  We present a detailed spectroscopic study of the optical counterpart of the
neutron star X-ray transient Aquila X-1 during its 2011, 2013 and 2016
outbursts. We use 65 intermediate resolution GTC-10.4m spectra with the aim of
detecting irradiation-induced Bowen blend emission from the donor star. While
Gaussian fitting does not yield conclusive results, our full phase coverage
allows us to exploit Doppler mapping techniques to independently constrain the
donor star radial velocity. By using the component N III 4640.64/4641.84 A we
measure Kem = 102 +-6 km/s. This highly significant detection ( >13 sigma) is
fully compatible with the true companion star radial velocity obtained from
near-infrared spectroscopy during quiescence. Combining these two velocities we
determine, for the first time, the accretion disc opening angle and its
associated error from direct spectroscopic measurements and detailed modelling,
obtaining alpha = 15.5 +2.5 -5 deg. This value is consistent with theoretical
work if significant X-ray irradiation is taken into account and is important in
the light of recent observations of GX339-4, where discrepant results were
obtained between the donor's intrinsic radial velocity and the Bowen-inferred
value. We also discuss the limitations of the Bowen technique when complete
phase coverage is not available.
