Phosphor and radiation image storage panel containing the same

PHOSPHOR AND RADIATION IMAGE STORAGE PANEL CONTAINING THE SAME

BACKGROUND OF THE INVENTION 5

1. Field of the Invention

The present invention relates to a novel phosphor, a process for the preparation of the same, a radiation image recording and reproducing method utilizing the ]() same, and a radiation image storage panel employing the same. More particularly, the invention relates to a novel cerium activated rare earth halophosphate phosphor.

2. Description of the Prior Art 15 There is well known a cerium activated rare earth

oxyhalide phosphor (LnOX:Ce, in which Ln is at least one rare earth element selected from the group consisting of Y, La, Gd and Lu; and X is at least one halogen selected from the group consisting of CI and Br) as a 20 cerium activated rare earth halide phosphor. As described in Japanese Patent Provisional Publication No. 55(1980)-12144 (which corresponds to U.S. Pat. No. 4,236,078), etc., the phosphor gives emission (stimulated emission) in the near ultraviolet region when excited 25 with an electromagnetic wave such as visible light or infrared rays after exposure to a radiation such as Xrays, cathode rays or ultraviolet rays. The phosphor is valuable as a stimulable phosphor employable for a radiation image recording and reproducing method. 30

The radiation image recording and reproducing method utilizing the stimulable phosphor can be employed in place of the conventional radiography utilizing a combination of a radiographic film having an emulsion layer containing a photosensitive silver salt and an intensifying screen as described, for instance, in U.S. Pat. No. 4,239,968. The method involves steps of causing a stimulable phosphor to absorb a radiation having passed through an object or having radiated ^ from an object; sequentially exciting (or scanning) the phosphor with an electromagnetic wave such as visible light or infrared rays (stimulating rays) to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting 4J the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.

In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient 50 amount of information by applying a radiation to the object at a considerably smaller dose, as compared with the conventional radiography. Accordingly, this method is of great value, especially when the method is used for medical diagnosis. 55

For other stimulable phosphors employable in the above-described method, there have been known a divalent europium activated alkaline earth metal fluorohalide phosphor (MwFX:Eu2+, in which M^is at least one alkaline earth metal selected from the group 60 consisting of Mg, Ca and Ba; and X is at least one halogen selected from the group consisting of CI, Br and I); an europium and samarium activated strontium sulfide phosphor (SrS:Eu,Sm); an europium and samarium activated lanthanum oxysulfide phosphor ... 65 :Eu,Sm); an europium activated barium aluminate phosphor (BaO.Ah03:Eu); an europium activated alkaline earth metal silicate phosphor (M2+O.Si02:Eu, in which

M2+ is at least one alkaline earth metal selected from the group consisting of Mg, Ca and Ba), and the like.

SUMMARY OF THE INVENTION

The present invention provides a novel cerium activated rare earth halide phosphor which is different from the above-mentioned cerium activated rare earth oxyhalide phosphor, and a process for the preparation of the same.

The present invention has researched for a stimulable phosphor and newly found that a cerium activated rare earth halophosphate phosphor gives stimulated emission as well as spontaneous emission, to accomplish the invention.

The phosphor of the invention is a cerium activated rare earth halophosphate phosphor having the formula (I):

35

in which Ln, X, a and x have the same meanings as defined above; and

firing the obtained mixture at a temperature within the range of 500°-1400° C. in a weak reducing atmosphere.

The cerium activated rare earth halophosphate phosphor having the formula (I) of the invention gives stimulated emission in the near ultraviolet to blue region when excited with an electromagnetic wave having a wavelength within the range of 500-850 nm after exposure to a radiation such as X-rays, ultraviolet rays and cathode rays.

The cerium activated rare earth halophosphate phosphor having the formula (I) of the invention also gives emission (spontaneous emission) in the near ultraviolet to blue region when exposed to a radiation such as X-rays, ultraviolet rays and cathode rays.

The present invention further provides a radiation image recording and reproducing method utilizing the novel stimulable phosphor and a radiation image storage panel using said phosphor.

