Dear George,
I thank you once again for taking the trouble of replying my queries. You are absolutely right that the absolute value of importance of a particle in a region does not mean anything, but the ratio of the particle importance value of the region and the region immediately after that. As I understand the default importance of a particle in every region is set to 1. So, when we start applying importance biasing from the first region and if the value is less than 1, then RR is played. If it is more than 1, then particles are split depending upon the ratio. To see this, I have made a small input file, attached herewith and find the neutron fluence at the first boundary and at the last boundary. The results are as follows:
1) Without any biasing Neutron fluence at first boundary: 4.113E-06 +/- 5.462% and at last boundary 2.034E-06 +/- 6.465%
2) With Imp=0.1 in Shield region, First boundary: 3.727E-06 +/- 5.6822% and at last boundary: 1.5366E-06 +/- 9.769%
3) With Imp=2 in Shield region, First boundary: 3.992E-06 +/- 4.196 % and at last boundary: 1.9014E-06 +/- 3.894%
So, we see that with same number of histories, applying Imp more than 1 decreases the error, where as applying Imp less than 1 increases the error. So, I guess, I have to start putting the Imp values 2,4 8,....for every region for my simulation job of deep penetration.
with regards,
डॉ. सुजय चटर्जी/Dr. Sujoy Chatterjee
भाभा परमाणु अनुसंधान केंद्र/Bhabha Atomic Research Centre
1/ए एफ विधान नगर/1/AF-Bidhannagar,
कोलकाता - 700064/Kolkata - 700064
Alt. Email: sujoy_at_barc.gov.in
-----George Kharashvili <georgek_at_jlab.org> wrote: -----
To: Sujoy Chatterjee <sujoy_at_vecc.gov.in>
From: George Kharashvili <georgek_at_jlab.org>
Date: 06/21/2018 11:18AM
Cc: FLUKA Discussion List <fluka-discuss_at_fluka.org>
Subject: Re: [fluka-discuss]: Regarding importance biasing
Dear Sujoy,
The absolute value of the importance biasing of a region does not matter, unless a particle crosses the boundary of this and another region with a different value. The only parameter that really matters is the ratio of importance biasing values of two neighboring regions. Therefore, to take advantage of the full range of values, it makes sense to assign the minimum allowed value to the region where your particles originate and keep increasing it as the particle fluence decreases throughout the shielding.
It is my understanding that biasing mean free path would not help here, because it biases inelastic hadronic interaction length.
I'm not sure about the rationale, but the manual seems clear that WHAT(1)=1.0 applies biasing to all hadrons, heavy ions and muons. May be someone else can comment.
Cheers,
George
----- Original Message -----
From: "Sujoy Chatterjee" <sujoy_at_vecc.gov.in>
To: "George Kharashvili" <georgek_at_jlab.org>
Cc: "FLUKA Discussion List" <fluka-discuss_at_fluka.org>
Sent: Thursday, June 21, 2018 8:17:08 AM
Subject: Re: [fluka-discuss]: Regarding importance biasing
Hi George,
First of all thank you for the reply. If I understand correctly (FLUKA experts, please correct me), if the importance of a region is less than 1, say 0.001, then Russian roulette is played at the boundary, and the surviving particle is assigned higher weight. But it reduces the particles fluence. Where as, if the importance is more than 1, say, 5, then the particles are split with reduced weight. As in my problem, I want to keep the particle fluence intact towards the detector, I have the choice of Importance values starting from 1 to 10^5.
I would also like to know, in such cases of deep penetration, does biasing mean free path helps ? Is it safe, without expertise ?
Regarding WHAT(1) of the biasing card, if the value of 1.0 takes care of all neutrons of all energies, why the value of 3 has been explicitly given for low energy neutrons ?
with regards,
डॉ. सुजय चटर्जी/Dr. Sujoy Chatterjee
भाभा परमाणु अनुसंधान केंद्र/Bhabha Atomic Research Centre
1/ए एफ विधान नगर/1/AF-Bidhannagar,
कोलकाता - 700064/Kolkata - 700064
Alt. Email: sujoy_at_barc.gov.in
-----owner-fluka-discuss_at_mi.infn.it wrote: -----
To: Sujoy Chatterjee <sujoy_at_vecc.gov.in>
From: George Kharashvili
Sent by: owner-fluka-discuss_at_mi.infn.it
Date: 06/21/2018 12:56AM
Cc: FLUKA Discussion List <fluka-discuss_at_fluka.org>
Subject: Re: [fluka-discuss]: Regarding importance biasing
Hello Sujoy,
Importance biasing can be set from 0.0001 to 100000. So the importance of all regions can be set to 0.0001 by the first BIASING card and then increase by a factor of 2 for every next layer, up to 100000. These are hard limits and, as far as I know, there is nothing you can do to change them.
WHAT(1)=3 will only apply to neutrons with energy less than 20 MeV, but you can use WHAT(1)=0.0 (all particles) or WHAT(1)=1.0 (hadrons, heavy ions, and muons), both of which will apply importance biasing to all neutrons and will most likely give the same, or very similar results for a typical neutron shielding problem.
Cheers,
George
----- Original Message -----
From: "Sujoy Chatterjee" <sujoy_at_vecc.gov.in>
To: "FLUKA Discussion List" <fluka-discuss_at_fluka.org>
Sent: Wednesday, June 20, 2018 4:02:08 PM
Subject: [fluka-discuss]: Regarding importance biasing
Hello FLUKA experts,
The importance biasing of a region can be set up to 10^5 as seen from the FLUKA courses lectures. Now for my problem of shielding neutrons, it has been observed that the DOSE-EQ due to neutrons becomes 1/2 after 10 cm of concrete. So, I started putting Imp=2, 4, 8 .... for every 10 cm of concrete. As I have to study upto a thickness of 600 cm of concrete, I donot know how to tackle this problem as for deeper thicknesses the Imp values are going beyond 10^5.
Another question is regarding the WHAT(1) parameter of the Biasing card. The neutrons which I am studying is a spectrum from thermal to 50 MeV. If I put WHAT(1)=3 for low energy neutrons, do I require separate Biasing card for neutrons beyond 20 MeV ? Can the value of WHAT(1)=1.0 take care of all the neutrons, i.e., below 20 MeV and above 20 MeV ?
Thank you.
with regards,
डॉ. सुजय चटर्जी/Dr. Sujoy Chatterjee
भाभा परमाणु अनुसंधान केंद्र/Bhabha Atomic Research Centre
1/ए एफ विधान नगर/1/AF-Bidhannagar,
कोलकाता - 700064/Kolkata - 700064
Alt. Email: sujoy_at_barc.gov.in
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Received on Thu Jun 21 2018 - 09:53:00 CEST