An X-ray image intensifier includes an envelope having an input window consisting of an aluminum alloy, and an input phosphor screen arranged in the envelope to oppose the input window, and including a substrate, an input phosphor layer formed on the substrate, and a photocathode formed on the input phosphor layer, wherein the aluminum alloy contains 3 to 6 wt % of Mg and 0.01 to 0.5 wt % of Zr. This aluminum alloy may further contain at least one metal element selected from the group consisting of 0.1 to 1 wt % of Mn, 0.01 to 0.5 wt % of Cr, 0.01 to 0.5 wt % of Sc, and 0.01 to 0.05 wt % of Ti.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an X-ray image intensifier and, more 
particularly, to an X-ray image intensifier suitable for low-energy X-ray 
photography. 
2. Description of the Related Art 
An X-ray image intensifier is used to obtain an X-ray transmitted image of 
an object to be examined in an X-ray diagnostic apparatus or the like. The 
material constituting an input window of the X-ray image intensifier is 
naturally required to transmit x-rays well and cause less scattering of 
x-rays. In addition, since the interior of the X-ray image intensifier is 
in a vacuum, the input window is required to have an enough strength to 
withstand this vacuum. The input window is also required to be well 
balanced in cost with other components that constitute the X-ray image 
intensifier. 
Aluminum or titanium has been used as the material of the input window, 
that can meet the above conditions. 
The X-ray image intensifiers are presently, widely used in the fields of, 
e.g., medical diagnoses and industrial applications (e.g., nondestructive 
testing). These X-ray image intensifiers have various excellent features 
that they can convert a moving x-ray image into a visible image in real 
time and can reduce the exposure dose compared to systems using films. 
This extends the range of applications of the X-ray image intensifiers. 
Recently, in the field of medical diagnoses, researchers have investigated 
the use of the X-ray image intensifier in diagnoses of breast cancers. 
Since the diagnosis of a breast cancer is testing on a soft tissue, 
relatively low-energy X-rays, such as those generated at an X-ray 
intensifier voltage of approximately 20 KV to 40 KV, are used. Also, in 
the field of industrial applications (e.g., nondestructive testing), 
researchers have investigated the use of low-energy X-rays in testing on 
products made of paper or resins. 
In applying low-energy X-rays, the transmittance of an input window of the 
X-ray image intensifier with respect to X-rays is of primary concern. The 
X-ray transmittance is determined by the material and the thickness of an 
input window and the energy of X-rays: the higher the energy of X-rays or 
the smaller the thickness of a window, the higher the transmittance. Also, 
an element having a smaller atomic number generally has a higher x-ray 
transmittance. 
FIG. 1 shows the X-ray transmittances of aluminum and titanium used as the 
material of the input window of the X-ray image intensifier. Referring to 
FIG. 1, the X-ray transmittance (%) is plotted on the ordinate, and the 
X-ray energy (KeV) is plotted on the abscissa. A broken curve a indicates 
the X-ray transmittance of aluminum (thickness 1 mm), and a solid curve b 
indicates that of titanium (thickness 250 .mu.m). 
When titanium is to be used as the material of the input window, the 
thickness of titanium must be decreased to about 250 .mu.m in order to 
obtain a practically sufficient X-ray transmittance. An input window 
consisting of a material with such a small thickness in a finished X-ray 
image intensifier assumes a recessed outer appearance owing to a 
differential pressure with respect to the atmospheric pressure, because 
the interior of the tube is in a vacuum. To assemble this titanium input 
window into the X-ray image intensifier, the input window is joined with a 
stainless-steel ring by spot welding. This stainless-steel ring is then 
welded to an Fe--Ni--Co alloy ring which is in turn joined with a glass 
container as a part of an envelope. 
When aluminum is used as the material of the input window, on the other 
hand, the input window is welded to a stainless-steel ring at a 
predetermined temperature and under a predetermined pressure. Since, 
however, the mechanical strength of aluminum is smaller than that of 
titanium, the thickness of aluminum is set to about 1 mm. In addition, in 
order to withstand the atmospheric pressure, the outer surface of the 
input window is projected. 
As shown in FIG. 1, the transmittance of aluminum is higher than that of 
titanium for x-rays with a low energy of 20 KeV to 40 KeV. Therefore, when 
the energy of X-rays is low, aluminum is a more suitable material of the 
input window than titanium. Aluminum materials are classified into various 
types in accordance with the types and the amounts of additive substances 
contained in them and the conditions of treatments. These materials are 
also different in mechanical and thermal characteristics. According to 
literature such as "Aluminum HandBook," the aluminum materials are 
classified as follows: (A.A. shows a grade determined by the Aluminum 
Association, Inc.) 
