Electroluminescence pulse shape and electron diffusion in liquid argon measured in a dual-phase TPC

TL;DR

This paper measures the longitudinal diffusion constant in liquid argon using a dual-phase TPC, deriving a pulse shape model for electroluminescence signals, and finds a value slightly lower than previous studies, with energy dependence observed.

Contribution

It introduces a new pulse shape model for electroluminescence signals in dual-phase TPCs and provides a precise measurement of the diffusion constant at various electric fields.

Findings

Measured diffusion constant DL = (4.09 ± 0.09) cm²/s at 200 V/cm.

Derived a new expression for the S2 pulse shape in dual-phase TPCs.

Observed energy dependence of the diffusion constant.

Abstract

We report the measurement of the longitudinal diffusion constant in liquid argon with the DarkSide-50 dual-phase time projection chamber. The measurement is performed at drift electric fields of 100 V/cm, 150 V/cm, and 200 V/cm using high statistics $^{39}$Ar decays from atmospheric argon. We derive an expression to describe the pulse shape of the electroluminescence signal (S2) in dual-phase TPCs. The derived S2 pulse shape is fit to events from the uppermost portion of the TPC in order to characterize the radial dependence of the signal. The results are provided as inputs to the measurement of the longitudinal diffusion constant DL, which we find to be (4.12 $\pm$ 0.04) cm$^2$/s for a selection of 140keV electron recoil events in 200V/cm drift field and 2.8kV/cm extraction field. To study the systematics of our measurement we examine datasets of varying event energy, field strength, and detector volume yielding a weighted average value for the diffusion constant of (4.09 $\pm$ 0.09) cm$^2$ /s. The measured longitudinal diffusion constant is observed to have an energy dependence, and within the studied energy range the result is systematically lower than other results in the literature.