JPH07118285B2 - Method of manufacturing X-ray image intensifier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application] The present invention relates to a method for manufacturing an X-ray image intensifier, particularly an input phosphor screen.

(Prior Art) The input phosphor screen of the X-ray image intensifier is, as shown in FIG. 7, formed on a substrate (1) made of aluminum, a vapor-deposited phosphor film (2) containing cesium iodide (2) and ( 3) has been deposited. As a vapor deposition source for the vapor deposited fluorescent film, for example, an activated granular phosphor containing cesium iodide and sodium iodide in a ratio of about 10 ^-1 mol% with respect to cesium iodide is used. The vapor-deposited film (2) has a film thickness of 20 by performing vapor deposition in an argon gas atmosphere of 3.0 × 10 ^-1 Pa.
Only 0 μm is formed. The vapor-deposited film (3) is formed on the vapor-deposited film (2) to a thickness of 10 μm by performing vapor deposition under a high vacuum of 1 × 10 ⁻³ Pa or less. Further, an insulating protective film (4) having a film thickness of 1000 Å is formed on the surface of the vapor deposition film (3) by vapor deposition of aluminum in an oxygen atmosphere. Such an input phosphor screen is incorporated into an image intensifier, and a multi-alkali photocathode is formed through a vacuum baking process at 250 ° C. for 10 hours. The formation of the photocathode is started at a temperature of 160 ° C., for example.
Then, first, Sb is vapor-deposited, and the vapor deposition is stopped when the predetermined transmissivity is reached while monitoring the light transmissivity of the film attached to the glass tube wall. Next, potassium is introduced while observing the photocurrent, and the introduction is stopped when the photocurrent reaches a peak. Next, Sb is vapor-deposited again, and the vapor deposition is stopped when the above-mentioned transmittance reaches a predetermined value. Next, sodium is introduced while monitoring the photocurrent, and the introduction is stopped when the photocurrent reaches a peak. Next, Sb was vapor-deposited while observing the photocurrent, the vapor deposition was stopped when the photocurrent became about half of the value immediately before the start of Sb vapor deposition, and then cesium was introduced while observing the photocurrent. , The introduction is stopped when the photocurrent shows a peak value. This last step of evaporating Sb cesium is repeated until the peak value of the photocurrent reached by the introduction of cesium becomes high, and a photocathode is obtained. The X-ray image intensifier manufactured in this way is irradiated with X-rays with a quality of Al half-value layer of 7 mm, and the average photocurrent per unit area at the center of the input surface (hereinafter referred to as input sensitivity) is measured. I did, 1
A value of 0.5 × 10 ⁻¹² A / mR · min ⁻¹ was obtained.

However, although a sufficiently high input sensitivity can be obtained by using the multi-alkali photocathode as described above, there is a drawback that the phosphor screen forming process requires a very long time. In the above example, it took about 8 hours.

In order to solve the above problems, a method of forming a bialkali photocathode instead of the multialkali photocathode is known. The manufacturing method will be described below.

Bialkali photocathode is 3 compared to multialkali photocathode
Since the surface resistance value is close to the order of magnitude, it is necessary to use the input phosphor screen shown in FIG. 8 instead of the input phosphor screen shown in FIG. (1) to (3) shown in FIG. 8 are the same as those shown in FIG. Reference numeral (5) is a transparent conductive film, which is made of an indium oxide film having a thickness of 2000 liters formed by depositing indium in an oxygen atmosphere on the surface of the deposited film (3) kept at 300 ° C. The input phosphor screen shown in FIG. 8 was incorporated into an image intensifier, which was subjected to a vacuum baking treatment at 250 ° C. for 10 hours to form a bialkali photocathode. The photocathode was formed at a temperature of 110 ° C. First, Sb was evaporated, and vapor deposition was stopped when the predetermined transmittance was reached while monitoring the light transmittance of the film attached to the glass tube wall. Next, potassium was introduced while observing the photocurrent, and the introduction was stopped when the photocurrent reached a peak. Then again Sb
Was evaporated, and the evaporation was stopped when the above-mentioned transmittance reached a predetermined value. Next, cesium was introduced while monitoring the photocurrent, and the introduction was stopped when the photocurrent reached a peak.

