Upgrade of the CMD-3 BGO Endcap Calorimeter

The BGO endcap calorimeter was successfully operated in the CMD-2 detector at the VEPP-2M collider. It consists of 680 BGO crystals, each with a size of 25×25×150 mm
³
, arranged in two identical endcaps, with a total weight of 450 kg. The light readout used vacuum phototriodes. The endcap calorimeter has been now upgraded to operate in the CMD-3 detector at the new VEPP-2000 collider. The major part of the endcap calorimeter, BGO crystals, remains the same. The main upgrade is the use of silicon PIN photodiodes and new electronics. The reason of changing a photosensitive device is a strong non-uniform magnetic field in the endcap calorimeter volume making the operation of the vacuum photosensitive devices impossible. We also had to reduce the space available for the endcap calorimeter. Hamamatsu PIN photodiodes S3590-08 were chosen as optimal devices as they are insensitive to magnetic fields and are both compact and stable. An order of magnitude larger capacitance of silicon photodiodes requires a design and production of the new custom electronics. The upgrade is expected to bring an overall improvement of the calorimeter parameters. The endcap calorimeter has been installed in the detector and operated during the data taking that started in 2010. Data analysis shows an improvement of the energy resolution compared to the CMD-2 detector.