# AW: Making klongs decay at a fixed position

From: Anna Ferrari <a.ferrari_at_hzdr.de>
Date: Mon, 24 Jun 2013 10:39:10 +0200

Dear Joe Comfort,

Making klongs decay all at a fixed position is obviously impossible and physically meaningless. "Biasing" means replacing a physical distribution (in this case the distance distribution of decays f(z)=exp(-L/z)) by a modified one (e.g. f'(z)=exp(-L'/z), with L'<L), and correcting for the modification by multiplying the weight of the particles (and of their daughters) by the ratio f(Z)/f'(Z), where Z is the sampled position.
In words, you obtain more decays at small z than it is physically probable, but the particles get a low weight, according to the basic rule: weight x probability = constant.
What you suggest is making f'(z) a Dirac delta function (infinite probability density at z=Z), which would result in particles with weight zero.
In my opinion the correct way to increase the statistics in your decay volume is to modify the physical decay length via LAM-BIAS with SDUM=GDECAY, as you already did. You should find the optimal decay length to 'fill' more your decay volume.

Concerning the second part of your question: what you say, that "klongs will remain along with the decay particles, at least part of the time" is again a statistical game (mathematically correct) ruled by the above formula (conservation of weight x probability). The manual (LAM-BIAS card) says: " At the decay point sampled according to the biased probability, Russian Roulette (i.e. random choice) decides whether the particle actually will survive or not after creation of the decay products. The latter are created in any case and their weight adjusted taking into account the ratio between biased and physical survival probability". This is correct if we want that the Monte Carlo provides correct average values in the user-defined phase space regions (the user-defined "detectors") . If you are not interested in physically meaningful phase space densities, you can still "play" with the code at your own risk...
In any case, you can select "the decay particles without the klongs" just intercepting the tracking: you can discriminate them according to their generation number (available as LLO in the calls to several user routines, and as LTRACK in the TRACKR COMMON)

Hope it helps,
Anna

-----Ursprüngliche Nachricht-----
Von: owner-fluka-discuss_at_mi.infn.it [mailto:owner-fluka-discuss_at_mi.infn.it] Im Auftrag von Joseph Comfort
Gesendet: Dienstag, 28. Mai 2013 07:46
An: fluka-discuss_at_fluka.org
Betreff: Making klongs decay at a fixed position

I need to look at the behavior of a detector system when a beam of klongs all decay at a fixed position (ZBEAM) along the beam line. The physical decay volume is too short for the klong decay length (only about 4% of the klongs will decay in it, and I need to bias the rate).

I thought LAM-BIAS could help, but I could not get it to work after trying several variations. Also, the manual implies that klongs will remain along with the decay particles, at least part of the time. I want the decay particles without the klongs.

I tried a workaround by redefining the klong lifetime (in PAPROP). It sort of works, but gets entangled with kshort. Is there a 'clean' way to make things work?

Thank you,
Joe Comfort
Received on Mon Jun 24 2013 - 11:27:21 CEST

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