Numerical artifact on the spatial distribution of the proton beam

Dear FLUKA experts,

I am simulating the magnet based spectrometer for proton beam. The magnetic field is homogeneous and equal to Bx = 0.2 T. The magnetic field is set by activating teh card MGNFIELD and assigning 0.2 T to Bx. In ASSIGMA for magnet refion the field is set to magnetic.
The proton beam is deflected vertically and detected on the detector screen in plane X-Y. The 0.5 MeV proton source is spatially extentend in X and Y axis with Gaussian shape with Gaussian energy spread of 0.001 GeV/c. I am checking the proton energy spectrum by using USRTRACK card in the thin region before entering the spectrometer and the beam has the energy bandwidth of interest. Then by using USRBIN card for PROTON I am detecting the proton beam distribution in the XY plane of detector, where Y axis is referring to energy (due to the deflection of magnetic field). In case of mono energetic beam I would get just a proton beam with the initial size in both X and Y axis centered on the Y pos that corresponds to the deflection of proton beam of such energy and in case of the beam with energy spread - something extended in Y axis. I am getting expected results up to 0.0008 GeV/c bandwidth, then I start to get kind of stripes in X direction. I have attached the image below. The number of stripes is increasing with increase of the bandwidth.
I used the following binning parameters: Nx:600, Ny:600 and Nz:1. I tried to increase the binning, but it did not help. What could be the reason of appearing of such numerical artifacts?


Dear Sophia,

I’m assuming, that you are using the BEAM card for your source, and not a source routine. If my assumption is correct, then it is highly unlikely, that it is a numerical artifact.

I would look into the geometry or magnetic field to see of something could cause the problem. You could try to use an Z-Y scoring, to see the tracks of the protons.

Also, if you can share your input file, we could take a look.


Dear David,

Thank you for your reply and suggestions. Yes, I am using BEAM card.
I just checked the geometry and I increased the width of the magnetic field region by several milimeters and the stripe dissapeared. I think it was a problem of cutting the lower energy protons by the magnet structure.

Thank you!