I'm reporting a bug I first noticed in February of 2020. At that time I was using Geant 10.6. I have not tried to reproduce the bug in a more recent version of Geant. Another user on the Geant4 forums recently reported they are having a similar issue so I am taking my post from the forums and copying it here:


"I’m cross activating DNA Physics and EMStandardPhysics_Option4 as shown in examples/extended/medical/dna/dnaphysics. I am then simulating decay of Rn-220.

Use of the macro commands:
/gun/particle ion
/gun/ion 86 220
/run/beamOn 1

Will only generate Rn220 primary particles without radioactive decay. If I explicitly state the initial ion energy is 0 through (/gun/energy 0 keV) I can get 1 decay in the decay chain down to Po-216. However if I deactivate DNA physics I get full decay through the decay chain as expected.

I have tracked this behavior to the “DeactivateNuclearStopping” function in G4EmDNAPhysicsActivator.cc file. If I set the low energy limit for the nuclear stopping model to 0 rather than 1 MeV I get the standard decay behavior. Is anyone aware of an appropriate way to use DNA physics and radioactive decay physics at the same time? (As I’m sure the change I’ve suggested above is not a solution. I know the low energy limit is there for a good reason).

To test this behavior from the dnaphysics example:
1.) Add radioactive decay physics to the physics list
2.) G4cout the particle name in stepping action (Filter out alpha, alpha+, helium, and e- to only get information on radionuclides)
3.) Modify the macro input to generate Rn-220"