Technical field
The present invention relates to an X-ray image
intensifier.
Background Art
Recently, an X-ray image intensifier has been
widely applied to medical diagnosis, non-destructive
examinations and the like, in which an X-ray image
obtained by a low-energy X-ray having an X-ray tube
voltage of 30 kV (a tube current of 1 mA) or less, or
by a high-energy X-ray having an X-ray tube voltage of
30 kV (a tube current of 1 mA) or more is converted to
a visible light image.
As shown in FIGS. 1 and 2, a convention (cf eg DE-A-21 37 392) X-ray
image intensifier is basically constituted of an input
screen 12, a focusing electrode 13, an anode 14, and an
output screen 15, all arranged in a vacuum envelope 11
(hereinafter referred to as an "envelope") in the order
mentioned from an X-ray source A. The envelope 11 has
an input window 11a made of metal, on which an X-ray is
incident, a body 11b made of glass for supporting the
focusing electrode, and an output portion 11c made of
optical glass serving as the output screen 15 or as a
support for the output screen 15.
The input screen 12 provided at a predetermined
distance from the input window 11a, functions as an
cathode. The input screen 12 is consisted of a curved
substrate 12a, for example, an aluminum metal substrate,
which is convex formed so as to project toward the
X-ray source A; a phosphor layer 12b for converting an
X-ray to a visible light, formed on the concave of the
metal substrate 12a; a transparent conductive film 12c
formed on the phosphor layer 12b; and a photocathode 12d
for converting the visible light from the phosphor layer
12b to electrons, formed on the transparent conductive
film 12c. The transparent conductive film 12c is
generally made of indium oxide, ITO (a compound made of
indium oxide and titanium oxide) or the like. The
transparent conductive film 12c is used for preventing
the reaction between an alkali halide such as sodium
iodide activated cesium iodide constituting the phosphor
layer 12 and a material constituting the photocathode
12d and for providing continuous conductivity on the
surface of the phosphor layer.
On the other hand, an anode 14 is disposed in the
opposed side to the input screen 12, namely, in the side
in which the output screen 15 disposed (the outer face
herein is constituted by a structure such that the optical
glass substrate supporting output phosphors serves
as part of the envelope). The anode 14 is supported by
the side in which an envelope output portion 11c is
formed. Between the anode 14 and the input screen 12
used as the cathode, a first focusing electrode 13a is
provided along the inner wall of the envelope body 11b.
Between the focusing electrode 13a and the output screen
15, a pipe-shape second focusing electrode 13b is provided.
The first and the second focusing electrodes 13a
and 13b constitute an electrostatic electron lens
system.
In the X-ray image intensifier thus constituted, an
X-ray B radiated from the X-ray source A is transmitted
through an object C, reaching the input window 11a. The
X-ray image reflected on the input window 11a is converted
to an electron image formed on the input face, as
will be described later. The electron image is accelerated
and focused through the electrostatic electron
lens system constituted of the first focusing electrode
13a and the second focusing electrode 13b. A tube
voltage, which is applied between the input screen 12 as
the cathode and the anode 14, e.g., 30 kV of a tube
voltage, is divided into two voltages and these voltages
are applied to the electrodes, 13a, 13b, respectively.
Thereafter, the electron image is converted back into
a visible light on the output screen 15. In this way,
a visible image can be intensified, for example, 1000
times or more, in proportional to the intensity of the
visible light entered the input screen 12.
As shown in an enlarged view of FIG. 2, the input
screen of the above-mentioned conventional X-ray image
intensifier has a problem in that the X-ray is more
scattered, lowering image contrast since the input
window 11a and the input screen 12 are separated at a
predetermined distance from each other. Hereinbelow, this
problem will be explained by way of example of an X-ray
image intensifier having an effective input-screen
diameter of 4 inches, with reference to FIG. 3.
To obtain data shown in FIG. 3, 50KV of a tube
voltage and 1 mA of a tube current were applied to the
X-ray tube. A contrast (%) and a contrast ratio of the
X-ray image intensifier are plotted on a vertical axis
and a diameter (mm) of a lead circular plate is plotted
on the horizontal axis. The contrast herein is indicated
in percentage of brightness in the effective input
visual field when a lead plate having a predetermined
diameter is positioned at the center of the effective
input visual field, based on the brightness in the
effective input visual field of the X-ray image
intensifier when no lead plate is positioned. The
contrast ratio is numerically calculated from the
contrast values (%).