The time required for photocathode vapor deposition was only 3 hours, which was significantly shorter than the formation time (8 hours) of the multi-alkali photocathode. When the input sensitivity of the X-ray image intensifier manufactured in this way was measured, it was 8.5 × 10 ^-12 A / mR.
・ The value of min ^-1 was obtained. However, this value is 20% lower than the input sensitivity of the X-ray image intensifier using the multi-alkali photocathode described above.

(Problems to be Solved by the Invention) As described above, by using a bialkali photocathode, the photocathode formation process is significantly shortened, but the input sensitivity is
There is a problem that it will be about 20% lower. One of the reasons for the low sensitivity is that the transmittance of the conductive film for the fluorescence of Na-activated CsI is about 90%, and the sensitivity of the bialkali photocathode to the fluorescence of CsI activated by Na is This is because it is about 90% of that of the multi-alkali photocathode.

It is an object of the present invention to provide a method for producing an X-ray image intensifier capable of suppressing a decrease in sensitivity of a photocathode caused by deposition of an active substance contained in a fluorescent layer on the surface.

[Structure of the Invention] (Means for Solving Problems) In the present invention, a fluorescent layer of cesium iodide containing a trace amount of an active substance is attached onto a substrate, and a photocathode is laminated on the fluorescent layer for input. In a method of manufacturing an X-ray image intensifier for forming a phosphor screen, a sputtering process is performed on the input phosphor screen before forming a photocathode.

(Operation) With respect to the input phosphor screen shown in FIGS. 7 and 8, the concentration distribution of various elements existing on the surface in the depth direction was measured by an ion microanalyzer (hereinafter referred to as IMA).
FIG. 9 shows the result of IMA analysis on the input phosphor screen of FIG. FIG. 10 shows the result of IMA analysis on the input phosphor screen of FIG. 9 and 10, the vertical axis represents the amount proportional to the number of moles of the element, and the horizontal axis represents the depth estimated from the sputtering time assuming that the sputter rate of each layer is the same as that of indium oxide. . As can be seen from FIG. 9 and FIG. 10, in both cases, a large amount of sodium element is deposited in the region within 200 Å from the surface. Also,
CsI directly under the indium oxide or aluminum oxide layer
It was also confirmed that Na contained less than 10 ^-2 mol% in the region of a depth of about 5 μm. In the CsI layer deeper than 5 μm, the Na content gradually increased, and when it became deeper than about 10 μm, the addition amount reached 10 ⁻¹ mol%. From the above results, it was confirmed that the Na element contained in the approximately 10 μm layer in the CsI vapor-deposited film on the surface was deposited on the surface. On the other hand, the chemical analysis revealed that the Na concentration in the CsI deposited film was 10 ^-1 mol%. Therefore, the amount of Na deposited on the surface is estimated to be 0.4 μg / cm ² . The amount of Na deposited is almost equal to the amount of alkali contained in the photocathode. Before being incorporated into the image intensifier,
Considering that the input phosphor screen is exposed to the atmosphere, the precipitated Na element forms a hydroxide, not as a simple metal,
It is considered that the presence of this hydroxide reduces the sensitivity of the photocathode. In view of the above point of view, the present invention aims to improve the sensitivity of the photocathode formed on the input phosphor screen by providing an input phosphor screen having no Na deposition amount on the surface.

(Example) The surface of the input phosphor screen shown in FIG. 8 was subjected to sputtering treatment using the vacuum apparatus shown in FIG. The input phosphor screen (6) and the counter electrode (7) have almost the same shape,
They are arranged facing each other with a gap of 20 mm. Since the surface of the input phosphor screen (6) is covered with the transparent conductive film (5),
By applying a DC voltage of 1 kv between the counter electrode (7) and the cathode side of the input phosphor screen, and introducing argon gas into the vacuum chamber (8) at a pressure of 0.1 Torr,
The surface of the input fluorescent screen is sputtered by the generated argon ions which start the discharge between the input fluorescent screen (6) and the counter electrode (7). After sputtering for about 3 minutes, the input phosphor screen was taken out and IMA analysis was performed. The result is shown in FIG. 2, which differs from the result of IMA analysis before performing sputter etching (FIG. 10) in that there is no Na precipitation layer on the surface. The input phosphor screen shown in FIG. 8 was subjected to the above-described sputter etching treatment and then incorporated into an image intensifier to form a bialkali photocathode by a method according to the conventional technique. When the input sensitivity of this image intensifier was measured, a value of 10.6 × 10 ^-12 A / mR · min ^-1 was obtained. A sensitivity improvement of + 25% was obtained compared to the conventional one. This input sensitivity is comparable to the input sensitivity of a conventional image intensifier using a multi-alkali photocathode.

