Aging phenomena in the LHCb outer tracker

The Outer Tracker (OT) consists of 53 760 straw tubes, covering in total an area of 360 m$^2$. The detector is foreseen to operate under large particle rates, up to 30 kHz/cm per straw in the region closest to the beam. Extensive aging tests conducted earlier on with prototype modules indicated excellent gain stability. In contrast, mass production modules show significant degradation under a rather model level of radiation. This paper presents the observed phenomenon, together with ongoing investigations to both prevent the effect, as well as to recover the gain loss.