IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 51, NO. 3, JUNE 2004 1191 CdTe and CdZnTe X-Ray and Gamma-Ray Detectors for Imaging Systems Yossi Eisen, Asher Shor, and Israel Mardor Abstract—This review paper presents the current state of the art imaging systems using room-temperature CdTe and CdZnTe X-ray and gamma-ray detectors. Imaging systems composed of both single elements and monolithic segmented CdTe and CdZnTe arrays are used in medical diagnostics, astronomy, research and industry. This paper is divided into two sections: The first section describes different types and configurations of CdTe or CdZnTe detectors used in imaging systems, with the emphasis on single charge carrier collection detectors and front-end read-out electronics in the form of application specific integrated circuits associated with these detectors. It also discusses the advantages, disadvantages and limitations with respect to the efficiency and area of the imaging systems using CdTe and CdZnTe detectors compared to other types of X and gamma ray detectors. The second section describes several examples of imaging systems utilizing CdZnTe detectors: 1) a large area imaging telescope for the detection of gamma ray bursts using a coded aperture collimator; 2) a large area medical nuclear camera for cardiology and scintimammography using a parallel hole collimator; 3) a large field of view gamma camera based on a rotating slat collimator; and 4) a prototype of an electronically collimated Compton camera. I. INTRODUCTION I MAGING systems utilizing CdTe or CdZnTe (CZT) detec- tors have great potential to replace currently used imaging systems composed of scintillating materials, such as, NaI(Tl) and CsI(Tl), coupled to either photomultiplier tubes (PMTs) or photodiodes. Photodiodes and PMTs can be position sensitive and scintillators may be segmented into independent elements. However, the main advantage of CdTe and CZT detectors over scintillator detectors is their superior energy resolution at room temperature, that today can reach values at high energies only a factor of 4 [1] broader than those of liquid nitrogen cooled high purity Ge detectors (HPGe). Fig. 1 shows a spectrum of for a segmented 1 CZT detector where an energy resolution of 1% full-width at half-maximum (FWHM) is ob- served [1]. The excellent energy resolution which stems from the direct conversion of the absorbed energy to charge carriers, enables to develop energy dispersive imaging systems that can better reject Compton scattered photons in single photon emis- sion computed tomography (SPECT) procedures, or discrimi- nate between gamma lines close to each other, as needed in dual energy SPECT [2]. CdTe and CZT detectors may also offer ex- cellent intrinsic spatial resolution that makes possible the de- velopment of X-ray mammography imaging systems with spa- tial resolutions better than 100 [3]. The development of ad- vanced charge sensing analog electronics in the form of applica- Manuscript received November 12, 2003; revised January 15, 2004. The authors are with Soreq NRC, Yavne 81800, Israel (e-mail: yosef@soreq.gov.il). Digital Object Identifier 10.1109/TNS.2004.829437 Fig. 1. Combined spectrum of all 16 pads from a CZT segmented pad detector of dimension 1 cm 1 cm 1 cm ([1]). Energy resolution 1% FWHM. tion specific integrated circuits (ASICs) by several vendors [4], [5] enables to build small and compact imaging systems. Those imaging systems can be placed at close proximity to the imaged object and may function at high counting rates. A closer distance to the imaged object results in improved spatial resolution. CdTe and CZT detectors have currently several shortcomings that prevent their broad use in imaging systems: poor hole col- lection, moderate electron collection and nonuniformity over large volumes. Methods to improve the energy resolution in thick detectors and to circumvent the large hole trapping can be divided into two categories: The first category includes elec- tronic techniques applied mainly to planar detectors [6], [7] and the second category utilizes various contact configurations that differ from a regular planar configuration [8]–[10]. Electronic techniques mainly determine the rise time of either the elec- trons or the holes and correlate them with the total energy de- posited [6], [7], [11]. Utilizing this technique enables to find the depth of interaction and correct for the lost charge. Detec- tors of the second category are known as single charge carrier detectors, since they all lead to a collected charge that depends primarily on the electron movement near the anode, and the in- duced charge is weakly dependent on the depth of interaction [10]. A method developed for CZT detectors by Luke [8] is based on the Frisch Grids principle. The anode is segmented into alternating strips whereas adjacent strips are set at slightly different voltage from each other. The difference of the induced charge between the two sets is due to movement of the charge at regions near the gridded electrode, thus eliminating effects caused by long traveling charge carriers. To achieve single charge collection, monolithic detectors are mainly segmented into pads [10], strips [12], [13] or utilize spe- 0018-9499/04$20.00 © 2004 IEEE