Development of vectorized Monte Carlo particle transport programs

Special developments of vectorized Monte Carlo simulation codes for particle interaction with matter were performed for applications in medium and high energy physics, namely simulation of particle shower development in particle detectors. Particle showers in such large pieces of matter comprise particle energies from the energy of the incident high energy particle down to thermal energies. Due to high multiplicity of cascade particles with low energy, the Monte Carlo programs for low energy particle transport (mainly neutrons and photons) are the most time consuming ones. Therefore, we developed a vectorized geometry package for use with Monte Carlo programs, since it was found that in many cases this part of the codes is the most time consuming one at all. The question how to gather vectors in Monte Carlo programs was solved by a scheme of particle parallel processing by the code. Recently we developed vectorized versions of a nuclear event generator and a routine for tracking of neutral particles in material. The nuclear event generator code uses tabulated partial cross sections for all reactions of interest and simulates single nuclear collision events in the CM system, conserving energy and momentum in each. The tracking routine and the nuclear event generator together with the geometry package form the main parts of a particle transport MC program. Thus, we were able to assemble such a program for neutral particles with most of the inner loops vectorized. A series of test runs was performed, which showed CPU time gains of about 4.