  Accurate flux measurement of low energy charged particles, trapped in the
magnetosphere, is necessary for Space Weather characterization and to study the
coupling between the lithosphere and magnetosphere, allowing the investigation
of the correlations between seismic events and particle precipitation from Van
Allen Belts. In this work, the project of a CubeSat space spectrometer, the Low
Energy Module (LEM), is shown. The detector will be able to perform an
event-based measurement of energy, arrival direction, and composition of
low-energy charged particles down to 0.1 MeV. Moreover, thanks to a CdZnTe
mini-calorimeter, the LEM spectrometer also allows photon detection in the
sub-MeV range, joining the quest for the investigation of the nature of Gamma
Ray Bursts. The particle identification of the LEM relies on the $\Delta E - E$
technique performed by thin silicon detectors. This multipurpose spectrometer
will fit within a 10x10x10 $\text{cm}^3$ CubeSat frame and it will be
constructed as a joining project between the University of Trento, FBK, and
INFN-TIFPA. To fulfil the size and mass requirements an innovative approach,
based on active particle collimation, was designed for the LEM, this avoids
heavy/bulky passive collimators of previous space detectors. In this paper, we
will present the LEM geometry, its detection concept, and the results from the
developed GEANT4 simulation.

Instrumentation and Detectors;Instrumentation and Methods for Astrophysics