JPH0754676B2 - X-ray image intensity - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Object of the Invention (Field of Industrial Application) The present invention relates to an X-ray image intensifier, and more particularly to the structure of an input fluorescent surface.

(Prior Art) An X-ray image intensifier is generally attached via a cylindrical glass envelope (1), a Kovar ring (2) and a stainless ring (3) as shown in FIG. An input window (4) made of Al, an output envelope (5) made of glass processed into a cylindrical output part with a bottom, and an input surface (6) positioned inside the envelope on the side of the Al input window. , The output surface (7) attached to the bottom of the bottomed tubular shape of the envelope (5)
And a focusing electrode (8) provided on the inner surface of the glass envelope and a funnel-shaped accelerating electrode (9) attached to the output part. By the way, as a conventional bialkali photocathode, an indium oxide film having a thickness of about 1000Å is known as an example of a transparent conductive film, but as shown in FIG. 6, a vapor deposition phosphor layer, for example, a CsI vapor deposition layer (72). It is composed of a polycrystalline transparent conductive film (74) formed of crystal grains (73) having an average grain size of 400 Å or less. This transparent conductive film (7
4) has innumerable grain boundaries (70).

On the other hand, the output surface (7) shown in FIG. 1 comprises a glass substrate, a fluorescent material layer formed on the glass substrate, and an aluminum back layer formed on the fluorescent material layer. This output surface (7)
In order to increase the contrast, usually, the transmittance T of the glass substrate for the fluorescence emitted from the output phosphor is set to about 80%. However, conventional X-ray image intensifiers that combine the input and output surfaces inherently have the following drawbacks.

(Problems to be Solved by the Invention) In the above X-ray image intensifier, the contrast characteristic is further improved by setting the transmittance T to a smaller value, but the luminance is decreased. There are conflicting problems. It is also known to provide a light absorbing layer on a glass substrate as a means for obtaining the above-mentioned transmittance T, but there are similar problems.

The present invention provides an X-ray image intensifier with improved contrast by compensating for the above-mentioned decrease in brightness by improving the input surface.

[Structure of the Invention] (Means for Solving the Problems) The present invention is a transparent conductive film having crystallinity in which a plane parallel to the film surface between the photocathode and the input fluorescent film has an average crystallite size of 500 Å or more. By providing the X-ray image intensifier, the X-ray image intensifier can obtain brightness equal to or higher than that of the conventional X-ray image intensifier and can significantly improve the contrast.

(Function) The present invention makes it difficult for the alkali metal forming the photocathode to diffuse into the transparent conductive film by reducing the crystal grain boundaries in the transparent conductive film, and improves the photocathode sensitivity as compared with the conventional case. Therefore, it is characterized in that the transmittance which should be originally reduced should be compensated by decreasing the transmittance of the glass substrate in order to improve the contrast of the output surface.

(Embodiment) An embodiment of the X-ray image intensifier of the present invention will be described below with reference to FIGS. 1 to 4 and 6.

First, the manufacturing method according to this embodiment will be described.

The input fluorescent surface, which is one of the constituent elements of the input surface (6) shown in FIG. 1, has cesium iodide as a host material on an Al substrate, and a fluorescent material activated by Na is vapor-deposited to a thickness of 200 μm. As described in JP-A-57-136744, a method for producing a cesium iodide phosphor has a columnar crystal size of 15 to obtain high resolution.
This is a method of setting the thickness to be μm or less. The transparent conductive film was directly formed on the surface of the fluorescent substance vapor deposition layer without forming the protective film.
The transparent conductive film was manufactured by an electron beam evaporation method in an oxygen atmosphere. The vapor deposition material used was a mixture of indium oxide (In ₂ O ₃ ) powder and tin oxide (SnO ₂ ) powder. Further, as a comparative example, the mixing ratio of SnO _{2 in} this example is 5 mol%, 10 mol
%, 16 mol%, 20 mol%, 100 mo; Deposition rate under oxygen atmosphere (pressure; 3 × 10 ^-2 pa)
A film with a film thickness of 800 Å was formed at a rate of 50 Å / min on a 9-inch size input fluorescent surface. Input fluorescent surface temperature is 300 ° C during film formation
I kept it. It was confirmed that the surface resistivity of all the obtained transparent conductive films was 100 kΩ / □ or less. FIG. 2 shows the results of measuring the average crystal size of the conductive film and the transmittance of the input phosphor for fluorescence in each case. Here, the average crystal size is defined as the arithmetic average value of the inscribed radius of about 100 crystal grains measured from a scanning electron microscope photograph. The crystal size measured by the method described above is the size in the direction parallel to the film surface of the microcrystal. The horizontal axis in FIG. _{2 represents} the content of SnO _{2 in} the target. From this result, it was found that as the SnO ₂ content increases, the fluorescence transmittance decreases with the same film thickness of 800 Å. The average crystal size of SnO ₂ is 20 to 20mo
It can be seen that in the range up to l%, the maximum is shown around 5 mol%. It was 510Å when the SnO ₂ content was 100%. X of the structure shown in FIG. 1 using these input substrates
After constructing a line image intensifier and forming a bialkali photocathode, the photocurrent when X-rays were incident was measured. FIG. 3 shows a value obtained by dividing this photocurrent by the fluorescence transmittance of the transparent conductive film shown in FIG. 2 and plotting it against SnO ₂ content. The vertical axis of FIG. 3 is the value of photocurrent / fluorescent transmittance normalized by displaying the conventional case where the SnO ₂ content is 0 as 1. From this result, the sensitivity of the photocathode itself was all SnO ₂ except when the SnO ₂ content was 20 mol%.
It can be seen that the content is higher than the conventional case where the content is 0 mol%.

