JPH0228219B2 - KODENMENOJUSURUDENSHIKANNOSEIZOHOHO - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Technical Field of the Invention The present invention relates to a method of manufacturing an electron tube having a photocathode such as an X-ray fluorescence multiplier, and particularly relates to a method of manufacturing the photocathode.

(2) Background technology When forming the photocathode of an electron tube with a photocathode such as an X-ray fluorescence multiplier, alkali metals such as cesium (Cs) and potassium (K), antimony (Sb), Metalloids such as bismuth (Bi) and tellurium (Te) are evaporated as photocathode materials to form these compounds. Since the sensitivity of the photocathode is affected by the amount of evaporation of these materials, the optimal amount of evaporation must be quantitatively controlled. Methods for forming such a photocathode can be roughly divided into two types, considering the vapor deposition method of semimetals. One method is to alternately deposit a metalloid and an alkali metal to reach the maximum sensitivity. Another method is to deposit a predetermined amount of a metalloid in advance, and then react with an alkali metal to achieve the highest sensitivity.

Conventionally, these two methods have been used depending on the shape of the electron tube with the photocathode and the stability of the resulting photocathode sensitivity. In particular, although the former method provides stable sensitivity, it does not reach the maximum sensitivity obtained with the latter method. On the other hand, with the latter method, high sensitivity can be stably obtained if the structure allows an appropriate amount of metalloid to be deposited, but if the structure is such that it is difficult or impossible to quantify the amount of metalloid deposited, becomes completely unusable. However, it also has the great advantage of shortening the time required to form the photocathode. In addition, semimetal quantification is achieved by injecting light from the outside into the substrate on which the photocathode is formed or in its vicinity, and measuring the extent to which the transmitted light passing through the substrate or its vicinity is reduced by the vapor deposition of the semimetal. Control. Therefore, this method inevitably has structural limitations.

Incidentally, in recent years, the internal structure of an electron tube having a photocathode for converging an electric field has become increasingly complex as the performance of the electron tube has improved.
For example, if we take an X-ray image intensifier tube, it used to be made up of three electrodes, but in recent years it has been made up of four electrodes to vary the field of view, or five electrodes to improve the distribution of resolution. There is a growing trend towards multiple electrodes.

In addition, in order to reduce scattered radiation and deal with increases in material and processing costs, X-ray image intensifier tubes have been made of metal, starting from the entrance window material and extending to the vacuum vessel surrounding the photocathode. It's here. As the internal electrode structure of X-ray image intensifier tubes has become more complex and the vacuum container has become more metallic, an optical path has been installed to pass light from the outside into the vacuum container near the photocathode and measure the light transmittance. Therefore, it has become difficult to quantitatively control the amount of metalloids deposited.

(3) Purpose of the Invention In view of the above-mentioned circumstances, the present invention provides a method for manufacturing an electron tube in which the vacuum container surrounding the photocathode is made of metal, which can maximize the sensitivity of the photocathode and achieve stable mass production. It is something to do.

(4) Structure of the Invention In order to achieve the above-mentioned object, the present invention includes at least one semiconductor optical sensor installed near the photocathode inside a vacuum container, and receives light from a light source placed inside or outside the vacuum container. to reach this sensor, and keep the photocathode forming substrate at a predetermined temperature.
First, alkali metals such as Cs and K are evaporated.
Then, the alkali metal is deposited until the photoelectron flow from the photocathode reaches a maximum value and exceeds this value. Next, metalloids such as Sb are evaporated. At this time, the amount of semimetal is controlled so that the output current of the semiconductor optical sensor exhibits a value within the range of 70 to 95%, preferably 75 to 90%, of the initial value. After that, the alkali metal is evaporated again and the process is slowly cooled until the photoelectron flow is stabilized.

(5) Embodiments of the invention An explanation will be given by taking an X-ray fluorescence multiplier tube as an example. Note that the same parts are represented by the same symbols.

