Reflection type photocathode and photomultiplier using it

Abstract: A high performance reflection type photocathode for use in a photomultiplier tube is formed by sequentially depositing three layers on a substrate (1) made of nickel. The first layer (2) is made of chromium, manganese or magnesium as a major component and is deposited over the substrate (1). The second layer (3) is made of aluminium as a major component and is deposited over the first layer (2). The third layer (4) is made of antimony and at least one kind of alkaline metal and is deposited over the second layer (3).

Description: The present invention relates to a reflection type photocathode and a photomultipler using the same. The photomultiplier is a very versatile and sensitive detector of radiant energy in the ultraviolet, visible, and near infrared regions of the electromagnetic spectrum. In the photomultiplier, the basic radiation sensor is the photocathode which is located inside a vacuum envelope. Photoelectrons are emitted and directed by an appropriate electric field to an electrode or dynode within the envelope. A number of secondary electrons are emitted at the dynode for each impinging primary photoelectron. These secondary electrons in turn are directed to a second dynode and so on until a satisfactory gain is achieved. The electrons from the last dynode are collected by an anode which provides the signal current that is read out. One type of the photomultipliers uses a reflection type photocathode and another type thereof uses a transmission type photocathode. The reflection type photocathode is typically made up of a nickel substrate, an aluminum layer deposited over the substrate, a layer of antimony and alkaline metal such as cesium (Cs), sodium (Na) deposited over the aluminum layer. Document US-A- 4 160 185 disclose a photocathode comprising a layer of Aluminium oxide interposed between a nickel substrate and Antimonium layers. Various properties of the reflection type photocathode changes considerably depending on how the layer structure is determined or what kind of materials is used for each layer. In view of the foregoing, the present inventors explored the properties of numerous photocathode materials to provide a higher sensitivity reflection type photocathode. According to the present invention, a reflection type photocathode for use in a photomultiplier tube, comprises a substrate; a first layer containing chromium, manganese or magnesium, as a major component and being deposited over the substrate; a second layer containing aluminium as a major component and being deposited over the first layer; and, a third layer containing antimony and at least one alkaline metal and being deposited over the second layer. It is preferred that the first layer has a thickness in a range of from 2 to 50 nm and the third layer is deposited in an amount in a range of from 5 to 15 µg/cm². The present invention also embraces a photomultiplier tube including such a photocathode. The particular features and advantages of the invention will now be described with reference to the accompanying drawings, in which: FIG. 1 is a cross-sectional view showing a reflection type photocathode made according to the present invention; FIG. 2 is a graphical representation showing quantum efficiency characteristics of a prior art and inventive photocathode; FIG. 3 is a graphical representation showing dependency of Sk value on the thickness of a chromium layer; FIGS. 4A through 4C show occurrence frequencies of Sk values of the photomultipliers manufactured according to the present invention and FIG. 4D shows an occurrence frequency of Sk values of the prior art photomultipler; and FIG. 5 is a cross-sectional view showing an arrangement of a photomultiplier tube according to the present invention. Referring to Figure 1, there is shown a reflection type photocathode according to a preferred embodiment of the present invention. As shown, the photocathode is made up of a substrate 1 serving as an electrode, a first layer 2 deposited over the substrate 1, a second layer 3 deposited over the first layer 2, and a third layer 4 deposited over the second layer 3. The electrode or substrate 1 is made of nickel. The electrode 1 may not necessarily be a pure nickel plate but it may be a plate-like member with a nickel plating on the surface thereof. Alternatively, the electrode 1 may be a plate-like member containing nickel such as stainless plate. The first layer 2 is made of chromium, manganese or magnesium. It is desirable that the first layer 2 be uniform in thickness ranging from 2 to 50 nm. The second layer 3 is made of aluminum. The thickness of the aluminum layer 3 remains essentially the same as that of a conventional aluminum layer, say 200 nm. No problem arises even if the aluminum layer 3 is oxidized and no matter what degree the aluminum layer 3 is oxidized during the manufacturing process. The third layer 4 is made of antimony and at least one kind of alkaline metal so as to be sensitive to electromagnetic spectrum radiation. In the experiment, the antimony is deposited in an amount in the range of from 5 to 15 µg/cm². Examples of the alkaline metals are cesium, rubidium (Rb), sodium or potassium (K). Two or more such alkaline metals may be contained in the third layer or radiation sensitive layer 4 so as to provide bialkali or multialkali structure. Manufacturing process of the reflection type photocathode will next be described. Firstly, the chromium layer 2 and the aluminum layer 3 are sequentially deposited on the nickel substrate 1 by way of vacuum evaporation or sputtering until the thickness of each layer comes to a pre-selected value. Thereafter, air or gaseous matters contained in the envelope of the photomultiplier is sucked out while heating the envelope for about 45 minutes at a temperature of 260°C, whereupon antimony, sodium and potassium are supplied into the envelope and are rendered active for the formation of the radiation sensitive layer 3 over the aluminum layer 3. The formation method of the layer 4 is essentially the same as has been practiced conventionally and is well known in the art. Therefore, further description thereof is omitted herein. Figure 2 shows quantum efficiency characteristics of a conventional photocathode and an improved photocathode manufactured in accordance with the present invention. The quantum efficiency refers to an average number of electrons photoelectrically emitted from a photocathode per incident photon of a given wavelength. Both the conventional and inventive photocathodes subject to measurement use pure nickel plate for the substrate 1, a 200 nm thick aluminum layer 1, and antimony, cesium, sodium and potassium for the radiation sensitive layer 4. In the inventive photocathode, a 10 nm thick chromium layer 2 is interposed between the nickel substrate 1 and the aluminum layer 3. As can be appreciated from Figure 2, the inventive photocathode exhibits excellent quantum efficiency over the entire wavelength range, particularly in the wavelength ranging from 600 to 900 nanometers. Figure 3 shows dependency of Sk value (photocathode's lumen sensitivity) on the thickness of chromium layer 2, where the Sk values plotted on the graph in relation to the thickness of the chromium layer 2 represent average Sk values of the number of photocathodes test conducted for the same chromium thickness. The number of the test conducted photocathodes are as follows: Five for 2 nm thickness chromium layer; Five for 3 nm thickness chromium layer; Thirty for 9 nm thickness chromium layer; Forty for 10 nm thickness chromium layer; Forty for 11 nm thickness chromium layer; Twenty five for 18nm thickness chromium layer; and Five for 50nm thickness chromium layer. While the above embodiment uses chromium for the first layer 2, manganese or magnesium may be used therefor instead of chromium. Figures 4A through 4D show occurrence frequency, i.e. number of photomultipliers, of the Sk value, where Figure 4A is of the case using chromium for the first layer 2 according to the present invention, Figure 4B is of the case using magnesium for the first layer 2 according to the present invention, Figure 4C is of the case using manganese for the first layer 1 according to the present invention, and Figure 4D is of the case using the conventional structure in which the chromium, magnesium or manganese layer is not provided unlike the present invention. According to the inventive layer structure, it can be appreciated that the reflection type photocathodes with high Sk value can be produced with excellent yield-ablity. The reflection type photocathode of the invention can be applied to, for example, a circular-cage structure photomultiplier with end-on photocathode as shown in Figure 5. In the illustrated photomultiplier, when light is incident on the photocathode through a glass envelope, photoelectrons are emitted from the photocathode and are directed to a first dynode. A number of secondary electrons are emitted at the first dynode for each impinging primary photoelectron. These secondary electrons in turn are directed to a second dynode and so on. The electrons from the last dynode are collected by an anode which provides the signal current that is read out. As described, with the use of the reflection type photocathode constructed in accordance with the present invention, the quantum efficiency is greatly improved and in addition, high Sk value can be effectively realized. Further, a large number of applications in the field of dark light measurement can be accomplished with the use of the photocathode of the present invention. Yet further, detection of extremely weak light which cannot be readily achieved with the prior art devices can be readily done with the photomultiplier constructed in accordance with the present invention.

