We describe the work carried out at MPIK to design, model, build and characterize
a prototype cell filled with a novel indium-loaded scintillator of interest for real-time
low energy solar neutrino spectroscopy. First, light propagation in optical modules
was studied with experiments and Monte Carlo simulations. Subsequently a 5 cm x 5
cm x 100 cm prototype detector was set up and the optical performances of several
samples were measured. We first tested a benchmark PXE-based scintillator, which performed
an attenuation length of ~ 4.2 m and a photo-electron yield of ~ 730 pe/MeV. Then
we measured three In-loaded samples. At an In-loading of 44 g/l, an energy resolution
of ~ 11.6 % and a spatial resolution of ~ 7 cm were attained for 477 keV recoil electrons.
The long-range attenuation length in the cell was ~1.3 m and the estimated photo-electron
yield ~ 200 pe/MeV. Light attenuation and relative light output of all tested samples
could be reproduced reasonably well by MC. All optical properties of this system have
remained stable over a period of > 1 y.