The structure of this X-ray fluorescence multiplier tube and the state of forming the photocathode will be explained with reference to FIGS. 1 to 4. The vacuum envelope 1 consists of an Al input window 1a, a stainless steel body 1b, a glass body 1c, and a glass output window 1d. Input window 1 of envelope 1
An input surface 2 is arranged inside a. This input surface 2 has an input fluorescent surface 4 formed on a spherical aluminum substrate 3, and a photocathode 5 formed on this input fluorescent surface 4. On the other hand, inside the output side of the envelope 1 , an anode body 6 is disposed, and an output fluorescent surface 8 is formed on an output substrate 7. Furthermore, a first focusing electrode 9 is disposed along the side wall inside the envelope 1 and is electrically connected to the metal body 1b.
A second focusing electrode 10a and a third focusing electrode 10b are sequentially arranged on the anode body 6 side of the first focusing electrode 9. Inside the recess 11 of the second focusing electrode 10a, antimony nonmetal evaporation sources 11a are arranged at three locations. In addition, the glass body 1c of the envelope 1
A protrusion 21 is provided at the rear of the protrusion 2.
An evaporation source 12 that generates alkali metals such as cesium (Cs) and potassium (K) is provided within the evaporation source 1 .

Therefore, in the present invention, a semiconductor optical sensor 13, for example, a photodiode made of a silicon PN junction, is installed at one location of an internal electrode, for example, the first focusing electrode 9, near the periphery of the photocathode 5, and an antimony evaporation source 11a.
The terminal 14 necessary for this operation is taken out from the semiconductor optical sensor 13 to the outside of the envelope 1 . That is, a through hole 23 having a size approximately corresponding to the size of the light input window of the optical sensor 13 is formed in a part of the cylindrical wall 22 of the first focusing electrode 9, and the sensor 13 is inserted from the outside of the through hole 23. The inner surface 22a and the outer surface 33a of the light input window are aligned and fixed. This optical sensor 13 is placed at a position where the difference in distance from each nonmetal evaporation source 11a is as small as possible. One of the pair of signal output terminals 24a of the optical sensor 13 is connected via a lead wire to the terminal 14 that hermetically penetrates the metal body 1b of the envelope via an insulator 25, and the other 24b Torso 1b
The first focusing electrode 9 is electrically short-circuited with the first focusing electrode 9 . During operation, this first focusing electrode 9
A conductor spring 9a is attached to the metal body 1b of the envelope.
The two terminals are short-circuited to ground potential, which is used as one terminal of the optical sensor 13. In addition, the reference numeral 26 in the figure is a spacer that electrically separates the input surface 2 and the first focusing electrode 9 and mechanically supports this electrode 9, 27 is a lead terminal of the input surface 2 , and 46 and 47 are fixing metal fittings. Generally speaking, semiconductor optical sensors include photodiodes, solar cells,
A phototransistor, a CdS light sensor, etc. can be used. Taking the example of a photodiode,
As shown in FIG. 3, a silicon element 31 having a PN junction is disposed inside an airtight container 32, and electrodes are led out to the outside by a pair of lead wires 24a and 24b. In the airtight container 32, a glass light input window 33 and a metal case 34 are hermetically joined by a fritted glass seal portion 35, and further, the lead wires 24a, 24b and the metal case 34 are hermetically sealed by a fritted glass stem 36. .
The airtight container 32 is filled with an antioxidant gas such as N ₂ . Such hermetically sealed photodiodes are widely available on the market today, but of course an airtight container is selected that does not lose its airtightness even at temperatures of about 300°C. However, the upper limit of the operating temperature is generally about 125°C from the viewpoint of electrical characteristics. Therefore, the present inventors compared the electrical characteristics of such a photodiode before and after holding it at a high temperature of 250°C for 7 hours in a vacuum, which is one guideline for the exhaust conditions of an X-ray fluorescence multiplier tube. When measured, the results shown in FIG. 5 were obtained. That is, in FIG. 5, the horizontal axis shows the amount of incident light (relative value), and the vertical axis shows the relative output current of the photodiode. The dotted curve A is the initial characteristic, and the solid curve B is the characteristic after going through the steps under the above exhaust conditions. From this result, the output characteristics, or sensitivity characteristics, of the photodiode are approximately 30
%, but the relative dependence of the output current on the amount of incident light remains almost unchanged. It has been found that the relative sensitivity characteristics of this photodiode are sufficient for practical use in quantitatively controlling the amount of non-metal such as antimony deposited in the process of forming a photocathode.

