Factors to Scale Highly Ionizing Particle Rates in MSGC Irradiation Tests to the LHC Radiation Environment

A thorough discussion of highly ionizing particle production in microstrip gas chambers, exposed to hadron irradiation, is given. It is shown that inelastic hadronic interactions are the dominant source of very highly ionizing particles. The heaviest ions expected in the MSGC gas are fission fragments from the gold strips. Since each ion has a characteristic maximum ionization, which cannot be exceeded, the choice of ions to test the chambers is critical. Detailed simulations for the predicted LHC radiation environment, two different beam test conditions and one alpha-irradiation test have been performed and the results have been used to derive the factors, which can be applied to relate the test beam or irradiation data to detector life-expectancy at LHC. The results verify that, of the three alternatives studied, the best fit to the LHC conditions is obtained with a 300 MeV/c pion bea m. It is also shown that chambers which can tolerate very high local ionization, cannot be reliably tested with alpha-particles since these do not reach the highest ionization densities expected from the heaviest fragments at LHC.