Process for forming phosphor powder layer

A phosphor powder layer is formed by a process comprising a step of mixing a phosphor powder with a polymer emulsion and drying the mixture to give the phosphor powder coated with the polymer and a step of subjecting the phosphor powder coated with the polymer to heating and pressing. The resulting powder layer is suitable as a scintillator used in a radiation detector, particularly in a radiation detector used in X-ray CT.

BACKGROUND OF THE INVENTION 
This invention relates to a process for forming a phosphor powder layer, 
more particularly to a process for forming a phosphor powder layer used as 
a scintillator. 
Heretofore, as a radiation detector used in X-ray CT (computerized 
tomography), there has been used a detector combining a xenon gas chamber 
or bismuth germanate with a photomultiplier. But when a large number of 
radiation detectors having equal performance were necessary as in the case 
of X-ray CT, it was difficult to adjust characteristics of individual 
radiation detectors in the case of using such detectors. Particularly in 
the case of a radiation detector combining bismuth germanate with a 
photomultiplier, it was very difficult to adjust characteristics of 
individual radiation detectors due to dispersion in characteristics of 
monocrystals of the bismuth germanate used as scintillator and dispersion 
in characteristics of the photomultipliers. 
In order to solve this problem, some of the present inventors previously 
proposed a radiation detector in which phosphor powders (particles) were 
used as scintillator (U.S. patent appln. Ser. No. 047,133, filed June 11, 
1979, now U.S. Pat. No. 4,317,037). For the purpose of obtaining a 
tomography of high accuracy in a radiation detector for conventional X-ray 
CT, the width of scintillator is about 1-10 mm and preferably about 1-3 mm 
and the length thereof is about 20 mm, for example. Accordingly, the 
number of phosphor particles in one radiation detector is about 300,000, 
though it may vary depending on the particle size. Although individual 
phosphor particles may possibly be slightly different from one another in 
characteristics, the dispersion in the characteristics as scintillator can 
be made about one divided by the square root of particle number, that is, 
about 0.01%, by sufficiently mixing them, whereby a satisfactory result 
can be obtained. A radiation detector resembling to the above-mentioned 
one is also disclosed in Japanese Patent Appln. Kokai (Laid-Open) No. 
90089/79. 
As a process for forming a thin layer of phosphor powder, there has been 
employed, for example, a process wherein a phosphor powder is suspended in 
a polymer solution, and the resulting suspension is placed in a mold 
having a desired shape and then dried. But this process had defects in 
that voids are easily formed in the resulting thin layer and a packing 
density of the thin layer cannot be increased. In the case of another 
process wherein a monomer solution was used in place of the polymer 
solution and the polymerization reaction was conducted at the time of 
molding, said process had almost the same defects as mentioned above as 
well as other defects such as a much more time being necessary for the 
polymerization reaction, the desired shape used in a radiation detector 
being not able to be formed in a mold from the beginning, and a large 
block having to be formed first, followed by cutting and polishing to give 
the desired shape, which results in making the cut and polished face lower 
in emission efficiency. There has also been employed a process wherein a 
phosphor powder and a polymer powder are mixed, charged in a molding 
machine and formed into a powder layer with heating and/or under pressure. 
This process was considerably good when a mixing ratio of the phosphor to 
the polymer was small, but when the mixing ratio of the phosphor to the 
polymer became large in order to increase the packing density of the 
phosphor powders, a uniform mixture could not be obtained due to a large 
difference in specific gravities of the two, which results in unfavorably 
giving ununiform molded articles having high polymer contents locally and 
having some weak portions. It has also been proposed a process wherein a 
phosphor powder was fired to give a thin film, but when a phosphor, 
Gd.sub.2 O.sub.2 S:Pr,Ce,F, which is suitable as scintillator, was fired 
in air, emission efficiency of the resulting thin layer was lowered 
undesirably. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a process for forming a 
phosphor powder layer which is high in packing density of the phosphor 
powder and is also stiff. 
This invention provides a process for forming a phosphor powder layer which 
comprises a step of mixing a phosphor powder with a polymer emulsion and 
drying the mixture to give the phosphor powder coated with the polymer and 
a step of subjecting the phosphor powder coated with the polymer to 
heating and pressing. 
