Dear FLUKA experts,
I want to bias the decay length of Pions to let them all decay in a distance of 300m and see their decay products, especially the neutrinos. I set the decay length to 6.E+03 cm using LAM-BIAS with the option GDCAY. However, I cannot see any difference in the results of my simulations with or without the LAM-BIAS card activated. To compare the results, I am using USRYIELDs (Total momentum, polar angle in the lab) at different distances along my beam line, I attach the flair file for more clarity.
Thank you very much for your support.
NuTag_beamline_compare.flair (15.5 KB)

Dear @abaratto,
In your input file, you’re increasing the decay length by a factor 6000. Of course I don’t know what exactly you want to do, but I would guess that you want to reduce the decay length rather than increasing it.
As a bonus comment, 6000 is a very large biasing factor, I would suggest not to go beyond a factor 100 (or better 1/100).

Thank you very much for your answer @amario.
However, according to the FLUKA CERN manual, with SDUM = GDCAY
WHAT1 > 0.0: The mean decay length (in cm) of the particle in the laboratory frame is set to |WHAT(1)| if smaller than the physical decay length (otherwise it is left unchanged). Let Pu = unbiased probability and Pb = biased probability: at the decay point sampled according to Pb, the particle always survives with a reduced weight W(1−Pu/Pb), where W is the current weight of the particle before the decay. Its daughters are given a weight WPu/Pb (as in case WHAT(1) < 0.0).

So I should be setting de decay length equal to 60 m, right?

Dear @abaratto,
You’re right, I didn’t read your post carefully enough.
I’ll look more carefully into your problem and get back to you.

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Just to make sure about your expectations @abaratto: your biasing is meant to improve the decay product statistics, decreasing related statistical uncertainties. As for the resulting yield, this is not supposed to necessarily increase, since the artificial augmentation of the decay products is duly compensated by their reduced statistical weight, which is properly taken into account in the USRYIELD scoring. So the results of the latter may not visibly change, if their statistical quality was already satisfactory without biasing.

Thank you for the reply. I understand, I was interpreting the results in the wrong way.

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