* Corresponding author. Fax: #1-734-763-4540. E-mail address: hezhong@umich.edu (Z. He) Nuclear Instruments and Methods in Physics Research A 441 (2000) 459}467 Measurement of material uniformity using 3-D position sensitive CdZnTe gamma-ray spectrometers Z. He*, W. Li, G.F. Knoll, D.K. Wehe, C.M. Stahle Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109, USA Detector Systems Branch, NASA Goddard Space Flight Center, Code 553, Greenbelt, MD 20771, USA Received 12 August 1999; accepted 12 August 1999 Abstract We present results from two 1 cm CdZnTe gamma-ray spectrometers with full 3-D position sensitivity. To our knowledge, these are the "rst reported semiconductor spectrometers that provide independent spectral data for each of over 2000 volume elements. Energy resolutions of 1.5}1.6% FWHM and position resolutions of 0.70.70.5 mm were obtained at 662 keV gamma-ray energy from the central region of both detectors for single-pixel events. With the 3-D position sensing capability, variations in spectral response over the detector volume were recorded using a Cs source. These measurements allow a study of full-energy peak e$ciency, mean ionization energy and electron trapping as a function of 3-D position. The e!ects of material non-uniformity on detector spectroscopic performance are dis- cussed.  2000 Elsevier Science B.V. All rights reserved. PACS: 07.85.Ne Keywords: Gamma-ray spectrometers; Semiconductor; CdZnTe; Position sensitive gamma-ray detectors 1. Introduction Since the single polarity charge sensing method was implemented on solid-state semiconductor gamma-ray detectors [1], and the depth sensing technique was employed [2], the spectroscopic per- formance of CdZnTe gamma-ray detectors has been improved dramatically compared with that obtained on conventional detectors using planar electrodes [3]. However, the best energy resolution of &1.8% FWHM obtained at 662 keV gamma- ray energy on a relatively large volume (1 cm) device [3] is still much worse than that predicted by the limit set by charge carrier statistics (&0.2% FWHM assuming a Fano factor of 0.1). Although experimental studies [3] indicated that the gamma-ray spectroscopic performance can be fur- ther improved by reducing the di!erence between weighting potentials of the collecting and the non- collecting anodes, other factors such as the vari- ation of ionization energy due to gradients in the Zn concentration, the variation of electron trap- ping, and the presence of material defects could also degrade detector performance. This work investi- gates the e!ects of some of these factors in CdZnTe detectors. 0168-9002/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 8 6 0 - 8