1. Field of the Invention
This invention relates generally to a system for spatially detecting x-rays of varying wavelengths and in particular to an integrated x-ray detection system which can efficiently detect x-rays with energies in the range from approximately 1 keV to approximately 150 keV with an energy resolution as low as 0.5 keV or less and a spatial resolution of less than 100 micrometers.
2. Background of the Related Art
The defining characteristics of x-ray imaging technologies include spatial resolution, contrast sensitivity, speed, and cost. In addition, recently developed techniques for quantifying material composition require x-ray energy sensitivity. (See Ting et al., "Using Energy Dispersive X-ray Measurements for Quantitative Determination of Material Loss Due to Corrosion", in Review of Progress in Quantitative Nondestructive Evaluation, Vol. 12B, 1963, eds. D. O. Thompson and D. E. Chimenti, Plenum Press, New York (1993).) Many practical applications also require imaging under severe environmental conditions or in restricted spaces. Since no single x-ray detector offers optimum performance in all of the above areas, compromises must often be made.
X-ray detection technologies exhibiting energy sensitivity generally fall into two broad categories: wavelength dispersive systems or energy dispersive systems. In the former, Bragg diffraction from either a natural or artificial crystal is combined with collimating optics such that only those x-rays within an energy band determined by the geometry of the collimating optics and the lattice spacing of the diffracting crystal are allowed to impinge on an x-ray sensitive element. In such a system, the x-ray detector need not have any intrinsic energy sensitivity, since the collimating optics and crystal act as a filter and only allow certain wavelengths to reach the detector. Such wavelength dispersive systems generally have very limited throughputs both because the geometry of the incident x-ray optical system must be changed to allow detection of different energy photons, and because they are only a single pixel or point detection system.
Energy dispersive x-ray detection systems, on the other hand, generally rely on the photoelectric interaction between the incident quantum of radiation and a medium that results in the production of a number of charged particles in proportion to the energy of the incident photon. When the medium is a semiconductor such as silicon, germanium, or cadmium telluride, the electrons and holes generated by the interaction can be collected, and the amount of charge is a direct measure of the energy given up by the incident photon. Alternatively, the medium may be a gas that is ionized by the radiation such as in a gas proportional tube. Because energy dispersive detectors are intrinsically capable of distinguishing different wavelength photons, they are capable of rapid throughput. In addition, if the interaction medium is compartmentalized in some fashion, these detectors can be made to have many simultaneously active pixels, further improving the throughput of the system.
Gas proportional counters have been used for many years to detect ionizing radiation. Familiar Geiger counters are a close relative of this detector. In its simplest form, proportional counter 102 consists of a cylindrical outer cathode 110 with a small diameter anode wire 114 along axis A as shown in FIG. 1A. FIG. 1B shows a cross-sectional view of proportional counter 102. Volume B is typically filled with a gas 118 such as argon or xenon plus a few percent of a quenching gas such as methane. Electrons liberated by interaction of an x-ray or charged particle in gas 118 are driven toward anode 114 by an electric field. The electric field is produced when a voltage is applied by power supply 122 with leads 126a and 126b connected to cathode 110 and anode 114, respectively. Near anode 114, this electric field varies as 1/r, where r is the distance from the center, as shown in FIG. 1A. The electric field must be strong enough such that electrons are accelerated to energies sufficient to ionize the gas molecules, thus generating an avalanche of electrons between anode 114 and cathode 110. The multiplicative gain in this process depends on the properties of the gas 118, the diameter d of anode 114, and the high voltage potential between anode 114 and cathode 110. This gain can be as high as 106. Anode 114 is typically connected to electronic circuitry 150 to amplify and digitize the signal. A pulse of height h is produced at anode 114, where h is proportional to the number of electrons initially liberated in the interaction with the gas 118. The number of electrons liberated in this initial interaction between the quantum of ionizing radiation and the gas 118 is in turn proportional to the energy of the incident quantum of radiation. This is why counter 102 is referred to as a "proportional counter".
Counter 102, however, has a drawback for x-ray imaging in that it provides very little spatial information. In 1968, Charpak improved on this with the introduction of a multiwire proportional chamber. (See G. Charpak et at., "The Use of Multiwire Proportional Counters to Select and Localize Charged Particles", Nucl. Instrum. Methods 62, 262 (1968).) In that device, many parallel anode wires are positioned in a common gas volume. Each anode wire behaves as a proportional counter and can be connected to a separate electronic circuit to give position information. The spatial resolution, however, of these multiwire proportional chambers is limited because the wires cannot be placed closer than about 1 millimeter apart without becoming unstable. Such multiwire proportional chambers are also quite fragile, which has limited their use even more.
A new technology related to multiwire proportional chambers which offers promise in improving both spatial resolution and mechanical ruggedness is the microstrip proportional chamber. (See A. Oed, "Position-Sensitive Detector with Microstrip Anode for Electron Multiplication with Gases", Nucl. Instrum. Methods, A263, 351 (1988).) This device has been developed for research in astrophysics and high-energy physics. Its properties make it an attractive choice for x-ray imaging applications. It is conceptually similar to a multiwire proportional counter, but instead of parallel anode wires stretched across a gas volume, the anodes are fabricated by patterning a thin metal layer which adheres to a solid substrate. The solid supporting substrate allows both narrower anodes and closer spacing of the anodes than is possible with freely suspended wires. In addition, the adherence of the metal anodes to a solid insulating substrate prevents mechanical vibration and shock from causing relative movement and consequent short-circuiting of the anodes, thus greatly improving reliability.
While several research groups have tested many different substrate materials for fabrication of microstrip gas proportional chambers, we know of only three groups that have explored the use of silicon. The first group (See F. Angelini, et at., "A microstrip gas chamber on a silicon substrate", Nucl. Instrum. Methods, A314, 450, (1992).) used a low resistivity (i.e., heavily doped) silicon substrate with a thermally grown oxide layer for electrical isolation of the anodes and a conductive contact to the back of the silicon. In this implementation, the silicon is a conductor and is used as one of the electrodes of the chamber. This heavy doping renders the silicon useless for active device fabrication. The second group. (See S. F. Biagi, et al., "Initial investigations of the performance of a microstrip gas-avalanche chamber fabricated on a thin silicon-dioxide substrate", Nucl. Instrum. Methods, A323, 258, (1993).) did not indicate the resistivity of their substrate, but used a combination of thermal oxidation and plasma enhanced chemical vapor deposition to build an insulating layer for the anodes and placed the silicon substrate between sets of electrodes that must be held at high voltages during operation. The high fields from the electrodes can easily deplete the silicon and render any active devices fabricated in the silicon useless. The third group (See E. F. Barasch, et al., "Gas Microstrip Detectors on Polymer, Silicon and Glass substrates", Nuclear Physics B (Proc. Suppl.) 32, 216, (1993).) used anisotropic etching of the silicon substrate to etch pedestals to support the anodes and oxidation of the resulting silicon surface to provide electrical isolation of the anodes. Depletion by the electric fields from the electrodes will inhibit active device function. In addition, the etched pedestals are incompatible with the planar fabrication techniques needed to build active devices.