Photomultiplier tube

Abstract

To provide a photomultiplier tube which has a photocathode and enables suppression of sensitivity deterioration of the photocathode and suppression of increase in the number of after-pluses even when used in high-temperature environment.SOLUTION: A photomultiplier tube 1 includes: a container 10 including a window portion 13 formed of light-transmitting material, and a tubular portion 14 connected to the window portion 13 and defining a vacuum space S in combination with the window portion 13; a photocathode 20 formed of photo-electron emission material composed of sodium, potassium, and antimony, and provided on a first surface 13a on the vacuum space S side of the window portion 13, the photocathode 20 emitting photo-electrons in correspondence with incident light; and an electron multiplier 40 that emits secondary electrons in correspondence with the incident of the photo-electrons emitted from the photocathode 20 and multiplies the secondary electrons. An aluminum oxide layers 60 is formed between the photocathode 20 and the window portion 13 and on a surface of the tubular portion 14.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a photomultiplier tube. [Background technology]

[0002] Patent Document 1 describes a photomultiplier tube in which a photocathode, a multiplier section, and an anode are disposed in an evacuated container. In this photomultiplier tube, photoelectrons are emitted from the photocathode in response to incidence of light. The photoelectrons emitted from the photocathode are multiplied as secondary electrons in the multiplier section and collected by the anode. In the photomultiplier tube described in Patent Document 1, the photocathode is formed from a photoelectron emitting material containing an alkali metal, and is provided on the inner surface of the container. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2021-044118 A Summary of the Invention [Problem to be solved by the invention]

[0004] The present inventors have found that when the above-mentioned photomultiplier tube is used in a high-temperature environment (for example, 175°C to 200°C), a specific problem may occur. That is, when the photomultiplier tube is operated in a high-temperature environment, the alkali metal contained in the photocathode may move into the container, and the sensitivity of the photocathode may deteriorate. In addition, in a high-temperature environment, hydrogen gas or helium gas easily permeates the container and enters the container. Therefore, afterpulses caused by hydrogen gas or helium gas may increase.

[0005] An object of the present invention is to provide a photomultiplier tube capable of suppressing deterioration in sensitivity of the photocathode and an increase in afterpulses even when used in a high-temperature environment. [Means for solving the problem]

[0006] The photomultiplier tube of the present invention is [1] "a photomultiplier tube comprising: a container having a window portion formed of a light-transmitting material and a tubular portion connected to the window portion and defining a vacuum space together with the window portion; a photocathode formed of a photoelectron emitting material consisting of sodium, potassium, and antimony and provided on the surface of the window portion facing the vacuum space, which emits photoelectrons in response to incident light; and an electron multiplier portion which emits secondary electrons in response to the incidence of the photoelectrons emitted from the photocathode and multiplies the secondary electrons, wherein a protective layer containing a trivalent metal oxide of Group 3 or Group 13 is formed between the photocathode and the window portion and on the surface of the tubular portion."

[0007] In this photomultiplier tube, a protective layer containing a trivalent metal oxide of Group 3 or Group 13 is formed between the photocathode and the window. This makes it possible to suppress the alkali metals (sodium and potassium) contained in the photocathode from migrating into the window even when the photomultiplier tube is operated in a high-temperature environment, and to suppress deterioration in the sensitivity of the photocathode caused by changes in the composition ratio of the alkali metals constituting the photocathode. In addition, the protective layer is also formed on the surface of the tubular part. This makes it possible to suppress hydrogen gas or helium gas from penetrating the tubular part and entering the container in a high-temperature environment, and to suppress an increase in after-pulses caused by hydrogen gas or helium gas. Therefore, with this photomultiplier tube, it is possible to suppress deterioration in the sensitivity of the photocathode and an increase in after-pulses even when used in a high-temperature environment.

[0008] The photomultiplier tube of the present invention may be [2] "the photomultiplier tube according to [1], in which the protective layer is formed on the surface of the tubular portion on the vacuum space side." In this case, in addition to preventing hydrogen gas or helium gas from penetrating the tubular portion from the outside and entering the container, it is possible to prevent hydrogen gas or helium gas present inside the tubular portion from leaking into the container, thereby further suppressing an increase in afterpulses.

[0009] The photomultiplier tube of the present invention may be [3] "the photomultiplier tube according to [1] or [2], wherein the protective layer is formed on a surface of the tubular portion opposite to the vacuum space." In this case, an increase in afterpulses can be more effectively suppressed.

[0010] The photomultiplier tube of the present invention may be [4] "the photomultiplier tube according to any one of [1] to [3], wherein the protective layer is further formed on a surface of the window portion opposite to the vacuum space." In this case, hydrogen gas or helium gas can be prevented from penetrating the window portion and entering the container.

