Photocathode and electron tube

Abstract

This photocathode comprises, in the following order: a substrate; a photoelectric conversion layer containing at least two alkali metals; and a surface layer containing tellurium and cesium. In the photocathode, the content of the tellurium, as measured by fluorescent X-ray analysis in the thickness direction from the outermost surface of the surface layer, is at most 0.15 μg / cm2.

Description

Photocathode and electron tube

[0001] The present invention relates to a photocathode and an electron tube.

[0002] As a photocathode used in an electron tube such as a photomultiplier tube, for example, a base layer, K, is formed on the base of a glass container. ２ A photocathode is known that has an inner layer containing CsSb and a surface layer containing CsTe in this order (see Patent Document 1 below). ３ N ２ a photoelectron reflective layer comprising K ２ a photogenerating layer comprising CsSb; and ２ A photocathode having, in this order, a doped surface layer containing SbTeCs is known (see Patent Document 1 below).

[0003] Chinese Patent No. 111261472 Chinese Patent No. 111816533

[0004] However, both of the photocathodes described in Patent Documents 1 and 2 have room for improvement in terms of sensitivity to light in the long wavelength region of 680 nm or more (red sensitivity).

[0005] An object of the present invention is to provide a photocathode and an electron tube that can improve red sensitivity.

[0006] As a result of investigations aimed at solving the above problems, the present inventors have found that the amount of tellurium from the surface of the surface layer of the photocathode along its thickness direction contributes greatly to improving red sensitivity. Specifically, the present inventors have found that if the amount of tellurium is greater than a specific value, no improvement in red sensitivity can be obtained, whereas if the amount of tellurium is equal to or less than a specific value, improvement in red sensitivity can be obtained, and have completed the present invention.

[0007] That is, the present invention provides [1] a photoelectric conversion layer comprising a substrate, a photoelectric conversion layer containing two or more alkali metals, and a surface layer containing tellurium and cesium in this order, wherein the tellurium content measured by X-ray fluorescence analysis in the thickness direction from the outermost surface of the surface layer is 0.15 μg / cm ２According to the photocathode of the present invention, the tellurium content measured by X-ray fluorescence analysis in the thickness direction from the outermost surface of the surface layer is 0.15 μg / cm or less. ２ or less, the red sensitivity can be improved.

[0008] The present invention may also be [2] "the photocathode according to [1]," in which the alkali metal contained in the photoelectric conversion layer is different from cesium." In this case, the surface layer and the photoelectric conversion layer together constitute a multi-alkali photocathode containing three or more alkali metals, and therefore the long wavelength region of 680 nm or more is included in the wavelength range of sensitivity to be measured. Therefore, improvement of red sensitivity is even more preferable.

[0009] The present invention may also be [3] "the photocathode according to [2]," in which the photoelectric conversion layer contains cesium. In this case, the photocathode can have improved sensitivity to light having a wavelength of 680 nm to 850 nm in particular.

[0010] The present invention may also be [4] "the photocathode according to any one of [1] to [3]," further comprising an underlayer between the substrate and the photoelectric conversion layer." In this case, migration of the alkali metal in the photoelectric conversion layer to the substrate can be suppressed, and a decrease in the photoelectric conversion efficiency of the photoelectric conversion layer can be suppressed.

[0011] The present invention may also be [5] "the photocathode according to any one of [1] to [4], wherein the photoelectric conversion layer includes a region containing two or more alkali metals and a region containing one alkali metal." In this case, the photoelectric conversion layer includes a region containing two or more alkali metals and a region containing one alkali metal, thereby enabling the photocathode to further improve red sensitivity.

[0012] The present invention may also be [6] "a photocathode according to any one of [1] to [5]," wherein in the photoelectric conversion layer, the two or more alkali metals include sodium and potassium." In this case, by including sodium and potassium as the two or more alkali metals in the photoelectric conversion layer, the red sensitivity of the photocathode can be effectively improved. The present invention may also be [7] "a photocathode according to any one of [1] to [6]," wherein in the surface layer, the atomic ratio of cesium is greater than the atomic ratio of tellurium." In this case, the photocathode is more likely to improve its sensitivity to light with wavelengths of around 680 nm to 850 nm. The present invention may also be [8] "a photocathode according to any one of [1] to [6]," wherein the tellurium content measured by X-ray fluorescence analysis in the thickness direction from the outermost surface of the surface layer is 0.03 μg / cm ２ The present invention may be a photocathode according to any one of [1] to [7], in which the above-mentioned conditions are satisfied. In this case, the red sensitivity of the photocathode can be further improved. The present invention may also be [9] a photocathode according to [4], in which the underlayer contains a metal oxide.

