| 48 | 48 | The efficiency loss was experienced in case of isotopes with high probability for fission around the starting energy. When the starting energy is low, neutrons released in fission take on much higher velocity than starters, thus leaking out of the system very fast. As a result, significant part of computational effort was spent on a few neutrons bouncing around in the system. Population drop caused vectorization gain to be cancelled due to the computational overhead of event-based tracking (particles need to be sorted by event type). On higher starting energies, no considerable speedup was observed in case of elements with low atomic numbers, the improvement from vectorization was more expressed when heavy elements were present. This is due to that the outgoing energy and angle of a neutron scattered on a light isotope are derived by simple laws of collision mechanics, while more complicated energy laws are applied when heavier isotopes are present . In GUARDYAN beside elastic scatter only ACE law 3 (inelastic discrete-level scattering) was used in the former case, and ACE law 4 (and 44) was additionally used in the latter. ACE law 4 represents a continuous tabular distribution, the outgoing energy is given as a probability distribution for every incoming energy. This sampling procedure takes considerably more time, contributing to thread divergence, and resulting in substantial efficiency boost for event-based tracking. |