[0011] The photomultiplier of the present invention may be [5] "the photomultiplier according to any one of [1] to [4], wherein the protective layer is an ALD layer." In this case, the protective layer can be formed densely, and deterioration in sensitivity of the photocathode and an increase in afterpulses can be effectively suppressed.

[0012] The photomultiplier of the present invention may be [6] "the photomultiplier according to any one of [1] to [5], wherein the thickness of the protective layer is 10 nm or more and 100 nm or less." In this case, it is possible to effectively suppress deterioration in sensitivity of the photocathode and an increase in afterpulses while suppressing the protective layer from interfering with light transmission through the window.

[0013] The photomultiplier of the present invention may be [7] "the photomultiplier according to any one of [1] to [6], wherein the protective layer is an aluminum oxide layer." In this case, it is possible to effectively suppress deterioration in sensitivity of the photocathode and an increase in afterpulses.

[0014] The photomultiplier tube of the present invention may be [8] "the photomultiplier tube according to [7], in which the thickness of the protective layer is 10 nm or more and 30 nm or less." In this case, it is possible to effectively suppress deterioration in sensitivity of the photocathode and an increase in afterpulses while suppressing the protective layer from interfering with light transmission through the window.

[0015] The photomultiplier of the present invention may be [9] "the photomultiplier according to any one of [1] to [6], wherein the protective layer is an yttrium oxide layer." In this case, it is possible to effectively suppress deterioration in sensitivity of the photocathode and an increase in afterpulses.

[0016] The photomultiplier tube of the present invention may be

[10] "the photomultiplier tube according to [9], in which the thickness of the protective layer is 10 nm or more and 100 nm or less." In this case, it is possible to more effectively suppress deterioration in sensitivity of the photocathode and an increase in afterpulses while suppressing the protective layer from interfering with light transmission through the window. Effect of the Invention

[0017] According to the present invention, it is possible to provide a photomultiplier tube capable of suppressing deterioration in sensitivity of the photocathode and an increase in afterpulses even when used in a high-temperature environment. [Brief description of the drawings]

[0018] [Figure 1] 1 is a cross-sectional view of a photomultiplier tube according to an embodiment. [Diagram 2] 13 is a graph showing the results of comparing life characteristics. [Diagram 3] 13 is a table showing a comparison result of afterpulses. [Figure 4] 11(a) to 11(c) are graphs showing the measurement results of afterpulses. [Diagram 5] FIG. 11 is a cross-sectional view of a photomultiplier tube according to a first modified example. [Figure 6] FIG. 11 is a cross-sectional view of a photomultiplier tube according to a second modified example. [Figure 7] 13(a) and 13(b) are diagrams for explaining the first and second experiments. [Figure 8] 1 is a table showing the experimental results of a first experiment and a second experiment. [Figure 9] Photographs (a) to (d) are photographs of Sample 1 to Sample 3 in the first experiment. [Figure 10]6(a) to 6(c) are photographs of sample 4 and sample 5 in the first experiment. [Figure 11] Photographs (a) to (d) are photographs of samples 6 to 9 in the first experiment. [Figure 12] Photographs (a) to (c) are photographs of Samples 10 to 12 in the first experiment. [Figure 13] Photographs (a) to (d) are photographs of Sample 1 to Sample 3 in the second experiment. [Figure 14] 6(a) to 6(c) are photographs of sample 4 and sample 5 in the second experiment. [Figure 15] Photographs (a) to (d) are photographs of samples 6 to 9 in the second experiment. [Figure 16] Photographs (a) to (c) are photographs of Samples 10 to 12 in the second experiment. [Figure 17] 13 is a table showing a comparison result of afterpulses for the third embodiment and the third comparative example. [Figure 18] 13(a) and 13(b) are graphs showing the measurement results of afterpulses for the third embodiment and the third comparative example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In the following description, the same or corresponding elements are designated by the same reference numerals, and duplicated description will be omitted.

[0020] 1, a photomultiplier tube 1 (PMT) includes a container 10, a photocathode 20, a focusing electrode 30, an electron multiplier section 40, and an anode 50. The photocathode 20, the focusing electrode 30, the electron multiplier section 40, and the anode 50 are disposed within the container 10. The container 10 has a stem section 11 and a bulb section 12, and the bulb section 12 is composed of a window section 13 and a tubular section 14.

[0021] In the photomultiplier tube 1, light L (incident light) transmitted through the window portion 13 is incident on the photocathode 20, and photoelectrons are emitted from the photocathode 20 in response to the incidence of the light L. The photoelectrons emitted from the photocathode 20 are focused by the focusing electrode 30, multiplied as secondary electrons in the electron multiplier section 40, and collected by the anode 50. This causes the photomultiplier tube 1 to function as a photodetector. The photomultiplier tube 1 in this example is a high-temperature PMT that can be used in high-temperature environments (e.g., 175°C to 200°C), and can be used, for example, in oil exploration. The configuration of each section will be described below.

