Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
1999-11-09
2003-02-18
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
Reexamination Certificate
active
06521894
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to radiation detection and, more specifically, to a radiation detector comprising a scintillator and a semiconductor drift photodetector wherein the scintillator decay time and drift time in the photodetector are matched in order to achieve the best signal-to-noise ratio.
For gamma-ray spectroscopy applications there are two primary detection systems from which one may choose, namely: the scintillator/photomultiplier tube (PMT) combination and germanium. Scintillator/PMT systems offer excellent efficiency but have relatively poor energy resolution. PMT's are also bulky and fragile components that are vulnerable to damage, have low and non-uniform quantum efficiency, require a stabilized high voltage for operation, and are undesirably affected by magnetic fields. Consequently, there has been an ongoing need for suitable alternatives to replace either the PMT, or the entire scintillator/PMT system, for gamma-ray spectrometry.
Germanium semiconductor detectors are excellent in terms of their energy resolution capabilities. They are also essentially insensitive to magnetic fields, a problem which must be considered when using PMTs. However, Ge devices do have very significant disadvantages, including a requirement that they be cooled to temperatures below 100° K for proper operation and for insuring adequate resistance to radiation damage.
There are areas of investigation aimed at finding solid state alternatives to replace the PMT in gamma-ray spectrometry. Silicon PIN semiconductor detectors have been studied for this application. The performance of large-area Si-PIN photo detectors in combination with scintillators is limited by their relatively high detector capacitance, which increases with the area of the detector, and a large leakage current which increases with the Si volume of the device. Enlarging the area of these devices above 200 mm
2
results in severe performance penalty due to the increased electronic noise [James et al 1992]. In view of the maturity of Si technology, it is difficult to expect dramatic breakthroughs in achievable active area and depletion thickness.
Silicon avalanche photodiodes (APD's) produced high expectations as a solid-state replacement for the PMT [McIntyre 1966, Webb et al 1974, Entine et al 1983, Iwanczyk 1991, James et al 1992]. However, even though good results continue to be reported [Moszynski 1998], reliability is a major obstacle, and availability of commercial large-area (>200 mm
2
) devices for this application are still not feasible.
A scintillation detector based on CsI(TI) scintillation crystals coupled with HgI
2
photodetectors has produced excellent results for volumetric scintillators for high energy gamma-rays [Wang, Patt & Iwanczyk 1994]. However, there is currently a lack of availability of HgI
2
crystals of sufficient quality and area to be suitable for photodetector fabrication.
A new photodetector device, the silicon drift photodetector (SDP), is derived from the silicon drift particle detector (SDD) [P. Rehak U.S. Pat. No. 4,688,067] which historically has been linked to the charge coupled device (CCD) and was first implemented for detection and tracking of high energy particles [W. Chen, H. Kraner, Z. Li, P. Rehak, E. Gatti, A. Longni, M. Sampietro, P. Holl, J. Kemmer, U. Faschingbauer, B. Schmitt, A. Woner and J. P. Wurm, IEEE Trans. Nucl. Sci. Vol. 39, No.4,1992,619].
SDD structures have also been used for x-ray spectroscopy [G. Bertuccio, A. Castoldi, A. Longoni, M. Sampietro & C. Gautheir, Nucl. Inst. & Meth. Phys. Res. A312 (1992) 613; J. S. Iwanczyk, B. E. Patt, et. al. “Simulation and Modeling of a New Silicon Drift Chamber X-ray Detector Design for Synchrotron Radiation Applications”, Nucl. Instr. & Meth. in Phys. Res. A380 (1996) 288-294; J. Segal, J. Plummer, B. E. Patt, J. S. Iwanczyk, and G. Vilkelis, “A New Structure for Controlling Dark Current Due to Surface Generation in Drift Detectors,” manuscript in preparation; J. Segal, C. Aw, J. Plummer, C. Kenney, S. Parker, B. E. Patt, J. S. Iwanczyk, and G. Vilkelis, “A Vertical High Voltage Termination Structure for High-Resistivity Silicon Detectors,” Submitted to IEEE Nucl. Science Symposium and Medical Imaging Conference, 1997 [2] J. S. Iwanczyk, B. E. Patt, C. R. Tull, C. Kenney, J. Segal, J. Bradley, B. Hedman, & K. Hodgson, “Large Area Silicon Drift Detectors for X-Rays—New Results,” Submitted to the 1998 IEEE Nuclear Science Symposium, Toronto, Canada, Nov 12-14, 1998].
Recently SDD structures for detection of light (hereinafter referred to as Silicon Drift Photodetectors (SDPs)) have also begun to appear. These include structures described by Hartman [Hartman, R., Struder L., Kemmer J., Lechner P., Lorenz E., & Mirzoyan R. Nuclear Instruments and Methods in Physics Research A387: 250-254 (1997); Olschner [Olschner F. IEEE Trans. Nucl. Sci. NS-43(3):1407-1410, (1996)] and Fiorini [Fiorini C., Perotti F., and Labanti C., IEEEE Transactions on Nuclear Science, V45, No3: 483-486 (1998)]. However, none of the Silicon Drift Photodetectors or combination of scintillator with the Silicon Drift Photodetector described were optimized, as they have not recognized nor taken into account the criteria of the present invention. Thus it is desirable to provide a means for implementing radiation detectors using scintillators with semiconductor drift photodetectors wherein the components are specifically constructed to achieve the best signal-to-noise ratio.
Radiation imaging systems typically are used to generate images of the distribution of radiation either transmitted through an object or emitted by an object. These images can be used to determine the structure and function of internal organs. The radiations are not of themselves visible to the naked eye. In emission imaging (“Nuclear Medicine”), radiation invisible to the naked eye is generated within an organ by radiopharmaceutical or other radiation bearing substance which passes through or in some cases is designed to accumulate within the organ.
Prior emission imaging applications include single photon planar imaging and Single Photon Emission Computed Tomography (SPECT) for imaging the structure or function of internal organs. Anger introduced one system which has remained largely unchanged since it was first described in the 1950's (Anger, HO “Scintillation camera,” Rev. Sci. Instr. 29, 27. 1958; Anger, HO “Scintillation camera with multichannel collimators,” J. Nucl. Med. 5, p515-531. 1964). These Anger-type gamma-ray cameras employed in single-photon emission imaging applications typically have a shielded enclosure, preferably made from lead or similar materials. Incident gamma-rays pass through the parallel-hole collimator which “focuses” the gamma-rays. The collimator limits the system sensitivity for typical medical imaging applications. Gama-rays received through the collimator enter a large scintillation crystal (typically NaI[TI], CsI[TI] or CsI[Na]), generating light photons which pass through an optical diffuser to neighboring photomultiplier tubes. The light photons are guided through the scintillator and the optical diffuser using reflectors along the sides of the scintillator and diffuser. The light photons strike an array of the photomultiplier tubes, each of which is between 1 inch and 2 inches in diameter, and signals pass to analog electronics which perform the position arithmetic and spectrometric functions. A device can be used to display the acquired images. Below the photomultiplier tubes is a position/pulse-height module (Webb S, In “The Physics of Medical Imaging,” Adam Hilger, Bristol, England p161 1996).
A second type of emission imaging in Nuclear Medicine is dual photon imaging, such as Positron Emission Tomography (PET). Positron Emission Tomography systems typically are used to generate images of the distribution of positron emitting radiopharmaceuticals or other positro
Iwanczyk Jan S.
Patt Bradley E.
Christie Parker & Hale LLP
Hannaher Constantine
Lee Shun
Photon Imaging, Inc.
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