Photoelectron-emitting cathode and electron tube

Abstract

To provide a photoemission cathode and electron tube that offer high sensitivity while improving cathode linearity characteristics. [Solution] A photoelectron-emitting cathode comprising a substrate, a conductive layer, and a photoelectric conversion layer in this order, wherein the conductive layer is a laminate comprising a first layer made of zinc oxide and a second layer containing a non-zinc metal, and the conductive layer is electrically connected to the photoelectric conversion layer. In the conductive layer, the atomic ratio of zinc to the non-zinc metal may be 6 or more. The non-zinc metal may be aluminum.

Description

Technical Field

[0001] The present invention relates to a photoelectron emission cathode and an electron tube.

Background Art

[0002] In photoelectron emission cathodes, it is desired to perform detection with linearity over a wide range from a very small light quantity to a large light quantity, that is, to improve the cathode linearity characteristic. Here, the cathode linearity characteristic means the linearity of the cathode output current with respect to the incident light quantity. To improve the cathode linearity characteristic, appropriate charge supply to the photoelectric conversion layer is required. For this purpose, for example, it is conceivable to provide a conductive layer (intermediate layer) between the substrate and the photoelectric conversion layer to reduce the sheet resistance of the photoelectric conversion layer.

[0003] For example, the following Patent Document 1 discloses a known photoelectron emission cathode in which a conductive layer made of carbon nanotubes, fullerenes or the like is provided between a substrate and a photocathode (see Patent Document 1 below).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of Invention

Problems to be Solved by the Invention

[0005] However, in the photoelectron emission cathode described in the above Patent Document 1, the carbon nanotubes and fullerenes cited as the intermediate layer are difficult to apply uniformly in a suspension containing them, so that a thick film is required to ensure conduction over the entire surface of the substrate, leaving room for improvement in terms of light transmittance and, in turn, sensitivity. There is also a demand for a photoelectron emission cathode having high sensitivity in the visible light region (for example, around 400 nm).

[0006] The present invention has been made in view of the above problems, and aims to provide a photoelectron-emitting cathode and an electron tube that can improve cathode linearity characteristics while having high sensitivity in the visible light range. [Means for solving the problem]

[0007] The present invention is a photoelectron-emitting cathode comprising a substrate, a conductive layer and a photoelectric conversion layer in this order, wherein the conductive layer is a laminate comprising a first layer made of zinc oxide and a second layer containing a non-zinc metal, and the conductive layer is electrically connected to the photoelectric conversion layer. In this photoemission cathode, the conductive layer consists of a laminate comprising a first layer made of zinc oxide and a second layer containing a non-zinc metal. The conductive layer, by comprising the first layer made of zinc oxide, has high transmittance in the visible light range. Furthermore, the conductive layer, by comprising the second layer containing a non-zinc metal, enables a stable charge supply to the first layer, making it possible to uniformly and sufficiently reduce the surface resistance of the conductive layer. In other words, in an electron tube equipped with a photoemission cathode and anode, when a voltage is applied between the conductive layer and the anode, it becomes easier to supply electrons from the conductive layer to the photoelectric conversion layer. Therefore, when light is incident on the photoelectric conversion layer, it becomes easier to emit electrons from the photoelectric conversion layer. Thus, this photoemission cathode can improve cathode linearity characteristics while maintaining high sensitivity in the visible light range.

[0008] The present invention may also be [2] "the photoelectron-emitting cathode described in [1], wherein the thickness of the first layer is greater than the thickness of the second layer." This photoemission cathode can achieve higher sensitivity in the visible light range while improving cathode linearity characteristics.

[0009] The present invention may also be [3] "a photoelectron-emitting cathode according to [1] or [2], wherein the atomic ratio of zinc to the non-zinc metal in the conductive layer is 6 or more." This photoemission cathode can achieve higher sensitivity in the visible light range while improving cathode linearity characteristics.

[0010] The present invention may also be [4] "a photoelectron-emitting cathode according to any one of [1] to [3], wherein the non-zinc metal is aluminum." This photoemission cathode allows for sufficient conductivity in the conductive layer, thereby more effectively improving cathode linearity characteristics.

[0011] The present invention may also be [5] "a photoelectron-emitting cathode according to any one of [1] to [4] having a conductive layer with a thickness of 230 Å or more." This photoemission cathode can effectively improve cathode linearity characteristics.