[0022] The container 10 is made of a light-transmitting material such as glass. The container 10 is hermetically sealed and defines a vacuum space S inside. The stem portion 11 is formed, for example, in a substantially circular disk shape. A substantially cylindrical protrusion 11a is formed in the center of the stem portion 11 and protrudes toward the opposite side to the vacuum space S. A plurality of stem pins 15 are fixed to the stem portion 11, penetrating the stem portion 11 along direction A (a direction parallel to the center line of the tubular portion 14).

[0023] As described above, the valve section 12 has the window section 13 and the tubular section 14. The window section 13 is a face plate (substrate) portion of the valve section 12, and is formed, for example, in a substantially circular plate shape. The window section 13 has a first surface 13a (inner surface) on the vacuum space S side, and a second surface 13b (outer surface) on the opposite side to the first surface 13a (vacuum space S). In this example, the first surface 13a is a curved surface that is curved in a substantially spherical shape so as to be recessed toward the opposite side to the vacuum space S, and the second surface 13b is a flat surface perpendicular to the direction A.

[0024] The tubular portion 14 is a side tube portion of the valve portion 12, and is formed, for example, in a cylindrical shape having a center line parallel to the direction A. One end of the tubular portion 14 is connected to the outer edge of the window portion 13, and the other end of the tubular portion 14 is connected to the outer edge of the stem portion 11. The stem portion 11, the window portion 13, and the tubular portion 14 define the above-mentioned vacuum space S.

[0025] The photocathode 20 is provided in the form of a thin film on the first surface 13a of the window portion 13. The photocathode 20 is formed over substantially the entire first surface 13a and extends in a curved manner along the first surface 13a. In this embodiment, as described below, an aluminum oxide layer 60 (protective layer) is formed between the first surface 13a and the photocathode 20, and the photocathode 20 is provided on the first surface 13a with the aluminum oxide layer 60 interposed therebetween.

[0026] The photocathode 20 is formed of a photoelectron emitting material that emits photoelectrons in response to the incidence of light. In this example, the photoelectron emitting material that constitutes the photocathode 20 contains antimony (Sb) and alkali metals sodium (Na) and potassium (K). In this case, the photocathode 20 is obtained by activating antimony by reacting it with sodium and potassium. The photocathode 20 is, for example, a Na2K photocathode that contains Na2K with a molar ratio of Na to K of 2:1. The Na2K photocathode exhibits low noise properties. The low noise properties mean that the current (dark current) that flows when no light L is incident on the photocathode 20 during the operation of the photomultiplier tube 1 is small during operation. The photocathode 20 functions as a photocathode that emits photoelectrons in response to the incidence of light L.

[0027] The focusing electrode 30 focuses the photoelectrons emitted from the photocathode 20. The focusing electrode 30 is disposed to face the window portion 13. An opening 30a is formed in the center of the focusing electrode 30. This opening 30a functions as an entrance for allowing the photoelectrons emitted from the photocathode 20 to enter the electron multiplier section 40.

[0028] The electron multiplier section 40 is provided between the focusing electrode 30 and the anode 50. The electron multiplier section 40 emits secondary electrons in response to the incidence of photoelectrons coming from the focusing electrode 30, and multiplies the secondary electrons. The electron multiplier section 40 is configured, for example, with a plurality of dynodes 41 (10 in this example). The plurality of dynodes 41 are arranged in multiple stages between the focusing electrode 30 and the anode 50. Each dynode 41 is formed in a curved or flat shape, and emits secondary electrons toward the next dynode 41 due to collision with secondary electrons emitted from the previous dynode 41. This causes the secondary electrons to be multiplied sequentially. The multiplied secondary electrons are collected by the anode 50. The electron multiplier section 40 may have any configuration as long as it is capable of emitting and multiplying secondary electrons in response to the incidence of photoelectrons.

[0029] The photomultiplier tube 1 further includes an aluminum oxide layer 60. The aluminum oxide layer 60 has a first portion 61, a second portion 62, a third portion 63, and a fourth portion 64. The first portion 61 is formed on the entire first surface 13a of the window portion 13 on the vacuum space S side. The first portion 61 is interposed between the first surface 13a of the window portion 13 and the photocathode 20. In other words, the photocathode 20 is provided on the first surface 13a via the first portion 61.

[0030] The second portion 62 is formed on the entire surface of the second surface 13b of the window portion 13 opposite the vacuum space S. The third portion 63 is formed on the entire surface of the surface 14a (inner surface) of the tubular portion 14 facing the vacuum space S. The fourth portion 64 is formed on the entire surface of the surface 14b (outer surface) of the tubular portion 14 opposite the vacuum space S. The first portion 61 and the third portion 63 are connected and formed integrally with each other. The second portion 62 and the fourth portion 64 are connected and formed integrally with each other. The aluminum oxide layer 60 is not formed on the stem portion 11.

