The Effect of Highly Ionising Events on the APV25 Readout Chip

Inelastic nuclear interactions in silicon sensors can produce highly ionising particles, which in turn can generate signals equivalent of up to >>1000 minimum ionising particles. These highly ionising events have been observed to cause measurable deadtime in the CMS Tracker APV25 front-end readout chip. An analysis of beam test data and a simulation of the effect in the laboratory have provided measurements of both the rate at which non-negligible deadtime is observed and the deadtime resulting from a highly ionising event. Laboratory studies also indicate that through a suitable choice of a frontend hybrid resistor, the deadtime may be reduced. The predicted inefficiency of the CMS Tracker due to highly ionising events is at the sub-percent level.