Impact of a cryogenic baffle system on the suppression of radon-induced background in the KATRIN Pre-Spectrometer

Abstract

A : The KATRIN experiment will determine the effective electron anti-neutrino mass with a sensitivity of 200 meV/c 2 at 90% CL. The energy analysis of tritium β-decay electrons will be performed by a tandem setup of electrostatic retarding spectrometers which have to be operated at very low background levels of < 10 −2 counts per second. This benchmark rate can be exceeded by background processes resulting from the emanation of single 219,220 Rn atoms from the inner spectrometer surface and an array of non-evaporable getter strips used as main vacuum pump. Here we report on a the impact of a cryogenic technique to reduce this radon-induced background in electrostatic spectrometers. It is based on installing a liquid nitrogen cooled copper baffle in the spectrometer pump port to block the direct line of sight between the getter pump, which is the main source of 219 Rn, and the sensitive flux tube volume. This cold surface traps a large fraction of emanated radon atoms in a region outside of the active flux tube, preventing background there. We outline important baffle design criteria to maximize the efficiency for the adsorption of radon atoms, describe the baffle implemented at the KATIRN Pre-Spectrometer test set-up, and report on its initial performance in suppressing radon-induced background. K : KATRIN; non-evaporable getter; radon emanation; baffle; cold trap 1deceased 2Corresponding author arXiv:1808.09168v1 [physics.ins-det]

Status of light sterile neutrino searches

“…Recent studies have shown that an elevated background can strongly affect the KATRIN sensitivity. KATRIN successfully applied methods to reduce radon-induced backgrounds [195], however, further measures are needed and are being explored to reach the design background rate of 10 mcps.…”

First operation of the KATRIN experiment with tritium

“…A fraction of the background arises from Rn-219 and Rn-220 decays in the volume of the main spectrometer and subsequently magnetically stored electrons. Through ionization of residual gas, this primary stored electrons creates numerous low-energy secondary electrons, which can reach the detector and create background [27,[54][55][56][57]. These background events are correlated in time, and hence the total background rate is not Poisson distributed.…”

Improved Upper Limit on the Neutrino Mass from a Direct Kinematic Method by KATRIN

“…Owing to its origin from a small number of 219 Rn decays, this background results in extended time periods where the background level is enhanced yielding a small non-Poissonian component [43]. Liquid-nitrogen cooled copper baffles at the inlet of the NEG pumps act as a countermeasure [44]. The coverage of the inner surface of the main spectrometer with a monolayer of H 2 O, originating from an imperfect bake out of the prespectrometer, has led to the formation of a thin layer of H 2 O covering the baffle surface.…”