  High pressure xenon Time Projection Chambers (TPC) based on secondary
scintillation (electroluminescence) signal amplification are being proposed for
rare event detection such as directional dark matter, double electron capture
and double beta decay detection. The discrimination of the rare event through
the topological signature of primary ionisation trails is a major asset for
this type of TPC when compared to single liquid or double-phase TPCs, limited
mainly by the high electron diffusion in pure xenon. Helium admixtures with
xenon can be an attractive solution to reduce the electron diffusion
significantly, improving the discrimination efficiency of these optical TPCs.
We have measured the electroluminescence (EL) yield of Xe-He mixtures, in the
range of 0 to 30% He and demonstrated the small impact on the EL yield of the
addition of helium to pure xenon. For a typical reduced electric field of 2.5
kV/cm/bar in the scintillation region, the EL yield is lowered by ~ 2%, 3%, 6%
and 10% for 10%, 15%, 20% and 30% of helium concentration, respectively. This
decrease is less than what has been obtained from the most recent simulation
framework in the literature. The impact of the addition of helium on EL
statistical fluctuations is negligible, within the experimental uncertainties.
The present results are an important benchmark for the simulation tools to be
applied to future optical TPCs based on Xe-He mixtures.
