Problem 1994

Currently, we use a parameterization for the density effect from [1], which is from 1971 and designed for easy calculation for people without computers.  In the case of certain materials, the tables from [2] are used instead, which are more accurate, but many (most?) detectors aren't made from these materials.  This can result in Geant4 using dE/dx values up to a few percent different from the best estimates.

According to [3], "Given the power of modern computers, experts now calculate the density effect from first principles rather than use these formulae".  It would be ideal if Geant4 could do this on the fly for arbitrary materials.  As a intermediate step, a smaller, but substantial, improvement would be some method to tell Geant4 that one's material is similar to one or more tabulated materials.  For instance, mineral oil (untabulated) is very similar chemically to paraffin wax (tabulated).  It would be more accurate to use the paraffin wax values than generic ones.

This is specifically motivated by studies of the muon dE/dx uncertainties in the NOvA detector, which is constructed out of untabulated hydrocarbons.  The density effect parameterization turns out to be a dominant effect.

I certainly recognize that this may be a significant effort and that it is unlikely that it is going to happen immediately, but I am hoping that it can be included on the TODO list for future versions.


[1] R. M. Sternheimer and R. F. Peierls. General expression for the density effect for the ionization loss of charged particles. Phys. Rev., B3:3681-­3692, 1971.

[2] R. M. Sternheimer, M. J. Berger, and S. M. Seltzer. Density effect for the ionization loss of charged particles in various substances. Atom. Data Nucl. Data Tabl., 30:261, 1984.

[3] D.E. Groom, N.V. Mokhov, and S.I. Striganov. Muon stopping power and range tables 10 MeV-100 TeV. Atomic Data and Nuclear Data Tables, 78(2):183-356, 2001.