What is the exact meanings of weight window?

For describing the question, the sample figure is below:

when a particle enters in region 2 from region 1 with the weight 1,
the lower bound weight is 0.5 in region 1, and the lower bound weight is 0.25 in region 2,
the factor the upper weight divided the lower weight is 5 in all the regions.
the questions are:

1. the number of split is WL1/WL2=2?
2. after the particle is splitted, the weight of the particle(W2) is larger than Wu2，the particle is splitted again?what is the the number of split?
3. after the particle is splitted, the weight of the particle(W2) is less than WL2，the particle is Russian roulette? what is survival weight?

Hello!
Thanks for posting this question on the forum.

In the figure, two regions are represented. For each of them, a weight window (WW) biasing is declared.

1. No, the number of splits is not defined in this way in WW (please, see references in this post). In this figure, no WW takes place in the two regions: in Region1, a particle with a weight of 1 falls within the weight window. The same holds for a particle with a weight of 1 (or even 0.5) in Region2.
2. For a particle in Region2 with a weight larger than Wu2, there would be splitting because of the WW. A similar conclusion holds for Region1.
• Splitting in WW: as in the FLUKA manual - note 2: “Splitting is performed if the particle weight is higher than the top window edge for that energy, particle and region. The particle is replaced by two identical ones with half its weight.”
3. For a particle in Region2 with a weight lower than Wl2, there would be Russian roulette because of the WW. A similar conclusion holds for Region1.
• Russian roulette in WW: As in the FLUKA manual - note 2: “Russian Roulette is played in a given region if the particle weight is lower than the bottom window edge for that energy, particle and region. The particle survives with a probability equal to the ratio between its weight and the RR edge, and is given a new weight equal to the RR edge itself”.

Other side comment: in the figure, a simple way to observe splitting (or Russian roulette) moving from Region1 to Region2 would be to set up a Region Importance Biasing. Region Importance Biasing is by far easier to implement than WW. Region Importance Biasing in these two regions would allow to have splitting or Russian roulette (depending on the relative importance of regions) when a particle crosses the region boundary.

I hope this helps.
Best regards

Tommaso