 |
Description  |
|
|
BACKGROUND OF THE INVENTION
This invention relates to an X-ray image observing device, specifically to
a device which comprises a vacuum chamber incorporating an X-ray sensitive
photocathode screen for emitting electrons in response to incident X-ray
photons.
RELATED BACKGROUND ART
X-rays enable thicker objects (specimens) to be observed whose thickness is
greater than about 1000 Angstroms (.ANG.), as compared to objects which
may be observed with an electron microscope. Because of their high
penetrating ability and short wavelength, X-rays permit wet biological
materials, for example, human cells, in an atmosphere or a liquid, to be
observed.
In the conventional X-ray image observing device, a magnified X-ray
absorption image is projected onto an X-ray film made with silver halides,
and, after the X-ray film is developed, its magnified image is observed.
In particular, in order to observe the image made by a soft X-ray, it is
necessary to install a glazing incidence mirror and an X-ray film in a
vacuum chamber. The X-ray film is exposed, fixed in the vacuum chamber and
then is taken out of the vacuum chamber to be developed. Such conventional
radiographic device has the following disadvantages: firstly, the
magnified image of a specimen, e.g. a living cell, cannot be transiently
observed on the move in a magnified image; secondly, in order to develop
the X-ray film, the vacuum chamber has to be broken or the vacuum in the
chamber has to be released; and, thirdly, the reproducibility of the
relationship between the amount of X-rays radiated onto the X-ray film and
the blackening thereof is poor, i.e. an exact linearity between the amount
of X-ray radiation and the blackening of the film is not obtained, with a
result that an exact, magnified image cannot be obtained for accurate
observation. Furthermore, in the above described conventional X-ray image
observing device, the developed X-ray film has to either be further
enlarged for observation, or else it has to be observed by means of an
optical microscope, and consequently, additional steps are required in
order to observe a sufficiently magnified image.
Japanese Patent Publication Kokai No. 59-101134, for example, describes a
device for observing an image in which an X-ray absorption image is
converted by a scintillator into a photoelectric convertible image, the
converted image is further converted into an electron image by a
photocathode screen, and the electron image is imaged on a phosphor
screen. In this device, the X-ray absorption image is not magnified in a
vacuum chamber. Accordingly the device can neither observe X-ray
absorption images of fine biological materials nor magnify X-ray
absorption images at such high magnifications as to be used as a
microscope.
"Photoelectron microscope for X-ray microscopy and microanalysis", (Rev.
Sci. Instrum 52(2), Feb., 1981, Ps. 207-212) by F. Ploack shows a method
comprising fixing a specimen to an X-ray incident window of a vacuum
chamber, converting the X-ray which has penetrated the specimen into
electrons by an X-ray cathode screen deposited on the inside surface of
the vacuum chamber at the opposing postion to the X-ray incident window,
and imaging the electron image on a film. This method requires that the
X-ray incident window be larger than a certain thickness for the purpose
of preventing the breakage of the window due to the pressure difference
between the atmosphere and the interior of the vacuum chamber.
Accordingly, the X-ray is absorbed by the window and attenuated. This
makes it difficult to obtain clear images. It is also difficult using this
method to magnify an image at such high magnification as to be used as a
microscope.
SUMMARY OF THE INVENTION
An object of this invention is to provide an X-ray image observing device
which makes it possible to use X-rays to observe clear magnified images at
high magnifications.
Another object of this invention is to provide an X-ray image observing
device which enables a specimen, such as living cells, to be observed
transiently on the move in magnified X-ray absorption images, continuously
or real time.
The X-ray image observing device according to one embodiment of this
invention comprises an X-ray source; a vacuum chamber having an input
window which permits an X-ray radiated from the X-ray source to penetrate
therethrough a first vacuum compartment provided on the side of the vacuum
chamber nearer to the input window, and a second vacuum compartment
provided on the side thereof farther from the input window; X-ray imaging
means for magnifying and focusing the X-ray incident from the input
window, at a set position on the boundary between the first and the second
vacuum compartments; a photocathode screen assembly for emitting electrons
in response to the incident X-ray, disposed at the X-ray focusing
position; and an electron imaging means for focusing the electrons emitted
from the photocathode screen into the second vacuum compartment, at a set
position in the second vacuum compartment.