[0012] The present invention may also be [6] "a photoelectron-emitting cathode according to any one of [1] to [5], wherein the substrate is a light-transmitting substrate and the conductive layer is a light-transmitting conductive layer." This photoemission cathode can be a transmissive photoemission cathode that has high sensitivity in the visible light range while improving cathode linearity characteristics.

[0013] The present invention may also be [7] "a photoelectron-emitting cathode according to any one of [1] to [6], wherein the photoelectric conversion layer is a bialkali photoelectric conversion layer." This photoemission cathode can further improve sensitivity in the visible light range.

[0014] The present invention may also be [8] "an electron tube comprising a photoelectron-emitting cathode as described in any of [1] to [7]". This electron tube includes the above-described photoelectron emission cathode. The photoelectron emission cathode can have high sensitivity in the visible light region while improving cathode linearity characteristics, and can improve anode linearity characteristics of the electron tube in the visible light region. For this reason, the electron tube of the present invention can improve quantitativeness for light amounts in a wide range from weak light to relatively strong light in the visible light region.

[0015] The present invention may be the electron tube according to [8], further comprising a conductive tube that supports the photoelectron emission cathode, and a second conductive layer provided on an inner surface of the conductive tube, wherein the second conductive layer contains zinc oxide and a non-zinc metal, and the second conductive layer is electrically connected to the conductive layer of the photoelectron emission cathode. According to this electron tube, the second conductive layer provided on the inner surface of the conductive tube ensures electrical continuity between the conductive tube and the conductive layer of the photoelectron emission cathode, and can reliably supply power to the photoelectric conversion layer. Effects of the Invention

[0016] According to the present invention, a photoelectron emission cathode and an electron tube are provided that can have high sensitivity in the visible light region while improving cathode linearity characteristics. Brief Description of the Drawings

[0017] [Figure 1] Figure 1 is a partial cross-sectional view showing a photomultiplier tube that is an embodiment of the electron tube of the present invention. [Figure 2] Figure 2 is a cross-sectional view showing a part of the photoelectron emission cathode of Figure 1. [Figure 3] Figure 3 is a cross-sectional view showing a first structure formed during the manufacture of the electron tube of Figure 1. [Figure 4] Figure 4 is a cross-sectional view showing a second structure obtained after the first structure during the manufacture of the electron tube of Figure 1. [Figure 5] Figure 5 is a cross-sectional view showing a third structure obtained after the second structure during the manufacture of the electron tube of Figure 1. [Figure 6]This is a graph showing the relationship between wavelength and quantum efficiency (QE) for the photoelectron emission cathodes of the photomultiplier tubes according to Examples 1 to 7. [Figure 7] This is a graph showing the relationship between the output current of the photoelectron emission cathode and the deviation rate in the photomultiplier tubes according to Example 1 and Comparative Example 1. [Figure 8] This is a graph showing the relationship between the anode output current and the deviation rate in the photomultiplier tube according to Example 1. [Modes for Carrying Out the Invention]

[0018] Hereinafter, embodiments of the photoelectron emission cathode according to the present invention will be described with reference to the drawings. In each drawing, the same or corresponding parts are denoted by the same reference numerals, and redundant descriptions will be omitted.

[0019] Figure 1 is a partial cross-sectional view showing a photomultiplier tube which is one embodiment of the electron tube of the present invention. The electron tube 100 shown in Figure 1 includes a transmission-type photoelectron emission cathode 10, a sealed container 20, a multiplier section 30, an anode 40, a focusing electrode 50, stem pins 60, and a tip tube 70.

[0020] The focusing electrode 50, the multiplier section 30, and the anode 40 are provided in the internal space S of the sealed container 20. The sealed container 20 includes a conductive tube 21, an incident window 22 provided at one end of the conductive tube 21, and a stem plate 23 provided at the other end of the conductive tube 21, and the conductive tube 21 is sealed by the incident window 22 and the stem plate 23. The tip tube 70 is also sealed. Therefore, the sealed container 20 constitutes a vacuum housing. That is, the internal space S inside the sealed container 20 is in a vacuum state.

