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On Fri, 27 Jun 2014 10:04:07 GMT, Debasish Datta wrote:
> I am quite a novice user of GEANT4 and trying to simulate the effect of > energetic ion (with keV - MeV energy) impact on metal nanoparticles. In > such cases, it is an well known fact that some atoms of the target > materials (here, the nanoparticles) will be sputtered out and the target > will get eroded. So my question is, can it be possible to incorporate > the effect of such erosion and subsequent target modification in the > GEANT simulation? In other words, I need to change the target > composition and geometry slightly after each ion impact. Can I do that?
As Andrea wrote, you may define a geometry which changes from one run (i.e., between /run/beamOn commands) to the next (the geometry may not be substantially changed during a run; it is said to be "locked").
GEANT4 does not do modification of materials as a result of irradiation, since each event is typically a single incident particle, and any change to the material composition is negligible at that scale (less than ~ 10^-24). In your case, the target is actually very small (a few thousands of atoms, I guess), so the erosion is measurable.
You could write your job to be C++ driven, rather than macro based: Have a loop where you invoke G4RunManager::BeamOn(n); collect scoring data, perhaps using a TrackingAction, where you count how many atoms were ejected; after each run, use the total atom count to reduce the target mass and volume appropriately; then iterate the loop. This can probably all be done with macros as well, but I haven't personally used those commands.
> In short...Is it possible to define the crystal structure (like Cubic, > Face-centered cubic etc.) of the targets in Geant4?
No. Materials in GEANT4 are treated as amorphous and continuous. The latter means that when a particle's path length is generated, whatever its value, a target atom is "guaranteed" to be at that location in the volume.
There are two examples in the GEANT4 10.1-beta release, which do provide examples of physics processes in defined crystal structures: extended/exoticphysics/phonons, where the lattice structure determines the phonon modes and speeds; and extended/exoticphysics/channeling, where the crystal structure affects the transportation of high energy particles through the volume. These are not part of the toolkit, but may provide some guidance on how you could add such functionality to your own simulation.
-- Michael Kelsey