Problem 2312

Summary:	Wrong calculation of maximum energy transfer to secondary e- in G4BetheBlochModel
Product:	Geant4	Reporter:	Konstantin <konstik>
Component:	processes/electromagnetic	Assignee:	Vladimir.Ivantchenko
Status:	RESOLVED FIXED
Severity:	major	CC:	daren.sawkey
Priority:	P4
Version:	10.7
Hardware:	All
OS:	All

Description Konstantin 2021-01-17 23:33:33 CET

/source/processes/electromagnetic/standard/src/G4BetheBlochModel.cc
Function G4BetheBlochModel::MaxSecondaryEnergy calculates 'tmax' and limits it with 'tlimit', calculated in G4BetheBlochModel::SetupParameters with the aid of 'formfact'.

For proton:
G4version    formfact     tlimit [MeV]
4.10.04.p03  1.43948e-06  1.38939e+06
4.10.05.p01  1.548e-06    516.637
4.10.07      1.548e-06    516.637

Value of 516.637 MeV sets too small limit for protons with energy of several tens GeV and higher. Form factor has low influence on max transferable energy (and dE/dx) for such protons. For example, according to [J.D.Jackson, Electromagnetic form factor corrections to collisional energy loss of pions and protons, and spin correction for muons, PHYSICAL REVIEW D, VOLUME 59, 017301, (https://escholarship.org/uc/item/15x944gk)] the reduction of dE/dx due to form factor is less than 1% for protons of energies less than 700 GeV. 

The limited tmax from MaxSecondaryEnergy is also used in ComputeDEDXPerVolume, ComputeCrossSectionPerElectron, SampleSecondaries not only limiting max transferable energy for secondary electron but also changing their spectra via -(beta^2)(T/Tmax).

Comment 1 Vladimir.Ivantchenko 2021-01-26 19:51:36 CET

Hello Konstantin,

thank you very much for this report. This is a real bug, which relativistic hadron/ion ionisation. In fact, there was the problem how tlimit is computed and used. The problem is now fixed and the fix will be available in the 1st patch to 10.7 and for new releases.

Vladimir