Radiation length calculation

[cdevanne]

Bug Report

I might have found an obsolete calculation for the radiation length in the current FairRoot code, in the file fairroot/geobase/FairGeoMedium.cxx. and the function Bool_t FairGeoMedium::calcRadiationLength()

The formula used in FairRoot is correct. For example it's the same given by the (Particle Data Group 2024 in formula (34.25)
in the section 34.4.2

However the fitted parameters used in FairRoot are from "GEANT manual CONS110" and those numbers are not correct/outdated.

Therefore, in FairRoot we have:

• log(183)
• log(1440)

instead of:

• log(184.15)
• log(1194)

Explanation for those two set of values can be found in the paper from Tsai 1973 in section III.B Radiation Length of Materials, see from equation 3.66 to 3.70

other remarks

For curiosity, may I also ask what is the reason for calculating the radiation length manually in FairRoot rather than taking it straight from G4. (By the way, G4 uses the calculation from Tsai: see https://geant4.kek.jp/lxr/ident?i=ComputeLradTsaiFactor)

Thank you
Clément Devanne