U.S. patent number 3,714,486 [Application Number 05/078,778] was granted by the patent office on 1973-01-30 for field emission x-ray tube.
Invention is credited to James H. McCrary.
United States Patent |
3,714,486 |
McCrary |
January 30, 1973 |
FIELD EMISSION X-RAY TUBE
Abstract
A miniature X-ray tube with D.C. power supply and a cold cathode
field emission electron beam for continuous or steady state X-ray
output. A tube a few centimeters in length with a needle cathode
along the axis of the tube and with an exit window at the end
behind the cathode for optimum X-ray output.
Inventors: |
McCrary; James H. (Las Vegas,
Clark County, NV) |
Family
ID: |
22146160 |
Appl.
No.: |
05/078,778 |
Filed: |
October 7, 1970 |
Current U.S.
Class: |
378/122; 378/143;
313/336 |
Current CPC
Class: |
H01J
35/02 (20130101); H01J 35/18 (20130101) |
Current International
Class: |
H01J
35/14 (20060101); H01J 35/00 (20060101); H01J
35/02 (20060101); H01j 035/00 () |
Field of
Search: |
;313/55,59,336 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Lake; Roy
Assistant Examiner: Hostetter; Darwin R.
Claims
I claim:
1. In a tube for continuously producing X-rays by field emission,
the combination of:
a housing;
an anode mounted in said housing;
an unheated cathode mounted in said housing and spaced and
electrically insulated from said anode;
an X-ray transmitting window in said housing; and
a high voltage DC electric power supply connected across said anode
and cathode for producing by field emission, a continuous stream of
electrons from said cathode to said anode producing a continuous
X-ray output through said window, with the electrical impedance
across said anode and cathode in excess of 1,000,000 ohms.
2. An X-ray tube as defined in claim 1 in which said housing
includes an insulating tube, an anode support ring at one end of
said tube and a cathode support ring at the other end of said
tube,
with said anode having a generally hemispherical end and carried in
said anode support ring along the axis of said tube, and
with said cathode being needle shaped and carried in said cathode
support ring along the axis of said insulating tube with the needle
tip pointing to and spaced from the anode end.
3. An X-ray tube as defined in claim 1 in which the anode-cathode
spacing is in the order of 0.2 to 2 mm. and with an anode-cathode
potential in the order of 10-30 kv.
4. An X-ray tube as defined in claim 1 in which said housing is
generally tubular in configuration and in the order of a few
centimeters in length, and in which the anode-cathode spacing is in
the order of a millimeter.
5. An X-ray tube as defined in claim 4 with said anode carried at
one end and said cathode carried at the other end of said tubular
housing with the electron beam from cathode to anode parallel with
the longitudinal axis of said housing, and with said window
disposed at said other end of said housing generally perpendicular
to said axis.
6. An X-ray tube as defined in claim 5 in which said anode has a
convex end, said cathode is needle shaped and pointing toward said
anode end, and including a bracket at said housing other end
supporting said cathode centrally of said window.
Description
This invention relates to X-ray tubes and in particular to steady
state field emission tubes which provide continuous output with low
power requirements and high efficiency and which can be provided in
very small sizes.
The X-ray tube in common use utilizes a heated cathode to provide
an electron beam directed toward an anode to generate the X-rays.
Typically the cathode comprises a filament in the form of a coil of
tungsten wire and is energized from a low voltage AC source which
produces resistive heating and an electron beam by thermionic
emission. A substantial amount of electrical power is required to
heat the cathode and the resultant tube and associated power supply
are large and cumbersome and expensive, usually weighing hundreds
of pounds and costing thousands of dollars.
An electron beam may be produced between a cathode and an anode by
field emission, that is, by emission of electrons from an unheated,
pointed metal cathode due to an intense electric field in the
vicinity of the cathode. Some field emission X-ray machines are
presently known for producing intense, fast pulses of X-rays. Such
machines utilize large banks of capacitors which are charged to
high voltage from a DC source, with the capacitors then being
discharged across the electrodes of the tube to provide a single
burst of X-rays. These pulse type field emission machines with
their capacitors are also large, cumbersome, and expensive,
typically weighing many hundreds of pounds and costing tens of
thousands of dollars and substantially filling a room.
The X-ray tube of the present invention utilizes a high voltage
power supply connected across the electrodes to provide a field
emission electron beam and a continuous X-ray output. This
instrument provides a number of significant advantages over the
presently known equipment. The efficiency is very high as most of
the electrical power delivered to the tube is converted into
X-rays, since a cold cathode is utilized. Also, operation with a
cold cathode results in no deposit of cathode material on the
anode. The simplicity of tube design and efficient power
utilization permit miniaturization of the system, with the tube
being a few centimeters long and with the tube and power supply
requiring a very small volume, typically in the order of 50 cubic
inches, weighing about 2 pounds and selling for less than a
thousand dollars. The instrument is quite small and is readily
operated from batteries, thereby making it readily portable.
The instrument has relatively high radiation output in terms of
electrical power consumption, has a very small anode focal spot,
and permits fabricating the anode from various metals to provide
different characteristic X-ray outputs.