A.A.#1000 type: Pure aluminum. Aluminum with a purity of 99% or more. 
Processability, corrosion resistance, and weldability are high, but 
strength is low. 
A.A.#2000 type: Al--Cu alloy. Duralumin. Strength is high, but corrosion 
resistance is poor. 
A.A.#3000 type: Al--Mn alloy. Strength is slightly increased by adding Mn 
to the #1000 type. 
A.A.#4000 type: Al--Si alloy. Abrasion resistance and heat resistance are 
improved by addition of Si. 
A.A.#5000 type: Al--Mg alloy. Strength is high. 
A.A.#6000 type: Al--Mg--Si alloy. Both strength and corrosion resistance 
are high. 
A.A.#7000 type: Al--Zn--Mg alloy. Strength is highest of all aluminum 
alloys, but formability is poor. 
Conditions required for the material constituting an input window of an 
X-ray image intensifier are as follows: 
(a) Having an enough strength to withstand the atmospheric pressure. 
(b) Having an enough strength to withstand the atmospheric pressure not 
only at room temperature but at the baking temperature (200.degree. C. to 
400.degree. C.) in an exhaust step as one of the steps of manufacturing 
X-ray image intensifiers. 
(c) Having a sufficient corrosion resistance. 
(d) Having a high formability in order to form the input window into a 
projecting shape. 
Aluminum materials meeting these conditions are those of the A.A.#5000 type 
and the A.A.#6000 type, and these materials are actually used. 
As described above, the input window is joined to the stainless-steel ring 
in the process of manufacturing the X-ray image intensifier. In this case, 
the aluminum material constituting the input window and the 
stainless-steel ring are joined together at a temperature of 400.degree. 
C. or more and under a predetermined pressure. This joining is performed 
by diffusing molecules of the aluminum material and the stainless steel 
into each other at a high temperature and a high pressure. Since the 
temperature of the aluminum material rises, a certain change occurs inside 
the aluminum material. As an example, in the case of the A.A.#6000 type 
aluminum material, an Mg.sub.2 Si precipitate forms while the temperature 
falls from the high temperature in the joining, and in the step of baking 
at about 250.degree. C. This state will be described with reference to 
FIGS. 2A and 2B. 
Generally, when aluminum is kept at a high temperature, Al crystal grains 
grow into coarse grains 21 as shown in FIG. 2A, and, in the middle of 
cooling from the high temperature, Mg.sub.2 Si phases 22 precipitate in 
grain boundaries as shown in FIG. 2B (especially in the A.A.#6000 type). 
This precipitate 22 is different in X-ray transmittance from aluminum. 
This difference in X-ray transmittance between the precipitate and 
aluminum is negligibly small when the energy of X-rays is 50 KV or more, 
if, however, the X-ray energy becomes 30 KV or less, the difference in 
X-ray transmittance between the two increases. Consequently, even when 
uniform X-rays are incident on the input window, the amount of transmitted 
X-rays changes in accordance with the presence/absence of the precipitate. 
When, therefore, an aluminum material containing the precipitates is 
directly formed into an input window, the presence of the precipitates 
gives rise to unevenness corresponding to the distribution of the 
precipitates in a visible-light image produced by the X-ray image 
intensifier. In addition, in the A.A.#5000 type aluminum material, 
recrystallization of aluminum occurs at the high temperature to produce 
coarse crystal grains of aluminum inside the aluminum material. The coarse 
crystal grain of aluminum is different in crystal orientation from the 
surrounding aluminum. Therefore, X-ray diffraction conditions vary in 
accordance with the incident direction of X-rays, and this produces a 
difference in X-ray transmittance between the two types of aluminum. Also 
in this case, when the energy of X-ray becomes low, a large difference 
arises in X-ray transmittance between a portion containing the coarse 
crystal grains and a portion not containing them, as in the A.A.#6000 type 
aluminum. 
Assuming that the thickness of aluminum is t.sub.1, an X-ray transmittance 
T.sub.1 is represented by: 
EQU T.sub.1 =exp(-.mu.t.sub.1) 
Likewise, assuming that the thickness of aluminum is t.sub.2, an X-ray 
transmittance T.sub.2 is given by: 
EQU T.sub.2 =exp(-.mu.t.sub.2) 
The ratio of one transmittance to the other is, therefore, T.sub.1 /T.sub.2 
=exp[.mu.(t.sub.2 -t.sub.1)]. 
In the above relation, .mu. is the transmittance coefficient corresponding 
to the energy of X-rays. 
Note that if t.sub.2 &gt;t1, the value of .mu.(t.sub.2 -t.sub.1) is positive, 
and so T.sub.1 /T.sub.2 &gt;1. In addition, since .mu. increases as the X-ray 
energy decreases, T.sub.1 /T.sub.2 increases when the X-ray energy 
decreases. That is, the lower the energy of X-rays, the larger the 
transmittance difference (ratio). It is assumed that this relationship 
produces unevenness in a visible-light image produced by the X-ray image 
intensifier. 
Recently, image processing apparatuses are widely used, and even a slight 
difference in X-ray transmittance is emphasized in these apparatuses. 
Hence, there is a high possibility that unevenness in a produced image, 
which is supposed to result from precipitates or coarse crystal grains as 
described above, becomes a more serious obstacle in the future. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an X-ray image 
intensifier capable of obtaining high-quality images free from unevenness 
even if low-energy X-rays are used. 
According to the present invention, there is provided an X-ray image 
intensifier comprising an envelope having an input window consisting 
essentially of an aluminum alloy, and an input phosphor screen arranged in 
the envelope to oppose the input window, and including a substrate, an 
input phosphor layer formed on the substrate, and a photocathode formed on 
the input phosphor layer, wherein the aluminum alloy contains 3 to 6 wt % 
of Mg and 0.01 to 0.5 wt % of Zr. 
In addition, according to the present invention, there is provided an X-ray 
image intensifier comprising an envelope having an input window consisting 
essentially of an aluminum alloy, and an input phosphor screen arranged in 
the envelope to oppose the input window, and including a substrate, an 
input phosphor layer formed on the substrate, and a photocathode formed on 
the input phosphor layer, wherein the aluminum alloy contains 3 to 6 wt % 
of Mg, 0.01 to 0.5 wt % of Zr, 0.1 to 0.4 wt % of Fe, 0.05 to 0.2 wt % of 
Si, and 0.1 to 1 wt % of Mn. 
Furthermore, according to the present invention, there is provided an X-ray 
image intensifier comprising an envelope having an input window consisting 
essentially of an aluminum alloy, and an input phosphor screen arranged in 
the envelope to oppose the input window, and including a substrate, an 
input phosphor layer formed on the substrate, and a photocathode formed on 
the input phosphor layer, wherein the aluminum alloy contains 3 to 6 wt % 
of Mg, 0.01 to 0.5 wt % of Zr, 0.1 to 0.4 wt % of Fe, 0.05 to 0.2 wt % of 
Si, and 0.1 to 1 wt % of Mn and at least one type of metal elements 
.selected from the group consisting of 0.01 to 0.5 wt % of Cr, 0.01 to 0.5 
wt % of Sc, and 0.01 to 0.05 wt % of Ti, contains five or more grains of 
intermetallic compound with an average diameter of 0.01 to 0.05 .mu.m per 
.mu.m.sup.3, and has material strength of 120 MPa or more at 250.degree. 
C. or less, and the crystal grain size is 30 .mu.m or less when a heat 
treatment is performed at 530.degree. C. for less than one hour. 
Furthermore, there is provided aluminum alloy having substantially uniform 
X-ray transmittance, and comprising 3 to 6 wt % of Mg, 0.01 to 0.5 wt % of 
Zr, at most 0.4 wt % of Fe, and at most 0.2 wt % of Si. 
Furthermore, there is provided a method of manufacturing aluminum alloy 
having substantially uniform X-ray transmittance, which comprises the 
steps of: 
nomogenizing an aluminum cast comprising 3 to 6 wt % of Mg, 0.01 to 0.5 wt 
% of Zr, at most 0.4 wt % of Fe, and at most 0.2 wt % of Si, at a 
temperature of 400.degree. to 530.degree. C. for 1 to 20 hours; 
hot rolling the aluminum alloy cast to form a aluminum alloy plate; 
thermal-treating the hot-rolled aluminum alloy plate for recrystallization; 
and 
cold rolling the thermal-treated aluminum alloy plate at a reduction ratio 
of 10% or more. 
The cold-rolled aluminum alloy plate may be thermal-treated for recovering 
an elongation. 
Additional objects and advantages of the invention will be set forth in the 
description which follows, and in part will be obvious from the 
description, or may be learned by practice of the invention. The objects 
and advantages of the invention may be realized and obtained by means of 
the instrumentalities and combinations particularly pointed out in the 
appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In an X-ray image intensifier of the present invention, an aluminum alloy 
containing 3 to 6 wt %, preferably 4 to 5 wt % of Mg and 0.01 to 0.5 wt %, 
preferably 0.05 to 0.2 wt % of Zr is used as the material of an input 
window. 
Mg is the most basic additive element of the aluminum alloy used in the 
present invention and serves to increase the strength. If the addition 
amount of Mg is less than 3 wt %, the material strength at 250.degree. C. 
becomes 120 MPa or less; i.e., strength required to decrease the thickness 
of the input window to 0.8 mm or less cannot be obtained. If the addition 
amount of Mg exceeds 6 wt %, the aluminum alloy becomes liable to crack 
during hot rolling. This makes industrial application of the aluminum 
alloy difficult. 
Zr precipitates uniformly and finely as Al.sub.3 Zr phases during 
homogenization of the aluminum alloy to prevent crystal grains from 
coarsening while they are held at a high temperature during diffusion 
joining. If the addition amount of Zr is less than 0.01 wt %, Zr cannot 
function in this way; if the addition amount of Zr exceeds 0.5 wt %, a 
coarse Al.sub.3 Zr-phase compound becomes easier to form, producing a 
density unevenness in a visible-light image obtained by the X-ray image 
intensifier. This density unevenness may be mistaken for defects in an 
object to be examined. 
The aluminum alloy used in the present invention may further contain one or 
more of Mn, Cr, Sc, and Ti as additive components. 
Mn serves to increase the strength as does Mg. In addition, similar to Zr, 
Mn has an effect of suppressing coarsening of crystal grains while they 
are held at a high temperature during diffusion joining. Using Mn in 
combination with Zr can further suppress coarsening of crystal grains than 
when Zr alone is used. The addition amount of Mn is 0.1 to 1.0 wt %, and 
preferably 0.3 to 0.6 wt %. If the addition amount is less than 0.1 wt %, 
Mn cannot achieve its function; if the addition amount exceeds 1.0 wt %, 
the aluminum alloy becomes liable to crack during hot rolling. This makes 
industrial application of the aluminum alloy difficult. Furthermore, if 
the addition amount exceeds 1.0 wt %, Mn bonds to Fe as an impurity during 
preparation of a bulk material to allow easy formation of a coarse 
Al--Fe--Mn compound. The result is a density unevenness in a visible-light 
image obtained by the X-ray image intensifier. This density unevenness may 
be mistaken for defects in an object to be examined. 
Similar to Zr, Cr precipitates uniformly and finely as Al.sub.7 Cr phases 
during homogenization of the aluminum alloy to prevent crystal grains from 
coarsening when they are kept at a high temperature during diffusion 
joining. Note that the Al.sub.7 Cr phase is easier to coarsen than the 
Al.sub.3 Zr phase. The addition amount of Cr is 0.01 to 0.5 wt %, and 
preferably 0.05 to 0.2 wt %. If the addition amount is less than 0.01 wt 
%, Cr cannot achieve its effect; if the addition amount exceeds 0.5 wt %, 
a coarse Al.sub.7 Cr-phase compound becomes easier to form, producing a 
density unevenness in a visible-light image obtained by the X-ray image 
intensifier. This density unevenness may be mistaken for defects in an 
object to be examined. 
Sc, similar to Zr and Cr, precipitates uniformly and finely as Al.sub.3 Sc 
phases during homogenization of the aluminum alloy to suppress coarsening 
of crystal grains when they are kept at a high temperature during 
diffusion joining. The addition amount of Sc is 0.01 to 0.5 wt %, and 
preferably 0.05 to 0.2 wt %. If the addition amount is less than 0.01 wt 
%, Sc cannot achieve its function; if the addition amount exceeds 0.5 wt 
%, a coarse Al.sub.3 Sc-phase compound becomes easier to form, producing a 
density unevenness in a visible-light image obtained by the X-ray image 
intensifier. This density unevenness may be mistaken for defects in an 
object to be examined. In addition, since Sc is an expensive material, the 
manufacturing cost is increased if the addition amount of Sc exceeds 0.5 
wt %. 
Ti has an effect of forming fine crystal grains in a cast mass texture. Ti 
also has an effect of uniformizing the sizes of crystal grains during hot 
rolling. The addition amount of Ti is 0.01 to 0.05 wt %, and preferably 
0.01 to 0.03 wt %. If the addition amount is less than 0.01 wt %, Ti 
cannot achieve its effects; if the addition amount exceeds 0.05 wt %, a 
coarse Al.sub.3 Ti-phase compound becomes easier to form, producing a 
density unevenness in a visible-light image obtained by the X-ray image 
intensifier. This density unevenness may be mistaken for defects in an 
object to be examined. 
The aluminum alloy for use in the present invention may contain 0.1 to 0.4 
wt % of Fe and 0.05 to 0.2 wt % of Si. Fe and Si are normally impurities 
contained in an aluminum base metal. If the content of Fe exceeds 0.4 wt 
%, a coarse Al--Fe--Mn compound undesirably becomes easier to form. 
Although the amount of Fe is preferably decreased, the purity of an Al 
base metal must be improved to do so, resulting in an increase in 
manufacturing cost. For this reason, an amount of 0.1 wt % is the lower 
limit for the Fe content. 
If the amount of Si exceeds 0.2 wt %, Mg.sub.2 Si phases become liable to 
precipitate nonuniformly, and this produces a density unevenness in a 
visible-light image obtained by the X-ray image intensifier. A smaller 
amount of Si is better as in the case of Fe, but the purity of an Al base 
metal must be improved for this purpose. Since this increases the 
manufacturing cost, an amount of 0.1 wt % is the lower limit for the Si 
content. 
The aluminum alloy constituting the input window of the X-ray image 
intensifier of the present invention is normally subjected to 
homogenization. The homogenization is performed to control the dispersion 
state of transition-element intermetallic compounds (Al.sub.3 Zr, Al.sub.7 
Cr, and Al.sub.3 Sc). 
The temperature of the homogenization process is preferably 400.degree. to 
530.degree. C. If the homogenization temperature is less than 400.degree. 
C., the precipitation rates of the above intermetallic compounds decrease 
significantly. This prolongs time required for the compounds to reach the 
target dispersion state, resulting in an industrial disadvantage. If the 
homogenization temperature exceeds 530.degree. C., the intermetallic 
compounds become easier to coarsen, and this impairs the effect of 
suppressing coarsening of crystal grains. 
In addition, if the Mg concentration is high, eutectic melting occurs 
during the homogenization, and consequently the aluminum alloy undesirably 
becomes liable to crack during hot rolling. 
When the homogenization temperature falls within the range from 400.degree. 
to 530.degree. C., the homogenization time is preferably one to 20 hours. 
If the homogenization time is less than one hour, the transition-element 
intermetallic compounds cannot be set in a desired dispersion state. This 
makes it difficult to suppress coarsening of crystal grains during 
diffusion joining. A homogenization time exceeding 20 hours is 
unpreferable because not only it is industrially disadvantageous but it 
leads to coarsening of the above intermetallic compounds. 
The dispersion state of the transition-element intermetallic compounds can 
be controlled by the conditions of the homogenization. To suppress 
coarsening of crystal grains during diffusion joining (a heat treatment at 
530.degree. C. or less for less than one hour), five or more compound 
particle of a size of 0.01 to 0.05 .mu.m need only exist per 1 
.mu.m.sup.3. If the number is less than five, the size of Al crystal 
grains becomes 100 .mu.m or more during diffusion joining, producing a 
density unevenness in a visible-light image obtained by the X-ray image 
intensifier. 
If crystal grains exceeding 100 .mu.m in diameter are present in the 
aluminum alloy used in the present invention, a density unevenness is 
undesirably produced in a visible-light image obtained by the X-ray image 
intensifier. No such problem arise if the crystal grain size is 30 .mu.m 
or less. 
An X-ray diagnostic apparatus using the X-ray image intensifier of the 
present invention will be described below with reference to FIG. 3. 
Referring to FIG. 3, X-rays 2 radiated from an X-ray image intensifier 1 
are transmitted through an object 3 to be examined. The X-rays 2 
transmitted through the object 3 are incident on an input phosphor screen 
through an input window 4 of an envelope constituting the X-ray image 
intensifier. Note that the input 10 window 4 of the X-ray image 
intensifier is made of a metal, and the input phosphor layer is on an 
input substrate 5, the input phosphor layer 6 converts X-rays into light 
and a photocathode 7 converts light into electrons. 
