Leonard J. Cirignano

Design studies of a high resolution PET detector using APD arrays

The authors evaluated a compact, high resolution PET detector module using avalanche photodiode (APD) arrays to replace bulky position sensitive PMTs. The newly developed APD array is a planar processed 4/spl times/4 array which has a 2/spl times/2 mm/sup 2/ pixel size with 0.4 mm gaps between pixels, about 60% quantum efficiency at 420 nm wavelength, and uniform high gain (>1000) across all channels. A 4/spl times/4 array of 2/spl times/2/spl times/10 mm/sup 3/ LSO crystals was coupled to an APD array. Different readout electronics and signal multiplexing schemes were explored. All crystals in the detector array were clearly identified in the flood source histogram, with average peak-to-valley ratios of about 12:1 using a charge sharing resistor network. The energy resolution was measured to be /spl sim/14% at 511 keV in the detector array. The measured timing resolution was 2.6 ns in coincidence with a LSO/PMT detector. By optimizing the readout electronics currently being used, it is likely that detector performance can be further improved. The authors have also determined depth-of-interaction (DOI) by reading out two APD arrays connected to the ends of a 2/spl times/2/spl times/22 mm/sup 3/ LSO crystal. Preliminary measurements show good DOI measurement capability with DOI positioning uncertainty between 4 and 6.5 mm.

Dual APD array readout of LSO crystals: optimization of crystal surface treatment

Summary form only received as follows: The authors are developing a compact PET detector module by coupling an LSO scintillator array with two APD arrays to achieve high sensitivity, and high and uniform spatial resolution. They report studies on improving the depth-of-interaction (DOI) resolution by optimizing the crystal surface treatment, and on the effect of crystal geometry on DOI resolution. Three 2/spl times/2/spl times/20 mm LSO crystals were treated with different surface finishes along their length: raw saw cut, polished with 12 mm grade AlO2 paper, and fine mirror polish. The 2/spl times/2 mm ends were fine mirror polished. The ratio of the signals from the APD arrays was used to measure DOI, and the sum of the signals to measure the total light output. Crystals finished with the 12 mm grade paper gave the best overall detector performance, with DOI resolutions ranging from 3.1 to 3.9 mm for all interactions with energy above /spl sim/150 keV threshold, and uniform light output for different DOI positions. The energy resolution averaged /spl sim/17%. A 1/spl times/1/spl times/20 mm and a 2/spl times/2/spl times/30 mm LSO crystals finished with saw-cut and 12 /spl mu/m grade paper were also measured, and gave DOI resolutions in the range of 2.7 to 4.4 mm, and 4.7 to 6.6 mm, respectively.

Developing Larger TlBr Detectors—Detector Performance

Thallium bromide (TlBr) is a high atomic number (81, 35), dense (7.56 g/cm3) wide band gap (2.68 eV) semiconductor. In addition, TlBr has a cubic crystal structure and melts congruently at a relatively low temperature (~460 C). Recently, mobility-lifetime product of electrons in TlBr has been reported to be greater than 0.001 cm2/V. These properties make TlBr a promising material for room temperature gamma radiation detection. Employing device designs such as small pixel arrays that depend primarily on the motion of a single carrier type allows fabrication of thicker devices with better energy resolution than planar devices of the same thickness. We report on our recent progress in developing larger TlBr detectors. Over the past several months we have increased the electron mobility-lifetime product of our TlBr by more than one order of magnitude. Electron mobility-lifetime values as high as 3.0 times 10-3 cm2/V have been measured. Devices with small pixel design have been built with 3, 5, and 10 mm thickness and pixel pitch of 1 mm, 1.5, and 2.0 mm respectively. Pulse height spectra have been recorded over a range of energies from 60 keV to 662 keV. Energy resolution (FWHM) as high as approximately 5% at 122 keV and 1.7% at 662 keV has been obtained without any 3-D corrections. Such arrays are well suited for 3-D correction techniques similar to those applied to CZT devices, indicating that further improvement in energy resolution should be achievable. These latest results demonstrate promise for TlBr as a room temperature semiconductor gamma ray detector.

