Feasibility Study of a Monolithic Silicon Telescope for BNCT Applications

A monolithic silicon telescope, consisting of a thin AE stage (of similar to 2 mu m in thickness) coupled to a residual-energy stage E (thickness 500 Am), was studied and tested for measuring the boron concentration in biological samples for boron neutron capture therapy (BNCT). At the Laboratorio di Energia Nucleare Applicata BNCT facility, Pavia, Italy, this information is derived by placing the tissue sample in front of a surface barrier diode and by irradiating the system with neutrons generated in the thermal column of the TRIGA Mark II reactor. The boron concentration is estimated through the measurement of the energy deposited in the detector by the products of the B-10(n,alpha)Li-7 reaction. However, the low-energy part Of the measured spectra is typically distorted by secondary electrons produced by photon background and by protons generated via the N-14(n,p)C-14 reaction in tissue. This work discusses the possibility of using a different silicon device, namely, the monolithic silicon telescope, for improving the accuracy of the method by exploiting its particle discrimination capabilities. A device with a sensitive area of I mm 2 was irradiated (in vacuo) bare, faced both with a certified boron implanted silicon wafer and with a thin sample of rat lung loaded with boron. The preliminary results showed that (a) alpha particles and lithium ions produced by neutron capture on boron are well identified, (b) the events due to protons generated in tissue by neutron capture on nitrogen can be well discriminated, and (c) the presence of nitrogen inside the detector dead layer gives rise to an additional contribution of protons from neutron absorption. These preliminary studies gave confidence about the possibility of improving the accuracy for the assessment of the B-10 concentration in biological samples for BNCT.