Different results using Geant4 10.6 and 10.7

Hello, I’ve just installed Geant4 10.7…I runned some simulations and I notice some differences between the results that I got using Geant4 10.6 and now using Geant4 10.7 (notice that I didn’t change any line of the code!) …for example I got 1540 muons produced using Geant4 10.6 and I’m getting 1476 muons using Geant4 10.6…

how is it possible?

Isn’t that just within the statistical fluctuation range? Repeating the same simulation with a new seed will change typically by that much.

Hi @Jerry_Nolen obviously the results are compatible, but unfortunatly I had plots showing a number and now I’ve other number…

Every time you run a new simulation starting with a new seed you should get a slightly different number of muons.

But I didn’t use any seed (I also don’t know hot to change the seed)…
look this is the simulation

You see…there aren’t seeds…

There are always seeds; that how random number generator work. In the Geant4 toolkit, the same seeds are used at the start of the job, to allow for reproducibility and debugging. Users who run very large samples, with multiple jobs, will have their own method for setting unique, and different, seeds for each job to avoid generate the same events over and over again.

When you switch Geant4 versions, obviously there will be changes to some of the physic processes, which means that the detailed sequence of steps taken by a track will be different. That means that the sequence of random numbers generated over the course of the job will be different: the second event will start with a different random number in your G4 10.7 job than it did in your 10.6 job.

Thus, at the end of the day you’ll have numerically different results, but those results should be statistically equivalent. A very, very crude test of that is simply +/-sqrt(N). You wrote:

sqrt(1508) = 38.83. Both of your individual results are within +/-1 sigma of their average, so I don’t see an obvious problem.

Hi @mkelsey I know that results are eequivalents, it was just to have the same numbers but…ok no problem!
Instead, more important, do you know how to solve this Geant4 step size ?