A STUDY OF THE FACTORS AFFECTING THE ELECTRON LIFETIME IN ULTRA-PURE LIQUID ARGON

As part of the development program for the ICARUS experiment, which consists of a very large time projection chamber (TPC) filled with ultra-pure liquid Ar (LAr), we have carried out tests with different purifier systems, in order to evaluate the performance of the various parts and to improve the purification techniques developed so far. Electron lifetime tau in LAr has been determined with an improved method based on the measurement of the attenuation of a current due to an electron cloud, photoproduced by a laser pulse impinging on a metallic cathode and moving in a small drift chamber filled with the purified LAr. Results of the above-mentioned tests are reported. During these tests, we observed repeatedly and reproducibly an increase of tau that took place over a period of 10 to 20 h after liquefaction. Several tests performed in an attempt to elucidate this effect, suggest that the increase in tau is due to adsorption of electron-attaching impurities on the walls of the stainless steel container, a process governed by thermal diffusion. The electron lifetime monitoring system reported here was used to measure the electric field dependence of tau in purified LAr doped with O2 and CO2, for fields 100 V/cm < E < 800 V/cm: The results for O2 are consistent with published data.