EM scattering at low density

Hello there,

I am simulating a deuteron beam at 0.5 MeV Coulomb scattering off a very low density (1e-9 g/cm3) proton gas target.
The standard EM single scatter physics list gives the kinematics and scatter rate I expect to observe however produces a higher than expected backscatter rate on my detector (Si). Using the Livermore EM, the rate and kinematics aren’t what I expect but the backscatter rate on the detector is much better.

Is it expected that multiple scattering physics processes aren’t applicable at such low densities, requiring the single scatter models and which model would be better for looking at my detector interactions?

Many thanks