[0013] The present invention may also be

[10] "an electron tube comprising the photocathode according to any one of [1] to [9]." In this case, an electron tube with improved red sensitivity can be obtained.

[0014] According to the present invention, a photocathode and an electron tube capable of improving red sensitivity are provided.

[0015] 1 is a schematic partial cross-sectional view showing one embodiment of an electron tube of the present invention; FIG. 2 is a cross-sectional view showing the photocathode of FIG. 1; FIG. 3 is a graph showing the relationship between the Te content and relative red sensitivity for the photocathodes of Examples and Comparative Examples; FIG. 4 is a graph showing the relationship between the atomic number ratio in the thickness direction and sputtering time for the photocathode of Example 1; FIG. 5 is a graph showing the relationship between the atomic number ratio in the thickness direction and sputtering time for the photocathode of Example 2; FIG. 6 is a graph showing the relationship between the atomic number ratio in the thickness direction and sputtering time for the photocathode of Comparative Example 1; FIG. 7 is a graph showing an enlarged view of a portion of the graph of FIG. 4 where the atomic number ratio is small; FIG. 8 is a graph showing an enlarged view of a portion of the graph of FIG. 5 where the atomic number ratio is small; and FIG. 9 is a graph showing an enlarged view of a portion of the graph of FIG. 6 where the atomic number ratio is small.

[0016] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In all the drawings, the same or corresponding elements are designated by the same reference numerals, and redundant explanations will be omitted.

[0017] 1 is a schematic partial cross-sectional view showing a photomultiplier tube as an embodiment of the electron tube of the present invention. The electron tube 100 shown in FIG. 1 includes a housing 10, a photocathode 20, an anode 30, a multiplier section 40, a focusing electrode 50, and a stem pin 60.

[0018] The focusing electrode 50, the anode 30, and the multiplication section 40 are provided in an internal space S of the housing 10. The housing 10 includes a cylindrical main body 11, an entrance window 13 provided at one end of the main body 11, and a stem plate 12 provided at the other end of the main body 11, and the main body 11 is sealed by the entrance window 13 and the stem plate 12. Therefore, the housing 10 constitutes a vacuum housing. That is, the space S within the housing 10 is in a vacuum state.

[0019] The photocathode 20 emits photoelectrons e in response to incident light hν. － The focusing electrode 50 has an opening, and the photoelectrons e emitted from the photocathode 20 are － The photoelectrons e are guided through an opening to the multiplication section 40. The multiplication section 40 includes a plurality of dynodes 42. － The anode 30 multiplies the secondary electrons generated in response to the incidence of the electrons. The anode 30 collects the secondary electrons generated by the multiplier 40. The stem pins 60 are provided so as to penetrate the stem plate 12. The corresponding focusing electrodes 50, anodes 30, and dynodes 42 are electrically connected to the stem pins 60.

[0020] FIG. 2 is a cross-sectional view showing the photocathode 20 of FIG. 1. The photocathode 20 includes, in this order, a substrate 21, a photoelectric conversion layer 22, and a surface layer 23 containing tellurium (Te) and cesium (Cs). The substrate 21 is configured with an entrance window 13 (see FIG. 1), and the entrance window 13 is configured with a material that transmits incident light hν (light-transmitting material). In other words, the photocathode 20 is configured as a transmissive photocathode. The surface layer 23 faces the inside of the housing 10, i.e., the space S. The tellurium content measured by X-ray fluorescence analysis in the thickness direction of the surface layer 23 from the outermost surface 23a (the surface not in contact with the photoelectric conversion layer 22) 23a of the surface layer 23 is 0.15 μg / cm ２ The details are as follows.

[0021] According to the photocathode 20, the Te content measured by fluorescent X-ray analysis in the thickness direction from the outermost surface 23a of the surface layer 23 is 0.15 μg / cm ２ or less, the red sensitivity can be improved. Therefore, the electron tube 100 is particularly suitable for detecting light with a wavelength of 680 nm or more, particularly light with a wavelength of 680 to 850 nm. Therefore, the electron tube 100 is useful as an electron tube for light detection used in devices that involve fluorescence detection, such as a flow cytometer (FCM).

