Ionization potential NIST Compounds

Dear Geant4 Forum,

I’m interested in the impact of the ionization potential on the position of the Bragg peak.

I know that the ionization potential is the energy required to remove an electron to an atom or a molecule and by default, the ionization potential is calculated thanks to the Bragg’s additivity rule.

But how was choose the ionization potential NIST Compounds?

For example, G4_Water is fixed at 78 eV when it is 75 eV at NIST.

More globally, this can be observed for ICRP materials.

Best regards,



Mean ionisation potential is a parameter which depend on atom composition of material, atomic and molecular shells. It is difficult to compute this parameter theoretically, so experimental values are used. For water 75 eV is an old recommendation (for example, ICRU37 report). Since INCU73 recommendations we are using 78 eV.


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Thank you very much for your response.

Indeed 78 eV also corresponds to the new ICRU 90 standard. But I was a bit confused because the report states that in this case the density is 0.9982 * (the impact is certainly negligible).

It is difficult to understand which report NIST Compound is based on because not all of them are commented on.



We have uploaded NIST data in 2005. NIST experts M.Trocme & S.Seltzer verified these data in 2013.

ICRU90 data are not yet Geant4 default, we will change water density if community will ask.