  The SoLid collaboration has developed a new detector technology to detect
electron anti-neutrinos at close proximity to the Belgian BR2 reactor at
surface level. A 288$\,$kg prototype detector was deployed in 2015 and
collected data during the operational period of the reactor and during reactor
shut-down. Dedicated calibration campaigns were also performed with gamma and
neutron sources.
  This paper describes the construction of the prototype detector with a high
control on its proton content and the stability of its operation over a period
of several months after deployment at the BR2 reactor site. All detector cells
provide sufficient light yields to achieve a target energy resolution of better
than 20%/$\sqrt{E(MeV)}$. The capability of the detector to track muons is
exploited to equalize the light response of a large number of channels to a
precision of 3% and to demonstrate the stability of the energy scale over time.
Particle identification based on pulse-shape discrimination is demonstrated
with calibration sources. Despite a lower neutron detection efficiency due to
triggering constraints, the main backgrounds at the reactor site were
determined and taken into account in the shielding strategy for the main
experiment. The results obtained with this prototype proved essential in the
design optimization of the final detector.
  This paper is dedicated to our SCK$\cdot$CEN colleague, Edgar Koonen, who
passed away unexpectedly in 2017. Edgar was part of the SoLid collaboration
since its inception and his efforts were vital to get the experiment started.
He will be duly missed.
