Additional shielding behind the LHCb muon detector

This note presents studies on the effect of backsplash hits in the last station of the LHCb muon detector (M5) and the results of Monte Carlo simulations of various additional shielding behind M5. These particles are delayed in comparison with normal muons coming from the interaction point, so the backsplashes are late by up to 50 ns. During the winter 2011/2012 shutdown, ~ 5 tons of iron were installed to reduce the backsplashes. The iron was placed on the top of the last muon filter (MF4), in front of the concrete beams above the corrector magnet. After Long Shutdown 1 (LS1), the beam energy will be increased to ~ 6.5 TeV (now is 4 TeV) and the spacing between bunches decreased to 25 ns (now is 50 ns). It will cause a big increase of number of backsplash hits (approximately 2.25 times more). This document focuses on possible methods of installing additional shielding behind M5. Various methods were tested using MC simulations. In each kind of simulated shielding, the distribution and the number of hits were analyzed. The most effective shielding requires ~ 42 tons of extra iron and reduce backsplashes by more than a factor 3.5 (comparing to estimated number of backsplashes after LS1 in case of no use additional shielding), bringing them after LS1 to a level ~ 40% less than during the 2012 run.