Abstract
The high uncertainty in the Relative Biological Effectiveness (RBE) values of particle therapy beams, which are used in combination with the quantity absorbed dose in radiotherapy, together with the increase in the number of particle therapy centres worldwide necessitate a better understating of the biological effect of such modalities. The present novel study is part of performance testing and development of a microcalorimeter based on Superconducting QUantum Interference Devices (SQUIDs). Unlike other microdosimetric detectors that are used for investigating the energy distribution, this detector provides a direct measurement of energy deposition at the micrometer scale, that can be used to improve our understanding of biological effects in particle therapy application, radiation protection and environmental dosimetry. Temperature rises of less than 1 μK are detectable and when combined with the low specific heat capacity of the absorber at cryogenic temperature, extremely high energy deposition sensitivity of approximately 0.4 eV can be achieved. The detector consists of three layers: a Tissue Equivalent (TE) absorber, a SuperConducting (SC) absorber and a silicon substrate. Ideally all energy would be deposited in the TE absorber and the heat rise in the SC layer would arise due to heat conduction from the TE layer. However, in practice direct particle absorption occurs in all three layers and must be corrected for. To investigate the thermal behavior within the detector, and quantify any possible correction, particle tracks were simulated employing Geant4 (v9.6) Monte Carlo simulations. The track information was then passed to the COMSOL Multiphysics (Finite Element Method) software. The 3D heat transfer within each layer was then evaluated in a time-dependent model. For a statistically reliable outcome, the simulations had to be repeated for a large number of particles. An automated system has been developed that couples Geant4 Monte Carlo output to COMSOL for determining the expected distribution of proton tracks and their thermal contribution within the detector. The percentage heat contribution from the TE absorber into the SC absorber was determined for mono-energetic proton pencil beams of 3.8, 10, 62 and 230 MeV. The corrected energy distribution is compared to the ideal energy distribution, exhibiting good agreement.