The X-ray image observing device, according to another embodiment of this
invention, comprises a vacuum chamber having a first vacuum compartment
formed in the middle thereof, a second vacuum compartment formed on one
side of the first vacuum compartment, and a third vacuum compartment
formed on the other side of the first vacuum compartment; an X-ray source
for radiating X-ray to the first vacuum compartment disposed in the third
vacuum compartment; X-ray imaging means for magnifying and focusing X-rays
radiated from the X-ray source on a set position on the boundary between
the first and the second vacuum compartments; a photocathode screen
assembly for emitting electrons in response to the incident X-ray,
disposed at the focusing position of the X-ray; and an electron imaging
means for focusing the electrons emitted from the photocathode screen to
the second vacuum compartment, at a set position in the second vacuum
compartment.
The devices according to these embodiments of the invention preferably have
imaging means for making a picture of the electron image produced by the
electron imaging means which comprises, e.g., converting means for
converting the electron image into an optical image, and optical imaging
means for taking a picture of the light image. The imaging means has
storing means for storing the data obtained by the optical imaging means
for a certain period of time.
The present invention will become more fully understood from the detailed
description given hereinbelow and the accompanying drawings which are
given by way of illustration only, and thus are not to be considered as
limiting the present invention.
Further scope of applicability of the present invention will become
apparent from the detailed description given hereinafter. However, it
should be understood that the detailed description and specific examples,
while indicating preferred embodiments of the invention, are given by way
of illustration only, since various changes and modifications within the
spirit and scope of the invention will become apparent to those skilled in
the art from this detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side view of an X-ray microscope incorporating one embodiment
of the invention;
FIGS. 2(a), 2(b) and 2(c) are sectional views of a mounting unit for the
specimen to be observed;
FIG. 3 is an enlarged side view of the electron imaging unit shown in FIG.
1;
FIG. 4(a) is a sectional view of the photocathode screen assembly shown in
FIG. 1;
FIG. 4(b) is a perspective view of the photocathode screen assembly shown
in FIGS. 1 and 4(a);
FIGS. 5(a)-(d) are sectional views, illustrating the process of forming the
photocathode screen and the support film for the embodiments of this
invention;
FIG. 6 is a sectional view of another embodiment of the photocathode screen
assembly of this invention;
FIG. 7 is a side view of an X-ray microscope incorporating another
embodiment of this invention;
FIG. 8 is an enlarged sectional view of the structure surrounding the
photocathode screen;
FIG. 9 is a side view of an X-ray microscope incorporating an embodiment of
this invention; and
FIG. 10 is a side view of an X-ray microscope incorporating one
modification of the embodiment of FIG. 9.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In the X-ray image observing device according to one embodiment of this
invention, an X-ray imaging unit for making the image of an X-ray which
has penetrated a specimen (object to be observed), and an electron imaging
unit for focusing the electrons emitted from an X-ray sensitive
photocathode screen in response to the incident X-ray onto a microchannel
plate (MCP) are disposed in one and the same vacuum chamber or are annexed
to the vacuum chamber, and an X-ray source is disposed outside the vacuum
chamber. Accordingly the device is characterized in that an X-ray from the
X-ray source is incident on the interior of the vacuum chamber through an
input window formed in the vacuum chamber. This will be explained in more
detail with reference to FIG. 1. An X-ray microscope comprises an X-ray
source 1, a specimen mounting unit (object mounting unit) 2 for
introducing a specimen 25, e.g. a living cell, in front of the X-ray
radiating surface 15 of the X-ray source 1, an X-ray imaging unit 3
disposed in a first vacuum compartment 31 of the vacuum chamber 100 on the
side thereof nearer to the specimen mounting unit 2, a second vacuum
compartment 41 disposed on the other side of the vacuum chamber 100 and an
electron imaging unit 4 disposed in and around the second vacuum
compartment 41, and a imaging unit 5 for taking a picture of a magnified
image produced by the electron imaging unit 4.
