Drift field limitations to the energy resolution in Time Projection Chambers for 136Xe neutrino-less double beta decay search

The effect of drift electric field in the degradation of the energy resolution of gaseous xenon Time Projection Chambers for the search of neutrino-less double beta decay of 136Xe is calculated with the PENELOPE code. Calculations are presented first for single electron emission with energies from 0.2 to 3 MeV and reduced electric fields in the 0.1–2 V cm−1 Torr−1 range, showing energy resolution degradations by as much as 12% (FWHM). Calculations are also presented for neutrino-less double beta decay of 136Xe assuming two decay mechanisms, the mass mechanism (MM) and the right-handed current due to the λ parameter (RHCλ) mechanism, for reduced drift electric fields in the 0.03–0.8 V cm−1 Torr−1 range. It is shown that the drift field degrades the energy resolution of the two electrons sum peak (2457.8 keV) by an amount that is significant even for reduced fields as low as 0.1 V cm−1 Torr−1. It is concluded that to reach the experimental target of 1% (FWHM) for the energy resolution of TPCs set-ups (like the NEXT collaboration set-up) the drift electric field should be weaker than about 0.1 V cm−1 Torr−1.