Transmission mode photocathode

A transmission mode photocathode comprises: an optically transparent substrate having an outside face to which light is incident, and an inside face from which the light incident to the outside face side is output; a photoelectric conversion layer disposed on the inside face side of the optically transparent substrate and configured to convert the light output from the inside face into a photoelectron or photoelectrons; and an optically-transparent electroconductive layer comprising graphene, and disposed between the optically transparent substrate and the photoelectric conversion layer.

TECHNICAL FIELD

The present invention relates to a transmission mode photocathode.

BACKGROUND ART

The transmission mode photocathode is desired to perform detection with linearity in a wide range of small to large light quantities, or, to improve its cathode linearity characteristic. The cathode linearity characteristic herein means linearity of cathode output current against incident light quantity. For improving the cathode linearity characteristic, it is necessary to implement appropriate charge supply to a photoelectric conversion layer and it can be considered that the necessity is met, for example, by placing an electroconductive layer (underlying layer) between an optically transparent substrate and the photoelectric conversion layer to reduce the surface resistance of the photoelectric conversion layer.

On the other hand, for a reflection photocathode, there is a known configuration wherein a layer of graphite and carbon nanotube or the like (intermediate layer) is placed between a substrate and a photoelectric surface (cf. Patent Literature 1 below).

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

However, such an intermediate layer absorbs a considerable amount of incident light in certain cases; for this reason, when it was applied to the transmission photoelectric surface, the quantity of light reaching the photoelectric conversion layer sometimes became insufficient, resulting in failure in detection with sufficient sensitivity. On the other hand, it is also possible to add an additive to the photoelectric conversion layer so as to reduce the surface resistance of the photoelectric conversion layer itself, thereby achieving appropriate charge supply to the photoelectric conversion layer, but the addition of the additive could lower a quantum efficiency of the photoelectric conversion layer, also resulting in failure in obtaining sufficient sensitivity. As described above, the transmission photoelectric surface had the problem that the attempt to improve the cathode linearity characteristic by reduction in surface resistance of the photoelectric conversion layer led to degradation of sensitivity at the same time.

The present invention has been accomplished in view of the above problem and it is an object of the present invention to provide a transmission mode photocathode capable of achieving an improvement in cathode linearity characteristic, while maintaining sufficient sensitivity.

Solution to Problem

A transmission mode photocathode according to one aspect of the present invention comprises: an optically transparent substrate having one face to which light is incident, and another face from which the light incident to the one face is output; a photoelectric conversion layer disposed on the other face side of the optically transparent substrate and configured to convert the light output from the other face into a photoelectron or photoelectrons; and an optically-transparent electroconductive layer comprising graphene, and disposed between the optically transparent substrate and the photoelectric conversion layer.

The transmission mode photocathode according to the one aspect of the present invention can reduce the surface resistance of the photoelectric conversion layer without impeding incidence of light to the photoelectric conversion layer because the optically-transparent electroconductive layer comprising graphene with high optical transparency and high electrical conductivity is disposed between the optically transparent substrate and the photoelectric conversion layer. This can achieve an improvement in cathode linearity characteristic, while maintaining sufficient sensitivity.

In the transmission mode photocathode, the optically-transparent electroconductive layer may be comprised of a single layer of graphene. When the optically-transparent electroconductive layer is formed of a single layer of graphene in this manner, the optical transmittance of the optically-transparent electroconductive layer can be made higher than in a case where the optically-transparent electroconductive layer is formed of multiple layers of graphene. This allows the light output from the other face of the optically transparent substrate to be more certainly guided to the photoelectric conversion layer, so as to more enhance the sensitivity.

In the transmission mode photocathode, the optically-transparent electroconductive layer may be comprised of multiple layers of graphene. When the optically-transparent electroconductive layer is formed of a stack of multiple layers of graphene with high electrical conductivity in this manner, the surface resistance of the photoelectric conversion layer can be reduced more certainly, so as to more improve the cathode linearity characteristic.

Advantageous Effects of Invention

The present invention has achieved the improvement in cathode linearity characteristic, while maintaining the sufficient sensitivity.

