First measurement of the positive volume charge in a 1 litre dual-phase argon detector

The dynamics of the positive ions created by particle interactions inside argon time projection chambers plays an important role in characterizing the next generation of massive detectors planned for the direct search for dark matter and the study of neutrino properties. Under the influence of an electric field, positively charged argon ions and negatively charged ionisation electrons move in opposite directions; however, the drift velocity of the argon ions in liquid is five orders of magnitude lower than that of electrons. A volume charge is created by the slowly drifting positive charges, which can significantly change the magnitude of the electric field and the shape of the drift lines, resulting in quenching of the electronic signal. Additionally, a constant photon emission, unrelated in time and space to the physical interactions, can occur due to the recombination of ionisation electrons with neighboring positive volume charge along the drift direction. In the case of a detector with charge amplification, this effect can be increased by the additional ions created at the anode. We have constructed a 1 L liquid argon chamber (ARION: ARgon ION experiment) with a high voltage pulse generator capable of injecting, in a controlled manner, a sizeable ion current into the drift region. This chamber allows studying the effects produced by the volume charge in large detectors. Recent experimental results regarding ion dynamics in liquid and the direct measurement of ion feedback from the gas into the liquid phase are discussed in this paper. In addition, a new measurement of the drift velocity of argon ions in the gas phase is presented.