Reflection-type photoelectric surface and photomultiplier

Abstract: The photocathode according to this invention is characterized in that an aluminium thin film (2) is formed on a substrate (1), and then an antimony thin layer (3) is deposited directly on the aluminium thin film (2) and is activated by an alkali metal. It is especially preferable that the antimony thin layer (2) is deposited in a thickness of 15 µg/cm² to 45 µg/cm² and is activated by an alkali metal. Such reflection-type photocathode is applicable to photomultipliers. Among functions which are considered to be done by the A l film (2), which is in direct contact with the Sb layer (3), a first one is to prevent the alloying between the Sb layer (3) and the substrate (1) (e. g. , Ni) , and a second one is to augment a reflectance of light to be detected.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a reflection-type photocathode (i.e. photoelectric surface), and a photomultiplier. Related Background Art Reflection-type photocathodes using nickel (Ni), etc. as the substrates are known in the art disclosed in a first literature, U.S. Patent No. 4,160,185, a second literature, Japanese Patent Laid-Open Publication No. 87274/1974 and a third literature, Japanese Patent Publication No. 47665/1977. The first literature discloses the art in which an aluminium oxide (Al 2 O 3 ) layer is formed on a Ni substrate, and antimony (Sb) is deposited on the Al 2 O 3 layer and is activated by alkali metals. The Al 2 O 3 layer is provided for the prevention of the alloying of the Ni and Sb. The second literature discloses the art in which a surface of an Al substrate (or a substrate having Al applied to a surface of a base) is oxidized to form an Al 2 O 3 layer, and a reflection-type photocathode containing Sb and alkali metal is formed. The base for Al to be applied to is exemplified by tantalum (Ta). In the third literature as well, a surface of an Al substrate is oxidized to form an Al 2 O 3 layer, and a photocathode containing Sb activated by alkali metals is formed. As described above, each of the conventional reflection-type photocathodes has the Al 2 O 3 layer below the activated Sb film which is a photosensitive layer. Therefore, their fabrication process essentially includes the step of oxidizing Al. Photomultipliers are used for the photometry of feeble light, and are effective especially at a limit where light to be detected is measured by counting photons. Accordingly, the sensitivity improvement by even some percentage is significant, and the process control is very difficult. A restrictive condition that the Al 2 O 3 layer is necessary not only lowers yields of their fabrication, but also makes it difficult to realize a stable sensitivity. Depending on characteristics of the Al 2 O 3 layer, the reflection-type photocathodes adversely have various sensitivities. The document EP-A-0 532 358 forms part of the state of the art as defined in Article 54(3) EPC. This document discloses a reflection type photocathode including a layer containing aluminium and a layer containing an amount of antimony in the range 5 to 15 µg/cm 2 deposited thereon. SUMMARY OF THE INVENTION In view of these disadvantages, the inventors have made studies and found that a good reflection-type photocathode can be realized without the step of forming an Al 2 O 3 layer. In addition, they have found optimum conditions for the fabrication of the reflection-type photocathode without the step of forming the Al 2 O 3 layer. According to one aspect of the present invention, there is provided a reflection-type photocathode as claimed in claim 1. According to another aspect of the present invention, there is provided a method of fabricating a reflection-type photocathode, as claimed in claim 5. Such a photocathode is applicable to photomultipliers. In the above description, the unit of the layer thickness is noted µg/cm 2 which is equivalent to the dimension of length. This notation is used in the following. The reflection-type photocathode according to this invention comprises the alkali metals-activated Sb thin layer directly formed on the Al thin film without the special step of forming an Al 2 O 3 layer. This is an innovation to the conventional reflection-type photocathodes. That is, even when the Sb layer is deposited directly on the Al film as long as the Sb layer is thin, satisfactory results can be obtained. Since the Sb layer has a thickness of 15 µg/cm 2 to 45 µg/cm 2 , this invention is especially significant. It is considered that the Al film, which is in direct contact with the Sb layer, has among various functions a first function of preventing the alloying of the Sb layer with the base substrate (e.g. Ni), and a second function of increasing a reflectivity of light to be detected. This invention has successfully achieved a reflection-type photocathode of high sensitivity and high yields. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of the reflection-type photocathode according to an embodiment of this invention. FIG. 2 is a graph of the spectral sensitivity characteristic of the reflection-type photocathode according to a first example. FIG. 3 is a view of the spectral sensitivity characteristic of the reflection-type photocathode according to a second example. An embodiment of this invention will be explained in good detail. As shown in FIG. 1, an Al thin film 2 is formed on, e.g., a base substrate of, e.g., Ni by, e.g., vacuum vaporization. A photosensitive layer 3 containing Sb activated by alkali metals, such as cesium (Cs), potassium (K), sodium (Na), etc., is formed on the Al film 2. When light hv is incident on the reflection-type photocathode of FIG. 1, in accordance with the energy of the incident light, a photoelectron e - is emitted from the photosensitive layer. A photomultiplier including such a reflection-type photocathode is fabricated as follows. First, a vacuum vessel is prepared. An Al film is formed by vacuum vaporization on a part for the reflection-type photocathode to be formed on. Subsequently Sb is vaporized directly on the Al film without the step of oxidizing the Al film. It is preferable that at this time the Sb is vaporized in a thin film or a porous film, of a 15 µg/cm 2 to 45 µg/cm 2 thickness. Then one or some of alkali metals, such as Cs, Na, K, etc. are introduced to activate and anneal the Sb layer. Temperature conditions, periods of time, etc. of the activation and annealing are optionally determined as known. The temperature is selected from 140°C to 220°C. The fabrication procedure of the other elements of the photomultiplier, e.g., dynodes, microchannel plates, anodes, etc. is the same as that for the conventional photomultipliers. When the formation of the reflection-type photocathodes and the fabrication of the elements are over, the vacuum vessel is sealed, and the photoelectric multiplier is completed. Next, examples of the photomultiplier according to this invention will be explained. In each example the base substrate 1 was a Ni plate, and the Al film 2 was formed on a surface of the substrate 1 in a thickness of hundreds Å (by vacuum vaporization) . The Sb layer 3 was directly formed on the Al film 2. The thickness of the Sb layer was about 180 µg/cm 2 in a first example and about 30 µg/cm 2 in a second example. Then Na, K and Cs were let in to activate the Sb layer, and a multialkali (Na-K-Cs-Sb) photocathode was prepared. The first example had the spectral sensitivity characteristic of FIG. 2. The dot line indicates its quantum efficiency, and the solid line indicates its cathode emission sensitivity. The average lumen sensitivity is 80 (µA/ℓm). The second example had the spectral sensitivity characteristic of FIG.3. Its average lumen sensitivity is as high as 200 (µA/ℓm). As seen from the comparison between FIGS. 2 and 3, the reduction of the Sb layer thickness can attain a great sensitivity improvement. A cause of this improvement is considered to be as follows. That is, since the Al film is in direct contact with the photosensitive layer 3, the reflectivity of the incident light (light to be detected) is improved, and more photoelectrons are generated in the photosensitive layer 3. In the case that the photosensitive layer 3 is too thick, the generated photoelectrons are adversely trapped by the photosensitive layer 3 itself before being emitted into a vacuum, with the result of low electron yields. But in the case that the photosensitive film 3 is thin, the photoelectron trapping ratio can be low, with the result of higher ratios of emitting photoelectrons into a vacuum. In the case that the photosensitive film 3 is too thin, even if more light is reflected on the Al film 2, the photosensitive layer 3 contributes less to the generation of photoelectrons. The Sb layer has the optimum thickness, and the inventors have found that the optimum thickness of the Sb layer is 15 µg/cm 2 ∼ 45 µg/cm 2 . The above-described embodiment has been explained by means of the multialkali photocathode, but Cs-Sb or Cs-K-Sb (bialkali) photocathodes may be used. The base substrate is not limited to Ni.

