An estimation of the effective number of electrons contributing to the coordinate measurement with a TPC

For time projection chambers (TPCs) the accuracy in measurement of track coordinates along the pad row direction deteriorates with the drift distance (z): σX2∼DX2.z/Neff, where DX is the diffusion constant and Neff is the effective number of electrons. Experimentally, it has been shown that Neff is smaller than the average number of drift electrons per pad row (N-bar ). We estimated Neff by means of a simple numerical simulation. The simulation shows that Neff can be as small as ∼30% of N-bar due to the combined effect of statistical fluctuations in the number of drift electrons (N) and in their multiplication in avalanches