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."

Comments

Re:High-energy photon detection

[Anonymous Coward]

GLAST uses inorganic scintillator for energy measurement (a.k.a. calorimetry). This would be optically transparent crystals made of something like CsI, NaI, or whatever, possibly grown with a dopant. I don't know the details for what they ended up using.

I think it may also use semiconductor detectors (probably a silicon microstrip detector?), but for determining directionality rather than energy measurement.

Re:High-energy photon detection

[Bananenrepublik]

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.

Re:High-energy photon detection

[niklask]

My guess is that the secondaries in turn generate photons. Incoming gamma yields neutrons and ions (for example), ions and neutrons make more lower energy gammas, etc etc. As gamma rays at these energies are not too common, it is possible that the detector even can resolve individual "showers" of secondaries. Correct me if I'm wrong, but this is my intuition. You are almost right. The dominant interaction in the high-energy regime is pair-production (as long as there is some material to interact with). When the gamma ray hits one of the interaction layers in the GLAST tracker, it produces an electron-positron pair. These secondaries will also interact (bremsstrahlung) and produce more secondary gammas and e-p pairs. This is a well known concept called electromagnetic showering in particle physics. By studying the shower one can determine incoming direction etc. However, energy measurement is better done with a calorimeter. Almost all high-energy particle experiment use calorimeters to measure particle energies. Someone else already described this in a post. The GLAST LAT has both a silicon microstrip tracker and a CsI calorimeter.

Re:Why not osmium then?

[Bananenrepublik]

Well, I don't know why they decided the way they did. But it is clear that even if a material were desirable from a physics point of view, it might be impossible to use it, due to chemical instability, mechanical instability, cost, prohibitve security requirements during manufacturing, etc.

BTW is there any slashdot story that attracted fewer comments?