2004/12/26 by A. Badertscher, Badertscher, A., M. Laffranchi +3
Physics and Astronomy · #FOS: Physical sciences #Instrumentation and Detectors (physics.ins-det) #physics.ins-det
paper · pdf · doi:10.48550/arxiv.physics/0412166
15 pages, 9 figures
arxiv created 2004/12/26 · arxiv updated 2009/12/01
A novel liquid argon (LAr) purity monitor was developed with a sensitivity to electronegative impurities of the order of ppb (O2 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 210Po chain daughter of an isotope 210Pb 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\rm 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.