Improving the performance of positron-emission tomography (PET) systems for small animal imaging requires the use of high-speed detectors, such as a Lutetium Oxyorthosilicate (LSO) crystal coupled to avalanche photodiodes (APDs). We have developed an application-specific integrated circuit (ASIC) for reading out the multiple channels of an LSO-APD detector module for use in a small animal PET system. In this work, we characterize the performance of the ASIC, referred to as the AE172 chip. The AE172 can accommodate both positive and negative input signals, with a programmable conversion gain that ranges from 2.8 mV/fC to 21 mV/fC. The ASIC channels exhibit a linear response and an equivalent noise charge (ENC) of ~2000 electrons, RMS. The timing jitter depends on the amplitude of the input signal; and is less than 1 ns for test signals greater than 100,000 electrons. Connecting the ASIC to a linear APD detector reduces the relative readout noise to 26 electrons referenced to the input of the APD, which is operated at a gain of ~200. An LSO-APD- ASIC detector achieves an energy resolution of 13% for the 511 keV annihilation photons from a 22 Na source, and a coincident timing resolution of ~13 ns.

Detection of single photons is crucial for a number of applications. Geiger photodiodes (GPD) provide large gains with an insignificant amount of multiplication noise exclusively from the diode. When the GPD is operated above the reverse bias breakdown voltage, the diode can avalanche due to charged pairs generated from random noise (typically thermal) or incident photons. The GPD is a binary device, as only one photon is needed to trigger an avalanche, regardless of the number of incident photons. A solid-state photomultiplier (SSPM) is an array of GPDs, and the output of the SSPM is proportional to the incident light intensity, providing a replacement for photomultiplier tubes.We have developed CMOS SSPMs using a commercial fabrication process for a myriad of applications. We present results on the operation of these devices for low intensity light pulses. The data analysis provides a measured of the junction capacitance (similar to 150 fF), which affects the rise time (similar to 2 ns), the fall time (similar to 32 ns), and gain (>10(6)). Multipliers for the cross talk and after pulsing are given, and a consistent picture within the theory of operation of the expected dark current and photodetection efficiency is demonstrate. Enhancement of the detection efficiency with respect to the quantum efficiency at unity gain for shallow UV photons is measured, indicating an effect due to fringe fields within the diode structure. The signal and noise terms have been deconvolved from each other, providing the fundamental model for characterizing the behavior at low-light intensities.

Exploration in nuclear physics may require extreme conditions, such as temperatures down to a few Kelvin, high magnetic fields of several Tesla, or the small physical dimensions of a few centimeters. As a standard technique for radiation detection using scintillation materials, it is desirable to develop photodetectors that can operate under these harsh conditions. Though photomultiplier tubes (PMTs) have been used for most applications for readout of scintillation materials, they are bulky, highly susceptible to magnetic fields, and present a large heat load in cryogenic environments. Avalanche photodiodes are a reasonable alternative to PMTs in that they are extremely compact and less susceptible to magnetic fields. Avalanche photodiodes have been developed in a commercial CMOS process for operation at temperatures below 100 Kelvin. Here we present the overall operation of the photodiodes at 5 Kelvin. The diodes show a quantum efficiency of at least 30% at 532 nm at 5 Kelvin. At about 30 Kelvin, the diodes exhibit an internal resistive term, which generates a second breakdown point. The prototype diode shows a proportional response to the intensity of light pulses down to 150 detected photons with a hole to electron ionization ratio, k, of 2.3x10(-13) at 5 Kelvin. The properties of the photodiodes and the readout electronics will be discussed for general photon detection below 100 K.