The X-ray tube of the X-ray source 1 generates X-rays of, for example,
about 23-44 .ANG. in order that carbon atoms and oxygen atoms are clearly
contrasted to each other in the biological material to be observed. A
specimen mount 23 is made of a material which the X-ray can penetrate,
specifically it is made of a film of an organic material, such as, for
example, poly-para-xylylene, etc. The specimen mount 23 has the structure
shown in FIGS. 2(a)-(c), for example. As shown in FIG. 2(a), the specimen
mount 23 is an assembly of two support plates 233,234 respectively having
recesses 231,232, and two organic thin films 235,236 and two metal meshes
235M, 236M. Opening or holes 237,238 are formed in the center of the
recesses 231,232 of the support plates 233,234. Each organic thin film
235,236 comprises an X-ray penetrative organic material, such as
poly-para-xylylene. The specimen mount 23 is assembled as in FIG. 2(b).
One of the organic films 235 is adhered to the convex side of the male
support plate 233 with one of the metal mesh 235M interposed between the
support plate 233 and the thin film 235 so as to close the opening or hole
237. The other organic film 236 is adhered to the concave side of the
female support plate 234 with the other metal mesh 236M interposed between
the support plate 234 and the thin film 236 so as to close the opening or
hole 238. As shown in FIG. 2(b), the specimen 25 containing a living cell
is attached to the organic film 236, and then the convex portion of the
male support plate 233 is inserted into the concave portion 232 of the
female support plate 234. Then, as shown in FIG. 2(c), the specimen 25 is
set. The metal mesh 235M interposed between the support plate 233 and the
thin film 235, and the metal mesh 236M interposed between the support
plate 234 and the thin film 236 improve the mechanical strength of the
films. It is also possible to use the meshes 235M, 236M for focussing. The
specimen mount 23 is supported by a manipulater 22, and the specimen mount
23 is moved in the plane perpendicular to optical axis.
As shown in FIG. 1, an input window 30 is formed in the wall of the vacuum
chamber 100 opposite to the specimen mount 23. The input window 30 is made
of an X-ray penetrative material. An X-ray is incident in the first vacuum
compartment 31. The plate is adhered to the opening or hole, which is
about 10 mm diameter, formed in the stainless steel vacuum chamber 100.
Accordingly the input window 30 includes the X-ray unpenetrative mesh in
addition to an X-ray penetrative organic material. However, since the
window 30 is placed several millimeters away from the specimen 25, the
input window 30 does not hinder imaging the specimen 25. The mesh
incorporated in the input window 30 improves the mechanical strength
thereof, which prevents the breakage of the input window due to the
difference of the atmospheric pressure on each side of the window 30.
An incident X-ray is reflected on a glazing incidence mirror 32, and the
reflected X-ray is focussed on the boundary between the first vacuum
compartment 31 and the second vacuum compartment 41. Accordingly, a
magnified X-ray image of the specimen 25 is produced on a photocathode
screen 42 disposed on the boundary between vacuum compartment 31 and 41. A
stopper 33 serves to shut off unnecessary X-rays. The first vacuum
compartment 31 is connected to a vacuum drawing system, such as a vacuum
pump, through a valve 34 so that a degree of vacuum of above about
10.sup.-5 -10.sup.-6 Torr may be obtained.
As shown in FIGS. 3 and 4, the photocathode screen 42 disposed on the
boundary between the first vacuum compartment 31 and the second vacuum
compartment 41 is evaporated on the side of a support film 43 opposite to
the second vacuum compartment 41. The support film 43 is formed so as to
close an aperture at the center of a support plate 44. Two openings or
holes 40 are formed in the support plate 44, and enable communication
between the first and the second vacuum compartments 31,41. In an electron
imaging unit 4, two electromagnetic coils 47,48 are wound on the exterior
of the vacuum chamber 100 which magnifies the electron image. An MCP 45 is
provided in the second vacuum compartment 41 on the side opposite to the
photocathode screen 42. A phosphor screen (display screen) 46 made of, for
example, ZnS(Ag) is formed by deposition on the inside wall of the vacuum
chamber 100 behind MCP 45.
