode


90


, which is set to a middle potential, has such a minimum size that does not interfere with the electron orbit and is set to the potential of a space near the electron orbit, whereby the influence of the charge in the inner peripheral wall face


12




a


of each of the first and second bulbs


12


A and


12


B upon the electron orbit can be suppressed.




In the electron tube


300


according to the fourth embodiment, as with the electron tube


1


in the first embodiment, a voltage of −150 V is applied to the electron entrance surface


44




a


of the semiconductor device


40


, whereby the electron entrance surface


44




a


is kept at a negative potential with respect to the anode


60


. Accordingly, regarding to the ion feedback, the effects similar to those of the above-mentioned first embodiment can also be obtained by the electron tube


300


according to the fourth embodiment.




In the following, the configuration of the electron tube


400


according to the fifth embodiment of the present invention will be explained with reference to FIG.


13


. Hereinafter, while its differences from the fourth embodiment will be explained, the constituent parts in the drawing identical or equivalent to those of the electron tubes


1


,


100


, and


300


according to the first, second and fourth embodiments will be referred to with the marks identical thereto without their overlapping explanations repeated. Also, the configuration and assembling process of the electron tube


400


according to the fifth embodiment are identical to those of the electron tube


100


according to the second embodiment except for the structure and assembling step of the body


12


. Further, the semiconductor device


80


in the electron tube


400


according to the fifth embodiment has the configuration shown in FIG.


7


.




As shown in

FIG. 13

, the electron tube


400


differs from the electron tube


300


of the fourth embodiment in that the cathode electrode has a length of 2 mm, the body is divided into four pieces of first to fourth bulbs (insulating members)


12


C to


12


F, three sheets of first to third disk electrodes


91


to


93


(included in the intermediate electrode


90


) are successively held between the bulbs


12


C to


12


F, and a PD (photodiode) is employed as the semiconductor device


80


. In this embodiment, the operation of the electron lens is changed as the length of the cathode electrode


11


is altered, whereby the extent of the photoelectrons emitted from the photocathode


22


having an effective diameter of 8 mm is converged to a diameter of about 5 mm and made incident on the semiconductor device


80


. Further, the anode


70


(part of the welded electrode


13


) is formed on an extension of the second flange section


13


C of the welded electrode


13


beforehand such that the welded electrode


13


can also serve as the anode


70


.




As with the electron tube


100


according to the second embodiment, thus configured electron tube


400


of the fifth embodiment is supposed to be usable in a strong magnetic field exceeding 1 T (tesla) as well. Since no electron lens formed by the electric field can operate in such a strong magnetic field, the substantial effective diameter of the photocathode


22


is restricted by the size of the electron entrance surface


84




a


of the semiconductor device


80


. Thus, in order to keep the effective diameter of the photocathode


22


as large as possible, the semiconductor device


80


having the large electron entrance surface


84




a


is required.




The configuration of the semiconductor device


80


employed in the electron tube


400


according to the fifth embodiment is shown in

FIG. 7

(as in the case of the second embodiment). The electron entrance surface


84




a


of the PD


80


is substantially defined by the inner diameter of the entrance surface electrode


87


and preferably has a diameter of 7.2 mm.




Here, in the electron tube


400


, a voltage of −16 kV is applied to the photocathode


22


and the cathode electrode


11


, whereas a voltage of +50 V is applied to the anode


70


. Respectively applied to the first to third disk electrodes


91


to


93


are predetermined voltages, between the photocathode


22


and the anode


70


, of −12 kV, −8 kV, and −4 kV. At this time, the cathode electrode


11


, the anode


70


, and the intermediate electrode


90


cooperate to form an electron lens. The photoelectrons emitted from the photocathode


22


having an effective diameter of 8 mm are reduced, in terms of their extent, to a diameter of 5 mm, which is smaller than the inner diameter of the opening section


71


of the anode


70


, and then are guided onto the electron entrance surface


84




a


of the semiconductor device


80


, i.e., PD. On the other hand, a reverse bias is applied to the PD


80


, such that a voltage of +50 V is applied to its cathode side by way of the stem


31


, whereas the ground potential of an external circuit (processing circuit) is applied to its anode side by way of the lead pin


32


and the wire


33


. Also, a DC signal component is outputted from the lead pin


32


.




