The GLAST team celebrated the installation of the 16th—and last—detector tower into the instrument this week. This telescope forms an image of the gamma ray sky by measuring the direction and energy of each gamma ray that passes through it (see http://www2.slac.stanford.edu/tip/2005/may6/glasttower.htm for an earlier story on the towers).

The direction is measured with a silicon strip detector called the tracker. The instrument has 800,000 of these strips to provide precise position information. It is the largest area silicon detector ever built, either for space or ground use. The tracker project was managed by Robert Johnson (UCSC). The modules were fabricated at INFN in Pisa, Italy. The Italian Space Agency (ASI) funded the fabrication effort and one half of the silicon (the other half was funded by the Japanese government). INFN funded the staff and much of the development work. SLAC personnel provided engineering and management support and supplied key components to the operation in Italy.

The energy of the gamma ray is measured with a cesium iodide detector called the calorimeter. Cesium iodide emits light when a particle passes through it and the light is measured by photodiodes at the end of the CsI logs. All of the logs together weigh approximately 1 and one-half tons (about half of the total weight of the instrument). The CsI was supplied by Sweden, the mechanical structure by France, and the units were completed and tested at the Naval Research Laboratory in Washington, DC. The calorimeter was managed by Neil Johnson of NRL.

The detectors were divided into 16 towers in the instrument for manufacturability. Now all of these towers have been assembled into the backbone of the instrument, the grid. This assembly and test work was done by the GLAST Integration and Test group at SLAC headed by Elliott Bloom.

Next the computers and software will be added the instrument and then a system test will be conducted. After that, the instrument will leave SLAC in January to go to NRL for environmental tests. The GLAST launch is scheduled for the fall of 2007.

RELATED STORY: Last Chance to See the Satellite