U.S. patent application number 09/797101 was filed with the patent office on 2001-09-06 for x-ray generator.
This patent application is currently assigned to Rigaku Corporation. Invention is credited to Kuribayashi, Masaru, Osaka, Naohisa, Tkahashi, Sadayuki.
Application Number | 20010019601 09/797101 |
Document ID | / |
Family ID | 26586803 |
Filed Date | 2001-09-06 |
United States Patent
Application |
20010019601 |
Kind Code |
A1 |
Tkahashi, Sadayuki ; et
al. |
September 6, 2001 |
X-ray generator
Abstract
An X-ray generator includes a cathode having an emitter made of
carbon nanotubes which emits electrons by field emission and thus
becomes a cold cathode electron emission source. In the invention
using the carbon nanotubes, any one of the following three forms is
adopted to control the tube current apart from the
electron-focusing control. The first form is that a takeoff
electrode is disposed near the cathode and the Wehnelt potential
and the takeoff electrode potential are controlled independently.
The second form is that an electron emission source is disposed
behind the cathode and the electron emission source emits electrons
which collide against the back of the cathode so that the cathode
temperature is controlled in a range of the room temperature to
about 100 degrees Celsius to regulate an amount of electron
emission from the cathode. The third form is that the cathode has a
heater so that the cathode temperature is controlled in a range of
the room temperature to about 100 degrees Celsius to regulate an
amount of electron emission from the cathode.
Inventors: |
Tkahashi, Sadayuki; (Tokyo,
JP) ; Kuribayashi, Masaru; (Tokyo, JP) ;
Osaka, Naohisa; (Tokyo, JP) |
Correspondence
Address: |
FRISHAUF, HOLTZ, GOODMAN &
LANGER & CHICK, PC
767 THIRD AVENUE
25TH AVE
NEW YORK
NY
10017-2023
US
|
Assignee: |
Rigaku Corporation
3-9-12-, Matsubara-cho Akishima-shi
Tokyo
JP
|
Family ID: |
26586803 |
Appl. No.: |
09/797101 |
Filed: |
March 1, 2001 |
Current U.S.
Class: |
378/119 ;
378/101; 378/114 |
Current CPC
Class: |
B82Y 10/00 20130101;
Y10S 977/742 20130101; H01J 35/147 20190501; H01J 2201/30469
20130101; H01J 35/065 20130101; Y10S 977/844 20130101 |
Class at
Publication: |
378/119 ;
378/101; 378/114 |
International
Class: |
H01J 035/06 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 6, 2000 |
JP |
2000/59916 |
Mar 24, 2000 |
JP |
2000/83390 |
Claims
1. An X-ray generator comprising: (a) a Wehnelt; (b) a cathode
disposed within said Wehnelt and having emitter means made of
carbon nanotubes; (c) a target disposed so as to face said cathode;
(d) a takeoff electrode disposed near said cathode; (e) a first
power supply connected between said cathode and said target so as
to control a target potential based on a cathode potential; (f) a
second power supply connected between said cathode and said Wehnelt
so as to control a Wehnelt potential based on the cathode
potential; and (g) a third power supply connected between said
cathode and said takeoff electrode so as to control a takeoff
electrode potential based on the cathode potential.
2. An X-ray generator comprising: (a) a Wehnelt; (b) a cathode
disposed within said Wehnelt and having emitter means made of
carbon nanotubes; (c) a target disposed so as to face said cathode;
(d) an electron emission source disposed behind said cathode so
that electrons emitted from said electron emission source collide
against a back of said cathode; (e) a first power supply connected
between said cathode and said target so as to control a target
potential based on a cathode potential; (f) a second power supply
connected between said cathode and said Wehnelt so as to control a
Wehnelt potential based on the cathode potential; and (g) a third
power supply connected between said cathode and said electron
emission source so as to control an electron emission source
potential based on the cathode potential.