FIG. 4 is a plot of the values of photocurrent / fluorescence transmittance with respect to the average crystal size. From the results shown in FIG. 4, it is understood that the larger the average crystal size, the higher the photocathode sensitivity.
FIG. 5 schematically shows the crystallinity of an indium tin oxide vapor deposition film containing SnO ₂ in an amount of 10 mol%. Since the size of the crystal size is close to the size of the film thickness, the grain boundaries are reduced as compared with the conventional transparent conductive film shown in FIG. As described above, when the crystal size of the transparent conductive film is large, the decrease in crystal grain boundaries has an effect of forming a photocathode having higher sensitivity.

The X-ray image intensifier constructed by using the input surface with the above-mentioned sensitivity can set the transmittance of the output surface to a smaller value compared to about 80% of the conventional one, thus improving the contrast. In addition, the brightness can be made equal to or higher than the conventional one.

For example, if a transparent conductive film with an average crystal size of 500Å is used and a glass substrate with a transmittance of 67% is used for the output surface, the 10% contrast ratio is 15: 1 for a conventional X-ray image intensifier. On the other hand, it is significantly improved to 20: 1, and the brightness is comparable to the conventional one. Furthermore, the average crystal size is 1500
When a transparent conductive film of Å is used and a glass substrate with a transmittance of 42% is used for the output surface, the 10% contrast ratio is 15: 1 in the case of the conventional X-ray image intensifier. : 1 greatly improved, and the brightness becomes the same as before.

As a method for producing these transparent conductive films, other than the embodiment of the present invention,
A sputtering method, a magnetron sputtering method, an ion beam sputter link method, an ion plating method or the like can also be applied.

Further, in the embodiment of the present invention, the case where the transparent conductive film is directly formed on the input fluorescent surface of the X-ray image intensifier has been described, but the same applies to the case where the protective film is previously formed on the input fluorescent surface. The effect of is obtained.

Even when glass is used as the substrate, the crystal size of the transparent conductive film is 500 Å or more not only when the glass easily reacts with the alkali metal vapor during the photocathode formation but also when the glass and the alkali metal vapor hardly react with each other. As a result, the influence of the diffusion of the alkali metal into the transparent conductive film itself can be suppressed, and thus the sensitivity of the photocathode can be improved.

[Advantages of the Invention] As described above, the present invention reduces crystal grain boundaries by increasing the average crystal size in the transparent conductive film, so that the alkali metal on the photocathode penetrates into the transparent conductive film to improve sensitivity. This has the effect of preventing the effect of lowering the X-ray image intensifier and improving the contrast and making the brightness equal to or higher than the conventional one.

[Brief description of drawings]

FIG. 1 is a sectional view showing an example of an X-ray image intensifier according to the present invention, FIGS. 2 to 4 are views for explaining the effect of an embodiment of the present invention, and FIG. FIG. 6 is a schematic diagram illustrating the crystal size of the transparent conductive film on the input fluorescent surface of one embodiment, and FIG. 6 illustrates the crystal size of the transparent conductive film on the input fluorescent surface used in the conventional X-ray image intensifier. It is a schematic diagram. (1) …… Glass envelope, (2) …… Kovar ring,
(3) …… Stainless steel ring, (4) …… Al input window,
(5) …… Output envelope, (6) …… Input surface, (7) ……
Output surface, (8) ... Focusing electrode, (9) ... Accelerating electrode,
(80) …… Crystal grain boundary, (82) …… Fluorescent layer, (84) …… Transparent conductive film

Claims (1)

[Claims] 1. An X-ray image intensifier in which a transparent conductive film is formed on an input fluorescent surface directly or via a protective film, and a photocathode is formed on the transparent conductive film. , Mainly containing indium oxide and containing tin oxide in an amount of 16 mol% or less (excluding 0%), and having a crystallinity of an average crystallite size of a plane parallel to the input fluorescent surface of 500 angstroms or more. X-ray image intensifier.