  We investigate the performance of large area radiation detectors, with high
energy- and spatial-resolution, intended for the development of a Total Energy
Detector with gamma-ray imaging capability, so-called i-TED. This new
development aims for an enhancement in detection sensitivity in time-of-flight
neutron capture measurements, versus the commonly used C6D6 liquid
scintillation total-energy detectors. In this work, we study in detail the
impact of the readout photosensor on the energy response of large area (5050
mm2) monolithic LaCl3(Ce) crystals, in particular when replacing a conventional
mono-cathode photomultiplier tube by an 88 pixelated silicon photomultiplier.
Using the largest commercially available monolithic SiPM array (25 cm2), with a
pixel size of 66 mm2, we have measured an average energy resolution of 3.92%
FWHM at 662 keV for crystal thicknesses of 10, 20 and 30 mm. The results are
confronted with detailed Monte Carlo (MC) calculations, where both optical
processes and properties have been included for the reliable tracking of the
scintillation photons. After the experimental validation of the MC model, se
use our MC code to explore the impact of different a photosensor segmentation
(pixel size and granularity) on the energy resolution. Our optical MC
simulations predict only a marginal deterioration of the spectroscopic
performance for pixels of 33 mm2.