[0022] The photocathode 20 will now be described in detail.

[0023] <Substrate> The substrate 21 supports the photoelectric conversion layer 22 and the surface layer 23. In the case of a transmission type photocathode, the substrate 21 may be made of a light-transmitting material, and examples of such light-transmitting materials include glass. Examples of glass include borosilicate glass and quartz glass. Among these, glass that can transmit at least near-infrared rays on the short wavelength side of the near-infrared rays (680 to 2500 nm) is preferred.

[0024] <Photoelectric Conversion Layer> The photoelectric conversion layer 22 contains two or more alkali metals. Examples of alkali metals include cesium (Cs), potassium (K), rubidium (Rb), and sodium (Na). The two or more alkali metals may be selected from among these, but are preferably alkali metals other than Cs contained in the surface layer 23. In this case, the surface layer 23 and the photoelectric conversion layer 22 together form a multi-alkali photocathode containing three or more alkali metals, so that the long wavelength region of 680 nm or more is included in the measurement sensitivity wavelength range. This further widens the wavelength range in which the red sensitivity is improved. The two or more alkali metals preferably include sodium and potassium. In this case, when the two or more alkali metals in the photoelectric conversion layer 22 include sodium and potassium, the red sensitivity of the photocathode 20 can be effectively improved. When the two or more alkali metals include sodium and potassium, the atomic ratio of sodium may be smaller than or equal to the atomic ratio of potassium in at least a portion of the region in the thickness direction of the photoelectric conversion layer 22, but is preferably smaller than the atomic ratio of potassium. In this specification, the "atomic ratio" is measured using the total atomic ratio of all elements as the reference (100%).

[0025] The thickness of the photoelectric conversion layer 22 is not particularly limited, but is, for example, 30 Å to 2500 Å.

[0026] The photoelectric conversion layer 22 may include a first layer 22a that is a region containing two or more alkali metals and a second layer 22b that is a region containing one alkali metal. In this case, the photoelectric conversion layer 22 further includes the second layer 22b, thereby further improving the red sensitivity of the photocathode 20. Here, the one alkali metal may or may not be any of the two or more alkali metals contained in the first layer 22a, but it is preferable that it be any of the alkali metals. Note that the second layer 22b, which is composed of only one alkali metal, has a lower photoelectric conversion efficiency than the first layer 22a, which is a region containing two or more alkali metals.

[0027] The first layer 22 a may further contain antimony. The atomic ratio of antimony in the first layer 22 a may be larger or smaller than the atomic ratio of each of the two or more alkali metals, but is preferably larger than the atomic ratio of each of the two or more alkali metals.

[0028] The second layer 22b may contain antimony. In at least a portion of the second layer 22b, the atomic ratio of antimony may be larger or smaller than the atomic ratio of the alkali metal, but is preferably smaller.

[0029] The photoelectric conversion layer 22 preferably contains Cs. In this case, the photocathode 20 can have improved sensitivity, particularly to light with wavelengths of 680 nm to 850 nm. Cs may be contained throughout the entire thickness of the photoelectric conversion layer 22. That is, it may be contained throughout both the first layer 22a and the second layer 22b in the photoelectric conversion layer 22. Alternatively, Cs may be contained throughout a portion of the thickness of the photoelectric conversion layer 22. In this case, it is preferable that Cs be contained in at least the first layer 22a in the photoelectric conversion layer 22. That is, assuming that a plane P exists between the interface (first interface) S1 between the photoelectric conversion layer 22 and the surface layer 23 and the interface (second interface) S2 between the photoelectric conversion layer 22 and the underlayer 24, Cs may be contained in a region A between the first interface S1 and the plane P in the thickness direction of the photoelectric conversion layer 22. Here, the ratio of the distance from the first interface S1 to the plane P to the thickness of the photoelectric conversion layer 22 is not particularly limited as long as it is greater than 0, but may be 0.5 or less, 0.4 or less, or 0.3 or less. The photoelectric conversion layer 22 may or may not contain Te.

