Background due to stored electrons following nuclear decays in the KATRIN spectrometers and its impact on the neutrino mass sensitivity

TL;DR

The paper investigates a novel background source from stored electrons due to radon and tritium decays in KATRIN spectrometers, assessing its impact on neutrino mass measurement sensitivity and proposing mitigation strategies.

Contribution

It identifies and characterizes a new background source in KATRIN, and suggests design improvements and active methods to reduce background levels.

Findings

Stored electrons significantly affect background levels.

Refined spectrometer design parameters reduce background.

Active background reduction methods are effective.

Abstract

The KATRIN experiment is designed to measure the absolute neutrino mass scale with a sensitivity of 200 meV at 90% C.L. by high resolution tritium beta-spectroscopy. A low background level of 10 mHz at the beta-decay endpoint is required in order to achieve the design sensitivity. In this paper we discuss a novel background source arising from magnetically trapped keV electrons in electrostatic retarding spectrometers. The main sources of these electrons are alpha-decays of the radon isotopes (219,220)Rn as well as beta-decays of tritium in the volume of the spectrometers. We characterize the expected background signal by extensive MC simulations and investigate the impact on the KATRIN neutrino mass sensitivity. From these results we refine design parameters for the spectrometer vacuum system and propose active background reduction methods to meet the stringent design limits for the overall background rate.