Pixel Electronics for a Hybrid X/Gamma – Ray Imager C. P. Lambropoulos * a , E. G. Zervakis a , A. Nikologiannis a , G. Theodoratos a , D. S. Hatzistratis a , I. Papadakis b , A. Papadimitriou b , D. Loukas b a Sensors, Electronics and Communications Laboratory, T.E.I. of Chalkida, Psahna-Evia, 34400 Greece; b Institute of Nuclear Physics, NCSR Demokritos, Aghia Paraskevi, Attiki, 15310 Greece ABSTRACT The Photon 4–dimensional Digital Information (P4DI) ASIC is a new generation of 2D imaging chips to be connected to a pixel sensor using the bump and flip chip technologies. It gives in digital format energy, time and position information for each recorded event. In pixel digitization and storage of the time and amplitude signal are performed. Circuit solutions for gain and offset variation compensation have been implemented. The ASIC works in sparse data scan mode. An 8x8 pixels prototype has been manufactured in UMC 0.18um CMOS technology and evaluated Keywords: pixel readout electronics, CMOS, ASIC, X/gamma-ray hybrid detectors, time tagging 1. INTRODUCTION Compound semiconductor sensors have emerged as leading candidates in room – temperature, large volume X/Gamma- ray imaging coupled with spectrometry for security 1 , medical 2,3 , industrial 4 and space applications 5,6 . Hybrid Imagers were the sensing layer is made of a pixilated compound semiconductor single crystal have entered not only in the field of scientific instrumentation but also in high market applications with driving force the medical sector 7,8 . The progress in position sensitive semiconductors has boosted the effort for the implementation of Compton imagers. A new field of application, the localization and identification of radioactive sources 1 , has been added to the other areas considered in the almost 40 years history of the “electronically collimated gamma cameras”. In the case of Compton imaging energy resolution and time tagging capability are critical for the performance of the system. The energy resolution is connected to the so called ARM (angular resolution measure). This is the FWHM of the distribution of the difference between the angle of direction of the photon emanating from a radioactive source calculated by using the Compton formula and the real one. The time tagging is necessary for the reduction of the combinatorial background in the determination of the event sequence. ESA, NASA, JAXA and other space agencies have included in their plans for the payload of future missions the development of hard x-ray / gamma – ray imagers. For most of them the identification of the photon conversion point in the crystal is required. Energy resolution and time tagging with pixilated compound semiconductor detectors can be achieved only with VLSI multichannel signal processing electronics. Many systems have been built with the aid of the VA-TA series of ASICs 9,10,11 , with most recent member of the family a new version of the XA 12 , while the BNL instrumentation department has developed an ASIC with low noise performance and time tagging capability in order to support the 3D position sensitive detection technique 13 . All these ASICs are one dimensional arrays. NOVARAD has announced DANA, a two dimensional version of RENA3 14 . P4DI is a 2-D array with 400um pixel pitch. Although in the first implementation, which is reported here, there are pads all around the ASIC periphery, its specifications target assemblies of large arrays with thousands of pixels. This is the reason for a stringent requirement on the pixel power consumption (200uW). There are two important features which differentiate P4DI from all the ASICs referred previously. The one is the use of a Wilkinson type A/D conversion scheme for the in pixel digitization of the dc levels which give the information of the peak amplitude and of the time the hit was recorded. The second is the readout mode: All the pixels are “open” in parallel to receive hits for a certain time interval adjusted externally. *lambrop@teihal.gr; phone +30 22280 99631; fax +30 22280 23766; Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XII, edited by Arnold Burger, Larry A. Franks, Ralph B. James, Proc. of SPIE Vol. 7805, 78050W · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.862712 Proc. of SPIE Vol. 7805 78050W-1 Downloaded from SPIE Digital Library on 02 Sep 2010 to 212.152.106.188. Terms of Use: http://spiedl.org/terms