High energy backscattering at 1.4GeV beam

Dear colleagues

In my simulation I am sending a 1.4 GeV proton beam on a thick iron target
(l=1.55m, r=50 cm). When looking at the result I see backscattered
protons between 1.2 GeV and 1.4 GeV. Within 6E6 protons sent on the
targets I found 3 protons with more than 1.2 GeV going backwards. I
separated these three tracks in a separate run introducing a proton
threshold of 1.2 GeV (see the picture in the attachment). Looks almost
like elastic scattering but I did not expect it at that energy with such
a large scattering angle. Although it concerns only a very minor
fraction of the final fluence, I would like to understand its reason. I
would be grateful for an explanation concerning the physics behind such
an event.

Thanks in advance

Cheers
Helmut