IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 50, NO. 3, JUNE 2003 307 Measurement of the Depth of Interaction of an LSO Scintillator Using a Planar Process APD Ernesto Gramsch, Member, IEEE, Ricardo E. Avila, and Peter Bui, Member, IEEE Abstract—The authors have evaluated the performance of an avalanche photodiodes (APD)/Lutetium oxyorthosilicate (LSO) module for use in positron emission tomography systems. They have used a recently developed APD detector in combination with a 3 3 30 mm LSO scintillator to measure the depth of interaction of 511 keV photons from a Na source. The detectors were built using standard planar technology for silicon devices. Photodiodes with 3 mm diameter active area have been produced by deep boron diffusion, followed by shallow boron and phosphor diffusion. Because the structure has not been optimized yet, the authors only obtained a gain of 6 at 1300 V. A simple noise analysis of the detector characteristics indicate that they are still capable of measuring 511 keV photons with sufficient energy resolution for depth of interaction position measurement from a long LSO scintillator. In spite of the low gain, the authors have obtained an energy resolution of 25% full-width at half-maximum (FWHM) with a single APD and the 3 3 30 mm scintillator, better resolution (23%) is obtained when the scintillator is coupled to two APDs and the spectrum is obtained from the sum of the detectors. DOI resolution varies with the position of interaction being 5.3 and 8.6 mm FWHM at 2 and 28 mm from the APD, respectively. Index Terms—Avalanche photodiode, depth of interaction, LSO scintillator, PET module. I. INTRODUCTION P OSITRON emission tomography (PET) has had important advances in the last few years, because of its potential for in vivo studies of tracer metabolism with a full-width at half-max- imum (FWHM) resolution up to 1 mm [1], [2]. In order to con- tinue improving the spatial resolution of PET systems, the radial elongation effect has to be solved [3]. This problem is caused by 511 photons that are generated far from the center of the ring. These photons strike the face of a crystal at an oblique angle and frequently penetrate and interact in an adjacent crystal, which causes the event to be assigned to the wrong cord. This effect can be removed by measuring the depth of interaction (DOI) of the photon in the scintillator crystal [4]. Avalanche photodiodes (APDs) are an attractive alternative for high resolution PET be- cause of its potential for making arrays with small pixel size [5]. APDs of the deep diffused type have had important techno- logical developments in the past few years. One important de- Manuscript received November 9, 2001. This work was supported in part by the Chilean Research Foundation, Fondecyt, under Grant 1990293 and by Dicyt under Grant 04-98-31-GL. E. Gramsch is with the Physics Department, Universidad de Santiago, San- tiago, Chile (e-mail: egramsch @lauca.usach.cl). R. E. Avila is with the Department Materiales Nucleares, CCHEN, Santiago, Chile (e-mail: ravila@cchen.cl). P. Bui is with UDT Sensors, Hawthorne, CA 90250 USA (e-mail: pbui@udt.com). Digital Object Identifier 10.1109/TNS.2003.812430 velopment was the use of neutron transmutation doped silicon to get improved doping uniformity across the wafer. Another improvement was the introduction of better beveling and passi- vation techniques that allowed improved reliability and stability [6], [7]. However, there are still reliability and cost issues that should be solved before these detectors can be used in PET sys- tems or consumer applications. A distinctive characteristic of deep diffused APDs is that they have a junction that extends 20 to 30 m inside the silicon. The structure is characterized by a wide depleted region with a field that varies slowly with depth. This region enables opera- tion of the device with high gain and low excess noise [6]–[8]. Reach through APDs are also widely used for scintillation de- tection, and a PET camera has been built using these detectors [9]. These detectors have a wide low-field region (without gain) in the front, and a narrow region in the back with high field. An advantage of the reach through design is the high quantum effi- ciency for long wavelength photons, and high efficiency for soft x-rays up to 10 keV [10]. A disadvantage of this design is that the gain region is very narrow with a very high electric field that generates high excess noise. Because of the high electric field, the gain uniformity is low making it hard to build large area APDs [10], [11]. Reach through APDs also have diffused, graded rings surrounding the sensitive area to reduce the elec- tric field at the surface [10], [11], making these detectors easy to fabricate with standard silicon processing. II. APD STRUCTURE We have designed and manufactured APDs of the deep-dif- fused type that do not need a bevel to reduce the electric field at the surface. By diffusing rings at the edge of the device we have achieved reduction of the electric field and avoided early break- down at the surface. The resistivity of the wafers was chosen to obtain a bulk breakdown voltage of 1800 V, with a wide de- pletion layer, and a multiplication region around the junction of about 30 m. The doping concentration in the zone is much higher than in the zone, thus the depletion region extends mostly into the n zone when reverse bias is applied. The width of the region width reaches 140 m. One of the difficulties inherent to APDs is that nonuniformities in the doping concen- tration can cause local variation in the gain and early breakdown when high voltage is applied. This problem has prevented the development of large area APDs of the reach-through type [11], [12]. Part of this problem has been solved by use of neutron transmutation doping (NDT), that has allowed to make large area APDs with improved reliability [7]. 0018-9499/03$17.00 © 2003 IEEE