That is, the radiation image recording and reproducing method comprises steps of:

(i) causing the cerium activated rare earth halophosphate phosphor having the formula (I) to absorb a radiation having passed through an object or having radiated from an object;

(ii) exciting said stimulable phosphor with an electromagnetic wave having a wavelength within the range of 500-850 nm to release the radiation energy stored therein as light emission; and

(iii) detecting the emitted light.

The radiation image storage panel of the invention comprises a support and a stimulable phosphor layer provided thereon which comprises a binder and a stimulable phosphor dispersed therein, in which said phos3

phor layer contains the cerium activated rare earth halophosphate phosphor having the formula (I).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a spontaneous emission spectrum and 5 an excitation spectrum of LaPO4.0.5LaBo:0.001Ce-H phosphor (Curves 1 and 2, respectively), which is an example of the cerium activated rare earth halophosphate phosphor according to the invention.

FIG. 2 shows a stimulation spectrum of the La- 10 PO4.0.5LaBrj:0.001 Ce3 + phosphor.

FIG. 3 shows a stimulated emission spectrum of the LaPO4.0.5LaBr3:0.001 Ce3 + phosphor.

FIG. 4 shows a relationship between a value and an intensity of stimulated emission with respect to LaPOj- 15 .aLaBr3:0.001Ce3+ phosphor, which is an example of the cerium activated rear earth halophosphate phosphor according to the invention.

FIG. 5 is a schematic view showing the radiation image recording and reproducing method according to 20 the invention.

DETAILED DESCRIPTION OF THE
INVENTION

The cerium activated rare earth halophosphate phos- 25 phor of the present invention can be prepared, for instance, by a process described below.

As starting materials, the following materials can be employed:

(1) at least one rare earth oxide selected from the 30 group consisting of Y9O3, ... ... and LU2O3;

(2) P205;

(3) at least one rare earth halide selected from the group consisting of YF3, YCI3, YBr3, YI3, LaF3, LaCl3, LaBr3, Lal3, GdF3, GdCl3, GdBr3, Gdl3, LuF3, LuCl3, 35 LuBr3 and LUI3; and

(4) at least one compound selected from the group consisting of cerium compounds such as cerium halide, cerium oxide, cerium nitrate and cerium sulfate.

Further, ammonium halide (NH4X', in which X' is 40 any one of CI, Br and I) may be employed as a flux.

In the process for the preparation of the phosphor of the invention, the above-mentioned rare earth oxide (1), phosphorus pentaoxide (2), rare earth halide (3) and cerium compound (4) are, in the first place, mixed in the 45 stoichiometric ratio corresponding to the formula (II):

LnP04.aLnX3:xCe (II)

in which Ln is at least one rare earth element selected 50 from the group consisting of Y, La, Gd and Lu; X is at least one halogen selected from the group consisting of F, CI, Br and I; and a and x are numbers satisfying the conditions of 0.1 ^aS 10.0 and 0<x^0.2, respectively.

From the viewpoint of enhancement in the intensity 55 of stimulated emission and in the intensity of spontaneous emission, Ln in the formula (II) which indicates rare earth element is preferably at least one element selected from the group consisting of Y and La. The halogen X is preferably at least one element selected from the 60 group consistig of CI and Br. The number for a which indicates the amount of rare earth halide (LnX3) is preferably within the range of 0.5=a=9.5, and more preferably of l.OSaSl 8.0. From the same viewpoint, the number for x which indicates the amount of cerium activa- 65 tor is preferably within the range of 10-5=lxSil 10~2.

The mixture of starting materials for the phosphor is prepared by any one of the following procedures:

4

(i) simply mixing the starting materials (1), (2), (3) and (4); and

(ii) mixing the starting materials (1), (2) and (3), heating the obtained mixture at a temperature of not lower than 100° C. for several hours and then mixing the heattreated mixture with the starting material (4).

Further, as a modification of the above procedure (ii), there may be mentioned a procedure comprising mixing the starting materials (1), (2), (3) and (4) and subjecting the obtained mixture to the heating treatment.

