  Plastic scintillators are widely used as particle detectors in many fields,
mainly, medicine, particle physics and astrophysics. Traditionally, they are
coupled to a photo-multplier (PMT) but now silicon photo-multipliers (SiPM) are
evolving as a promising robust alternative, specially in space born experiments
since plastic scintillators may be a light option for low Earth orbit missions.
Therefore it is timely to make a new analysis of the optimal design for
experiments based on plastic scintillators in realistic conditions in such a
configuration.
  We analyze here their response to an isotropic flux of electron and proton
primaries in the energy range from 1 MeV to 1 GeV, a typical scenario for
cosmic ray or space weather experiments, through detailed GEANT4 simulations.
First, we focus on the effect of increasing the ratio between the plastic
volume and the area of the photo-detector itself and, second, on the benefits
of using a reflective coating around the plastic, the most common technique to
increase light collection efficiency. In order to achieve a general approach,
it is necessary to consider several detector setups. Therefore, we have
performed a full set of simulations using the highly tested GEANT4 simulation
tool: several parameters have been analyzed such as the energy lost in the
coating, the deposited energy in the scintillator, the optical absorption, the
fraction of scintillation photons that are not detected, the light collection
at the photo-detector, the pulse shape and its time parameters and finally,
other design parameters as the surface roughness, the coating reflectivity and
the case of a scintillator with two decay components. This work could serve as
a guide on the design of future experiments based on the use of plastic
scintillators.
