Radiation image converting panel

1. Field of the Invention

The present invention relates to a radiation image converting panel comprising a radiation converting film having a columnar crystal structure, which converts an incident radiation ray to a visible light.

2. Related Background Art

Radiation images typified by X-ray images have conventionally been widely used for a purposes such as disease diagnosis. As a technique for obtaining such a radiation image, for example, a radiation image recording and reproducing technique using a radiation converting film that accumulates and records irradiated radiation energy, and also emits a visible light according to radiation energy accumulated and recorded as a result of irradiating an excitation light has been widely put into practical use.

A radiation image converting panel to be applied to such a radiation image recording and reproducing technique as this includes a support body and a radiation converting film provided on the support body. As the radiation converting film, a photostimulable phosphor layer having a columnar crystal structure formed by vapor-phase growth (deposition) has been known. When the photostimulable phosphor layer has a columnar crystal structure, since a photostimulable excitation light or photostimulable emission is effectively suppressed from diffusing in the horizontal direction (reaches the support body surface while repeating reflection at crack (columnar crystal) interfaces), this allows remarkably increasing the sharpness of an image by photostimulable emission.

For example, Japanese Patent Application Laid-Open No. 2003-028994 (Document 1) describes a technique that reduces a luminance unevenness by uniforming a concentration distribution of activator along a film thickness direction of phosphor layer. On the other hand, Japanese Patent Application Laid-Open No. 2005-091146 (Document 2) describes a technique that reduces a luminance unevenness by uniforming a concentration distribution of activator in a phosphor layer.

The present inventors have examined the conventional radiation image converting panels in detail, and as a result, have discovered the following problems.

Namely, the conventional radiation image converting panels are manufactured by a moisture-resistant protective film covering a surface of a phosphor layer formed on a support body. At the time of forming the phosphor layer, an activator with a concentration most appropriate to an emission is added in the phosphor layer to be manufactured, but a change of luminance distribution occurs after formation of the moisture-resistant protective film. In concrete terms, since the luminance of the periphery of the phosphor layer relatively increases with respect to the luminance of the vicinity of center of the phosphor layer, a luminance unevenness occurs in the entire panel.

Both Documents 1 and 2 has a problem such that a luminance distribution is changed after formation of a moisture-resistant protective film due to an non-uniformity of columnar crystals. In addition, a technique for arbitrarily controlling a luminance distribution of an entire panel surface after formation of a moisture-resistant protective film is not established.

The present invention has been developed to eliminate the problems described above. It is an object of the present invention to provide a radiation image converting panel with a structure capable of arbitrarily controlling a luminance distribution of an entire panel surface after formation of a moisture-resistant protective film to be provided on the surface of a radiation converting film, by using a change of the luminance distribution that is occurred due to the formation of the moisture-resistant protective film.

A radiation image converting panel according to the present invention has been completed by the inventors\' focusing to the characteristics of the radiation image converting panel such that a luminance ditribution of the entire panel is changed after formation of a moisture-resistant protective film. In concrete terms, a radiation image converting panel comprises a support body, a radiation converting film formed on the support body, and a moisture-resistant protective film covering the radiation converting film. The support body includes a parallel plate having a first main surface and a second main surface opposing the first main surface. The radiation converting film is formed on a film forming region which exists within the first main surface of the support body and includes at least a gravity center position of the first main surface. The radiation converting film is a photostimulable phosphor layer doped with Eu as an activator, and is constituted by columnar crystals which are coincident or tilted at a predetermined angle with respect to a normal direction of the first main surface. The moisture-resistant protective film is preferably a transparent organic film that covers an exposed surface of said radiation converting film without a surface that is covered by the first main surface of the support body.

Particularly, over the entire radiation converting film, the Eu concentration falls within the range of 0.01 wt % or more but 0.5 wt % or less, preferably the range of 0.01 wt % or more but 0.3 wt % or less. In addition, the Eu concentration distribution in the radiation converting film has a concentration gradient along the direction from the radiation converting film (central portion) located on the vicinity of gravity center position toward the peripheral portion of the film.

In concrete terms, in the film forming region of the first main surface, the Eu concentration of the radiation converting film, which locates on a central area, is set in an optimal range capable of obtaining a sufficient emission, or the Eu concentration of the radiation converting film, which locates on a peripheral area, is set in the optimal range. At this time, the optimal range is 0.01 wt % or more but 0.07 wt % or less. Also, in the case that the Eu concentration of the radiation converting film located on the central area is set in the optimal range, the Eu concentration distribution is provided with a concentration gradient by setting the Eu concentration of the radiation converting film located on the peripheral area so as to become higher than the optimal Eu concentration (first concentration pattern), or inversely setting it so as to become lower than the optimal Eu concentration (second concentration pattern). On the other hand, in the case that the Eu concentration of the radiation converting film located on the peripheral area is set in the optimal range, the Eu concentration distribution may be provided with a concentration gradient by setting the Eu concentration of the radiation converting film located on the central area so as to become higher than the optimal Eu concentration (third concentration pattern), or inversely setting it so as to become lower than the optimal Eu concentration (fourth concentration pattern).

Here, in the case that the Eu concentration of the radiation converting film located on the central area is relatively lower than the Eu concentration of the radiation converting film located on the peripheral area, it is preferable that the Eu concentration distribution, which locates on a middle area sandwiched by the central area and the peripheral area, monotonically decreases along a direction directing from the gravity center position to the edge of the film forming region, in the film forming region of the first main surface. Reversely, in the case that the Eu concentration of the radiation converting film located on the central area is relatively higher than the Eu concentration of the radiation converting film located on the peripheral area, it is preferable that the Eu concentration distribution, which locates on a middle area sandwiched by the central area and the peripheral area, monotonically increases along a direction directing from the gravity center position to the edge of the film forming region, in the film forming region of the first main surface.

Among the above described first to fourth concentration distribution patterns, the first and second concentration distribution patterns are an effective concentration distribution pattern when pointing up the luminance of the radiation converting film located on the central area. On the other hand, the third and fourth concentration distribution patterns are an effective concentration distribution pattern when pointing up the luminance of the radiation converting film located on the peripheral area.

Furthermore, in the first and third concentration distribution patterns, the Eu concentration of the radiation converting film located on the peripheral area may be set at the value of 0.3 times or more but 0.8 times or less than the Eu concentration of the radiation converting film located on the central area. In this case, by the moisture-resistant protective film formed so as to cover the radiation converting film (formed on the film formatting region in the first main surface of the support body), the entire luminance distribution of the manufactured radiation image converting panel can be made be uniform from a panel gravity center toward a panel edge.

Here, the central area is defined as an area around the gravity center position whose radius equals 5% or less of a minimum distance from the gravity center position to an edge of the film forming region, in the film forming region of the first main surface. The peripheral area is defined as an area sandwiched by the edge of the film forming region and a circumference of a reference circle centering the gravity center position whose radius equals 40% or more but 80% or less of the minimum distance from the gravity center position to the edge of the film forming region.

The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will be apparent to those skilled in the art from this detailed description.