Abstract
A key feature for the success of the liquid Argon imaging TPC (LAr-TPC) technology is the industrial purification against electro-negative impurities, especially Oxygen and Nitrogen remnants, which have to be continuously kept at an exceptionally low level by filtering and recirculating liquid Argon. Improved purification techniques have been applied to a 120 liters LAr-TPC test facility in the INFN-LNL laboratory. Through-going muon tracks have been used to determine the free electron lifetime in liquid Argon against electro-negative impurities. The short path length here observed (30 cm) is compensated by the high accuracy in the observation of the specific ionization of cosmic ray muons at sea level as a function of the drift distance. A free electron lifetime of τ ∼ (21.4+7.3−4.3) ms, namely > 15.8 ms at 90% C.L. has been observed over several weeks under stable conditions, corresponding to a residual Oxygen equivalent of ≈ 15 ppt (part per trillion). At 500 V/cm, the free electron speed is 1.5 mm/μs. In a LAr-TPC a free electron lifetime in excess of 15 ms corresponds for instance to an attenuation of less than 20% after a drift path of 5 m, opening the way to the operation of the LAr-TPC with exceptionally long drift distances.
Highlights
Some of the main features of the liquid Argon imaging Time projection Chambers (TPC) (LAr-TPC) are hereby briefly summarized
The present paper reports new results on the attainable drift length in a practical detector
The liquid Argon imaging TPC (LAr-TPC) detector has been realized in the framework of the ICARUS R&D activity at the Istituto Nazionale di Fisica Nucleare (INFN)-LNL laboratories in Legnaro (Italy)
Summary
Some of the main features of the LAr-TPC are hereby briefly summarized. The minimum ionization particle yield is of about 8400 electrons/mm, reduced to about 4600 e/mm at 500 V/cm because of columnar recombination. According to the kinetic theory, the densities (particle/cm3) of the (gaseous) contaminating impurities are the same in the liquid and in the gas regions. Since the density of the (cold) Argon gas (GAr) is some 200 times smaller than the one of the liquid, the drift speed of diffusion of impurities is very slow, 1 m/hour and the establishing of the equilibrium relays mostly on convective motions rather than on diffusion. The present paper reports new results on the attainable drift length in a practical detector. The longest drift distance at which one can operate with free electrons is inversely proportional to the residual contamination due to several electro-negative gases, far more demanding since the free electron path in a liquid is ≈ 600 times shorter than in a gas. Considerable progress over the last few years has permitted to reach industrial purification techniques that are capable of a much better performance reported in the present paper
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