We have designed a 5×5mm2 position sensitive solid-state photomultiplier (PS-SSPM) using a complementary metal-oxide-semiconductor (CMOS) process that provides imaging capability on the micro-pixel level. The PS-SSPM has 11,664 micro-pixels total, with each having an active area and micro-pixel pitch of 30×30μm2 and 44.3μm, respectively. The PS-SSPM was then examined for its performance characteristics such as its energy and spatial resolution, and LYSO scintillator array imaging capabilities. When coupled to 5×5×3mm3 LYSO, the energy resolution at 511keV (22Na) was measured as a function of bias, and corrected for the PS-SSPM non-linear output. The resolution is 14% (FWHM) at 511keV with 30V bias. The LYSO coincidence timing resolution was 9.4ns (FWHM) at 511keV. Spatial resolution studies were conducted using a focused (∼30μm beam spot diameter) pulsed 635nm diode laser. Scintillator array imaging studies were conducted at 511keV using a 6×6 LYSO array, having 500μm pixels (530μm pitch) and 5mm tall.

High-energy, gamma-ray calorimetry typically employs large scintillation crystals coupled to photomultiplier tubes. These calorimeters are segmented to the limits associated with the costs of the crystals, photomultiplier tubes, and support electronics. A cost-effective means for construction of a calorimeter system is to use solid-state photomultipliers (SSPM) with front-end electronics, which is at least half the cost, but the SSPM must provide the necessary energy resolution defined by the physics goals. One experiment with plans to exploit this advantage is an upgrade to the PRIMEX experiment at Jefferson Laboratories. We have developed a large-area SSPM (1 cm x 1 cm) for readout of large scintillation crystals. As PbWO4 has excellent properties (small Moliere radius and radiation hard) for high-energy gamma-rays (>1 GeV) but low light yields (similar to 150 photons/MeV at 0 degrees C), evaluation of the SSPM and support readout electronics with LaBr3 provides a measure of the device performance. Using the known detection efficiency and dark current of the SSPM, an excess noise factor associated with after pulsing and cross talk is determined. The contribution to the energy resolution from the detector module is calculated as <1% for gamma rays greater than similar to 2.5 GeV (0.7% at 4.5 GeV).

To replace PMTs for readout of optical photons from scintillation materials in harsh environments, prototype solid-state photodetectors has been developed to operate in harsh environments, which implies temperatures down to 4 degrees K and magnetic fields up to 7 T. A photodiode structure developed out of RMD's CMOS SSPM technology shows avalanche multiplication at temperatures down to 4 degrees K when operated in a proportional mode (below breakdown). The diode and photodetector characteristics are discussed, providing information on the implementation of these devices for the HIFROST target, which will be used to study the proton spin polarizabilities. The photodetector shows a quantum efficiency of similar to 45% at 532 nm for 4 degrees K operation with a dark current of similar to 0.1 mu A/mm(2) and a gain of similar to 100 for an operating bias of 31.8 V. Light pulses of 500 photons were easily read out using amplifiers operated at room temperature.

Based on CMOS Geiger photodiode (GPD) pixels developed at RMD Inc. and fabricated with MOSIS consortium, we present a detector design that is capable of providing highspeed and single optical photon sensitivity in the near infrared region. The integration of the electronic reset with these CMOS pixels enables their use in a cost-effective large imaging array to provide a substantial increase in the signal-to-noise ratio and facilitate the high-speed, pixel-level signal processing required for photon-counting Diffuse Correlation Tomography (DCT). DCT is an imaging technique in progress that promises to provide a method for routine, non-invasive cancer screening and treatment by imaging blood flow, tissue oxygenation, and the distinct vasculature associated with the angiogenesis of cancerous growth using non-ionizing radiation. In this work, we present results of the design, fabrication and characterization of the CMOS Geiger pixels and their associated integrated reset circuits for DCT measurements.