[0031] The aluminum oxide layer 60 is, for example, an ALD layer. The ALD layer is a layer formed by atomic layer deposition (ALD). The atomic layer deposition is a technique for obtaining a thin film by depositing atomic layers one by one by repeatedly performing a process of adsorption of compound molecules, a process of film formation by reaction, and a process of purging to remove excess molecules. The aluminum oxide layer 60 is formed to a uniform thickness by the ALD process.

[0032] As an example, when manufacturing the photomultiplier tube 1, an aluminum oxide layer 60 is formed by ALD on the entire surfaces of the window portion 13 and the tubular portion 14 (i.e., the entire surfaces of the first surface 13a and the second surface 13b, and the surfaces 14a and 14b). Thereafter, the stem portion 11 is fused to the end of the tubular portion 14 to seal the container 10. The thickness of the aluminum oxide layer 60 is, for example, 10 nm to 100 nm (100 Å to 1000 Å), and preferably 10 nm to 30 nm (100 Å to 300 Å). The aluminum oxide layer 60 is made of, for example, aluminum oxide (Al2O3, alumina). [Action and Effects]

[0033] In the photomultiplier tube 1, a first portion 61 of an aluminum oxide layer 60 (protective layer) is formed between the photocathode 20 and the window portion 13. This makes it possible to suppress the alkali metal contained in the photocathode 20 from moving into the window portion 13 (alkali blocking) even when the photomultiplier tube 1 is operated in a high-temperature environment, and to suppress deterioration in sensitivity of the photocathode 20 due to a change in the composition ratio of the alkali metals constituting the photocathode 20. In addition, a third portion 63 and a fourth portion 64 of the aluminum oxide layer 60 are formed on the surface of the tubular portion 14. This makes it possible to suppress hydrogen gas or helium gas from penetrating the tubular portion 14 and entering the container 10 in a high-temperature environment (enhancing the gas barrier properties), and to suppress an increase in afterpulses caused by hydrogen gas or helium gas. Therefore, according to the photomultiplier tube 1, even when used in a high-temperature environment, it is possible to suppress deterioration in sensitivity of the photocathode 20 and to suppress an increase in afterpulses.

[0034] The third portion 63 of the aluminum oxide layer 60 is formed on the surface 14a of the tubular portion 14 on the vacuum space S side. This not only prevents hydrogen gas or helium gas from penetrating the tubular portion 14 from entering the container 10 from the outside, but also prevents the hydrogen gas or helium gas present inside the tubular portion 14 from leaking into the container 10, thereby further suppressing an increase in afterpulses.

[0035] The fourth portion 64 of the aluminum oxide layer 60 is formed on the surface 14b of the tubular portion 14 opposite to the vacuum space S. This makes it possible to more effectively suppress an increase in afterpulses.

[0036] The second portion 62 of the aluminum oxide layer 60 is formed on the second surface 13b of the window portion 13 opposite the vacuum space S. This makes it possible to prevent hydrogen gas or helium gas from penetrating the window portion 13 and entering the container 10.

[0037] The aluminum oxide layer 60 is an ALD layer. This allows the aluminum oxide layer 60 to be formed densely, and makes it possible to effectively suppress deterioration in sensitivity of the photocathode 20 and an increase in after-pulses.

[0038] The thickness of the aluminum oxide layer 60 is 10 nm or more and 30 nm or less. This makes it possible to effectively suppress deterioration in sensitivity of the photocathode 20 and an increase in after pulsing while suppressing the aluminum oxide layer 60 from blocking the transmission of light through the window portion 13. That is, when the thickness of the aluminum oxide layer 60 is 10 nm or more, it is possible to effectively suppress deterioration in sensitivity of the photocathode 20 and an increase in after pulsing. Furthermore, when the thickness of the aluminum oxide layer 60 is 30 nm or less, it is possible to suppress the first portion 61 and the fourth portion 64 of the aluminum oxide layer 60 from blocking the transmission of light L through the window portion 13.

[0039] The aluminum oxide layer 60 is not formed on the stem portion 11. This makes it possible to omit the man-hours and materials required for the process of forming the aluminum oxide layer 60 on the stem portion 11, and facilitates manufacturing. The aluminum oxide layer 60 may also be formed on the stem portion 11, in which case the gas barrier properties can be further improved. In this case, the aluminum oxide layer 60 may be formed, for example, on the entire inner and outer surfaces of the container 10.

