Solid-State Photomultiplier in CMOS Technology for Gamma-Ray Detection and Imaging Applications Christopher Stapels, Member, IEEE, William G. Lawrence, James Christian, Member, IEEE, Michael R. Squillante, Member, IEEE, Gerald Entine, Frank L. Augustine, Member, IEEE, Purushottam Dokhale, and Mickel McClish Abstract 1 -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- μm-diameter pixel. Pair-wise cross talk was measured to be less than 2% for 150 μm pixel spacing. I. INTRODUCTION Although scintillating materials are ideal for detecting and measuring high-energy radiation, the limitations of existing optical detectors drastically reduces their functionality. Replacing the photomultiplier tube (PMT) with an appropriate CMOS technology would provide a fully integrated, low-cost solution to optimize the functionality of scintillation materials, which is essential for applications such as the development of deployable digital dosimeters and medical imaging modalities. A solid-state photomultiplier (SSPM), or an array of avalanche photodiodes operating in Geiger mode (GPDs), produces a device that can achieve the low noise of a PMT at a lower cost, but retain the high quantum efficiency of a silicon device without the deterioration of signal from Manuscript received Oct 11, 2005. This work was supported in part by DTRA under Grant No. HDTRA 1-05-P0093. Christopher Stapels (email: Cstapels@rmdinc.com), William G. Lawrence, Michael R. Squillante, Gerald Entine, P. Dokhale, M. McClish, and James Christian are with Radiation Monitoring Devices, Inc.; 44 Hunt Street; Watertown, MA 02472 Frank L. Augustine is with Augustine Engineering; 2115 Park Dale Lane; Encinitas, CA 92024 thermal noise. The SSPM provides a basis for radiation spectrometers with a wide range of applications. Since the light produced in the scintillator is proportional to the energy of the absorbed event, the number of pixels that fire will provide the energy of the incident photon when the SSPM is uniformly illuminated. Fig. 1 illustrates the principle of operation of the SSPM. High energy incident radiation Low energy incident radiation Few pixels fire Scintillator Many pixels fire GPD array High energy incident radiation Low energy incident radiation Few pixels fire Scintillator Many pixels fire GPD array Fig. 1. SSPM principal of operation. Nuclear photons strike the scintillation crystal and produce visible light proportional to their energy. The number of pixels that fire in the GPD array is thus a function of the incident energy. P. Buzhan et al. have shown that this method approaches and exceeds the performance of a standard PMT for detection of the scintillator optical photons in certain applications.[1,2] Implementing this approach in a CMOS compatible process will allow high precision, low-cost, sensors with the additional benefit of integration of signal processing electronics right on the chip. We have fabricated several CMOS-based test arrays of these pixel sensors and characterized their performance as individual detectors, and as arrays. Borrowing a technique used in large area, position sensitive APD’s, the SSPM can be used as a position sensitive detector, (PS-SSPM). The position sensitive SSPM pixels are coupled together in a resistive network to produce four signal outputs at the corners of the device to provide signals that determine the position of the event in the scintillation crystal, as demonstrated in Fig. 2. The PS- SSPM provides a fast, simple approach to reading out solid or even segmented scintillator devices [3]. In addition, the