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None How best to simulate radiation 

Forum: Physics List
Date: 13 Jul, 2018
From: <mandragara>

Firstly, apologies if this is the wrong forum and apologies for the long post.

I am trying to simulate the interactions between low energy beta radiation and nanoparticles suspended in a water medium. These nanoparticles are small, with diameters ranging from 5 to 25 nm.

I am unsure of how best to set up my physics for this simulation.

Of particular concern is how the radiation will interact with the boundaries of my nanoparticles. Geant4 does a lot of clever things at boundaries, which I imagine doesn't impact larger scale simulations, however in my nanoscale simulations the physics seems heavily dominated by these boundary effects. I am aware of the fact that, for example, only single scattering and not multiple scattering occurs at boundaries.

I've outlined a more detailed description of my simulation setup and the issues that concern me below. I really value your input.

Simulation Geometry:

I have a cubic volume of side length 200 nm, filled with water. Within it I have spherical daughter volumes which represent nanoparticles, they range from 5 to 25 nm in diameter. I have a point source located on the surface of each nanoparticle. The point source emits e- particles isotropically with energies ranging from 100 eV to 1 MeV according to a predefined spectrum.

To demonstrate the effect I'm concerned about more clearly, I've replaced the aforementioned geometries with a box nested within another box, both filled with water. The attached figure is a 2D histogram of the number of hits recorded, one can clearly see the outline of the boundary.

One can see that the effect is more intense at the corners of the inner box.

Because my world is small, and I really care about the accuracy of the nanoscale interactions, my cuts should be as small as possible. So I have set them to 1 nm.

The effect of concern occurs when I just I have electron ionisation enabled (Penelope), so I'll just leave it at that for now. I've varied the eIoni parameters finalRange, linLossLimit and dRoverRange over a range of values but can't seem to minimise the boundary effect.

If it's relevant, my intended msc parameters are:

/process/msc/StepLimit UseSafety
/process/msc/LateralDisplacement true
/process/msc/MuHadLateralDisplacement false
/process/msc/DisplacementBeyondSafety true
/process/msc/RangeFactor 0.00001
/process/msc/GeomFactor 2.5
/process/msc/Skin 1 // my program won't accept a value of 0

I'm also using 500 bins for DEDX and Lamba binning, ranging from 100 eV to 1 GeV

My questions are:

1) Can I, and also should I, try and disable Geant4's clever treatment of boundaries to try and more consistently model the nanoscale interactions occurring?

2) How would you set up the physics for my nanoparticle geometry? Do my choices seem reasonable?


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1 Idea: Re: How best to simulate radiation   (Vladimir Ivanchenko - 07 Aug, 2018)
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