Biasing in FLUKA

Advanced Features and User Routines
2

Dear Riya,

the region importance biasing only applied during the transport of the particles, not for the source generation. It will only increase or decrease the particle weight.

By default each primary particle starts with the weight of 1.0.

What they did is to change the initial weight of the primary particle.

To understand the reason why they did it, imagine a surface (volume) where we want to start our primaries uniformly, and we split it into two equal halves A and B.

To get an uniform distribution, the we start the primaries from A 50% if the cases, and so from B. In this case we don’t change their weight.

But, what if we want to start more particles from B, because it is closer to our region of interest? We want to reduce the statistic there, without wasting extra resources to A.

Then we can start more particles from B that from A. But this would mean our source not uniform any more. To keep the uniformity, we need to reduce their weight.

Let’s start twice as many particles from B, than from A (The probability of primary starting in A is ~33.3% and from B is ~66.7%). If the primary starts from A we keep their weight as 1.0, but if they start in B, we set the weight to 0.5, so the total weight of the primaries in A and B is the same → we got our uniform source back.

But now we 1.5 times the primaries to get the same total weigh, as with the original source. However, you don’t need to worry, in FLUKA the results are not normalized to the number of primaries, but to the primary weight. (This is the same if all primaries have the weight of 1.0)

To change the weight of a primary particle you need to implement it in a source routine.

I hope this is clear enough.

Now back to your other questions related to importance biasing:

  1. The region importance biasing is applies to particles regardless to their generation. However it is possible to apply the biasing only to
    a. hardons and muons
    b. electron, positrons and gammas,
    c. low energy neutrons

  2. There is no such statistical check, you need to be aware of this kind of issues.

Cheers,
David