  In microdosimetry, lineal energies y are calculated from energy depositions
$\epsilon$ inside the microdosimeter divided by the mean chord length, whose
value is based on geometrical assumptions on both the detector and the
radiation field. This work presents an innovative two-stages hybrid detector
(HDM: hybrid detector for microdosimetry) composed by a Tissue Equivalent
Proportional Counter (TEPC) and a silicon tracker made of 4 Low Gain Avalanche
Diode (LGAD). This design provides a direct measurement of energy deposition in
tissue as well as particles tracking with a submillimeter spatial resolution.
The data collected by the detector allow to obtain the real track length
traversed by each particle in the TEPC and thus estimates microdosimetry
spectra without the mean chord length approximation. Using Geant4 toolkit, we
investigated HDM performances in terms of detection and tracking efficiencies
when placed in water and exposed to protons and carbon ions in the therapeutic
energy range. The results indicate that the mean chord length approximation
underestimate particles with short track, which often are characterized by a
high energy deposition and thus can be biologically relevant. Tracking
efficiency depends on the LGAD configurations: 34 strips sensors have a higher
detection efficiency but lower spatial resolution than 71 strips sensors.
Further studies will be performed both with Geant4 and experimentally to
optimize the detector design on the bases of the radiation field of interest.
The main purpose of HDM is to improve the assessment of the radiation
biological effectiveness via microdosimetric measurements, exploiting a new
definition of the lineal energy ($y_{T}$), defined as the energy deposition
$\epsilon$ inside the microdosimeter divided by the real track length of the
particle.

Instrumentation and Detectors;Applied Physics;Biological Physics;Medical Physics