ISPA Front End Integrated Circuit for Gamma Imaging Application Valentino Orsolini Cencelli, Member, IEEE, Enrico D’Abramo, Student Member, IEEE, Francesco de Notaristefani, Andrea Fabbri, Luca Zerilli Abstract— In recent works we presented the γ0, a chip expressly designed for gamma imaging application based on the ISPA tube (i.e. a vacuum sealed tube with a quartz or crystal window in which electrons, generated by the scintillating light on a photocatode, are accelerated over a 20kV potential to hit a pixelated silicon detector matrix). We now made a new version that includes new features, including a more complete test subsystem. The chip is a matrix of 1024 square pixels 135μm in side, and is designed to be bump bonded to the pin diode matrix. In each pixel, the analog part, includes a Charge Sensitive Amplifier, a shaper, a transconductance amplifier, that converts the voltage output of the shaper to a current and a discriminator. The digital part has a ten event buffer and five configuration flip flops. In this work we present the test of the device with pulses equivalent to 2000 e - , that is well below the 5500 e - that are given in the ISPA tube at a 20kV supply, obtaining a very clear response and a measured noise below 200 e - . We will also present the tests of the integrated energy discrimination system. Index Terms— ISPA, Gamma Imaging, ASIC I. I NTRODUCTION G AMMA Imaging, both for clinical application and for small animal imaging, has a major importance for cancer therapy, development of new pharmaceuticals and the develop- ment of animal models of disease, among others. In recent years many gamma camera were proposed either based on flat panel PMT or on other and most advanced detectors like CdTe/CdZnTE. Beside to these detectors the ISPA (Imaging Silicon Pixel Array) is still competitive due to a very good energy discrimination capability and to a very low gain fluctuation. In recent years we have demonstrated an ISPA gamma camera based on the Omega 3 chip developed at CERN [1]– [3]. This device could take advantage of the Omega 3 chip but was limited by the nature of the readout electronics specifically designed for High Energy Physics experiments. Recently we also realized another ISPA based on the Alice1 ASIC [4]–[6]. In this paper we present the development of an ASIC designed to match the requirements found in nuclear medicine. In fact the characteristics of the source in the two cases are very different: in High Energy Physics experiments the source has usually a very high rate, in the megahertz range, and is synchronized with a beam clock at a fixed rate. The clock is available to the designer that can use it to drive the readout system, data selection is done using different levels of triggers, with different time response, that determine if what is read has to be retained or not. Moreover the readout electronics is usually inserted in huge detectors with hundred thousands of channels and there are constraints due to material budget, power consumption and radiation hardness, that have to be taken into account. In nuclear medicine, vice versa, a radio-pharmaceutical tracer, i.e. a sugar marked with meta-stable radioactive atoms like Tc 99m , is injected in the blood flow in proximity of the area under study, cells in such area absorb the sugar with an intensity proportional to the metabolic activity of tissues, giving rise to a concentration of the radio-pharmaceutical tracer that is proportional to such activity. Of course, in this case the nature of particle flow is asynchronous, being generated by natural decay of the radio-pharmaceutical tracer, and a large background is present due to natural radioactivity, cosmic rays and Compton emission from other organs. Of course the radioactivity dose must be kept as low as possible, this takes the overall counting rate in the range of hundreds of kilohertz, while the signal stays well below 1 kHz. This condition makes energy discrimination very important, in order to save, in the gamma image, the information related to the metabolic activity that is the most meaningful. The system must be also as compact as possible to be easily moved and used on field and power consumption or radiation hardness are usually not an issue. In previous works we presented the first version of the γ 0 [8], [9], In this work we present a new version of the ASIC that includes new features, including a more complete test subsystem. In Section II we will summarize how the ISPA tube based works; in Section III we will give an overall description of the γ 0 ; in Section IV we will present the test results and particularly the on–chip energy discrimination system details, in Section V we will draw some conclusion. II. THE ISPA TUBE The ISPA tube is a valid alternative to common image intensifiers like PSPMTs (Position Sensitive Photomultiplier Tube) used in gamma imaging. It basically consists of a vacuum tube equipped with a photocatode and a pixellated silicon detector. The energy gain, needed to detect the extremely low light produced in the scintillation process, does not rely on the electron number multiplication like in PMTs, but is given by a strong electric field that accelerates electrons while they are crossing the tube.