Microdosimetry measurements for low-energy particles using a mini TEPC with removable plug

Knowledge about the single-event energy deposition in a subcellular biological target, e.g. cell nucleus, is required in order to understand the radiation action and the biological effectiveness of ionizing radiation. This energy deposition can be measured using a tissue equivalent proportional counter (TEPC). Such measurements are particularly useful to find the microdosimetric spectra for mixed radiation fields comprising different radiation types and energies. In the present work, several mixed radiation fields were generated by irradiating a mini TEPC, containing a small plug inserted into the hole on the counter wall, with reactor neutrons. The plug, made of A150, boron, nitrogen, lithium or cadmium mixture, was designed to generate different ionizing particles under the neutron irradiation. The measured spectra separated the contributions to the absorbed dose from electrons (photons), protons (neutrons) and heavy ions. The lineal energy spectra demonstrated that three distinct regions of lineal energy y, were identified, i.e. the region from heavy ions at y>100 keV/µm, the region for recoil protons at 10<y<100 keV/µm, and the region due to electrons at y<10 keV/µm. In all measurements, the proton edge and the electron edge were used as marking points to calibrate the lineal energies. The relative magnitudes of the measured spectra in each y region depended on the yields and stopping powers of nuclear reaction products. For instance, lineal energy peaks were identified at y>100 keV/µm due to the ¹⁰B (n, α)⁷Li reaction. If one substitutes the plug material by a radionuclide, microdosimetry spectra could be measured for internal dosimetry applications.