Phosphor materials for radiation intensifying screens and radiation intensifying screens containing the phosphor materials

A phosphor material for a radiation intensifying screen includes phosphor particles which emit light upon radiation excitation, and alkaline-earth metal fluoride. The alkaline-earth metal fluoride is formed in situ on the surface of the phosphor particles through a reaction of a first reactant containing the alkaline-earth metal with a second reactant containing the fluoride to improve the dispersibility of the phosphor particles. The phosphor material is dispersed in a binder and coated on a base support as a phosphor layer to provide a radiation intensifying screen.

BACKGROUND OF THE INVENTION 
This invention relates to phosphor materials for radiation intensifying 
screens and intensifying screens containing the same. 
DESCRIPTION OF THE PRIOR ART 
In various fields of radiography, such as a medical radiography, such as a 
medical roentogenography for medical diagnosis, and an industrial 
radiography for the nondestructive examination of materials, a radiation 
intensifying screen is employed such that it is attached to a photographic 
film in order to enhance the utilization factor of the radiation and hence 
to improve the photographic sensitivity. The intensifying screen contains 
a phosphor layer of high radiation absorptivity which is supported on a 
base support. A light beam which is emitted from a phosphor with radiation 
exposure is recorded on the photographic film. 
In general, the intensifying screen is formed through the mixing of a 
radiation phosphor with a binder, preparation of a coating liquid with a 
solvent added to the mixture and coating of the liquid on the base support 
followed by a drying step. However, the conventional phosphor, such as a 
calcium tungstate phosphor, europium-activated barium halide phosphor, or 
thulium-activated yttrium tantalate phosphor reveals poor compatibility 
with a binder, such as a cellulose derivative, poly (alkyl methacrylate), 
polyurethane or linear polyester, and with a solvent, such as an ester of 
a lower fatty acid with a lower alcohol, a ketone or an ether, or poor 
dispersibility in a binder and a solvent. Thus, the packing density of the 
phosphor is low in the resultant phosphor layer. In order to obtain a 
predetermined emission luminance it is necessary to increase the thickness 
of the phosphor layer by that extent. Furthermore, because of the poor 
compatibility of the conventional phosphor with the binder and the solvent 
the coating solution becomes highly viscous, resulting in low operability. 
In order to attain improvement, a considerably greater amount of solvent 
needs to be added to the aforementioned mixture. 
SUMMARY OF THE INVENTION 
A principal object of this invention is to provide phosphor materials 
excellent in dispersibility in a binder and a solvent, and intensifying 
screens containing the phosphor material. 
Another object of this invention is to provide phosphor materials, and 
intensifying screens containing the material, which can reduce the 
thickness of the phosphor layer of the intensifying screen and hence lower 
the structure mottle of the intensifying screen. 
The aforementioned and other objects which will become apparent from the 
following descriptions are achieved by a phosphor material for a radiation 
intensifying screen, which comprises: 
phosphor particles which emit light upon radiation excitation; and 
an alkaline-earth metal fluoride formed in situ on the surface of the 
phosphor particles through a reaction of a first reactant containing the 
alkaline-earth metal with a second reactant containing fluorine and 
attached to the surface of the phosphor particles to thereby improve the 
dispersibility of the phosphor particles.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The inventors have made an extensive research in an effort to develop 
phosphor materials for intensifying screens which manifest excellent 
dispersibility in a binder and a solvent, and have found that the 
dispersibility of the phosphor is improved by attaching an alkaline-earth 
metal fluoride to the surface of the phosphor particles through a reaction 
of a reactant containing the alkaline-earth metal with a reactant 
containing fluorine. 
The phosphor as employed in this invention emits light upon radiation 
excitation, for example, through an X-ray excitation, and includes calcium 
tungstate, yttrium strontium tantalate, europium-activated barium halide 
(for example, europium-activated barium chloride fluoride), and 
thulium-activated yttrium tantalate phosphors. As the phosphor of this 
invention, use may be made of a phosphor which is disclosed and claimed 
in: 
(1) Nakajima et al. U.S. Ser. No. 823,299, filed Jan. 28, 1986, entitled 
"PHOSPHOR WHICH EMITS LIGHT BY THE EXCITATION OF X-RAY, AND AN X-RAY 
INTENSIFYING SCREEN USING THE PHOSPHOR" now abandoned; and 
(2) Nakajima et al. U.S. Ser. No. 931,066, filed Nov. 17, 1986, entitled 
"X-RAY PHOSPHORS AND X-RAY INTENSIFYING SCREEN USING THE PHOSPHOR". 
