Mickel McClish

Position sensitive APDs for small animal PET imaging

In this paper, investigation of position sensitive avalanche photodiodes (PSAPDs) as optical detectors for reading out segmented scintillation arrays of LSO in high resolution PET modules is reported. PSAPDs with 8/spl times/8 mm/sup 2/ and 14/spl times/14 mm/sup 2/ area have been characterized with single LSO crystals and arrays. Energy resolution of 19% (FWHM) for 511 keV /spl gamma/-rays and coincidence timing resolution of /spl sim/3 ns (FWHM) have been recorded with PSAPD coupled to 1/spl times/1/spl times/20 mm/sup 3/ LSO detectors. Flood histogram studies have been successfully conducted by coupling multi-element element LSO arrays (1 mm pixels, 20 mm tall) to the PSAPDs. Finally, depth of interaction (DOI) resolution of <4.5 mm (FWHM) has been measured by coupling two PSAPDs on opposite ends of a 20 mm long LSO crystal with a 1/spl times/1 mm/sup 2/ cross section. Based on these results, PSAPDs appear to be promising for high resolution PET. An important advantage of these PSAPDs is significant reduction in electronic readout requirements.

Performance Measurements of CMOS Position Sensitive Solid-State Photomultipliers

We have designed position sensitive solid-state photomultipliers (PS-SSPM) using a complementary metal-oxide-semiconductor (CMOS) process. Four variations of the PS-SSPM design were fabricated, however, one of the variations did not function properly. The remaining three functional variations were characterized for their energy and coincidence timing resolution, spatial resolution, and scintillator array imaging. Each PS-SSPM is 1.5 × 1.5 mm2, however, each device has different micro-pixel geometries and different micro-pixel electrical readout for event position sensing. When coupled to 1 × 1 × 20 mm3 LYSO, the energy resolution at 511 keV was measured as a function of bias. The same LYSO scintillator was used to measure the coincidence timing resolution. Results between the PS-SSPMs varied from 2.0 ns to 0.9 ns (FWHM) at 511 keV. Spatial resolution studies were conducted using a focused (15 μm beam spot diameter) pulsed 635 nm diode laser. For each PS-SSPM, the X and Y spatial resolution was measured between 70 and 75 μm (FWHM). Lastly, scintillator array images were generated using a CsI:Tl and LYSO array having 300 × 300 μm2 and 500 × 500 μm2 pixels respectively.

Solid-state photomultiplier in CMOS technology for gamma-ray detection and imaging applications

A CMOS solid-state photomultiplier (SSPM) coupled to a scintillation crystal uses an array of CMOS Geiger-mode avalanche photodiode (GPD) pixels to collect light and produce a signal proportional to the energy of the radiation. Each pixel acts as a binary photon detector, but the summed output is an analog representation of the total photon intensity. 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. In this work, we compare designs for the SSPM detector and present preliminary results in constructing a position-sensitive solid-state photomultiplier (PS-SSPM) using a commercially available CMOS process. The prototype arrays utilize a resistor network to provide a position-sensitive readout of the array. One pixel design achieves maximum detection efficiency for 632-nm photons approaching 20% with a room temperature dark count rate of less then 1 kHz for a 30-mum-diameter pixel. Pair-wise cross talk was measured to be less than 2% for 150 mum pixel spacing

Modeling and Correction of Spatial Distortion in Position-Sensitive Avalanche Photodiodes

Position-sensitive avalanche photodiodes (PSAPDs) are a promising alternative to photomultiplier tubes for the development of a new generation of gamma imagers. They offer compactness, high gain and superior quantum efficiency. PSAPDs having a sensitive surface of up to 28times28 mm2 have been fabricated. However, unlike pixellated imaging devices having a similar configuration, each PSAPD can achieve submillimeter position sensing over its surface with only four readout channels. This key feature is obtained by Anger-logic event positioning from the signals of four corner anodes printed on a resistive layer covering one of the PSAPD surfaces. This readout scheme provides high degree of multiplexing for reading position and energy information from the device, but leads to pincushion distortion in the spatial information due to the nonlinear charge sharing pattern associated with the potential across the resistive layer. We have developed a method to reproduce and correct this distortion based on finite-element simulations of the readout configuration. The resistive layer and the anodes are represented by a two-dimensional array of resistors and this circuit is numerically solved to obtain the signal on the four anodes for different current injection nodes. The relation between the injection positions and the resulting Anger positions is modeled and then used to correct experimental data. The algorithm was tested on 99mTc flood images of a 16times16 array of 0.4times0.4times4 mm3 CsI(Tl) crystals and successfully restored the regular pattern. The correction procedure is fast and robust, and constitutes a step toward the realization of a low-cost, high-resolution gamma camera based on PSAPDs

A Reexamination of Deep Diffused Silicon Avalanche Photodiode Gain and Quantum Efficiency

