Radon induced background processes in the KATRIN pre-spectrometer

TL;DR

This paper investigates how radon decay inside the KATRIN pre-spectrometer generates background events, affecting the experiment's sensitivity to neutrino mass measurements by producing secondary electrons that increase background noise.

Contribution

It provides detailed measurements and analysis of radon-induced background processes in the KATRIN pre-spectrometer, highlighting the mechanisms of electron emission and storage that impact background levels.

Findings

Radon decay produces electrons that contribute to background noise.

Stored high-energy electrons can generate thousands of secondary electrons.

Radon emanation from materials significantly affects background levels.

Abstract

The KArlsruhe TRItium Neutrino (KATRIN) experiment is a next generation, model independent, large scale tritium beta-decay experiment to determine the effective electron anti-neutrino mass by investigating the kinematics of tritium beta-decay with a sensitivity of 200 meV/c2 using the MAC-E filter technique. In order to reach this sensitivity, a low background level of 0.01 counts per second (cps) is required. This paper describes how the decay of radon in a MAC-E filter generates background events, based on measurements performed at the KATRIN pre-spectrometer test setup. Radon (Rn) atoms, which emanate from materials inside the vacuum region of the KATRIN spectrometers, are able to penetrate deep into the magnetic flux tube so that the alpha-decay of Rn contributes to the background. Of particular importance are electrons emitted in processes accompanying the Rn alpha-decay, such as shake-off, internal conversion of excited levels in the Rn daughter atoms and Auger electrons. While low-energy electrons (< 100 eV) directly contribute to the background in the signal region, higher energy electrons can be stored magnetically inside the volume of the spectrometer. Depending on their initial energy, they are able to create thousands of secondary electrons via subsequent ionization processes with residual gas molecules and, since the detector is not able to distinguish these secondary electrons from the signal electrons, an increased background rate over an extended period of time is generated.