DESCRIPTION OF EMBODIMENTS

An embodiment of the transmission mode photocathode according to the present invention will be described below with reference to the drawings. It should be noted that the terms “upper,” “lower,” etc. in the description hereinbelow are used for descriptive purposes based on the states shown in the drawings. Throughout the drawings identical or equivalent portions are denoted by the same reference signs, while avoiding redundant description. The drawings include emphasized portions in part in order to facilitate understanding of the description of the features of the present invention, which are different in size from actual corresponding portions. The present embodiment will be described with an example of transmission mode photocathode2which is used as a photocathode of a transmission type in a photomultiplier tube1.

As shown inFIG. 1toFIG. 3, the photomultiplier tube1being an electron tube has a side tube3made of metal in a substantially cylindrical shape. As shown inFIG. 3, a flange portion3aextending inward is formed at the upper end of the cylindrical side tube3. An optically transparent substrate4with optical transparency is hermetically fixed to this flange portion3awhile being kept in contact therewith. On the side where an inside face (other face)4bof the optically transparent substrate4lies, a photoelectric conversion layer5is formed through an optically-transparent electroconductive layer6with optical transparency and, a contact portion7comprised of an electroconductive material. The photoelectric conversion layer5converts light incident thereto through the optically transparent substrate4into a photoelectron or photoelectrons. The contact portion7and the side tube3are electrically connected by a bonding wire8. The transmission mode photocathode2of the present embodiment is composed of the optically transparent substrate4, optically-transparent electroconductive layer6, contact portion7, and bonding wire8. The details of the configuration of the transmission mode photocathode2will be described after description of the overall configuration of the photomultiplier tube1.

As shown inFIG. 2andFIG. 3, a stem9of a circular disk shape is disposed at the lower opening end of the side tube3. A plurality of (fifteen) electroconductive stem pins10, which are disposed at respective positions along a substantially circular shape while circumferentially separated from each other, are hermetically arranged so as to penetrate through this stem9. A ring-shaped side tube11made of metal is hermetically fixed to this stem9so as to surround it from its side. A flange portion3bformed at the lower end of the upper side tube3and a flange portion11awith the same diameter formed at the upper end of the lower ring-shaped side tube11are welded, as shown inFIG. 3, whereby the side tube3and the ring-shaped side tube11are hermetically fixed to each other. In this configuration, a hermetic vessel12is formed as composed of the side tube3, optically transparent substrate4, and stem9, while the inside thereof is maintained in a vacuum state.

An electron multiplication unit13for multiplying the photoelectrons emitted from the photoelectric conversion layer5is housed in the hermetic vessel12formed as described above. This electron multiplication unit13is configured in a block form by stacking multiple stages (ten stages in the present embodiment) of dynode plates14of a thin plate shape having a large number of electron multiplication holes with secondary electron faces, and is installed on the top surface of the stem9. A dynode plate connection piece14cprojecting outward is formed, as shown inFIG. 1, at a predetermined edge of each dynode plate14. A tip portion of a predetermined stem pin10penetrating through the stem9is fixed as welded to the lower face side of each dynode plate connection piece14c. This establishes electrical connection between each dynode plate14and each stem pin10.

Furthermore, as shown inFIG. 3, a focusing electrode15of a flat plate shape for guiding the photoelectrons emitted from the photoelectric conversion layer5, to the electron multiplication unit13while focusing them is installed between the electron multiplication unit13and the photoelectric conversion layer5in the hermetic vessel12. An anode (positive electrode)16of a flat plate shape for extracting as output signal, secondary electrons emitted from the final-stage dynode14bthrough multiplication by the electron multiplication unit13is arranged as laminated at a stage one step up the final-stage dynode14b. As shown inFIG. 1, projection pieces15aprojecting outward are formed respectively at the four corners of the focusing electrode15. A predetermined stem pin10is fixed as welded to each of the projection pieces15a, thereby establishing electrical connection between the stem pins10and the focusing electrode15. An anode connection piece16aprojecting outward is also formed at a predetermined edge of the anode16. An anode pin17, which is one of the stem pins10, is fixed as welded to this anode connection piece16a, thereby establishing electrical connection between the anode pin17and anode16. When a predetermined voltage is applied to the electron multiplication unit13and the anode16through the stem pins10connected to an unillustrated power supply circuit, the photoelectric conversion layer5and focusing electrode15are set at the same potential and the dynode plates14are set at respective potentials so as to become higher in the stacked order from top to bottom. The anode16is set at a higher potential than the final-stage dynode plate14b.