3. An X-ray generator comprising: (a) a Wehnelt; (b) a cathode
disposed within said Wehnelt and having emitter means made of
carbon nanotubes; (c) a target disposed so as to face said cathode;
(d) heater means attached to said cathode; (e) a first power supply
connected between said cathode and said target so as to control a
target potential based on a cathode potential; (f) a second power
supply connected between said cathode and said Wehnelt so as to
control a Wehnelt potential based on the cathode potential; and (g)
a third power supply connected to said heater means so as to
control a heating temperature of said heater means.
4. An X-ray generator comprising: (a) a cathode having emitter
means made of carbon nanotubes; and (b) a target disposed so as to
face said cathode.
5. An X-ray generator comprising: (a) a Wehnelt; (b) a cathode
disposed within said Wehnelt and having emitter means made of
fullerenes; (c) a target disposed so as to face said cathode; (d) a
takeoff electrode disposed near said cathode; (e) a first power
supply connected between said cathode and said target so as to
control a target potential based on a cathode potential; (f) a
second power supply connected between said cathode and said Wehnelt
so as to control a Wehnelt potential based on the cathode
potential; and (g) a third power supply connected between said
cathode and said takeoff electrode so as to control a takeoff
electrode potential based on the cathode potential.
6. An X-ray generator comprising: (a) a Wehnelt; (b) a cathode
disposed within said Wehnelt and having emitter means made of
fullerenes; (c) a target disposed so as to face said cathode; (d)
an electron emission source disposed behind said cathode so that
electrons emitted from said electron emission source collide
against a back of said cathode; (e) a first power supply connected
between said cathode and said target so as to control a target
potential based on a cathode potential; (f) a second power supply
connected between said cathode and said Wehnelt so as to control a
Wehnelt potential based on the cathode potential; and (g) a third
power supply connected between said cathode and said electron
emission source so as to control an electron emission source
potential based on the cathode potential.
7. An X-ray generator comprising: (a) a Wehnelt; (b) a cathode
disposed within said Wehnelt and having emitter means made of
fullerenes; (c) a target disposed so as to face said cathode; (d)
heater means attached to said cathode; (e) a first power supply
connected between said cathode and said target so as to control a
target potential based on a cathode potential; (f) a second power
supply connected between said cathode and said Wehnelt so as to
control a Wehnelt potential based on the cathode potential; and (g)
a third power supply connected to said heater means so as to
control a heating temperature of said heater means.
8. An X-ray generator comprising: (a) a cathode having emitter
means made of fullerenes; and (b) a target disposed so as to face
said cathode.
9. An X-ray generator comprising: (a) a Wehnelt; (b) a hot cathode
disposed within said Wehnelt and being not a direct-heating type;
(c) a target disposed so as to face said hot cathode; (d) an
electron emission electrode disposed behind and apart from said hot
cathode and having emitter means made of carbon nanotubes; (e) a
first power supply connected between said hot cathode and said
target so as to control a target potential based on a hot cathode
potential; (f) a second power supply connected between said hot
cathode and said Wehnelt so as to control a Wehnelt potential based
on the hot cathode potential; and (g) a third power supply
connected between said hot cathode and said electron emission
electrode to provide said electron emission electrode with a
negative potential based on the hot cathode potential so that said
electron emission electrode emits electrons which collide against
said hot cathode to heat it.
10. An X-ray generator according to claim 9, wherein said negative
potential is controlled so as to regulate a tube current.
11. An X-ray generator according to claim 9, wherein at least an
electron emission region of said hot cathode is made of lanthanum
hexaboride.
12. An X-ray generator comprising: (a) a hot cathode which is not a
direct-heating type; (b) a target disposed so as to face said hot
cathode; (d) an electron emission electrode disposed behind and
apart from said hot cathode and having emitter means made of carbon
nanotubes; and (e) a power supply connected between said hot
cathode and said electron emission electrode to provide said
electron emission electrode with a negative potential based on a
hot cathode potential so that said electron emission electrode
emits electrons which collide against said hot cathode to heat
it.