In order to obtain the optimum photoelectric conversion sensitivity of the photocathode, the output current of the optical sensor 13 is detected by connecting an ammeter 41 between its external terminal 14 and the grounded first focusing electrode 9 and body 1b. . Further, the photoelectron flow from the photocathode is detected by an ammeter 42 and a DC power source 43 connected between the terminal 27 and the first focusing electrode 9. Further, light sources 20, 20 are arranged outside the glass body 1c of the envelope 1 , and light is introduced into the tube from this glass body. Note that in the case of a structure in which there is no light incident from outside the envelope or the incidence of light is extremely weak, a small light source may be provided at an appropriate position within the envelope. Furthermore, a plurality of optical sensors may be arranged near the outer periphery of the photocathode, and in this way, the uniformity of the metalloid deposition over the entire area of the photocathode can be detected. Furthermore, the optical sensor may be attached to the inner surface of the metal envelope.

Next, an embodiment of the method for forming a photocathode according to the present invention will be described. X-ray fluorescence multiplier tube is approximately 250℃7
After evacuation for an hour, the temperature is maintained within the range of 80-120°C. As shown in FIG. 6, in the first step, an alkali metal such as cesium is evaporated until the photoelectric current generated from the photocathode reaches its maximum value as indicated by the dotted curve C. Next, in a second step, a non-metal such as antimony is evaporated, and the output current of the semiconductor optical sensor including the evaporated film of the alkali metal is raised to a predetermined level d ₂ with respect to the initial value d ₁ as shown by the solid curve D. Evaporate it so that This level d ₂ is approximately 70 to 95% of the initial value d ₁ in the case of an X-ray fluoromultiplier tube using a CsI phosphor, preferably 75 to 90%.
Control to within % range. Next, as a third step, alkali metal vapor is introduced again. At this stage, a compound of the previously deposited antimony and alkali metal is formed, so the photoelectron flow from the photocathode rapidly increases. When this current value reaches its maximum value again, the alkali introduction is stopped. At this time, the output current of the semiconductor optical sensor further decreases to level _d3 due to compound formation. The ambient temperature of the X-ray fluorescence multiplier tube from the first step to the third step is maintained within the range of 80 to 120 DEG C., as described above.
Thereafter, as a fourth step, slow cooling is performed to room temperature, and at this stage the photoelectric current c from the photocathode further increases and finally reaches a stable value. Here, the ratio of the output current in the second step to the initial value of 100% of the output current is applicable to general electron tubes having a photocathode. In other words, the change in output current is mainly determined by the Sb film thickness, and the most sensitive practical value is 100~
The optical sensor output current value is 70~ corresponding to about 500Å
This falls within the 95% range. When forming a photocathode in which a compound is created by alternately evaporating alkali metals and antimony, quantitative control of the amount of antimony deposited is particularly important in order to provide good and stable photoelectric conversion sensitivity. Therefore, in the present invention, the relative output current of the semiconductor optical sensor is detected and controlled to be 70 to 95% of the initial value, thereby increasing the optimum and stable reproducibility of the sensitivity. In particular, when the vacuum envelope surrounding the photocathode, that is, the X-ray entrance window and the envelope wall around the photocathode are made of metal, a semiconductor optical sensor is disposed inside the envelope. Since this method first evaporates an alkali metal and then evaporates a predetermined amount of a nonmetal, it is possible to reliably and easily form a photocathode with optimal photoelectric sensitivity.

If the amount of antimony evaporated is controlled to be less than or too much than the above relative value of the optical sensor output, the final sensitivity of the fourth step will remain at a low level, so the above value should be appropriate. It was confirmed.

In this example, since the outer surface of the optical input window of the semiconductor optical sensor is installed so as to be substantially flush with the inner surface of the cylindrical wall of the internal electrode to which it is attached, there is a risk of disturbing the focused electric field by the internal electrode. There is no. This is because the optical input window of an optical sensor is made of an insulating material such as glass, but when antimony is deposited on its outer surface, it becomes conductive. It is electrically connected to the internal electrode through. For this reason, the inner surface of the electrode is electrically equivalent to a virtually closed hole, and the focused electric field in this vicinity is hardly disturbed. This prevents deterioration of image quality.

