Re: Question about low energy neutron propagation

From: Alberto Fasso' (
Date: Fri May 02 2003 - 09:47:20 CEST

Photons from neutron capture are generated and tracked
explicitely, provided the relevant cross section sets include
gamma generation. Most cross sections in the FLUKA neutron library
do include gamma generation, but not all. You can check which of
them do by looking at the list in Chapter 16} of the Manual
(it will be Chapter 15} in the next release which will take place
in a few days).
Example, for the elements mentioned in your mail:

     Material Temp. Origin RN Name Identifiers Gam

  He Natural Helium * 293 K JEF-1 Y HELIUM 2 -2 293 N
27Al Aluminium 27 293 K ENDF/B6R8 Y ALUMINUM 13 -2 293 Y
  Ag Natural Silver(see #3)* 293 K JENDL-3.2 Y SILVER 47 -2 293 N
The "Y" and "N" in the last column "Gam" mean that gamma generation
respectively is or is not included.
So, photons are explicitely generated in Aluminium, and tracked down to
1 keV or to any higher energy cutoff set by the user. In this case,
kerma factors provide energy deposition only by heavy charged particles
(alpha,d, etc). In the case of Helium and Silver, however, gamma generation
is not available and the corresponding energy is deposited locally as part
of kerma.

  Alberto FassÚ

On Thu, 1 May 2003, Deborah Harris wrote:

> Dear FLUKA development team,
> I am interested in studying the signal due to few MeV neutrons
> as they pass through muon monitoring chambers that will be used in the
> MINOS experiment. The chambers consist of a 2 or 5mm active gap which is
> filled with helium gas, surrounded by 10micron silver electrodes, 1mm
> ceramic, more gas, then another 3mm of aluminum for a gas box. I'd
> expect some of the energy deposition is due to upstream materials, and
> some is due to neutron interactions in the helium itself. The FLUKA
> pages say that electrons, positrons, and photons can be tracked down to
> 1keV, which would be low enough, but on the other hand, the pages also
> say that energy deposition for low energy neutrons in anything besides
> hydrogen is treated by kerma factors.
> So does FLUKA track any of the photons which come from neutron
> capture in the ceramic or aluminum and scatter to make electrons which
> might make it to the helium gap?
> Thanks for your help,
> Debbie Harris

Alberto FassÚ
CERN-EP/AIP, CH-1211 Geneve 23 (Switzerland)
Phone: (41 22) 767 2398    Fax: (41 22) 767 9480

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