Method and apparatus for controlling electron beam current

Abstract

An x-ray generating device includes a field emission cathode formed at least partially from a nanostructure-containing material having an emitted electron current density of at least 4 A / cm2. High energy conversion efficiency and compact design are achieved due to easy focusing of cold cathode emitted electrons and dramatic reduction of heating at the anode. In addition, by pulsing the field between the cathode and the gate or anode and focusing the electron beams at different anode materials, pulsed x-ray radiation with varying energy can be generated from a single device. Methods and apparatus for independent control of electron emission current and x-ray energy in x-ray tubes are also provided. The independent control can be accomplished by adjusting the distance between the cathode and anode. The independent control can also be accomplished by adjusting the temperature of the cathode. The independent control can also be accomplished by optical excitation of the cathode. The cathode can include field emissive materials such as carbon nanotubes.

Description

RELATED APPLICATIONS [0001] This application is a divisional patent application which claims the benefit of the filing date of U.S. patent application Ser. No. 10 / 358,160, filed Feb. 5, 2003, which was a continuation-in-part of U.S. Pat. No. 6,553,096, filed on Oct. 6, 2000, the disclosure of which is incorporated herein by reference in its entirety.GOVERNMENT INTEREST [0002] At least some aspects of this subject matter were made with Government support under the sponsorship of the Office of Naval Research, contract no. N00014-98-1-0597. The Government may have certain rights in this subject matter.BACKGROUND ART [0003] 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 subject m...

AI Technical Summary

Benefits of technology

[0028] The present subject matter avoids the undesirable features of current x-ray generating devices by incorporating a field emission nanostructure cathode material into an x-ray generating device. The nanostructure field emission material of the present subject matter is capable of producing, in a controlled and reliable manner, a high emitted electron current density through the application of a relatively small control electrical field. Therefore, a substantially higher electron beam current can be achieved compared with that of thermionic emission. The nanostructure field emission cathode of the present subject matter is capable of providing precise step-function initiation and termination of the emission of electrons in a pulse of varying duration simply by varying the applied voltage. The problem of residual emission during thermal decay experienced in thermionic emission is avoided. Using x-ray tubes with nanostructure based field-emission cathodes of the present subject matter, it is also possible to construct portable x-ray machines for use in the field.

[0029] In addition to being advantageously pulsed, field emission electrons may be directed to particular areas on the anode target region by the use of either mechanical and / or electrical means that control the orientation of the beam within the x-ray generating device. The orientation feature allows for the use of multiple anode target materials within one x-ray device, thus generating a larger range of characteristic x-rays in a single device. Additionally, the reduced bombardment time on the anode results in a lower anode cooling requirement with attendant reduction in device peripherals.

[0030] The use of a nanostructure cathode material enables the emission of a high electron beam current which is stable and easy to control and focus. Thus, x-ray generating devices of the present subject matter provide for a variety of medical applications that make capable dramatic improvements in imaging quality and speed over current designs. The main characteristics required of x-ray beams for medical applications are high intensity, precise control of x-ray generation, and small “apparent” focus spot.

Problems solved by technology

The efficiency of such emission of x-rays is very low causing the anode to be heated.

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).

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.

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 thennionic cathode may emit electrons which may cause unwanted x-ray emission from the target.

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.

Therefore, a principle limitation in these applications is the amount of electron beam current generated by the cathode.

However, the incorporation of conventional field emission cathode materials into x-ray generating devices presents certain challenges.

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.

However, the emission is unstable above a current density of 30 mA / cm2 and the fabrication of uniform sharp tips is difficult and costly.

Moreover, the stability of these materials in a real device environment is of concern, partially due to ion bombardment, reaction with chemically active species and high temperatures.

The use of diamond materials as field emitters also suffers from the problem that the diamond produces lower than desired current densities.

The higher reported electron emission data are difficult to interpret and are unreliable because, for example, the data is independent of the distance between the emitting cathode and target anode material (U.S. Pat. No. 6,057,637).

This is not possible due to constraints of power supplied to the cathode filament.

Another difficulty with a conventional thermionic emitter is the space charge effect.

Thus, it is difficult to achieve independent control of electron beam current (mA) and kV unless the tube is operating in the so called saturation limit.

Due to the inherent difficulty of initiation and termination of thermionic electron sources, the process is slow and patients are exposed to unnecessary higher dosages of x-rays.

Achieving uniformity in conventional tube design is difficult because emission of x-rays from the target surface is anisotropic (namely, dependent on the emission direction relative to the surface).

Thus, even when the focus spot is bombarded with a uniform electron beam the resulting x-ray beam is non-uniform.

However, these conventional methods do not allow for independent controls of the electron emission current and the x-ray energy.

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