Re: On biasing

From: Paola Sala <>
Date: Wed, 11 May 2011 12:20:58 +0200 (CEST)

Dear beatrice,

> Dear Dr. Sala,
> thank you for your help; I see one has to assign different
> importance factors to different regions, but I can not understand the
> criterion to do that.
Very roughly : for deep penetration studies ( NOT your case) the aim of
importance biasing is to keep an high number of tracked particles even
after a large amount of absorbing material, since the statistical error
depend on how many different tracks are followed. To achieve this,
particles are cloned at boundaries, according to particle importance. A
rough criterion is to keep the fluence constant, achieved for instance by
doubling or 3-pling the importance every interaction length.
Note that paricles going into a region with smoller importance are
randomly killed according to the importance ratio.
Please have a look at the lectures from the fluka courses for more info.

> On the other hand, would you, please, explain to me better how to do
> practically point 1) use higher e.m. thresholds in regions far from
> the scoring one
EMFCUT works by region. You can segment your geometry, and put high
electron-photon thresholds in regions far from the aggregate. How far and
how high? have a look at the range of electrons and photons in concrete.
Keep a region around the aggregate with the same transport threshold as
the aggregate, to preserve electronic equilibrium.

and point 3) use biasing to kill particles that
> travel deep in the shield.

It is very unlikely, for instance, that a neutron that is 2 m deep in the
part of the shield oopposite to the aggregate will ever reach the
aggregate itself. So: if -again- you segmant your hgeometry, you can
either to eliminate part of your shiled, or to assign a small importance
to "far" regions (with the BIASING card) , so that only a fraction of the
particles will be transported there.
Hope this helps

> Thank you again,
> Sincerely
> Beatrice
> At 15.48 10/05/2011, Paola Sala wrote:
>>Dear Beatrice
>>sorry, but I do not have brilliant, or even simple, ideas for your case.
>>If I understand correctly your geometry, your region of interest is on
>> the
>>surface of the shielding, directly in front of the target. seen the beam
>>energy, I assume that the beam is stopped in the target.
>>Therefore, you do not have a deep penetration problem, where the biasing
>>card (or equivalent) would be useful.
>> By the way, how the biasing card is used in your input is meaningless:
>>importanmce biasing works by splitting ok killing particles according to
>>the relative importance of regions: if all regions have the same
>>importance nothing happens.
>>My only suggestion is to try to speed up the problem: use higher e.m.
>>thresholds in regions far from the scoring one, use a thinner shielding
>> or
>>use biasing to kill particles that travel deep in the shield.
>> >
>> > Dear Fluka experts,
>> > I would like to put forward a problem in biasing. My
>> problem
>> > is in attachment: the geometry is a concrete room around a UC2
>> > target. Near the point of coordinates (400, 500, 370) a small
>> portion
>> > of the concrete shielding is created at a mesoscale: aggregates and
>> > paste. Results of neutron flux and energy deposition are reported as
>> > well. I guess with biasing I can better my results, that is I can
>> get
>> > no more neutron tracks but more homogeneous contours; which biasing
>> > technique should I apply? I tried to use the card but maybe you can
>> > suggest which is the most suitable way to do that because I am not
>> > that expert on biasing results.
>> > Thank you for your help,
>> > my best regards,
>> > Beatrice
>> >
>> > **************************************************
>> > Ing. Beatrice Pomaro
>> >
>> > Universita' degli Studi di Padova
>> > Dipartimento di Costruzioni e Trasporti
>> > Via F. Marzolo, 9 - 35131 Padova (Italy)
>> > tel.: +39 049 8275605
>> > e-mail:
>> >
>> >
Received on Wed May 11 2011 - 15:48:21 CEST

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