The mixing is carried out using a conventional mixing apparatus such as a variety of mixers, a V-type blender, a ball mill and a rod mill in any case of the abovedescribed procedures (i) and (ii).

Then, the resulting mixture of the starting materials is placed in a heat-resistant container such as a quartz boat, an alumina crucible or a quartz crucible, and fired in an electric furnace. The temperature for the firing suitably ranges from 500° to 1400° C, and preferably ranges from 700° to 1200° C. The firing period is determined depending upon the amount of the mixture of starting materials, the firing temperature, etc., and suitably ranges from 0.5 to 6 hours. As the firing atmosphere, there can be employed a weak reducing atmosphere such as a nitrogen gas atmosphere containing a small amount of hydrogen gas or a carbon dioxide gas atmosphere containing carbon monoxide gas. In the case of using a tetravalent cerium compound as the above-mentioned starting material (4), the tetravalent cerium contained in the mixture is reduced into trivalent cerium by the weak reducing atmosphere in the firing stage.

Through the firing procedure, a powdery phosphor of the present invention is produced. The powdery phosphor thus obtained may be processed in a conventional manner involving a variety of procedures for the preparation of phosphors such as a washing procedure, a drying procedure and a sieving procedure.

The phosphor of the invention prepared in accordance with the above-described process is a cerium activated rare earth halophosphate phosphor having the formula (I):

LnP04.aLnX3:xCe3+ (I)

in which Ln is at least one rare earth element selected from the group consisting of Y, La, Gd and Lu; X is at least one halogen selected from the group consisting of F, CI, Br and I; and a and x are numbers satisfying the conditions of 0.15ia= 10.0 and 0<x=0.2, respectively.

The cerium activated rare earth halophosphate phosphor of the present invention gives spontaneous emission in the near ultraviolet to blue region (peak wavelength of the emission; approx. 420 nm) upon excitation with a radiation such as X-rays, ultraviolet rays and cathode rays.

FIG. 1 shows a spontaneous emission spectrum and an excitation spectrum of ... phosphor which is an example of the cerium activated rare earth halophosphate phosphor of the invention.

Curve 1: spontaneous emission spectrum

Curve 2: excitation spectrum

As is clear from FIG. 1, the phosphor of the invention gives spontaneous emission in the near ultraviolet to blue region upon excitation with ultraviolet rays.

The spontaneous emission spectra upon excitation with ultraviolet rays and excitation spectra of the phosphor of the invention are illustrated above. It has been confirmed that the spontaneous emission spectrum of the phosphor of the invention given upon excitation with X-rays or cathode rays are almost the same as those given upon excitation with ultraviolet rays which are shown in FIG. 1 5

The cerium activated rare earth halophosphate phosphor of the invention also gives stimulated emission in the near ultraviolet to blue region when excited with an electromagnetic wave having a wavelength within the region of 500-850 nm such as visible-light or infrared 10 rays after exposure to a radiation such as X-rays, ultraviolet rays and cathode rays.

FIG. 2 shows a stimulation spectrum of LaPCU.O.5LaBr3:0.001Ce3+ phosphor which is an example of the cerium activated rare earth halophosphate phosphor of 15 the invention.

As is clear from FIG. 2, the phosphor of the invention gives stimulated emission upon excitation with an electromagnetic wave in the wavelength region of 500-850 nm after exposure to X-rays. Particularly, the 20 phosphor exhibits stimulated emission of high intensity upon excitation with an electromagnetic wave in the wavelength region of 500-700 nm. Based on this fact, the wavelength region of the electromagnetic wave employed as stimulating rays, namely 500-850 nm, has 25 been decided in the radiation image recording and reproducing method of the present invention.

FIG. 3 shows a stimulated emission spectrum of La ... phosphor which is an example of the cerium activated rare earth halophosphate phos- 30 phor of the invention.

As is clear from FIG. 3, the phosphor of the invention gives stimulated emission in the near ultraviolet to blue region. The stimulated emission spectrum of the phosphor is in good accordance with the spontaneous 35 emission spectrum thereof shown in FIG. 1.