[0040] When the third portion 63 of the aluminum oxide layer 60 is formed on the surface 14a of the tubular portion 14 facing the vacuum space S, some dark current may occur, but the increase in after pulses can be suppressed. Specifically, when the aluminum oxide layer 60 does not have the third portion 63, the alkali metal introduced in excess into the vacuum space S during the formation of the photocathode 20 is absorbed by the tubular portion 14, so that the generation of dark current caused by the excess alkali metal introduced can be suppressed. On the other hand, when the third portion 63 of the aluminum oxide layer 60 is formed on the surface 14a of the tubular portion 14 facing the vacuum space S, the alkali metal introduced in excess into the vacuum space S is less likely to be absorbed by the tubular portion 14, so that some of the alkali metal may remain in the vacuum space S, and a dark current caused by the remaining alkali metal may occur. However, since it is important to suppress the increase in after pulses when used in a high-temperature environment, in the photomultiplier tube 1, the aluminum oxide layer 60 is formed on the surface 14a of the tubular portion 14 facing the vacuum space S to suppress the increase in after pulses.

[0041] The results of the confirmation experiment will be described below with reference to Figs. 2 to 4. Fig. 2 is a graph showing the results of comparing life characteristics. In this experiment, the life characteristics were evaluated for the photomultiplier tubes of Examples 1 to 4 and Comparative Examples 1 to 3. Examples 1 to 4 correspond to the case where the aluminum oxide layer 60 is 20 nm thick in the photomultiplier tube 1 of the embodiment, and Comparative Examples 1 to 3 correspond to the case where the aluminum oxide layer 60 is not provided in the photomultiplier tube 1 of the embodiment. Each photomultiplier tube was operated in a state where the ambient temperature was 200°C, and the output value was measured. The vertical axis in the graph of Fig. 2 represents the output value (%) when the output value one hour after the start of the measurement is set to 100%, and the horizontal axis represents the elapsed time from the start of the measurement. In this experiment, the measurement was continued until the output value of one of the photomultiplier tubes (here, Comparative Example 1) reached 50%.

[0042] From FIG. 2, when comparing the output values ​​of Examples 1 to 4 and Comparative Examples 1 to 3 at the end of the measurement (after about 100 hours), it can be seen that the output values ​​of Examples 1 to 4 were higher than the output values ​​of Comparative Examples 1 to 3, and Examples 1 to 4 had better life characteristics than Comparative Examples 1 to 4.

[0043] FIG. 3 is a table showing the comparison results of afterpulses. In this experiment, the increase rate of afterpulses was measured for the photomultiplier tubes of the first example, the second example, and the comparative example. The first example and the second example correspond to the photomultiplier tube 1 of the embodiment in which the thickness of the aluminum oxide layer 60 is 10 nm and 20 nm, respectively, and the comparative example corresponds to the photomultiplier tube 1 of the embodiment in which the aluminum oxide layer 60 is not provided. Each photomultiplier tube was stored in a non-operating state for 20 hours in a state where the ambient temperature was 200° C., and the increase rate of afterpulses was measured when the photomultiplier tube was operated 20 hours after the start of storage. The number of samples (N) for the first example, the second example, and the comparative example was 5, and the average value of the five samples was taken as the increase rate of afterpulses.

[0044] For the comparative example, the afterpulse increase rates of the five samples were 165%, 200%, 194%, 239%, and 213%, respectively, with an average of 202%. For the first embodiment, the afterpulse increase rates of the five samples were 184%, 202%, 161%, 177%, and 167%, respectively, with an average of 178%. For the second embodiment, the afterpulse increase rates of the five samples were 165%, 156%, 143%, 145%, and 171%, respectively, with an average of 156%.

[0045] 3, it can be seen that the after pulse increase rates of the first and second examples were smaller than that of the comparative example. This shows that the formation of the aluminum oxide layer 60 can suppress the increase in after pulses. Furthermore, the after pulse increase rate of the second example was smaller than that of the first example. This shows that the thicker the aluminum oxide layer 60 is, the more effectively the after pulse increase can be suppressed.

[0046] An afterpulse is a pseudo pulse observed after an output pulse corresponding to the incidence of light to be detected, and occurs when hydrogen gas, helium gas, or the like is present in the container 10. The greater the amount of gas present in the container 10, the larger the peak observed as an afterpulse. Furthermore, the longer the photomultiplier tube is stored in an inoperative state, the greater the afterpulse. Since the afterpulse can affect the detection accuracy of the photomultiplier tube, it is necessary to suppress it.

[0047] Fig. 4 is a graph showing the afterpulse measurement results for the first embodiment, the second embodiment, and the comparative example. The first embodiment, the second embodiment, and the comparative example, as well as the operating conditions in this experiment, are the same as those in the experiment in Fig. 3. The vertical axis represents the output value, and the horizontal axis represents the elapsed time.

