The weight window is a very powerful biasing technique, not based on relative importances, but on the absolute value of particle weight. The user sets an upper and a lower limit for the particle weight in each geometry region, possibly tuned per type of particle and energy. Splitting and RR will be applied so that the weight of all relevant particles will have a value between the two limits. In addition to controlling the particle population, this technique helps also to "damp" excessive weight fluctuations due to other biasing options. Its use is not as easy as that of importance biasing, because it is necessary to have at least a rough idea of what are the average weights in different regions. Special splitting and RR counters can be printed on request to help setting the window parameters settingSDUM= PRINT in command BIASING. An explanation about the meaning of the counters can be found in Chap. 9}. Weight window setting is done in FLUKA by three input commands: WW-FACTOr, WW-THRESh and WW-PROFIle. The first two commands must be used together: WW-FACTOr sets the upper and lower weight limits per region, while WW-THRESh defines energy limits within which the weight window must be applied, and the particles to which it is to be applied. The third option is reserved to low-energy neutrons, whose transport characteristics often require a more detailed biasing pattern: WW-PROFIle allows indeed to tune the weight window by neutron energy group.
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