Claims: Claims for the following Contracting States : FR, GB, NL A reflection-type photocathode comprising: a reflective layer of aluminium (2) formed on the upper surface of a substrate (1); and a photosensitive layer (3) formed directly on the reflective layer, and formed of antimony activated with at least one kind of alkali metal; characterised in that the area density of the antimony is greater than 15µg/cm 2 and less than or equal to 45µg/cm 2 . A reflection-type photocathode as claimed in claim 1, wherein the alkali metal includes one or more of cesium, potassium, sodium and rubidium. A reflection-type photocathode as claimed in claim 1 or 2, wherein the substrate (1) is formed of nickel. A photomultiplier comprising a vacuum vessel accommodating a reflection-type photocathode as claimed in any preceding claim; photomultiplying means for multiplying photoelectrons emitted from the reflection-type photocathode; and an anode for receiving the multiplied photoelectrons. A method of fabricating a reflection-type photocathode, comprising: the step of depositing a reflective layer (2) of aluminium on the upper surface of a substrate (1); and the step of forming a photosensitive layer (3) by depositing an antimony layer directly on the reflective layer and activating the antimony with an alkali metal; characterised in that the antimony is deposited at an area density greater than 15µg/cm 2 and less than or equal to 45µg/cm 2 . A method of fabricating a photocathode according to claim 5, wherein the photosensitive layer (3) is formed by activating with the alkali metal the antimony layer deposited directly on the reflective layer, and then annealing the activated antimony layer. Claims for the following Contracting State : DE A reflection-type photocathode comprising: a reflective layer of aluminium (2) formed on the upper surface of a substrate (1); and a photosensitive layer (3) formed directly on the reflective layer, and formed of antimony activated with at least one kind of alkali metal; characterised in that the area density of the antimony is greater than or equal to 15µg/cm 2 and less than or equal to 45µg/cm 2 . A reflection-type photocathode as claimed in claim 1, wherein the alkali metal includes one or more of cesium, potassium, sodium and rubidium. A reflection-type photocathode as claimed in claim 1 or 2, wherein the substrate (1) is formed of nickel. A photomultiplier comprising a vacuum vessel accommodating a reflection-type photocathode as claimed in any preceding claim; photomultiplying means for multiplying photoelectrons emitted from the reflection-type photocathode; and an anode for receiving the multiplied photoelectrons. A method of fabricating a reflection-type photocathode, comprising: the step of depositing a reflective layer (2) of aluminium on the upper surface of a substrate (1); and the step of forming a photosensitive layer (3) by depositing an antimony layer directly on the reflective layer and activating the antimony with an alkali metal; characterised in that the antimony is deposited at an area density greater than or equal to 15µg/cm 2 and less than or equal to 45µg/cm 2 . A method of fabricating a photocathode according to claim 5, wherein the photosensitive layer (3) is formed by activating with the alkali metal the antimony layer deposited directly on the reflective layer, and then annealing the activated antimony layer.

Inventors: IKUMA TOSHIO, WASHIYAMI HIROAKI, WATASE YASUSHI, IKUMA, TOSHIO, WASHIYAMI, HIROAKI, WATASE, YASUSHI
Applicants: HAMAMATSU PHOTONICS KK, HAMAMATSU PHOTONICS K.K.
Application year: 1993
Main classification: H01J1
IPC codes: H01J40, H01J43, H01J1