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 0.2eV/c<sup loc="post">2</sup> at 90% C.L.<sup loc="post">1</sup>1C.L. - confidence level.. This goal can only be achieved with a very low background level in the order of 10mcps<sup loc="post">2</sup>2mcps - milli count per second. in the detector region of interest. A possible background source are <math display="inline" id="d1e777" altimg="si33.svg"><mi>α</mi></math>-decays on the inner surface of the KATRIN Main Spectrometer. Rydberg atoms, produced in sputtering processes accompanying the <math display="inline" id="d1e782" altimg="si33.svg"><mi>α</mi></math>-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 <math display="inline" id="d1e787" altimg="si33.svg"><mi>α</mi></math>-sources, <sup loc="pre">223</sup>Ra and <sup loc="pre">228</sup>Th, were installed at the Main Spectrometer with the purpose of temporarily increasing the background in order to study <math display="inline" id="d1e799" altimg="si33.svg"><mi>α</mi></math>-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 <math display="inline" id="d1e804" altimg="si33.svg"><mi>α</mi></math>-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 <math display="inline" id="d1e810" altimg="si33.svg"><mi>α</mi></math>-decay induced events to the residual KATRIN background.