Re: FLUKA: FLUKA questions

[Alberto Fasso' <Alberto.Fasso@cern.ch>]

Dear Baerbel,

your first question is difficult. FLUKA uses only the 
total cross section to decide that a photonuclear interaction
must occur, and then calls the PEANUT event generator
to simulate the interaction. As a result, sometimes you get one 
neutron, sometimes 2, sometimes none at all. There is no 
(gamma,2n) cross section implemented explicitely, but of course
you can always simulate an experiment and "count" how many
times you get a gamma nuclear interaction and how many times
you have 2 neutrons as final particles. 
It should be possible to write an MGDRAW routine (under ENTRY USDRAW) 
to do this, but I am not completely sure because this part is
not yet well documented. I think (Alfredo please correct me if I 
am wrong) that you should do the following:
- check for calls with ICODE = 101 (inelastic interactions)
- check that the interacting particle was a photon (JTRACK = 7)
- check how many secondaries have been produced (NP) and find
  out which kind of particles they are. For instance, if it is a
  (gamma,2n) reaction, you should have:
          ICODE = 101  (it was an inelastic nuclear interaction)
          JTRACK = 7   (incident particle was a photon) 
          NP = 2       (there are two secondaries produced)  
          KPART(1) = 8 (the first secondary is a neutron)
          KPART(2) = 8 (the 2nd secondary is a neutron)


The second question is easier.
To stop ALL particles at some particular region boundary,
the easiest thing is to define at that position a blackhole 
region (even very thin). Just don't assign any material to that
region, and it becomes automatically a blackhole. Normally you 
have a blackhole all around your geometry, but you can have some 
blackhole regions anywhere. 
To stop only a few selected particles, set instead a very high 
threshold in that region for the type of particles you want to stop.
To do this, use PART-THR for hadrons and muons, and EMFCUT for
electrons and photons.

Best regards,

       Alberto

On Wed, 11 Apr 2001, Baerbel Naumann wrote:

> Dear FLUKA users and FLUKA authors,
> 
> I have 2 questions concerning the use of the FLUKA code :
> 
> 1)
> I want to check the implementation of special nuclear reactions
> inside the FLUKA code during the tracking. For instance, I want
> to know the cross section of the 181Ta(gamma,2n) reaction at
> photon energies between 15 and 22 MeV. How can I do it in an 
> effective way ? Can I use the USRYIELD option and which parameters 
> I have to give in this case ?
> 
> 2)
> Is there a possibility to stop the tracking of particles inside
> the code ? This would be good for optimization of the CPU-time,
> if I want to simulate the particle flux inside a small detector, 
> which is in a distance of some meters from the target.
> 
> Many thanks in advance, regards
> 
> Baerbel
> 

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