Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Utility Patent
1998-09-03
2001-01-02
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370130, C250S370090
Utility Patent
active
06169287
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of detectors for x-ray or gamma ray photons and, more particularly, to methods for processing the signals generated by semiconductor detectors in response to absorbing such photons so as to promptly obtain information about the absorbed photon, such as its energy or the location in the detector where it was absorbed.
All references mentioned in this application are incorporated by reference in their entirety for all purposes.
2. Background and Prior Art
2.1. X-ray Detector Applications
In this application the term “x-ray” is used in a generic sense—to denote photons of energies typically above 3-4 keV, specifically including such photons generated from nuclear decays (i.e., gamma rays).
There are applications in a variety of fields, including medicine, astronomy, and non-destructive testing, wherein it would be useful to have x-ray detectors capable of resolving not only the location of interaction of the x-ray within the detector but also the energy of the interacting x-ray. Being able to resolve location allows, for example, the formation of x-ray images. Being able to resolve energy then allows the image either to be formed at different x-ray energies (when using a polychromatic source) or to be formed at the same x-ray energy as the source by rejecting scattered, image degrading x-rays at other energies.
It would be additionally useful if the output of such a detector were digital: the result of each detected x-ray being a set of N+1 numbers (where N is the spatial dimension of the detector) representing the spatial coordinates of the x-ray's interaction location and its energy. This would allow the detector's image to be directly accumulated by a digital electronics system, processed by digital computer, stored on digital storage media, and/or digitally transmitted to remote locations. These capabilities are particularly important in medical imaging, where they greatly simplify the enhancement, analysis, and archiving of patient data.
The availability of energy resolved digital imaging detectors (ERDIDs) would enhance a variety of existing imaging detector applications as well as stimulating a variety of new ones. In medical imaging applications, for example, obtaining x-ray images as a function of energy would be to allow images of features having a particular absorption coefficient (e.g., bones) to be either extracted or suppressed. Another ERDID use would be to reject Compton scattered radiation, which commonly reduces contrast, from images without having to resort to grids, which reduce efficiency. In both medicine and astronomy, ERDIDs could also be employed in Compton cameras, allowing them to generate energy resolved images. In other areas of research, an ERDID would significantly increase the efficiency of energy dispersed diffraction experiments by removing the need for exit detector pinholes to define the exit x-rays' scattering angles. This would allow the technique to be practiced effectively with much weaker, and hence cheaper, x-ray sources than is currently possible.
In many applications it would further be beneficial if the necessity for cryogenic cooling could be dispensed with and, when working with higher energy x-rays, materials with greater absorption cross section could be employed. These desires have fueled considerable research and development in the area of compound semiconductor detectors, including such materials as HgI
2
, GaAs, CdTe, and CdZnTe (CZT). These materials have generally been found to possess poor hole collection properties, however, which in turn precludes good energy resolution except in very thin detectors. It would thus be beneficial to have a semiconductor detector technology wherein the energy resolution was insensitive to hole collection properties or, more generally, a technology capable of generating x-ray position and energy information using charge carriers of only a single polarity.
2.2. Brief Survey of Existing Art
The field of x-ray detection is highly developed. A fairly comprehensive introduction to the state of the art may be found in the volume “Radiation Detection and Measurement, 2nd Ed.” by Glenn F. Knoll (J. Wiley, New York, 1989). Below we note only lines of development relevant to the present invention.
2.2.1. X-ray Energy Resolution
The common classes of commercially successful x-ray detectors include film, scintillator plus secondary light detector, gas detectors, and solid state semiconductor detectors, in order of increasing energy resolution. Film has essentially no energy resolution unless used with absorbing layers. The energy resolution of scintillator-based systems depends upon both the efficiency of the scintillator and upon the nature of the secondary detector. They can easily operate as single photon counters and if carefully optimized may have energy resolutions of 40% at 10 keV and better at higher energies. Gas detectors, including such variants as proportional counters and microstrip gas chambers, can approach 5-10% resolution at 10 keV, but as the x-ray energy rises they tend to become inefficient due to their low cross sections for x-ray absorption. Semiconductor detectors have the best commercial energy resolution, approaching 1.5% energy resolution at 10 keV and improving with energy. Germanium detectors in particular have a fairly good cross section even at energies in excess of 100 keV and can be made thick enough to absorb x-rays efficiently at these energies. For the proposed applications, the energy resolution of semiconductor detectors will generally be required. Therefore the following discussion will be so limited.
2.2.2. Spatially Resolved, Semiconductor X-ray Detectors
The common categories of spatially resolved, semiconductor x-ray detectors are silicon (Si), germanium (Ge) and compound semiconductor.
2.2.2.1. Si Detectors and Integrated Electronics
There are three primary Si detector technologies: cryogenically cooled spectrometers, CCDs, and pixel detectors with integrated electronics.
Cryocooled spectrometers: In this technology a Si diode is operated fully depleted at a low temperature to reduce leakage currents from thermally generated charge carriers (typically about 100% K). The charges produced in the diode from an x-ray absorption are integrated by a charge sensitive preamplifier, generating (after appropriate pulse shaping) an electrical pulse signal whose amplitude is proportional to the x-ray's energy. While this technique can give state of the art energy resolution for low energy x-rays it suffers from several problems. First, the cryogenic cooling makes the technique difficult to apply to complex imaging detectors. Second, Si has a low cross section and does not absorb x-rays well above 20 keV. Third, it is difficult to fabricate arrays of these detectors. To date only small numbers of elements have been successfully grouped in commercial instruments.
CCD detectors: This is the most common Si imaging detector, whose various forms range from inexpensive devices for telecommunications to ultra-low noise devices for scientific research, particularly in astronomy. Typical pixel sizes range from 12 to 50 &mgr;m, with overall array sizes of from 512 to 2096 pixels squared. CCDs can be used as direct x-ray detectors (see, for example [J. R. Janesick, “Open Pinned-Phase CCD Technology” in PROC. SPIE Vol. 1159, pp. 363-371 (1989)]), but have very low efficiencies because their active volumes are so thin. They are therefore typically coupled to a scintillator screen by a fiber optic, thereby reducing their energy resolution to that of other scintillation detectors.
Pixel detectors with integrated electronics: Because Si technology can also be used to produce integrated signal processing circuits, there have also been efforts to create pixelated imaging detectors with integrated processing electronics, that is, a more-or-less complete set of processing (or pre-processing) associated with each pixel in the detector array. There have bee
Gagliardi Albert
Hannaher Constantine
Townsend and Townsend / and Crew LLP
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