13. An X-ray generator comprising: (a) a Wehnelt; (b) a hot cathode
disposed within said Wehnelt and being not a direct-heating type;
(c) a target disposed so as to face said hot cathode; (d) an
electron emission electrode disposed behind and apart from said hot
cathode and having emitter means made of fullerenes; (e) a first
power supply connected between said hot cathode and said target so
as to control a target potential based on a hot cathode potential;
(f) a second power supply connected between said hot cathode and
said Wehnelt so as to control a Wehnelt potential based on the hot
cathode potential; and (g) a third power supply connected between
said hot cathode and said electron emission electrode to provide
said electron emission electrode with a negative potential based on
the hot cathode potential so that said electron emission electrode
emits electrons which collide against said hot cathode to heat
it.
14. An X-ray generator according to claim 13, wherein said negative
potential is controlled so as to regulate a tube current.
15. An X-ray generator according to claim 13, wherein at least an
electron emission region of said hot cathode is made of lanthanum
hexaboride.
16. An X-ray generator comprising: (a) a hot cathode which is not a
direct-heating type; (b) a target disposed so as to face said hot
cathode; (d) an electron emission electrode disposed behind and
apart from said hot cathode and having emitter means made of
fullerenes; and (e) a power supply connected between said hot
cathode and said electron emission electrode to provide said
electron emission electrode with a negative potential based on a
hot cathode potential so that said electron emission electrode
emits electrons which collide against said hot cathode to heat it.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to an X-ray generator having an
improved cathode.
[0002] The conventional X-ray generator has a hot cathode which is
typically made of tungsten whose operational temperature is very
high as 2000 to 2300 degrees Celsius. Other than the tungsten,
thorium-added tungsten or lanthanum hexaboride also have been used
for the hot cathode materials. The operational temperature of those
materials is 1000 to 1500 degrees Celsius which is lower than that
of the tungsten but is a relatively high temperature. The hot
cathode made of any materials described above requires a relatively
high-power heating power supply. The hot cathode made of
thorium-added tungsten or lanthanum hexaboride requires high vacuum
to obtain a steady emission current. The tungsten filament requires
vacuum under 1.times.10.sup.-3 Pa, while the hot cathode made of
thorium-added tungsten or lanthanum hexaboride requires vacuum
under 1.times.10.sup.-5 Pa.
[0003] Since the conventional X-ray generator has a hot cathode as
described above, it has the following disadvantages: (1) With the
hot cathode, a high-power heating power supply is required. A large
current (e.g., ten and several amperes) must flow through the hot
cathode to emit hot electrons and thus a large-current cable is
required. Since a negative high voltage of several tens kV based on
the ground potential is supplied to the cathode of the X-ray
generator, a cable connected to the X-ray generator must bear not
only a high-voltage but also a large current and heat generation.
Such a large-current high-voltage cable is expensive, thick, rigid
and difficult to handle. (2) Since the cathode becomes a very high
temperature, the surrounding parts must be designed to bear the
high temperature. (3) The cathode made of lanthanum hexaboride and
so on requires high vacuum. (4) The hot cathode becomes a high
temperature to discharge gas which affects the X-ray generator.
Therefore, before the use of the X-ray generator, the hot cathode
must be heated for a period of time to discharge gas so as to
reduce gas discharge in the actual use. (5) The cathode material
would slightly evaporate and scatter from the hot cathode, so that
such material adheres to the target surface and causes
contamination with which the characteristic X-ray of the adhering
material (i.e., cathode material) is generated
inadvantageously.