When light is incident on thus configured electron tube


400


, a photoelectron is emitted from the photocathode


22


into the vacuum (within the electron tube


400


). Through the electron lens formed by the cathode electrode


11


, intermediate electrode


90


, and anode


70


, thus emitted photoelectron is accelerated with its orbit converged. After passing through an opening section


90


Aa of the intermediate electrode


90


and the opening section


71


of the anode


70


, the photoelectron is made incident on the PD


80


with an energy of 16 keV. As this photoelectron generates one piece of electron-hole pair each time it loses 3.6 eV of energy within the PD


80


, it is multiplied by about 4,000, which becomes the gain of the electron tube


400


.




The body


12


is divided into four pieces of the ceramic bulbs


12


C to


12


F by way of the intermediate electrode


90


(first to third disk electrodes


91


to


93


). Predetermined voltages between the photocathode


22


and the anode


70


are respectively applied to the first to third disk electrodes


91


to


93


. Accordingly, dielectric breakdown does not occur even when a strong negative voltage is applied to the photocathode


22


, whereby a high implanting gain can be obtained. Further, since the intermediate electrode


90


is set to the potential in the space near the electron orbit, even when the inner peripheral wall face


12




a


of each of the bulbs


12


C to


12


F is charged, the electron orbit is not influenced thereby.




As with the second embodiment, the electron tube


400


according to the fifth embodiment can also be employed in high-energy experiments using an accelerator. In general, in a strong magnetic field of about 4 T, no electron lens made by an electric field can operate, whereby the photoelectron emitted from the photocathode


22


cannot be converged into a small area by means of an electric field. Accordingly, as with the second embodiment, in the electron tube


400


of the fifth embodiment, which is tolerant to such a use, the photocathode


22


having an effective diameter of 8 mm and the semiconductor device


80


having the electron entrance surface


84




a


with an effective diameter of 7.2 mm which is substantially equivalent to the former are disposed, whereas only the anode


70


(part of the welded electrode


13


) having the opening section with a diameter of 7 mm is disposed therebetween. When the electron tube


400


is operated in a strong magnetic field of 4 T having the same direction as the incident light (coinciding with AX shown in FIG.


2


), the photoelectron emitted from the center region of the photocathode


22


(portion with a diameter of 7 mm) is made incident on the semiconductor device


80


without being blocked. Accordingly, in the electron tube


400


, an effective diameter of 7 mm can be obtained in a strong magnetic field. It is needless to mention that a typical photomultiplier (PMT) cannot be used in such a strong magnetic field.




Further, in the electron tube


400


according to the fifth embodiment, as with the second embodiment, as shown in

FIGS. 8 and 9

, the grid-shaped mesh electrode


72


can be disposed at the opening section


71


of the anode


70


(part of the welded electrode


13


). In order to form the mesh electrode


72


, the anode


70


made of stainless is partially etched. In this case, the line width and pitch of the mesh electrode


72


are 50 microns and 1.5 mm, respectively. Electrons are transmitted through the mesh electrode


72


at a rate corresponding to the open area ratio (93%) of the mesh electrode


72


.




