KATRIN background due to surface radioimpurities

F.M. Fränkle, A. Schaller (nee Pollithy), K. Blaum, L. Bornschein, G. Drexlin, F. Glück, V. Hannen, F. Harms, D. Hinz, K. Johnston, J. Karthein, U. Köster, A. Lokhov, S. Mertens, F. Müller, A. Osipowicz, P.C.-O. Ranitzsch, K. Schlösser, T. Thümmler, N. Trost, C. Weinheimer, J. Wolf

Zusammenfassung

The goal of the KArlsruhe TRItrium Neutrino (KATRIN) experiment is the determination of the effectiveelectron antineutrino mass with a sensitivity of 0.2 eV∕c2 at 90 % C.L.1. This goal can only be achieved with avery low background level in the order of 10 mcps2 in the detector region of interest. A possible backgroundsource are 𝛼-decays on the inner surface of the KATRIN Main Spectrometer. Rydberg atoms, produced insputtering processes accompanying the 𝛼-decays, are not influenced by electric or magnetic fields and freelypropagate inside the vacuum of the Main Spectrometer. Here, they can be ionized by thermal radiation andthe released electrons directly contribute to the KATRIN background. Two 𝛼-sources, 223Ra and 228Th, wereinstalled at the Main Spectrometer with the purpose of temporarily increasing the background in order to study𝛼-decay induced background processes. In this paper, we present a possible background generation mechanismand measurements performed with these two radioactive sources. Our results show a clear correlation between𝛼-activity on the inner spectrometer surface and background from the volume of the spectrometer. Two keycharacteristics 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 findingsindicate a high contribution of 𝛼-decay induced events to the residual KATRIN background.