Photocathode and electron tube

Abstract

The photocathode comprises, in this order, a substrate, a photoelectric conversion layer containing two or more alkali metals, and a surface layer containing tellurium and cesium. In the photocathode, 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 The following is the result.

Description

[Technical Field]

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

[0002] Known photocathodes for use in electron tubes such as photomultiplier tubes include, for example, a photocathode that includes, in this order, a base layer, an inner layer containing K2CsSb, and a surface layer containing CsTe on the base of a glass container (see Patent Document 1 below).Also known photocathodes include, in this order, a photoelectron reflecting layer containing Be3N2, a photoelectron generating layer containing K2CsSb, and a surface doped layer containing K2SbTeCs on a glass substrate (see Patent Document 1 below). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Chinese Patent No. 111261472 [Patent Document 2] Chinese Patent No. 111816533 Summary of the Invention [Problem to be solved by the invention]

[0004] However, both of the photocathodes described in Patent Documents 1 and 2 above have room for improvement in terms of improving 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. [Means for solving the problem]

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

[0008] The present invention may also be [2] "the photocathode according to [1]," wherein 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 wavelength range of sensitivity to be measured also includes the long wavelength region of 680 nm or more, making an improvement in red sensitivity even more desirable.

[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 improve its sensitivity to light with wavelengths 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, it is possible to prevent the alkali metal in the photoelectric conversion layer from migrating to the substrate, and to prevent a decrease in the photoelectric conversion efficiency in the photoelectric conversion layer.

[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 including a region containing two or more alkali metals and a region containing one alkali metal can further improve the red sensitivity of the photocathode.

[0012] The present invention may also be [6] "the 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, when the two or more alkali metals in the photoelectric conversion layer include sodium and potassium, 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 the atomic ratio of cesium is greater than the atomic ratio of tellurium in the surface layer." In this case, the photocathode is likely to have improved sensitivity to light with a wavelength of approximately 680 nm to 850 nm. The present invention relates to [8] "a surface layer having a tellurium content of 0.03 μg / cm 3 measured by fluorescent X-ray analysis in the thickness direction from the outermost surface of the surface layer." 2 The photocathode may be the photocathode according to any one of [1] to [7] above. 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. [Effects of the Invention]

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

[0015] [Figure 1] 1 is a schematic partial cross-sectional view showing an embodiment of an electron tube of the present invention. [Figure 2] FIG. 2 is a cross-sectional view showing the photocathode of FIG. [Figure 3] 1 is a graph showing the relationship between the Te content and the relative red sensitivity for the photocathodes of Examples and Comparative Examples. [Figure 4] 1 is a graph showing the relationship between the atomic ratio in the thickness direction of the photocathode of Example 1 and the sputtering time. [Figure 5] 10 is a graph showing the relationship between the atomic ratio in the thickness direction of the photocathode of Example 2 and the sputtering time. [Figure 6] 10 is a graph showing the relationship between the atomic ratio in the thickness direction of the photocathode of Comparative Example 1 and the sputtering time. [Figure 7] 5 is a graph showing an enlarged view of a portion of the graph in FIG. 4 where the atomic ratio is small. [Figure 8] 6 is a graph showing an enlarged view of a portion of the graph in FIG. 5 where the atomic ratio is small. [Figure 9] 7 is a graph showing an enlarged view of a portion of the graph in FIG. 6 where the atomic ratio is small. DETAILED DESCRIPTION OF THE INVENTION

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

[0020] FIG. 2 is a cross-sectional view showing the photocathode 20 of FIG. 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). That is, 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 fluorescent X-ray 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 was 0.15 μg / cm 2 The details are as follows.