The mesh electrode


72


is disposed at the opening section


71


of the anode


70


since the opening section


71


of the anode


70


is increased in view of the electron entrance surface


84




a


of the semiconductor device


80


. Namely, it is due to the fact that, when the opening section


71


of the anode


70


is made large, the valley of minus potential on the side of the photocathode


22


penetrates through the anode


70


from the opening section


70


, thereby lowering the effect for suppressing the feedback of the positive ion generated at the electron entrance surface


84




a


of the semiconductor device


80


. When the mesh electrode


72


is additionally provided, the minus potential from the photocathode


22


can be prevented from intruding into the electron entrance surface


84


, whereby the effect for suppressing the ion feedback can be maintained. Here, the maximum diameter of the opening section


71


of the anode


70


is preferably equal to or smaller than the electron entrance surface


84




a


of the PD


80


. These effects are similar to those of the above-mentioned second embodiment.




As explained in the foregoing, in accordance with the present invention (fourth and fifth embodiments), the case is configured to comprise a ring-shaped cathode electrode integrally made of a conductive material, which is disposed on the photocathode side so as to form, together with an anode, an electron lens for irradiating a semiconductor device with a photoelectron emitted from the photocathode, and is connected to a face plate by way of a bonding member made of indium or the like; a ring-shaped welded electrode, positioned on the stem side, having an outer end secured to the stem; and a body made of an electrical insulating material, positioned between the cathode electrode and the welded electrode, having one end secured to an end face of the cathode electrode and the other end secured to an end face of the welded electrode. In this body, at least two insulating members and a ring-shaped intermediate electrode (including a plurality of disk electrodes) inserted between the insulating members are mounted on each other with their center axes coinciding with each other. With this configuration, an electron tube can be made smaller such that a number of electron tubes can be arranged densely within a limited narrow space, and an electron tube having a very high workability in its assembling process can be obtained.




From the invention thus described, it will be obvious that the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims.




The basic Japanese Application No. 186387/1996 filed on Jul. 16, 1996 and No. 186392/1996 filed on Jul. 16, 1996 are hereby incorporated by reference.