Claims: A reflection type photocathode for use in a photomultiplier tube, comprising: a substrate (1) made of nickel or containing nickel such as stainless plate; a first layer (2) made of chromium, manganese or magnesium, and being deposited over the substrate (1); a second layer (3) made of aluminium and being deposited over the first layer (2); and, a third layer (4) made of antimony and at least one alkaline metal and being deposited over the second layer (3). A photocathode according to claim 1, wherein the first layer (2) has a thickness in a range of from 2 to 50nm. A photocathode according to claim 1 or 2, wherein the third layer (4) is deposited in an amount of from 5 to 15 µg/cm². A photomultiplier comprising: a glass envelope; a photocathode in accordance with any one of the preceding claims, disposed within the glass envelope; at least one dynode disposed within the glass envelope to receive photoelectrons produced from said photocathode; and, an anode disposed within the glass envelope to collect secondary electrons emitted from the dynode, a signal current being derived from said anode.

Inventors: NAKATSUGAWA KIYOSHI C O HAMAMA, OGURI KAZUYOSHI C O HAMAMATSU, ONDA HIROYUKI C O HAMAMATSU PH, WATANABE HIROYUKI C O HAMAMATS, NAKATSUGAWA, KIYOSHI, C/O HAMAMATSU PHOTONICS K.K., OGURI, KAZUYOSHI, C/O HAMAMATSU PHOTONICS K.K., ONDA, HIROYUKI, C/O HAMAMATSU PHOTONICS K.K., WATANABE, HIROYUKI, C/O HAMAMATSU PHOTONICS K.K.
Applicants: HAMAMATSU PHOTONICS KK, HAMAMATSU PHOTONICS K.K.
Application year: 1992
Main classification: H01J1
IPC codes: H01J9, H01J43, H01J40, H01J29, H01J1