Re: About precision and credibility

From: Alberto Fasso' <fasso@SLAC.Stanford.EDU>
Date: Mon Apr 14 2008 - 20:24:52 CEST

It is very difficult to answer your question.
There is no sharp boundary: the performance of the code will degrade as you
make the scale smaller and smaller, but how fast it does so depends on the
As a crude simplification, I would say that a scale of um is still acceptable
in many cases, but one of nm certainly not.
Let's try to see why.
- Geometry: objects at a very small scale do not present smooth and
well defined surfaces as can be described by FLUKA combinatorial geometry.
(However, this problem is less important if you are interested in what happens
in a gas). But when you speak about nm scale, we cannot even talk about an
"object". This is the size of molecules, which do not even have a "surface"
and do not occupy a precise and static position in space.
- Physical interactions: first of all, at the small scales you are mentioning
the use of single scattering is mandatory, since multiple scattering theories
do not work for small steps. This is possible with FLUKA. But you are talking
about "dose calculations". Dose is deposited by charged particles,
especially by electrons, and charged particles are transported only at
energies > 1 keV. But already at 5 keV, the energy loss theory used in FLUKA
becomes rather shaky (see
As you can see at the above link, the residual range of 5 keV electrons is of
the order of 0.5 um in materials such as collodion and polystyrene. Therefore,
I guess that dose calculations would not be so good in solids of that size.
In a gas, as I said above, it could be better, but not much.
Similar considerations can be made for neutron interactions where recoil
energy is deposited on the spot as "kerma", disregarding the actual range
of the heavy recoil, which can exceed 1 um for Oxygen and Carbon recoils
from interactions of neutrons of a few MeV (see Broerse and Zoetelief
Rad. Prot. Dosim. 112, 509, 2004)


On Sun, 13 Apr 2008, wrote:

> Dear FLUKA authors and users,
> I am a new user of FLUKA and now interested in the precision of the results
> of FLUKA.
> To put it in a simple way, I am asking: what is the minimum scale for dose
> calculation? For example, when the calculation is performed in the scale of
> um or even nm, what is the extent to which the results are credible?
> I tried to look for related answers in the discuss-list, but failed.I would
> appreciate it very much if anyone can tell me the answers.
> Thank you all for noticing this mail.
> Regards,
> Kyan

Alberto Fasso`
SLAC-RP, MS 48, 2575 Sand Hill Road, Menlo Park CA 94025
Phone: (1 650) 926 4762   Fax: (1 650) 926 3569
Received on Mon Apr 14 22:48:49 2008

This archive was generated by hypermail 2.1.8 : Thu Apr 17 2008 - 11:19:21 CEST