Claims
  • 1. An electron tube comprising:a face plate on which a photocathode for emitting an electron in response to incident light is provided; an electron entrance surface on which the electron emitted from said photocathode is incident, said electron entrance surface being provided so as to face said photocathode; a cathode electrode provided between said face plate and said electron entrance surface, said cathode electrode having a through-hole for transmitting therethrough the electron from said photocathode toward said electron entrance surface; and a bonding member, disposed between said face plate and said cathode electrode, for bonding said face plate and said cathode electrode together, said bonding member being made of a metal material selected from the group consisting of In, Au, Pb, alloys containing In, and alloys containing Pb.
  • 2. An electron tube according to claim 1, further comprising:a body being a hollow member which has a first opening and a second opening opposing said first opening, said cathode electrode being provided on the first opening side of said body; and a stem, provided on the second opening side of said body, for defining a distance between said photocathode and an electron entrance surface on which the electron emitted from said photocathode is incident.
  • 3. An electron tube according to claim 2, further comprising a semiconductor device having said electron entrance surface,wherein said stem has a mounter section for holding said semiconductor device, said mounter section projecting toward said photocathode.
  • 4. An electron tube according to claim 1, further comprising a container accommodating said electron entrance surface, said container having:an opening for supporting said cathode electrode; and a bottom portion supporting said electron entrance surface and defining a distance between said photocathode and said electron entrance surface.
  • 5. An electron tube according to claim 4, further comprising a semiconductor device having said electron entrance surface and mounted on said bottom portion while being accommodated in said container.
  • 6. An electron tube according to claim 1, further comprising a conductive auxiliary member for sealing a gap between said cathode electrode and said face plate, said bonding member being positioned in said gap.
  • 7. An electron tube according to claim 2, further comprising a welded electrode provided on the second opening side of said body and positioned between said body and said stem, said welded electrode having a through-hole for transmitting therethrough the electron transmitted through the through-hole of said cathode electrode toward said electron entrance surface.
  • 8. An electron tube according to claim 7, further comprising an anode having a through-hole for transmitting therethrough the electron transmitted through said cathode electrode, said anode being supported by said welded electrode such that at least part of said anode is positioned between said cathode electrode and said electron entrance surface.
  • 9. An electron tube according to claim 2, wherein the through-hole of said cathode electrode has an area smaller than that of each of said first and second openings of said body.
  • 10. An electron tube according to claim 7, wherein the through-hole of said welded electrode has an area equal to or smaller than that of said electron entrance surface.
  • 11. An electron tube according to claim 8, wherein the through-hole of said anode has an area equal to or smaller than that of said electron entrance surface.
  • 12. An electron tube according to claim 7, wherein said welded electrode has a portion to be resistance-welded to said stem.
  • 13. An electron tube according to claim 2, wherein said body comprises:at least two insulating members each having a through-hole extending from said photocathode toward said electron entrance surface; and at least one conductive member provided between said insulating members adjacent to each other, said conductive member having a through-hole extending from said first opening toward said second opening.
  • 14. An electron tube according to claim 13, wherein the through-hole of said conductive member has an area smaller than that of the through-hole of each of said insulating members.
  • 15. An electron tube comprising:a body having a first opening and a second opening opposing said first opening, said body comprising: at least two insulating members each having a through-hole extending from said first opening toward said second opening, and at least one conductive member provided between said insulating members adjacent to each other, said conductive member having a through-hole extending from said first opening toward said second opening; a face plate which is provided on the first opening side of said body and on which a photocathode for emitting an electron in response to incident light is provided; a stem, provided on the second opening side of said body, for defining a distance between said photocathode and an electron entrance surface on which the electron emitted from said photocathode is incident; a cathode electrode provided on the first opening side of said body and positioned between said body and said face plate, said cathode electrode having a through-hole for transmitting therethrough the electron from said photocathode toward said electron entrance surface; and a bonding ring, provided between said face plate and said cathode electrode, for bonding said face plate and said cathode electrode together, said bonding ring being made of a metal material selected from the group consisting of In, Au, Pb, alloys containing In, and alloys containing Pb.
  • 16. An electron tube according to claim 15, further comprising a conductive auxiliary member for sealing a gap between said cathode electrode and said face plate, said bonding member being positioned in said gap.
  • 17. An electron tube according to claim 15, further comprising a welded electrode provided on the second opening side of said body and positioned between said body and said stem, said welded electrode having a through-hole for transmitting therethrough the electron transmitted through the through-hole of said cathode electrode toward said electron entrance surface.
  • 18. An electron tube according to claim 17, further comprising an anode having a through-hole for transmitting therethrough the electron transmitted through said cathode electrode, said anode being supported by said welded electrode such that at least part of said anode is positioned between said cathode electrode and said electron entrance surface.
  • 19. An electron tube according to claim 15, wherein the through-hole of said cathode electrode has an area smaller than that of each of said first and second openings.
  • 20. An electron tube according to claim 17, wherein the through-hole of said welded electrode has an area equal to or smaller than that of said electron entrance surface.
  • 21. An electron tube according to claim 18, wherein the through-hole of said anode has an area equal to or smaller than that of said electron entrance surface.
  • 22. An electron tube according to claim 17, wherein said welded electrode has a portion to be resistance-welded to said stem.
  • 23. An electron tube according to claim 15, further comprising a semiconductor device having said electron entrance surface,wherein said stem has a mounter section for holding said semiconductor device, said mounter section projecting toward said photocathode.
  • 24. An electron tube according to claim 15, wherein the through-hole of said conductive member has an area smaller than that of the through-hole of each of said insulating members.
Priority Claims (2)
Number Date Country Kind
8-186387 Jul 1996 JP
8-186392 Jul 1996 JP
RELATED APPLICATION

This is a continuation-in-part application of application Ser. No. 08/891,840 filed on Jul. 14, 1997, now U.S. Pat. No. 5,883,466.

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Continuation in Parts (1)
Number Date Country
Parent 08/891840 Jul 1997 US
Child 09/207667 US