  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.