[0048] As shown in Fig. 4(a), in the case of the comparative example, afterpulses due to hydrogen gas (H2) and methane gas (CH4) were detected after 20 hours. Afterpulses due to these afterpulses were also detected. On the other hand, as shown in Fig. 4(b) and Fig. 4(c), in the cases of the first and second examples, afterpulses due to hydrogen gas were detected after 20 hours, but the afterpulses were smaller than those in the comparative example. This shows that the increase in afterpulses can be suppressed by forming the aluminum oxide layer 60.

[0049] Furthermore, comparing the first and second examples, the after-pulse in the second example was smaller than that in the first example. This shows that the thicker the aluminum oxide layer 60 is, the more effectively the increase in after-pulse can be suppressed. Note that while FIG. 4 shows the effect of suppressing the after-pulse caused by hydrogen gas, it is believed that the after-pulse caused by helium gas can also be suppressed in the same way. [Variations]

[0050] In a first modified example shown in Fig. 5, the aluminum oxide layer 60 does not have the second portion 62 and the fourth portion 64, but only has the first portion 61 and the third portion 63. In a second modified example shown in Fig. 6, the aluminum oxide layer 60 does not have the second portion 62, but only has the first portion 61, the third portion 63, and the fourth portion 64. As with the above embodiment, these first and second modified examples can also suppress deterioration in sensitivity of the photocathode and an increase in afterpulses even when used in a high-temperature environment.

[0051] Moreover, in the first and second modified examples, the aluminum oxide layer 60 does not have the second portion 62. That is, the aluminum oxide layer 60 is not formed on the second surface 13b of the window portion 13 opposite to the vacuum space S. In this case, it is possible to suppress the aluminum oxide layer 60 from impeding the transmission of light through the window portion 13. On the other hand, from the viewpoint of manufacturing, it is preferable that the aluminum oxide layer 60 has the second portion 62 as in the above embodiment. That is, when forming the aluminum oxide layer 60 by the ALD method, it is easier to form the aluminum oxide layer 60 on the entire surface of the valve portion 12 than to form the aluminum oxide layer 60 excluding the second portion 62, so from the viewpoint of manufacturing, it is preferable that the aluminum oxide layer 60 has the second portion 62.

[0052] The present invention is not limited to the above-mentioned embodiment and modified examples. For example, the material and shape of each component are not limited to the above-mentioned material and shape, and various materials and shapes can be adopted. The aluminum oxide layer 60 does not have to be an ALD layer. For example, the aluminum oxide layer 60 may be formed by electron beam deposition, sputter deposition, coating, etc. An underlayer may be provided between the aluminum oxide layer 60 and the photocathode 20. The underlayer is, for example, a beryllium oxide (BeO) layer.

[0053] Instead of the aluminum oxide layer 60, a protective layer formed of a material other than aluminum oxide may be provided. Examples of the material for this protective layer include oxides and oxynitrides of trivalent metal elements of Group 3 or Group 13, such as aluminum oxynitride (AlON), yttrium oxide (Y2O3), and lanthanum oxide (La2O3). Oxynitrides are a type of oxide. The protective layer is not limited to a layer made of a single material, and may be a layer made of multiple materials, such as an alternate laminate film of aluminum oxide and silicon oxide (SiO2). On the other hand, examples of materials that are not suitable for this protective layer include titanium oxide (TiO2), silicon oxide (SiO2), magnesium fluoride (MgF2), magnesium oxide (MgO), tantalum oxynitride (TaON), and hafnium oxide (HfO2).

[0054] Hereinafter, the results of the effect confirmation experiment will be described with reference to Figs. 7 to 16. In this experiment, the alkali blocking effect (life characteristics) of the protective layer was confirmed. As shown in Fig. 7, the sample 100 was prepared by coating the protective film 102 on the quartz glass 101. After the sample 100 was exposed to sodium vapor at 400 to 500 degrees, the degree of alkali (sodium) penetration into the sample 100 was confirmed. The quartz glass 101 and the protective film 102 turn black when alkali penetrates, so the alkali blocking effect of the protective film 102 can be evaluated by checking the degree of discoloration. In other words, there is a correlation between the appearance of the sample 100 and the alkali blocking effect. The experiment was conducted on eleven types of samples 1 to 11 (No. 1 to No. 11) with different materials for the protective film 102, and on sample 12 (No. 12) without the protective film 102. The configuration of each sample will be described later with reference to Fig. 8.

[0055] Two experiments, a first experiment and a second experiment, were conducted. In the first experiment, as shown in FIG. 7(a), the sample 100 after being exposed to sodium vapor was placed on a white paper 103, and the sample 100 was observed from a direction D1 (the opposite side to the paper 103). This confirmed how the sample 100 looked when light L1 was incident on the sample 100 and reflected light L2. The light L1 was light from a fluorescent lamp. If the sample 100 looked transparent, it was determined that no alkali had penetrated and the alkali blocking effect of the protective film 102 was good. If the sample 100 looked black and opaque, it was determined that alkali had penetrated and the alkali blocking effect of the protective film 102 was poor.

