Ponente
Descripción
Proton therapy is one of the radiotherapy options currently available, which has significant advantages over conventional photon therapy. In order to assess the beam dose delivered to the patients at the cellular level during the hadrontherapy treatments, the Instituto de Microelectrónica de Barcelona (IMB-CNM) has designed and manufactured a new generation of silicon microdosimeters. The microdetectors are cylinder-shaped with a size comparable to that of human cell nuclei (20 $\mu m$-thick, 26 $\mu m$-diameter). These sensors have been characterized at the microbeam line of the Centro Nacional de Aceleradores (CNA) using the Ion Beam Induced Charge (IBIC) technique with a 1.17 MeV focused proton beam.
Taking advantage of the fact that our system allows the acquisition of data in "event-by-event" format (List mode), a program has been developed to obtain energy spectra from any part of the detector and for any desired dose range. By means of a model proposed in a previous work [1], based on the shape of the energy spectrum to evaluate the radial dependence of the Charge Collection Efficiency (CCE), the evolution of the CCE as a function of proton fluence has been studied for proton fluences up to $1.7 \times 10^{12} p \cdot cm^{-2}$ (~ 16 kGy).
The pristine microdosimeter presents a nearly perfect CCE profile, showing 100% CCE from the center to a radius of 10 microns, and dropping sharply to zero as the edge of the detector is approached. Moreover, the active volume of the sensor is ≈ 95%. This behaviour has been observed up to doses about two orders of magnitude higher than those used in a clinical protontherapy treatment (~ 50 Gy), which indicates that the lifetime of these sensors is long enough for this application. For higher doses the transport properties of the microdosimeters show a progressive degradation, with the CCE drop being more pronounced at the edge of the detector, probably due to the presence of a lower electric field in this area, and propagating towards the center as the proton fluence increases.
[1] Bachiller-Perea et al. IEEE Trans. Instrum. Meas. 70, pp. 1-11, 2021, Art no. 6005211
