Contribution of gaseous tritium source rear wall to tritium beta-spectrum in Troitsk nu-mass experiment

We avaluate the contribution of electron scattering on the rear wall of a windowless gaseous tritium source in the Troitsk nu-mass setup. There is a finite probability for them to scatter, return back and reach the detector. A such additional component distorts the measured electron spectrum. We calculate the scattering on the rear side of the vacuum pipe by the GEANT4 simulation and trace the electrons back through the magnetic fields to the spectrometer and registration system. The contribution is a few orders of magnitude smaller compared to the original spectrum at the end of the beta-spectrum of 18.6 keV but reaches 2\% at 11 keV. Taking into account this rear wall scattering we can fit well the measured spectrum in search for a sterile neutrino component in the tritium beta decay.
Comment: 7 pages, 5 figures