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v3.3.3-pre
The subject of space charge in HEP particle detectors became relevant first with large gaseous TPCs, while new interest came with liquid ionisation detectors, namely liquid krypton or liquid argon calorimeters operated at high intensity, and large liquid argon TPCs for neutrino experiments. In liquid media, positive ions are produced over the full detector volume, and the approach of grids as ion gates used in gas-multiplication detectors is not suitable. The case of the NA48 calorimeter is reviewed, where local response variation for the shower axis position within the readout cell and the interplay of ion collection time and beam cycle were taken into account. An analytical description of space charge effects was developed in this occasion, defining a scale parameter and critical conditions. The latter were the subject of R&D for liquid argon sampling calorimeters, in the framework of high intensity hadron colliders. In liquid argon TPCs operated on the Earth surface, space charge from cosmic rays alone affects the detector response. Besides effects on the charge yield and on the drift time, distortions along the transverse directions, in proximity of the detector boundaries, are typically the most visible effects. Further complexity can be added by fluid motion due to thermal and recirculation flow. Numerical evaluations, mitigation and calibration approaches are discussed.
Burkhard Schmidt (EP-DT)