JPH0143810Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to an improvement in an electron tube having a photocathode, such as an X-ray image intensifier.

Generally, electron tubes with a photocathode, such as X-ray image intensifiers and gamma-ray image intensifiers, are used to detect high-energy rays, but the entrance window material of these image intensifiers has traditionally been glass. has been used. but,
Recently, due to improvements in various technologies and the rising cost of raw materials for glass, thin metal is being used for the entrance window of these image intensifier tubes, as metal is actually cheaper when you consider the process until completion. It has become like that.
However, the use of a metal entrance window led to the use of metal in other parts as well.

On the other hand, the photocathode is generally made of Sb, Bi, or Te.
It is composed of a reaction product between a metalloid such as a metalloid and an alkali metal. 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.

However, 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, in the case of an X-ray image intensifier tube, which conventionally consisted of three electrodes, it has been changed to four poles to vary the field of view, or to five poles to improve the distribution of resolution. It is headed in the direction of multipolarization.

In addition, in order to reduce scattered radiation and deal with increased material and processing costs, X-ray image intensifier tubes are being made of metal, starting from the entrance window material and extending to other parts of the vacuum envelope. I'm getting old. As X-ray image intensifier tubes have become more complex in structure and metalized vacuum envelopes, it has become difficult to provide an optical path for measurement near the photocathode, making it difficult to quantitatively measure the amount of metalloid deposited. The restrictions have become difficult.

This invention was made in view of the above circumstances, and the purpose is to provide an electron tube having a photocathode with a convenient structure for forming a photocathode having a desired photocathode sensitivity and stable photocathode sensitivity. .

Hereinafter, embodiments of this invention will be described in detail with reference to the drawings. Taking an X-ray fluorescence multiplier tube, which is a type of X-ray image intensifier tube, as an example of an electron tube, the X-ray fluorescence multiplier tube according to this invention is constructed as shown in Figures 1 and 2. There is. That is, the vacuum envelope 1 is made of Al
It consists of an entrance window 1a, a stainless steel body 1b, a glass body 1c, and a glass output window 1d. An input surface 2 is arranged inside the entrance window 1a of the envelope 1 . The input surface 2 has an input fluorescent surface 4 formed on a spherical aluminum substrate 3, and a photocathode 5 formed on the 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. Second focusing electrode 1
An alkali metal evaporation source is arranged inside the recess 11 of Oa. In addition, the glass body 1 of the envelope 1
A protrusion 21 is provided at the rear part of c, and a boat 12 for generating a metalloid such as antimony is provided within this protrusion 21.

Conventionally, one method for forming a photocathode in such an X-ray fluorescence multiplier tube is to alternately use cesium, for example, from one alkali metal evaporation source and antimony, for example, from the other boat 12, in order to obtain the highest sensitivity. This is a way to continue until. However, as already mentioned, this method has the disadvantage that it is difficult to obtain a highly sensitive product. Another method is to place a phototube outside a position where antimony can fly onto the inner surface of the envelope 1 , for example in a portion of the input surface 2 between the substrate 3 and the focusing electrode 9. A light source is placed on the opposite side of the phototube and the X-ray fluorescence multiplier tube, and the light from this light source enters the phototube and the photocurrent that flows through the tube is quantified by reading the value that changes due to antimony deposition. After that, cesium is evaporated from an alkali metal evaporation source to form a photocathode.

In the latter method, if the quantification of antimony is performed with good reproducibility, the photocathode sensitivity is high and stable. However, as described above, this method cannot be used when the number of substrates 3, anode bodies 6, focusing electrodes 10, etc. is large, or when the envelope 1 is made of a metal container.

Therefore, in this invention, near the periphery of the photocathode 5,
In other words, a semiconductor optical sensor 13, for example, a photodiode made of a silicon PN junction, is placed at one location on the internal electrode, for example, the first focusing electrode 9, near the part where the photocathode 5 is functionally required, in such a way as not to greatly disturb the electric field of the electrode. The terminal 14 necessary for this operation is connected from the semiconductor optical sensor 13 to the envelope 1.
I'm taking it outside. That is, a through hole 23 having a size approximately corresponding to the size of the input window of the optical sensor 13 is formed in a part of the cylindrical wall 22 of the first focusing electrode 9,
An optical sensor 13 is fixed on the outside thereof. One of the pair of signal output terminals 24a of the optical sensor 13
is connected via a lead wire to the terminal 14 which hermetically passes through the metal body 1b of the envelope through an insulator 25, and the other 24b is electrically short-circuited with the metal body 1b. It is connected to electrode 9.
In operation, the first focusing electrode 9 is short-circuited to the metal body 1b of the envelope via the conductor spring 9a, and both are at ground potential, which connects the optical sensor 13.
It is used as one terminal of Note that the reference numeral 26 in the figure represents a spacer that electrically separates the input surface 2 and the first focusing electrode 9 and also mechanically supports this electrode 9, and 27 represents a lead terminal of the input surface 2.

