Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
1998-07-27
2001-09-04
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370100
Reexamination Certificate
active
06285029
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to semiconductor gamma-ray detectors, and especially to arrays of such detectors for high-energy gamma-ray imaging.
BACKGROUND OF THE INVENTION
The method of annihilation detection coincidence (ADC) is a very attractive detection technique in the field of nuclear imaging for medical purposes. This method makes use of he physical principle of electron-positron annihilation, which produces a pair of high-energy (511 Kev) photons. These photons propagate along a common line but in opposite directions. A two-headed gamma-ray camera is used to detect the locations where the photon-pair absorbed. An image reconstruction is accomplished by determining the liens along which the photons propagate from their point of production to their point of absorption. In order to reject scattered photons or stray photons not related to the pair being recorded, which if counted would produce a distorted or incorrect image, the energy of the photons and their timing (coincidence) is also measured. Since the method does not require a collimator, it is known as a collimator less method or a method of electronic collimation. Instruments designed according to this method have the advantages of improved sensitivity and of much reduced weight.
Imaging technologies based on Positron Emission Tomography (PET) that include multiple detectors and PET-like cameras having two detector-heads, require the use of detectors with high stopping power. The high stopping power is needed for efficient absorption of the high-energy photons. High stopping power is achieved by using thick detectors made of materials having a high atomic number, Z.
The traditional gamma-ray imaging technology presently used in nuclear medicine, including PET-like machines, uses Anger cameras, such as the type described in U.S. Pat. No. 3,011,057 to Anger. In this technology, the detectors are made of thick scintillators (such as sodium iodide NaI) combined with photo-multipliers. The more recently introduced semiconductor radiation detectors, such as those made of CdTe and CdZnTe, have the advantages of improved performance over scintillation detectors, in terms of improved energy and spatial resolution, count rate, stopping power and compactness. Accordingly such detectors have great potential to replace the traditional current technology of the Anger camera.
The idea of using a pixelated imaging-plane detector, consisting of multiple cells of semiconductor detector arrays is known in the art, as for instance described by H. H. Barrett, J. D. Eskin and H. B. Barber in their article “Charge transport in arrays of semiconductor gamma-ray detectors”, published in Physical Review Letters, Vol. 75, pp. 156-159, 1995. Until recently, the very low yield associated with the growth of high quality semiconductor crystals, meant that the manufacturing process was costly and time consuming, which caused the above-mentioned idea to be unsuitable for implementation on a commercial basis.
Recent advances in crystal growth technology has improved the yield, enabling the production of relatively large modular pixelated detector arrays, which can be combined to form the complete imaging plane for gamma-ray and X-ray cameras. The current yield enables the economic production of pixelated detector arrays with typical module sizes of about 20×20 mm at the electrode surfaces, and several millimeters thick. Such relatively large modules of detectors have provided the commercial justification for the production of semiconductor gamma and X-ray cameras.
However, because the thickness of these detectors is limited to several millimeters, such cameras are suitable only or use in the energy range between X-ray and medium energy gamma-rays. In order to make such cameras suitable for proper operation with the method ADC, the thickness of the detectors must be increased to provide the high stopping power needed for high-energy photons (511 Kev). However, since such detectors would have higher volume, they would also have higher levels of grain boundaries, defects, traps and included non-uniformity in electric field, all of which degrade detector performance. The manufacturing yield thus goes down with the detector volume.
If the probability for producing a good module having a specific area of pixelated electrodes and of thickness d, is P, then the probability p of producing a good module having the same area, but of thickenss D, is given by:
p=p
(D/d)
(1)
This means that increasing the detector thickness for use with high-energy photons, while maintaining the same surface area, causes a significant reduction in the probability P of producing a good detector module. Alternatively, the same probability P of producing a good thick detector module would mean the reduction of the surface area of the detector modules, resulting in an area which is impractical for use.
There therefore exists a serious need for a detector module having a thickness with stopping power sufficient for use with high-energy photons, but which can be produced at a yield similar to that of thinner detector modules of similar detection area.
The disclosures of all publications and patents mentioned in this section, and in the other sections of the specification, and the disclosures of all documents cited in the above publications, are hereby incorporated by reference.
SUMMARY OF THE INVENTION
The present invention seeks to provide a new semiconductor high-energy gamma-ray detector module, capable of being manufactured with a high process yield, which overcomes the drawbacks and disadvantages of existing semiconductor detector modules.
There is thus provided in accordance with a preferred embodiment of the present invention, a novel semiconductor detector device, consisting of several years layers of two dimensional detector modules, each module being divided into an array of separate pixelated detector cells, by means of the pixelation of the electrodes on the surfaces of the modules. The bottom surface electrode is generally left continuous and serves as a common electrode to all of the detector cells in the two dimensional array. The superimposed detector cells in equivalent positions in each layer are joined electrically to those in the two immediately adjacent layers by means of good Ohmic contact. In this way, the whole device effectively becomes a two dimensional array of stacks of individual detector cells, with a common bottom electrode. The bottom electrode is generally made the cathode, b the application of a negative bias voltage. Current in each detector cell stack, induced by the absorption of a high energy photon in that stack, is measured by means of an integrating charge sensitive amplifier attached to each anode at the top of each cell stack.
A primary advantage of the detector device, constructed and operative according to the present invention, is that it becomes possible to obtain a large area gamma-ray detector, sufficiently thick to absorb the high energy photons arising from electron-position annihilation events, but without the very high expense associated with the production of a single detector crystal of the required thickness.
According to a further preferred embodiment of the present invention, the performance of the detector device can be improved by the use of a “nail head like structure” for the contact electrodes, which conduct the current into and out of each semiconductor detector. This structure is achieved by depositing an insulating layer on the detector surface, before deposition of the electrode layer. The insulating layer has holes at the center of each detector cell, and the electrodes thus make contact with the detector only through the limited area of the holes.
According to yet another preferred embodiment of the present invention, it is possible to provide a larger area detector device, for use, for instance, in medical imaging applications, by building up a two dimensional array of devices from individual modules constructed according to the present invention.
El-Hanany Uri
Halberthal Eldan
Klier Shimon
Shahar Arie
Tsigelman Alex
Darby & Darby
Gabor Otilia
Hannaher Constantine
Imarad Imaging Systems Ltd.
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