Radiant energy – Source with recording detector
Patent
1995-12-01
1997-06-10
Fields, Carolyn E.
Radiant energy
Source with recording detector
25037009, 250591, G01N 2304
Patent
active
056378823
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates generally to radiation imaging systems and, in particular, to an improved detector plate for use in such systems.
BACKGROUND OF THE INVENTION
Conventional radiation imaging systems may utilize photoconductive materials to absorb incident radiation representative of an object. U.S. Pat. No. 4,176,275 discloses a digital x-ray imaging system in which a radiation source is positioned to direct a radiation image of an object onto the upper surface of a detector plate. The detector plate includes a suitable photoconductive material that absorbs the radiation and produces electron-hole pairs (first charge carriers) which may be separated from each other by an electric field applied across the photoconductor, creating a latent image of the object at the surface of the photoconductor which is typically a thin planar layer within the detector plate. A narrow beam of scanning radiation substantially completes discharge of the photoconductor by creating the motion of a second set of charge carders. The distribution of these second charge carders in the plane of the photoconductor is affected by the distribution of the first charge carriers, i.e., by the latent image. The motion of the second charge carriers is detected and digitized in an appropriate circuit, thereby capturing the latent image in digital form.
The detector plate is a multi-layered device having a plane parallel stack of first conductive, dielectric (insulative), photoconductive and second conductive layers. The first conductive layer provides the surface to which the radiation image is directed, and therefore both the first conductive layer and the dielectric layer must be substantially transparent to the radiation energy produced by the radiation source so that it can reach the photoconductive layer. A D.C. voltage source is connected between the first and second conductive layers, with the polarity typically being that the first conductive layer is positive with respect to the second conductive layer.
During use, large voltages of up to 10 kilovolts are applied across the sandwich structure of the detector plate, resulting in electric fields as high as 10 v/micron across the dielectric. Under the application of this high voltage and repeated use of the detector plate, the first conductive layer tends to fail electrically due to (1) cracking, and/or (2) arcing from the first conductive layer to ground which is typically the second conductive layer of the detecter plate or possibly the cassette in which the detector plate is housed. This type of arcing not only breaks down the first conductive layer but also could potentially damage the rest of the detector plate. Cracking typically occurs after repeated application of the high voltage, and can be considered as a surface "brush discharge" whereby the first conductive layer is ablated in the discharge area leaving the dielectric layer exposed.
SUMMARY OF THE INVENTION
A first aspect of the present invention provides a detector plate for use in a radiation imaging system, including a first conductive layer, a dielectric layer, a photoconductive layer and a second conductive layer, arranged as a stack in that order. The first conductive layer and the dielectric layer are substantially transparent to radiation energy so as to allow said energy to pass therethrough to be received by the photoconductive layer. The first conductive layer has a periphery defined by a first edge and the dielectric layer has a periphery defined by a second edge. The first edge is offset inward of the second edge defining a margin between the first and second edges.
A preferred embodiment of the invention includes art insulative barrier, for example silicone based Sylgard, in the margin around the periphery of the first (transparent) conductive layer, in the form of a "dam" to further prevent arcing and the resulting detector plate failure.
Instead of Sylgard, Kapton which is a polyamide film could be used as the insulative barrier. A specific brand of Kapton film suitable fo
REFERENCES:
patent: 3757069 (1973-09-01), Reuting
patent: 3780288 (1973-12-01), Dryden
patent: 4176275 (1979-11-01), Korn et al.
Cook Earl L.
Disanayaka Bimsara W.
Divigalpitiya Ranjith
McLaughlin Keith K.
Robbins William B.
Bates Carolyn A.
Bauer William D.
Fields Carolyn E.
Minnesota Mining and Manufacturing Company
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