As shown inFIG. 3, the stem9has a three-layer structure consisting of a base member18, an upper retainer19joined to the top (inside) of the base member18, and a lower retainer20joined to the bottom (outside) of the base member18, and the aforementioned ring-shaped side tube11is fixed to the side face thereof. In the present embodiment, the side face of the base member18forming the stem9is joined to the inner wall surface of the ring-shaped side tube11, whereby the stem9is fixed to the ring-shaped side tube11.

The transmission mode photocathode2will be described usingFIG. 4.FIG. 4(a)is a schematic side sectional view of the transmission mode photocathode2.FIG. 4(b)is a schematic plan view of the transmission mode photocathode2viewed from the side where the optically transparent substrate4is disposed. However, illustration of the optically transparent substrate4is omitted inFIG. 4(b).

As described above, the optically transparent substrate4, which has good optical transparency to light of wavelengths to be detected by the photoelectric conversion layer5, e.g., ultraviolet light, is provided in the circular disk shape on the top face of the upper flange portion3aof the side tube3. The optically transparent substrate4is, for example, a faceplate comprised of glass such as quartz. The optically transparent substrate4has an outside face (one face)4ato which light is incident, and an inside face4bprovided opposite to the outside face4awith respect to the main body of the substrate. The light incident from the outside face4aside passes through the interior of the substrate main body to be output from the inside face4b.

The optically-transparent electroconductive layer6comprised of graphene is formed as separated from the edge of the flange portion3a, on the surface of a circular region out of contact with the flange portion3aon the inside face4bof the optically transparent substrate4. Furthermore, the contact portion7comprised of an electroconductive material (e.g., aluminum (Al)) is formed in an annular shape as kept in contact with the flange portion3aso as to be interposed between the optically-transparent electroconductive layer6and the edge of the flange portion3aand as covering the edge portion6aof the optically-transparent electroconductive layer6, in order to establish electrical connection between the optically-transparent electroconductive layer6and the flange portion3a(metal side tube3). As the contact portion7is formed in this configuration, the side tube3can be securely electrically connected through the contact portion7to the optically-transparent electroconductive layer6and the photoelectric conversion layer5. It is noted that the contact portion7may be formed so as to extend up onto the lower face of the flange portion3a.

Furthermore, in the present embodiment, the bonding wire8, one end of which is connected to the lower face7aof the contact portion7and the other end of which is connected to the lower face of the flange portion3a, is provided, thereby establishing securer electrical connection of the side tube3to the optically-transparent electroconductive layer6and the photoelectric conversion layer5.

The photoelectric conversion layer5is formed so as to cover the lower face of the flange portion3a, the contact portion7, and the lower face of the optically-transparent electroconductive layer6. The photoelectric conversion layer5converts the light output from the inside face4bof the optically transparent substrate4into a photoelectron or photoelectrons. The photoelectric conversion layer5is configured, for example, so as to contain antimony (Sb), potassium (K), and cesium (Cs), or the like.

The below will describe an example of a method for manufacturing the above-described transmission mode photocathode2. First, the optically transparent substrate4is prepared and the optically-transparent electroconductive layer6comprised of graphene is deposited on the surface of this optically transparent substrate4. A method of this deposition will be described below in detail. First, a layer of graphene is formed on the surface of copper foil31by a thermal CVD method. For example, the copper foil is placed under high pressure and high temperature of 1000 Pa and about 1000° C. and methane (CH4) and hydrogen (H2) are supplied thereto at a ratio of 9:1 (e.g., CH4=450 sccm and H2=50 sccm), to form a graphene layer (optically-transparent electroconductive layer6) on the surface of the copper foil31(cf.FIG. 5(a)). Subsequently, PMMA (polymethylmethacrylate resin) is applied to the surface of the optically-transparent electroconductive layer6to form a resin layer32(cf.FIG. 5(b)). Thereafter, the copper foil31is removed by etching (cf.FIG. 5(c)). Then, the film33consisting of the optically-transparent electroconductive layer6and resin layer32obtained as described above is made to float on water and thereafter this film33is scooped up by the optically transparent substrate4(cf.FIG. 5(d)). After that, water34remaining between the film33and the optically transparent substrate4is vaporized by drying (cf.FIG. 5(e)). Finally, the resin layer32is removed with acetone to obtain the optically transparent substrate4on which the optically-transparent electroconductive layer6is formed in a desired region (central region) on the surface (inside face4b).

