Question on DELTARAY + PRINT output and stopping power vs. proton energy

Physics, Transport and Magnetic Fields
1

Versions

FLUKA: 4-5.1

Flair: 3.4-5.2

Description

Hello everyone,

I am studying the stopping power of protons in a water medium using FLUKA, with no external fields applied. For this purpose, I am using the DELTARAY card with the PRINT option, aiming to understand the correlation between the proton stopping power and the beam energy.

However, when analyzing the *.out file, I noticed that some reported energy values are higher than the initial beam energy, which should not be physically possible, since the beam should not gain energy while propagating through the medium.

Therefore, I would like to clarify a few points:

  1. Do the energy values reported by DELTARAY with PRINT correspond directly to the actual kinetic energy of the primary proton along its transport, or can they be associated with other effects (e.g. delta-ray production, locally transferred energy, or internal averaging procedures)?

  2. In this context, does FLUKA provide a fitted or reconstructed correlation between stopping power and beam energy based on its internal databases, rather than reporting the instantaneous kinetic energy of the primary particle?

  3. Is it possible to obtain a physically meaningful stopping power vs. proton energy curve using the USRBIN estimator?

    • If so, what would be the recommended setup (e.g. choice of quantity, binning, and post-processing) to extract this correlation correctly?

  4. Alternatively, is there another estimator or methodology within FLUKA that would be more appropriate to directly evaluate stopping power as a function of proton energy?

Any clarification or guidance would be greatly appreciated.

Best regards,
Raul

3

Dear Raul,

The tables you get from DELTARAY with print option enabled are the tables (i.e. the ‘database’) FLUKA uses internally. They are not the result of any actual tracking, they are the basis for said tracking.

It should be noted that the output tables are the RESTRICTED stopping power, excluding the explicitly tracked deltarays themselves. To obtain the unrestricted stopping power, you will have to disable deltaray production (by setting the deltaray production threshold <0.0). See here for details.

Best regards
Roman

4

Dear Raul,

I take the liberty of complementing Roman’s reply.

While the stopping power table is evaluated at fixed grid of energies, if you’re familiar with user routines, you can benefit from the ready-to-use solution reported in this post, which exploits the GETLET function to extract the stopping power (restricted or unrestricted) at any energy you like, without interpolating in the table.

Here’s the input/output argument documentation for the GETLET function:

      DOUBLE PRECISION FUNCTION GETLET ( IJ, EKIN, PLA, TDELTA, MATLET )

*         Input variables:                                             *
*                                                                      *
*               Ij = particle index (Paprop)                           *
*             Ekin = particle kinetic energy (GeV)                     *
*              Pla = particle momentum (GeV/c)                         *
*           Tdelta = maximum secondary electron energy (GeV)           *
*                   (unrestricted if =< 0)                             *
*           Matlet = material index for which LET is requested         *
*                                                                      *
*         Output variables:                                            *
*                                                                      *
*           Getlet = (un)restricted LET (keV/(um g/cm3))               *

NB: the material index indicated via the MATLET argument must be assigned to a geometry region. You can fish out the material index from the respective table in the output file.

Cheers,

Cesc