In the electron tube of this invention, when forming the photocathode 5, a semi-metal such as antimony is applied to the outer surface of the light input window of the optical sensor 13 by using a small amount of light that enters the interior from the glass portion 1c of the envelope. The amount of deposition can be controlled while detecting the signal from the optical sensor 13, which changes depending on the amount. Since the optical sensor 13 can be installed near the photocathode 5, it is possible to more accurately detect the amount of coating. Note that in the case of a structure in which there is no light incident from outside the envelope 1 or the light is extremely weak, a small light source may be provided at an appropriate position within the envelope 1 . Furthermore, the optical sensor 13
A plurality of them may be arranged near the outer periphery of the photocathode, and in this way, the uniformity of the metalloid deposition over the entire photocathode can be detected. Furthermore, the optical sensor 13 may be attached to the inner surface of the metal envelope.

Note that the silicon photodiode mentioned above is originally a semiconductor, and repeated use of the same photodiode is not suitable for the heat applied to exhaust a vacuum tube such as an X-ray fluorescence multiplier tube and to form a photocathode. However, this idea only requires attaching one photodiode to the X-ray fluorescence multiplier tube, and this photocathode part is removed during tube manufacturing.
Because it does not reach extremely high temperatures of 300℃ or higher,
It is fully practical and presents no problems for this purpose. In fact, by arranging photodiodes, the reproducibility of the amount of antimony deposited is much better than with conventional methods, and even in the case of a metal envelope where it is almost impossible for external light to enter, this structure can be used. Now it can be applied. Considering the X-ray fluorescence multiplier tube as in the above example, the exhaust temperature of this bulb is 250℃.
Conducted in a furnace for 7 ^Hrs . Therefore, a photodiode must be able to withstand this temperature, but in recent years, a type of silicone encased in a metal case and sealed with a glass solvent has been developed, and such a photodiode meets the purpose of this invention. .

FIG. 3 shows an example of a photodiode that can be used for this purpose. The semiconductor optical sensor 13 consisting of this photodiode is a PN junction silicon element 3.
1 is disposed inside an airtight container 32, and electrodes are drawn out to the outside by a pair of lead wires 24a and 24b. The airtight container 32 has a glass light input window 33 and a metal case 34 hermetically joined by a fritted glass seal 35, and further has a lead wire 2.
4a, 24b, and a metal case 34 are hermetically sealed by a fritted glass system 36. The inside of this airtight container 32 is sealed with anti-oxidation glass such as N ₂ .

Such hermetically sealed photodiodes are widely commercially available today, but of course the hermetically sealed container 32 is
Select one that does not lose its airtightness even at temperatures as low as ℃. However, the upper temperature limit for its use is generally
Based on electrical characteristics, it is estimated to be approximately 125°C. Therefore, the present inventors investigated the electrical characteristics of such a photodiode before and after holding it in a vacuum at 250°C for 7 hours, which is one guideline for the evacuation conditions of an X-ray fluorescence multiplier tube. When I compared and measured the 4th
The results shown in the figure were obtained. That is, in FIG. 4, the horizontal axis shows the amount of incident light (relative value), and the vertical axis shows the output current of the photodiode. The dotted curve A is the initial characteristic, and the solid curve B is the characteristic after passing through the steps under the above exhaust conditions. From this result, the output characteristics, or sensitivity characteristics, of the photodiode are 250°C.
It was found that the relative dependence of the output current on the amount of incident light remained almost unchanged, although it decreased by about 30% after a 7-hour electron tube evacuation process. It was 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 the photocathode.

Next, an example of forming a photocathode after the above-described evacuation step will be described with reference to FIG. That is,
First, 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 the second step, a non-metal such as antimony is evaporated so that the output current of the semiconductor optical sensor becomes a certain predetermined level _d2 with respect to the initial value _d1 , as shown by the solid curve D. . this level d ₂
In the case of an X-ray fluorescence multiplier tube using a CsI phosphor, the optimum value is within a range of approximately 70 to 90% of the initial value _d1 .

