From: Alberto Fasso' <fasso_at_slac.stanford.edu>

Date: Mon, 19 Nov 2012 03:09:11 -0800 (PST)

Date: Mon, 19 Nov 2012 03:09:11 -0800 (PST)

Dear Michele,

you are not contradicting me at all! On the contrary, the quotes you

are reporting from the MCNP/X and the Geant4/GAMOS manuals are confirming

exactly the description I gave of the next event estimator.

I wrote:

"The next event estimator produces a contribution at every collision point,

by calculating the probability that the particle would scatter in the

direction of the scoring point and would reach it without further

interaction. THAT DIRECTION IS DIFFERENT FROM THE ACTUAL DIRECTION

FOLLOWED BY THE PARTICLE".

To be compared with the GAMOS manual:

"...for each neutron or gamma interaction is calculated the probability

that it would be deviated in the direction of the point detector

(INSTEAD OF THE REAL DIRECTION TOWARDS WHICH IT IS DEVIATED)"

You wrote instead:

"it is a fluence tally at a point if the next event is a trajectory without

further collision directly to the point detector".

I didn't see in your definition any mention of the fact that the "trajectory

without further collision directly to the point detector" is NOT the actual

trajectory of the particle, a point that I have stressed above by putting the

relevant text in upper case. And since you seemed to be interested in electrons

and not in neutrons or gammas, your definition seemed to me to coincide with

what is called "range rejection" in electron transport codes such as EGS.

Notice that the next event estimator can only be applied to neutrons and

gammas, as it is correctly stated in the GAMOS manual. Also in the two papers

you quote, that estimator is used for gammas but not for electrons.

Sorry for the misunderstanding.

Alberto

On Sat, 17 Nov 2012, Michele Marziani wrote:

*> Sender: owner-fluka-discuss_at_mi.infn.it
*

*>
*

*> Dear Alberto,
*

*>
*

*> it's not my intention to contradict you, however the F5 tally in MCNP/X
*

*> is called "point detector". Every MCNP/X manual describes it as a next
*

*> event estimator, with the following meaning (the quote is from the
*

*> MCNP4B manual):
*

*>
*

*>> A point detector is a deterministic estimate (from the current event
*

*>> point) of the flux at a point in space. Contributions to the point detec-
*

*>> tor tally are made at source and collision events throughout the random
*

*>> walk.
*

*>
*

*> The same technique is also available in Geant4 through the GAMOS
*

*> interface. The situation is best described in the GAMOS User's Guide:
*

*>
*

*>> The point detector scorer covers the problems where the quantity to be
*

*>> calculated is the ?ux or the dose of neutral particles (neutrons or
*

*>> gammas) in a very small detector (small with respect to the setup
*

*>> dimensions) situated far from the primary particles source. In this
*

*>> kind of problems the fraction of particles that reach the detector is
*

*>> very reduced, and therefore if one wants to calculate the ?ux or dose
*

*>> by conventional methods the statistics needed would be prohibitive.
*

*>>
*

*>> The technique implemented in GAMOS is similar to the F5 tally
*

*>> implemented in MCNP. It is based on the following idea: normal tracks
*

*>> are propagated and for each neutron or gamma interaction is calculated
*

*>> the probability that it would be deviated in the direction of the point
*

*>> detector (instead of the real direction towards which it is deviated)
*

*>> and the probability that it reaches the point detector without any
*

*>> further interaction. The ?rst probability is based on a precalculated
*

*>> table of angle probabilities [...]
*

*>
*

*> As far as MCNP/X is concerned, validation of the above technique can be
*

*> found e.g. in the paper by J.J. DeMarco et al. (Med. Phys. 22, 11?16,
*

*> 1995) and in the LANL Report LA-UR-98-363, 1999 by D.P. Gierga and K.J.
*

*> Adams
*

*> (http://www.osti.gov/bridge/servlets/purl/663200-owvwwk/webviewable/663200.pdf).
*

*>
*

*> So, even though undesirable aspects are present, sometimes it's the only
*

*> way of estimating gamma fluxes using feasible computing times.
*

*>
*

*> Anyway, I take note that the technique is not implemented in Fluka and
*

*> likely never will be.
*

*>
*

*> Cheers,
*

*> Michele
*

*>
*

*>
*

*> On 17/11/2012 12:24, Alberto Fasso' wrote:
*

*>>
*

*>> A point detector (or next event estimator) is not what you describe,
*

*>> which is
*

*>> what is commonly called "range rejection". Neither of them is available in
*

*>> FLUKA, as a deliberate choice of the developers. Both techniques in fact
*

*>> present a number of undesirable aspects, which are discussed at length in
*

*>> Monte Carlo textbooks.
*

*>
*

*>
*

*>
*

-- Alberto Fasso` SLAC-RP, MS 48, 2575 Sand Hill Road, Menlo Park CA 94025 Phone: (1 650) 926 4762 Fax: (1 650) 926 3569 fasso_at_slac.stanford.eduReceived on Mon Nov 19 2012 - 12:52:25 CET

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