Inhomogenous fluence - why?

Scoring
1

Dear FLUKA Experts,

please excuse me if this is a silly question. Being aware that fluence is evaluated as the density of particle tracks, I am still not able to understand why the fluence of a homogenoeus beam becomes inhomogeneous in presence of regions with different densities, being higher in the region with higher density, even in setups for which I’d expect a homogeneous fluence scoring. For instance, consider the case of a 2 MeV electron beam which perpendicularly (along the z axis) impacts on the circular face of a copper cylinder, the transversal size of the beam beam larger than this face. Why is xy-fluence higher inside the central area even though the USRBIN detector is placed before the cylinder and does not include it? In the project of the example picture, a 5x5 cm2 flat beam starts from z = -1 cm, the copper cylinder of radius 1 cm is placed along z from z = 0 to z = 1 cm, and the xyz USRBIN is placed between z = -5 mm and z = -1 mm. I upload the project of this example setup.

I’m grateful for your help,
Enrico

fluence test.inp (1.2 KB)
fluence test.flair (3.2 KB)

image

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Dear Enrico,

what you are seeing is the effect of the elastically backscattered electrons. Since these electrons don’t lose energy, they are still considered BEAMPART (their generation number remains 1).

Cheers,
David

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3

Dear David,

that indeed explains what I see. Thank you very much!

Cheers,
Enrico

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P.S. Out of curiosity, does that mean that some electrons are scored twice?

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Dear Enrico,

Yes. Since the backscattered electrons pass through the volume twice, their track length is scored in both cases.

Cheers,
David

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