To be brief, the phosphor disclosed in U.S. Ser. No. 823,299 is given by 
the formula: 
EQU M.sub.y Ln.sub.1-x-(2/3)y DO.sub.4 :R.sup.3+ 
where 
M: at least one divalent metal selected from the group consisting of 
beryllium, magnesium, calcium, strontium, barium, zinc and cadmium; 
Ln: at least one metal selected from the group consisting of yttrium, 
gadolinium, lanthanum and lutetium; 
D: tantalum or niobium; 
R: at least one activator metal selected from the group consisting of 
thulium, praseodymium, samarium, europium, terbium, dysprosium and 
ytterbium; 
X: 0 to 0.05 
Y: 1.times.10.sup.-5 to 1 
The phosphor disclosed in U.S. Ser. No. 931,066 is given by the formula: 
EQU Ln.sub.1-x(2/3)y-(1/3)2 M".sub.y M'.sub.2 DO.sub.4 :xR.sup.3+ 
where 
M": at least one divalent metal selected from the group consisting of 
beryllium, magnesium, calcium, strontium, barium, zinc and cadmium; 
M': at least one alkali metal selected from the group consisting of 
lithium, sodium and potassium; 
Ln: at least one metal selected from the group consisting of yttrium, 
gadolinium, lanthanum and lutetium; 
D: tantalum or niobium; 
R: at least one activator metal selected from the group consisting of 
thulium, praseodymium, samarium, europium, terbium, dysprosium ytterbium; 
x: 0 to 0.05 
y: 1.times.10.sup.-5 to 1 
z: 1.times.10.sup.-4 to 0.1 
The phosphor particles employed in this invention have normally an average 
particle size of 2 to 20 .mu.m. 
In the phosphor materials of this invention, an alkaline-earth metal 
fluoride is attached to the surface of the phosphor particles. As the 
fluoride use may be made of calcium fluoride, magnesium fluoride, barium 
fluoride and strontium fluoride. Among them use may be made of preferably 
calcium fluoride and barium fluoride and more preferably calcium fluoride. 
The following method is adopted to attach the fluoride to the surface of 
the phosphor particles. 
First, phosphor particles are suspended or dispersed in a reaction medium 
which does not substantially dissolves the phosphor and alkaline-earth 
metal fluoride therein. A first reactant containing alkaline-earth metal 
and a second reactant containing fluoride are added to the suspension or 
dispersion, and reacted there to form or deposit an alkaline-earth metal 
fluoride in situ on the surface of the phosphor particles. The first and 
second reactants are preferably added each in the form of solution in the 
reaction medium used. The reaction between the first and second reactants 
is usually carried out at 5.degree. to 30.degree. C. If the reaction 
temperature is higher, the particle size of the resultant fluoride 
particles become too large. 
It is preferred that the reaction medium employed be water if the phosphor 
is not water-soluble and an organic medium, such as alcohol, if the 
phosphor is water-soluble. It is preferable to select first and second 
reactants of such a type that no insoluble byproduct is produced in the 
reaction medium during the production of the alkaline-earth metal fluoride 
yielded through the reaction of the first reactant with the second 
reactant. From this viewpoint it is preferable that alkaline metal halide 
(excluding fluoride), preferably a chloride, and ammonium fluoride be used 
as the first and second reactants, respectively. Most conveniently, an 
alkaline earth metal halide solution is added to the suspension of the 
phosphor and then an ammonium fluoride solution is added dropwise to the 
mix ture. The addition of ammonium fluoride is preferably conducted at a 
rate of 0.2 g/min. to 5 g/min. The reaction is substantially completed 
when the addition of ammonium fluoride is completed. 
By so doing, the alkaline-earth metal fluoride in the form of particles of 
the order of 0.5 .mu.m or less in size is relatively firmly and uniformly 
deposited on the surface of the phosphor particles. FIG. 1 is an SEM 
photograph showing a phosphor material of this invention. It is seen that 
very small particles of the fluoride are attached to the surfaces of the 
phosphor particles. In FIG. 1, "60*2NM" means 60.times.10.sup.2 NM (6 
.mu.m). 
