X-ray or gamma ray systems or devices – Source – Electron tube
Reexamination Certificate
2000-05-05
2001-12-25
Dunn, Drew (Department: 2882)
X-ray or gamma ray systems or devices
Source
Electron tube
C378S122000, C313S327000, C445S046000
Reexamination Certificate
active
06333968
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This device pertains generally to a device for generating X rays and more specifically to an X-ray transmission cathode wherein the X rays produced in an evacuated X-ray tube by an anode or sample are allowed to exit the tube through the cathode.
2. Description of the Related Art
The typical configuration for a sealed X-ray tube involves a resistively heated, drawn wire filament cathode for generating free electrons in vacuum, and a metallic anode held at high voltage with respect to the cathode. The emitted electrons are electrostatically accelerated to high energy and made to collide with the anode, which then emits the X rays. The voltages required for economical X-ray emission exceed the binding energy of inner electrons in the atoms of the anode, typically kilovolts. The anode emits continuum bremstrahlung X rays as well as characteristic X rays. Emission occurs in all directions, but the intensity in any direction is modified by the absorption of the X rays as they depart their points of origin. The characteristic rays are distinctive for each of the chemical elements, and form the basis of the well known elemental analysis by X-ray emission. Selective detection, processing, and display techniques have been used to record the characteristic rays and analyze the spatial variations of composition in X-ray emitting materials.
As used herein, an x ray photon is a photon with sufficient energy to ionize a neutral atom by photoelectric absorption. There is a wide variation in the energy range of ionizing photons.
The usual geometry for sealed X-ray tubes
10
, as shown in
FIG. 1
a
, includes a filament cathode
12
, an anode
14
, and a separate X-ray “window”
16
made of thin material, usually metal, through which the X rays
18
exit the vacuum sealed X-ray tube
10
. It is well understood in the art that a fraction of incident x rays
18
are absorbed by any X-ray transmitting window
16
material such as a window
16
, and that the suitability of a material as a window
16
is enhanced for smaller magnitudes of that fraction. The filament cathode
12
is connected between a pair of terminals
13
and
17
, to a cathode low voltage power supply
22
which supplies current to the cathode
12
to heat the filament cathode
12
and excite electron flow
15
. A high voltage power supply
24
is connected to the anode
14
to accelerate the flow of the emitted electrons
15
. In this design, the anode
14
placement and shape is subject to two major geometric constraints, (1) maintaining sufficient distance between the anode
14
and other items that the electric fields within the vacuum sealed X-ray tube
21
remain low enough to preclude breakdown and surface currents, and (2) insuring that the window
16
placement is such that X rays
18
are afforded sufficient solid angle to reach the outside of the vacuum sealed X-ray tube
21
with acceptable levels of absorption.
Typical vacuum sealed X-ray tube
10
design of the prior art places the sample or X-ray target
23
and window
16
such that X rays
18
are emitted at or near 90 degrees from the path of the incident electrons. Because X rays are less strongly absorbed than the electrons, angles are commonly chosen such that the electron penetration distance in the anode
14
is shorter than the exit path for emitted X rays
18
. X-ray
18
takeoff angles of 6 to 30 degrees (from the surface of the anode
14
) are not uncommon; appreciable X-ray absorption in the anode
14
occurs at these low angles.
A variant among tube designs of the prior art is the transmission anode, end window, tube
20
, as shown in
FIG. 1
b
, commonly known as the end-window tube , in which the transmission anode
26
functionalities of an anode and a window are combined in a single member. A transmission anode
26
must allow the electrons
29
to strike the anode
26
to produce X rays
31
, dissipate charge and heat from its surfaces and from throughout its volume, and permit the X rays
31
to pass through to the outside; these requirements are usually achieved with transmission anodes
26
made of thin metal foils. The transmission anode, end-window tube
20
is advantageous in some applications, but the requirement for a thin anode
26
results in lower X-ray
31
output power. It is quite common for the end-window anode
26
, an exterior component, to be held at ground potential, which leads to the requirement for the cathode
33
portion to be at high voltage. The cathode filament current power supply
34
must float at high negative voltage while the anode
26
is connected to a tube high voltage power supply
32
to accelerate the flow of emitted electrons
31
.
In contrast to tube designs of the prior art shown in
FIGS. 1
a
and
1
b
, the transmission cathode, end-window, tube discussed below, enables the X rays
31
from the transmission cathode
33
to exit the anode
26
at the same angle that the electrons
29
are incident, thus reducing the X-ray absorption and enhancing tube
2
output and permitting grounded exterior components. The transmission cathode
33
is not bombarded by high energy electrons and need not dissipate as much charge or heat from within its volume, thus it need not be as good a volume conductor of either.
While the hot filament cathode based on thermionic emission is very common, alternative technologies based on field emission, photo emission, and plasma emission have been investigated as well. Field emission tips have been used for X-ray production in the past on radiography machines to produce nanosecond pulses of X rays by accelerating electrons from an array of emitters into a metal foil end-window anode. Photoemission involves irradiating the cathode with suitable light sources capable of stimulating the cathode to emit electrons. SEE, U.S. Pat. No. 5,042,058, Rentzepis, issued Aug. 20, 1991, entitled ULTRASHORT TIME-RESOLVED X-ray SOURCE. Plasma emission cathodes involve locally heating the cathode surface to temperatures sufficient to produce a plasma, from which electrons are emitted. SEE, U.S. Pat. No. 5,335,258, Whitlock, issued Aug. 2, 1994, entitled SUBMICROSECOND, SYNCHRONIZABLE X-ray SOURCE.
Spatial resolution based on direct X-ray emission has been practiced with the electron microprobe and scanning electron microscope. Fluorescent X-ray emission has also been used for compositional mapping. SEE, U.S Pat. No. 5,742,658, Tiffin et al., issued Apr. 21, 1998, entitled APPARATUS AND METHOD FOR DETERMINING THE ELEMENTAL COMPOSITIONS AND RELATIVE LOCATIONS OF PARTICLES ON THE SURFACE OF A SEMICONDUCTOR WAFER.
The focusing and collimation of arrays of micro electron sources has been well documented. SEE, U.S. Pat. No. 4,874,981, Spindt, entitled AUTOMATICALLY FOCUSING FIELD EMISSION ELECTRODE, and Cha-mei Tang ET AL.; PLANAR LENSES FOR FIELD-EMITTER ARRAYS; J. Vac. Sci. Technol. B
13
(2), March/April 1995, pp. 571-575.
Due to the unavailability of lenses for X rays, geometric imaging means are commonly used to generate X-ray images. X radiography
30
, as shown in
FIG. 1
c
, in which a sample
42
is imaged with X-rays
38
, typically uses point projection imaging. A small (“point”) source of X-rays
36
emits X-rays
38
spherically outward through the exit window of the tube (not shown). The sample
42
to be radiographed is placed between the X-ray source
36
and the imaging detector
44
, e.g., an X-ray film plate used for medical imaging. The spatial resolution of the image is limited by the size of the X-ray point source
36
. The achievable X-ray output power cannot exceed the ability of the X-ray tube
37
to absorb the heat load of its internal electron beam within the small focal point from which the X-rays
38
emanate. Where the sample is in close proximity to or contacting the imaging detector (typically X-ray film), the arrangement is called contact radiography and unit magnification is achieved. In typical applications where a magnified image is required, this can be obtained by moving the image plane further from the source
Bell Michael I.
Davidson Jimmy L.
Kang Weng Poo
Kerns David V.
Kerns Sherra
Dunn Drew
Karasek John J.
Mills John G.
The United States of America as represented by the Secretary of
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