Problem 2568

Summary:	Outdated values for the mean excitation energy of carbon
Product:	Geant4	Reporter:	Andrew Sutton <asutton>
Component:	materials	Assignee:	Vladimir.Ivantchenko
Status:	RESOLVED FIXED
Severity:	normal
Priority:	P4
Version:	other
Hardware:	All
OS:	All

Description Andrew Sutton 2023-10-23 19:44:14 CEST

In bug #2474 it was reported that the mean excitation energy of carbon in G4NistMaterialBuilder differed from the NIST database. Additionally, it was noted that in G4DensityEffectData the value was set to 78. As a result, this value was revised from 81 eV down to 78 eV. 

Section 5.2 of ICRU report 90 has updated the recommended value of the carbon (graphite) mean excitation energy. Based on the combination of new measurements, the current ICRU recommendation is 81 ± 1.8 eV. 

Altering this value leads to a change in muon dE/dx of about ~0.5% for energies between 0.1 and ~10 MeV. While this value is small, it is a notable source of uncertainty for length-based estimation of muon energy. Therefore, I believe the most up-to-date recommendation should be implemented in G4NistMaterialBuilder.cc and G4DensityEffectData.cc

Comment 1 Vladimir.Ivantchenko 2023-10-25 19:55:49 CEST

Hello Andrew,

Thanks for this problem report. The value of mean ionisation potential (IP) for G4_C and G4_GRAPHITE will be set to 81 eV inside G4NistMaterialBuilder. NIST data on IP are propagated to G4Material and to all involved classes, so density effect data are overwritten. We prefer not to change the last, because these data were taken from the original publication.

Vladimir

Comment 2 Vladimir.Ivantchenko 2023-11-07 12:33:39 CET

Hello,

the fix will be available with the new public release 11.2 (December, 2023). It will not be included in the patches to previous versions, because the fix may slightly change results of mass productions already done by experiment groups.

VI