Electrons radiated from the photocathode 7 are accelerated and converged by 
electron lenses consisting of electrodes 8, reaching an output phosphor 
layer 9. The output phosphor layer 9 converts the electrons into visible 
light. This visible light is photographed by using a film or a TV camera 
to perform an X-ray diagnosis. 
FIG. 4 shows a portion of the input window of the X-ray image intensifier 
according to the present invention. Reference numeral 10 denotes the input 
window of the X-ray image intensifier, which is joined to a 
stainless-steel ring 11. 
The material of the input window 10 is an aluminum alloy produced by adding 
0.15 wt % of chromium and 0.15 wt % of zirconium, both in weight ratio, to 
an Al--Mg alloy of the A.A.#5000 group. An aluminum material consisting of 
this aluminum alloy is stretched by about 20% into a plate beforehand and 
projected as shown in FIG. 4. 
Since the aluminum material contains 0.15 wt % of chromium and 0.15 wt % of 
zirconium, precipitates of these contents are dispersed in the material. 
When the precipitates of the contents are exposed to a high temperature of 
400.degree. to 500.degree. C. in order to join the input window consisting 
of this aluminum material to the stainless-steel ring, the precipitates 
function to suppress coarsening of aluminum crystals formed inside the 
aluminum material. Therefore, no nonuniformity in X-ray transmittance 
arises in the input window consisting of the aluminum material in which 
growth of coarse aluminum crystal grains is suppressed. 
This state will be described with reference to FIG. 2B. When Zr or the like 
is added to an Al--Mg alloy, coarsening of Al crystal grains 21 is 
suppressed by fine Al.sub.3 Zr precipitates 23 even if the alloy is held 
at a high temperature. This effect prevents easy occurrence of unevenness. 
FIG. 5 is a graph showing the correlation among the growing tendency of 
coarse crystals (Al crystal grains) plotted on the ordinate (left), the 
growing tendency of coase cryatals (intermetallic compound) on the 
ordinate (right), and the content (wt %) of Zr on the abscissa. As can be 
seen from this graph, the growing tendency of Al crystals increases at a 
welding when the content of Zr is less than 0.01 wt %. 
On the other hand, when the content of Zr exceeds 0.5 wt %, a coarse 
Al.sub.3 Zr phase undensirably becomes easier to form. 
In addition, in the step of processing the aluminum material into the input 
window of the X-ray image intensifier, pressing and drawing are performed 
to form the aluminum material into a projecting shape in order to obtain a 
structure that can withstand the atmospheric pressure. In this processing, 
the aluminum material is stretched by about 20%. In this case, the growing 
tendency of coarse crystal grains at a high temperature changes in 
accordance with the degree of stretch of the aluminum material: when the 
aluminum material is stretched, internal strain accumulates in the 
material, and the density of accumulation of this internal strain 
increases as the degree of stretch increases. 
A sufficiently low strain has no large effect on growth of coarse crystal 
grains inside aluminum. If, however, the strain increases, this 
accelerates growth of coarse crystal grains as is well known to those 
skilled in the art. The growth of coarse crystal grains is maximized when 
the degree of stretch is 20 to 30% and decreases as the degree increases. 
Therefore, when the aluminum material is formed into a projecting shape in 
order to constitute the input window of the X-ray image intensifier, the 
material is stretched by approximately 20%, accelerating growth of coarse 
crystal grains. 
In the present invention, therefore, an aluminum material already stretched 
by about 20% is processed into a projecting shape. Consequently, since the 
aluminum material is stretched by a total of about 40% before and after 
the processing, formation of coarse crystal grains can be suppressed. 
EXAMPLE 
A cast mass of an aluminum alloy having the composition to be presented 
later in a table was homogenized at 480.degree. C. for five hours and 
subjected to hot rolling at a temperature from 300.degree. to 480.degree. 
C., forming a plate 2 mm thick. Subsequently, cold rolling was performed 
for this plate until the thickness became 0.75 mm, and an intermediate 
heat treatment was performed for the resultant plate at 350.degree. C. for 
two hours. Final cold rolling was then performed to form a plate 0.6 mm 
thick. Thereafter, in order to improve the ductility (tensile strength) of 
the plate, a heat treatment was performed at 150.degree. C. for four 
hours, yielding a final plate. 