Next, as a third step, alkali is introduced again. At this stage, a compound of the previously deposited antimony and alkali is formed, so that the photoelectric current c 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.

Note that 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°C. Thereafter, as a fourth step, slow cooling to room temperature is performed, and at this stage the photoelectric current c from the photocathode further increases and finally reaches a stable value.

When forming a photocathode in which a compound is created by alternately evaporating alkali metals and antimony, it is especially important to quantitatively control the amount of antimony deposited in order to have good and stable photoelectric conversion sensitivity. Therefore, in this invention, reproducibility is improved by detecting the relative output current by a semiconductor optical sensor. 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. This control is reliable and easy.

The embodiment shown in FIG. 6 includes a photocathode 5, a fluorescent screen 4,
A through hole 23 is formed in a part of a photocathode support 37 having an L-shaped half 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 is installed with its optical input window 33 aligned with the through hole 23, and one of its pair of lead wires 2
4a is electrically connected to a support 37 which also serves as an electrode of the photocathode 5 and an 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. Furthermore, a second second focusing electrode 40 is provided below.

Thus, in this embodiment, unlike the embodiments shown in FIGS. 1 and 2, the optical sensor 13
The unique output terminal (numeral 14 in the previous figure) is omitted, and these external terminals are commonly used by connecting to the internal electrode to which the optical sensor 13 is attached. This eliminates the need for an extra airtight electrode terminal provided on the metal body 1b, which is convenient.

In the 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. Then, when the use of the optical sensor 13 is no longer necessary, 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.

Since the electron tube having the photocathode of this invention is constructed as described above and shown in the drawings, it has the following excellent effects. In other words, conventionally, it was unthinkable to incorporate a semiconductor optical sensor into an electron tube manufactured through such a high-temperature treatment process from the viewpoint of the electrical characteristics of the semiconductor optical sensor. It has become possible to control the amount with high precision, and the high-sensitivity photocathode formation method can be applied to X-ray image intensifier tubes that have multiple poles and have a metal container for higher performance.There are many advantages. be.

In the above embodiment, a photodiode is used as an example of the semiconductor optical sensor 13, but the present invention is not limited to this, and similar effects can be obtained even if a phototransistor or a CdS optical sensor is used as the semiconductor optical sensor.

Furthermore, in the above embodiment, an X-ray fluorescence multiplier tube was used as an example of the electron tube, but it goes without saying that this invention can be widely applied to image intensifier tubes for gamma ray detection and other electron tubes with photocathode. Nor.

As explained above, according to this invention, it is possible to provide an electron tube having a photocathode of great industrial value.

[Brief explanation of the drawing]

FIG. 1 is a schematic sectional view showing an electron tube (X-ray fluorescence multiplier tube) having a photocathode according to an embodiment of the invention, and FIG. 2 is a vertical sectional view showing an enlarged main part of FIG. 1. , FIG. 3 is a cross-sectional view showing an example of an optical sensor, FIG. 4 is a characteristic diagram thereof, FIG. 5 is a characteristic diagram of the photocathode forming process, and FIG. 6 is a longitudinal cross-section of main parts showing another embodiment of this invention. It is a front view. 1 ... Envelope, 1a... Entrance window, 1b... Body, 5... Photocathode, 13... Semiconductor optical sensor, 2
3...Through hole, 33...Light input window.

Claims (1)

[Claims for Utility Model Registration] (1) In 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, the photocathode A semiconductor optical sensor consisting of a semiconductor element housed in an airtight container with an optical input window is disposed in a vacuum envelope near the periphery of the photocathode and at a position where the constituent materials of the photocathode can fly. An electron tube having a photocathode characterized by: (2) A through hole is provided in a part of the tube electrode provided near the periphery of the photocathode, and the semiconductor optical sensor is fixed on the back side of the electrode by aligning the through hole with the light input window of the semiconductor optical sensor. An electron tube having a photocathode as claimed in claim 1 of the utility model registration. (3) An electron tube having a photocathode as set forth in claim 1, which is a utility model registration, wherein output lead wires of the semiconductor optical sensor are commonly electrically connected to external lead terminals of electrodes to which the semiconductor optical sensor is fixed.