Next, the inside face4bof the optically transparent substrate4is hermetically fixed to the flange portion3aof the side tube3so that the flange portion3aof the side tube3surrounds the optically-transparent electroconductive layer6as separated therefrom. Subsequently, from the inside of the side tube3, aluminum (Al) is evaporated in an annular shape so as to cover the gap between the optically-transparent electroconductive layer6and the flange portion3aand cover the edge portion6aof the optically-transparent electroconductive layer6, thereby to form the contact portion.7. Then, the lower face7aof the contact portion7and the lower face of the flange portion3aof the side tube3are electrically connected by the bonding wire8. Next, from the inside of the side tube3, antimony (Sb) is evaporated onto the lower face of the flange portion3a, the contact portion7, and the lower face of the optically-transparent electroconductive layer6. Furthermore, potassium (K) and cesium (Cs) are made to react with antimony (Sb) by means of a transfer device to form a bialkali photoelectric surface (photoelectric conversion layer5). Thereafter, the flange portion11aof the ring-shaped side tube11to which the stem9with the electron multiplication unit13installed thereon is hermetically fixed is welded to the flange portion3bof the side tube3, thereby forming the hermetic vessel12. It is also possible to preliminarily hermetically fix the inside face4bof the optically transparent substrate4to the flange portion3aof the side tube3and then form the optically-transparent electroconductive layer6on the inside face4bof the optically transparent substrate4.

The following will describe the superiority of use of the optically-transparent electroconductive layer6comprised of graphene as an underlayer for the photoelectric conversion layer5, usingFIGS. 6 and 7. The graph ofFIG. 6shows the measurement results of spectral transmittances of respective cases where graphene is used and where carbon, nanotube (CNT) mixed with graphite is used, as an underlayer for the photoelectric conversion layer5. The graph ofFIG. 6also shows the spectral transmittances of transparent electroconductive film materials used in electron tubes for reference. The transparent electroconductive film materials herein are indium tin oxide (ITO), aluminum-added zinc oxide (Al—ZnO), and nickel (Ni).

A sample of CNT mixed with graphite is one prepared by a procedure as described in 1 to 6 below.

1. Mixed powder of CNT and graphite is solved in alcohol and stirred.

2. The mixture is kept still until graphite flakes are precipitated.

3. A supernatant solution is collected.

4. A sample substrate (Φ1-inch quartz plate) is heated to 200° C. by a heater.

5. A drop of the supernatant solution collected in 3 is placed onto the quartz plate with a pipette.

6. 5 is executed again after evaporation of alcohol is confirmed.

As shown inFIG. 6, CNT mixed with graphite used as a conventional underlayer has lower transmittances overall across a wide wavelength range than graphene and the difference thereof from graphene is prominent, particularly, in the range from ultraviolet light to visible light. For this reason, it can be said that graphene with higher optical transparency than conventional CNT mixed with graphite is suitable, particularly, as an underlayer for the photoelectric conversion layer5with sensitivity to the range from ultraviolet light to visible light. Furthermore, ITO and Al—ZnO have lower transmittances in the ultraviolet region than graphene and Ni has lower transmittances overall than graphene. In this manner, graphene has the higher optical transparency across the wide wavelength range, particularly, from ultraviolet light to visible light, not only than CNT mixed with graphite used conventionally as an underlying layer, but also than the other electroconductive materials. Therefore, the optically-transparent electroconductive layer6comprised of graphene can be said to be better suited for the underlayer for the photoelectric conversion layer5in the transmission mode photocathode2.