[0021] The conductive tube 21 is made of metal, for example, and the stem plate 23 is made of an insulating material, for example. The electron tube 100 may further include a second conductive layer 110 on the inner surface of the conductive tube 21. The photoelectron emission cathode 10 emits photoelectrons in response to incident light. The focusing electrode 50 has an opening, which guides the photoelectrons emitted from the photoelectron-emitting cathode 10 to the multiplier unit 30 through the opening. The multiplication unit 30 includes a plurality of dynodes 31 and multiplies the secondary electrons generated in response to the incidence of photoelectrons. The anode 40 collects the secondary electrons generated by the multiplier unit 30. The stem pin 60 is provided so as to penetrate the stem plate 23. The corresponding focusing electrode 50, anode 40, and dynode 31 are electrically connected to the stem pin 60.

[0022] Figure 2 is a cross-sectional view showing a portion of the photoelectron-emitting cathode 10 in Figure 1. The photoelectron-emitting cathode 10 comprises a substrate 11, a conductive layer 13, and a photoelectric conversion layer 12 in this order. The substrate 11 is composed of an incident window 22, and the incident window 22 is made of a material that transmits incident light (a light-transmitting material). The conductive layer 13 contains zinc oxide and a non-zinc metal. Specifically, it consists of a laminate comprising a first layer 13a made of zinc oxide and a second layer 13b containing a non-zinc metal. The conductive layer 13 is electrically connected to the photoelectric conversion layer 12. That is, when a voltage is applied between the conductive layer 13 and the anode 40, electrons can be supplied from the conductive layer 13 to the photoelectric conversion layer 12. The photoelectric conversion layer 12 is a layer that converts incident light into photoelectrons. The photoelectron-emitting cathode 10 may further include an underlayer 14 between the conductive layer 13 and the photoelectric conversion layer 12.

[0023] In the photoelectron-emitting cathode 10, the conductive layer 13 is made of a laminate comprising a first layer 13a made of zinc oxide and a second layer 13b containing a non-zinc metal. The conductive layer 13 has high transmittance in the visible light range due to the inclusion of the first layer 13a made of zinc oxide. Furthermore, the conductive layer 13 has a second layer 13b containing a non-zinc metal, which enables a stable charge supply to the first layer 13a, and makes it possible to uniformly and sufficiently reduce the surface resistance of the conductive layer 13. In other words, when a voltage is applied between the conductive layer 13 and the anode 40 in the electron tube 100, it becomes easier to supply electrons from the conductive layer 13 to the photoelectric conversion layer 12. Therefore, it is possible to make it easier to emit electrons when light is incident on the photoelectric conversion layer 12. In this way, the photoelectron-emitting cathode 10 can improve cathode linearity characteristics while having high sensitivity in the visible light range. Therefore, the photoelectron-emitting cathode 10 can improve the anode linearity characteristics of the electron tube 100 in the visible light range. As a result, the electron tube 100 can improve the quantitative accuracy of light intensity over a wide range, from weak to relatively strong light, in the visible light range. Therefore, the electron tube 100 is useful as a photomultiplier tube for wide dynamic range applications.

[0024] The photoelectron-emitting cathode 10 will be described in detail below.

[0025] <Circuit board> The substrate 11 supports the conductive layer 13 and the photoelectric conversion layer 12. The substrate 11 is a light-transmitting substrate that transmits light, including at least visible light. Examples of such a light-transmitting substrate include glass. Examples of glass include borosilicate glass (Kovar glass) and quartz glass. When the glass is made of borosilicate glass, the airtightness of the sealed container 20 can be improved.

[0026] <Photoelectric conversion layer> The photoelectric conversion layer 12 contains alkali metals. Examples of alkali metals include cesium (Cs), potassium (K), rubidium (Rb), and sodium (Na). The photoelectric conversion layer 12 may contain each of these individually or two or more of them. The photoelectric conversion layer 12 may also further contain antimony. Antimony chemically bonds with alkali metals to form semiconductor compounds such as K2CsSb. The photoelectric conversion layer 12 may be a layer containing two alkali metals and antimony, that is, a bialkali photoelectric conversion layer. In this case, the photoelectron-emitting cathode 10 can have improved sensitivity in the visible light range compared to the case where a multi-alkali photoelectric conversion layer is included. When the photoelectric conversion layer 12 is a bialkali photoelectric conversion layer, the two alkali metals are preferably cesium and potassium. In this case, the photoelectron-emitting cathode 10 can effectively improve sensitivity.

