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The KATRIN neutrino mass experiment: a new challenge for precision high voltage at the ppm level

Kolloquium der Abteilung 2

The KATRIN (KArlsruhe TRitium Neutrino-) experiment located at the Karlsruher Institute of Technology (KIT) is measuring the endpoint region of the tritium-β-decay spectrum to determine the neutrino mass with a sensitivity of 0.2 eV/c2.

The retarding voltage of -18.6 kV of the electrostatic spectrometer of MAC-E-filter type has to be monitored with a precision of 60 mV 3 ppm over a measurement period of two month. The potential will be measured directly via two custom made ppm-precision high-voltage dividers, which were developed and built in cooperation with the PTB. In order to determine the absolute values and the stability of the scale factors of the voltage dividers, regular calibration measurements with ppm-precision are essential. To guarantee a redundant monitoring system two independent high-voltage calibration methods applicable directly at the KATRIN experiment are used:

Firstly, a new absolute calibration method based on a differential voltage measurement at high voltage has been developed. Here a voltage of 1 kV is added to a base high-voltage potential. This additional voltage and the high voltage divider output change are traceable to reference systems. The base voltage is monitored with a reference measuring system, which does not require ppm-precision. First results have demonstrated an absolute calibration with expanded measurement uncertainties of about 1 ppm.

Secondly, the high-voltage will be compared to a natural standard given by monoenergetic conversion electrons from the decay of 83mKr. This is done with three independent sources (implanted, condensed and gaseous) distributed over different locations of the experiment. During commissioning measurements of KATRIN last year a high-voltage calibration with expanded measurement uncertainties of about 5 ppm could be demonstrated, currently limited by the uncertainty of atomic binding energies of 83mKr.

 

The talk will give an overview of the high-voltage calibration of the KATRIN experiment with a

focus on the new developed calibration methods. Especially the first method is not restricted to the KATRIN experiment but is in principle a general and transportable method to perform absolute calibrations of high-voltage dividers and measurements of their linearities with ppm-precision in a wide range of applications.