The primary interaction of photons in the Fermi energy range with matter is pair conversion. This process forms the basis for the underlying measurement principle by providing a unique signature for gamma rays, which distinguishes them from charged cosmic rays whose flux is as much as 105 times larger, and allowing a determination of the incident photon directions via the reconstruction of the trajectories of the resulting e+e- pairs.
Incident radiation first passes through an anticoincidence shield, which is sensitive to charged particles, then through thin layers of high-Z material called conversion foils. Photon conversions are facilitated in the field of a heavy nucleus. After a conversion, the trajectories of the resulting electron and positron are measured by particle tracking detectors, and their energies are then measured by a calorimeter. The characteristic gamma-ray signature in the LAT is therefore (1) no signal in the anticoincidence shield, (2) more than one track starting from the same location within the volume of the tracker, and (3) an electromagnetic shower in the calorimeter.
The baseline LAT is modular, consisting of a 4 x 4 array of identical towers. Each 40 × 40 cm2 tower comprises a tracker, calorimeter and data acquisition module. The tracking detector consists of 18 xy layers of silicon strip detectors. This detector technology has a long and successful history of application in accelerator-based high-energy physics. It is well-matched to the requirements of high detection efficiency (>99%), excellent position resolution (<60 μm in this design),large signal:noise (>20:1), negligible cross-talk, and ease of trigger and readout with no consumables. The calorimeter in each tower consists of eight layers of 12 CsI bars in a hodoscopic arrangement, read out by photodiodes, for a total thickness of 10 radiation lengths. Owing to the hodoscopic configuration, the calorimeter can measure the three-dimensional profiles of showers, which permits corrections for energy leakage and enhances the capability to discriminate hadronic cosmic rays. The anticoincidence shield, which covers the array of towers, employs segmented tiles of scintillator, read out by wavelength-shifting fibers and miniature phototubes.
The instrument design is based on detailed computer simulations, validated with tests of prototype towers at particle accelerators. A complete software model of the instrument, including gaps, uninstrumented structural material, noise, inefficiencies and other real-world effects, was constructed using the object-oriented C++ GISMO toolkit. The computer model was used to generate simulated instrument data, and then to develop realistic reconstruction algorithms. The simulations were used to (1) demonstrate the necessary background rejection performance of the instrument, (2) produce realistic triggering and readout schemes, and (3) evaluate and optimize the performance of the instrument (effective area, angular resolution, etc.) after all background rejection cuts and instrumental effects have been taken into account. Accelerator tests of increasingly sophisticated prototype towers were made at the Stanford Linear Accelerator Center (1997 and 1999/2000) and the European Organization for Nuclear Research (CERN) in 1999.
The LAT is self triggered; events that cause detector hits in three planes automatically trigger readouts of each tower and the anticoincidence system. Efficient rejection of the charged particle background, which is thousands of times more intense than the celestial gamma-ray radiation, is essential for Fermi to function. (The expected raw trigger rate in orbit will average a few kHz, and the rate of celestial gamma rays will be a few Hz.) The anticoincidence system is only the first line of defense in identifying cosmic rays that trigger the telescope. As described above, from simulations, other discriminators against charged particles have been developed to further reduce the background level. Some of the discriminators will be applied onboard to reduce the trigger rate to the ~30 Hz rate that can be stored and downlinked.