X-ray or gamma ray systems or devices – Electronic circuit – With display or signaling
Reexamination Certificate
1999-12-21
2001-12-04
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Electronic circuit
With display or signaling
C378S164000
Reexamination Certificate
active
06327337
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method and apparatus for determining the incident position of an x-ray detected by use of a charge-coupled device (hereinafter referred to as a “CCD”).
BACKGROUND ART
CCDs for directly detecting X-rays are now widely used, particularly in the field of astronomy (see G. W. Frazer: “X-Ray Detectors in Astronomy,” Cambridge University press (1989) 208; and B. E. Burke, R. W. Mountain, P. J. Deniels, M. J. Cooper, and V. S. Dolat: IEEE Trans. Nuclear Sci. 41 (1994) 375).
A CCD is an integral-type sensor which can increase the resolution of an image to a level corresponding to pixel size. If the flux of incident X-rays can be reduced to such a level that primary electron clouds produced by X-ray photons do not overlap each other, in addition to an X-ray image, an X-ray energy spectrum can be obtained.
At present, a CCD is widely used as an X-ray photon counter, particularly in the field of astronomy. When the flux of incident X-rays is reduced to such a level that primary electron clouds produced by X-ray photons do not overlap each other, in addition to an X-ray image, an X-ray energy spectrum can be obtained.
Recently, the present inventors proposed a new technique, by which the X-ray response of a CCD can be obtained with subpixel resolution (Japanese Patent Application No. 9-82444). The new technique employs a metal mesh placed in proximity to a CCD, and a parallel X-ray beam. The mesh has a large number of small holes with periodic spacing, the spacing being equal to the CCD pixel size. Hereinafter, such a mesh is referred to as a “single-pitch mesh.” Since the position of an input X-ray can be determined with subpixel resolution, X-ray response can be investigated in detail.
FIG. 1
shows the overall structure of an apparatus for measuring X-rays detected by use of a CCD.
FIG. 2
is a schematic view showing an experiment performed by use of a single-pitch mesh.
In
FIGS. 1 and 2
, reference numeral
1
denotes an X-ray generator,
2
denotes a CCD camera,
3
denotes a CCD,
4
denotes a single-pitch metal mesh, and
4
a
denotes holes of the single-pitch metal mesh.
As shown in
FIG. 2
, the metal mesh
4
having periodically spaced holes is placed in close proximity to the surface of the CCD
3
. The mesh
4
is placed parallel to the surface of the CCD
3
and tilted by a small angle &thgr; with respect the pixel axis. A parallel X-ray beam is radiated normal to the surface of the CCD
3
. The X-rays passing through the mesh holes
4
a
reach the CCD
3
. The position of X-ray interaction (hereinafter referred to as an “X-ray interaction position”) is restricted by the mesh hole
4
a.
An X-ray event whose interaction position is far from a pixel boundary should take place in a pixel where a mesh hole
4
a
is located at the center thereof. In such a situation, a single-pixel event usually occurs.
Meanwhile, an X-ray event whose interaction position is close to a pixel boundary takes place in a pixel which has more than two hole shadows. In this case, a split event occurs. When a single-pitch mesh is used, the single-pixel event and the split event cannot be distinguished from each other, because the mesh holes
4
a
have a fixed size, and spacing between holes is equal to the inter-pixel distance. Therefore, even when a pixel at which an X-ray event occurs can be determined, the hole through which the X-ray enters cannot be determined.
DISCLOSURE OF THE INVENTION
When X-rays are detected by use of a CCD, the X-ray energy can be measured as well, if the X-rays are measured on a ray-by-ray basis. In this case, a primary electron cloud (consisting of several hundreds to several thousands of electrons) formed due to an incident X-ray spreads to some degree. If a pattern of such a spread can be determined so as to determine the shape, it becomes possible to find how a signal is determined on the basis of the incident position of an X-ray, to thereby enable precise determination of the incident position.
In view of the foregoing, an object of the present invention is to provide a method and apparatus for determining an incident position of an X-ray detected by use of a CCD, which method and apparatus enable the incident position of an X-ray to be determined with sub-pixel resolution.
To achieve the above object, the present invention provides the following methods and apparatuses.
(1) A method of determining an incident position of an X-ray detected by use of a CCD of a CCD camera onto which a parallel X-ray beam generated from an X-ray generator is radiated via a metal mesh disposed in front of the CCD, characterized in that a multi-pitch metal mesh is disposed as the metal mesh; and the incident position of the X-ray is determined on the basis of a ratio between a primary electron cloud produced by the X-ray spread over a plurality of pixels and a portion of the cloud existing in each of the plurality of pixels.
(2) The method as described in (1) above, further characterized in that the accuracy in determining the incident position of an X-ray is about one-tenth of the pixel size of the CCD.
(3) The method as described in (1) above, further characterized in that an interaction position is determined from an event pattern on the basis of the center of gravity of an X-ray event.
(4) The method as described in (3) above, further characterized in that the center of gravity of an X-ray event is obtained on the basis of an output signal of a corresponding pixel.
(5) An apparatus for determining an incident position of an X-ray detected by use of a CCD of a CCD camera onto which a parallel X-ray beam generated from an X-ray generator is radiated via a metal mesh disposed in front of the CCD, characterized by comprising: an X-ray generator for obtaining a parallel X-ray beam; a multi-pitch metal mesh disposed in front of the CCD and adapted to receive the parallel X-ray beam; a CCD camera for photographing the CCD; and means for determining the incident position of the X-ray on the basis of a ratio between a primary electron cloud produced by the X-ray spread over a plurality of pixels and a portion of the cloud existing in each of the plurality of pixels.
(6) The apparatus as described in (5) above, further characterized in that the multi-pitch metal mesh has a large number of circular holes disposed at a pitch four times the pixel size of the CCD.
In the present invention, when an X-ray signal spreads over two pixels, the signal is distributed according to the incident position of the X-ray and the shape of the electron cloud. Through an experiment using a mesh, the present inventors investigated how an X-ray signal spreads over two pixels depending on the incident position of the X-ray, and found a method of accurately determining the incident position of the X-ray on the basis of the ratios of portions spreading over the two pixels. As a result, the accuracy in determining the incident position can be improved to one-tenth of the pixel size (variance).
REFERENCES:
Hiraga et al., “How big are charge clouds inside the charge-coupled device produced by x-ray photons?,” Japanese Journal of Applied Physics, Part 1: Regular Papers, Short Notes & Review Papers—Aug. 1998—vol. 37, Issue 8 pp. 4627-4631.*
Pivaroff, M. et al., “Measurement of the Subpixel Structure of AXAF CCD's”, IEEE Transactions on Nuclear Science, vol. 45, No. 2, Apr. 1998.*
Tsunemi, H. et al., “Direct Measurements at the Sub-pixel Level of the X-ray Detection Efficiency of the CCD on board the ASCA satellite, ” Nuclear Science Symposium, 1997. IEEE, 1997 pp.: 592-596 vol. 1.
Japan Science and Technology Corporation
Kim Robert H.
Lorusso & Loud
Thomas Courtney
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