X-ray or gamma ray systems or devices – Source – Electron tube
Reexamination Certificate
2000-10-06
2003-04-22
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Source
Electron tube
C378S124000
Reexamination Certificate
active
06553096
ABSTRACT:
BACKGROUND OF THE INVENTION
In the description that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicants expressly reserve the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention.
X-rays occupy that portion of the electromagnetic spectrum between approximately 10
−8
and 10
−12
m. Atoms emit x-rays through two separate processes when bombarded with energetic electrons.
In the first process, high-speed electrons are decelerated as they pass through matter. If an individual electron is abruptly decelerated, but not necessarily stopped, when passing through or near the nuclear field of a target atom, the electron will lose some of its energy which, through Plank's law, will be emitted as an x-ray photon. An electron may experience several such decelerations before it is finally stopped, emitting x-ray photons of widely different energies and wavelengths. This process produces the bulk of x-ray radiation and results in a continuous-type spectrum, also called Bremsstrahlung.
In the second process, an incident electron collides with and ejects an orbital electron of a target atom. If the ejected electron is from an inner shell orbit, then an electron in an outer shell orbit will fall to the inner vacant orbit with an attendant emission of an x-ray photon. In this process energy is emitted in the form of an x-ray whose energy or wavelength represents the orbital transition involved. Because the energies of orbital electrons are quantized, the x-ray photons emitted are also quantized and can only have discrete wavelengths characteristic of the atom. This gives rise to their classification as characteristic x-rays.
Several methods have been used to produce the incident electrons at a cathode and accelerate them into a target anode. One traditional approach has been the use of an x-ray tube. Depending upon the method used in generating the electrons, x-ray tubes may be classified in two general groups, gas tubes and high-vacuum tubes.
FIG. 1
shows a conventional gas x-ray tube. The x-ray generating device
110
is substantially made of a glass envelope
120
into which is disposed a cathode
125
which produces a beam of electrons
140
which strike an anode
130
thereby causing x-rays to be emitted
150
which can be used for sundry purposes including medical and scientific. The cathode is powered by a high voltage power supply via electrical leads
135
. In addition, a gas pressure regulator
115
regulates the gas pressure in this type of x-ray device.
High vacuum tubes, an example of which is shown in
FIG. 2
, are a second type of x-ray tube.
FIG. 2
shows a vacuum x-ray tube device with a thermionic cathode. In this type of device
210
a glass envelope
220
serves as the vacuum body. The cathode
225
is deposed within this vacuum and is provided with electrical leads
235
. Electrons
240
are emitted by thermionic emission from the cathode
225
and strike an anode target
230
. The efficiency of such emission of x-rays is very low causing the anode to be heated. To increase the lifetime of this device, it has been necessary to provide a cooling mechanism. One embodiment of a cooling mechanism is a chamber
260
through which water is circulated by the use of an inlet
265
and an outlet
270
. To improve the efficiency of the emitted beam of electrons a focusing shield
245
is often utilized. The focusing shield
245
collimates the thermionically emitted electrons and directs them to the anode 230. However, the thermionic origin of the electrons makes focusing to a small spot size difficult. This, in part, limits the resolution of modern x-ray imaging (see, for example, Radiologic Science For Technologist, S. C. Bushong, Mosby-Year Book, 1997). X-rays
250
emitted from the anode
230
pass through a window
255
and are subsequently available for sundry purposes, including medical and scientific. An additional feature of this type of device is an exterior shutter
275
. It has been found necessary to incorporate such a shutter to prevent the incidental emission of x-rays associated with the heating decay of the cathode. This is because even though the application of power to the cathode may be terminated, residual heating may be such that electrons continue to be emitted towards the target and continue to produce x-rays.
This process of x-ray generation is not very efficient since about 98 percent of the kinetic energy of the electron stream is converted upon impact with the anode into thermal energy. Thus, the focus spot temperature can be very high if the electron current is high or continuous exposure is required. In order to avoid damage to the anode it is essential to remove this heat as rapidly as possible. This can be done by introducing a rotating anode structure.
As noted above, a shutter (e.g.
275
) is necessary in such devices because thermionic emission of electrons from a cathode does not allow for precise step function initiation and termination of the resulting electron beam. Indeed, while still at elevated temperatures and subsequent to removal of power, a thermionic cathode may emit electrons which may cause unwanted x-ray emission from the target. In operation the shutter is held open either mechanically or by means of a microswitch.
Moreover, due to high temperature heating, the cathode filament has a limited lifetime, typically around a few hundred hours in medical applications and thousand hours in analytical applications. Under normal usage, the principle factor determining the lifetime of the x-ray tube is often damage to the cathode filament.
The amount of useful x-rays generated in the anode is proportional to the electron beam current striking at the anode. In thermionic emission, the electron beam current is only a small fraction of the current passing through the cathode filament (typically 1/20). In modern medical applications such as digital radiography and Computed Tomography (CT), very high x-ray intensity is required concomitantly requiring a very high thermionic emission cathode current. Therefore, a principle limitation in these applications is the amount of electron beam current generated by the cathode.
A possible improvement in the generation of x-rays is the introduction of field emission cathode materials. Field emission is the emission of electrons under the influence of a strong electric field. However, the incorporation of conventional field emission cathode materials into x-ray generating devices presents certain challenges. For instance, the field emission cathode materials must be capable of generating an emitted electron current density of a sufficiently high level (can be as high as 2000 mA on the target for medical applications) such that, upon striking the anode target material, the desired x-ray intensity is produced.
Many conventional field emission materials are incapable of producing the desired emitted electron circuit density absent the application of a relatively high electrical field to the cathode. Moreover, many of the conventional field emission materials cannot produce stable emissions at high current densities under high applied electrical fields. The use of high control voltages increases the likelihood of damaging the cathode material, and requires the use of high powered devices which are costly to procure and operate.
Conventional field emission materials such as metals (such as Mo) or a semiconducting materials (such as Si), with sharp tips in nanometer sizes have been utilized. Although useful emission characteristics have been demonstrated for these materials, the turn-on electric field is relatively high, typically on the order of 50-100 V/&mgr;m at a current density of 10 mA/cm
2
. (See, for example, W. Zhu et al.,
Science,
Vol. 282, 1471, (1998)).
Carbon materials, in the form of diamond and carbon nanotubes, have emerged as potentially useful for electron field
Lu Jianping
Zhou Otto Z.
Burns Doane Swecker & Mathis L.L.P.
Song Hoon K.
The University of North Carolina - Chapel Hill
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