Characterization of polycrystalline TlBr films for radiographic detectors

Vapor deposited films of thallium bromide are evaluated as potential photoconductive layers in new large-area radiographic detectors. The attractiveness of the material lies in its inherent high effective atomic number and high density. Polycrystalline films up to 200 /spl mu/m have been grown and show a columnar structure with grains reaching 100 /spl mu/m in diameter. Current-voltage (IV) tests indicate a bulk resistivity of 10/sup 9/-10/sup 10/ /spl Omega//spl middot/cm, limited by ionic conduction. The instability of current with time is also observed, but it can be minimized with cooling. The films demonstrate high gain at relatively low field strengths as compared to other photoconductive layers. Benefits and drawbacks of TIBr are compared to other materials, and possible solutions are discussed.

Thallium Bromide Nuclear Radiation Detector Development

Thallium bromide (TlBr) is a dense, high-Z, wide bandgap semiconductor that has potential as an efficient, compact, room temperature nuclear radiation detector. In this paper we report on our recent progress in TlBr nuclear detector development. In particular, improvements in material purification have led to an order of magnitude increase in the mobility-lifetime product of electrons, (mutau)e, to as high as 5 times 10-3 cm2/V. This has enabled much thicker detectors with good charge collection to be fabricated. We fabricated and tested small pixel TlBr arrays up to 10 mm thick. The energy resolution ~2% FWHM at 662 keV was recorded with 5-10 mm thick devices without 3-D spectral correction. We also investigated the long-term detector stability and were able to constantly operate a thin (0.5 mm) detector for five months at -18degC, under an electric field and with irradiation.

Large area APDs and monolithic APD arrays

In this paper, development of large area planar APDs and monolithic APD arrays for scintillation detection is discussed. Single APDs with area as large as 10 cm2 have been fabricated and tested with CsI(Tl) scintillator (3.8 cm diameter, 2.5 cm high). The resolution of the 662 keV photopeak has been measured to be 9% (FWHM). Multi-element APD arrays have also been fabricated in various formats such as 4/spl times/4 to 14/spl times/14 elements (2 mm pixels) and their gain, noise and dark current performance has been characterized. Scintillation and timing studies have also been performed by coupling these arrays to LSO and CsI(Tl) scintillators. Packaging, and electronic readout issues related to these APD devices are discussed.

Room-temperature semiconductor device and array configurations

The use of high atomic number, room-temperature semiconductors for radiation detectors always involves making compromises to optimize the performance for specific applications. In recent years, a number of sophisticated device configurations and a variety of new read-out methods have been developed to extract the desired information from the detector signal. These approaches have significantly mitigated the effects of the inherent deficiencies found in available materials and have opened the way for promising new applications. The benefits of such approaches and their limitations are reviewed for CdTe and Cd1−xZnxTe (CZT) devices.

Bismuth iodide crystals as a detector material: some optical and electrical properties

This paper describes the preliminary results obtained from our study of optical and electrical properties of BiI3 crystals. The bismuth iodine polycrystals were grown using commercial starting material by vertical Bridgman method. For our measurements we used only single crystal samples that were cut out from grown crystals perpendicular to C6-axis.

CdTe Strip Detector Characterization for High Resolution Small Animal PET

Excellent spatial resolution is a requirement for preclinical PET imaging. In order to achieve spatial resolution of significantly better than one millimeter, an appealing possibility is to employ direct detector materials, such as cadmium telluride (CdTe). Prototype thin orthogonal strip detectors have been developed for testing. They have dimensions of 20 mm by 20 mm and are 0.5 mm thick, and have strips of 0.5 mm pitch on one side and 2.5 mm pitch on the other. Results are presented for the energy resolution (3% at 511 keV), intrinsic position resolution (equal to the 0.5 mm strip pitch), and timing resolution (3 ns FWHM in coincidence with an LSO detector, 8 ns FWHM for coincidence of two CdTe detectors) of the detectors. A PET scanner design is proposed using blocks made of the CdTe strip detectors, oriented in the blocks with their thin edges toward the center of the scanner. Simulation results suggest that this scanner, using a threshold of 250 keV, would have a sensitivity of 3.4% for a point source at its center.

An improvement in growing large, oriented lead iodide single crystals for detector applications

Abstract Recent improvements have been made in growing large, oriented lead iodide (PbI 2 ) single crystals for room temperature X-ray detector applications. The effect of ampoule design on the growth of crystals by the Bridgman-Stockbarger technique was studied. Crystals, optimal for detector labrication, were obtained using a necked ampoule with a 60° bend. This ampoule produced single crystals with the c -axis parallel to the axis of the ampoule.