[0030] <Surface Layer> The surface layer 23 is a layer containing Te and Cs, and is a layer that facilitates the emission of photoelectrons generated in the photoelectric conversion layer 22 to the outside of the photocathode 20. The surface layer 23 may or may not contain elements other than Te and Cs. Examples of elements other than Te and Cs include Na, K, Sb, and O. The atomic ratio of Cs may be greater than or equal to the atomic ratio of Te, but is preferably greater than the atomic ratio of Te. In this case, the photocathode 20 is more likely to have improved sensitivity to light with a wavelength of approximately 680 nm to 850 nm. The Te content is the total mass of Te per unit area in the depth direction from the outermost surface 23a of the surface layer 23, and can reflect not only the amount of Te in the surface layer 23 but also the amount of Te in the photoelectric conversion layer 22, the base layer 24, and the substrate 21. However, if the photoelectric conversion layer 22, the base layer 24, and the substrate 21 do not contain Te, the Te content is essentially the total mass of Te per unit area of ​​the surface layer 23.

[0031] <Underlayer> The photocathode 20 may further include an underlayer 24 between the photoelectric conversion layer 22 and the substrate 21. In this case, migration of alkali metals in the photoelectric conversion layer 22 to the substrate 21 can be suppressed, and a decrease in the photoelectric conversion efficiency of the photoelectric conversion layer 22 can be suppressed. As a result, the characteristics of the photocathode 20 (improved red sensitivity) can be maintained. In addition, in the case of a transmissive photocathode 20, the reflectance at a desired wavelength can be reduced by appropriately selecting the material and thickness of the underlayer 24. In other words, by reducing the reflectance for light with a wavelength of 680 nm or more, the amount of light of that wavelength incident on the photoelectric conversion layer 22 can be increased, resulting in a further improvement in red sensitivity. The material constituting the underlayer 24 is not particularly limited as long as it is a material that can act as a barrier against alkali metals. Examples of such materials include metal oxides, such as alumina (Al ２ O ３ ), beryllium oxide (BeO), and manganese oxide (MnO). The thickness of the underlayer 24 is not particularly limited, and is, for example, 5 Å to 3000 Å.

[0032] <Te Content> The Te content measured by X-ray fluorescence analysis (XRF) in the thickness direction of the surface layer 23 from the outermost surface 23a of the surface layer 23 was 0.15 μg / cm ２ In this case, the content of Te is 0.15 μg / cm ２ In order to further improve the red sensitivity of the photocathode 20, the Te content is preferably 0.13 μg / cm ２ or less, more preferably 0.1 μg / cm ２ From the viewpoint of further improving the red sensitivity of the photocathode 20, the content of Te is preferably 0.03 μg / cm ２ More preferably, it is 0.05 μg / cm ２ The content of Te is preferably 0.03 μg / cm ２ 0.13 μg / cm or more ２ or less, more preferably 0.05 μg / cm ２ 0.1 μg / cm or more ２ The Te content can be measured by irradiating X-rays onto the outermost surface 23 a of the surface layer 23 of the photocathode 20 and detecting fluorescent X-rays from the photocathode 20 .

[0033] Next, a method for manufacturing the photocathode 20 will be described.

[0034] First, the base layer 24 is formed on the substrate 21. When the base layer 24 is made of, for example, alumina, first, the housing main body in which the substrate 21 and the main body 11 are integrated is placed in a chamber, and the alumina is placed in the chamber so as to face the substrate 21 of the housing main body. Then, the base layer 24 is formed on the substrate 21 by vapor deposition of alumina or the like.

[0035] As the deposition method, physical vapor deposition, chemical vapor deposition, electron beam deposition, sputter deposition, or the like can be used.

[0036] Next, the focusing electrode 50, the multiplier section 40, and the anode 30 are placed within the housing body, and the open end of the housing body is sealed with the stem plate 12 through which the stem pin 60 passes. The tip tube (not shown) on the stem plate 12 is not sealed, allowing gas to be introduced into and discharged from the internal space via the tip tube. Prior to sealing, for example, an Sb evaporation source and a Te evaporation source are fixed to the focusing electrode 50, and a Cs evaporation source is fixed to the portion of the stem pin 60 located within the housing 10. Next, the photoelectric conversion layer 22 is formed on the base layer 24. More specifically, an intermediate layer is formed on the surface of the base layer 24 by evaporating antimony using an evaporation source made of antimony. Subsequently, the intermediate layer is activated by supplying alkali metal vapor from an alkali metal evaporation source to the intermediate layer. In this case, the supply of alkali metal vapor from the alkali metal vapor deposition source may be performed by preparing two or more vapor deposition sources made of different alkali metals as the alkali metal vapor deposition source, and for example, first supplying vapor of one type of alkali metal and then supplying vapor of two or more types of alkali metals. In this way, the second layer 22b made of a compound of one type of alkali metal and antimony and the first layer 22a made of a compound of two or more types of alkali metal and antimony are formed, and the photoelectric conversion layer 22 is obtained.