[0056] In the second experiment, as shown in Fig. 7(b), sample 100 after exposure to sodium vapor was placed in front of fluorescent lamp 105, and sample 100 was observed from direction D2 (the opposite side to fluorescent lamp 105). This confirmed how sample 100 looked when light L3 from fluorescent lamp 105 was transmitted through it. If sample 100 looked bright, it was determined that no alkali had penetrated and the alkali blocking effect was good. If sample 100 looked dark, it was determined that alkali had penetrated and the alkali blocking effect of protective film 102 was poor.

[0057] Figure 8 is a table showing the experimental results of Experiments 1 and 2. In the column "Alkaline blocking effect", materials marked with "◯" have a good alkali blocking effect, and materials marked with "X" have a poor alkali blocking effect.

[0058] 8, the protective films 102 of Samples 1, 3, and 5 to 11 are made of aluminum oxide, yttrium oxide, lanthanum oxide, magnesium oxide, silicon oxide, titanium oxide, hafnium oxide, tantalum oxynitride, and magnesium fluoride, respectively. The protective film 102 of Sample 2 is made of aluminum oxynitride, which is a mixture of aluminum oxide and aluminum nitride. The protective film 102 of Sample 4 has a structure in which three pairs of aluminum oxide and silicon oxide are laminated. The protective films 102 of Samples 1, 2, 4, 6 to 8, 10, and 11 are ALD films, and the protective films 102 of Samples 3, 5, and 9 are electron beam deposition films.

[0059] The materials of the protective film 102 of Samples 1 to 5 (aluminum oxide, aluminum oxynitride, yttrium oxide, silicon oxide, lanthanum oxide) are trivalent metal oxides. The materials of Samples 3 and 5 (yttrium oxide, lanthanum oxide) are also rare earth metal oxides. The material of the protective film 102 of Sample 6 (magnesium oxide) is a divalent metal oxide. The material of the protective film 102 of Sample 7 (silicon oxide) is a tetravalent non-metal oxide. The materials of Samples 8 and 9 (titanium oxide, hafnium oxide) are tetravalent metal oxides. The material of the protective film 102 of Sample 10 (tantalum oxynitride) is a pentavalent metal oxide. The material of the protective film 102 of Sample 11 (magnesium fluoride) is not an oxide.

[0060] 8, the alkali blocking effect was good for Samples 1 to 5, but was poor for Samples 6 to 12. This shows that the materials of the protective films 102 of Samples 1 to 5, which had good alkali blocking effect, were all trivalent metal oxides (trivalent metal oxides of Group 3 or Group 13) (including oxynitrides).

[0061] 9 to 16 are photographs of the experimental results in the first and second experiments. FIG. 9 to FIG. 12 are photographs of the experimental results in the first experiment, and FIG. 13 to FIG. 16 are photographs of the experimental results in the second experiment. As shown in FIG. 9 to FIG. 16, Samples 1 to 5 were good in visibility in the first and second experiments. On the other hand, Samples 6 to 12 were not good in visibility in the first and second experiments. Although the photographs in FIG. 15 and FIG. 16 are black and white, Samples 6 to 12 in the second experiment actually appeared in a color including red. In addition to the first and second experiments, a protective film was formed on the inner wall of a vacuum tube and gas analysis was performed to confirm that two samples corresponding to the above-mentioned Samples 1 and 4 (a sample in which the protective film is made of aluminum oxide and a sample in which the protective film has a structure in which three pairs of aluminum oxide and silicon oxide are laminated) have good gas barrier properties.

[0062] In the above embodiment, the aluminum oxide layer 60 is formed to have a uniform thickness throughout, but it may be formed to have different thicknesses in parts. For example, the thicknesses of the third portion 63 and the fourth portion 64 formed on the tubular portion 14 may be greater than the thicknesses of the first portion 61 and the second portion 62 of the aluminum oxide layer 60 formed on the window portion 13. In this case, it is possible to prevent the first portion 61 and the second portion 62 from impeding the transmission of light through the window portion 13, while the third portion 63 and the fourth portion 64 can prevent hydrogen gas or helium gas from penetrating the tubular portion 14 and entering the container 10.

[0063] The thickness of the first portion 61 of the aluminum oxide layer 60 may be 5 nm (50 Å) or more. In this case, an effect of suppressing deterioration in sensitivity of the photocathode 20 (alkali blocking effect) can be obtained. The thickness of the first portion 61 may be 300 nm or less. In this case, since the refractive index of the aluminum oxide layer 60 is a value between the refractive index of the window portion 13 and the refractive index of the photocathode 20, it is possible to suppress reflection of the light L incident from the window portion 13 by the first portion 61.

