  The direct observation of high-energy cosmic rays, up to the PeV energy
region, will increasingly rely on highly performing calorimeters, and the
physics performance will be primarily determined by their geometrical
acceptance and energy resolution. Thus, it is extremely important to optimize
their geometrical design, granularity and absorption depth, with respect to the
totalmass of the apparatus, which is amongst the most important constraints for
a space mission. CaloCube is an homogeneous calorimeter whose basic geometry is
cubic and isotropic, obtained by filling the cubic volume with small cubic
scintillating crystals. In this way it is possible to detect particles arriving
from every direction in space, thus maximizing the acceptance. This design
summarizes a three-year R&D activity, aiming to both optimize and study the
full-scale performance of the calorimeter, in the perspective of a cosmic-ray
space mission, and investigate a viable technical design by means of the
construction of several sizable prototypes. A large scale prototype, made of a
mesh of 5x5x18 CsI(Tl) crystals, has been constructed and tested on high-energy
particle beams at CERN SPS accelerator. In this paper we describe the CaloCube
design and present the results relative to the response of the large scale
prototype to electrons.
