Hall sensors need to calibrated against a precisely known magnetic field. The most accurate technique available to determine the strength of a magnetic field is to use the NMR (nuclear magnetic resonance) signal of, for example, a water or rubber sample. The nuclear magnetic moment of the hydrogen or carbon in the sample material is first perturbed and then precesses about the external field direction. The frequency of precession is well know for a number of elements and changes linearly with magnetic field. By precisely measuring the precession frequency, one can determine the applied magnetic field. The Hall sensor is placed right next to the NMR sample and Hall voltage is recorded simultaneously with NMR frequency and corresponding field.
DESY possesses a 2 T-electro magnet with temperature control which is equipped with a NMR Teslameter. Commercially bought Hall sensors are calibrated in this set-up with the magnetic field applied in three different directions for three-dimensional mapping of the undulator field.
Further literature: James A. Clarke: “The Science and Technology of Undulators and Wigglers”, Oxford University Press, 2004
Contact: P. Vagin
