  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 \mu$m in LAr and the ionization density is very
high, typically between $750\div 1500\rm MeV/cm$, leading to a high
recombination rate. To suppress the recombination of the argon ions with the
electrons, the $\alpha$-source was put in a strong electric field of $40\div
150$ 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.
