Effects of ion drift on time-over-threshold charge estimates using first-order integrators in thermal neutron MWPC

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.