2 00 4 Development of a novel high - sensivitiy LAr purity monitor based on an α - source

A novel liquid argon (LAr) purity monitor was developed with a sensitivity to elec-tronegative impurities of the order of ppb (O 2 equivalent). Such a high purity is e.g. needed in a LAr drift chamber. The principle is to measure the lifetime of quasifree electrons in LAr, since this is the important parameter for the operation of a drift chamber. Free electrons are produced by ionizing the LAr with α-particles emitted by the 210 P o chain daughter of an isotope 210 P b source. From a measurement of the charge of the electron cloud at the beginning and at the end of a drift path, together with the drift time, the lifetime of the electrons is obtained. The α-particles have a very short range of about 50 µm in LAr and the ionization density is very high, typically between 750 ÷ 1500 MeV/cm, leading to a high recombination rate. To suppress the recombination of the argon ions with the electrons, the α-source was put in a strong electric field of 40 ÷ 150 kV/cm. This was achieved by depositing the source on the surface of a spherical high voltage cathode with a diameter of about 0.5 mm. The anode was also made as a sphere of about the same diameter as the cathode, thus, close to the axis between the two electrodes the electric drift field was approximately a dipole field.