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

For time projection chambers (TPCs) the accuracy in measurement of the track coordinates along the pad-row direction deteriorates with the drift distance (z): σ X 2 ∼ D 2 · z / N eff , where D is the diffusion constant and N eff is the effective number of electrons. Experimentally it has been shown that N eff is smaller than the average number of drift electrons per pad row ( 〈 N 〉 ). In the previous work we estimated N eff by means of a simple numerical simulation for argon-based gas mixtures, taking into account the diffusion of electrons only in the pad-row direction (Kobayashi, 2006 [1]). The simulation showed that N eff could be as small as ∼ 30% of 〈 N 〉 because of the combined effect of statistical fluctuations in the number of drift electrons (N) and in their multiplication in avalanches. In this paper, we evaluate the influence of the diffusion normal to the pad-row direction on the effective number of electrons. The de-clustering of the drift electrons due to the diffusion makes N eff drift-distance dependent. However, its effect was found to be too small to explain the discrepancy between the values of N eff measured with two TPC prototypes different in size.