[0064] The results of the experiment to confirm the gas barrier properties will be described with reference to Figs. 17 and 18. Fig. 17 is a table showing the comparison results of the after pulses for the third example and the third comparative example. In this experiment, the increase rate of the after pulses was measured for the photomultiplier tubes of the third example and the third comparative example. The third example corresponds to the case where a protective layer (yttrium oxide layer) made of yttrium oxide is provided instead of the aluminum oxide layer 60 in the photomultiplier tube 1 of the embodiment. In the third example, the yttrium oxide layer was formed by electron beam deposition. The thickness of the yttrium oxide layer was 100 nm (1000 Å). The third comparative example corresponds to the case where the aluminum oxide layer 60 (protective layer) is not provided in the photomultiplier tube 1 of the embodiment. As in the experiment of Fig. 3 described above, each photomultiplier tube was stored in a non-operated state for 20 hours at an ambient temperature of 200°C, and the increase rate of the after pulses was measured when the photomultiplier tube was operated 20 hours after the start of storage. The number of samples (N) in the third example and the third comparative example was set to 5, and the average value for the five samples was taken as the rate of increase in afterpulses.

[0065] For the third comparative example, the afterpulse increase rates of the five samples were 276%, 219%, 236%, 299%, and 319%, respectively, with an average of 270%.For the third example, the afterpulse increase rates of the five samples were 274%, 202%, 247%, 228%, and 255%, respectively, with an average of 241%.

[0066] 17, it can be seen that the increase rate of the after pulses in Example 3 was smaller than that in Comparative Example 3. This shows that the increase in the after pulses can be suppressed by providing an yttrium oxide layer.

[0067] Fig. 18 is a graph showing the measurement results of afterpulses for the third embodiment and the third comparative example. The third embodiment and the third comparative example, and the operating conditions in this experiment are the same as those in the experiment in Fig. 17. The vertical axis represents the output value, and the horizontal axis represents the elapsed time.

[0068] As shown in Fig. 18(a), in the case of the third comparative example, after pulsing due to hydrogen gas (H2) was detected after 20 hours. On the other hand, as shown in Fig. 18(b), in the case of the third embodiment, after pulsing due to hydrogen gas was detected after 20 hours, but the after pulsing was smaller than that in the case of the third comparative example. This shows that the increase in after pulsing can be suppressed by providing an yttrium oxide layer.

[0069] 17 and 18, it can be seen that in the photomultiplier tube 1 of the embodiment, an increase in afterpulses can be suppressed (gas barrier properties can be improved) even when an yttrium oxide layer is provided instead of the aluminum oxide layer 60. Note that, although the thickness of the yttrium oxide layer was 100 nm in the above Example 3, the thickness of the yttrium oxide layer may be 10 nm or more and 100 nm or less, and in this case too, the gas barrier properties can be improved as in Example 3. [Explanation of symbols]

[0070] 1...photomultiplier tube, 10...container, 13...window portion, 13a...first surface, 13b...second surface, 14...tubular portion, 14a...surface, 14b...surface, 20...photocathode, 40...electron multiplier portion, 60...aluminum oxide layer (protective layer), S...vacuum space.

Claims

1. a container having a window portion formed of a light-transmitting material and a tubular portion connected to the window portion and defining a vacuum space together with the window portion; a photocathode formed of a photoelectron emitting material containing sodium, potassium and antimony, provided on a surface of the window portion facing the vacuum space, and emitting photoelectrons in response to incident light; an electron multiplier section that emits secondary electrons in response to the incidence of the photoelectrons emitted from the photocathode and multiplies the secondary electrons, A photomultiplier tube, comprising: a protective layer formed between the photocathode and the window portion and on the surface of the tubular portion, the protective layer containing an oxide of a trivalent metal of Group 3 or Group 13.

2. 2. The photomultiplier tube according to claim 1, wherein the protective layer is formed on a surface of the tubular portion on the side of the vacuum space.

3. 3. The photomultiplier tube according to claim 2, wherein the protective layer is formed on a surface of the tubular portion opposite to the vacuum space.

4. 3. The photomultiplier tube according to claim 1, wherein the protective layer is further formed on a surface of the window portion opposite to the vacuum space.

5. 3. The photomultiplier tube of claim 1, wherein the protective layer is an ALD layer.

6. 3. The photomultiplier tube according to claim 1, wherein the protective layer has a thickness of 10 nm or more and 100 nm or less.

7. 3. The photomultiplier tube according to claim 1, wherein the protective layer is an aluminum oxide layer.

8. 8. The photomultiplier tube according to claim 7, wherein the protective layer has a thickness of 10 nm or more and 30 nm or less.

9. 3. The photomultiplier tube according to claim 1, wherein the protective layer is an yttrium oxide layer.

10. 10. The photomultiplier tube according to claim 9, wherein the protective layer has a thickness of 10 nm or more and 100 nm or less.

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