Effects of Nitrogen contamination in liquid Argon

TL;DR

This study investigates how nitrogen contamination affects liquid argon scintillation light, revealing significant reductions in light yield and changes in emission characteristics at various contamination levels, crucial for optimizing detector performance.

Contribution

The paper provides the first detailed measurement of nitrogen's quenching effect on liquid argon scintillation, including the rate constant and emission behavior across a wide concentration range.

Findings

Scintillation yield decreases with increasing nitrogen concentration.

The rate constant for nitrogen quenching is 0.11 micros^-1 ppm^-1.

Significant changes in lifetime and amplitude of slow scintillation component at ~1 ppm N2.

Abstract

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. 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 about 10^-1 ppm up to about 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. The rate constant of the light quenching process induced by Nitrogen in liquid Ar has been found to be k(N2)=0.11 micros^-1 ppm^-1. Direct PMT signals acquisition at high time resolution by fast Waveform recording allowed to extract with high precision the main characteristics of the scintillation light emission in pure and contaminated LAr. In particular, the decreasing behavior in lifetime and relative amplitude of the slow component is found to be appreciable from O(1 ppm) of Nitrogen concentrations.