(6) Modified Example of Implementation of the Invention The embodiment shown in FIG. 7 includes a photocathode 5, a fluorescent surface 4,
A through hole 23 is formed in a part of the tapered portion of a photocathode support 37 having a dogleg-shaped half cross section that supports the substrate 3, and a semiconductor optical sensor 13 is fixed to the back side of the hole 23. The optical sensor 13 connects its light input window to the through hole 2.
3, and one of the pair of lead wires 24a is electrically connected to a support 37 which also serves as an electrode of the photocathode, and its external lead terminal 27 for common use. . The other terminal 24b of the optical sensor 13 is electrically connected to a first focusing electrode 39 integrally connected to the metal body 1b via a fuse 38 that is easily blown. Further below, a second focusing electrode 40 is provided. In this way, unlike the previous embodiment, in this embodiment, the unique output terminal of the optical sensor (reference numeral 14 in the previous figure) is omitted and is connected to the photocathode support and internal electrode on which the optical sensor is mounted. and use their external terminals in common. This eliminates the need for an extra airtight electrode terminal provided on the metal body 1b, which is convenient.

In manufacturing an electron tube having this structure, the output current of the optical sensor 13 is detected by connecting an ammeter 41 between the external terminal 27 of the photocathode, the grounded first focusing electrode 39, and the body 1b. Further, the photoelectric current from the photocathode is detected by an ammeter 42 and a DC power source 43 connected between the terminal 27 and the second focusing electrode 40. When the optical sensor 13 no longer needs to be used, the fuse 38 is blown by passing a large current through it. The power supply 44 and switch 45 are for this purpose. This ultimately makes it possible to apply a predetermined operating potential to each electrode within the tube. In this embodiment, since the light input window of the optical sensor and the photocathode face the antimony evaporation source, there is an advantage that the amount of evaporation can be controlled with higher precision.

Note that without using the fuse 38, a large current may be applied directly to the semiconductor optical sensor to cut the bonding wire within the optical sensor.

(7) Effects of the Invention The present invention provides an electron tube in which the vacuum envelope near the photocathode is formed of a non-light-transmitting metal, and a semiconductor optical sensor is provided in the tube near the photocathode in order to form the photocathode. , the alkali metal is evaporated until the photoelectron flow reaches its maximum value, and then the nonmetallic photocathode material is heated until the detection value of the optical sensor is 70 to 95% of the initial value.
The photocathode forming method is characterized by evaporating the alkali metal in an amount that preferably ranges from 75 to 90%, and then evaporating the alkali metal. Thereby, a highly sensitive photocathode can be stably obtained with good reproducibility. This is an extremely effective method for mass production.

In this respect, it would have been completely unthinkable to incorporate a semiconductor optical sensor into an electron tube manufactured through such a high-temperature treatment process in view of the electrical characteristics of the semiconductor optical sensor. We discovered that it is possible to skillfully utilize this to control the amount of semimetal evaporated with high precision, and applied it to X-ray image intensifier tubes that have multiple poles and have a metal container for higher performance. There are many advantages that can be achieved.

In the above embodiments, an X-ray fluorescence multiplier tube was used as an example of the electron tube, but it should be noted that the present invention can be widely applied to the manufacture of image intensifier tubes for gamma ray detection and other electron tubes having photocathode surfaces. Not until now.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional view showing an electron tube (X-ray fluorescence multiplier tube) having a photocathode according to an embodiment of the present invention, and FIG. 2 is a cross-sectional view at - in FIG.
Fig. 3 is a sectional view showing an example of an optical sensor, Fig. 4 is a longitudinal sectional view at - in Fig. 2, Fig. 5 is its characteristic diagram, Fig. 6 is a characteristic diagram of the photocathode forming process, and Fig. 7. FIG. 3 is a vertical cross-sectional view of main parts showing another embodiment of the present invention. 1 ... Envelope, 1a... Entrance window, 1b... Body, 5... Photocathode, 9, 39... Internal electrode, 13
... Semiconductor optical sensor, 23 ... Through hole, 33 ... Light input window.

Claims (1)

[Claims] 1. Forming the photocathode by vapor deposition of an alkali metal and a metalloid of an electron tube in which a high-energy ray entrance window in which a photocathode is disposed and a vacuum envelope portion around the photocathode are made of metal. In a method for manufacturing an electron tube having a photocathode, a semiconductor optical sensor is placed in advance near the photocathode, and when forming the photocathode, an alkali metal as a material for the photocathode is first heated until the photoelectron current reaches its maximum value. Then, the non-metal of the photocathode material is deposited until the output current of the semiconductor optical sensor is 70 to 70% of the initial value.
The above manufacturing method is characterized in that vapor deposition is performed until a value within the range of 95% is exhibited, and then an alkali metal is further vapor deposited.