Radon induced background processes in the KATRIN pre-spectrometer

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

Simulation and measurement of the suppression of radon induced background in the KATRIN experiment

“…Currently only two of the three getterpumps are installed, each equipped with 1000 m of SAES St707 NEG strips [8]. Although the NEG material has been selected for its low intrinsic activity, it still emanates small amounts of 219 Rn [9]. With their short half-live of T 1/2 = 3.96 s the radon atoms are likely to decay inside the MS volume.…”

Status of the KATRIN experiment

“…The pre-spectrometer has been operated as a prototype for systematic investigations and hardware developments which are also relevant for the main spectrometer. A radon induced background signal, emerging from the material of vacuum getter strips, has been identified and suitable experimental measures for suppression were found [11]. The test operation of the prespectrometer is finished and it is ready for its final integration into the KATRIN setup.…”

Direct probes of neutrino mass

“…The spectrometer has run at up to 35 kV, has delivered sub-eV energy resolution, and has shown background rates of a fraction of 1 c/s. One surprise was the discovery of 219 Rn decaying in the active volume of the spectrometers [24]. This unusual isotope in the 235 U series is emanating from the Zr-rich getter strips used to pump hydrogen isotopes, water, and air.…”