[0021] According to the photocathode 20, the content of Te 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, such as glass. Examples of such a light-transmitting material include borosilicate glass and quartz glass. Among these, glass that can transmit at least short-wavelength 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 different from the 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 wavelength range of sensitivity to be measured. This further widens the wavelength range in which 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, in at least a portion of the region in the thickness direction of the photoelectric conversion layer 22, the atomic ratio of sodium may be smaller than or equal to the atomic ratio of potassium, but is preferably smaller than the atomic ratio of potassium. In this specification, the "atomic ratio" is measured with 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 a single 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 22a may further include antimony. The atomic ratio of antimony in the first layer 22a 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, which can improve the sensitivity of the photocathode 20 to light with a wavelength of 680 nm to 850 nm in particular. Cs may be contained throughout the entire thickness of the photoelectric conversion layer 22. That is, Cs may be contained in the photoelectric conversion layer 22 throughout both the first layer 22a and the second layer 22b. Alternatively, Cs may be contained over a portion of the thickness direction of the photoelectric conversion layer 22. In this case, Cs is preferably contained in at least the first layer 22a of the photoelectric conversion layer 22. That is, when a plane P exists between an interface (first interface) S1 between the photoelectric conversion layer 22 and the surface layer 23 and an 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 sensitivity of the photocathode 20 to light with a wavelength of approximately 680 nm to 850 nm is easily improved. 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 underlayer 24, and the substrate 21. However, when the photoelectric conversion layer 22, the underlayer 24, and the substrate 21 do not contain Te, the Te content is substantially 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, it is possible to suppress the movement of the alkali metal in the photoelectric conversion layer 22 to the substrate 21, and it is possible to suppress a decrease in the photoelectric conversion efficiency in the photoelectric conversion layer 22. As a result, the characteristics (improvement of red sensitivity) of the photocathode 20 can be maintained. Further, in the case of the transmissive photocathode 20, by appropriately selecting the material and thickness of the underlayer 24, the reflectance at a desired wavelength can be reduced. That is, by reducing the reflectance of light having a wavelength of 680 nm or more, the amount of incident light of light having that wavelength to the photoelectric conversion layer 22 can be increased, and as a result, the red sensitivity can be further improved. The material constituting the underlayer 24 is not particularly limited as long as it can be a barrier against alkali metals. Examples of such materials include metal oxides. Examples of metal oxides include alumina (Al2O3), 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 is such that 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 is 0.15 μg / cm 2 or less. In this case, when the Te content exceeds 0.15 μg / cm 2 no improvement in red sensitivity can be obtained, whereas an improvement in red sensitivity can be obtained. The content of Te is preferably 0.13 μg / cm 3 from the viewpoint of further improving the red sensitivity of the photocathode 20. 2 or less, more preferably 0.1 μg / cm 2 The following is the result. The content of Te is preferably 0.03 μg / cm 3 from the viewpoint of further improving the red sensitivity of the photocathode 20. 2 More preferably, it is 0.05 μg / cm 2 That's all. The content of Te is preferably 0.03 μg / cm 2 More than 0.13μg / cm 2 or less, more preferably 0.05 μg / cm 2 More than 0.1μg / cm 2 The following is the result. 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 part 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 unit 40, and the anode 30 are placed inside 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. Note that the tip tube (not shown) provided on the stem plate 12 is not sealed, so gas can be introduced into and discharged from the internal space through the tip tube. Furthermore, before 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 part of the stem pin 60 that is located inside the housing 10. Next, the photoelectric conversion layer 22 is formed on the underlayer 24. More specifically, an intermediate layer is first formed on the surface of the underlayer 24 by vapor deposition of antimony using a vapor deposition source made of antimony. Subsequently, alkali metal vapor is supplied to the intermediate layer from an alkali metal vapor deposition source to activate the intermediate layer. The supply of alkali metal vapor from the alkali metal vapor deposition source can be performed by preparing two or more vapor deposition sources made of different alkali metals, for example, by first supplying vapor of one type of alkali metal and then supplying vapor of two or more types of alkali metals. In this way, a second layer 22b made of a compound of one type of alkali metal and antimony and a first layer 22a made of compounds of two or more types of alkali metals and antimony are formed, thereby obtaining the photoelectric conversion layer 22.

[0037] Next, the 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 content of Te 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 has been described as a photomultiplier tube, but the electron tube is not limited to a photomultiplier tube, and may be a phototube, an image intensifier (II), or an HPD (Hybird Photo Detector). [Example]

[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 a base layer 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, a 700 Å-thick base layer of alumina was formed on the substrate by electron beam evaporation.

