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Parallax error during coincidence imaging with dual-head cameras causes infidelity in tomographic images, worsening as one increases crystal thickness to combat poor detection efficiency. We are integrating wavelength-shifting (WLS) scintillating fibers onto a thick NaI(Tl) crystal to create an Anger detector capable of measuring depth-of-interaction (DOI). Our studies target eventual implementation on the large-area detectors of commercial multihead coincidence imaging systems. The prototype detector uses a 127 127 25.4 mm 3 NaI(Tl) crystal. The crystal is sealed in a two-windowed aluminum enclosure-both faces of the crystal are coupled optically to glass windows. A ribbon of 125 square 1-mm WLS fibers is coupled lengthwise to the entrance window; enhanced-green-response photomultiplier tubes (PMTs) detect the light from the fibers. The number of fibers exhibiting signals above a threshold value determines the DOI. For interactions near the ribbon, light localizes in only a few fibers; light from distant interactions illuminates many fibers. Four PMTs, for position centroid calculation and energy windowing, are coupled through a light-guide to the exit window of the detector. This paper summarizes the concept of the depth-encoding Anger detector, describes the design of the detector prototype, and details on-going feasibility testing and optimization of the detector.Index Terms-Coincidence imaging, depth of interaction (DOI), parallax error, wavelength-shifting scintillating fibers.

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A unique photomultiplier tube has been designed, fabricated and studied. The quadrant tube has four transmission-mode K2CsSb (bialkali) photocathodes in a single 81mm square glass envelope. A novel electron multiplier structure with four independent anodes supplies secondary emission gains of 105 or more. Detailed measurements indicate that the tube performs like four individual photomul ti pl i ers .Data are presented on crosstalk, energy resolution, linearity and spatial uniformity. The use of the tube in Anger scintillation cameras and other applications requiring high detection efficiency, position-sensitive photomultipliers is discussed.

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