Search

Motivated by the ongoing study of a possible directional signal in liquid argon dark matter detectors, we introduce a new model describing the recombination of electron-ion pairs in ionizing tracks in liquid argon in the presence of a drift field. The emphasis is on the three-dimensional distribution of electrons and ions and on their orientation relative to that of the electric field. We successfully apply our model to describe the angular dependence of the ionization signal of protons recently reported in measurements performed by the ArgoNeuT Collaboration with a liquid argon time projection chamber.

A dedicated test of the effects of Nitrogen contamination in liquid Argon has been performed at the INFN-Gran Sasso Laboratory (LNGS, Italy) within the WArP R&D program. A detector has been designed and assembled for this specific task and connected to a system for the injection of controlled amounts of gaseous Nitrogen into the liquid Argon. The purpose of the test is to detect the reduction of the Ar scintillation light emission as a function of the amount of the Nitrogen contaminant injected in the Argon volume. A wide concentration range, spanning from similar to 10(-1) ppm up to similar to 10(3) ppm, has been explored. Measurements have been done with electrons in the energy range of minimum ionizing particles (gamma-conversion from radioactive sources). Source spectra at different Nitrogen contaminations are analyzed, showing sensitive reduction of the scintillation yield at increasing concentrations. Direct PMT signal acquisition exploiting high time resolution by fast waveform recording allowed high precision extraction of the main characteristics of the scintillation light emission in contaminated LAr. In particular, the decreasing behavior in lifetime and relative amplitude of the slow component is found to be appreciable starting from O(1 ppm) of Nitrogen concentrations. The rate constant of the quenching process induced by Nitrogen in liquid Ar has been found to be k(Q)(N(2)) = 0.11 +/- 0.01 mu s(-1) ppm(-1), consistent with a previous measurement of this quantity but with significant improvement in precision. On the other hand, no evidence for absorption by N(2) impurities has been found up to the higher concentrations here explored.