Strange behavior with biasing

From: Steffen Mueller <steffen.mueller_at_cern.ch>
Date: Mon, 26 Jan 2009 19:02:58 +0100

Dear Fluka users,

I'm getting strange results as soon as I turn biasing on. To make it
immediately clear what I mean, I prepared some plots:
https://test-stemu.web.cern.ch/test-stemu/fluka_stuff/

For testing I used a 10GeV pion beam starting at 0,0,0.001 going into
the negative Z direction inside a beampipe vacuum (R002, R003, R004,
R005). One run were 50 primaries. The relevant parts of the geometry,
including the region names can be seen in "ip_regions.eps".

With disabled biasing I get a result, which is shown in
"pions_bias_off.eps" . This is as expected, beam is going all the way
downstream.

After turning biasing on (nothing else changed), I get the result which
is shown in "pion.eps". If you compare this with the region boundaries
you can clearly see that after each boundary several particles get
killed, which shouldn't happen, given that the bias settings for these
regions are the same. Here are the relevant BIASING cards:

BIASING 0.0 1.0 1.0 R001 @LASTREG 0.0
BIASING 0.0 1.0 1.0 R002 0.0
BIASING 0.0 1.0 1.0 R003 0.0
BIASING 0.0 1.0 1.0 R004 0.0
BIASING 0.0 1.0 1.0 R005 0.0
* ------------------ ADDITIONAL BIASING --------------------------*
BIASING -1.0 0.0 MUON+ MUON-
BIASING -1.0 0.4 ELECTRON POSITRON
BIASING -1.0 0.4 PHOTON
BIASING -1.0 8.0 PROTON APROTON
BIASING -1.0 2. NEUTRON
BIASING -1.0 2. ANEUTRON
BIASING -1. 8.0 KAONLONG
BIASING -1. 8.0 PION+
BIASING -1. 8.0 PION-
BIASING -1. 8.0 KAON+
BIASING -1. 8.0 KAON-
BIASING -1.0 8.0 KAONZERO
BIASING -1.0 8.0 AKAONZER

The full input file is also copied to the above mentioned web page.

Looking into the manual I also have a question regarding BIASING: For
the case M>1 it seems to me that RR and splitting becomes asymmetric,e.g:

I2=0.015, I1=0.0125, M=8:
RR: I1/(M*I2)=0.104
Splitting: 1+M*(I2/I1 -1) = 1+8*(1.2 - 1) = 2.6

So about 4times more RR than splitting. Therefore I don't understand
this sentence a bit further down: "If the only biasing is via region
importances set by WHAT(3),only limited fluctuations arise (all
particles of a given kind have about the same weight in the same
region), and no window is needed."

As far as I can see it is possible that same particles in same regions
might have different weights because of that asymmetry. Please let me
know where I didn't get the point.

Last point is, that the manual clearly splits up between I1>I2 and
I1<I2, so I wonder what really happens for I1=I2 M=8:
a) 1 + M* (I2/I1 -1) = 1.0 (great!)
b) I2/(M*I1) = 1/8 = 0.125 (not so great!)
c) internal check puts biasing directly to 1.0 (also great!)

Please let me know if you need more information.

Cheers,
  Steffen
Received on Mon Jan 26 2009 - 23:47:33 CET

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