X-ray or gamma ray systems or devices – Electronic circuit – With display or signaling
Reexamination Certificate
1998-09-09
2001-03-06
Font, Frank G. (Department: 2877)
X-ray or gamma ray systems or devices
Electronic circuit
With display or signaling
C250S385100
Reexamination Certificate
active
06198798
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a planispherical parallax-free X-ray imager more particularly adapted to industrial and/or medical application.
2. Brief Description of the Invention
Planispherical X-ray imaging devices have been up to now investigated. Most important work concerning that particular subject matter was developed by Georges CHARPAK at the EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH in Geneva (Switzerland).
A first development concerned the properties of proportional chambers with spherical drift spaces.
A proportional wire chamber equipped with a resistive divider adapted to generate appropriate spherical equipotential surfaces within the drift space of the wire chamber has been first disclosed by G. CHARPAK, Z. HAJDUK, A. JEAVONS, R. STUBBS—CERN, Geneva, Switzerland, and R. KAHN, Centre Multidisciplinaire Paris XII, av. General de Gaulle, Créteil, France, and edited by NUCLEAR INSTRUMENTS AND METHODS 307 (1974)—Geneva, Jul. 29, 1974.
A proportional wire chamber embodied as a large aperture X-ray imaging chamber equipped with a spherical drift space has been also disclosed by G. CHARPAK, C. DEMIERRE, R. KAHN, J-C. STANDIARD and F. SAULI at the CERN in Geneva. See NUCLEAR INSTRUMENTS AND METHODS 141 (1977) 449-455, North-Holland Publishing Co. A spherical drift space is disclosed as to embodying entrance and exit electrodes of spherical shape with an angular acceptance for X-rays to 90°. Coupling of spherical drift space and readout proportional chamber is disclosed to consist of a transfer space T, the lateral wall of which comprises a resistive divider adapted to generate spherical equipotential surfaces of increasing radius up to the first cathode electrode of the readout proportional chamber.
A general survey on various methods of correction for parallax errors on gaseous detectors for X-rays and UV has been published by G. CHARPAK, CERN, Geneva, Switzerland. See NUCLEAR INSTRUMENTS AND METHODS 201 (1982) 181-192, North Holland Publishing Company.
More recently, P. REHAK, G. C. SMITH and B. YU, Brookhaven National Laboratory, Uptown N.Y. 11973 presented a method for reduction of parallax broadening in gas-based position sensitive detectors at the 1996 IEEE Nuclear Science Symposium, Anaheim, Calif., Nov. 2-9, 1996 and published as IEEE Transactions on Nuclear Science, vol.44, No. 3, 1997, 651-655.
Although the drift space for photons is confined within an entrance electrode and the cathode wire plane of the readout chamber are plane and parallel, entrance window of the readout chamber is further provided with a particular conductive pattern adapted to introduce progressive bending of the equipotential surfaces, electric field lines crossing thus this equipotential surfaces at right angle, whichever the impinging direction of X-rays emanating from the focal point, so as to correct and reduce any parallax error.
In a general point of view, the above mentioned X-ray imagers may prove satisfactory to the extent that the parallax error is now reduced to a few percent. Embodying the entrance window of the readout chamber with conductive pattern adapted to provide full correction of parallax error is quite difficult to implement, since actual pattern and corresponding voltage which is to be applied to these conductive patterns are such that the electric field is approximately radial only close to the ring patterned entrance window, while it becomes substantially parallel in approaching the equipotential second electrode which defines the conversion volume. As a consequence, parallax error is thus increasing with penetration of the converting X-rays.
OBJECTS OF THE INVENTION
An object of the present invention is therefore to provide a planispherical parallax-free X-ray imager in which any image distorsion is suppressed thanks to its full symmetrical structure with respect to a symmetry axis orthogonal to an entrance window of the imager.
Another object of the invention is further to provide a planispherical parallax-free X-ray imager of very high performance that overcomes the above mentioned drawbacks of corresponding X-ray imagers of the prior art and however mechanically much simpler to implement.
SUMMARY OF THE INVENTION
More particularly, in accordance with the present invention, there is provided a planispherical parallax-free X-ray imager in which a parallel X-ray beam is directed to a crystal so as to generate a conical X-ray beam for illuminating an entrance window of the X-ray imager. The X-ray imager at least comprises a vessel containing a ionizing gas through the entrance window. The X-ray imager further comprises within the vessel, a spherical conversion volume chamber which is associated with the entrance window. The conversion volume chamber comprises a first and a second parallel electrodes adapted to generate in operation electrical equipotential surfaces of spherical shape and corresponding radial electric field lines within this spherical conversion volume chamber with these electrical equipotential surfaces of spherical shape being thus each centred at a focus common centre point substantially corresponding to the location of the crystal so as to allow any primary electron generated within the spherical conversion volume chamber to drift along the radial field lines. A third electrode substantially parallel with the second electrode is provided so as to form together a gas electron multiplier structure which comprises at least one matrix of electric field condensing areas distributed within a solid surface. Each of the electric field condensing areas is adapted to produce a local electric field amplitude enhancement proper to generate within the gas an electron avalanche from one of the primary electrons so as to allow the gas electron multiplier structure to operate as an amplifier of given gain for the primary electrons. A readout electrode is further provided with an array of elementary electrodes which is formed onto a wall of the vessel and is laid parallel to the third electrode.
The X-ray imager also comprises, outside the vessel, an electrical bias circuit which is connected to the first, second and third electrodes and thus adapted to deliver adequate voltage potentials so as to drift the primary electrons within the spherical conversion volume chamber and then multiply corresponding drifted primary electrons through an avalanche phenomenon within the gas electron multiplier structure. A detection circuit is further provided and connected to the readout electrode so as to allow a bi-dimensional readout of the position of any generated avalanche phenomenon thanks to the gas electron multiplier structure in the absence of a substantial parallax readout phenomenon.
The objects, advantages and other particular features of the parallax-free X-ray imager of the invention will become more apparent upon reading of the subsequent non restrictive description of the preferred embodiments thereof which are given by way of example only with reference to the accompanying drawings.
REFERENCES:
patent: 4317038 (1982-02-01), Charpak
patent: 4595834 (1986-06-01), Burns
patent: 4954710 (1990-09-01), Comparat et al.
patent: 5521956 (1996-05-01), Charpak
patent: 6011265 (2000-01-01), Sauli
Charpak et al., “The Spherical Drift Chamber for X-Ray Imaging Applications”, Nuclear Instruments and Methods, Geneva—Jul. 29, 1974.
Charpak et al., “Some Properties of Spherical Drift Chambers”, Nuclear Instruments and Methods, 141 (1977), pp. 449-455.
Charpak, “Parallax-Free High-Accuracy Gaseous Detectors for X-Ray and Vuv Localization”, Nuclear Instruments and Methods, 201 (1982), pp, 181-192.
Rehak et al., “A Method for Reduction of Parallax Broadening in Gas-Based Position Sensitive Detectors”, IEEE Transactions on Nuclear Science, vol. 44, No. 3 (1997), pp. 651-655.
European Organization For Nuclear Research
Font Frank G.
Larson & Taylor PLC
Rodriguez Armando
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