  The Accelerator Neutrino Neutron Interaction Experiment (ANNIE) is a 26-ton
water Cherenkov neutrino detector installed on the Booster Neutrino Beam (BNB)
at Fermilab. Its main physics goals are to perform a measurement of the neutron
yield from neutrino-nucleus interactions, as well as a measurement of the
charged-current cross section of muon neutrinos. An equally important focus is
placed on the research and development of new detector technologies and target
media. Specifically water-based liquid scintillator (WbLS) is of interest as a
novel detector medium, as it allows for the simultaneous detection of
scintillation and Cherenkov light. This paper presents the deployment of a 366L
WbLS vessel in ANNIE in March 2023 and the subsequent detection of both
Cherenkov light and scintillation from the WbLS. This proof-of-concept allows
for the future development of reconstruction and particle identification
algorithms in ANNIE, as well as dedicated analyses, such as the search for
neutral current events and the hadronic scintillation component within the WbLS
volume.