A curve c of FIG. 3 shows the characteristics of
the X-ray image intensifier having the conventional
structure shown in FIG. 2. As is apparent from the
curve c, as the diameter of the lead circular plate used
in measuring contrast becomes smaller than 40 mm, the
image contrast significantly reduces. This fact implies
that the contrast of a small object image is significantly
inferior to that of a large object. From the
industrial point of view, this fact leads to a drawback
in that it is difficult to find defects of fine portions.
FIG. 4 shows the contrast data obtained from an
experiment conducted in the same manner as above except
that a tube voltage of the X-ray tube is changed to
30 kV, using the same X-ray intensifier. According to
the straight line e of FIG. 4, in the same fashion as in
the curve c of FIG. 3, as the diameter (mm) of the lead
circular plate becomes smaller than 40 mm, the contrast
significantly reduces. However, the degree of the image
contrast reduction in this case is larger than in the
case of FIG. 3.
On the other hand, Jpn. UM Appln. KOKOKU
Publication No. 34-20832 and some other publications (cf eg FR-A-1 557 119)
disclose an X-ray image intensifier comprising an input
screen directly formed on the inner surface of an aluminum
input-window. Nonetheless, such an X-ray image
intensifier comprising an input screen directly formed
on an inner surface of an input window made of aluminum
has not yet been put into practical use. If an X-ray
image intensifier comprising the input window made of
such a thin material is fabricated and then evacuated,
the input window will be distorted by the pressure difference
between the inside and the outside of the tube.
As a consequence, the input screen will be distorted.
Hence, a desired photocathode cannot be obtained and the
output image is distorted.
Disclosure of Invention
The object of the present invention is to provide
an X-ray image intensifier which overcomes the aforementioned
drawbacks, maintains high brightness of an image,
and provides high image contrast.
According to the present invention, there is provided
an X-ray image intensifier which comprises
a vacuum envelope having an X-ray input window
; an input screen including a phosphor layer,
said input screen being directly formed on an inner
surface of said X-ray input window,
and a photocathode formed on the phosphor layer;
a focusing electrode ; an anode and an
output screen ; said focusing electrode, said
anode and said output screen being arranged in turn
in said vacuum envelope along traveling direction of
electrons generated from said input screen, and which
is characterized in that
said X-ray input window is formed of one of
aluminum and aluminum alloy and has a rough, surface-hardened
layer with a Vickers hardness of 120 to 250 on a side
on which said input screen is formed.
In the X-ray image intensifier of the present
invention, a material possibly used as the metal X-ray
input window, is a substance, for example, aluminum or
an aluminum alloy, which has a high X-ray transmissivity,
good workability, and sufficient strength enough
to tolerate the pressure difference between the outside
and the inside of the X-ray image intensifier, due to
the surface hardening.
The rough, surface-hardened layer of the metal
X-ray input window can be formed by applying the
surface-hardening to a metal plate constituting the
metal X-ray input window. The treatment for forming the
rough, surface-hardened layer can be performed as
follows:
Hard spherical particles such as glass beads having
a particle diameter of 50 to 200 µm are impinged onto
the metal plate at a pressure of 98,1 to 392,4 kPa (1 to 4 kg/cm2) for a
processing time of 1 to 5 minutes, thereby completing
the surface hardening. As a result, the surface of the
metal plate becomes rough, providing a surface-hardened
layer with a rough surface.
The X-ray image intensifier of the present
invention is effective particularly in the case where
a low-energy X-ray having a X-ray tube voltage of 30 kV
(1 mA in a tube current) or less is used.
Brief description of Drawings
FIG. 1 is a schematic view of a conventional X-ray
image intensifier used for explaining an X-ray
photography;
FIG. 2 is a sectional view of part of the conventional
X-ray image intensifier shown in FIG. 1;
FIG. 3 is a graph showing image contrast
characteristics obtained under the application of a
high-energy X-ray in the embodiment of the X-ray image
intensifier according to the present invention and
a conventional X-ray image intensifier;
FIG. 4 is a graph showing contrast characteristics
obtained under the application of a low-energy X-ray in
an embodiment of the X-ray image intensifier according
to the present invention and a conventional X-ray image
intensifier;
FIG. 5 is an partially enlarged sectional view of a
main portion of the X-ray image intensifier according to
an embodiment of the present invention;
FIG. 6 is a partially enlarged sectional view of
the portion shown in FIG. 5; and
FIG. 7 is a graph showing the relationship between
the surface roughness of an Al plate obtained by the
surface hardening and the hardness of the surface-hardened
layer.
