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.