[0037] Next, a surface layer 23 is formed on the photoelectric conversion layer 22 by vapor deposition of Te and Cs using a Te vapor deposition source and a Cs vapor deposition source. At this time, the Te content measured by fluorescent X-ray analysis in the thickness direction (or depth direction) of the surface layer 23 from the outermost surface 23a of the surface layer 23 is 0.15 μg / cm ２ The surface layer 23 is formed as follows: In this way, the surface layer 23 containing Te and Cs is formed, the photocathode 20 is formed, and the electron tube 100 is obtained.

[0038] The present invention is not limited to the above-described embodiment. For example, in the above-described embodiment, the photocathode 20 is described as a transmissive photocathode, but the photocathode 20 may be a reflective photocathode. In this case, the substrate 21 may be made of a light-transmitting material, or may be made of a light-impermeable material such as a metal.

[0039] Furthermore, in the above embodiment, the electron tube of the present invention is described as a photomultiplier tube, but the electron tube is not limited to a photomultiplier tube, and may be a phototube, an image intensifier (I.I.), or an HPD (Hybrid Photo Detector).

[0040] The present invention will now be described in more detail with reference to examples.

[0041] (Examples 1 to 4 and Comparative Examples 1 to 10) First, a substrate made of borosilicate glass was prepared, and an underlayer was formed on the substrate. Specifically, a housing body, which integrated the substrate and a cylindrical main body, was placed in a chamber, and an alumina evaporation source was placed in the chamber so as to face the substrate of the housing body. Then, an underlayer with a thickness of 700 Å was formed on the substrate by electron beam evaporation of alumina.

[0042] Next, a focusing electrode, a multiplier, and an anode were placed inside the housing body, and the open end of the housing body was sealed with a stem plate through which a stem pin passed. The tip tubes on the stem plate were not sealed, allowing gas to be introduced into and discharged from the internal space through the tip tubes. Furthermore, before sealing, Sb and Te evaporation sources were fixed to the focusing electrode, and a Cs evaporation source was fixed to the portion of the stem pin located inside the housing. Next, a photoelectric conversion layer was formed on the base layer. More specifically, an intermediate layer was formed on the surface of the base layer by evaporation of Sb using an evaporation source made of antimony (Sb). Subsequently, Na vapor from a sodium (Na) evaporation source and K vapor from a potassium (K) evaporation source were supplied to form a photoelectric conversion layer containing a compound of Na, K, and Sb. The thickness of the photoelectric conversion layer was 1400 Å.

[0043] Next, a surface layer was formed on the photoelectric conversion layer by vapor deposition of Te and Cs using a Te vapor deposition source and a Cs vapor deposition source. The amounts of Te and Cs vapor deposited were varied in each example and comparative example. In this way, a surface layer made of a compound of Te and Cs was formed, and photocathodes with different Te contents were formed as shown in Table 1, and electron tubes were obtained at the same time.

[0044] <Preparation of Photocathodes> Photocathodes were prepared by cutting out the photocathode from the photomultiplier tubes of Examples 1 to 4 and Comparative Examples 1 to 10 by making cuts in the outer periphery of the main body of the housing.

[0045] <Measurement of Te Content> For the photocathode prepared as described above, an X-ray fluorescence analyzer (manufactured by Rigaku Corporation, product name: ZSX Primus) was used to irradiate X-rays onto the surface of the surface layer of the photocathode, and the fluorescent X-rays from the photocathode were detected to determine the Te content (μg / cm ２ The results are shown in Table 1.

[0046] <Red Sensitivity> The Ik68 red sensitivity (A1) of the photomultiplier tubes of Examples 1 to 4 and Comparative Examples 2 to 10 was measured. The Ik68 red sensitivity refers to the lumen sensitivity measured by irradiating the photocathode with light that has passed through an "R-68" (manufactured by Toshiba Corporation) filter (a filter having a transmittance of 80% or more in the wavelength range of at least 700 to 1200 nm). The lumen sensitivity refers to the output current measured by the photomultiplier tube for an incident luminous flux of 1 lumen (lm) from a standard tungsten lamp with a distribution temperature of 2856 K. Meanwhile, photomultiplier tubes were prepared in the same manner as in Examples 1 to 4 and Comparative Examples 2 to 10, except that the surface layer of the photocathode was changed to a layer that did not contain Te, and the Ik68 red sensitivity (A2) of these photomultiplier tubes was measured. The Ik68 red sensitivity (A2) of the photomultiplier tube was designated as the Ik68 red sensitivity (A2) of the comparative photocathode. The ratio of the Ik68 red sensitivity (A1) of the photomultiplier tube to the Ik68 red sensitivity (A2) of the comparative photocathode was calculated as relative red sensitivity based on the following formula. The results are shown in Table 1. Relative red sensitivity (%) = 100 × A1 / A2. Note that the photocathode of Comparative Example 1 was also a comparative photocathode, and A2 = A1, so A2 was not measured. The relationship between relative red sensitivity and Te content is shown in Figure 3.