[0004] Incidentally, in the field other than the X-ray generator,
carbon nanotubes have lately attracted attention as a cold cathode
electron emission source. The carbon nanotube is one form of carbon
material which has a cylindrical structure with a diameter of
nanometer order. The carbon nanotubes can emit electrons by field
emission under the room temperature even with the flat surface of
the electron emission region (i.e., requiring no needle shape). It
is known that the cold cathode electron emission source made of
carbon nanotubes may be used for the electron source of the flat
display, as disclosed in Japanese patent publication Nos. JP
11-194134 A (1999) JP 10-199398 A (1998), JP 10-149760 A (1998) and
JP 10-12124 A (1998). The cold cathode electron emission source
emits electrons which collide against fluorescent substance to make
a light-emitting display. Also it is known that the carbon
nanotubes may be used for the electron gun of the cathode ray tube,
as disclosed in Japanese patent publication Nos. JP 11-260244 A
(1999) and JP 11-111158 A (1999).
[0005] Furthermore, it is known that, other than the carbon
nanotubes, fullerenes may be used for the cold cathode electron
emission source, as disclosed in Japanese patent publication No. JP
10-149760 A (1998), the fullerene being another form of carbon
material.
SUMMARY OF THE INVENTION
[0006] Accordingly it is an object of the invention to provide an
X-ray generator in which a cold cathode electron emission source
made of carbon material is used as the cold cathode so that various
problems caused by the hot cathode can be solved.
[0007] It is another object of the invention to provide an X-ray
generator in which a cold cathode electron emission source made of
carbon material emits electrons which heat a hot cathode so that a
high-voltage cable is given no large current.
[0008] An X-ray generator according to the first aspect of the
invention includes a cathode having an emitter made of carbon
nanotubes which emits electrons by field emission and thus becomes
a cold cathode electron emission source. In the invention using the
carbon nanotubes, any one of the following three forms is adopted
to control the tube current apart from the electron-focusing
control. The first form is that a takeoff electrode is disposed
near the cathode and the Wehnelt potential and the takeoff
electrode potential are controlled independently. The second form
is that an electron emission source is disposed behind the cathode
and the electron emission source emits electrons which collide
against the back of the cathode so that the cathode temperature is
controlled in a range of the room temperature to about 100 degrees
Celsius to regulate an amount of electron emission from the
cathode. The third form is that the cathode has a heater so that
the cathode temperature is controlled in a range of the room
temperature to about 100 degrees Celsius to regulate an amount of
electron emission from the cathode.
[0009] The emitter made of carbon nanotubes has the following
advantages as compared with the conventional hot cathode: (1) Since
the cathode requires no high-temperature heating, it saves power.
(2) The cathode requires no large-current cable which is used for
high-temperature heating. (3) Since the cathode temperature is near
the room temperature, the surrounding parts requires no
countermeasure for a high temperature. (4) Since the cathode has no
high-temperature region, it requires no heating operation for
outgassing before the use so that the X-ray generator can be used
soon. (5) If the cathode becomes a high temperature, the cathode
material would evaporate and adhere to the target surface. The
cathode of this invention has no such a problem and the target
contamination is reduced. (6) A steady emission current is obtained
under a pressure of about 1.times.10.sup.-3 Pa so that the X-ray
generator requires no high vacuum.
[0010] Fullerenes may be used instead of the carbon nanotubes. The
fullerene has a polyhedral structure including pentagons and
hexagons made of carbon atoms, the typical one being a spherical
structure including 60 carbon atoms. Such fullerenes may be used
for the cathode emitter of the X-ray generator.
[0011] An X-ray generator according to the second aspect of the
invention includes a hot cathode and a cold cathode electron
emission source made of carbon material (e.g., carbon nanotubes)
for heating the hot cathode. The hot cathode is not a
direct-heating type in which a current directly flows through the
cathode to heat it by resistance, but a type in which electrons
from the electron emission source collide against the cathode to
heat it. The carbon nanotubes are used as the emitter of the
electron emission electrode. The electron emission electrode is
disposed behind and apart from the hot cathode. The electron
emission electrode is given a negative potential based on the hot
cathode potential so that the electron emission electrode emits
electrons which collide against the hot cathode to heat it. The
negative potential is controlled to regulate the tube current of
the X-ray generator. The hot cathode material is not limited to
specific ones, but at least an electron emission region is made of
lanthanum hexaboride preferably. Fullerenes may be used instead of
the carbon nanotubes.