Other objects, advantages and results will more fully appear in the
course of the following description. The drawing merely shows and
the description merely describes a preferred embodiment of the
present invention which is given by way of illustration or
example.
In the drawing:
FIG. 1 is a longitudinal sectional view of an X-ray tube
incorporating the preferred embodiment of the invention;
FIG. 2 is a sectional view taken along the line 2--2 of FIG. 1;
and
FIG. 3 is a sectional view taken along the line 3--3 of FIG. 1.
In the preferred embodiment, a housing 10 is formed of a glass tube
11 with an anode end ring or anode holder 12 and a cathode end ring
or cathode holder 13. An anode 15 preferably is formed as part of
the ring 12 and projects into the tube 11. A plurality of openings
17 in the anode communicate with the interior of a hollow stem 16
and provide a passageway for evacuation of the interior of the
tube. A needle-shaped cathode 20 is supported at the center of the
end ring 13. The end of the tube is closed by a window 22 formed of
a material which passes X-rays, typically beryllium.
A DC power supply 25 is connected to the anode and cathode, as by
conductors 26, 27. The power supply provides a high voltage for
producing a high intensity electric field in the vicinity of the
cathode. The voltage desirably should be variable, permitting
variation of the X-ray intensity. While a well regulated DC power
supply is preferred, an AC supply can be used. The output intensity
would be less since X-rays are produced only when the anode is at a
high positive voltage with respect to the cathode. Operation at
60H.sub.z would provide the desired continuous X-ray output.
The cathode end ring 13 typically may be of brass and the cathode
20 typically may be a conventional nickel-plated steel sewing
needle pressed into an opening in the center of the holder 13. The
anode 15 may be made of any metal, although metals which are easily
machined and plated are preferred. Copper and aluminum are
presently being used. The stem 16 may be pressed into the end ring
12, the parts may be soldered or welded together, or the parts may
be made as a single piece, if desired. The tube 11, end rings 12,
13, and window 22 may be assembled using a vacuum compatible epoxy.
The glass tube 11 provides electrical insulation between the anode
and cathode.
The assembled housing may be evacuated following conventional
techniques and typically is evacuated to a pressure of about
10.sup.-.sup.7 Torr and the stem 16 is sealed at 28. Conventional
gettering material may be used if desired. The cathode end ring 13
provides a support for the window 22 and the openings 29, 30 in the
cathode end ring define the X-ray beam from the tube.
The X-ray tube may be made quite small and one unit assembled as
illustrated in the drawing is 5.5 centimeters long over the end
rings with an outside diameter of 2.54 centimeters. The end of the
anode 15 preferably is hemispherical and in this embodiment has a
radius of 0.64 centimeters. The tip of the cathode needle has a
radius approximately 0.03 millimeters. The window 22 is formed of
beryllium in the order of 0.025 to 0.05 mm thick.
Electrode spacing is determined by the desired operating voltage
and beam current, with the X-ray intensity being a function of the
applied voltage. The electrode spacing is in the order of a few
millimeters and preferably in the range of 0.2 to 2 mm. The applied
voltage preferably is in the range of about 10 kv to 30 kv.
In an instrument manufactured as shown in the drawing, with a
copper anode, and a space of 1.5 mm between the electrodes, an
applied voltage of 25 kv DC produces a beam current of about 20
microamperes, indicating anode-cathode impedance to be in excess of
one million ohms. Moving the electrodes closer together would
provide the same current with a lower voltage. With 25 kv dc
applied, the X-ray intensity measured 30 cm from the window 22 is
100 mR/h. Increasing the voltage to 30 kv dc increases the X-ray
intensity to 1,000 mR/h. The power consumption is less than 1 watt,
with little heating, thereby eliminating the requirement for anode
cooling. Analysis of cathode shadows observed in radiographs
utilizing the tube indicate that the effective diameter of the
radiation source is 0.1 mm or less. The shape of the X-ray spectrum
as measured with a Si(Li) spectrometer shows the typical
bremsstrahlung spectrum cutting off at 25 keV with intense Cu
K.sub..alpha. and K lines at 8.0 keV and 8.9 keV respectively.
The electron beam trajectories between cathode and anode are
parallel with the axis of the tube 11 and the housing construction
permits locating the outlet window 22 behind the cathode and
perpendicular to the trajectories. This results in a more efficient
utilization of X-rays than in the conventional design where the
exit window is located at the side of the housing parallel to the
trajectories. Exiting X-rays traverse less anode material in the
present design.
The X-ray tube illustrated and described above is extremely small,
light and simple in design. It has low power consumption and high
efficiency and does not have any heat dissipation problems, since
it is a cold cathode device. The power supply requirements are low
resulting in an inexpensive, reliable, small and portable unit.
In operation, a high voltage is connected across the anode and
cathode, producing a field emission electron beam from the tip of
the cathode to the anode. Impingement of electrons on the anode
produce X-rays which exit through the window 22 providing the
desired output of the tube.
Although an exemplary embodiment of the invention has been
disclosed and discussed, it will be understood that other
applications of the invention are possible and that the embodiment
disclosed may be subjected to various changes, modifications and
substitutions without necessarily departing from the spirit of the
invention.
* * * * *