  Due to its angular dependency, Cerenkov light has long been considered a
contamination signal in plastic scintillating dosimeters. In this study, we
propose a novel approach designed to take advantage of this angular dependency
to perform a direct measurement of an external beam radiation angle of
incidence. A Cerenkov probe composed of a 10-mm long filtered sensitive volume
of clear PMMA optical fibre was built. Both filtered and raw Cerenkov signals
from the transport fibre were collected through a single 1-mm diameter
transport fibre. An independent PSD composed of 10-mm BCF12 scintillating fibre
was also used for simultaneous dose measurements. A first series of
measurements aimed at validating the ability to account for the Cerenkov
electron energy spectrum dependency by simultaneously measuring the deposited
dose. A cylindrical phantom was then used to obtain an angular calibration
curve for fixed dose irradiations and perform incident angle measurements using
electron and photon beams. The beam nominal energy was found to have a
significant impact on the shapes of the angular calibration curves obtained for
irradiation angle measurements. This can be linked to the electron energy
spectrum dependency of the Cerenkov emission cone. Irradiation angle
measurements exhibit an absolute mean error of 1.86{\deg} and 1.02{\deg} at 6
and 18 MV, respectively. Similar results were obtained with electron beams and
the absolute mean error reaches 1.97, 1.66, 1.45 and 0.95 degrees at 9, 12, 16
and 20 MeV, respectively. Reducing the numerical aperture of the Cerenkov probe
leads to an increased angular dependency for the lowest energy while no major
changes were observed at higher energy. This allowed irradiation angle
measurements at 6 MeV with a mean absolute error of 4.82{\deg}. The detector
offers promising perspectives for external beam radiotherapy and brachytherapy
applications.