[0012] The X-ray generator according to the second aspect of the
invention includes a hot cathode heated by electrons which are
emitted by a cold cathode electron emission source made of carbon
material (carbon nanotubes or fullerenes), so that a high-voltage
cable is given no large current.
[0013] The X-ray generator according to the second aspect has the
advantage described below as compared with that according to the
first aspect. It is known that an electron emission surface made of
carbon nanotubes generates uneven brightness and its hourly
fluctuation, the uneven brightness of the emitter being that an
electron emission strength depends upon locations on the electron
emission surface. It is desirable in the X-ray generator that
uneven brightness on the target is reduced as much as possible and
hourly fluctuation of the X-ray intensity is reduced as much as
possible, the uneven brightness on the target being that a strength
of electron current colliding against the target depends upon
locations on the target surface. Therefore, if the carbon nanotubes
are used as the cathode as in the X-ray generator according to the
first aspect, the above-described uneven brightness of the emitter
and its hourly fluctuation would affect the performance of the
X-ray generator. The X-ray generator according to the second aspect
has no such problem.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a cross-sectional view of an electron gun unit in
the first embodiment of the X-ray generator according to the first
aspect of this invention;
[0015] FIG. 2 is an elevation view of a takeoff electrode of the
electron gun unit shown in FIG. 1;
[0016] FIG. 3 is a cross-sectional view of an electron gun unit in
the second embodiment of the X-ray generator according to the first
aspect of this invention;
[0017] FIG. 4 is a cross-sectional view of an electron gun unit in
the third embodiment of the X-ray generator according to the first
aspect of this invention;
[0018] FIG. 5 is a cross-sectional view of an electron gun unit in
one embodiment of the X-ray generator according to the second
aspect of this invention;
[0019] FIG. 6 is an enlarged cross-sectional view illustrating the
vicinity of a hot cathode of the electron gun unit shown in FIG. 5;
and
[0020] FIG. 7 is an elevation view of a hot cathode of the electron
gun unit shown in FIG. 5.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] FIG. 1 is a cross-sectional view of an electron gun unit in
the first embodiment of the X-ray generator according to the first
aspect of this invention. This cross-sectional view illustrates the
electron gun unit 10 seen from its lateral side. The electron gun
unit 10 has a Wehnelt 12 within which a cathode 14 is disposed. The
cathode 14 includes a cathode base 22 having a surface to which an
emitter 24 is fixed. The cathode base 22 is made of a nickel plate
0.5 mm thick. The emitter 24 is for electron emission and made of
carbon nanotubes. A target 16 is disposed so as to face this
electron gun unit 10. The cathode 14, the takeoff electrode 18 and
the target 16 make a specific electric field under which the
emitter 24 emits electrons. The electrons are focused by a window
20 of the Wehnelt 12 and accelerated by the electric field between
the cathode 14 and the target 16 so as to collide against the
surface of the target 16 to generate X-rays. The distance L1
between the emitter 24 and the front surface of the Wehnelt 12 is 6
to 7 mm. The distance L2 between the front surface of the Wehnelt
12 and the surface of the target 16 is about 12 mm.
[0022] FIG. 2 is an elevation view of the takeoff electrode 18 seen
from the target side. The takeoff electrode 18 has a substantially
rectangular shape and a rectangular opening 38. The takeoff
electrode 18 further has two legs 40 supported by insulators 32
(see FIG. 1). Seen from the target side, the takeoff electrode 18
appears inside the Wehnelt window 20 and the emitter 24 appears
inside the opening 38 of the takeoff electrode 18.
[0023] The Wehnelt window 20 has a size of about 10 mm square. The
takeoff electrode opening 38 has a size of about 7 mm square. The
takeoff electrode 18 is made of a plate 0.5 to 1.0 mm thick. The
emitter 24 has an electrode emission region having a size of about
5 mm square.