Traditionally the measured gain of an avalanche photodiode (APD) has been considered the product of two parameters. The electron multiplication process being one and the quantum efficiency (QE) the other. The multiplication process is often considered wavelength dependent and the QE being bias independent. We propose a new examination of these related parameters where the APD gain is considered intrinsic, defined as being the amount of electron multiplication each photoelectron undergoes based only on the applied bias and is thus independent of the incident photon wavelength, and where the QE, which can be defined as the number of generated photoelectrons per number of incident photons, is considered intrinsic being a function of not only wavelength, but also bias. This is a more logical, and physically real, perspective of APD behavior. We introduce a technique to measure the intrinsic gain and the intrinsic QE of deep diffused silicon APDs. Once the intrinsic gain vs. bias of the APD is measured, it becomes possible to use this measurement as an absolute parameter. With the intrinsic gain known, we show how the intrinsic QE of an APD, for a given wavelength, changes as a function of bias. We show that when the APD is operated from the low to high gain regime, light at 400 nm to 800 nm experiences an increase in QE. These fundamental, dynamic and operational properties of APDs are critical when considering the wavelength(s) that are of interest for a given application

New developments for CMOS SSPMs

A high fill factor SSPM built using a standard CMOS fabrication process can provide an energy resolution of 12.4% at 511 keV using CsI(Tl) crystals. The SSPM was operated at an excess bias of 2 V and 0 °C. The magnitude of the noise terms of the SSPM under these conditions are provided. This is compared to the energy resolution of 11.7% using a PMT at room temperature and the identical crystal. CMOS SSPMs can provide PMT-like energy resolution. Additional developments in back-illuminated and position-sensitive SSPMs devices are provided. A back-illuminated device has the promise of a low-noise, high fill-factor design, and the initial results of the quantum efficiency of back-illuminated, thinned devices, fabricated with an existing SSPM design, are provided. For position-sensitive SSPMs, an image of a 3 × 3 CsI array has been made with an SSPM based on a resistive-network configuration to provide position information has been made with minimal distortions.

Development of a hard X-ray polarimeter for solar flares and gamma-ray bursts

We describe recent work on the development of a Compton scatter polarimeter for measuring the polarization of hard X-rays (100-300 keV) from astrophysical sources. Results from measurements with a laboratory prototype are summarized, along with comparisons to Monte Carlo simulations. We also present our latest design concept, which envisions a complete polarimeter module on the front end of a 5-inch position-sensitive PMT. Although the emphasis of our development effort is towards measuring hard X-rays from solar flares, our latest design has the advantage that it is sensitive over a rather large field-of-view (>1 steradian), a feature that makes the design especially attractive for /spl gamma/-ray burst studies.

Recent laboratory tests of a hard x-ray solar flare polarimeter

We report on the development of a Compton scatter polarimeter for measuring the linear polarization of hard X-rays (50 - 300 keV) from solar flares. Such measurements would be useful for studying the directivity (or beaming) of the electrons that are accelerated in solar flares. We initially used a simple prototype polarimeter to successfully demonstrate the reliability of our Monte Carlo simulation code and to demonstrate our ability to generate a polarized photon source in the lab. We have recently fabricated a science model based on a modular design concept that places a self-contained polarimeter module on the front-end of a 5-inch position- sensitive PMT (PSPMT). The PSPMT is used to determine the Compton interaction location within an annular array of small plastic scintillator elements. Some of the photons that scatter within the plastic scintillator array are subsequently absorbed by a small centrally-located array of CsI(Tl) crystals that is read out by an independent multi-anode PMT. The independence of the two PMT readout schemes provides appropriate timing information for event triggering. We are currently testing this new polarimeter design in the laboratory to evaluate the performance characteristics of this design. Here we present the initial results from these laboratory tests. The modular nature of this design lends itself toward its accommodation on a balloon or spacecraft platform. A small array of such modules can provide a minimum detectable polarization (MDP) of less than 1% in the integrated 50 - 300 keV energy range for X-class solar flares.

Development of a hard X-ray polarimeter for astrophysics

We have been developing a Compton scatter polarimeter for measuring the linear polarization of hard X-rays (100-300 keV) from astrophysical sources. A laboratory prototype polarimeter has been used to successfully demonstrate the reliability of our Monte Carlo simulation code and to demonstrate our ability to generate a polarized photon source in the lab. Our design concept places a self-contained polarimeter module on the front-end of a a 5-inch position sensitive PMT (PSPMT). We are currently working on the fabrication of a science model based on this PSPMT concept. Although the emphasis of our development effort is towards measuring hard X-rays from solar flares, our design has the advantage that it is sensitive over a rather large field-of-view (>1 steradian), a feature that makes it especially attractive for /spl gamma/-ray burst studies.

Characterization and Scintillation Studies of a Solid-State Photomultiplier

The solid-state photomultiplier (SSPM) is a relatively new semiconductor based photodetector that, by using hundreds of micro silicon Geiger-APDs, possesses high gain (105 to 106) and low noise while needing only low voltage (40 - 60 V) to operate. The fast response and high intrinsic gain of SSPMs makes them attractive for timing applications, in particular for positron emission tomography (PET). Here we present the results from the characterization of a 1 times 1 mm2 SSPM (SSPM-050701GR-TO18, Photonique SA). Several intrinsic SSPM characteristics were measured such as gain, noise, and linearity. Single photon spectra were also collected. Additionally, scintillation studies were performed. The SSPM was coupled to CsI:Tl and LSO scintillation crystals and exposed to various common laboratory radionuclides to measure photopeak energy resolutions. The linearity of the SSPM, when coupled with a scintillator, was also measured. Using a 22Na source (511 keV annihilation photons), the coincidence timing resolution was measured with the SSPM coupled to LSO. Lastly, we introduce our complementary metal oxide semiconductor (CMOS) based SSPM and show preliminary characterizations.