KATRIN background due to surface radioimpurities

The goal of the KArlsruhe TRItrium Neutrino (KATRIN) experiment is the determination of the effective electron antineutrino mass with a sensitivity of 0.2 eV/c(2) at 90% C.L.(1). This goal can only be achieved with a very low background level in the order of 10 mcps(2) in the detector region of interest. A possible background source are a-decays on the inner surface of the KATRIN Main Spectrometer. Rydberg atoms, produced in sputtering processes accompanying the a-decays, are not influenced by electric or magnetic fields and freely propagate inside the vacuum of the Main Spectrometer. Here, they can be ionized by thermal radiation and the released electrons directly contribute to the KATRIN background. Two a-sources, Ra-223 and Th-228, were installed at the Main Spectrometer with the purpose of temporarily increasing the background in order to study a-decay induced background processes. In this paper, we present a possible background generation mechanism and measurements performed with these two radioactive sources. Our results show a clear correlation between a-activity on the inner spectrometer surface and background from the volume of the spectrometer. Two key characteristics of the Main Spectrometer background - the dependency on the inner electrode offset potential, and the radial distribution - could be reproduced with this artificially induced background. These findings indicate a high contribution of a-decay induced events to the residual KATRIN background.

Automated summary

The goal of the KATRIN experiment is to determine the effective electron antineutrino mass with high sensitivity, which requires a very low background level. This paper introduces the possibility of alpha decays on the inner surface of the KATRIN Main Spectrometer as a background source and presents measurements performed to study alpha decay induced background processes. The results show a clear correlation between the alpha activity on the inner spectrometer surface and the background from the volume of the spectrometer, indicating a significant contribution of alpha decay induced events to the residual KATRIN background.