This paper examines the performance of CMOS avalanche photodiode pixels operated in a Geiger mode. The pixels, called Geiger photodiode (GPD) pixels, convert an incident analog photon flux into a digital count rate. The maximum detection efficiency of the characterized GPD pixel for 632-nm light is 22%. The passively quenched GPD pixel exhibits an after pulsing at excess bias voltages above 2 V, and a minimum in the after-pulsing correction factor, of 0.53, occurs at an excess bias of 5.8 V. The after pulsing increases the fluctuations, or noise, in the count-rate signal. The following expression accurately describes the noise in the characterized GPD pixel, which exhibits a relatively low after-pulsing probability: /spl sigma//sup 2/=n~/sub dp/+n~/sub ap/+2/spl middot/[n~/sub dp//spl middot/n~/sub ap/]/sup 1/2/, where /spl sigma/ represents the count-rate fluctuations, the "dp" subscript stands for "detected photons," the "ap" subscript stands for "after pulses," and the n~ represents the "average count rate of" dp, or ap. The noise-equivalent illumination exhibits a minimum of 300 Hz at an operating voltage of 28 V. The best operating voltage for the GPD pixel increases from 28 V with increasing signal intensity.

An approach to detect the light from scintillation materials using an array of small photon counting detectors, referred to as a "solid-state photomultiplier" (SSPM), detects light from a scintillation material to provide a sensitive radiation monitor. Each pixel acts as a binary photon detector, but the summed output is an analog representation of the total photon intensity, which is proportional to the energy of the gamma ray. We have successfully fabricated arrays of GPD pixels in a CMOS environment, which makes possible the production of miniaturized arrays integrated with the detector electronics in a small silicon chip. This detector technology allows for a substantial cost reduction while preserving the energy resolution needed for radiological measurements. In this work, we demonstrate the operation of a 100-pixel array in reading an LSO scintillation crystal. In addition, we present the energy spectra corrected for saturation effects and examine the effects of cross talk on the performance of the SSPM.

Improving the performance of positron-emission tomography (PET) systems for small animal imaging requires the use of high-speed detectors, such as a lutetium oxyorthosilicate (LSO) crystal coupled to avalanche photodiodes (APDs). We have developed an application-specific integrated circuit (ASIC) for reading out the multiple channels of an LSO-APD detector module for use in a small animal PET system. In this work, we characterize the performance of the ASIC, referred to as the AE172 chip. The AE172 can accommodate both positive and negative input signals, with a programmable conversion gain that ranges from 2.8 mV/fC to 21 mV/fC. The ASIC channels exhibit a linear response and an equivalent noise charge (ENC) of ~2000 electrons, RMS. The timing jitter depends on the amplitude of the input signal; and is less than 1 ns for test signals greater than 100,000 electrons. Connecting the ASIC to a linear APD detector reduces the relative readout noise to 26 electrons referenced to the input of the APD, which is operated at a gain of ~200. An LSO-APD-ASIC detector achieves an energy resolution of 13% for the 511 keV annihilation photons from a 22Na source, and a coincident timing resolution of ~13 ns

Avalanche photodiode (APD) arrays fabricated by using complementary metal-oxide-semiconductor (CMOS) fabrication technology offer the possibility of combining these high sensitivity detectors with cost effective, on-board, complementary circuitry. Using CMOS techniques, Radiation Monitoring Devices has developed prototype pixels with active diameters ranging from 5 to 60 microns and with measured quantum efficiencies of up to 65%. The prototype CMOS APD pixel designs support both proportional and Geiger modes of photo-detection. When operating in Geiger mode, these APDis act as single-optical-photon-counting detectors that can be used for time-resolved measurements under signal-starved conditions. We have also designed and fabricated CMOS chips that contain not only the APD pixels, but also associated circuitry for both actively and passively quenching the self-propagating Geiger avalanche.This report presents the noise and timing performance for the prototype CMOS APD pixels in both the proportional and Geiger modes of operation. It compares the quantum efficiency and dark-count rate of different pixel designs as a function of the applied bias and presents a discussion of the maximum count rates that is obtained with each of the two types of quenching circuits for operating the pixel in Geiger mode. Preliminary data on the application of the APD pixels to laser ranging and fluorescent lifetime measurement is also presented.

