  The Miniaturized Detector for Application in Space (MIDAS) is a compact
device with dimensions 5 x 5 x 1 cm3 which combines position sensitive Si
detectors and a fast neutrons spectrometer. MIDAS is developed with purpose to
act as a linear energy transfer (LET) spectrometer for the charged particles
and measure dose and dose equivalent from both charged particles and neutrons.
It is based on fully depleted monolithic active Si pixel sensors for the
charged track and energy deposition measurements, while a plastic scintillator
read out by a silicon photomultiplier is used to determine energy depositions
from fast neutrons. A simulation study of the detector response in galactic
cosmic ray (GCR) radiation fields with the aid of GEANT4 has been performed.
Energy depositions and hit pixel addresses have been used to reconstruct tracks
and calculate LET spectra. A method to calculate $LET\infty$ in water from the
measured LET has been elaborated. Dose rate in water and dose equivalent rate
have been calculated. The energy and particle composition of the radiation
field produced by the interaction of GCR with the Al walls of a spacecraft
model has been determined and the response of MIDAS in this radiation field has
been investigated.