Best Mode of Carrying Out the Invention
The X-ray image intensifier of the present
invention has the same constitution as that of the
conventional X-ray image intensifier shown in FIG. 1,
except that an input screen is formed directly on the
inner surface of an input window and the surface-hardening
is applied onto the inner surface of the
input window.
To be more specific, as shown in FIG. 1, the X-ray
image intensifier of the present invention is basically
constituted of the input screen 12, the electrode 13,
the anode 14, the output screen 15, all disposed in the
vacuum envelope 11 in the order mentioned from the X-ray
source A. The envelope 11 is further constituted of the
metal input window 11a, to which an X-ray is radiated,
the glass body 11b supporting the focusing electrode,
and the glass output portion 11c serving as the output
screen 15 or a support for the output screen 15.
As shown in FIG. 5, the surface-hardened layer 11d
having a rough surface obtained by the surface hardening
is formed on the inner surface of the input window 11a.
An aluminum alloy, in particular, an ASTM 5000 series
Al-Mg alloy is used as the material of the input window
11a. Such an aluminum alloy plate is press-molded into
a dish shape and the aforementioned surface-hardening is
applied thereto, thereby obtaining the input window 11a.
The input screen 12 is formed directly onto the
rough-surface of the surface-hardened layer 11d. The
input screen 12 is constituted of the optically reflective
substance layer 12a formed on the rough-surface of
the surface-hardened layer 11d; the phosphor layer 12b
for converting an X-ray to a visible light, formed on
the layer 12a; the transparent conductive film 12c
formed on the phosphor layer 12b; and the photocathode
12d for converting the visible light from the phosphor
layer 12b into electrons, formed on the transparent conductive
film 12c. The transparent conductive film 12c
is generally made of indium oxide, ITO (a compound made
of indium oxide and titanium oxide) or the like. The
transparent conductive film 12c is used for preventing
the reaction between an alkali halide such as sodium
iodide activated cesium iodide constituting the phosphor
layer 12 and a material forming photocathode 12d, and
for providing continuous conductivity on the surface of
the phosphor layer.
On the other hand, an anode 14 is disposed in the
opposed side to the input screen 12, namely, in the side
in which the output screen 15 disposed (the output
screen is constituted by the structure such that an
optical glass substrate supporting output phosphors serves
as part of the envelope). The anode 14 is supported
by the side in which an envelope output portion 11c is
placed. Between the anode 14 and the input screen 12
serving as the cathode, a first focusing electrode 13a
is provided along the inner wall of the envelope body
11b. Between the focusing electrode 13a and the output
screen 15, a pipe-shape second focusing electrode 13b is
provided. The first and the second focusing electrodes
13a and 13b constitute an electrostatic electron lens
system in the same fashion as in the structure of the
X-ray image intensifier shown in FIG. 1.
As described above, according to the X-ray image
intensifier of the present invention, the rough,
surface-hardened layer 11d is formed on the inner
surface of the input window 11a, and the input screen 12
is formed directly on the rough, surface-hardened layer.
The Vickers hardness of the rough, surface-hardened
layer 11d is preferably in a range of 120 to 250 as
described above. If the Vickers hardness is less than
120, since the layer 11d is not strong enough to
tolerate the pressure difference between the inside and
the outside of the X-ray image intensifier, the X-ray
input window will be distorted. In contrast, if the
Vickers hardness exceeds 250, the moldability of the
layer lid will unfavorably deteriorate.
The surface roughness of the rough, surface-hardened
layer 11d is preferably in a range of 1 to
10 µm. If the roughness is less than 2 µm, the hardness
of the rough, surface-hardened layer 11d is too low to
tolerate the pressure difference between the inside and
the outside of the X-ray image intensifier, with the
result that the X-ray input window 11a will be
distorted. In contrast, if the roughness exceeds 10 µm,
phosphors to be formed on the layer 11d exhibit weak
adhesiveness and has disadvantages in its film quality.
The present inventors have conducted an experiment
in the following manner with a view to find the relationship
between the surface roughness of the surface-hardened
Al alloy, the hardness of the treated surface,
the adhesiveness of phosphors to the treated surface,
and the phosphor-film quality.
The aforementioned Al-Mg alloy plate of 0.5 mm in
thickness was molded into the shape of the input window,
and then subjected to the surface-hardening by use of
glass beads of 100 µm in diameter. Al-alloy input-window
samples having a wide variety of surfaces roughness
were obtained by varying the pressure and the
processing time.
