The Tkr multiplicity has been observed to exceed significantly that predicted by the Monte-Carlo simulation. This effect has been attributed to low-energy secondary electrons
produced in the upstream material, whose thickness is underestimated in the current simulation. The figure below compares the distributions for a run with the second arm
of the spectrometer in the beam amounting to more 10% X0 (red, run 811), for a run with the arm out of the beam (purple, run 916) and for the Monte-Carlo simulation (blue, run 122).
!Tkrhits.gif!