Re: about scoring

From: Alberto Fasso' (Alberto.Fasso@cern.ch)
Date: Thu Mar 06 2003 - 16:50:34 CET


I sent already an answer to your question about fluence and current
on February 27. Didn't you see it?

When scoring fluence, the weight of the particle crossing the boundary is
divided by the cosine of the angle between the particle direction and the vector
normal to the boundary. With current, the weight of the particle is scored as it
is. A cosine has no dimensions, so the units are the same.
If all particles have a direction perpendicular to the boundary, their cosine
is always = 1 and current = fluence. In all other cases, current < fluence.
I suggest that you read the attachments to my previous mail for more
information. As a rule, current should never be used, at least in shielding
and dosimetry problems, because it is not a good predictor of radiation effects.

Both collision and track-length estimators can be used to estimate the average
fluence inside a region. The fluence can be defined as track length density:
this definition indeed is much better than the usual one you find in dosimetry
textbooks. Again, in the attachments to my previous mail you can find a proof
that the two definitions are equivalent.

The collision estimator is also used to estimate the average fluence
inside a region, using the formula:

                           C = FSV
where:
C = number of collisions (each of course weighted with particle's weight)
S = macroscopic cross section = inverse of mean free path
V = volume of the region

from which we obtain the fluence as:
                           F = C/(SV)

it is easy to see that C/S = number of collisions multiplied by mean free path
= total track length: therefore, this formula is consistent with the one which
uses the track length density. However, the collision estimator cannot be used
in vacuum, while the track length estimator can.

  Alberto

On Thu, 6 Mar 2003, wuzhen wrote:

> 1) What is the difference between the fluence and current? In my example, the
> results of the two quantities are same. And in the manual, they both are given
> as cm-2 Gev-1 sr-1 per incident primary unit weight.

> 2) I don't understand the meaning of collision and track-length estimators
> coincident with regions. Who can explain it to me?
>
> Best regards,
>
> Zhen wu
>
> 03/06/2003

-- 
Alberto Fassò
CERN-EP/AIP, CH-1211 Geneve 23 (Switzerland)
Phone: (41 22) 767 2398    Fax: (41 22) 767 9480   Alberto.Fasso@cern.ch


This archive was generated by hypermail 2.1.6 : Tue Jun 10 2003 - 14:47:42 CEST