The Vickers harnesses of the treated surfaces of
the Al input-window samples were measured. The results
are shown in FIG. 7. From the graph of FIG. 7, it is
found that the surface of the input window must have the
roughness of 2 µm or more in order to attain the Vickers
hardness of 120 or more at which the input window exhibits
tolerance to the pressure difference between the
inside and the outside of the X-ray image intensifier.
Next, phosphor layers were formed on the treated
surfaces of the Al alloy input window samples by the
vapor-deposition. The adhesiveness and the film quality
of the phosphor layers were checked. The results are
shown in the following Table 1.
Roughness of the treated surface (µm) | x<2 | 2<x<5 | x=5 | 5<x<10 | 10<x |
Hardness | × | ▵ | ○ | o ○ | o ○ |
Adhesiveness | ○ | ○ | o ○ | ○ | ▵ |
Phosphor film quality | o ○ | o ○ | o ○ | ▵ | × |
o ○ : very good
○ : good
▵ : slightly good
× : not good |
Table 1 demonstrates that the surface roughness of
the Al-alloy input window plate is preferably 5 or more
to obtain sufficient hardness thereof; that the surface
roughness of the Al-alloy input window is preferably
10 µm or less to obtain sufficient adhesiveness of
phosphors; and that the surface roughness of the Al-alloy
input window is preferably 2 to 10 µm to obtain
sufficient quality of the phosphor film.
As far as only adhesiveness and the film quality of
phosphors are concerned, the Al-alloy plate having the
surface roughness of 5 µm is the most preferable,
whereas, the hardness thereof is not the most preferable.
However, even if the surface roughness is
5 µm, it is possible to obtain the most preferable
hardness depending on the manner of the surface-hardening.
More specifically, to obtain the most preferable
hardness, the Al-Mg alloy plate (ASTM 5000 series) is
molded into the shape identical to the input window, and
then subjected to the rough-surface treatment with high
pressure, thereby obtaining the surface roughness of
10 µm or more. Second, the Al-plate is subjected to the
surface-hardening with low pressure to smooth the
projections and recesses which have been formed, thereby
attaining the surface roughness of about 5 µm. In this
way, it is possible to impart the Vickers hardness of
approximately 250 µm to the surface even if the surface
roughness thereof is 5 µm.
As described in the foregoing, according to the X-ray
image intensifier of the present invention, since
the rough, surface-hardened layer is formed on the inner
surface of the X-ray input window, the X-ray input window
is less distorted by the pressure difference, caused
by evacuation, between the inside and the outside of the
X-ray image intensifier. Due to the presence of an
reflective substance layer formed on the rough surface-hardened
layer, light generated from the input screen
travels toward the photocathode, bringing a high
contrast output-image.
In the case where Al or an Al alloy is used as a
material of the X-ray input window, an X-ray image
intensifier having an X-ray input window excellent in
moldability can be obtained with advantage in cost.
Further, it is possible to obtain an output image
of a small object with higher contrast when the X-ray
image intensifier of the present invention employs a
low-energy X-ray source.
Hereinbelow, Examples of the present invention will
be described.
(Example 1)
The X-ray image intensifier according to this
example is characterized in that it has an input screen
of a specific structure. To be more specific, the
surface-hardening is applied to the concave of the
X-ray input window 11a, which is made of an aluminum
alloy (or aluminum) of 0.5 mm in thickness. Due to
surface-hardening, the surface becomes rough having
projections of several microns tall and pits of several
microns deep; and simultaneously the surface becomes
hard. In this way, a rough surface-hardened layer 11d
is formed on the concave of the X-ray input window 11a
thus treated.
On the rough surface of the rough, surface-hardened
layer 11d, an aluminum thin film 12a of approximately 200 nm
(2000 Å), namely, a reflective substance layer is formed.
The aluminum thin film is formed by the vapor-deposition
under reduced pressure of approximately 2 × 10-5 Pa.
On the reflective substance layer 12a, a phosphor layer
12b of 400 µm in thickness is formed by the vapor-deposition.
The phosphor layer 12b is manufactured by
the two steps; the first layer is formed of CsI/Na
phosphors in a thickness of approximately 380 µm under
a pressure of 4.5 × 10-1 Pa at a substrate temperature
of 180°C, and then, a second layer is formed of CsI/Na
phosphors by the vapor-deposition in a thickness of
approximately 200 µm (microns) under a pressure of 10-3 Pa.
