LAr purification and purity monitor systems
The number of the ionization electrons can decrease due to the attachment to the electronegatives impurities in LAr (oxygen and polar molecules - H2O, CO2 and fluorinated or chlorinated compounds). In order to achieve long enough electron dirft path, the concentration of impurities must be kept, below 0.1 part per billion (ppb). The concentration in standard commercial LAr is typically a few parts per million oxygen equivalents. Moreover, LAr can be contaminated inside the cryostat from outgassing of the walls and TPC components or by micro-leaks. The required purity is achieved with continuous purification system of commercial cartridges.
The purity of the LAr inside the detector is monitored by a set of dedicated purity monitors. The devices are small double-gridded drift chambers placed in the LAr volume outside the sensitive volume. The cathode is illuminated with the light transported over quartz fibres from a source placed outside the cryostat. The electrons are extracted from the cathode via the photo-electric effect and drift towards the anode crossing a drift region between two parallel, transparent grids. During the drift over the distance of 16 cm, the number of electrons is reduced due to the attachment to impurities. The charge collected by the anode is reduced comparing to the charge extracted. The ratio of these charges allows to estimate the free electron lifetime.