The role of electron scattering from registration detector in the “Troitsk nu-mass” MAC-E type spectrometer

Abstract

There is a proposal to search for a sterile neutrino in a few keV mass range by the "Troitsk nu-mass" facility. In order to estimate sterile neutrino mixing one needs to make precision spectrum measurements well below the endpoint using the existing electrostatic spectrometer with a magnetic adiabatic collimation, or MAC-E filter. The expected signature will be a kink in the electron energy spectrum in tritium beta-decay. In this article we consider the systematic effect of electron backscattering on the detector used in the spectrometer. For this purpose we provide a set of Monte-Carlo simulation results of electron backscattering on a silicon detector with a thin golden window with realistic electric and magnetic fields in the spectrometer. We have found that the probability of such an effect reaches up to 20-30%. The scattered electron could be reflected backwards to the detector by electrostatic field or by magnetic mirror. There is also a few percent probability to escape from the spectrometer through its entrance. A time delay between the scattering moment on the detector and the return of the reflected electron can reach a couple of microseconds in the Troitsk spectrometer. Such estimations are critical for the planning upgrades of the detector and the registration electronics.

First measurements in search for keV sterile neutrino in tritium beta-decay in the Troitsk nu-mass experiment

“…Transmission function deviates from the step function characterizing an ideal spectrometer due to effect described in [5], and stronger effect arises at potentials lower than 14 kV (in the range which we do not employ in the present measurements) due to break up of adiabaticity. We used e-gun calibration results to describe and calibrate spectrometer response function.…”

“…1. Simulation of the back scattering processes from the detector with reflection of the scattered particles back to the detector by electrostatic and magnetic mirrors [5], which dominate C o u n t r a t e p e r 2 0 c h a n n e l s A D C ( c h a n n e l s ) E x p e r i m e n t S i m u l a t i o n : i n i t i a l r e g i s t e r e d p i l e u p in this region of amplitudes, qualitatively shows similar exponential shape. The measured slope of this exponent is almost the same for different spectrometer potential or the electron energy.…”

“…Relevant details of this feature of the MAC-E filter are described in Ref. [5], while systematic errors related to the detection efficiency are described, in Section 4 below.…”

Measurements with a TRISTAN prototype detector system at the “Troitsk nu-mass” experiment in integral and differential mode

“…n 0 is the normalization and cannot be inferred from electrode measurements. Instead, a simulation was performed to determine the amount of backscattered electrons that are not backreflected and get lost in the WGTS depending on their initial energy [31]. n 3 does not show a slope and is described by a single constant c n 3 .…”