KATRIN background due to surface radioimpurities

TL;DR

This paper investigates how surface radioimpurities, specifically alpha-decays from implanted sources, contribute significantly to the background noise in the KATRIN neutrino mass experiment, affecting its sensitivity.

Contribution

It demonstrates a correlation between alpha-decay activity on the spectrometer surface and background levels, highlighting a key background source for KATRIN.

Findings

Alpha-decay induced events significantly contribute to KATRIN background.

Background characteristics depend on inner electrode potential and radial position.

Artificially induced alpha activity reproduces key background features.

Abstract

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. This goal can only be achieved with a very low background level in the order of 0.01 counts per second. A possible background source is $\alpha$-decays on the inner surface of the KATRIN Main Spectrometer. Two $\alpha$-sources, $^{223}$Ra and $^{228}$Th, were installed at the KATRIN Main Spectrometer with the purpose of temporarily increasing the background in order to study $\alpha$-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 $\alpha$-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 $\alpha$-decay induced events to the residual KATRIN background.