Precise temperature measurement at 30 K in the KATRIN source cryostat

The source cryostat of the Karlsruhe Tritium Neutrino Experiment (KATRIN) contains in its centre a 10 m long beam tube of 90 mm inner diameter, operated in a temperature range of 30-550 K and surrounded by superconducting solenoids with a field of 3.6 T. During standard operation at 30 K, the temperature must be measured continuously with an uncertainty better than 0.03 K. This can be achieved with Pt500 sensors distributed along the beam tube. In order to correct their deviations from a Pt500 characteristic curve, their magnetic field dependence and possible degradation, each Pt500 is in situ calibrated with an adjacent vapour pressure sensor. The in situ calibrations are carried out under various conditions, and will be partly repeated over the lifetime of the system. Starting from a temperature sensor overview, the Pt500 preparation and tests are explained, and the measurement uncertainties are derived. The design of vapour pressure sensors is presented next together with experimental results and the uncertainty estimation for the final installation. This is followed by descriptions of both the Pt500 and the vapour pressure measurement system, and the Pt500 in situ calibration procedure. Finally, an outlook is given to upcoming experiments.