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Since the polymer emulsion is used in the process of this invention, the 
mixing of the phosphor powder with the polymer as a binder is uniform, the 
kind and amount of polymer emulsion can be selected freely, and 
pulverizing of the mixture after drying can be conducted easily. 
As the polymer emulsion, there can be used emulsions of polymers such as 
polyvinyl chloride, polyvinyl acetate, polystyrene, polyacrylates, 
polyvinylpyridine, polybutadienes, etc. Since these emulsions are mainly 
aqueous polymer emulsions, there are no danger in working and no injury to 
health. It is also possible to use two or more polymer emulsions 
simultaneously. But it is not preferable to use two or more polymer 
emulsions which break emulsion by mixing them, for example, mixing a 
cationic polymer emulsion and an anionic polymer emulsion. Therefore, when 
the two or more polymer emulsion are used simultaneously, the resulting 
mixed system should maintain the emulsified state. Among various polymer 
emulsions mentioned above, the use of an emulsion of polystyrene is more 
preferable. Since polystyrene shows little deterioration when exposed to 
X-rays, there is no problem in the life when used as scintillator. 
The mixture of phosphor powder and polymer emulsion is then dried by a 
conventional method to give the phosphor powder coated with the polymer. 
The phosphor powder coated with the polymer is placed in a mold machine and 
molded under a pressure of from 4-5 kg/cm.sup.2 to 500-600 kg/cm.sup.2 
with heating at a temperature of the glass transition temperature or 
melting point of the polymer coating the phosphor powder or higher to form 
a phosphor powder layer. In order to prevent the phosphor from 
deterioration, it is preferable to keep the heating temperature at 
300.degree. C. or lower, more preferably 200.degree. C. or lower. It is 
also possible to mold the phosphor powder under pressure with heating or 
by giving pressure first, followed by heating without pressure. 
According to this invention, the molding can be conducted in a short time 
and the resulting molded layer is uniform in quality without voids. In 
this invention, a conventional molding machine can be used. 
The amount of polymer emulsion is preferably in the range of 4 to 33% by 
weight, more preferably 6 to 15% by weight, in terms of solid content 
based on the weight of the phosphor powder. If the solid content is less 
than 4% by weight, the effect as a binder is insufficient and sufficient 
strength cannot be obtained. On the other hand, if the solid content is 
more than 33% by weight, the packing density is lowered and it becomes 
difficult to release a molded layer from the molding machine. In order to 
increase the packing density, the solid content of 15% by weight or less 
is particularly preferable. 
As the phosphor powder used as scintillator in a radiation detector, 
particularly in a radiation detector used in X-ray CT, it is preferable to 
use phosphor powder having an average particle size of 5 to 50 .mu.m. In 
such a case, there can easily be obtained a phosphor powder layer with a 
high packing density and desired strength. The above-mentioned particle 
size of 5 to 50 .mu.m is not limitative one but only shows a practically 
preferable range. It is also possible to use phosphor powders having an 
average particle size in the range of 1 to 200 .mu.m. 
As the phosphor powder, there can be used a powder of at least one phosphor 
selected from the group consisting of phosphors of Y.sub.2 O.sub.2 S, 
La.sub.2 O.sub.2 S, Gd.sub.2 O.sub.2 S, or BaSO.sub.4 activated with at 
least one of Eu, Pr and Tb, and other phosphors such as CdS:Cu,Al, 
CdS:Ag,Cl, ZnS:Cu,Al, ZnCdS:Cu,Al and ZnCdS:Ag,Cl. Among them, Gd.sub.2 
O.sub.2 S:Pr,Ce,F is more preferable. 
The phosphor powder layer formed by the process of this invention has many 
advantages compared with that obtained by the known resin mold method in 
that complicated procedures are not necessary, cutting and polishing for 
giving a suitable shape is not necessary for use in a radiation detector, 
and thus the emission efficiency is not lowered, since polystyrene can be 
used, deterioration of the scintillator layer due to X-rays can be reduced 
remarkably, and the like.

This invention is illustrated by way of Examples, in which all percents are 
by weight unless otherwise specified, and which do not limit this 
invention. 