[0024] Referring back to FIG. 1, an electric circuit of the X-ray
generator will be explained. The cathode 14 is supported by an
insulator 28 and connected to a cathode line 30. The takeoff
electrode 18 is supported by the insulators 32 and connected to a
takeoff electrode line 33. The Wehnelt 12 is connected to a Wehnelt
line 34. The target 16 is connected to a target line 36.
[0025] The first power supply 42 is connected between the cathode
line 30 and the target line 36, so that the target potential (i.e.,
tube voltage) is set zero to 60 kV based on the cathode potential.
The emitter 24 of the cathode 14 emits electrons which are
accelerated by the tube voltage and collide against the target 16.
The second power supply 44 is connected between the cathode line 30
and the Wehnelt line 34, so that the Wehnelt potential is set zero
to minus 1000 V based on the cathode potential. The emitter 24 of
the cathode 14 emits electrons which are focused by the Wehnelt
window 20 (its potential is negative based on the cathode 14) on
the predetermined region of the surface of the target 16. The third
power supply 46 is connected between the cathode line 30 and the
takeoff electrode line 33, so that the takeoff electrode potential
is set minus 1000 to plus 1000 V based on the cathode potential.
The takeoff electrode potential is controlled so as to regulate an
emitter-emitting electron current (i.e., tube current). The takeoff
electrode 18 potential is ordinarily set positive based on the
cathode 14 potential to regulate the tube current, while in some
cases it may be set negative to restrain the tube current. With the
emitter 24 made of carbon nanotubes, electron density can be high
as 100 mA to 1 A per square centimeters.
[0026] The second power supply 44 controls the Wehnelt 12 potential
based on the cathode 14 potential so that the electron beam can be
focused on the target 16 and the focus size on it can be adjusted.
On the other hand, the third power supply 46 controls the takeoff
electrode 18 potential based on the cathode 14 potential so that
the tube current can be regulated. With the cathode having the
conventional hot filament "a filament current" is controlled to
regulate the tube current, while with the emitter 24 made of carbon
nanotubes "the takeoff electrode 18 potential" is controlled as
described above to regulate the tube current because the cathode 14
per se has no function of controlling the tube current.
[0027] With the cathode having the emitter made of carbon
nanotubes, the emitter is not required to be heated to a high
temperature, so that no high-power heating power supply is needed
and reduced energy consumption is obtained as compared with the
conventional hot cathode.
[0028] It is required in the X-ray generator to stabilize the tube
current with high accuracy, its allowable fluctuation being about
0.1 percent. To stabilize the tube current it is required to
control independently the Wehnelt 12 potential and the takeoff
electrode 18 potential with the use of the second power supply 44
and the third power supply 46 respectively. If the takeoff
electrode 18 would be omitted, the Wehnelt 12 potential must be
controlled to regulate both the focus size on the target and the
tube current, in such a case the tube current can not be precisely
controlled independently of the focus size.
[0029] Next, the second embodiment of the X-ray generator according
to the first aspect of this invention will be explained. FIG. 3 is
a cross-sectional view of an electron gun unit of the second
embodiment. This embodiment differs from the first embodiment in
that there is no takeoff electrode while a tungsten filament 48 for
electron emission is disposed behind the cathode 14. The filament
48 has both ends connected to a filament heating power supply 50
which supplies a low voltage of about zero to 2 V between the both
ends of the filament 48 to control the heating temperature of the
filament 48. Between such a filament-heating circuit 52 and the
cathode line 30 is connected the third power supply 54, so that the
filament 48 potential is set zero to minus 300 V based on the
cathode 14 potential. The filament 48 potential is controlled based
on the cathode 14 potential to regulate the collision energy of the
electrons emitted from the filament 48 against the back of the
cathode 14, so that the heating temperature of the cathode 14 is
controlled and an amount of electron emission from the emitter 24
is regulated to adjust the tube current. It is noted that the
cathode temperature caused by the electron collision is not so
high, the temperature being in a range of the room temperature to
about 100 degrees Celsius at most. The first power supply 42 and
the second power supply 44 are the same as those in the first
embodiment.