As shown in FIG. 3, X-rays penetrate the support film 43 to reach the
photocathode screen 42, and, in response to the incident X-ray,
photo-electrons are emitted by the photocathode screen 42 to the side of
the second vacuum compartment 41. The support film 43 has to be made of an
X-ray penetrative material, for example, an organic material such as
poly-para-xylylene, poly-propylene, etc, or silicon nitride (Si.sub.3
N.sub.4) which does not include carbon. The support film 43 has to be thin
enough not to hinder the penetration of soft X-rays therethrough and
preferably has a thickness of less than about three microns(.mu.m).
Specifically, in the case of a penetration of above 20% through the
support film 43 for a wavelength of 30-40 .ANG., the support film 43 has a
thickness of below 0.5 .mu.m for poly-para-xylylene and below 0.25 .mu.m
for silicon nitride. It is possible to increase the thickness of the
support film 43 when the X-ray incident on the support film 43 has higher
intensity, or where a highly penetrative X-ray having a short wavelength
(for example, less than about 10 .ANG.) is used. In this embodiment, since
the holes 40 in the support plate 44 provide communication with the first
vacuum compartment 31 and the second vacuum compartment 41, the degree of
difference of the vacuum between the first and the second vacuum
compartments 31,41 can be substantially compensated. Consequently, even
though the support film 43 is made sufficiently thinner, it never breaks
due to a pressure difference. The photocathode screen 42 is made of gold
(Au) film, which is able to convert X-ray photons directly into electrons
but may be made of a two layer film comprising a cesium iodide and
antimony cesium.
When an electrons are focussed on the front surface of MCP 45 by the
electron imaging unit 4, the incident electrons are multiplied by MCP 45
and impact onto the phosphor screen 46. Consequently, a light image
corresponding to the electron beam on MCP 45 is produced on the phosphor
screen 46. In the case that the magnification of the glazing incidence
mirror 32 is 20 times, the resolving power of the photocathode screen 42
is 1 .mu.m, and the magnification of an electron lens comprising
electromagnetic coils 47,48 is 100 times, the resolving power on the
specimen 25 is 1 .mu.m/20=50 nano meters (nm), and on the phosphor screen
46 light image of 0.1 mm can be obtained for 50 nm on the specimen 25.
A light image produced on the phosphor screen 46 is caught by a TV camera
52 through a relay lens 51, and the magnified light image caught by the TV
camera 52 is converted into an electrical video signal, and the signal is
sent to a video frame memory 53. The video frame memory 53 converts the
analog electric video signal to a digital signal and integrates the
digital video signals for a certain period of time. The integration result
is supplied to a monitor 54. The monitor 54 produces a visible image on
the screen, based on the integration result. The TV camera 52 takes a
picture of the visible image produced on the phosphor screen 46, so that
the specimen can be visualized at the resolving power of 50 nm thereon
easily on the monitor 54. That is, in the case that the magnification of
the relay lens 51 is once, and the size of the input surface of the TV
camera 52 is 10 mm.times.10 mm, and the screen of the monitor is 20
cm.times.20 cm, the X-ray microscope itself provides a magnification of
20.times.100.times.20=40000. The integration of signals by the video frame
memory 53 is effective especially when the X-ray absorption image is
faint. In this case the magnified image cannot be observed real time, but
can be observed continuously. In contrast, in the case that the X-ray
absorption image has a sufficient intensity, the video frame memory 53
does not have to be used. In this case, the resolution power of the TV
camera 52 in terms of time allows one sheet of picture to be taken every
1/30 seconds. A substantially real time X-ray shadow image can be
observed.
Next, the methods of making the photocathode screen and the support film
will be explained with reference to FIG. 5.
As described above, the support film 43 for supporting the photocathode
screen 42 has to be made thin enough so as not to hinder the penetration
of the X-ray. First, as shown in FIG. 5(a), a polycrystal silicon (Si) 44b
is formed on a silicon substrate 44a by, for example, the epitaxial
growth. Further, a thermally oxidized layer 43' of SiO.sub.2 is formed
thereon by thermal oxidation. Instead of the thermally oxidized layer, a
silicon nitride (Si.sub.3 N.sub.4) layer may be formed thereon. Since the
uppermost layer 43' functions as the support film 43 of the photocathode
screen 42, it is made very thin, such as, for example less than about
three hundred Angstroms.
