Truncation. MCNP can operate energy or time cut-offs; the particle is killed below an energy threshold. This is good because it saves time, but it is dangerous because low-energy particles can produce highly energetic ones (fissions) and thus lead to erroneous results. Similar time cut-offs exist when the particle time exceeds a given cut-off.

Population control methods use particle splitting and Russian roulette to control the number of particles in various regions. Each MCNP cell is given an importance I. For example, if a neutron of weight W passes from a cell of importance 2 to one of importance 8, it is split into 8=2 = 4 identical neutrons each with a weight W=4. Conversely, if the neutron passes from a cell of importance 8 to one of importance 2, a Russian roulette is played and the neutron is killed with the probability 1 — | = 75% or, worded differently, followed with a 25% probability and a weight W x 4. This type of popula­tion control can be applied to the energy range.