The amount of the alkaline-earth metal fluoride to be deposited on the 
surface of the phosphor particles is determined, taking into consideration 
the dispersibility of the phosphor particles as well as the luminance. If 
the alkaline-earth metal fluoride is too small in its amount, it is not 
possible to obtain adequate dispersibility. If, on the other hand, the 
alkaline-earth metal fluoride is too large in its amount, the luminance as 
a whole will be lowered since the alkaline-earth metal fluoride itself 
does not emit luminescence. In general, the alkaline-earth metal fluoride 
is deposited on the surface of the phosphor particles in an amount of 0.01 
to 30%, most preferably 0.3 to 5%, based on the weight of the phosphor 
particles. The amount of the alkaline-earth metal fluoride to be deposited 
can be controlled by the amounts of the first and second reactants fed, 
since the reaction of the first reactant with the second reactant 
progresses substantially quantitatively and the alkaline-earth metal 
fluoride produced during the reaction substantially all precipitates. 
Incidentally, an alkaline-earth metal fluoride cannot be deposited on the 
surface of the phosphor in an amount of 0.01% or more by dipping the 
phosphor particles in an aqueous solution of the alkaline-earth metal 
fluoride followed by filtering and drying, since the solubility of the 
fluoride in water is very small and the phosphor particles can contain 
water at most in an amount of several tens of grams per 100 grams of the 
phosphor particles after filtering. 
FIG. 2 shows a structure of an intensifying screen. As seen from FIG. 2, 
intensifying screen 10 includes a sheet- or film-like base support 11 and 
phosphor layer 12 containing a phosphor material of this invention is 
formed on base support 11. Support 11 is normally formed of paper or 
plastics material, such as polyethylene terephthalate. 
Phosphor layer 12 can be formed in the same way as in the conventional 
method as set forth above, except that the phosphor material of this 
invention is used instead of the conventional phosphor. That is, the 
phosphor material of this invention and binder are dispersed in a solvent. 
Examples of the binder are cellulose derivative, such as nitrocellulose or 
cellulose acetate; poly (alkyl methacrylate), such as poly (methyl 
methacrylate); polyurethane, and linear polyester. Examples of the solvent 
are an ester of lower fatty acid with lower alcohol, such as ethyl acetate 
or butyl acetate; ketone, such as acetone or methylethylketone; dioxane; 
ether, such as ethyleneglycol monoethylether; or a mixture thereof. The 
resultant mixture is coated on base support 11 by means of, for example, a 
doctor blade, roll coater or knife coater and dried up there to obtain an 
intensifying screen. 
The phosphor material of this invention has better dispersibility in the 
associated binder and the solvent, and, even if somewhat greater in its 
amounts, low viscosity and hence a free- or smooth-flowing property in the 
form of the coating solution. It is, therefore, possible to enhance the 
packing density of the phosphor material in phosphor layer 12. As a 
result, the phosphor layer can be made small in thickness by about 10 to 
40% as compared to a phosphor layer using a conventional phosphor not 
subjected to surface treatment, in order to obtain the same luminance. 
Thus, the structure mottle of the intensifying screen can be lowered when 
the phosphor material of this invention is used. Therefore, the use of the 
phosphor material of this invention can enhance the image characteristic, 
such as the granularity and sharpness. It is preferable to use the 
phosphor material of this invention and binder in a weight ratio of 1:50 
to 1:300. 
It is to be noted that phosphor layer 12 can be covered with protective 
layer 13 formed of, for example, cellulose material, polyethylene 
terephthalate. 
This invention will now be explained below in conjunction with the 
following examples. 
EXAMPLE 1 
80 g of calcium tungstate phosphor particles (average particle size: 5 
.mu.m) was suspended in 500 ml of pure water, and 10 ml of 32.5% aqueous 
calcium chloride solution was added to the suspension. Then, while 
stirring the resultant mixture, 100 ml of 1% aqueous ammonium fluoride 
solution was added dropwise at 18.degree. C. In this way, the calcium 
fluoride particles were substantially uniformly deposited on the surface 
of the phosphor particles. The suspension was separated into solid and 
liquid phase by decantation and the liquid phase (phosphor material) was 
dried for 5 hours at 120.degree. C. The phosphor material contained 
calcium fluoride in an amount of 2.6% of the weight of the phosphor. 
