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GLAST Reaches Orbit, Set To Begin Observations
Btarlinian writes "GLAST (the Gamma-ray Large Area Space Telescope) was launched Wednesday at 1605 GMT. GLAST was built in a rather interesting manner, in that much of the work was funded by the Department of Energy. In fact, the main instrument on GLAST, the Large Area Telescope was assembled at the Stanford Linear Accelerator Center. It can detect gamma rays at energies between 20 MeV and 300 GeV. Researchers will use GLAST to study some of the most massive and energetic objects known to science."
I'm currently building a detector for photons (= gamma rays) and pions in the range of ~10MeV to a ~100GeV. Our implementation is different, due to our interest in particles other than photons, but the approach is similar. The important point to realize is that you're not detecting the photons, you're detecting secondary particles created by the photons. That's why they have the Tungsten layers in GLAST (people with a smaller budget usually uses lead). Photons passing through it will undergo pair conversion, producing pairs of electrons and positron. You need a heavy material for this purpose, as the interaction probability strongly increases with the charge of the nucleus (Z^2) and its density (proportionally). These pairs are then detected in the silicon microstrip detectors, not the photons themselves.
Since these electron-positron pairs carry most of the energy of the photon (some of it is transferred to the recoiling heavy core), they will in turn radiate of gamma rays of lower energy in a process called Bremsstrahlung. These Bremsstrahlung photons will undergo pair prodution again until the end of detector or until all energy has been absorbed, whatever comes first. This process is called showering. Since GLAST is inside a space vessel it can't be large enough to contain the whole shower, and this is where the Caesium Iodide calorimeter comes in: the charged shower particles leaking out of the first part of the detector will produce light flashes whose intensity and duration which allow the GLAST people to determine the number of shower particles (and maybe rough estimates of their energy) and in turn this will allow them to estimate the energy of the original incident particle.
The constraint of low mass really works against a precise enrgy measurement, but looking at shower shapes the way GLAST does may reveal enough information to obtain halfway reliable numbers.
I'm definitely looking forward to seeing their results. Go GLAST.