In 2005 there was an in vacuum water leak located in the crotch absorber of cell 15. It resulted in a 5-day loss of USM. Soon afterwards, we realised that all other crotches were developing the same type of damage and were expected to break over the coming years. The problem is linked to the presence of a high flux of X-rays in the water producing very active radicals that combine with the copper and slowly erode the copper tube walls.

A modified version was quickly developed with twice the copper thickness in order to reduce the X-ray flux. A new series of crotch absorbers was ordered making use of glidcop (dispersion strengthened copper) in order to keep good mechanical resistance at high temperatures. Seven out of the thirty-two cells of the ring were let to atmospheric pressure in 2006 to replace the older crotches with new ones. The work was performed during the scheduled shutdown and there were no consequences for the user programme. The systematic replacement will continue throughout 2007 and should be completed in early 2008. Other activities of the vacuum group included the installation of another set of four narrow aperture ID vessels equipped with a non-evaporable getter (NEG) coating, a new in-vacuum undulator in cell 9 as well as the replacement of a few other chambers.