doctoral thesis

iv
the University of Graz [2] and were published in [3, 4]. The simulation model presented
in [1] is completed by the dynamic calculation of the space charge field using
these formulas. Since the gas parameters like drift velocity and the Townsend and attachment
coefficients depend on the electric field, they are calculated dynamically as
well. The functional dependence of these parameters on the field is obtained with the
simulation programs MAGBOLTZ and IMONTE. For the primary ionization parameters,
we use the values that are predicted by the program HEED. While the described
procedure only simulates the longitudinal avalanche development towards the anode
of the RPC, we also present more dimensional models that allow a careful study of
the transverse repulsive and attractive forces of the space charge fields, and of the
consequences for the avalanche propagation.
We shall show that the efficiencies of single gap Timing RPCs is indeed explained
by the high primary ionization density (about 9.5 /cm as predicted by HEED) and a
large effective Townsend coefficient (around 113 /mm as predicted by IMONTE). We
show that the space charge field reaches the same magnitude as the applied electric
field in avalanches at large gas gain. This strong space charge effect effectively suppresses
large values for the avalanche charges. The shape of the simulated charge
spectra is very similar to the measurements. Also the simulated average charges are
close to the experimental results. RPCs are operated in a strong space charge regime
over a large range of applied voltage, contrary to wire chambers.
We apply only standard detector physics simulations to RPCs. The performance of
Timing and Trigger RPCs is well reproduced by our simulations. The results concerning
the space charge effect were presented and discussed at the ’RPC 2001’ workshop
[5] and on the ’2002 NSS/MIC’ conference [6].