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1 Introduction
The goal of the Troitsk nu-mass experiment is a search for sterile neutrinos in the tritium beta decay, ref. [1]. One of the main parts of the setup is the windowless gaseous tritium source (WGTS) about 5 meters long. A set of solenoids in WGTS forms a magnetic bottle . Electrons at small pitch angle to the magnet axis will leave the WGTS towards the spectrometer or in the opposite direction. The spot of electrons that escape from the WGTS in the direction opposite to the spectrometer (rear wall) increases in size in the fringe field and then scatters on the vacuum pipe. At some combination of scattered angle and final energy, electrons may return to the WGTS and spectrometer and reach the detector. This contamination modifies the measured spectrum. In addition to direct penetration to the rear wall, there is also a finite probability for trapped electrons to reach the rear wall after multiple electron scattering in the working gas, reference [2]. In this article we estimate contribution of the the rear wall to the measured integrated tritium spectrum and demonstrate its influence to the spectrum. We also want to stress that the rear wall problem is crucial for measurements similar to ours, such as the TRISTAN project, which is an extension of the KATRIN experiment, reference.
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This paper is available on arxiv under CC by 4.0 Deed (Attribution 4.0 International) license.
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