IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 58, NO. 1, FEBRUARY 2011 17 The Inﬂuence of Pixel Pitch and Electrode Pad Size on the Spectroscopic Performance of a Photon Counting Pixel Detector With CdTe Sensor Ewald Guni, Juergen Durst, Bjoern Kreisler, Thilo Michel, Gisela Anton, Michael Fiederle, Alex Fauler, and Andreas Zwerger Abstract—The quality and availability of room temperature CdTe sensor material for X-ray detection has improved signiﬁ- cantly in the last years. A CdTe sensor with different pixel pitches and electrode pad sizes was bump-bonded to a Medipix2-MXR ASIC. With this photon counting detector we were able to in- vestigate the inﬂuence of pixel pitch and electrode pad size on the energy response functions. The accurate knowledge of the energy response is crucial for energy resolving X-ray imaging. Therefore we exposed the detector to gamma rays of and . In the following analysis of the energy response spectra we determined the number of events in the photo peak, the energy resolution and the threshold energy where the photo peak is found (photo peak position) caused by the absorption of the 59.5 keV photons of . For the energy calibration we used the 59.5 keV photons of and the 122 keV photons of . Concerning energy resolution, energy threshold calibration and photo peak position our measurements show good agreement with the expected behaviour. The number of events in the photo peak strongly depends on the pixel pitch and the electrode pad conﬁguration. Index Terms—CdTe, electrode pad, medipix, photon counting, pixel detector, pixel pitch, X-ray. I. INTRODUCTION P HOTON counting X-ray detectors are gaining increasing interest in medical imaging. In contrast to conventional systems which integrate the charge released in the sensor by many photons they are able to determine the number of photons. The Medipix2 [1] detector is such a detector. With this imaging detector it is possible to measure the energy of the incoming X-ray photons which provides additional valuable information. This detector enables us to determine the composition of mate- rials (type and quantity) which can be used for the detection of contrast agents in medical imaging [2]. Manuscript received March 04, 2010; revised August 03, 2010 and October 27, 2010; accepted November 21, 2010. Date of publication December 30, 2010; date of current version February 09, 2011. This work was carried out within the Medipix collaboration and was supported by the International Max-Planck Research School (IMPRS) for Optics and Imaging. E. Guni, J. Durst, B. Kreisler, T. Michel, and G. Anton are with the Er- langen Centre for Astroparticle Physics, 91058 Erlangen, Germany (e-mail: ewald.guni@physik.uni-erlangen.de). M. Fiederle, A. Fauler, and A. Zwerger are with the Freiburg Materials Re- search Center, 79104 Freiburg, Germany. Color versions of one or more of the ﬁgures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identiﬁer 10.1109/TNS.2010.2095883 Fig. 1. Cross section of the detector for a pixel pitch of 330 . Fig. 2. Distribution of pixel pitch and electrode pad size combinations on the sensor together with the number of respective pixels. Notation: (pixel pitch)- (electrode pad size)(in ), ﬁll factor, available number of pixels. The hybid design of the Medipix2 detector enables the usage of different sensor materials. Silicon (Si), being the common sensor material for the detection of charged particles in high en- ergy physics, can not be used in most X-ray imaging systems due to its low atomic number . Therefore there is a 0018-9499/$26.00 © 2010 IEEE