Public defense of the doctoral dissertation will be held on June 9th 2007 at 12 o'clock in auditorium D101 of the Department of Physical Sciences of the University of Helsinki, Finland.
VTT's MultiTrans programme enables modelling of radiation transport in arbitrary 3D geometry. The computational geometry is generated directly from a CAD-model, which makes it possible to use modern design tools. The computational grid is tree-structured and self-adaptive at the material boundaries, where the mesh automatically becomes the finest. With this method, even a complicated geometry can be represented in fine detail without an excessive number of grid points compared to equidistant mesh.
The tree-structure makes it possible to always find a coarser representation for the problem. This enables the use of multigrid method in iterative solution of the transport equation: the problem can be quickly solved on a much coarser grid, and this solution can then be used as an initial guess for the solution on finer grids. Multigrid method accelerates the iterative solution significantly. In addition, the tree structure leads to a smaller number of grid points, which also makes the iterative solution faster. To VTT's knowledge, this is the first application of the tree-multigrid technique to the radiation transport modelling.
The MultiTrans programme has been tested for different radiotherapy, such as boron neutron capture therapy (BNCT) given at VTT's nuclear research reactor, and for reactor physics applications. So far, the MultiTrans programme has been in use only at VTT.
When high accuracy is required, the simplified spherical harmonics approximation of the radiation transport used in MultiTrans has, in some cases, turned out to be problematic. More accurate methods will be studied further.
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