[0030] Next, the third embodiment of the X-ray generator according
to the first aspect of this invention will be explained. FIG. 4 is
a cross-sectional view of an electron gun unit of the third
embodiment. This embodiment differs from the first embodiment in
that there is no takeoff electrode while the cathode 14 has a
heater 56 fixed on the back of the cathode 14. The heater 56 has
both ends connected to a heater power supply 58 which controls the
heating temperature of the heater 56 to regulate the temperature of
the cathode 14, so that an amount of electron emission is
controlled to adjust the tube current. It is noted that the cathode
temperature is not so high, the temperature being in a range of the
room temperature to about 100 degrees Celsius at most. The first
power supply 42 and the second power supply 44 are the same as
those in the first embodiment.
[0031] Although the three embodiments described above use the
emitter 24 made of carbon nanotubes, an emitter made of fullerenes
may be used instead.
[0032] Although the three embodiments described above use the
emitter having a flat surface, the emitter may have a concave or
convex surface facing the target so that electron beam focusing may
be improved.
[0033] Next, an X-ray generator according to the second aspect of
this invention will be explained. FIG. 5 is a cross-sectional view
of an electron gun unit 60 in one embodiment of the X-ray generator
according to the second aspect of this invention. This
cross-sectional view illustrates the electron gun unit 60 seen from
its lateral side. The electron gun unit 60 has a Wehnelt 62 within
which a hot cathode 64 is disposed. An electron emission electrode
68 is disposed behind and apart from the hot cathode 64. The
electron emission electrode 68 functions as a bombarding electrode
to heat the hot cathode 64.
[0034] A target 66 is disposed to face the electron gun unit 60.
The hot cathode 64 emits hot electrons 76 which are focused by the
window 78 of the Wehnelt 62 and accelerated by an electric field
between the hot cathode 64 and the target 66 so as to collide
against the surface of the target 66 to generate X-rays. The
distance L3 between the front surface of the Wehnelt 62 and the
surface of the target 66 is about 12 mm.
[0035] FIG. 6 is an enlarged cross-sectional view illustrating the
vicinity of the hot cathode 64. The electron emission electrode 68
includes a electrode base 70 having a surface on which an emitter
72 is fixed. The electrode base 70 is made of a nickel plate 0.5 mm
thick. The emitter 72 is for electron emission and made of carbon
nanotubes. The distance L4 between the hot cathode 64 and the
electron emission electrode 68 is set 0.5 to 3.0 mm. The hot
cathode 64 is made of lanthanum hexaboride 0.5 mm thick. The hot
cathode 64 may have a carbon base having an electron emission
surface made of lanthanum hexaboride. The hot cathode 64 may be
made of other materials such as (1) tungsten only, (2) tantalum
only, (3) impregnated tungsten, i.e., tungsten impregnated with
barium for an improved emission characteristic or (4) thorium-added
tungsten.
[0036] A voltage is supplied between the hot cathode 64 and the
electron emission electrode 68 so that the potential of the
electron emission electrode 68 becomes negative based on the hot
cathode 64 potential, so that the emitter 72 of the electron
emission electrode 68 emits electrons 74 by the field emission.
With the emitter 72 made of carbon nanotubes, electron density can
be high as 100 mA to 1 A per square centimeters. The electrons 74
are accelerated by the electric field to collide against the back
of the hot cathode 64, so that the hot cathode 64 is heated to emit
other hot electrons 76 which collide against the target 66. A
preferable heating temperature of the hot cathode 64 is 1000 to
1600 degrees Celsius for lanthanum hexaboride, impregnated tungsten
or thorium-added tungsten, and 2000 to 2300 degrees Celsius for
tungsten or tantalum.