EXAMPLE 1 
A phosphor powder, Gd.sub.2 O.sub.2 S:Pr,Ce,F in an amount of 20 g was 
sufficiently mixed with 3.5 g of a polystyrene emulsion (Nipol LX-303, a 
trade name, manufactured by The Japanese Geon Co., Ltd., solid content 
45%) and dried in vacuum. The amount of polystyrene was 7.9% based on the 
weight of the phosphor powder. Subsequently, the dried product was 
pulverized by softly pressing with a pestle. The resulting powder was 
filled in a frame made of ceramic (size 2 mm.times.2 mm.times.40 mm) and 
pressed with a punch made of ceramic at a pressure of about 10 
kg/cm.sup.2, followed by heating on a hot plate at 120.degree. C. for 5 
minutes. The resulting phosphor powder layer had no voids and sufficient 
strength and was uniform in quality. 
EXAMPLE 2 
A phosphor powder, Gd.sub.2 O.sub.2 S:Pr,Ce,F, in an amount of 20 g was 
sufficiently mixed with 6 g of a polystyrene emulsion (Nipol LX-303, solid 
content 45%) and dried in vacuum. The amount of polystyrene was 13.5% 
based on the weight of the phosphor powder. Subsequently, the dried 
product was pulverized by softly pressing with a pestle. The resulting 
powder was filled in a frame made of ceramic (size 2 mm.times.2 
mm.times.40 mm) and pressed with a punch made of ceramic at a pressure of 
about 10 kg/cm.sup.2, followed by heating on a hot plate at 120.degree. C. 
for 5 minutes. The resulting phosphor powder (scintillator) layer was 
uniform in quality without voids and had sufficient strength. 
EXAMPLE 3 
A phosphor powder, Gd.sub.2 O.sub.2 S:Pr,Ce,F, in an amount of 20 g was 
sufficiently mixed with 1.8 g of a polystyrene emulsion (NiPol LX-303, 
solid content 45%) and dried in vacuum. The amount of polystyrene was 4% 
based on the weight of the phosphor powder. Subsequently, a scintillator 
layer was obtained in the same manner as described in Example 2. The 
resulting scintillator layer was uniform in quality without voids and had 
sufficient strength. 
EXAMPLE 4 
A phosphor powder, Gd.sub.2 O.sub.2 S:Pr,Ce,F, in an amount of 20 g was 
sufficiently mixed with 8 g of a polystyrene emulsion (Nipol LX-303, solid 
content 45%) and dried in vacuum. The amount of polystyrene was 18% based 
on the weight of the phosphor powder. Subsequently, a scintillator layer 
was obtained in the same manner as described in Example 2. The resulting 
scintillator layer was uniform in quality without voids and had sufficient 
strength. 
EXAMPLE 5 
A phosphor powder, Gd.sub.2 O.sub.2 S:Pr,F,Ce, in an amount of 20 g was 
sufficiently mixed with 6 g of a polyacrylate emulsion (Nipol LX-851, a 
trade name, manufactured by The Japanese Geon Co., Ltd., solid content 
45%) and dried in vacuum. The amount of polyacrylate was 13.5% based on 
the weight of the phosphor. Subsequently, a scintillator layer was 
obtained in the same manner as described in Example 2. The resulting 
scintillator layer was uniform in quality without voids and had sufficient 
strength. 
EXAMPLE 6 
A phosphor powder, Gd.sub.2 O.sub.2 S:Pr,F,Ce, in an amount of 20 g was 
sufficiently mixed with 5.1 g of a polyvinyl chloride emulsion (Geon 151, 
a trade name, manufactured by The Japanese Geon Co., Ltd., solid content 
53%) and dried in vacuum. The amount of polyvinyl chloride was 13.5% based 
on the weight of the phosphor. Subsequently, the dried product was 
pulverized by softly pressing with a pestle. The resulting powder was 
filled in a frame made of ceramic (size 2 mm.times.2 mm.times.40 mm) and 
heated on a hot plate at 200.degree. C. for 5 minutes, while pressing with 
a punch made of ceramic at a pressure of about 10 kg/cm.sup.2. The 
resulting scintillator layer was uniform in quality without voids and had 
sufficient strength.