This final plate was observed by using a transmission electron microscope, 
and an image analysis was performed to examine the dispersion state of 
transition-element intermetallic compounds 0.01 to 0.05 .mu.m in diameter. 
In addition, the tensile strength of the final plate was checked at 
250.degree. C., and the crystal grain size after diffusion joining was 
also checked. Subsequently, an input window of an X-ray image intensifier 
was formed from this plate, and an X-ray image intensifier was 
manufactured by assembling the input window into it. The quality of a 
visible-light image obtained by this X-ray image intensifier was also 
evaluated. The method of evaluating the image quality was as follows. 
The image quality evaluation was performed by using a system shown in FIG. 
6. X-rays emitted from an X-ray tube 31 are guided into an X-ray image 
intensifier 32 and converted into a visible-light image in it. This 
visible-light image is imaged by a TV camera 34 through a tandem optical 
system 33. An image signal from the TV camera 34 is processed by a digital 
image processor 35 and displayed on a TV monitor 36. Although several 
different types of digital image processing were used, two of them are 
explained herein. 
(1) Averaging 
A plurality of images are successively input and averaged in units of 
pixels. When TV signals of still images are averaged in this fashion, 
random noise components that the signals carry cancel one another out, 
resulting in images with less noise. 
(2) Image emphasis 
A certain portion of an image having gradation is enlarged and displayed on 
the TV monitor. This makes it possible to visually check small changes in 
gradation. 
By performing the above image processing, smaller gradation changes can be 
detected without being covered with noise. When observation was performed 
in accordance with the above method by using a conventional X-ray image 
intensifier using X-rays with an energy of 50 KeV, no "unevenness" was 
observed even after the image processing was performed. When X-rays with 
an energy of 20 KeV were used, on the other hand, "unevenness" was 
observed after the image processing. FIG. 7 shows the "unevenness" 
appearing on the TV monitor. 
In addition, when the brightness of a scan line running in a central 
portion of the TV monitor is plotted, the consequent curve is nonuniform 
as shown in FIG. 8. 
The test results are summarized in the table below. 
TABLE 
__________________________________________________________________________ 
Dispersion 
state of Crystal 
transition- 
grain 
Alloy components 
element size Strength 
Quality 
(wt %) compounds (average: 
at 250.degree. C. 
of X-ray 
No. 
Mg Zr Mn Cr (number/.mu.m.sup.3) 
.mu.m) 
(MPa) image 
Remarks 
__________________________________________________________________________ 
1. 3.0 
0.15 
-- -- 15 20 120 o Present 
2. 5.0 
0.05 
0.4 
0.01 
8 30 170 o invention 
3. 3.5 
0.20 
0.2 
-- 20 20 127 o 
4. 5.7 
0.10 
-- 0.3 
18 20 150 o 
5. 5.0 
-- 0.4 
-- 0 2000 170 x Comparative 
6. 5.0 
0.007 
0.4 
0.25 
3 1000 170 x example 
7. 2.2 
0.15 
0.8 
-- 15 30 110 -- 
8. 5.0 
0.6 
-- -- 39 15 170 x 
*Al.sub.3 Zr compounds 
of size exceeding 
100 .mu.m were found 
__________________________________________________________________________ 
The above table reveals the following. That is, each of sample Nos. 1 to 4 
had a crystal grain size of 30 .mu.m or less after diffusion joining, and 
the quality of each resulting visible-light image obtained by the X-ray 
image intensifier was high. In addition, each sample had a material 
strength of 120 MPa or more and hence could be formed into a thin plate. 
In contrast, since sample No. 5 was not added with Zr, crystal grains grew 
into coarse grains after diffusion joining to impair the consequent image 
quality. Sample No. 6 was added with only a small amount of Zr, and so the 
same problem as with sample No. 5 arose. Since the addition amount of Mg 
was small in sample No. 7, its material strength at 250.degree. C. was 
low, and consequently the input window deformed upon evacuation of the 
X-ray image intensifier. Also, since the addition amount of Zr was too 
large in sample No. 8, Al.sub.3 Zr compounds of a size of 100 .mu.m or 
more were formed. As a result a density unevenness was found in a 
visible-light image obtained by the X-ray image intensifier. 
According to the present invention as has been described above, it is 
possible to realize an X-ray image intensifier capable of forming 
high-quality images free from unevenness even with low-energy X-rays.