The surface of the input phosphor screen shown in FIG. 7 was sputtered by using the same device as the vacuum device shown in FIG. Since the surface of the input phosphor screen shown in FIG. 7 is covered with a high-resistance aluminum oxide layer (4), an AC voltage of 1 kv was applied between it and the counter electrode (7). After sputtering for about 6 minutes, the input fluorescent screen was taken out and IMA analysis was performed.

As a result, the Na analysis layer on the surface was lost, similar to the result of the above-mentioned example.

The input phosphor screen shown in FIG. 7 was subjected to the above-described sputter etching treatment, and then incorporated into an image intensifier to form a multi-alkali photocathode by a conventional method. When the input sensitivity of this image intensifier was measured, a value of 11.6 × 10 ^-12 A / mR · min ^-1 was obtained, and a sensitivity improvement of + 10% was obtained compared to the conventional one.

FIG. 3 shows another embodiment. (1) and (2) are the 7th
(3a) is the same as shown in the figure, sodium iodide
Cesium iodide containing only 10 ⁻³ mol% was vapor-deposited under a high vacuum of 1 × 10 ⁻³ Pa or less to form a film having a thickness of 10 μm. The relationship between the amount of sodium iodide added to cesium iodide and the fluorescence intensity of the deposited film is shown in FIG. As can be seen from FIG. 4, the fluorescent layer (3a) containing sodium iodide only at 10 ⁻³ mol% hardly emits fluorescence due to X-ray absorption. However, its film thickness is about the same as that of the fluorescent layer (3).
Since it is as thin as 1/10, the fluorescence intensity of the input phosphor screen is about 3%
There was only a slight decrease. (5) is a transparent conductive film of indium oxide.

The result of IMA analysis of the input phosphor screen shown in FIG. 3 is shown in FIG. There is no Na precipitate layer on the surface. In addition, CsI under the indium oxide layer was also analyzed up to a depth of 5 μm, and Na was contained in an amount of 10 ⁻² mol% or less with respect to CsI.

The input phosphor screen shown in FIG. 3 was incorporated into an image intensifier and a bialkali photocathode was formed by a conventional method. When the input sensitivity of this image intensifier was measured, a value of 10.4 × 10 ⁻¹² A / mR · min ⁻¹ was obtained.
The sensitivity has increased by + 22% compared to the conventional one, which is comparable to the input sensitivity of the conventional image intensifier that uses a multi-alkali photocathode.

FIG. 6 shows another embodiment. (1) to (3a) are the same as those shown in FIG. (4) is an insulating protective film made of aluminum oxide.

As a result of IMA analysis of the input phosphor screen shown in Fig. 6, as in Fig. 5, the Na precipitate layer on the surface disappeared, and at the same time, CsI under the aluminum oxide layer was analyzed to a depth of 5 µm. Na contained less than 10 ^-2 mol% relative to CsI.

The input phosphor screen shown in FIG. 6 was incorporated into an image intensifier to form a multi-alkali by a conventional method. When the input sensitivity of this image intensifier was measured, a value of 11.0 × 10 ^-2 A / mR · min ^-1 was obtained, and a sensitivity improvement of + 5% was obtained compared with the conventional one.