[0027] The thickness of the photoelectric conversion layer 12 is not particularly limited and may be, for example, 30 to 2500 Å.

[0028] <Conductive layer> The conductive layer 13 is a laminate of multiple layers, and as shown in Figure 2, it consists of a first layer 13a made of zinc oxide and a second layer 13b containing a non-zinc metal. Non-zinc metals are metals other than zinc (Zn), and examples of non-zinc metals include aluminum (Al), gallium (Ga), indium (In), and tin (Sn). These can be used individually or in combination of two or more. Among these, aluminum is preferred. In this case, the photoemission cathode 10 can effectively improve its cathode linearity characteristics. The conductive layer 13 is a light-transmitting conductive layer that includes at least visible light.

[0029] In the conductive layer 13, the atomic ratio R1 of zinc to non-zinc metal is not particularly limited, but may be 6 or more, 8 or more, or 10 or more. When the atomic ratio R1 is 6 or more, the photoelectron-emitting cathode 10 can more reliably achieve high sensitivity in the visible light range while effectively improving its cathode linearity characteristics. The atomic ratio R1 may be 20 or less, 18 or less, or 15 or less. The atomic ratio R1 may be 6-20, 8-18, or 10-15. The atomic ratio R1 can be determined by measuring the X-ray intensity of Al and Zn from the conductive layer 13 using an EPMA (Electron Probe Microanalyzer) device (product name "EPMA 8050G", manufactured by Shimadzu Corporation), calculating the molar ratio (Atomic%) of Al and Zn from these measurements using ZAF correction, converting the molar ratio of Al and Zn to the molar ratio of oxides, and determining the ratio of the molar ratio of zinc oxide to the molar ratio of aluminum oxide. The measurement conditions for the EPMA device are as follows. (Measurement conditions) • Acceleration voltage: 15kV • Beam current: 100nA • Beam size: 50 μm • Standard samples: Al (metal) and Zn (metal)

[0030] In the conductive layer 13, the first layer 13a and the second layer 13b may be arranged alternately. The first layer 13a may consist of at least one layer, and the second layer 13b may also consist of at least one layer. The thickness of the first layer 13a is greater than the thickness of the second layer 13b. The ratio R2 of the thickness of the first layer 13a to the thickness of the second layer 13b is not particularly limited and may be 4 or more and 40 or less, more preferably 5 or more and 30 or less, and even more preferably 6 or more and 20 or less.

[0031] The thickness of the conductive layer 13 is not particularly limited and may be 230 Å or more, 250 Å or more, 270 Å or more, or 300 Å or more. If the thickness of the conductive layer 13 is 230 Å or more, the photoelectron-emitting cathode 10 can provide sufficient conductivity to the conductive layer 13, thereby effectively improving the cathode linearity characteristics. On the other hand, as the thickness of the conductive layer 13 increases, the transmittance in the visible light range gradually decreases. Therefore, the thickness of the conductive layer 13 may be 500 Å or less, 450 Å or less, 400 Å or less, or 350 Å or less. The thickness of the conductive layer 13 may be 230-500 Å, 250-450 Å, 270-400 Å, or 300-350 Å. When the thickness of the conductive layer 13 is 300-350 Å, a good balance is achieved between transmittance and conductivity in the visible light range, and sensitivity can be particularly improved.

[0032] The conductive layer 13 may or may not be electrically connected to the second conductive layer 110 provided on the inner surface of the conductive tube 21. When the conductive layer 13 is electrically connected to the second conductive layer 110, the electron tube 100 can ensure electrical continuity between the conductive tube 21 and the conductive layer 13 of the photoelectron emission cathode 10 through the second conductive layer 110 provided on the inner surface of the conductive tube 21, thereby enabling reliable power supply to the photoelectric conversion layer 12. The configuration of the second conductive layer 110 may be the same as or different from that of the conductive layer 3. That is, the second conductive layer 110 may consist of a laminate comprising a first layer 13a made of zinc oxide and a second layer 13b containing a non-zinc metal, or it may have a configuration different from such a laminate.

