As Paola pointed out, mean excitation potential <I> of liquid water that
was suggested as 75.0eV by protocols and worked reasonably well for photon
and electron beams may not be suitable for protons or ions.
Accordong to our recent research combining theoretical and simulations
approaches along with comparisons with experimental data (in water
ionizations for proton beams within clinical energy range), a suitable
value for <I> is (77+-1)eV.
FLUKA main code has the great advantage of allowing user to (carefully)
change <I> value, so I may suggest to repeat your calculations modifying
this parameter that strongly affect stopping power.
best regards,
Il giorno mer 27 giu 2018 alle 08:39 paola sala <paola.sala_at_cern.ch> ha
scritto:
> Hi Ryan,
> on your slab dimensions, stopping power is constant up to the 4th digit.
> The difference you see is due to nuclear interactions . You can see in
> the .out file that there are some (few) of them. You can check by
> yourself, using the THRESHOL card to turn off inelastic interactions.
> If you are interested in precision, ICRU recently re-evaluated the
> stopping powers for some materials, including water. This essentially
> because the mean excitation energy (I) has been reevaluated to be 78 eV
> instead of 75.
>
> https://www.nist.gov/sites/default/files/documents/2017/04/26/newstar.pdf
>
> To get these new values with FLUKA one should use the MAT-PROP card and
> change the value for I.
>
> Hope this helps
> Paola
> On 06/27/2018 11:07 AM, Francesco Cerutti wrote:
> >
> > Hi Ryan,
> >
> > you are missing the fundamental concept that stopping power is not
> > constant. It's increasing for decreasing energy (your proton is losing
> > energy through the slab), which explains the higher energy deposition
> > you found.
> >
> > Moreover, note that your expectation misses also the fact that delta
> > electrons, generated by proton induced ionization and contributing to
> > the proton stopping power, may actually leave your reference volume
> > and deposit some energy outside.
> >
> > Kind regards
> >
> > Francesco
> >
> > **************************************************
> > Francesco Cerutti
> > CERN-EN/STI
> > CH-1211 Geneva 23
> > Switzerland
> > tel. +41 22 7678962
> > fax +41 22 7668854
> >
> > On Wed, 27 Jun 2018, Ryan Brosch wrote:
> >
> >> Dear FLUKA experts,
> >> I have a simple setup of a 200 MeV proton beam incident on a 1mm slab of
> >> water. I am scoring energy deposition with SCORE.
> >>
> >> The stopping power for a 200 MeV proton is 4.491 MeV/cm or 449 KeV in
> >> a 1mm
> >> slab.
> >>
> >> However, SCORE shows about 463 KeV of energy deposition, 14 KeV more
> >> than
> >> expected. This is to much energy to be from nuclear stopping power.
> >>
> >> Am I missing some physics here? Surely the energy deposition should be
> >> almost entirely due to inelastic coulomb scattering and should just be
> >> dE=(dE/dz)dz where dE/dz is the stopping power.
> >>
> >> Thanks very much,
> >>
> >> --
> >> Ryan Brosch
> >> Ph.D Candidate
> >> Department of Physics
> >> Arizona State University
> >>
> >>
>
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> --
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Received on Wed Jun 27 2018 - 22:46:07 CEST