  Time-over-threshold measurements of the output of a first order integration
amplifier can be used as a cheap way to estimate the charge deposited on a wire
in an MWPC. Even if the time scale of the first order system is seemingly much
larger than the signal development time, we noticed experimentally a
significant deviation from the relationship of time-over-threshold and the
charge, as would be naively expected from the exponential response of the
amplifier. We strongly suspect the current induced by the residual ion movement
to be at least partially responsible for this observation. It is of course
well-known that the ion movement is responsible for the bulk of the signal
generation in a wire chamber, this is not the point ; however, the surprise is
rather that this movement has influences on time scales which are so long that
the finite ion movement could be thought negligible. In this paper we will
treat theoretically the influence of ion movement and of AC coupling on the
expected time-over-threshold relationship and confront this to the measurements
on a small test MWPC in thermal neutron detection.
