CZT & ASIC development

1. ProtoEXIST1
The principal technology challenge is the development of the very large array of CZT imaging detectors. The small pixels on a single CZT crystal allow more efficient detector and readout connections to a single multi-pixel readout ASIC (application specific integrated circuit), which provides charge preamp, shaping and peak detect on each pixel above a programmable threshold. The CZT-ASIC readout must allow for an inter-connect and packaging system that permits tiling of the CZT-ASIC modules in a semi-infinite detector plane without (significant) gaps to achieve maximum packing efficiency without dead spaces on a motherboard for interface to a digital data system.

Over the past year of our current SRT program, we have carried out an extensive test program for the detector and RADNET ASIC module for the prototype module of 2 x 2 CZT crystals and ASICS for ProtoEXIST1. Many of the features of this ASIC (approximate gain, and particularly its multi-pixel readout capability) are appropriate for ProtoEXIST1. Thus, we employ the RADNET detector packaging architecture to be fundamental detector module - a Detector Crystal Module (DCM) consisting of a 20 x 20 x 5mm CZT crystal with 8 x 8 pixel anodes connected through an Interposer Board (IPB) to the ASIC underneath the IPB. The figure shows views of the DCM (yellow - IPB and pink - RADNET ASIC), and the photo shows the actual modules.

This has been in close collaboration with LLNL and Caltech, who have developed a 64 channel ASIC derived from the Caltech-HEFT design (Cook et al 1998) that was optimized for higher energy range (~100-1000 keV) for Homeland Security application as a cellphone-based radiation detector (RADNET). The DCM mounts to a motherboard via a pingrid array and 4 such DCMs will be mounted to a common Detector Crystal Array (DCA) board containing the FPGA to control and readout the 4 ASICs. We combine 4 x 4 DCAs onto a Detector Module (DM, 256 cm2) and 4 DMs complete the detector plane (10242cm) for ProtoEXIST1.

2. ProtoEXIST2
The desired 5' resolution for the primary high energy telescopes on EXIST, and the constraints on mask-detector focal length as 1.5m, require 1.25mm pixels for Nyquist sampling of the mask shadow. Thus the transition to ProtoEXIST2 is a factor of 2 reduction in CZT pixel size and corresponding factor of 4 in ASIC pixel number and corresponding increased complexity of connecting the ASIC to the CZT through the IPB. The desired smaller pixels are more subject to charge spreading, especially at higher energies, so multipixel events are more likely. Hence the need for a higher resolution, multipixel (at least peak plus neighbors, but commandable) CZT-ASIC combination. Significant work has already been done on the design concept for this ASIC during our current SRT grant. However, final layout of this ASIC has not yet begun due to uncertainty about the best way to couple the ASIC to the CZT: either with an IPB (as for RADNET and ProtoEXIST1) or with direct-bonding as for HEFT. The figure shows packaging conceprt using an IPB for "de-magnification" of pixel pitch from the CZT (1.25mm pitch) to an ASIC array input (0.9 mm pitch).