[0033] <Underlayer> The photoelectron-emitting cathode 10 may further include an underlayer 14 between the photoelectric conversion layer 12 and the substrate 11. In this case, the migration of alkali metals in the photoelectric conversion layer 12 to the substrate 11 can be suppressed, and a decrease in the photoelectric conversion efficiency in the photoelectric conversion layer 12 can be suppressed. As a result, the characteristics of the photoelectron-emitting cathode 10 can be maintained. Furthermore, by appropriately selecting the material and thickness of the underlayer 14, the reflectance at wavelengths in the visible light range can be reduced. In other words, by reducing the reflectance to light at wavelengths in the visible light range, the amount of light of those wavelengths incident on the photoelectric conversion layer 12 can be increased, and as a result, the sensitivity can be further improved.

[0034] The materials constituting the underlayer 14 are not particularly limited as long as they can act as a barrier against alkali metals, and such materials include, for example, metal oxides. Examples of metal oxides include aluminum oxide, hafnium oxide, beryllium oxide, and magnesium oxide. The base layer 14 may consist of a single layer or a laminate of multiple layers. If the base layer 14 consists of a laminate of multiple layers, for example, the base layer 14 may be a laminate comprising a layer made of hafnium oxide and a layer made of magnesium oxide, in that order from the conductive layer 13 side.

[0035] The thickness of the sublayer 14 is not particularly limited and may be, for example, 5-3000 Å, 20-1000 Å, 30-500 Å, 50-300 Å, 70-250 Å, or 80-200 Å.

[0036] Next, the manufacturing method of the electron tube 100 will be explained with reference to Figures 2 to 5. Figure 3 is a cross-sectional view showing the first structure formed during the manufacturing of the electron tube 100, Figure 4 is a cross-sectional view showing the second structure obtained after the first structure during the manufacturing of the electron tube 100, and Figure 5 is a cross-sectional view showing the third structure obtained after the second structure during the manufacturing of the electron tube 100.

[0037] First, prepare the conductive tube 21. Then, fix the substrate 11 so as to close one end opening of the conductive tube 21. In this way, obtain the cap for the photomultiplier tube (hereinafter also referred to as the "PMT cap").

[0038] Next, the PMT cap is placed inside the chamber, and a conductive layer 13 containing zinc oxide and a non-zinc metal (e.g., aluminum) is deposited on the inner surface of the substrate 11 of the PMT cap using atomic layer deposition (ALD).

[0039] The film deposition temperature when forming the conductive layer 13 is not particularly limited and may be, for example, 80 to 250°C.

[0040] The conductive layer 13 is formed specifically as follows. First, a first layer 13a made of zinc oxide is formed (see Figure 2). The first layer 13a can be obtained, for example, by supplying diethylzinc (DEZ) to form a zinc precursor on the surface of the substrate 11, and then supplying an oxidizing agent to oxidize the zinc precursor.

[0041] Next, a second layer 13b containing aluminum and oxygen is formed (see Figure 2). The second layer 13b can be obtained by supplying trimethylaluminum (TMA) to form an aluminum precursor on the surface of the first layer 13a, and then supplying an oxidizing agent to oxidize the aluminum precursor. Therefore, the second layer 13b is also a layer made of aluminum oxide.

[0042] Furthermore, the chamber may be evacuated after the zinc precursor is formed, after an oxidizing agent is supplied to oxidize the zinc precursor, or after the aluminum precursor is formed, after an oxidizing agent is supplied to oxidize the aluminum precursor, or after all of these processes.

[0043] After forming the first layer 13a and the second layer 13b in this order on the inner surface of the substrate 11, the operation of further forming the first layer 13a and the second layer 13b may be repeated until the conductive layer 13 reaches a desired thickness.

[0044] As described above, a conductive layer 13 is formed on the inner surface of the substrate 11, consisting of a laminate formed by stacking multiple layers of a first layer 13a made of zinc oxide and a second layer 13b made of aluminum oxide. At this time, a second conductive layer 110, having the same configuration as the conductive layer 13, is formed integrally with the conductive layer 13 not only on the inner surface of the substrate 11, but also on the inner and outer surfaces of the conductive tube 21 and the outer surface of the substrate 11. Therefore, the conductive layer 13 is electrically connected to the second conductive layer 110. Furthermore, by forming the conductive layer 13 as a laminate comprising the first layer 13a and the second layer 13b as described above, it is possible to arrange the aluminum to have substantially uniform conductivity in both the planar and depth directions, compared to, for example, directly adding aluminum to the first layer 13a made of zinc oxide, thus reducing the surface resistance of the conductive layer 13 in a more uniform manner.

