Effect of sense wire resistance on readout noise of large liquid argon time projection chambers

Large Liquid Argon (LAr) Time Projection Chambers (TPCs) are becoming very attractive for long baseline neutrino and proton decay experiments because of their imaging capabilities and their excellent energy resolution. In the design of large LTPCs, with wire lengths of several meters (~5 m, and up to 10-20 m for proposed multi-kiloton detectors), the noise generated by the resistance of the wire needs to be taken into account. The wire cannot be modeled simply as a capacitance connected to the preamplifier: its resistance, coupled capacitively to adjacent wires constitutes a distributed RC diffusive line, which injects a noise current into the preamplifier. It will be shown that for most practical cases the noise source can be modeled as an equivalent noise generator corresponding to a resistance of R_wire/3, in series to an impedance equal to the wire capacitance. This simple models allows the use the familiar formulas for the noise analysis of capacitive sources. The wire resistance noise for a typical 5 m long, 150 ¿m stainless steel wire (3 mm wire spacing, and 3 mm wire planes separation) increases the total noise by about 20% at 1 ¿s shaping time, and becomes dominant for wire lengths >10 m. It can be reduced by plating the stainless steel wire with high electrical conductivity metals.