Irradiation of an IBL stave in a 10MeV beta beam

The new IBL detector (Insertable B-Layer), due to be integrated into the ATLAS detector as the closest Pixel layer to the beam pipe during the first LHC long-shutdown (2013), is composed of fourteen stave sub-assemblies. These staves have a triangular cross section carbon foam core, sandwiched between a triangular carbon fibre plate (omega) and a flat top plate (faceplate) onto which the silicon detector modules and services are glued. The assembly is strongly dependant on glue and thermal grease interfaces; high density amorphous materials prone to degradation under high radiation doses. In order to evaluate the mechanical stability and integrity of one of these staves it was decided to impose a very high radiation dose upon it, representative of the full dose IBL will receive in its life-cycle. Because of the lack of availability of proton beams - the closest approximation to the radiation received in operation - with a large enough sweep area and dose it was decided to undertake the experiment using an industrial electron beam. The ionization itself was performed at Ionisos in France, while the pre and post-irradiation tests were shared between CERN and the University of Geneva. After having been exposed to a beta dose equivalent to twice that expected in the IBL lifetime, a series of metrology surveys and thermal cycles were performed. It appears from metrology comparisons between the various steps in the experiment that the stave's stability was only minimally affected, with a maximum faceplate movement of 60 $\mu$m between the pre and post-irradiation metrologies. However it appears that the heating effects were of importance considering the grease pads had ran slightly when inspected after the irradiation. The temperature monitoring confirms that the stave experienced excessive temperatures, about 20$^o$C higher than the maximum temperature experienced in operation, during the experiment. It is therefore complicated to separate the consequences of thermal and ionization effects on the stave. A series of peel samples irradiated with the stave in a beta beam as well as in a proton beam were used to compare the effect of these two types of irradiation on the glue interfaces.