As shown in FIG. 5(b), a photoresist is subsequently applied to the
underside of the silicon substrate 44a, and the photoresist is partially
exposed and then developed to form a mask 71. Then the silicon substrate
44a is selectively wet etched into the structure shown in FIG. 5(b). Next,
without removing the mask 71 of the photoresist, the polycrystal Si 44b is
selectively phase etched into the structure of FIG. 5(c) in which the
uppermost layer 43' is left. Layer 43' has an even thickness and a
sufficient intensity. As shown in FIG. 5(d), Au(gold) is evaporated at a
certain position in a hole formed beforehand from the side of the silicon
substrate 44a to form the photocathode screen 42. The photocathode screen
assembly having the thus formed photocathode screen 42 and the support
film 43 is disposed on the boundary between the first and the second
vacuum compartments 31,41.
The photocathode screen assembly may be formed as shown in FIG. 6. An
aperture 44d is formed in a support body 44c comprising glass, metal,
silicon, etc., and the support film 43, made of, for example,
poly-para-xylylene is adhered thereto so as to close the aperture 44d.
Then the photocathode screen 42 made of, for example, Au(gold) is
evaporated on the support film 43.
Another embodiment of the X-ray microscope will be explained, with
reference to FIG. 7.
As shown in FIG. 7, the specimen mounting unit 2 is disposed in a vacuum
chamber 100. That is, specimen compartment 21 is attached on the vacuum
chamber 100 at one end. The specimen compartment 21 is in communication
with the first vacuum compartment 31 through a gate valve 24 which can be
opened or closed. When the specimen 25 is mounted, the gate valve 24 is
closed as indicated by the dotted line in FIG. 7 to release the vacuum of
the specimen compartment 21. In this condition, the manipulater 22, and
the specimen mount 23 are accommodated in the specimen compartment 21 as
indicated by the solid line in FIG. 7, and the door (not shown) is opened
to set the specimen 25 on the specimen mount 23. Then the door is closed,
and a valve 26 is opened to create a vacuum in the specimen compartment
21. When the vacuum of the specimen compartment 21 becomes about 10.sup.-5
-10.sup.-6 Torr, the gate valve 24 is opened, as indicated by the solid
line in FIG. 7, so as to operate the manipulater 22 to move the specimen
mount 23 to an observation position. Thus the specimen 25 is mounted on a
set position in the first vacuum compartment 31. Accordingly the X-ray
which has penetrated the specimen 25 is incident on the glazing incidence
mirror 32 without being attenuated.
FIG. 8 shows an enlarged diagrammatic view of the vicinity of the
photocathode screen 42 in FIG. 7. As shown in FIG. 8, the first vacuum
compartment 31 and the second vacuum compartment 41 are partitioned by a
support member 44', and the support member 44' is secured at the proximal
end to the inside surface of the vacuum chamber 100. The support member
44' is in the form of a cylinder projected into the side of the electron
imaging unit 4, and the support film 43 is fixed to the forward end
thereof. The support film 43 is evaporated on the end of the photocathode
screen 42. The support film 43 is thin enough for the X-ray to penetrate
(less than about three .mu.m) and is made of an X-ray penetrative
material. As shown in FIG. 8. when an X-ray is incident on the support
film 43, photoelectrons are emitted to the opposite side. The cylinder of
the support member 44' accommodates a number of through holes 40 in the
side wall of the support member 44'. Accordingly the through holes 40
permit a larger amount of a gas to flow in the first and the second vacuum
compartments 31,41, compared with the above described embodiment.
Consequently, even when the degree of the vacuum in the second vacuum
compartment 41 decreases due to insufficient release of the gas of the
phosphor screen 46, or even when there is a difference between evacuation
capability through the valve 34 and that through the valve 41, the
difference in the pressure between the first and the second vacuum
compartments 31, 41 is promptly compensated. Accordingly this enables the
support film 43 to be made as thin as possible with a result that the
attenuation of the X-ray can be sufficiently lowered.