Semiconductor detector arrays made of CdTe/CdZnTe are expected to be the main components of future high-performance, clinical nuclear medicine imaging systems. Such systems will require small pixel-pitch and much larger numbers of pixels than are available in current semiconductor-detector cameras. We describe the motivation for developing a new readout integrated circuit, AEGIS, for use in hybrid semiconductor detector arrays, that may help spur the development of future cameras. A basic design for AEGIS is presented together with results of an HSPICETM simulation of the performance of its unit cell. AEGIS will have a shaper-amplifier unit cell and neighbor pixel readout. Other features include the use of a single input power line with other biases generated on-board, a control register that allows digital control of all thresholds and chip configurations and an output approach that is compatible with list-mode data acquisition. An 8x8 prototype version of AEGIS is currently under development; the full AEGIS will be a 64x64 array with 300 μm pitch.

Development of an advanced, application-specific integrated circuit (ASIC) for use With high-density, avalanche photodiode (APD) detector arrays has been undertaken.APD technology recently developed at Radiation Monitoring Devices (RMD) has demonstrated great promise in the form of compact arrays. While APD arrays are capable of providing the basis for a new generation of high-resolution, photon-imaging systems, it is extraordinarily difficult to use conventional pulse-processing circuitry to support the many signals generated by these arrays. Due to the high detector density, small size, and unique electronic features of APD arrays, conventional readout electronics quickly become problematic for the implementation to APD arrays in positron-emission tomography (PET) systems.As the focus of NIH Phase I Small Business Innovation Research (SBIR) research, we designed and developed an ASIC to enable the implementation of APD arrays in PET. The circuit incorporates preamplifier, timing, shaping, and sample-and-hold capabilities that are necessary for APD use in PET. The ASIC has been designed, fabricated, and has undergone preliminary evaluation. We report on the development process, operational requirements, and performance results.

A 3cm3 multichannel gamma spectrometer for DOE applications is under development by Digirad Corporation. The device is based on a position sensitive detector packaged in a compact multi-chip module (MCM) with integrated readout circuitry. The modular, multichannel design will enable identification and quantitative analysis of radionuclides in extended sources, or sources containing low levels of activity. The MCM approach has the advantages that the modules are designed for imaging applications, and the sensitivity can be arbitrarily increased by increasing the number of pixels, i.e. adding modules to the instrument. For a high sensitivity probe, the outputs for each pixel can be corrected for gain and offset variations, and summed digitally. Single pixel results obtained with discrete low noise readout indicate energy resolution of 3keV can be approached with currently available CdZnTe. The energy resolution demonstrated to date with MCMs for 511keV gamma rays is 10keV.

We report the development of a CdZnTe gamma ray imager whose advantages over the conventional Anger camera include: 1) improved contrast and spatial resolution resulting from superior energy resolution and unambiguous position determination and 2) the small size and weight and high reliability inherent in an all-solid-state construction. It provides high energy resolution and peak efficiency by incorporating Digirad's new SpectrumPlus/sup TM/ detector technology. The new imager employs a modular design, in which each 1"/spl times/1" module incorporates a monolithic 64-element CdZnTe detector array and ASIC-based circuitry that provides signal conditioning for every channel, identification of valid events and addressing functions. Imagers with a wide range of sizes and shapes can be assembled by timing modules together on a specially designed signal routing board.

Previous efforts by our group have demonstrated the potential of hybrid semiconductor detector arrays for use in gamma-ray imaging applications. In this paper, we describe progress in the development of a prototype imaging system consisting of a 64/spl times/64-pixel CdZnTe detector array mated to a multiplexer readout circuit that was custom designed for our nuclear medicine application. The detector array consists of a 0.15 cm thick slab of CdZnTe which has a 64/spl times/64 array of 380 /spl mu/m square pixel electrodes on one side produced by photolithography; the other side has a continuous electrode biased at -150 V. Electrical connections between the detector electrodes and corresponding multiplexer bump pads are made with indium bump bonds. Although the CdZnTe detector arrays characterized in this paper are room-temperature devices, a slight amount of cooling is necessary to reduce thermally generated dark current in the detectors. Initial tests show that this prototype imager functions well with more than 90% of its pixels operating. The device is an excellent imager; phantom images have a spatial resolution of 1.5 mm, limited by the collimator bore.