  In the double detonation scenario the ignition of a surface He detonation on
a sub-Chandrasekhar mass white dwarf leads to a secondary core detonation.
Double detonation models have shown promise for explaining Type Ia supernovae
(SNe Ia) with a variety of luminosities. A key feature of such models is
unburnt He in the ejecta, which can show significant variation in both its mass
and velocity distribution. Many previous radiative transfer simulations for
double detonation models have neglected treatment of non-thermal ionization and
excitation, preventing them from robustly assessing whether He spectral
features are expected to form. We present a non local thermodynamic equilibrium
radiative transfer simulation, including treatment for non-thermal electrons,
for a double detonation model with a modest mass of He (${\sim}$0.04
M${\odot}$) ejected at reasonably low velocities
(${\sim}$12000$\,\mathrm{km}\,\mathrm{s}^{-1}$). Despite our simulation
predicting no clear optical He features, a strong and persistent He I
10830$\,\r{A}$ absorption feature forms that is significantly blended with the
spectral contribution of Mg II 10927$\,\r{A}$. For some normal SNe Ia the Mg
feature shows an extended blue wing, previously attributed to C I, however the
simulated He feature shows its strongest absorption at wavelengths consistent
with this wing. We therefore suggest this extended wing is instead a spectral
signature of He. The He feature predicted by this particular model is too
strong and persistent to be consistent with normal SNe Ia, however, this
motivates further work to use this observable signature to test the parameter
space for double detonation models.
