Accuracy of LHC proton loss rate determination by the BLM system

Most of the monitors of the LHC beam loss monitoring (BLM) system are installed on the outside of the magnet cryostats, around the quadrupole magnets. Their aim is to prevent quenches and to protect the superconducting magnets from damage. The lost beam particles initiate hadronic showers through the magnets and deposit energy in the coils. The gas filled BLM ionization chambers probe the very far transverse tail of the showers. The BLM system relies on GEANT simulations and control measurements to determine the relation between the chamber signal, the number of lost beam particles and the energy deposited in the magnet coil. The specification of the BLM system includes a factor of two in absolute precision on the final prediction of the quench levels. As the shower tails are not necessarily well represented by particle simulation codes, it is crucial to experimentally determine the accuracy of these simulations. An LHC type BLM system was installed at the internal beam dump of HERA at DESY since 2005. The hadronic showers created by the impacting 39 GeV and 920 GeV protons have been simulated with GEANT4. The far transverse tails of the showers on the outside of the dump have been measured by ionization chambers. This paper will present the comparison of simulation to measurement and the conclusions drawn for the LHC BLM system.