The stimulated emission spectrum and stimulation spectrum of the cerium activated rare earth phosphor according to the present invention are illustrated above with respect to the specific phosphor. It has been con- 40 firmed that other phosphorus according to the invention show the similar stimulated emission characteristics to those of the above-mentioned specific phosphor, and further confirmed that they give stimulated emission in the near ultraviolet to blue region when excited with an 45 electromagnetic wave having a wavelength within the range of 500-850 nm after exposure to a radiation.

FIG. 4 graphically shows a relationship between a value and an intensity of stimulated emission [emission intensity upon excitation with a He-Ne laser (wave- 50 length: 632.8 nm) after exposure to X-rays at 80 KVp] with respect to ... phosphor.

As is evident from FIG. 4, the ... lCe3+ phosphor having a value within a range of 0.1 =a= 10.0 gives stimulated emission. On the basis of 55 this fact, the value range (0.1 =a= 10.0) of the phosphor of the invention has been decided. Particularly, the emission intensity of the phosphor is high in the a value range of 0.5=a=9.5, and is further high in the range of 1.0=laS8.0. 60

The phosphor has almost the same tendency as shown in FIG. 4 with respect to the relationship between a value and an intensity of spontaneous emission. It has been further confirmed that other cerium activated rare earth halophosphate phosphors according to 65 the invention than the above-mentioned phosphor have the same tendencies on the relationships between a value and the intensity of stimulated emission and be

tween a value and the intensity of spontaneous emission as shown in FIG. 4.

From the viewpoint of emission properties described hereinbefore, the phosphor of the invention is very useful as a phosphor for a radiation image storage panel employed in the radiation image recording and reproducing method, or for a radiographic intensifying screen employed in the conventional radiography, both panel and screen being used in medical radiography such as X-ray photography for medical diagnosis and industrial radiography for non-destructive inspection.

The cerium activated rare earth halophosphate phosphor having the formula (I) is preferably employed in the form of a radiation image storage panel (also referred to as a stimulable phosphor sheet) in the radiation image recording and reproducing method of the invention.

The radiation image storage panel comprises a support and at least one phosphor layer provided on one surface of the support. The phosphor layer comprises a binder and a stimulable phosphor dispersed therein. Further, a transparent protective film is generally provided on the free surface of the phosphor layer (surface not facing the support) to keep the phosphor layer from chemical deterioration or physical shock.

The radiation image recording and reproducing method of the invention is desired to be performed employing the radiation image storage panel comprising a phosphor layer which contains the cerium activated rare earth halophosphate phosphor having the formula (I).

In the radiation image recording and reproducing method employing the stimulable phosphor having the formula (I) in the form of a radiation image storage panel, a radiation having passed through an object or radiated from an object is absorbed by the phosphor layer of the panel to form a radiation image as a radiation energy-stored image on the panel. The panel is then irradiated (e.g., scanned) with an electromagnetic wave in the wavelength region of 500-850 nm to release the stored image as stimulated emission. The emitted light is photoelectrical^ detected to obtain electric signals so that the radiation image of the object can be reproduced as a visible image from the obtained electric signals.

The radiation image recording and reproducing method of the present invention will be described more in detail with respect to an example of a radiation image storage panel containing the stimulable phosphor having the formula (I), by referring to a schematic view shown in FIG. 5.

In FIG. 5 which shows the total system of the radiation image recording and reproducing method of the invention, a radiation generating device 11 such as an X-ray source provides a radiation for irradiating an object 12 therewith; a radiation image storage panel 13 containing the stimulable phosphor having the formula (I) absorbs and stores the radiation having passed through the object 12; a source of stimulating rays 14 provides an electromagnetic wave for releasing the radiation energy stored in the panel 13 as light emission; a photosensor 15 such as a photomultiplier faces the panel 13 for detecting the light emitted by the panel 13 and converting it to electric signals; an image reproducing device 16 is connected with the photosensor 15 to reproduce a radiation image from the electric signals detected by the photosensor 15; a display device 17 is connected with the reproducing device 16 to display the reproduced image in the form of a visible image on