Unexpected Activity Changes

Physics Processes, Models and Cross Sections
1

  • Greetings Fellow Users,

    I am working with the STCyclotron example (from the advanced category of the Geant4 distribution) to simulate radionuclide production. The code runs, and I can generate output files and ROOT plots without issue. Before relying on the Monte Carlo results, however, I am trying to validate the configuration against basic physical expectations.

    I’ve run into a puzzling inconsistency that I have not been able to resolve.

    ISSUE:

    For simulations where the beam radius ≤ target radius, and the total proton current remains fixed, I obtain significantly different end-of-bombardment activities when I change either:

    • the target radius (beam radius fixed), or

    • the beam radius (target radius fixed).

    These changes produce large differences in the integrated activity reported in Output_ParentIsotopes.txt, even though the total number of incident protons is the same in all runs. For example, given a fixed target radius of 3.0 mm, this is what I get when I vary the spot beam radius:
    3.0 mm radius → 292.6 mCi
    2.5 mm radius → 348.9 mCi
    2.0 mm radius → 431.2 mCi

    From my understanding, in the ideal case, for a sufficiently thick, uniform target that stops all protons, the total number of nuclear reactions should not depend on the beam spot size. A smaller beam should simply produce a more concentrated activation distribution, but the total number of reactions should be the same as long as:

    • the beam is fully intercepted by the target,

    • the beam current and irradiation time are unchanged, and

    • proton stopping and nuclear cross sections do not depend on lateral position.

    I had in mind results similar to that found in Figure 4 of (Yu and Chen 2025) https://doi.org/10.1016/j.radphyschem.2025.112695

  • figure 4
    figure 41722×1303 185 KB

    Could it be that activity integration in STCyclotron might be scaling with the entire target geometry rather than the irradiated area, but I have not yet identified where in the code this occurs.

    Thank you in advance for any insight.

Geant4 Version: 11.3.2
Operating System: Linux
Compiler/Version: Ubuntu
CMake Version: