Development of a novel high-sensivitiy LAr purity monitor based on an $\alpha$-source

Abstract

A novel liquid argon (LAr) purity monitor was developed with a sensitivity to electronegative 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 $\alpha$-particles emitted by the ${}^{210}Po$ chain daughter of an isotope ${}^{210}Pb$ 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 $\alpha$-particles have a very short range of about 50 μ$minLArandtheionizationdensityisveryhigh,typicallybetween$750÷1500\rm MeV/cm$,leadingtoahighrecombinationrate.Tosuppresstherecombinationoftheargonionswiththeelectrons,the$α$-sourcewasputinastrongelectricfieldof$40÷150$ 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.

15 pages, 9 figures