How to understand ABSLENGTH of material properties table (MPT)

Physics Processes, Models and Cross Sections
1

In the Book for application developers,

The absorption length is the average distance traveled by a photon before being absorbed by the medium; i.e., it is the mean free path returned by the GetMeanFreePath method. (Physics Processes — Book For Application Developers 11.3 documentation)

As far as I understand, the absorption length (λabs) is equal to the attenuation length (λatt) if the scattering process is negligible, and there is I(x) = I0 e-x/λabs (Eq. 1).
The live track length distribution is expected to be like Eq. 1. Here is the live track length distribution I got. It does look like e-x/λabs but the λabs is different from the value I set (280 vs. 380). Is there anything I misunderstood?

image
image496×387 3.18 KB

P.S. The track length is obtained by

        if (processName == "OpAbsorption")
        {
            std::ofstream outFile;
            outFile.open("OpAbsorptionLength.txt", std::ios_base::app);
            outFile << step->GetTrack()->GetTrackLength() / cm << G4endl;
            outFile.close();
        }

Geant4 Version: 11.2.1
Operating System: Windows 11
Compiler/Version: VS 2022
CMake Version: 3.31.1

2

I think your understanding is correct.

I can’t tell from the plot what lambda is, or what the uncertainty might be.

And, strange things might get swept up in GetTrackLength. What if the optical photon leaves the volume? Probably you’ve checked that, but maybe there’s something else. I’d set up a simple geometry, make sure there is no other scattering, shoot photons along an axis, and record where they end up. (OpNovice2 could be modified easily for this purpose.)

Whichever application you use, look at the verbose text output and visualization to make sure nothing strange is happening.

3

Thanks for your response.

I tested ABSLENGTH using OpNovice2 as you suggested. The distribution of the optical photon’s endpoint satisfies Eq. 1 with a good λabs.

My question now is, why does it not satisfy Eq. 1 in my application? Is it because of the reflection at the boundary in the scintillator?