[0042] Next, a focusing electrode, a multiplier section, and an anode were arranged inside the housing main body, and the open end portion of the housing main body was sealed with a stem plate through which the stem pins passed. Note that the chip tube provided on the stem plate was not sealed, and gas could be introduced into and discharged from the internal space through the chip tube. Also, before sealing, an Sb evaporation source and a Te evaporation source were fixed to the focusing electrode, and a Cs evaporation source was fixed to the portion of the stem pin disposed inside the housing. Next, a photoelectric conversion layer was formed on the underlying layer. More specifically, first, an intermediate layer was formed on the surface of the underlying 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. At this time, the thickness of the photoelectric conversion layer was 1400 Å.

[0043] Next, a surface layer was formed on the photoelectric conversion layer by evaporation of Te and Cs using a Te evaporation source and a Cs evaporation source. At this time, the evaporation amounts of Te and Cs were varied in each of the examples and comparative examples. Thus, a surface layer made of a compound of Te and Cs was formed, and as shown in Table 1, a photoelectric cathode having different Te contents was formed and an electron tube was obtained at the same time.

[0044] <Preparation of Photoelectric Cathode> From the photomultiplier tubes of Examples 1 to 4 and Comparative Examples 1 to 10, a cut was made on the outer peripheral surface of the main body portion of the housing to cut out the photoelectric cathode, and the photoelectric cathode was prepared.

[0045] <Measurement of Te Content> Regarding the photoelectric cathode prepared as described above, by using a fluorescent X-ray analyzer (manufactured by Rigaku Corporation, product name: ZSX Primus), X-rays were incident on the surface of the surface layer of the photoelectric cathode, and the fluorescence X-rays from the photoelectric cathode were detected to measure the content of Te (μg / cm 2 ). The results are shown in Table 1.

[0046] <Red Sensitivity> The Ik68 red sensitivity (A1) was measured for the photomultiplier tubes of Examples 1 to 4 and Comparative Examples 2 to 10. Ik68 red sensitivity refers to the lumen sensitivity measured by irradiating the photocathode with light that has passed through an "R-68" (Toshiba Corporation) filter (a filter having a transmittance of 80% or more in the wavelength range of at least 700 to 1200 nm). 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. On the other hand, 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 the relative red sensitivity based on the following formula. The results are shown in Table 1. Relative red sensitivity (%) = 100 x A1 / A2 The photocathode according to Comparative Example 1 was also a comparative photocathode, and A2 was not measured because A2=A1. The relationship between the relative red sensitivity and the Te content is shown in FIG.

[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, the photocathodes cut out from Example 2, and the photocathodes cut out from 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 in the thickness direction of the photocathode using an X-ray photoelectron spectroscopy (XPS) analyzer (manufactured by ULVAC-PHI, Inc., product name: PHI5000 VersaProbeII). The atomic ratios were measured using the total atomic ratio of all elements as the reference (100%). Specifically, X-rays were incident on the surface of the photocathode, and photoelectrons from the photocathode were detected to perform elemental analysis and determine the atomic ratio (%). The sensitivity coefficients of the analytical equipment manufacturer were used to calculate the atomic ratio of elements. The atoms in the surface layer were then removed by sputtering with Ar ions, after which 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. Enlarged graphs of the portions of the graphs in Figures 4 to 6 with small atomic ratios are shown in Figures 7 to 9. 4 to 9, the sputtering time corresponds to the thickness from the outermost surface of the surface layer 23, with 0 min corresponding to the outermost surface of the surface layer and 45 min reaching the underlayer. The sputtering conditions during sputtering were kept constant.

[0048] [Table 1]

[0049] From the results shown in Table 1 and Figure 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 red sensitivity. [Explanation of symbols]

[0050] 10...housing, 20...photocathode, 21...substrate, 24...underlying layer, 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 substrate; a photoelectric conversion layer containing two or more alkali metals; a surface layer containing tellurium and cesium, in this order; The tellurium content measured by fluorescent X-ray 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 the photoelectric conversion layer comprises cesium.

4. The photocathode according to claim 1 , further comprising an underlayer between the substrate and the 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 the photoelectric conversion layer, the two or more alkali metals include sodium and potassium.

7. 2. The photocathode according to claim 1, wherein the atomic ratio of cesium is greater than the atomic ratio of tellurium in the 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 the underlayer comprises a metal oxide.

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

Images