In the input phosphor screen shown in FIGS. 3 and 6, (3a) is Na
Although a CsI vapor deposition film containing no I was used, similar effects can be obtained by using a vapor deposition substance having a relatively columnar structure other than CsI as (3a). Alkali halides such as cesium chloride are preferred. The film thickness of the surface layer that does not contain Na is for cesium iodide.
A sufficient effect was obtained at 10 μm. This film thickness value is a value necessary for normal medical diagnosis, and is the film thickness value of the cesium iodide fluorescent film.
It corresponds to a film thickness of 1/20 or less as compared with 200 μm to 500 μm, and has little influence on the resolution property of the fluorescent film.

Instead of the indium oxide conductive film (5) used for the conventional input phosphor screen shown in FIG. 8, indium tin oxide containing 16 mol% of tin was used as a target, and indium oxide was deposited by electron beam evaporation in an oxygen atmosphere. A conductive film of tin was formed. The temperature of the deposited cesium iodide film during formation is 300 ° C.
Kept at.

As a result of IMA analysis of this input phosphor screen, it was confirmed that the concentration of Na precipitate layer on the surface was reduced to about 1/5 of the conventional concentration. In addition, the conductive film of indium tin oxide peeled off by immersing this input fluorescent screen in water was examined by TEM using a transmission electron microscope.
As a result of observing the image, it was confirmed that the average crystallite size was about 500Å, which was larger than the average crystallite size of about 380Å observed by the same method for the conventional indium oxide film.

This input phosphor film was incorporated into an X-ray image intensifier to form a bialkali photocathode as in the conventional case. When the input sensitivity of this X-ray image intensifier was measured, the value was 10.2 × 10 ^-2 A / mR · min, which was a + 20% increase in sensitivity compared with the conventional one.

It is considered that the reason for the decrease in the Na precipitation concentration is that the diffusion coefficient of Na in the conductive film decreases. By making the conductive film of indium tin oxide to be 500 Å or more, a sensitivity improvement of + 20% or more can be obtained compared to the conventional one, but part of the reason for this sensitivity improvement is that it reduces the concentration of the Na precipitation layer. it is conceivable that.

Further, from the above results, the sensitivity of the conventional input phosphor screen shown in FIG. 7 is also improved by using a crystal film having a smaller diffusion coefficient for Na than the conventional insulating protective film, in other words, having a larger crystal size. Is obtained.

EFFECTS OF THE INVENTION The present invention can improve the sensitivity of the photocathode as a result of substantially eliminating the Na element deposition layer on the surface on which the photocathode of the input phosphor screen is formed. The effect is great.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of a method for manufacturing an input phosphor screen of the present invention, and FIG. 2 is a diagram showing a surface analysis result by an ion microanalyzer of an embodiment of the input phosphor screen of the present invention.
FIG. 4 is a sectional view showing an embodiment of an input phosphor screen of the present invention.
The figure shows the relationship between the brightness of the cesium iodide phosphor and the content of the added sodium iodide. FIG. 5 shows the analysis of the input phosphor screen of the present invention shown in FIG. 3 by the ion microanalyzer. FIG. 6 shows a result, FIG. 6 is a sectional view showing another embodiment of the input phosphor screen of the present invention, FIG. 7 is a sectional view showing an embodiment of a conventional input phosphor screen, and FIG. 8 is a conventional input phosphor screen. FIG. 9 is a sectional view showing another embodiment of the phosphor screen, FIG. 9 is a diagram showing the result of surface analysis of the conventional input phosphor screen shown in FIG. 7 by an ion microanalyzer, and FIG. 10 is shown in FIG. It is a figure which shows the surface analysis result by the ion microanalyzer of the conventional input fluorescent surface. (1) …… Aluminum substrate, (2) …… Vapor-deposited fluorescent film,
(3) ... Vapor-deposited fluorescent film, (3a) ... Cesium iodide vapor-deposited film (intermediate film), (4) ... Aluminum oxide film, (5) ...
… Indium oxide film, (6) …… Input phosphor screen, (7)…
... Counter electrode, (8) ... vacuum chamber.

Claims (1)

[Claims] 1. An X-ray image in which a phosphor layer of cesium iodide containing a trace amount of sodium as an active substance is adhered on a substrate and a photocathode is laminated on the phosphor layer to form an input phosphor screen. A method of manufacturing an X-ray image intensifier, which comprises subjecting the input phosphor screen to a sputter etching treatment before forming the photocathode.