The γ0 chip: a new front end for the gamma camera based on the ISPA tube

In this paper we present an IC chip, the γ₀, specifically developed for gamma imaging in medical applications. The chip is designed to be used inside an ISPA (integrated silicon pixel array) tube, in simple description, a one stage opto-electronic camera consisting of sealed vacuum tube in which a photocathode, evaporated on a scintillating crystal window, is comforted to a pixelated electronic imaging device. The γ₀, that has a total area of 0.5 × 0.5 cm², is composed by a matrix of 32 by 32 square 135μ pixels. Each silicon pixel is designed to be bonded by a Pb-Sn bump of 50μ diameter to a PIN detector. The pixel has a front-end channel, a shaper, an individual 3-bit threshold adjustment and a discriminator. Every cell has a ten bit event buffer. The γ₀ is equipped with a novel on chip energy discrimination system that will allow the ISPA camera to self trigger on the gamma event with no extra electronics. The chip is self biased by a set of configurable internal DAC devices. For maximum simplicity in the design of the readout electronic system, every pixel and DAC device is memory mapped.