CMOS Charge amplifier for liquid argon Time Projection Chamber detectors

1. INTRODUCTION We developed an integrated circuit (IC) for the readout of LAr (Liquid Argon) TPC (Time Projection Chamber) neutrino detectors. These detectors employ LAr simultaneously as massive target and detection medium. The detection of secondary particles produced in neutrino interactions is achieved by collecting, on a system of wires at the sides of the detector, the electric charges from the ionization losses. This is obtained by drifting at constant speed the electrons with a high intensity electric field. Two planar coordinates are measured by the wires geometry, the third, perpendicular, coordinate is obtained by measuring the drift time. Typical signals are around 3 fC per particle per wire but they can go up to 120fC. They necessitate of low noise electronics for the readout. The goal is to achieve 1000e-ENC (Equivalent Noise Charge) with a detector capacitance of 250pF. The distribution of charges collected by the wires is then representative of a 3D image of the tracks of the secondary particles from the neutrino interaction. Normally the readout electronics for these detectors is located externally to the cryostat, implying the use of long flat cables to bring outside the signals collected by the wires. In order to limit cable capacitances (and therefore the noise), we studied the possibility to install the front-end electronics into the Argon vapours, above the LAr filled volume. Argon is liquid between 83 K and 87 K. A typical temperature around 120K is reached in the Argon vapours. Such environment may further improve the electronic noise which decreases with temperature. However, the circuit should dissipate as little heat as possible to prevent additional warming of the detector. The requested characteristics of the circuit are: capability of working at low temperatures, low noise, low heat dissipation, high reliability, low cost. In order to optimize the cost per channel ratio we selected a standard 0.35µ CMOS technology: AMSC35B4.