FIG. 7is a drawing showing the cathode linearity measurement results of the transmission mode photocathode2of the photomultiplier tube1(Example 1) according to the present embodiment, and a transmission mode photocathode of a photomultiplier tube (Comparative Example) without the underlayer (part corresponding to the optically-transparent electroconductive layer6) for the photoelectric conversion layer. In the graph ofFIG. 7the axis of abscissa represents cathode output current values and the axis of ordinate does change rates indicative of degrees of deviation of cathode output current values from current values in an ideal linearity case (ideal values). Namely, linearity becomes better as the change rate becomes closer to 0%. The results obtained were as shown inFIG. 7: Comparative Example becomes off the standard of cathode linearity (within ±5%) at about 0.1 μA, whereas Example 1 remains within the standard even over 10 μA. Therefore, the optically-transparent electroconductive layer6comprised of graphene is said to be suitable as an underlayer for the photoelectric conversion layer5in the transmission mode photocathode2, in terms of the cathode linearity characteristic as well.

FIG. 8is a graph showing estimations of quantum efficiencies with variation in the number of graphene layers forming the optically-transparent electroconductive layer6in the transmission mode photocathode2. As shown inFIG. 8, it is expected that, with increase in the number of graphene layers forming the optically-transparent electroconductive layer6, the quantum efficiency decreases because of decrease in optical transmittance. Namely, the optical transmittance of the optically-transparent electroconductive layer6can be made higher when the optically-transparent electroconductive layer6is formed of a single layer (monolayer) of graphene than when the optically-transparent electroconductive layer6is formed of multiple layers of graphene. This allows the light output from the inside face4bof the optically transparent substrate4to be more certainly guided to the photoelectric conversion layer5, so as to increase the quantum efficiency and enhance the spectral sensitivity more.

On the other hand, as shown inFIG. 8, as long as the graphene layers forming the optically-transparent electroconductive layer6are a stack of only several layers, the decrease in quantum efficiency, or degradation of spectral sensitivity is restrained to some extent and thus we can expect that the transmission mode photocathode2has sufficient sensitivity. Therefore, the optically-transparent electroconductive layer6may be composed of multiple layers of graphene in situations such as a case where the light quantity is sufficient and the output current from the photomultiplier tube1is desired to be made large. In this case, the surface resistance of the photoelectric conversion layer5is reduced more certainly and the cathode linearity characteristic is more improved. When the number of graphene layers is a certain number (e.g., six or more), the optically transparent substrate4with the optically-transparent electroconductive layer6thereon can be readily manufactured by applying an ink-like material onto the inside face4bof the optically transparent substrate4.

Since the transmission mode photocathode2described above has the optically-transparent electroconductive layer6of graphene with high optical transparency and high electrical conductivity between the optically transparent substrate4and the photoelectric conversion layer5, the surface resistance of the photoelectric conversion layer5can be reduced without impeding incidence of light to the photoelectric conversion layer5. This can achieve the improvement in cathode linearity characteristic, while maintaining the sufficient sensitivity.

The present invention does not have to be limited only to the above-described embodiment. For example, the transmission mode photocathode according to the present invention can be used as a transmission mode photocathode, for example, in electron tubes such as phototubes, image intensifiers, streak tubes, and X-ray image intensifiers.

The following will describe the fact that the transmission mode photocathode according to the present invention can also be suitably applied to the transmission mode photocathode of the image intensifier, with reference toFIG. 9.FIG. 9is a drawing showing the measurement results of quantum efficiencies of an image intensifier with a CeTe photoelectric surface (photoelectric conversion layer) wherein the optically-transparent electroconductive layer consisting of a single layer of graphene is formed as an underlayer between the optically transparent substrate and the CeTe photoelectric surface (Example 2); and an image intensifier manufactured using a conventional metal (Ni) underlayer (Conventional Example). In comparison between quantum efficiencies at the wavelength of 280 nm, the quantum efficiency of Example 2 is 17.41%, whereas that of Conventional Example is 12.76%, thereby confirming the sensitivity improvement of about 1.36 times.

It is noted that the photoelectric conversion layer5does not have to be limited only to the one consisting primarily of the alkali metals, but may be one consisting of a semiconductor crystal containing gallium or the like. The optically transparent substrate4, which does not have to be limited only to quartz, can also be selected from various optically transparent materials in accordance with conditions such as the wavelength range to be detected. Furthermore, the side tube3may also be comprised of an insulating material such as glass or ceramic, without having to be limited only to the electroconductive materials such as metal.

REFERENCE SIGNS LIST