[0045] Next, a base layer 14 and an antimony layer 12a made of antimony (Sb) are formed on the conductive layer 13, for example, by vapor deposition. Vapor deposition methods such as physical vapor deposition, chemical vapor deposition, electron beam deposition, and sputter deposition can be used. Thus, the first structure 200 is obtained (see Figure 3).

[0046] Next, the focusing electrode 50, the amplification unit 30, and the anode 40 are placed inside the conductive tube 21 of the first structure 200, and the open end of the conductive tube 21 is sealed with a stem plate 23 through which the stem pin 60 passes, thereby obtaining the second structure 210 (see Figure 4).

[0047] In this case, the tip tube 70 provided on the stem plate 23 is not sealed, allowing for the introduction and discharge of gas into and out of the internal space S via the tip tube 70.

[0048] Then, the second structure 210 is moved to an exhaust stand (not shown), and gas is discharged through the tip pipe 70. Subsequently, alkaline vapor is supplied from the tip tube 70 to activate the antimony layer 12a.

[0049] In this way, a photoelectric conversion layer 12 containing alkali metals and antimony is formed, and a third structure 220 is obtained (see Figure 5).

[0050] Next, the tip tube 70 is sealed to obtain the photoelectron-emitting cathode 10.

[0051] Finally, the second conductive layer 110 deposited on the outside of the substrate 11 of the third structure 220 is removed. At this time, the second conductive layer 110 deposited on the outer surface of the conductive tube 21 of the third structure 220 may also be removed. The second conductive layer 110 deposited on the inner surface of the conductive tube 21 may be removed, or it may be left in place. The second conductive layer 110 can be removed, for example, by hydrochloric acid treatment. This is how we obtain the electron tube 100.

[0052] In the manufacturing method of the electron tube 100 described above, the conductive layer 13 is formed using the ALD method. Therefore, uniform film formation of the conductive layer 13 is possible. As a result, stable characteristics are obtained for the entire photoelectron emission cathode 10, and the yield during mass production of the photoelectron emission cathode 10 is improved. In particular, since the conductive layer 13 is formed by the ALD method, it becomes possible to precisely control the thickness of the first layer 13a and the second layer 13b. As a result, it is possible to form a laminate in which the transmittance and conductivity of the conductive layer 13 in the visible light range are optimized. Furthermore, since the conductive layer 13 is formed by the ALD method, the adhesion between the conductive tube 21 and the conductive layer 13 is good, the electrical connection between the conductive tube 21 and the conductive layer 13 is stable, and a highly reliable electron tube 10 is obtained. Furthermore, the conductive layer 13 is formed by the ALD method, and since the ALD method has high process controllability and excellent reproducibility, it is possible to mass-produce electron tubes 10 with stable quality.

[0053] The present invention is not limited to the embodiments described above. For example, although the above embodiments described the case in which the electron tube of the present invention is a photomultiplier tube, the electron tube is not limited to a photomultiplier tube and may be a photoelectric tube, an image intensifier, a streak tube, or an HPD (Hybrid Photo Detector). [Examples]

[0054] The present invention will be described in more detail below with reference to examples.

[0055] (Example 1) First, a rectangular conductive tube (also called a "metal side tube") with sides of 25 mm and a length of 15 mm was prepared, and a substrate made of Kovar glass was fixed to it so as to block one end opening of the metal side tube. In this way, a cap for the PMT was obtained.

[0056] Next, the PMT cap was placed inside the chamber, and a conductive layer with a thickness of approximately 300 Å was deposited on the inner surface of the substrate of the PMT cap using the ALD method. This film deposition was carried out under conditions of a deposition temperature of 230°C. Specifically, diethylzinc (DEZ) was first supplied with a pulse time of 0.05 seconds, followed by 20 seconds of evacuating. Then, water (H2O) was supplied as an oxidizing agent with a pulse time of 0.05 seconds, followed by 20 seconds of evacuating. This series of operations was repeated to form a zinc oxide layer with a thickness of approximately 30 Å.

