Re: [fluka-discuss]: Alpha tracks and secondaries

From: Francesco Cerutti <Francesco.Cerutti_at_cern.ch>
Date: Wed, 3 May 2017 11:16:02 +0200

Hallo Jean-Emmanuel,

your findings are full of sense and well predictable.

1. The *physical* upper cutoff of the secondary electron (delta) spectrum,
for a non-relativistic particle much heavier than a electron, is

T_max= 2 * 511 keV * 2T / (M c**2)

where T is the particle kinetic energy and (M c**2) its mass.

Hence your numbers follow and one easily concludes that for protons below
500 keV and alphas below 2 MeV, T_max is indeed less than 1 keV.
Note that this is a natural limit.

Now, there is no way to further lower below 1 keV the electron transport
threshold, since the physical assumptions of the condensed history
approach adopted by general purpose Monte Carlo codes do no longer apply.
This is a limit of FLUKA and analogous tools.

2. MULSOPT allows to activate single scattering. This has an impact on
particle trajectory, shortening steps as you noticed in order to improve
accuracy in the Coulomb deflection computation. The latter is dominated by
interactions with atomic nuclei, while energy loss (and delta production)
is mostly (exclusively) related to interactions with atomic electrons.
Therefore, no surprise at all in ascertaining that MULSOPT - by
construction - has no effect on delta generation.

Best regards

Francesco

**************************************************
Francesco Cerutti
CERN-EN/STI
CH-1211 Geneva 23
Switzerland
tel. ++41 22 7678962
fax ++41 22 7668854

On Tue, 2 May 2017, Jean-Emmanuel Groetz wrote:

> Hello,
>
> Since my request one month ago about alpha particle tracking in water
> (thanks to Mary for giving me some advices), I used the mgdraw
> subroutine to get the energy deposition and the secondary electrons
> that could be produced.
>
> If my primaries are alpha at 5.87 MeV, the delta ray secondaries
> (ICODE=103 from usdraw entry in mgdraw.f) are generated by alpha
> particles from 5.87 MeV to 2 MeV, never below this limit. This
> threshold is lowered at 500 keV for protons as primaries.
> Another thing that I would like enlighten is the influence of the
> MULSOPT card. If this one is active, the number of alpha tracks is
> significantly higher, with shorter track segments, but the number of
> secondaries is unchanged compared to the case where the MULSOPT card
> was disable.
> I have checked that the energy of every alpha particle matches to the
> sum of the deposited energy along every track segment + the sum of
> energies for all secondaries produced.
> Particle thresholds were lowered to 1 keV, as well as the delta-ray
> production.
>
> Is there a way to lower this energy limit, i.e that alpha particles
> below 2 MeV could also produce secondaries?
>
> Many thanks.
>
> Jean-Emmanuel
>
>
> --
> _______________________________________________
> Jean-Emmanuel Groetz
> Laboratoire Chrono-Environnement UMR CNRS 6249
> UniversitÚ de Franche-ComtÚ
> 16 route de Gray
> 25030 Besanšon Cedex
> France
>
> mailto : jegroetz_at_univ-fcomte.fr
> tel : +33 (0) 3 81 66 65 07
> fax : +33 (0) 3 81 66 65 22
> http://chrono-environnement.univ-fcomte.fr/spip.php?article1537
> _______________________________________________
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Received on Wed May 03 2017 - 12:10:16 CEST

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