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

Abstract

For time projection chambers (TPCs) the accuracy in measurement of the track coordinates along the pad-row direction deteriorates with the drift distance (z):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 [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.

Measurement of the electron transmission rate of the gating foil for the TPC of the ILC experiment

“…13 shows an example of the electric field configuration in the proximity of the gating foil. 12 A significant fraction ( > ∼ 60%) of the signals are actually samples for which no drift electrons are detected since a single drift electron corresponds to ∼ 4 ADC counts. 13 Equivalently, the mean of the sum of pulse heights in each subset is N times larger than the average pulse height for single laser shots.…”

“…where σ X0 is a constant term, D T denotes the transverse diffusion constant of drift electrons, z is the drift distance, and N eff , the effective number of electrons [9][10][11][12][13][14]. For tracks normal to the pad row Eq.…”

Cosmic ray tests of a GEM-based TPC prototype operated in Ar–CF4–isobutane gas mixtures: II

“…where the relative variance F ′ ≡ σ Q 2 / Q 2 with σ Q being the standard deviation of the cluster charge, including the fluctuations in cluster size, and in avalanche gain for each electron in the cluster. Actually because F (∼ 2000 for argon [5]) is much greater than f (∼ 1).…”

“…3 In Refs. [1,4,5], n eff is denoted as N eff , which is reserved for the effective number of clusters per pad row (see below) in the present paper. 4 More precisely h should be understood as the pad-row pitch, which is usually slightly larger than the pad height when the readout plane is covered over with pads.…”

“…The pad-row pitch and the pad height (h) are not distinguished in the present paper. 5 θ is defined to be 0 • when the track is parallel to the readout plane. 6 The value of σ X0 given by Eq.…”