RUMP is ok. However, I used SIMNRA several times and I am quite sure that its calculations are correct. So if you "can not simulate correctly" with SIMNRA it's probably a simple matter with the configuration and I would encourage you to try SIMNRA until it fits well. Maybe you just missed a simple thing and your understanding will deepen once you get it right.
As additional info, there is a program called RbX which we used several times and which is used from the guys at the Dynamitron at the RUBION, Bochum, Germany.
see also Kótai, Nuclear Instruments and methods B 85 (1994) 588
However, Erik is right, all of these programs should do the same. So I also encourage you to find out what the problems with SIMNRA are!
As additional info, there is a program called RbX which we used several times and which is used from the guys at the Dynamitron at the RUBION, Bochum, Germany.
see also Kótai, Nuclear Instruments and methods B 85 (1994) 588
However, Erik is right, all of these programs should do the same. So I also encourage you to find out what the problems with SIMNRA are!