Electrostatic spectrometers utilized in high-resolution β-spectroscopy studies such as in the Karlsruhe Tritium Neutrino (KATRIN) experiment have to operate with a background level of less than 10−2 ...counts per second. This limit can be exceeded by even a small number of 219,220Rn atoms being emanated into the volume and undergoing α-decay there. In this paper we present a detailed model of the underlying background-generating processes via electron emission by internal conversion, shake-off and relaxation processes in the atomic shells of the 215,216Po daughters. The model yields electron energy spectra up to 400 keV and electron multiplicities of up to 20 which are compared to experimental data.
► This article describes background measurements at the KATRIN pre-spectrometer. ► Electrons released in Radon decay are magnetically trapped in the spectrometer. ► Trapped electrons can produce ...thousands of background events at the detector.
The
KArlsruhe
TRItium
Neutrino (KATRIN) experiment is a next generation, model independent, large scale tritium
β-decay experiment to determine the effective electron anti-neutrino mass by investigating the kinematics of tritium
β-decay with a sensitivity of 200
meV/c
2 using the MAC-E filter technique. In order to reach this sensitivity, a low background level of 10
−2 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
α-decay of Rn contributes to the background. Of particular importance are electrons emitted in processes accompanying the Rn
α-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.
The KATRIN experiment is designed to measure the absolute neutrino mass scale with a sensitivity of 200meV at 90% C.L. by high resolution tritium β-spectroscopy. A low background level of 10−2 counts ...per second (cps) at the β-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 α-decays of the radon isotopes 219,220Rn as well as β-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.
The KATRIN experiment aims to determine the effective electron neutrino mass with a sensitivity of 0.2eV/c2 (90\% C.L.) by precision measurement of the shape of the tritium beta-spectrum in the ...endpoint region. The energy analysis of the decay electrons is achieved by a MAC-E filter spectrometer. A common background source in this setup is the decay of short-lived isotopes, such as 219Rn and 220Rn, in the spectrometer volume. Active and passive countermeasures have been implemented and tested at the KATRIN main spectrometer. One of these is the magnetic pulse method, which employs the existing air coil system to reduce the magnetic guiding field in the spectrometer on a short timescale in order to remove low- and high-energy stored electrons. Here we describe the working principle of this method and present results from commissioning measurements at the main spectrometer. Simulations with the particle-tracking software Kassiopeia were carried out to gain a detailed understanding of the electron storage conditions and removal processes.
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 \(\beta\)-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.
The KATRIN experiment is a next-generation direct neutrino mass experiment with a sensitivity of 0.2 eV (90% C.L.) to the effective mass of the electron neutrino. It measures the tritium ...\(\beta\)-decay spectrum close to its endpoint with a spectrometer based on the MAC-E filter technique. The \(\beta\)-decay electrons are guided by a magnetic field that operates in the mT range in the central spectrometer volume; it is fine-tuned by a large-volume air coil system surrounding the spectrometer vessel. The purpose of the system is to provide optimal transmission properties for signal electrons and to achieve efficient magnetic shielding against background. In this paper we describe the technical design of the air coil system, including its mechanical and electrical properties. We outline the importance of its versatile operation modes in background investigation and suppression techniques. We compare magnetic field measurements in the inner spectrometer volume during system commissioning with corresponding simulations, which allows to verify the system's functionality in fine-tuning the magnetic field configuration. This is of major importance for a successful neutrino mass measurement at KATRIN.
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