[0047] <Measurement of atomic ratio in the thickness direction of the photocathode> Of the photocathodes prepared as described above, the photocathodes cut out from Example 1, Example 2, and Comparative Example 1 were used to measure the atomic ratios of Na, K, Sb, Al, O, Te, and Cs from the outermost surface of the surface layer of the photocathode to the thickness direction of the photocathode using an X-ray photoelectron spectroscopy (XPS) analyzer (manufactured by ULVAC-PHI, Inc., product name: PHI5000 VersaProbe II). The atomic ratios were measured using the sum of the atomic ratios of all elements as the reference (100%). Specifically, X-rays were incident on the surface of the surface layer of the photocathode, and photoelectrons from the photocathode were detected to perform elemental analysis and determine the atomic ratios (%). The atomic ratios of the elements were calculated using the sensitivity coefficients of the analyzer manufacturer. Then, after removing the atoms in the surface layer by sputtering with Ar ions, elemental analysis was performed again to determine the atomic ratio (%). In this way, the atomic ratio (%) was calculated for each sputtering time until the sputtering time reached 45 minutes. The results are shown in Figures 4 to 6. Also, Figures 7 to 9 show enlarged graphs of the portions of the graphs in Figures 4 to 6 where the atomic ratio is small. Note that in Figures 4 to 9, the sputtering time corresponds to the thickness from the outermost surface of the surface layer 23, with 0 minutes corresponding to the outermost surface of the surface layer and 45 minutes having reached the base layer. Note that the sputtering conditions during sputtering were constant.

[0048]

[0049] From the results shown in Table 1 and FIG. 3, the Te content measured by XRF analysis in the thickness direction from the outermost surface of the surface layer was 0.15 μg / cm ２ The photocathodes of Examples 1 to 4 below have a Te content of 0.15 μg / cm ２ It was found that the relative red sensitivity was greater than that of Comparative Examples 2 to 10, which exceeded 10. Therefore, it was confirmed that the photocathode of the present invention can improve the red sensitivity.

[0050] 10...housing, 20...photocathode, 21...substrate, 24...underlayer, 22...photoelectric conversion layer, 22a...first layer (region containing two or more types of alkali metals), 22b...second layer (region containing one type of alkali metal), 23...surface layer, 23a...outermost surface, 30...anode, 100...electron tube.

Claims

1. A photoelectric conversion element comprising a substrate, a photoelectric conversion layer containing two or more alkali metals, and a surface layer containing tellurium and cesium, in this order, wherein the tellurium content measured by X-ray fluorescence analysis in the thickness direction from the outermost surface of the surface layer is 0.15 μg / cm ２ Below is a photocathode.

2. The photocathode of claim 1, wherein the alkali metal contained in the photoelectric conversion layer is different from cesium.

3. The photocathode of claim 2, wherein said photoelectric conversion layer comprises cesium.

4. The photocathode according to claim 1, further comprising an underlayer between said substrate and said photoelectric conversion layer.

5. The photocathode according to claim 1, wherein the photoelectric conversion layer includes a region containing two or more types of alkali metals and a region containing one type of alkali metal.

6. The photocathode according to claim 1, wherein in said photoelectric conversion layer, said two or more alkali metals include sodium and potassium.

7. The photocathode according to claim 1, wherein the atomic ratio of cesium is greater than the atomic ratio of tellurium in said surface layer.

8. The tellurium content measured by fluorescent X-ray analysis in the thickness direction from the outermost surface of the surface layer is 0.03 μg / cm ２ The photocathode according to claim 1 .

9. The photocathode of claim 4, wherein said underlayer comprises a metal oxide.

10. An electron tube comprising a photocathode according to any one of claims 1 to 9.

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