Attenuation coefficient isn't consistent with NIST tables

Hi Guys,

I hope I’m in the right forum for this. I’m trying to perform computed tomography using projections made with Geant4. I’m creating materials based on NIST elements.

When running the CT algorithm using projections I got for Geant4, I’m able to recover with good accuracy attenuation coefficients (when comparing to the attenuation coefficients given in NIST tables) of elements with low z (i.e. H, C, N, O) but for elements with higher Z values (P and Ca) I’m not recovering the right values, the values I recover are about half of what I expected.

From what do you think this can arise? I’m using the FTFP_BERT_LIV physics list and use gamma particles with up to 120keV energy. Could I be using a physics list that isn’t a good fit?


please could you provide more details on the simulation configuration you adopt?

Thank you

Thank you for responding!

I have simplified the testing but still seem to find a mismatch between the results I get and what I expect.
My setup consists of a single gamma-ray source with a spectrum of up to 150keV, a single voxel of Calcium with a density of 1 g/cm3 and a detector.
I shoot 1E7 particles through the voxel, and 1E7 with no voxel to get I and I0 and use the formula:
mu = -log(I/I0) / L to recover the attenuation coefficient, L is the length of the voxel.
From this simulation I get an attenuation coefficient of 0.833 cm^-1.

I compare this result to the values I find at:

According to the table I expect to get a result of 1.549 cm^-1. I get this when averaging the according to the spectrum of the source I use.

I attach the spectrum I use, and also the physics list file I use to construct the physics of the simulation.Figure_1|640x480 (4.5 KB)

I hope this is enough information, and thank you very much.

Hi, yonigat,

You can easily check what coefficient is used for defined materials by compiling Geant4 example

It will print a stopping power table for specified energy and particle.

Hi, voronin,

Your answer was a great help, thank you!
There wasn’t really a problem, just had to change the dimension of the geometry a bit for lower energies.