Characterization of MOSFET Dosimeters for Alpha Particle Therapy

Alpha particle therapy, such as diffusing alpha-emitters radiation therapy (DaRT) and targeted alpha-particle therapy (TAT), exploits the short range and high linear energy transfer (LET) of alpha particles to destroy cancer cells locally with minimal damage to surrounding healthy cells. Dosimetry for DaRT and TAT is challenging, as their radiation sources produce mixed radiation fields of $\alpha $ particles, $\beta $ particles, and $\gamma $ rays. There is currently no dosimeter for real-time in vivo dosimetry of DaRT or TAT. Metal-oxide-semiconductor field-effect transistors (MOSFETs) have features that are ideal for this scenario. They can be read out in real-time nondestructively, store information permanently, and their sensitivity to radiation can be varied by changing the bias on the gate during irradiation. Moreover, owing to their compactness, MOSFETs can fit into fine-gauge needle applicators, such as those used to carry radioactive seeds into the tumor. This study characterized the response of MOSFETs designed at the Centre for Medical and Radiation Physics, University of Wollongong. Irradiations were performed with a mono-energetic helium ion beam (He ²⁺ ) of 5.5 MeV and an Americium-241 ( ²⁴¹ Am) source, for MOSFETs with three different gate oxide thicknesses (0.55, 0.68, and 1.0 $\mu \text{m}$ ). The results showed that the response was linear with alpha dose up to 25 Gy. Also, it was found that a gate bias of between 15 and 60 V would optimize the sensitivity to alpha particles with energy of 5.5 MeV.