Semiconductor radiation detector with internal gain

Details Semiconductor radiation detector with internal gain Semiconductor radiation detector with internal gain

2001-04-16

2003-04-01

Whitehead, Jr., Carl (Department: 2813)

Active solid-state devices (e.g., transistors, solid-state diode

Responsive to non-electrical signal

Electromagnetic or particle radiation

C257S438000

Reexamination Certificate

active

06541836

ABSTRACT:

FIELD OF THE INVENTION
The present invention is related to detection of radiation, and particularly, to a semiconductor radiation detector having internal gain.
BACKGROUND OF THE INVENTION
Gamma ray spectroscopy using scintillation detectors typically employs scintillators coupled to photomultiplier tubes (PMTs). Scintillator/PMT systems offer excellent efficiency with moderate energy resolution. These systems have drawbacks, however, particularly regarding the PMT light sensor. The PMT is a bulky and fragile component. It is vulnerable to damage, has low and non-uniform quantum efficiency, requires a stabilized high voltage for operation, and lacks immunity from magnetic fields. Consequently, it has been an ongoing quest to find suitable alternatives to replace either the PMT, or the entire scintillator/PMT system, for gamma-ray spectrometry. Such detectors are needed for applications such as gamma ray spectroscopy, photon counting, gamma ray counting, and gamma ray imaging.
Gamma ray imaging systems employ a large number of PMTs coupled to one or more scintillators.
FIG. 1
shows a schematic diagram (right) of a conventional PMT-based gamma camera
100
. The PMT-based gamma camera
100
includes a parallel-hole collimator
104
, a scintillation crystal (typically CsI[Tl] or NaI[Tl])
106
, a light diffuser
108
, an array of photomultiplier tubes (PMTs)
112
, and a position/pulse-height module
114
which performs position arithmetic and spectrometric functions, all enclosed by a lead shield
102
. This system has been largely unchanged since it was first described by Hal Anger in the 1950's. The position/pulse-height module
114
may include analog electronics and may also include one or more analog-to-digital converters (ADCs) for ADC conversion of analog signals at the front end.
Recently there has been an intensive effort by several groups to develop solid state photodetector-based gamma cameras based on scintillators coupled to silicon-PIN (Si-PIN) photodetectors, Si avalanche photodetectors (APDs), and Si drift photodetectors (SDPs), and non-scintillation cameras based on direct conversion of gamma rays in high-Z compound semiconductors such as cadmium zinc telluride (CZT) and mercuric iodide.
FIG. 1
also shows a schematic diagram (left) of a solid state photodetector-based gamma camera
120
. The solid state photodetector-based gamma camera
120
includes a parallel-hole collimator
124
, a scintillation crystal (typically CsI[Tl] or NaI[TL])
126
, a light diffuser
128
, solid state photodetectors
132
on substrate
134
, and a position/pulse-height module
136
, all enclosed by a lead shield
122
. The solid state photodetector-based gamma camera
120
may also be referred to as a solid state gamma camera.
Solid state gamma cameras have drawn a lot of interest because they may offer potential improvements in performance. Since the energy resolution of scintillation spectrometers depends primarily on the number of scintillation photons collected, the high quantum efficiency, high response uniformity, and low electronic noise of the solid state detectors should provide better energy resolution than current technologies.
In addition, because the spatial resolution of an Anger type scintillation camera typically depends on the spacing between the photodetectors and the signal-to-noise ratio (SNR) of the photodetectors, the use of smaller devices coupled with larger signal (due to high quantum efficiency) and lower variance (due to high response uniformity in the signal) should provide improved spatial resolution.
Further, the replacement of the photomultiplier tube with a solid state photodetector of just a few hundred microns thickness could lead to a more compact system with a significant reduction in shielding mass. For example, the lead shield
122
of the solid state gamma camera
120
is significantly smaller in size than the lead shield
102
of the PMT based gamma camera
100
. The reduction in shielding mass would allow the development of lighter scintillation cameras that could be accommodated in new types of gantrys or frames. This could open up the possibility of other uses in surgeries to help locate lesion sites or in research laboratories.
Solid state photodetector signals are typically more naturally interfaced to the digital domain leading to integrated/smart detectors with much more up-front processing capability resulting in improved performance and lower cost. They are typically amenable to digital signal processing, leading to improved energy performance afforded by more optimal signal shaping such as real-time adaptive filter pulse shaping, which can optimize shaping time based on instantaneous throughput. They may also provide higher throughput due to capability of replacing analog pulse shaping with digital filters incorporating schemes such as real-time adaptive filter pulse shaping described above.
Perhaps one of the most compelling reasons for the continued development of a solid-state replacement for the PMT based cameras is the expected reduction in cost afforded by high volume planar technology silicon processing compared to expensive PMT technology. With high-volume production, some of the mentioned solid state gamma camera technologies may allow significant (e.g., 3-5 times) reduction in cost compared to PMTs, particularly given that a significant portion of the processing electronics can be integrated with the photodetectors in the case of solid-state cameras.
Although laboratory prototypes of gamma cameras based on various solid-state technologies have now become a reality, there are still major limitations based on specific problems with the detector technologies. For example, a commercial gamma camera based on arrays of Si-PIN diodes coupled to a pixelated scintillator array is available on the market today. However, price and performance of the Si-PIN based camera is still worse than those of PMT-based cameras. The segmented scintillation crystal, used to optimize the resolution, typically adds significant cost to the system.
An even greater challenge to the widespread use of the Si-PIN photodetector-based gamma cameras may be that the intrinsic detector noise is quite high for photodetectors above a few square millimeters in area. Thus the Si-PIN photodetectors are usually pixelated to just a few square mm per pixel, and each pixel typically requires a separate electronic readout channel. For example, a 25 cm by 20 cm gamma camera based on an array of 3×3 mm
2
pixels requires 5555 channels of readout electronics. This is a major disadvantage compared to any detector architecture that can take advantage of sparse readout schemes, such as Anger logic. Because of the large number of channels of detector and preamp/shaper, the need for multiplexers, the complexity of the readout hardware and the need for higher cost segmented scintillators, the cost of a full-size gamma camera based on Si-PIN photodetectors is typically not competitive with PMT-based systems.
Laboratory gamma cameras based on large-area silicon avalanche photodetectors (APD) have also been developed. An advantage of the APD arises because the surface leakage current of semiconductor photodetectors is significantly larger than their bulk leakage current. Typically, only the signal and the bulk leakage current component see the very high gain in the avalanche multiplication stage, while the surface leakage current, which generally dominates, is not multiplied. Thus, the APD has an improved SNR compared to a Si-PIN photodetector that has no internal gain, because all of the leakage current in such a device is typically amplified together with the signal in the external gain stage. However, avalanche structures are difficult to control in production when the diode area is large (more than a few square mm in area). For this and other reasons, the devices still are not commercially available with sufficiently large areas and they suffer from manufacturability and reliability problems, which have not been completely sol

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