The X-ray input window 11a including the phosphor
layer 12b is welded to the envelop body 11c via a ring
11e made of metal, e.g. steel. The envelope body 11c
connected to the X-ray input window 11a is then connected
to the envelope output portion 11c. Thereafter,
the photocathode 12d is formed on the phosphor layer
12b, directly or via the transparent conductive layer
12c.
In the X-ray image intensifier constituted as
above, when the X-ray B from the X-ray source A is
transmitted through the object C and entered into the
input window 11a, light is generated, for example, at a
point a in the phosphor layer 12b as shown in FIG. 6.
The generated light is divided into two, light b which
travels toward the output screen and light c which heads
for the input window 11a. The light c heading for the
input window when reaches the rough surface 12d of the
rough, surface-hardened layer which is a surface of the
input window 11a, causes irregular reflection. In
general, the resultant irregular reflection light d is
a cause of reducing brightness. However, due to the
presence of the aluminum thin film 12a, namely, the
reflective substance layer formed on the rough,
surface-hardened layer 11d, the light c is reflected
by the aluminum thin film 12a and then travels toward
the output screen 15 instead of entering the rough,
surface-hardened layer 11d, thereby preventing the
decrease of brightness.
Hereinbelow, the data of the image contrast characteristics
obtained by the X-ray image intensifier of
this embodiment and the X-ray image intensifier of the
conventional structure are compared to each other by way
of example of the effective input screen diameter of 10,16 cm
(4 inches) with reference to FIG. 3.
In FIG. 3, as described above, a contrast (%) and
a contrast ratio are plotted on the vertical axis and
a diameter of a lead circular plate is plotted on the
horizontal axis. The experiment of FIG. 3 is conducted
using the X-ray image intensifier having an effective
input screen diameter of 10,16 cm (4 inches) at a tube voltage of
50 kV and a tube current of 1 mA.
In the case where the input window of this embodiment
is used, lines a and b of FIG. 3 are obtained.
More specifically, the line a is obtained in the case of
aluminum input window. The line b is obtained in the
case of beryllium input window, whose size is the same
as that made of aluminum. The line c exhibits the image
contrast which is obtained by a conventional X-ray image
intensifier shown in FIG. 1.
According to the results shown in FIG. 3, the image
contrast obtained by the conventional X-ray image intensifier
drastically decrease as the diameter (mm) of the
lead circular plate becomes smaller than 40 mm. In
contrast, the contrast obtained by the X-ray image
intensifier of this example linearly increases in proportional
to and depending on the diameter (mm) of the
lead circular plate, as shown in the lines a and b.
This fact means that it is very easy to detect a smaller
object, in particular, in the case where a light and
shade are discriminated by coloring.
As described in the foregoing, the present
invention made it possible to realize the X-ray image
intensifier having an input window serving as an input
screen, which has been considered difficult to attain.
Example 2
In this example, the image contrast characteristics
were determined by applying a low-energy X-ray to the
X-ray image intensifier (a material of the input window
is an Al-Mg alloy) which is identical X-ray image intensifier
to that used in the Example 1. In FIG. 4, as
described above, a contrast (%) and a contrast ratio are
plotted on the vertical axis, and the diameter of the
lead circular plate on the horizontal axis. An experiment
is carried out at a X-ray tube voltage of 30 kV and
a tube current of 1 mA.
In FIG. 4, line d shows the change in the contrast
obtained by the X-ray image intensifier of this Example.
Line e shows the same data as above obtained in the case
of the conventional X-ray image intensifier shown in
FIG. 1.
As shown in FIG. 4, compared to the case were a
high-energy X-ray (an X-ray tube voltage of 50 kV, a
tube current of 1 mA) is applied, the image contrast
obtained by the conventional X-ray image intensifier
significantly decreases as the lead circular plate
becomes smaller than 40 mm, whereas the contrast of the
X-ray image intensifier of this example increases
linearly in proportional to and depending on the
diameter of the lead circular plate a shown in line d.
Hence, in the X-ray image intensifier of the present
invention, it is demonstrated that a smaller object can
be examined with a high level of accuracy.
As explained in the foregoing, the present invention
accomplished an X-ray image intensifier having a
structure, which has been difficult to attain, such that
an input screen is formed directly on the inner surface
of an input window, and realized an X-ray image intensifier
which can provide higher image contrast.
For reference, even in the case where an Al-Mg-Si
series alloy (ASTM 6000 series) is employed instead of
the Al-Mg alloy used in the above Examples, similar
effects can be expected.