Re: On biasing

Dear Beatrice,
if I understand correctly your routine, it seems to me that it is doing
the opposite of what I suggested. My suggestion was to kill particles far
from the aggregate : "assign a small importance to "far" regions " , but
you are instead increasing the importance as the radius increases.

Maybe I was not clear : in your case, the usual way of biasing, that is
incrementing artificially the number of particles, has no meaning,
because you are interested in a zone on the surface of the shielding, a
zone that is reached without attenuation. In your case, you can try to
gain speed, reduce cpu time per primary, by reducing the time spent in
particles far from the aggregate, therefore reducing the importance at
large radii.

If you succeed, for a given number of primaries you should get the same
statistivcal error but with a reduced cpu time
Hope it helps, and I hope I did not misunderstand your setup

Paola

> Dear Dr. Sala,
> thank you for your last explanations on the problem I
> brought on Flukadiscuss; I have used the biasing card by modifying
> the USIMBS routine, because my geometry is quite complicated in the
> investigated area, due to the presence of small aggregates in the
> concrete shielding.
> In the subroutine I ask biasing to be applied only at regions from 7
> to 67, which correspond to the regions of the investigated small area
> in front of the target, where I modelled concrete with its aggregates
> and cement paste. Also I ask to use biasing only for neutrons (JTRACK=8).
> In the subroutine, the importance factor between two regions, FIMP,
> is defined by an exponential law depending on:
> FRADF radial coordinate at the end of the track of a particle
> FRADI radial coordinate at the beginning of the track of the particle
> the normalizing length is the absorption length for concrete (assumed
> value: 40cm)
> However, after biasing this way, I do not envisage a significant
> change in the results.
> I send enclose the project files without and with biasing; in the
> Word file are the results in both cases in terms of neutron fluence
> and deposited energy; neutron fluence is reported also in
> 1-dimensional graph in direction of the beam into the shielding.
> Can you suggest me how to better results in the small investigated area?
>
> Any help would be appreciated,
> Thank you and kind regards,
> Beatrice Pomaro
>
> At 12.20 11/05/2011, Paola Sala wrote:
>>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
>>choose
>>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
>>Paola
>>
>> > 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.
>> >>Ciao
>> >>Paola
>> >>
>> >>
>> >>
>> >> >
>> >> > 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: pomaro_at_dic.unipd.it
>> >> >
>> >> >
>> >
>>
>>
>>Paola Sala
>>INFN Milano
>>tel. Milano +39-0250317374
>>tel. CERN +41-227679148
>
> **************************************************
> 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: pomaro_at_dic.unipd.it
>

Paola Sala
INFN Milano
tel. Milano +39-0250317374
tel. CERN +41-227679148
Received on Fri Jul 22 2011 - 16:51:08 CEST