[0037] FIG. 7 is an elevation view of the hot cathode 64 seen from
the target side. The hot cathode 64 has a square surface and is
disposed within a Wehnelt opening 80. The Wehnelt has a front
surface formed with a window 78 which is larger than the opening
80. The window 78 has a size of about 7 mm square while the opening
80 has a size of about 5 mm square. The hot cathode 14 has a
surface whose size is about 3 mm square.
[0038] Referring back to FIG. 5, an electric circuit of the X-ray
generator will be explained. The first power supply 92 is connected
between the hot cathode 64 and the target 66, so that the hot
cathode 64 potential (i.e., tube voltage) is set, for example,
minus 60 kV based on the target 66 potential (ordinarily grounded).
The hot cathode 64 emits hot electrons 76 which is accelerated by
the tube voltage to collide against the target 66. The tube current
is about several tens to 300 mA. The second power supply 94 is
connected between the hot cathode 64 and the Wehnelt 62, so that
the Wehnelt 62 potential is set zero to minus 1000 V based on the
hot cathode 64 potential. The hot cathode 64 emits hot electrons 76
which are focused by the Wehnelt window 78, whose potential is
negative based on the hot cathode 64 potential, on the
predetermined region of the surface of the target 66. Thus the
second power supply 94 is for controlling the Wehnelt 62 potential
based on the hot cathode 64 to focus the hot electrons 76 from the
hot cathode 64 on the target 66. The second power supply 94 is
controlled to adjust the focus size on the target 66.
[0039] The third power supply 96 is connected between the hot
cathode 64 and the electron emission electrode 68, so that the
potential of the electron emission electrode 68 is set a suitable
potential within a range of zero to 1000 V, for example minus 500
V, based on the hot cathode 64 potential. The electric field
between the hot cathode 64 and the electron emission electrode 68
is set preferably 1000 to 6000 V/mm. The potential of the electron
emission electrode 68 is controlled based on the hot cathode 64
potential so as to regulate the current of the electrons 74 (see
FIG. 6) from the hot cathode 64. The current is about 1 to 10 mA.
The current value determines the heating temperature of the hot
cathode 64, and the heating temperature determines the current of
the hot electrons 76 from the hot cathode 64 (i.e., tube current).
Therefore, the third power supply 96 is controlled so as to
regulate the tube current of the X-ray generator.
[0040] The hot cathode of this X-ray generator is not the
direct-heating type in which a current flows directly through the
hot cathode to cause self-heating by resistance, so that it
requires no conventional large-current high-voltage cable. The
X-ray generator has a closed circuit comprised of the electron
emission electrode 68, the hot cathode 64 and the third power
supply 96. Through the closed circuit flows a current (hereinafter
referred to as heating current) which is very small as compared
with the conventional filament current. Through the high-voltage
cable connected to the X-ray generator flows a current which is the
sum of the tube current and the heating current described above.
The tube current is about several tens to 300 mA and the heating
current is about 1 to 10 mA, so that through the high-voltage cable
flows a current of about 300 mA at most. Therefore, no thick cable,
for a high-voltage and a large-current, is required and thin
high-voltage cables on the market are sufficient. Such a thin
high-voltage cable is not expensive and easy to handle. The X-ray
generator does not receive a large force from the thin high-voltage
cable. Since a large current does not flow the high-voltage cable,
the connector between the X-ray generator and the high-voltage
cable may not be a design for bearing heat generation. The
high-voltage cables connected to the X-ray generator include three
cables which are a cable connected to the electron emission
electrode 68, a cable connected to the hot cathode 64 and a cable
connected to the Wehnelt 62. The target 66 is grounded along with
the casing of the X-ray generator, so that no high-voltage cable is
required for the target 66.
[0041] Although the above-described embodiment of the second aspect
of this invention uses the emitter 72 made of carbon nanotubes, an
emitter made of fullerenes may be used instead.
* * * * *