As shown in FIG. 7, a solid-state image sensor 56 is fixed to the outside
surface of the vacuum chamber 100 at the position opposite the phosphor
screen 46. The solid-state image sensor 56 consists of, for example, a
charge coupled device (CCD) and has a scanning circuit built in. The
output data of the solid-state image sensor 56 is temporarily stored by a
data memory 53' having the same function as the video frame memory 53 and
is then supplied to the monitor 54 to be displayed on the screen. In the
case where the solid-state image sensor 56 is fixed to the inside surface
of the vacuum chamber 100 in place of the phosphor screen 46 in FIG. 7,
the electron image can be directly pictured without converting the
electron image into a light image on the phosphor screen 46.
The device according to the second embodiment of this invention differs
from that of the first embodiment of this invention in that the X-ray
source 1 is incorporated in the vacuum chamber 100. The device according
to the second embodiment of this invention will be explained in more
detail with reference to FIG. 9. The X-ray source 1 is disposed in a third
vacuum compartment 11 defined in the vacuum chamber 100 by a partitioning
film 10 and comprises a hot cathode 12 for emitting thermoelectrons, and a
target 13 fixedly formed on the partitioning film 10 so as to radiate the
X-ray to the first vacuum compartment 31 in response to incident electrons
thereto. The third vacuum compartment 11 is in communication with the
vacuum draw unit through a valve 14. The partitioning film 10 is made of
an X-ray penetrative material (for example, poly-para-xylylene, silicon
nitride, etc) and is made thin enough so as not to attenuate very much the
X-ray radiated into the first vacuum compartment 31. In this embodiment,
unlike the first embodiment of this invention, since the X-ray source 1 is
disposed within the vacuum chamber 100, no atmospheric pressure is applied
to the partitioning film 10. It is thus possible to make the partitioning
film 10 as thin as possible. The provision of vent holes across the first
and the third vacuum compartments 31,11 enables the partitioning film 10
to be made thinner without being broken by the difference in degree of the
vacuum. The target 13 may be made of, for example, carbon or other
similarly acting material. Since in the embodiment of FIG. 9, the specimen
mounting unit 2, the X-ray imaging unit 3 and the electron imaging unit 4
have the same structures as in the embodiment of FIG. 7, and the light
imaging unit 5 has the same structure as in the embodiment of FIG. 1, a
detail explanation of each is omitted.
FIG. 10 shows a modification of the device according to the second
embodiment of this invention. In the modification of FIG. 10, no
partitioning film is provided between the first and the third vacuum
compartments 31,11. The third vacuum compartment 11 provides a synchrotron
radiation source (SOR source). A reflecting mirror 17 is provided to
converge an X-ray from SOR source onto the specimen 25. In this
modification, since the first vacuum compartment 31 is connected to the
SOR source, the vacuum degree of the vacuum chamber 100 has to be about
10.sup.-8 Torr. In the other portions this modification is the same as the
embodiment of FIG. 9.
The X-ray image observing device is not limited to the above described
embodiments and includes its modifications and variations without
departing from the scope of the claims.
To give examples, in the embodiment of FIG. 1, the X-ray source 1 is not
limited to the one which radiates only the X-ray but may be, e.g. a laser
plasma source which simultaneously radiates an X-ray and an ultra violet
ray. In this case a filter of, for example, poly-para-xylylene, suitable
for shutting off the ultra violet rays is provided on the input window 30,
so that only the X-rays are permitted to be incident in the first vacuum
compartment 31. A gas plasma source may be used, but since the source, in
operation, generates gases, a partitioning film is necessary, different
from the structure of FIG. 10. The means for magnifying the X-ray
absorption image is not limited to the glazing incidence mirror 32 but may
be, for example, an X-ray zone plate or a multi layer screen X-ray
reflecting mirror. In the case that the radiated X-ray has high intensity,
MCP 45 is not required in the electron imaging unit 4.
From the invention thus described, it will be obvious that the invention
may be varied in many ways. Such variations are not to be regarded as a
departure from the spirit and scope of the invention, and all such
modifications as would be obvious to one skilled in the art are intended
to be included within the scope of the following claims.
* * * * *
|
|
 |
|
 |
Description |
|