50 g of the phosphor material thus obtained was dispersed in a solution of 
0.2 g of nitrocellulose in 9.8 g of butyl acetate, uniformly coated by a 
doctor blade on a polyethylene terephthalate film and naturally dried to 
give an intensifying screen. Similarly, various intensifying screens were 
prepared when phosphor layers had different thicknesses. 
For comparison, intensifying screens wherein phosphor layers had different 
thickness were prepared in the same manner as described above except that 
calcium tantalate phosphor which had not been surface-treated was used 
instead of the phosphor material of this invention. 
The intensifying screens thus prepared were measured for luminance by X-ray 
excitation. In the measurement, an aluminum foil was inserted between the 
intensifying screen and an X-ray tube with 50 kV of tube voltage and 2 mA 
of tube current. 
FIG. 3 shows the result of measurement thus conducted. As evident from FIG. 
3, the luminance of the intensifying screen using the phosphor of this 
invention with the calcium fluoride particles deposited thereon is about 
10% higher than that of the intensifying screen using a conventional 
phosphor, provided that these screens have the same thickness of the 
phosphor layer. The intensifying screen using the phosphor material of 
this invention can be made 1.0 to 60 .mu.m thinner than the conventional 
counter part, provided that the same luminance is obtained. Since the 
intensifying screen of this invention is less liable to suffer a structure 
mottle due to the reduction of the screen film thickness, thus improving 
the image characteristic, such as the granularity and sharpness. 
EXAMPLE 2 
Using 80 g of yttrium strontium tantalate phosphor particles (average 
particle size: 4 .mu.m), 500 ml of pure water, 10 ml of 32.5% aqueous 
calcium chloride solution and 100 ml of 1% aqueous ammonium fluoride 
solution, calcium fluoride particles were deposited on the surface of the 
phosphor particles by the same manner as in Example 1. The phosphor 
material contained calcium fluoride in an amount of 2.6% based on the 
weight of the phosphor. An intensifying screen was prepared with the use 
of the phosphor material thus obtained, as in Example 1. This intensifying 
screen was compared in its luminance with the conventional intensifying 
screen having yttrium strontium tantalate phosphor particles not 
surface-treated with calcium fluoride, as in the case of Example 1 (see 
FIG. 4). 
As evident from FIG. 4, the intensifying screen using the phosphor material 
of this invention which was surface-treated with calcium fluoride has a 
luminance 3 to 15% higher than that of the counterpart using a 
conventional phosphor, provided that the phosphor layers have the same 
thickness. According to this invention it is possible to improve the image 
characteristic of the intensifying screen because the phosphor layer 
thickness can be made 25 to 60 .mu.m thinner than the conventional 
counterpart, provided that both have the same luminance. 
EXAMPLE 3 
80 g of europium (II)-activated barium chloride fluoride phosphor particles 
(average particle size: 2.4 .mu.m) was suspended in 300 ml of methanol, 6 
ml of 19% calcium chloride methanol solution was added to the suspension 
and, while stirring the solution, 76 ml of 1% ammonium fluoride methanol 
solution was added dropwise. In this way, the calcium fluoride particles 
were substantially uniformly deposited on the surface of the phosphor 
particles. Then the suspension was separated into solid and liquid phases 
by decantation, followed by drying the solid phase for 5 hours at 
120.degree. C. The phosphor material contained calcium fluoride in an 
amount of 2.0% based on the weight of phosphor. 
Using the surface-treated phosphor of this invention and the conventional 
phosphor not surface-treated, intensifying screens were prepared, and 
compared for their luminance as in Example 1 (FIG. 5). 
As evident from FIG. 5, when the intensifying screen using a phosphor of 
this invention, which was surface-treated with calcium fluoride, is 
compared with the counterpart using the conventional phosphor, the former 
is about 18% higher in luminance than the latter. It is, therefore, 
possible to improve the image characteristic, such as the granularity and 
sharpness, of the intensifying screen because the thickness of the 
phosphor layer can be reduced, provided that both the phosphors have the 
same luminance. 
As set out above, a phosphor material for a radiation intensifying screen 
can be obtained according to this invention which, owing to the attachment 
of the particles of alkaline earth metal fluoride to the surface of the 
phosphor, can be formed as a coated film of high packing density on the 
base support with the phosphor particles very adequately dispersed in an 
associated binder and a solvent.