[Gate-users] Potential bug in material definition?

[David Leibold]

Dear Gaters,

(Note: I’m using Gate on the vGate machine 9.0, including GATE version 9.0 and geant4-10-06-patch-02.)

I checked the physics in Gate by simulating the attenuation of a monoenergetic beam through phantoms of different compositions. So far I have checked water, LuAP, LYSO, aluminium, silicon, calcium and iodine. For all materials (except LuAP), the relative error to the NIST reference data is below 2 % in the energy range 10 – 150 keV.

In the case of LuAP, the results were initially off by about a factor of about 2 (not a constant factor, though).

This is the spectra I got:

[cid:3F136832-F645-468E-A35A-119B27288818]



The graph labelled with "n=..." is obtained by defining LuAP in Gate’s material database with the following definition, using integer ratios:

    LuAP: d=8.34 g/cm3; n=3 ; state=Solid
        +el: name=Lutetium ; n=1
        +el: name=Aluminium; n=1
        +el: name=Oxygen; n=3

Please note that this is the definition as shipped with the Gate examples.

The graph labelled with "f=..." is obtained by defining LuAP in Gate’s material database with the following definition, using float ratios of 1:

    LuAP: d=8.34 g/cm3; n=3 ; state=Solid
        +el: name=Lutetium ; f=0.2
        +el: name=Aluminium; f=0.2
        +el: name=Oxygen; f=0.6

The graph labelled "NIST" denotes the attenuation coefficient data as obtained via NIST, which perfectly aligns with the graph labelled “f=…”, i.e. defining the material as fractions of 1.

Except for how the LuAP material was defined in the Gate database, the Gate scripts and the evaluation code are the same.

As a sanity check and to see what material composition Gate is using, I decided to fit the individual attenuation coefficients for lutetium, aluminum and oxygen as obtained by NIST to the output data "n=...", i.e. the data that is off. Here's the fit result:


[cid:DC607FE3-CC31-4736-8D8F-6D68DAD45607]

The fit coefficients are as follows:

    0.69914713*NISTdata_Lu +
    0.13694515*NISTdata_Al +
    0.1097257 *NISTdata_O

As can be seen, the composition as used by Gate has little in common with the composition as defined in the database…

So is this a bug or am I doing something wrong?

Thanks in advance for your help!

Kind regards,
David Leibold
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