Message: Re: Electric field and "local time goes back" issue Not Logged In (login)

## Re: Electric field and "local time goes back" issue

Forum: Fields: Magnetic and Otherwise
Re: Mixed Electromagnetic field and ChordFinder (Francois Mauger)
Re: Electric field and "local time goes back" issue (Francois Mauger)
Re: Re: Electric field and "local time goes back" issue (John Apostolakis)
Date: 25 Apr, 2015
From: Francois Mauger <Francois Mauger>

 ```Dear John, Finally I was able to install G4.9.6.p04 and make it work with my SW framework. The geometry setup is rather simple: a box of STP air with various simple daughter volumes of solid materials (lead copper, plastic...) in it. I put a E_y electric field of 3e5 V/m in the world volume (filled with STP air) and I ran again my G4 simulation with 1 MeV electrons shot in 4pi from a thin film representing a radioactive source in front of what represents an "optical module" (scintillator+light guide+PMT). Then let G4 track the electrons through the setup. Unfortunately, the bug is still here, despite the patch. So I made some investigations. First I have changed the intensity of the field to see if the mean number of "invalid steps" (those with backward times) is changing. The results are in the table below, where E_y is the intensity of the electric field and P(path#) is the proportion of events that have at a "bad step":
------------------------------------------------------ E_y (V/m)| 3e5  | 3e4  | 3e3 | 3e2 |  30 |  3  |  0  | ------------------------------------------------------ P(p01)   |  ~2% | idem | idem| idem| idem| idem| idem| ------------------------------------------------------ P(p04)   |  ~2% | idem | idem| idem| idem| idem| idem| ------------------------------------------------------
Conclusion: - no difference between p01 and p04 - no dependance with the intensity of the E field More, even if the E field is strictly zero (i.e. no field) the bug occurs with the same intensity! Then, using the "G4ParticleChange Information" dumped at each bad step, I have built the histogram of the kinetic energy of the electrons that are involved in the "bug". The conclusion is that most of them are rather low energy, below 50 keV. So I suspect these are delta-rays, randomly scattered along the primary tracks. Next I have plotted the positions of the "bad steps". They can occur everywhere in the geometry, even in the gas where no strong stepping algorithm constraints exist (MS at gas->solid transition), but with a larger probability in high density materials, particularly near the surfaces of solid volumes where particles leave the gas to enter a solid detector. So again, this is not a surprise because, I expect a rather big amount of generated delta-rays as soon as a primary electron enters a solid volume, so near its surface (a few mm). I consider this does not give ant valuable hint to understand the nature of the problem. This is all for now. I envisage to investigate the role of the tuning parameters provided by G4. My current setup uses the following (G4 default) values:
double delta_chord = 0.25 * CLHEP::mm; double min_step = 0.01 * CLHEP::mm; double delta_one_step = 0.01 * CLHEP::mm; double delta_intersection = 1.0 * CLHEP::micrometer; double eps_max = 0.001; double eps_min = 5e-5;