[0057] Next, trimethylaluminum (TMA) was supplied with a pulse time of 0.05 seconds, followed by 20 seconds of exhaust. Then, water (H2O) was supplied as an oxidizing agent with a pulse time of 0.05 seconds, followed by 20 seconds of exhaust, to form one layer of aluminum oxide. The series of steps described above for forming the zinc oxide layer and the aluminum oxide layer constitutes one cycle, and the conductive layer was formed by repeating this cycle.

[0058] This film deposition process resulted in the formation of a conductive layer not only on the inner surface of the substrate, but also on the inner and outer surfaces of the conductive tube, as well as on the outer surface of the substrate.

[0059] Next, using a high-vacuum deposition apparatus, an MgO layer (80 Å thick) and an antimony (Sb) layer (50 Å thick) were sequentially deposited on the conductive layer in that order. Thus, the first structure was obtained.

[0060] Next, a focusing electrode, a multiplier, and an anode were placed inside the conductive tube of the first structure, and the open end of the conductive tube was sealed with a stem plate through which a stem pin passed. The tip tube provided on the stem plate was not sealed, allowing for gas discharge and alkaline vapor introduction into the internal space S through the tip tube, thereby obtaining the second structure. The second structure was then moved to an exhaust stand, and after gas discharge treatment was performed through the tip tube, Cs vapor from a cesium (Cs) deposition source and K vapor from a potassium (K) deposition source were supplied through the same tip tube to activate the Sb layer, forming a bi-alkali photoelectric conversion layer containing potassium, cesium, and Sb. At this time, the thickness of the photoelectric conversion layer was 150 Å. Next, the chip tube was sealed to obtain a photoelectron-emitting cathode. Finally, the second conductive layer, which was deposited on the outside of the substrate of the first structure, was removed by hydrochloric acid treatment. In this way, a photomultiplier tube was obtained.

[0061] (Example 2) As shown in Table 1, a photomultiplier tube was obtained in the same manner as in Example 1, except that the thickness of the conductive layer was changed from 300 Å to 247 Å and an HfO2 layer (thickness 50 Å) was deposited between the conductive layer and the MgO layer.

[0062] (Example 3) As shown in Table 1, a photomultiplier tube was obtained in the same manner as in Example 1, except that the thickness of the conductive layer was changed from 300 Å to 277 Å and an HfO2 layer (thickness 50 Å) was deposited between the conductive layer and the MgO layer.

[0063] (Example 4) As shown in Table 1, a photomultiplier tube was obtained in the same manner as in Example 1, except that the thickness of the conductive layer was changed from 300 Å to 248 Å and an HfO2 layer (thickness 150 Å) was deposited between the conductive layer and the MgO layer.

[0064] (Example 5) As shown in Table 1, a photomultiplier tube was obtained in the same manner as in Example 1, except that the thickness of the conductive layer was changed from 300 Å to 248 Å.

[0065] (Example 6) As shown in Table 1, a photomultiplier tube was obtained in the same manner as in Example 1, except that the thickness of the conductive layer was changed from 300 Å to 324 Å.

[0066] (Example 7) As shown in Table 1, a photomultiplier tube was obtained in the same manner as in Example 1, except that the thickness of the conductive layer was changed from 300 Å to 269 Å and the thickness of the MgO layer was changed from 80 Å to 160 Å.

[0067] (Comparative Example 1) A photomultiplier tube was obtained in the same manner as in Example 1, except that a conductive layer was not formed, the thickness of the substrate and MgO layer was changed from 80 Å to 90 Å, and an HfO2 layer (thickness 160 Å) was deposited between the conductive layer and the MgO layer.

[0068] <Rating> (1) Sensitivity For the photomultiplier tubes obtained in the examples, the spectral curve characteristics showing the relationship between wavelength and quantum efficiency (QE), which is an indicator of sensitivity, were investigated using a spectral sensitivity measuring device. The QE (%) at 400 nm was then determined. The results are shown in Table 1. For Examples 1 to 7, the relationship between wavelength and QE is shown in Figure 6.

[0069] (2) Cathode linearity characteristics The cathode linearity characteristics of the photomultiplier tubes obtained in the examples and comparative examples were investigated as follows. Specifically, a shuttered aperture with four shutters was placed between the light source (tungsten lamp) and the photomultiplier tube. The four shutters were opened sequentially, and the output currents Ip1, Ip2, Ip3, and Ip4 of the photoemission cathode were measured each time. After that, all four shutters were opened, and the output current Ip0 of the photoemission cathode was measured. Subsequently, the deviation rate expressed by the following formula (A) was calculated. Deviation rate= (Ip0 / (Ip1+Ip2+Ip3+Ip4)-1)×100(%)...(A) Here, if there is linearity between the output current of the photoelectron-emitting cathode and the amount of incident light, then equation (B) below holds. Ip0 = Ip1 + Ip2 + Ip3 + Ip4 ... (B) In this case, the deviation rate will be zero, meaning there is no deviation from linearity. Next, the measurement was repeated by changing the light intensity of the light source, that is, by changing the output current of the photoelectron emission cathode of the photomultiplier tube, and the deviation rate expressed by the above formula (A) was calculated. Thus, the relationship between the output current of the photoemission cathode and the deviation rate was determined. At this time, the applied voltage between the photoemission cathode and the anode was set to 100V. Then, the output current of the photoemission cathode was increased, and the output current Ix was determined when the deviation rate reached -2%. The results for Examples 1 to 7 are shown in Table 1. Figure 7 shows the relationship between the output current of the photoelectron-emitting cathode and the deviation rate for Example 1 and Comparative Example 1.

[0070] (3) Anode linearity characteristics For the photomultiplier tube obtained in Example 1, the anode linearity characteristics were investigated in the same manner as the cathode linearity characteristics in (2) above, except that the output current of the photoemission cathode was changed to the output current of the anode of the photomultiplier tube, and the applied voltage between the photoemission cathode and the anode was set to -500V. The relationship between the anode output current and the deviation rate is shown in Figure 8.

[0071] [Table 1]

[0072] As shown in Table 1 and Figure 6, the photoemission cathodes of the photomultiplier tubes in Examples 1 to 7 had a QE at 400 nm exceeding 30%. Furthermore, the photoemission cathodes of the photomultiplier tubes in Examples 1 to 7 reached an output current Ix of 10 μA (10,000 nA), which is an indicator of cathode linearity characteristics (Table 1). In contrast, the output current Ix of the photoemission cathode of the photomultiplier tube in Comparative Example 1 was 80 nA, which is an indicator of cathode linearity characteristics, and did not reach 10 μA. From the above, it has been confirmed that the photoelectron-emitting cathode of the present invention can improve cathode linearity characteristics while maintaining high sensitivity in the visible light range. [Explanation of Symbols]

[0073] 10...Photoelectron-emitting cathode, 11...Substrate, 12...Photoelectric conversion layer, 13...Conductive layer, 13a...First layer, 13b...Second layer, 14...Underlayment, 20...Sealed container, 21...Conductive tube, 22...Inlet window, 23...Stem plate, 30...Multiplication section, 31...Dynode, 40...Anode, 50...Focusing electrode, 60...Stem pin, 70...Tip tube.

Claims

1. The substrate, conductive layer, and photoelectric conversion layer are provided in this order. The conductive layer comprises a laminate comprising a first layer made of zinc oxide and a second layer containing a non-zinc metal. A photoelectron-emitting cathode in which the conductive layer is electrically connected to the photoelectric conversion layer.

2. The photoelectron-emitting cathode according to claim 1, wherein the thickness of the first layer is greater than the thickness of the second layer.

3. The photoelectron-emitting cathode according to claim 1, wherein the atomic ratio of zinc to the non-zinc metal in the conductive layer is 6 or more.

4. The photoelectron-emitting cathode according to claim 1, wherein the non-zinc metal is aluminum.

5. The photoelectron-emitting cathode according to claim 1, wherein the conductive layer has a thickness of 230 Å or more.

6. The photoelectron-emitting cathode according to claim 1, wherein the substrate is a light-transmitting substrate and the conductive layer is a light-transmitting conductive layer.

7. The photoelectron-emitting cathode according to claim 1, wherein the photoelectric conversion layer is a bialkali photoelectric conversion layer.

8. An electron tube comprising a photoelectron-emitting cathode according to any one of claims 1 to 7.

9. A conductive tube supporting the aforementioned photoelectron-emitting cathode, The conductive tube further comprises a second conductive layer provided on the inner surface of the conductive tube, The second conductive layer comprises zinc oxide and a non-zinc metal, The electron tube according to claim 8, wherein the second conductive layer is electrically connected to the conductive layer of the photoelectron-emitting cathode.

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